On the Evolution of Energy Dissipation Devices for Seismic Design

On the Evolution of Energy DissipationDevices for Seismic Design

Juan Enrique Martınez-Ruedaa)

A review of the evolution of energy dissipation devices of hysteretic typeis presented. Both friction and yielding devices are included in the paper cov-ering a worldwide range of applications. Due to the increasingly large num-ber of available devices, the paper does not attempt to present a state-of-the-art on the subject, but to focus on discussing the main original researchefforts that have paved the way of the current technology of energy dissipa-tion devices. Relevant past applications of devices are briefly described mak-ing particular emphasis on important issues such as experimental assessment,effectiveness of their modeling by nonlinear analysis techniques, materialsand constructability. Devices selected for discussion in the paper are pre-sented in a historical perspective and are considered pioneer original steps orresearch efforts directed towards an efficient and rational use of energy dis-sipation technology. [DOI: 10.1193/1.1494434]

INTRODUCTION

Conventional methods of earthquake resistant design rely on the ductile behavior ofthe structural members for energy dissipation. When energy dissipation is achieved byinelastic response, the structure is damaged and hence an adequate seismic design re-quires that the structure yields and experiences damage without collapse under a cata-strophic event.

During the last decade, seismic design codes have been increasingly adopting a ca-pacity design approach, which aims to control both the location and the form of inelasticbehavior. The main advantage of this approach is that failure mode control results inmore reliable and predictable sources of energy dissipation while the final design is lesssensitive to the uncertainties associated with ground motion characteristics.

The main disadvantage of conventional seismic design methods is that the structureis susceptible to damage under the action of major earthquakes. The inflicted damagemay be repairable or may even be so serious that the structure must be demolished.

As a response to the shortcomings inherent in the philosophy of conventional seis-mic design, a number of innovative approaches rely on the incorporation of energy dis-sipation devices in the structure. The devices are used to protect the structure from dam-age by absorbing energy in elements that are designed to be accessible, easilyreplaceable or returnable after a major event. By following the latter approach, the load-

a) Facultad de Ingenierıa de la UAEMex, Manuel Doblado 13, Pilares, Toluca, Estado de Mexico, Mexico 52140

309Earthquake Spectra, Volume 18, No. 2, pages 309–346, May 2002; © 2002, Earthquake Engineering Research Institute

310 J. E. MARTINEZ-RUEDA

carrying function of the structure can be separated from the energy dissipation function.Furthermore, the energy dissipation characteristics of the structure can be more easilydetailed and optimized.

In general, the aim of including energy dissipation devices in a seismic structure is toconcentrate the dissipation of hysteretic energy in specially designed and detailed re-gions of the structure and to avoid inelastic behavior in members of the lateral forceresisting system (LFRS), except perhaps under a catastrophic event (Aiken et al. 1992).However, in the case of the redesign of an existing structure, the onset of inelastic be-havior in members of the LFRS may be unavoidable. Hence, for seismic redesign theincorporation of energy dissipation devices will aim at minimizing the inelastic behaviorof the existing LFRS.

New trends in seismic design have resulted in proposals of several innovative earth-quake protection strategies. To adopt any of these strategies, the designer has to choosefrom an increasingly large number of options. The final decision is usually controlled byeconomic considerations and by contrasting several alternatives from different perspec-tives, including the comparisons active devices vs. passive devices and base isolation vs.energy dissipation.

The main objective of this paper is to review the evolution of available technology onenergy dissipation devices of hysteretic type. The review is not intended to be exhaustiveas new devices are introduced on almost a daily basis. Instead, relevant worldwide pastapplications of devices are briefly described making particular emphasis on importantissues such as experimental assessment, effectiveness of nonlinear modeling techniques,materials and constructability. On the other hand, some state-of-the-art literature on en-ergy dissipation devices limited to North America or to specific countries is listed else-where (Hanson 1993, Ciampi 1995, Ruiz 1998), hence the current review may be con-sidered as a complement to that literature. Devices selected for discussion in the paperare presented with a historical perspective and, according to the author, represent pioneeroriginal steps or research efforts towards an efficient and rational use of energy dissipa-tion technology.

SEISMIC DESIGN PHILOSOPHY USING ENERGY DISSIPATION DEVICES

Traditionally, seismic design has been performed following a ductility-based ap-proach that relies on the application of force reduction factors (or behavior factors) tospecify the design seismic forces. These factors account in an approximate fashion forthe nonlinear behavior of the structure under the design earthquake, and implicitly con-sider the energy dissipation capacity of the structure as well as the deformation capacityof the structure. Therefore, ductility-based design aims at ensuring that the seismic duc-tility demand does not exceed the ductility supply.

Alternatively, energy-based design methods have been proposed over the last fortyyears. In general, these methods are based on the assumption that the energy demandduring an earthquake can be predicted and that the energy supply of a structure can alsobe assessed. Following this approach, a satisfactory design implies that the energy sup-ply should be larger than the energy demand.

ON THE EVOLUTION OF ENERGY DISSIPATION DEVICES FOR SEISMIC DESIGN 311

After the pioneering work of Housner (1956), many authors have investigated thepossibility of developing design procedures based on an energy balance approach. Al-though significant insight has been gained, a satisfactory design procedure based purelyon energy concepts has not been achieved; this is mainly because energy dissipation ca-pacity cannot be used as univocal measure of damage (Serino 1995). Nevertheless, forthe study of the seismic response of structural systems it is becoming customary to ana-lyze the balance of the energy input and to use energy considerations to improve seismicdesign. This is particularly important in the case of structural systems incorporating en-ergy dissipation devices, where a comparison of the energy balance of the structure withand without devices provides a way of evaluating the efficiency of both systems in termsof the energy dissipated by the primary members of the LFRS.

A number of devices with high energy-dissipation capacity have been developedover the last two decades. Generally, the devices are linked to the main members of theLFRS and perform as enhanced semi-rigid connections or joining elements with highdamping characteristics. Friction-damped devices, yielding steel inner frames, ADAS(added damping and stiffness) elements are examples of devices introduced at criticaljoints of steel bracing members. Devices used in this way are designed to deform duringsevere seismic excitation before yielding or buckling in the primary members takesplace. Therefore, the main members of the structure remain practically undamaged andenergy dissipation is achieved through the hysteretic behavior of the devices. This resultsin a significant increase of damping and consequently lower displacements and forcesexperienced by the structure.

A successful application of energy dissipation devices reduces the amount of energythat enters the structure. This can be achieved by

1. Limiting the energy entry at the source by making use of energy-avoiding de-vices at foundation level, i.e., by making use of a base isolation system.

2. Adding energy dissipation devices to the structure, from the foundation levelupwards.

The hysteretic energy is associated with the inelastic behavior of the structure, henceit may be considered as seismic damage energy. Detailed analyses of the energy balanceof earthquake-resistant structures have shown that the greatest contribution comes fromhysteretic energy (Dowrick 1987). Accordingly, special care should be taken in develop-ing adequate hysteretic damping although, as indicated earlier, the development of hys-teretic damping is associated with a certain amount of damage. However, most of thisdamage in structures with energy dissipation devices can be directed towards the de-vices.

Source-sink analogies are particularly useful to understand the way the seismic inputenergy is dissipated. Figures 1a and b show the source-sink analogies proposed by Popovet al. (1993) to explain the different energy paths encountered in conventional structuresand those incorporating a base isolation system. For the conventional system, the seis-mic input energy Ei beyond the sum of elastic strain energy, Es , kinetic energy, Ek , andviscous damping energy, Ej , is entirely dissipated as hysteretic energy Eh . In contrast,for a base isolation system, the Ek of the first rigid body mode is damped out by theisolator.


The analogy of Figure 1c illustrates how the seismic damage energy is shared by theenergy dissipated by the structure Eh and that dissipated by the devices Ehd . As pointedout later, for the design of new structures Eh can be minimized to zero, whereas in thecase of an existing one the inherent ductility and strength of the structure will dictatehow much Eh can be reduced.

HYSTERETIC DEVICES AND TYPICAL APPLICATIONS

As mentioned earlier, most of the input energy is dissipated by hysteresis in an earth-quake resistant structure. Accordingly, a large number of energy dissipation devices withhigh hysteretic damping have been developed during the last two decades. It should benoticed that most of the energy dissipation devices were originally conceived as integralparts of base-isolation systems and then adopted for braced systems. A considerableamount of experience has been achieved in the use of passive devices of yielding or fric-tion type. Early developments on energy dissipation devices of yield type started in Ja-pan in the late 1960s (Muto 1969) and in New Zealand in the early 1970s (Kelly et al.1972), whereas pioneering work on friction devices occurred in Canada in the early1980s (Pall et al. 1980). During the last decade the development of yield devices hasreceived considerable attention in Italy (Ciampi 1995). Further adaptations of technol-ogy on energy dissipation devices have occurred mainly in the United States (Whittakeret al. 1991, Grigorian et al. 1992).

In general, hysteretic devices developed to date may be classified as either yieldingor friction devices. In general, these devices are fabricated from traditional materials andif properly designed require little maintenance, and offer an economic and reliable al-ternative for energy dissipation in passive control systems.

Another important issue is that of device efficiency affected by high temperature in-crements. In contrast with the case of viscoelastic devices, a number of experimental

Figure 1. Source-sink analogies for seismic input energy dissipation.


studies have shown that hysteretic devices are virtually unaffected in their energy dissi-pation capacities as they transform energy into heat under seismic response.

FRICTION DEVICES

During the last two decades, the behavior of friction devices has shown great poten-tial and has become the subject of extensive experimental and analytical studies.

DEVICES ATTACHED TO WALL ELEMENTS

Tyler (1977a) pioneered the application of friction devices in seismic design. He pro-posed a method to reduce damage in infill panels while providing the building with anacceptable level of damping. As shown in Figure 2, the system introduces sliding ele-ments of polytetrafluoroethylene (PTFE) that join infill panels with frame members(PTFE is more commonly known under the trademark Teflon). The infill panels are thenallowed to take load within their capacity when the joints slip within a known forcerange. In this way it is possible to provide supplemental damping and at the same timemake a reasonable allowance for the effect of secondary elements in the design of themain structure. PTFE sliding elements consist of layers of PTFE joint by a high-strengthbolt to steel plates and infill panels. Teflon was selected as the friction material becauseof the good energy absorption characteristics as indicated by a near-rectangular hyster-esis loop observed during tests of the joints under harmonic excitation (Tyler 1977b).

Although the application of friction devices proposed by Tyler appears very promis-ing, he did not undertake studies of his proposal using analytical or experimental modelsof a framed building under seismic loading. Consequently, no guidance was providedwith respect to how the proposed devices should be calibrated to achieve acceptablebuilding performance.

Pall et al. (1980) developed friction devices for the passive seismic control of precastand cast-in-place concrete walls. Figure 3 shows possible locations of friction joints inshear walls, as well as typical details for the friction joints. The slipping friction joints

Figure 2. PTFE sliding elements (Tyler 1977a).


consist of heavy-duty brake lining pads (Ferrodo) inserted between sliding steel platesjointed by high-strength bolts. The friction joints must be carefully engineered to ap-proach ideal elastoplastic behavior. The slip load of the joint is determined by the coef-ficient of friction and the clamping force on the joint. The desired clamping force isprovided using high-strength bolts. Experimental results have shown (Pall and Marsh1981) that the hysteretic behavior of the slipping friction joints is in fact reliable andrepeatable, and approaches a rectangular hysteretic loop with negligible degradation overmany more cycles than encountered in successive earthquakes.

By creating vertical joint lines inside concrete walls and using carefully engineeredfriction joints to couple them together, the following beneficial characteristics of theproposed system emerge:

• The walls act monolithically during service load conditions including wind andmoderate earthquakes.

• The flexibility of the system increases as the friction joints slip during majorearthquakes resulting in effective period elongation which in general results inreduced seismic accelerations.

• The friction joints dissipate a large portion of the seismic input and delay thepossible onset of inelasticity in the LFRS.

Pall and Marsh (1981) performed a series of nonlinear time-history analyses of aconcrete wall of an apartment building of 20 stories for several earthquake intensities.The earthquake record of El Centro 1940 NS component was used in the study. Theanalyses showed that the friction joints should be tuned in order to optimize seismic per-formance. Because of the very low displacements encountered in wall systems, the cri-terion followed to define optimum device tuning was the minimization of stresses at thebase of the walls. Comparisons of the building performance assuming full monolithicwall action (i.e., equivalent to the case of friction joints with infinite strength) and with

Figure 3. Friction joints in concrete walls (Pall and Marsh 1981).


slipping friction joints showed that these joints reduced effectively maximum values ofshear, bending, deflections and overturning moments of the walls in as much as 25%,30%, 40%, and 20%, respectively.

The studies of Pall mentioned above introduced the energy dissipation devices as aneffective way to control failure and optimize hysteretic damping. However, the studyconsidered only one earthquake record and hence the influence of the seismic excitationon the efficiency of the proposed structural system was not assessed. Furthermore, noguidance can be derived from the study in relation to a practical way to tune the devicesto optimize seismic response.

DEVICES INSTALLED IN BRACING SYSTEMS

Further research carried out by Pall led to the development of friction devices thatare assembled in the intersection of steel bracing. Figure 4 shows several friction devicesproposed by Pall (1983) for tension-only and tension-compression bracing systems. Theproposed devices aim at solving the drawbacks encountered in the performance of steelbracing.

Figure 4. Friction devices for bracing systems (Pall 1983).


Braced frames are an economical solution for the control of lateral deflections due towind and moderate earthquakes. However, during major earthquakes, these structures donot perform well. The main deficiency of these frames is their very limited energy dis-sipation capacity as indicated by the extremely pinched hysteretic behavior of the brac-ing system. This deficiency is even worse when the brace is designed to be effective intension. A tension-only brace elongates during high interstory displacements and buck-les in compression during load reversal. On the next application of load in the same di-rection, the elongated brace is not effective even in tension until it is taut again and isbeing stretched further. As a result, energy dissipation degrades very quickly.

In contrast, in the bracing system proposed by Pall (1983) each brace is providedwith a friction device. The device is designed not to slip under normal service loads andmoderate earthquakes. During a severe earthquake, the device slips at a predeterminedload, before yielding occurs in the other structural elements of the frame. If the bracesare designed not to buckle in compression then a simple slotted friction connection canbe used to slip in tension and compression. More commonly used tension-only bracingsystems require special mechanisms as shown in Figure 4a. When tension in one of thebraces forces the joint to slip, it activates the four links that force the joint in the otherbrace to slip simultaneously.

Analytical studies reported by Pall (1983) compared the effectiveness of differentLFRS for a ten-story steel frame. The systems included were a moment-resisting frame,a braced moment-resisting frame and a friction-damped braced frame. Inelastic dynamicanalyses were performed using the earthquake record of El Centro 1940, NS component.Results showed that the incorporation of friction devices in the bracing system dramati-cally improves the overall seismic response. For the earthquake considered, deflections,beam moments, column moments and base shears of the friction-damped frame wereabout 40%, 70%, and 70%, respectively, of the other systems. The moment-resistingframe of the friction-damped structure showed elastic behavior over the entire responsehistory. In contrast, the rest of the systems experienced yielding in a large number ofmembers.

Baktash and Marsh (1986a) proposed a simple friction-damped bracing system. Asshown in Figure 5 the braces are connected to the structure by bolting through steel gus-

Figure 5. Friction device for bracing systems (Baktash and Marsh 1986a).


set plates with slotted holes. Brake lining pads are inserted at both sides of the plates.The pressure to control the slip force in the friction joints is provided by a system ofspring plates. A four-story large-scale steel frame with the proposed friction devices wastested under sinusoidal excitation on a shake table. Different slip forces were consideredin the study. The excitation was based on a constant acceleration amplitude sweepingthrough a frequency domain of one to ten cycles per second. For this type of excitationthe occurrence of the resonant frequency was passed through for all values of the slipforce. The analysis of the test results revealed that the experimental optimum slip forcewas very close to the optimum slip force derived from analytical studies (Baktash andMarsh 1986b). The analytical studies included the nonlinear time-history analysis of aten-story friction-damped braced frame subjected to the earthquake record of El Centro1940 NS component scaled by factors 0.5, 1.0, and 2.0. The condition of optimum slipforce was found to be that when the shear force is shared equally between the columnsand the braces, thus the story shear force causing the braces to slip is equal to the shearforce causing the rigid frame to yield.

The studies of friction-damped braced frames by Baktash and Marsh reported aboveprovided guidance on the tuning of friction devices. Because optimum device strengthdetected in experimental studies under harmonic excitation coincided with that of ana-lytical studies for seismic excitation, Baktash and Marsh suggested that optimum devicestrength in friction-braced frames is insensitive to ground motion. However, it should benoticed that only one record was used for the study.

Filiatrault and Cherry (1987, 1988, 1990) conducted further experimental and ana-lytical studies on the application of the friction device for cross-bracing proposed byPall. Seismic tests of a 1/3-scale model of a three-story steel frame were conducted on ashaking table under the action of various accelerograms with different intensities ex-pressed in terms of peak acceleration. The Newmark-Blume-Kapur (NBK) artificialrecord (Newmark et al. 1973) and the earthquake record of Taft 1952 S69E componentwere used in the experimental program. The beam-column connections of the modelwere designed in a way that allowed the structural model to be transformed into afriction-damped braced frame, a braced moment-resisting frame or a moment-resistingframe as needed. Figure 6 shows comparisons of overall experimental performance forthe NBK and the Taft records, respectively. It is observed that test results compare wellwith analytical predictions and show the enhanced performance of the friction-dampedbraced frames (FDBF) when compared to conventional moment-resisting frames (MRF)and braced moment-resisting frames (BMRF).

The response of a single-story braced structure equipped with the friction device wasanalytically studied under sinusoidal ground motion. For this type of excitation, the con-dition of minimum displacement response was found to be

2Pop cos a

mag5FSTbr

Tun,Thg

TunD (1)

where Pop is the optimum local slip load of friction damper, a is the angle of inclinationof diagonal cross braces with the horizontal, m is the total mass of the structure, ag is the


peak ground acceleration, Tbr is the natural period of the fully braced structure (no slip-page in devices), Tun is the natural period of the unbraced structure, Thg is the period ofthe harmonic ground motion, and F is an unknown function.

Equation 1 indicates that the optimum slip load depends on the frequency and am-plitude of the ground motion and is not strictly a structural property. Under earthquakeexcitation, Filiatrault and Cherry (1990) proposed that the optimum slip-load distribu-tion will be dependent on the characteristics of the earthquake ground motion antici-pated at the building construction site. This proposal was investigated in the parametricstudy described below.

A parametric study using 45 structural models, which included one-, three-, five-,and ten-story steel frames, was conducted. A stochastic representation of earthquakeground motion accounting for a wide range of peak ground accelerations ag and pre-dominant ground period Tg was used in the study. The combinations of artificial earth-quake accelerograms and structural parameters resulted in 4,880 nonlinear time-historyanalyses. From these analyses an approximate design equation for the total optimum slipshear Vo was derived based on statistical regressions.

It is important to notice that the total optimum slip shear Vo is based on a uniformdistribution of device slip load. Filiatrault and Cherry (1988) observed that very littlebenefit is derived from the use of an optimum distribution of device strength when com-pared with the use of the simpler uniform distribution. Therefore Vo is given as:

Vo5(i51

Ns

(j51

Ndi

2Poij cos aij (2)

where Poij is the local slip load for the jth friction device in the ith story, aij is the angleof the inclination from the horizontal of the jth braces, Ndi is the number of friction de-vices in the ith story and Ns is the number of stories in the frame. A closed-form solutionfor Vo was derived and is given by Equations 3 through 6:

Figure 6. Summary of analytical results obtained by Filiatrault and Cherry (1987, 1988, 1990)on friction-damped braced frames under seismic loading.


Vo

mag5a

Tg

Tunfor

Tg

Tun<1 (3)

Vo

mag5a1

b2a

14 F Tg

Tun21G for

Tg

Tun.1 (4)

a5~21.24Ns20.31!Tbr

Tun11.04Ns10.43 (5)

b5~21.07Ns20.10!Tbr

Tun11.01Ns10.45 (6)

The studies of Cherry and Filiatrault summarized above culminated in a design spec-trum that takes into account the properties of the structure and of the ground motionanticipated at the building site. This spectrum provides an effective simplified methodfor the design of braced frames incorporating friction devices and hence it may lead togreater acceptance by the engineering profession of this novel seismic design concept.

Anagnostides et al. (1989, 1990) proposed a new type of friction device for tension-only cross-bracing. Two variants of the proposed device are shown in Figure 7. In con-trast with the friction device proposed by Pall (1983) in which the friction joints slipfollowing a linear trajectory, the proposed device presents a simpler design based on theuse of rotational friction joints. These joints consist of frictional washers bolted to steelplates and distribution washers by high-strength bolts. The strength of the device de-pends upon the material and dimensions of the washers and the pressure applied by thebolts. Several friction materials were tested under cyclic friction using a rotational fric-tional joint. Materials tested included cast iron, Nitroy 40B, stainless steel, ground flatstock, FF Ferrodo friction material, and 3501F friction material. In terms of hystereticbehavior 3701F Ferrodo was found to be the best from all the friction materials tested,showing predictable and constant slipping load for a number of cycles expected duringearthquake excitation. However, 3701F Ferrodo friction material experiences creep un-

Figure 7. Friction devices for cross-bracing proposed by Anagnostides et al. (1989, 1990).


der permanent compressive stress and, hence, long-term performance of friction deviceshas to be carefully considered in the design. It was found that by retightening the devicesafter two periods of 14 days, creep was eliminated as a potential problem.

Two reduced-scale structural models were tested under seismic loading imposed by ashake table. The input of the shake table were artificially generated earthquake recordshaving dominant frequencies close to the fundamental frequency of the models. The firstmodel tested was a 1/3-scale model representative of the first story of a multistory steelbraced frame (Anagnostides and Hargreaves 1990). The second model tested was a 1/6-scale model representative of the first two stories of a multistory steel braced frame(Anagnostides et al. 1989). Figure 8 compares the force-displacement response of thetop story for the two-story model with and without devices under the same earthquakerecord. These results confirmed the enhanced performance of friction-damped cross-bracing observed previously by Filiatrault and Cherry (1987, 1988).

The main contribution of Anagnostides et al. for the further development of friction-damped bracing systems was the adoption of rotational friction as opposed to transla-tional friction used by previous authors. It is expected that rotational friction devices areeasier to construct and, consequently, cheaper. In addition, more consistent hystereticfriction behavior may be achieved in rotational friction devices because the geometry ofthe frictional sources favors the application of a more uniform clamping pressure on thefrictional material.

FURTHER DEVELOPMENTS IN ROTATIONAL FRICTION

Takai et al. (1988) proposed a rotational friction device for base isolation. The deviceconsists of a rather sophisticated assembly of four friction dampers such as the oneshown in Figure 9. The axial force in the clamping bolt is transmitted to the frictionsurfaces through coned disk springs to keep pressure constant. Rotational friction isachieved between brake lining disk pads and stainless steel disks welded to the innersurface of upper and lower plates. Extensive analytical and theoretical studies allowedthe assessment of the efficiency of the device and the validation of theoretical models topredict the dynamic response of isolated structures incorporating the proposed device.Dynamic tests of the device under harmonic excitation revealed that the friction force is

Figure 8. Response for a two-story frame with and without devices (Anagnostides et al. 1989).


virtually unaffected by the velocity of sliding. Shake table tests of the device using amodel of a base isolated structure were performed using sinusoidal waves and the earth-quake records of El Centro 1940 NS, Taft 1952 EW, Tokachioki-Hachinohe 1968 NS,and Miyagiken-oki-Tohoku 1978 NS. The analysis of results indicated good agreementbetween experimental and analytical results obtained by a simple bilinear model. Thetests also showed the importance of a good connection between the device and the struc-ture in order to have total control of the hysteretic behavior of the structure.

The study of Takai et al. confirmed the adequacy of simple bilinear models to pre-dict the hysteretic behavior of well-designed friction devices. Also this study showed thatthe use a constant coefficient of friction, i.e., independent of velocity of sliding, may beconsidered as a reasonable assumption in the design of friction devices.

A DEVICE FOR WALL STRUCTURES

Filiatrault (1990) proposed a technique to enhance the seismic performance of woodstructures. As illustrated by Figure 10, the technique relays on the incorporation of fric-tion devices in the corners of timber-sheathed wall structures. The proposed techniquewas studied using an analytical model of a three-story apartment wood building. Non-linear analysis techniques were used to study the model under the earthquake records ofEl Centro 1940 NS, San Fernando 1971 N21E, and Bucharest 1977 NS. Results clearlyshowed the improvements in the hysteretic behavior of timber wall structures incorpo-rating the proposed devices. Pinching in the hysteretic loops is eliminated allowing largeenergy-dissipation capacity at small deflection amplitudes. About 60% of the hystereticenergy is dissipated by friction in the wall devices. Furthermore, because of the devel-opment of smaller inertia forces and deflections, the seismic energy input is substan-tially reduced for the friction-damped wall.

The study of Filiatrault reported above may be considered as a step forward in ex-tending the application of friction devices for the case of wood structures. The proposed

Figure 9. Friction device for base isolation (Takai et al. 1988).


technique does not interfere with architectural or construction requirements and mayalso be applied for the seismic upgrading of existing wood structures.

DISSIPATIVE STRUTS

A sophisticated Japanese friction device known as a Sumitomo device is shown inFigure 11. It consists of friction pads that slide directly on the inner surface of a cylin-drical steel casing. This device was originally conceived as a shock absorber in railwayrolling stock and it was later applied as a friction damper for steel bracing. Large-scaletests of friction-damped braced frames incorporating Sumitomo devices have been re-ported by Aiken et al. (1992). A 1/4-scale nine-story steel frame was tested on a shaketable under the action of fourteen natural earthquake records. Test configurations of themodel structure included a moment-resisting frame and moment-resisting frames withchevron bracing with and without friction devices. Preliminary tests on individual damp-ers showed that the device is independent of loading frequency, amplitude, number ofloading cycles, and temperature. As expected, the Sumitomo device showed regular andrepeatable hysteretic behavior with no variation in slip load during earthquake motion.The friction-damped model did not exhibit yielding or buckling in any of its membersfor the entire ensemble of records. In contrast, several plastic hinges were formed in theother two models and several braces buckled in the braced frame model without devices.In general, drifts in the friction-damped braced model were reduced by 10 to 60% overthose of the moment-resisting frame model, while story accelerations were reduced by25 to 60%. Analytical predictions for the response of the friction-damped frame agreed

Figure 10. Friction device for wooden panels (Filiatrault 1990).

Figure 11. Sumitomo friction device (Aiken et al. 1992).


well with experimental results. This confirmed once more the fact that the stable hys-teretic behavior of the friction devices makes them particularly amenable for accuratemodeling.

An energy-dissipating strut originally developed as a seismic restraint device for thesupport of piping systems in nuclear power plants was also studied by Aiken et al.(1992). As shown in Figure 12, the mechanism of this device is sliding friction througha range of motion with a stop at the end of the range. The outstanding features of thedevice are its self-centering capability and that the frictional force is proportional to thedisplacement. In fact, this self-centering behavior would tend to reduce permanent off-sets if the structure were deformed inelastically. Figure 13 shows typical experimentalhysteresis loops for different adjustments of the device. A 1/5-scale model of a two-storysteel frame was tested on a shake table under the earthquake records of El Centro 1940NS and Michoacan, Zacatula 1985 NS. Results indicated that the addition of energy-dissipating struts reduces the model deformations and interstory drifts consistently withincreasing slip load. A greater reduction of response was observed in the case of theZacatula record. Hence, it was suggested that bracing systems incorporating energy-dissipating struts are more effective in reducing the response of structures to relativelyharmonic excitations than for impulsive excitations.

More recently, Filiatrault et al. (2000) have proposed a new version of a dissipativestrut referred by the authors as a friction-based ring spring damper. As shown in Figure14 this device consists of a friction spring created by the assembly of cylindrical wedgesor rings connected in series. As the assembly is loaded in compression the axial dis-placement is accompanied by sliding friction between the conical contact surfaces of therings. The assembly is retained at both ends by cylindrical cups. Under compression, theleft cup compresses the rings while the tie-bar-head slides in the slot of the right cup.

Figure 12. Energy-dissipating strut (Aiken et al. 1992).

Figure 13. Hysteresis loops for different adjustments of the dissipating strut (Aiken et al.1992).


Under tensile load reversal, the right cup is pulled by the tie-bar-head and the frictionsprings are subjected to compression again. This complex interaction between the ele-ments of the device produces a stable symmetrical flag-shaped hysteretic behavior simi-lar to that shown in Figure 13. Shake table tests on a one-story half-scaled moment-resisting steel frame with the device installed in a diagonal brace have been conducted.Results revealed that the damper was effective in reducing the lateral displacements ofthe frame, reducing the acceleration levels under the earthquake excitation that causedyielding of the structure without a damper.

The above studies of Aiken et al. (1992) and Filiatrault et al. (2000) were focused onextending the application of commercially available hardware on energy dissipation de-vices. Although the proposed new applications proved to be effective in enhancing theseismic behavior of braced frames, the hardware utilized appears to be rather sophisti-cated and, hence, more expensive. This would limit the applicability of the bracing sys-tems proposed when compared with available energy dissipation devices with simplerdesigns.

THE SEARCH FOR AN EFFECTIVE FRICTIONAL MATERIAL OF LOW COST

The manufacture of most of the friction devices described earlier requires precisionwork or special materials and their installation demands specialized training. In conse-quence, the additional expense in using such devices has prevented their wide accep-tance in engineering practice. In an attempt to overcome these shortcomings, Grigorianet al. (1992) developed an improved and simpler type of friction device referred to asslotted bolted connection (SBC). As shown in Figure 15 this device consists of a bolted

Figure 14. Friction-based ring spring damper (Filiatrault et al. 2000).

Figure 15. Slotted bolted connection (Grigorian et al. 1992).


connection where the elongated holes or slots in the main connecting plate are parallel tothe line of loading. The main plate is sandwiched between brass shim plates and thestrength of the device is controlled by the tightening of the bolts. In addition, a Bellevillewasher is placed under the nut and direct tension indicator (DTI) washers are placedunder each bolt head. Belleville washers are initially cone-shaped annular disk springsthat flatten when compressed. Past experience on the design of SBCs has shown thatunder large cyclic displacements the lack of Belleville washers results in a quick loss ofbolt tension, which in turn triggers a quick degeneration of the device hysteretic behav-ior. The DTI washers are used to achieve the desired initial bolt tension. These washershave protrusions pressed out of the flat surface. As the bolt is tightened, the compressiveforce exerted on the DTI flattens the protrusions and reduces the gaps between the flatportions of the DTI and the head of the bolt. The gaps can easily be measured with asupplied feeler gauge. When the feeler gauge fails to enter a specified number of gaps,the desired load in the bolt has been reached.

As illustrated in Figure 16, tests of SBCs with brass shims under reversed cyclicloading demonstrated the repeatable and reliable hysteretic behavior of these friction de-vices. These connections were also subjected to displacement histories derived fromsimulated earthquake responses. The earthquake record chosen first was the 1971 Pa-coima Dam S16E record, followed by the 1952 Taft N21E record with magnificationfactor of five, the 1940 El Centro S00E record with a magnification factor of two, andthe 1987 Sylmar N00E record of the Whittier Narrows earthquake with a magnificationfactor of forty. It was observed that the slip force drops insignificantly for the next ap-plied displacement history. The rectangular shape of the hysteresis loops, coupled withthe reasonably constant slip force, indicates that the assumption of elastic-perfectly-plastic behavior for SBCs with brass insert plates is a valid one.

The final verification of the SBCs was performed by testing a three-story, one-baysteel frame with chevron braces connected with SBCs at each level. The model was sub-jected to over 40 inputs of table motion for different earthquake records and severe sinu-

Figure 16. Hysteretic behavior of slotted bolted connections (Grigorian et al. 1992).


soidal motions. In all cases the performance of the SBCs was highly uniform with nodegeneration of the slip force, corresponding to wear of the brass shims. The experi-ments also showed that the residual deflections of the frame were very tolerable.

The research work of Grigorian and Popov has its major merit in that it resulted inthe development of a reliable friction device with intrinsic simplicity and low cost. Thevalidated good behavior of brass plates as frictional material and the use of availabletechnology to control the tension force of the bolts make the SBCs very attractive be-cause all these materials are easily available. Grigorian and Popov acknowledged that theconcept of frictional slotted bolted connection had been previously proposed by Pall andMarsh (1982) and devoted their research effort to improve the performance of an exist-ing friction device. This pinpoints the significance of adapting and exploring the use ofsimpler materials and manufacture techniques for the fabrication of energy dissipationdevices, in order to make them more attractive in seismic design and retrofit applica-tions.

THE LOW INVASIVITY CONCEPT

A large number of existing structures may exhibit inadequate deformation capacityunder earthquake loading. As a result of this, many conventional and innovative redesigntechniques have been developed and proved effective in reducing the vulnerability of ex-isting structures with anticipated deficient seismic behavior. However, these techniquesmay become so structurally invasive that they give rise to undesired ‘‘side effects’’ suchas significant amount of construction work, large increments in building weight andbase shear, critical alterations to building layout, and severe disturbance to building oc-cupants. In extreme cases, the redesign scheme requires the retrofitting of the foundationsystem due to the large increments of base shear.

In an attempt to provide alternative solutions to the above problems, the author hasproposed low-invasivity redesign techniques based on the local or global incorporationof energy dissipation devices into framed structures. A large number of nonlinear seis-mic analyses have suggested that devices used in this way introduce significant hyster-etic damping as the structure responds to seismic excitation.

The local approach exemplified in Figure 17a was developed as an alternative to theuse of steel bracing with or without devices. When a bracing system is installed in anexisting building several problems may arise. These include significant disturbance tothe occupants and loss of income due to major alterations to the space availability andfunctioning of the building after the intervention. Consider for example the problemsthat would arise if steel bracing with supplemental damping were installed in an openfloor that was explicitly considered in the original architectural design, say a groundlevel used as a parking facility or a department store.

As shown in Figure 17a, for the local approach (Martınez-Rueda 1992, 1996, 1997,1998; Mulas et al. 2000) rotational hysteretic devices are incorporated around expectedplastic hinges. The calibration of the devices is quite simple and consists of assessing themaximum rotational strength of the device that preserves the original collapse mecha-nism of the structure without devices. In general, the increments of strength, stiffness,and base shear are below 20%. Figure 17b shows a detail of the rotational friction de-


vice, which consists of brass and steel plates jointed to steel sections and connectionplates by a high-strength bolt. As in the case of the SBCs of Grigorian et al. (1992), forpractical reasons brass has been chosen as the frictional material and a Belleville washerbeneath the nut may be used as a mechanism to counteract wear due to friction, prevent-ing the loss of the clamping force.

Although the local approach seems effective from the conceptual and economicalview points, it is visualized that in some cases the technique may require the installationof a large number of devices. As a consequence of this, the global approach (Martınez-Rueda 1999, 2000) illustrated in Figures 17c and d has been proposed. The bracing sys-tems of the figure combine the benefits of imposing a passive control of the initial pe-riod of the structure (before devices are activated) with that of a brace with stable

Figure 17. Some alternatives of low invasivity incorporation of hysteretic devices.


hysteretic response and high energy-dissipation capacity (when devices are activated). Itcan be easily visualized that the proposed bracing systems deliberately adopt a geometrythat favors the activation of rotational hysteretic devices at discrete locations of thebraces. The adopted inclination angle of the struts or the geometry of the bracing archmust be decided in accordance with client requirements for aesthetics and space. Hence,with the adopted geometry the loss of space associated with the installation of the bracescan be controlled by the designer. In contrast, the adoption of traditional brace geom-etries such as cross- or chevron bracing is less flexible in terms of allowing for the in-stallation of doors or circulation through the open space originally available in the struc-ture.

For a redesign application using the global approach, it is not enough to calibratedevices for a rotational strength that guarantees stable and elastic response of the steelstruts or arch sectors. To enforce beneficial device activation, very large device strengthsmust be avoided, as they result in excessive gain in lateral strength incompatible with thestrength of the frame members, or the existing foundation. Figure 17e shows some ideason device global incorporation for the case of bridge structures (Martınez-Rueda 1999).

It is important to be aware that the application of the above low-invasivity redesigntechniques modifies the flexural and shear demands of the surrounding regions of theconnections between the assembly of devices and the frame members. Therefore, someof these regions have to be reinforced when needed. In particular, for the case of con-crete frames, the installation of either post-tensioned stirrups (Kahn 1980, UNIDO1983) or local steel jackets (Aboutaha and Jirsa 1996) are examples of effective andsimple techniques to achieve this purpose.

It is also clear, that the low-invasivity techniques can be applied to either new orexisting structures using either friction or yielding devices. Although, these techniqueshave been introduced using friction devices, a number of yielding devices presented be-low can be easily adapted to work in a low invasivity fashion.

YIELDING DEVICES

DEVICES ATTACHED TO INFILL WALLS

As in the case of friction devices, early proposals of yielding devices were directedtowards the enhancement of infilled frame systems. Newmark and Rosenblueth (1971)describe a system of peripheral metal bands to protect infill partitions from earthquakedamage. This system was proposed by Guerrero (1965) and is shown in Figure 18a. Ifthe shearing and normal forces required to make the band yield are chosen properly, it ispossible to have a passive control on the lateral forces that the structure will transmit tothe infill partition and to make use of the energy absorption capacity of the metal bands.

Although the above proposal of Guerrero (1965) may be effective in protecting infillwall partitions and in making a significant contribution to the energy dissipation capac-ity of the structure, the implementation in practice of this system appears to be rathercumbersome and hence expensive. A very large number of connections between metalbands and concrete members are required. This would imply a very expensive construc-tion procedure that includes the provision of steel inserts in the columns, beams, andinfill wall.


Muto (1969) introduced a nonconventional seismic design approach that has beenused in Japan, where a number of tall buildings have been designed with reinforced con-crete infill panels. As shown in Figure 18b, each panel has a set of vertical slits as aresult of which the panel acts as a series of RC columns. When a panel is deformed byinterstory deflection, plastic hinges are formed at the top and bottom of each effectivecolumn, thus absorbing energy.

The slitted RC wall of Muto appears to be the first documented application in builtstructures of energy-dissipating elements based on inelastic behavior. These elementscan be sacrificed during a major event and, in terms of their limited ductility and repair-ability, they may be considered as poor energy dissipation devices for today standards. Infact, while constituting an advance in seismic design, the use of slitted walls may resultin some disadvantages particularly if the panels are used in the redesign of an existingbuilding. The slitted walls add substantial weight to the structure and consequently to theinertial forces generated by the earthquake motion. Moreover, reinforced concrete suf-fers rapid deterioration under cyclic plastic deformation. It would seem more logical touse reinforced concrete to carry vertical loads and use plastically deforming steel to dis-sipate energy. Additionally, in the event of a damaging earthquake, the removal and re-placement of failed slitted walls will certainly involve significant construction work af-fecting considerably the functioning of the building.

DEVICES RELYING ON BAR YIELDING

The development of hysteretic yielding devices for base isolation systems was initi-ated in New Zealand in the early 1970s with the Physics and Engineering Laboratory(PEL) being responsible for pioneering and advanced research. An overview of some ofthese energy dissipators has been presented by Kelly et al. (1972) and Skinner et al.(1975, 1980). Because of the inherent minimum maintenance requirements, the devel-opment of simple devices involving either steel plasticity or lead plasticity has been fa-vored. In particular, research efforts have been directed towards the development of hys-teretic devices of solid cross section with stable behavior at high levels of plastic strain.Because of its composition, mild steel was found to be the most suitable material, pref-erably heat-treated for five hours at 620 7C after fabrication. In design, welding is keptwell away from highly strained zones; otherwise rapid failure takes place.

Figure 18. Incorporation of yielding devices into infill walls.


The first yielding device developed at PEL was the torsion beam hysteretic damper,shown in Figure 19a, for the Rangitikei Bridge, the design of which was carried out in1971. In the device, the sections of the beam between the loading arms are overstrainedin torsion and bending. Tests on a full-scale device of 450 kN capacity with a range ofmovement of up to 80 mm showed that the device provides an effective way of achievinga comparatively large dissipating force using welded plates.

The tapered round steel cantilever shown in Figure 19b was developed to providedamping in any direction in a horizontal plane of a base-isolation system. By adopting atapered section, a large volume of steel in the device is utilized for energy absorption asyield spreads over the entire length of the device. Damping forces of up to 100 kN for amovement of 675 mm can be achieved using steel rounds commercially available.

A flexural beam damper is shown in Figure 20. Horizontal application of loads to theends of the cranked arms cause the circular beam element to behave as an eccentricallyloaded strut. The cranked loading arms are made of cast steel and produce a favorable

Figure 19. Devices based on yielding by torsion or flexure (Kelly et al. 1972).

Figure 20. Flexural beam damper (Kelly et al. 1972).


geometry in that the alternate bowing up and down is compensated for by the inclinationof the arms. This results in a beneficial geometrical effect (smaller P-d effects at largebeam deformations) that produces a near rectangular hysteresis loop. This device hasbeen used in the design of the Cromwell Bridge in New Zealand, for which the bridgedeck was horizontally linked to the fixed abutment using flexural beam dampers. Thisallows damped relative movement during severe earthquake excitation.

One of the main concerns with respect to the performance of the above yield deviceswas the possible embrittlement of the steel following aging after overstrain. In order toassess this problem, some mild steel specimens were subjected to inelastic cyclic defor-mations and allowed to age both naturally and artificially. Tests indicated that for thematerials and strain levels adopted in the design of steel dissipators, embrittlement is notlikely to be a problem following inelastic excursions.

Figure 21 shows a base-isolation system for which energy dissipation capacity isachieved by the yielding of bent mild steel bars. This yielding device was inspired byfield observations of heavily damaged columns due to severe earthquake loading. Theobserved ability of ordinary reinforcing bars in continuing to resist earthquake loadingafter concrete has spalled away suggested that plain mild steel round bars could be usedto provide damping in a base isolation system, provided a bend is introduced into thebar, to allow for extension without premature tensile failure, during excursions in thehorizontal plane. In general, a combination of inelastic bending and torsion occurs inbars with this shape. Test results of bars of various diameters and for varying directionsof imposed horizontal displacement indicate that the energy dissipation of this device isprimarily associated with bending at the fixities region.

One positive attribute of the bent bars is the occurrence of a progressive locking-upmechanism as horizontal deflection increases and the bars straighten. In addition, in theevent of uplift under a catastrophic event, energy will still be dissipated as the barsstraighten vertically. This could be particularly advantageous at the corners of buildings.

Figure 21. Bent mild steel bars as energy dissipation device in base isolation (Kelly et al.1972).


DEVICES RELYING ON PLATE YIELDING

A flexural damper that utilizes a U-shaped steel strip rolled between two surfaces inparallel relative motion has been studied by Kelly et al. (1972). This device was origi-nally considered to be located between flexibly based shear walls as shown in Figure 22.

The mechanism of energy absorption in the U-shaped strips is very simple. The stripis initially in a semicircular form with two equal straight sections on either side. Whenone side is moved relative to the other, the semicircular portion rolls along the strip andwork is done at the two points where the radius of curvature is changed from straight tothe radius of the semicircle and then from this radius to straight again. Thus at any in-stant the energy dissipation is concentrated at two transverse surfaces, but these two sur-faces move along the strip. In general, this device is comparatively flexible in the elasticrange and can be operated with very large displacements in the inelastic range. Testscarried out on a U-shaped strip device under reversed cyclic loading showed that themode of failure is characterized by a localized kincking of the strip followed rapidly bycomplete transverse fracture. Although the use of rollers and stainless steel led to life-times longer than those obtained with mild steel, it was concluded that the expense ofthe rollers and stainless steel is the major disadvantage in comparison to mild steel.Properly designed mild steel U-shaped strips can reliably produce lifetimes in excess of100 cycles.

The tapered-plate device was suggested as a simpler alternative to the torsion-beamtype when space constraints are not too severe. Figure 23 shows a tapered-plate deviceproviding energy dissipation capacity to the base isolation connection of a concretebridge. Another interesting application of the tapered-plate device is shown in Figure 24,which corresponds to a chimney at Christchurch designed to rock on its base. This de-vice was fabricated economically mainly by plate cutting.

The yielding devices primarily developed for base isolation described above stimu-lated the engineering community to use and extend the application of these devices for

Figure 22. Application of U-shaped steel strip as energy dissipation device (Kelly et al. 1972).


the case of non-isolated structures. As indicated in the following paragraphs, the use oftapered yielding devices to protect bracing systems from buckling and yielding has re-ceived considerable attention.

One of the most popular steel-plate energy dissipation devices introduced in the in-tersection of chevron braces are the added damping and stiffness (ADAS) elements.These elements may be considered as an extended application of the tapered yieldingplates that have been successfully applied in base-isolation systems, as discussed earlier.Figure 25 shows a MRF incorporating ADAS elements in the connection between chev-ron braces and floor system. When properly designed and implemented, ADAS elementscan increase the strength, stiffness and energy dissipation capacity of moment-resistingframes. In the case of concentric-braced frames, the introduction of ADAS elements re-sults in a substantial increase in the energy dissipation capacity per unit-story drift.Commonly, ADAS elements consist of mild steel plates of triangular or X shape, as

Figure 23. Tapered-plate device for base isolation (Kelly et al. 1972).

Figure 24. Tapered-plate device for base isolation (Kelly et al. 1972).


shown in Figure 25. The tapered section of the devices allows a uniform distribution ofplastic deformations along the height of the devices. This results in maximum curvaturesand strains in the plates that will be significantly smaller than those in a rectangularplate for similar lateral displacements.

Whittaker et al. (1991) conducted a research program to assess the suitability ofADAS elements for upgrading moment-resisting frames. Several four-plate, six-plate,and seven-plate ADAS elements were tested under monotonic and reversed cyclic load-ing. The mechanical characteristics that most significantly affected the response of anADAS element were found to be the elastic stiffness, yield strength and yield displace-ment Dy . The appropriate design displacement of the ADAS element, compatible withthe demands imposed by the design earthquake, was found to be 3Dy . For a region ofhigh seismic risk, it was proposed that ADAS elements should be able to sustain theirmechanical characteristics for at least 15 to 20 cycles. All of the ADAS elements testedexhibited stable hysteretic behavior up to a displacement amplitude of 3Dy . A seven-plate ADAS element was subjected to a large number of yielding cycles with increasingamplitude between Dy and 14Dy . The strength and stiffness of this element did not de-grade and satisfied the maximum credible earthquake limit state requirements. It wassuggested that the susceptibility of ADAS elements to failure by low-cycle fatigue isnegligible unless their lateral strength is so low that they are subjected to more than 100cycles at a displacement level exceeding 10Dy .

The seismic performance of the ADAS elements was also investigated through a se-ries of shake table tests of a three-story steel moment-resisting frame. The input motionsused in the study included El Centro 1940 NS, Chile, Llolleo 1985 N10E, and a syn-thetic record consistent with the soft soil response spectrum of the 1988 UBC. The framewas tested both without braces and with ADAS elements in the intersection of chevronbraces. Results indicated that the incorporation of ADAS elements improved the behav-ior of the bare frame. The testing program also demonstrated the three major advantagesof braced frames with ADAS elements: (1) inelastic deformations and hysteretic energydissipation can be confined to a limited number of predetermined, easily replaceable el-ements; (2) for minor and moderate levels of earthquake shaking, yielding of the ADAS

Figure 25. Moment-resisting frame with ADAS elements (Whittaker et al. 1991).


elements can result in a significant reduction in interstory deformations, and (3) stablehysteretic behavior of the bracing system can be maintained throughout a severe earth-quake.

An interesting variation of the ADAS elements is the triangular-plate added dampingand stiffness (TADAS) devices developed by Tsai et al. (1992, 1993). As shown in Fig-ure 26, TADAS elements consist of a series of triangular steel plates that connect chev-ron braces to beams of steel frames. In contrast with the ADAS elements where theX-shaped plates are bolted together through two ends of each plate, the triangular platesof the TADAS elements are connected to a base plate using a welded connection as in-dicated in Figure 26. Experimental results have shown that despite the closeness of lightfillet welds to the region of maximum stress in the triangular plates, the TADAS ele-ments exhibit stable hysteretic behavior under cyclically increasing load, with no signsof stiffness or strength degradation for rotation amplitudes of more than 0.29 radians.Following the appropriate cyclic behavior observed in the tests of TADAS elements, atwo-story large-scale steel frame was tested under seismic loading following a pseudo-dynamic procedure. As expected, the observed behavior was very similar to that of steelframes with ADAS elements. Also, analytical predictions using nonlinear analysis tech-niques agreed well with experimental results.

Tsai (1995) points out that the most attractive features in the proposed TADAS ele-ment is that the effects of gravity load in the frame can be completely separated from thedevice by using the slotted holes in the connection details shown in Figure 26. Underlarge deformations of the device, the vertical displacements at the end of the triangularplate can be easily accommodated. Therefore, the plasticity within the triangular plate isgenerated by bending only, and the inelastic response of the proposed TADAS device ishighly predictable. However, it appears that the construction procedure of the TADASelements is far more elaborate than that required to fabricate ADAS elements. Further-more, the dimensions of the ADAS elements may be selected in such a way that the be-havior of the X-shaped plates is primarily controlled by flexure. It is also noted that if

Figure 26. TADAS elements (Tsai et al. 1992).


the need for replacing plates arises after a damaging event, this can be done easier in thecase of ADAS elements for which the plates are fixed by means of a bolted connection.

Martınez-Romero (1993) describes the redesign of three RC-framed buildings inMexico City. In two of the buildings ADAS elements were introduced in the same wayas new buildings, i.e., the ADAS elements are located in the intersection between chev-ron steel bracing and the beam of the existing frame. In the third building (a hospitalbuilding), ADAS elements were introduced in a way that departs significantly from pre-vious applications of these elements. As shown in Figure 27 the basic retrofit schemeconsists of external buttresses linked to building floors through ADAS elements. Soilconditions were so adverse that the use of separate foundations for the buttresses wasineffective in reducing significant soil-structure interaction effects. Hence, the buildingfoundation was extended to support the buttresses as well. Extensive nonlinear time-history analyses were conducted to verify the seismic behavior of the building with ex-ternal buttresses. Results showed a substantial reduction of both base shear and inter-story deflections. This was thought to be the result of the combined effect of the externalstiffening action from the buttresses and the additional damping from the ADAS. Theshear force that the building transmitted to the buttresses demonstrated the effectivenessof this retrofit scheme. In fact, the base shear of the retrofitted building calculated fromthe horizontal reactions on the building columns and the buttresses was considerably lessthan that for the original structure. The reduction in base shear on the building withADAS retrofit was about 50 percent, as compared to the original building. Thus it waspossible to upgrade the building to comply with the new seismic code without additionalstrengthening, i.e., avoiding alterations to the hospital that would disrupt its activities.Interestingly, the use of external buttresses rigidly connected to the building would have

Figure 27. Simplified representation of a retrofitting scheme based on the use of external me-tallic buttresses and ADAS elements proposed by Martınez-Romero (1993).


resulted in considerably larger base shears. This situation would have imposed substan-tial increases in the foundation, as well as in the buttress sections and materials, andlarge forces in the buttress chords.

The above retrofitting scheme proposed by Martınez-Romero is an interesting ex-ample of how the adaptation of existing devices can result in new approaches for theseismic redesign of existing structures. In this particular case, the adopted retrofittingscheme was selected because of its simplicity of execution and minimum interferencewith the day-to-day functioning of the hospital. The applicability of the retrofitting tech-nique based on external buttresses with ADAS elements appears to be constrained tolow-rise buildings with available peripheral space.

The retrofitting scheme of Martınez-Romero is, in a way, an extension of an oldstructural system first proposed in New Zealand for new RC buildings (Skinner et al.1975). The system is shown in Figure 28a and consists of a structural wall linked to aframed structure through hysteretic devices at each floor level. A decade after its pro-posal, Key (1984) studied the performance of the system using simplified nonlinearanalysis techniques based on the global building models shown in Figure 28b and usingfive earthquake records. It was found that owing to the incorporation of hysteretic de-vices, it was possible to introduce an RC wall without paying the penalty of increasedbase shear. Furthermore, as illustrated in Figure 28c, the effectiveness of the system wasfound to be independent of fine device tuning.

An interesting proposal of an enhanced bracing system that combines the economyof tension-only cross-bracing with the good hysteretic behavior of ADAS elements hasbeen proposed by Pocanschi et al. (1990). The proposed system is shown in Figure 29and consists of a closed cross-bracing system working only in tension, fixed at the bot-tom of the columns, and capable of moving at the top corners of the frame. A yieldingdevice very similar to an ADAS element is connected between the frame and the cablebracing in order to control their relative motion. Because of the continuity of the bracing

Figure 28. Simplified analytical study of a frame-wall system with yielding devices (Key1984).


mechanism, the increase in length of a crossbar corresponds to the same length reduc-tion of the other crossbar. So if the cable remains elastic, under cyclic loading both crossbraces are permanently under tension.

The closed circuit-bracing system as proposed by Pocanschi et al. ignores the advan-tages of using X-shaped steel plates such as those used in ADAS elements in order tooptimize energy dissipation and deformation capacity of the plates. Furthermore, as dis-cussed earlier, in order to achieve stable hysteretic behavior, welded connections shouldbe detailed in such a way that welding occurs far from the regions with anticipated maxi-mum ductility demands.

Aguirre and Sanchez (1992) adapted the U-shaped yielding device studied by Kellyet al. (1972) in order to develop a yield device for tension-compression steel bracing.The device, referred to as oval element, is fabricated from mild steel strips and is sche-matically shown in Figure 30. Several cyclic tests to determine optimum device dimen-sions as well as fatigue life have been conducted in the device. Test results show that itis possible to select the dimensions of the device in order to guarantee 100 cycles for adesired maximum displacement. The analysis of the oval elements is rather complex dueto the complex interaction between yielding and rolling mechanisms experienced by thesteel strips.

Figure 29. Closed cross-bracing with yield device (Pocanschi et al. 1990).

Figure 30. Application of oval element as yield device for bracing (Aguirre and Sanchez1992).


The study of Kelly et al. (1972) described earlier suggests that U-shaped yieldingdevices possess low stiffness compared to other yield devices. Although Aguirre andSanchez showed experimentally that the U-shaped devices can be easily detailed to pro-mote stable hysteretic behavior, the authors did not assess the efficiency of the proposeddevice using a structural model under earthquake excitation. This study is needed in or-der to validate the applicability of the proposed device for the design or the retrofit of anearthquake-resistant structure.

YIELDING FRAMES

According to Tyler (1985), the yielding device shown in Figure 31a was first sug-gested in the late 1970s by David Smith and Robert Henry of Auckland, New Zealand.The device, referred to as yielding frame, achieves energy absorption through yieldingin a rectangular frame made of round bars. The yielding frame is geometrically similarto the framework in which it is incorporated and hence the two parts of each diagonalare collinear. Under lateral cyclic loading the device distorts to a parallelogram that pro-motes a stable cyclic behavior without the progressive slack that normally develops incross-bracing under severe seismic loading. A device of this type will only perform sat-isfactorily if the braces are loaded by dynamic horizontal forces oscillating about a zeroload. If there is a permanent dead load component in the horizontal forces then the effectof an earthquake will be to cause a steady movement in one direction with the conse-quent locking up of the device in that direction. Cyclic tests of a cross-bracing systemincorporating the yielding device showed that many hundreds of cycles of loading couldbe completed without the development of slack in the bracing rods. A locking-up effectoccurs for very large deformations but these will normally be outside the range contem-plated in the design. As an example, consider a bracing system tested by Tyler that per-formed well for up to 200 cycles at 1 Hz for lateral displacements equivalent to a drift of3.5% with no development of slack.

Figure 31. Yielding frames as devices for cross-bracing.


One of the major limitations of the yield devices tested by Tyler is that the transversesection of the yielding frame is constant, thus the device favors high concentration ofductility demands around the corners of the device only. Hence, a small volume of thedevice contributes to energy dissipation. Furthermore, the study of Tyler is also limitedin the sense that no tests were performed under earthquake loading.

Ciampi and Samuelli-Ferretti (1990) proposed several yielding devices for cross-bracing. As shown in Figures 31b and 32, the devices consist of a yielding inner framefabricated from steel plates. It can be shown that when forces are applied along the di-agonals, the bending moment diagram in the device varies linearly, with maximum val-ues at nodes and zero values at the center. Hence, to promote a condition of uniformplastification in bending, the member sections of the yielding frame should vary accord-ingly. This can be accomplished by varying the width or the depth of the section, in alinear or parabolic fashion, respectively, as in the case of devices shown in Figures 31band 32, respectively. In order to ensure that plastification occurs primarily due to bend-ing and also to resist shear and axial force, a transition region of constant section is usedin the central part of the device members. The device shown in Figure 32 is lighter andrequires a simple fabrication procedure. In fact, this device can be constructed by cuttinga single plate, avoiding a welding process.

The design of the devices is done assuming a maximum conventional steel strain of3%. The devices proposed by Ciampi and Samuelli-Ferretti have been tested under cy-clic loading. The applied displacement history consisted of uniform cyclic displacementscorresponding to a maximum steel strain equal to 3%. Experimental results for the de-vice with variable width (Figure 31b) showed that the effect of geometric nonlinearitiesproduce an apparent strain hardening in the response of the device. After 30 cycles ofstable behavior, the response progressively degraded until the device broke down in the51st cycle. The failure was attributed to excessive closeness of the welded connections tozones of maximum plastic deformations and to local brittleness by thermal shock due towelding. Tests on a device with variable depth (Figure 32a) showed undesirable cyclic

Figure 32. Yielding frames with cross section of variable depth (Ciampi and Samuelli-Ferretti1990).


behavior associated with the accumulation of axial plastic deformation due to tension.These deformations resulted in the expansion of the size of the device. In real applica-tions, the expansion of the device may compromise the behavior of the entire bracingsystem. In view of this, Ciampi and Samuelli-Ferretti suggested that the performance ofthe device could be easily improved using pinned steel plates parallel to the sides of thedevice as shown in Figure 32b. Such complementary plates work under axial force onlyand are designed in such a way that they remain elastic, assuring that plastic deforma-tions in the device occur primarily in bending. Cyclic tests performed on the modifieddevice have shown that the elongation of one diagonal returns to zero as the other one isput back to zero tensile load. This results in stable hysteretic behavior, proving the ef-fectiveness of the modification of the device.

Nonlinear numerical analyses of steel-braced frames incorporating the devices men-tioned above have shown good agreement with experimental results. On the basis of thisvalidation of the numerical tools to predict the actual behavior of the devices, the designof a real structure incorporating the modified yield device has been conducted. Thestructure is a steel braced frame to be used in one of the buildings of the Laboratories ofthe National Research Council in Frascati, Italy. A series of nonlinear time-historyanalyses using an artificial record compatible with a design spectrum were conductedfor different PGA levels. Under the action of the design earthquake, a comparison be-tween conventional bracing and the proposed dissipative bracing showed the advantagesof the latter ones, in terms of reduction of top displacements, interstory drifts, baseshears, and number of excursions of large amplitude.

The main contribution of Ciampi and Samuelli-Ferretti for the design of dissipativecross-bracing using yielding devices is the attention they paid to the detailing of the de-vices. The device of Ciampi and Samuelli-Ferretti is a better option when compared withthat of Tyler (1985), because it optimizes the sources of energy dissipation and has re-dundancy. Even with the eventual fracture of the yielding frame, the complementarypinned plates guarantee that a bracing system would still be available under the action ofa catastrophic event.

Other types of yield devices proposed by Ciampi and his research group (Ciampiet al. 1993a and 1993b; Ciampi 1995) are those shown in Figure 33. Because of theirshape, they are referred to as E-shaped and C-shaped devices. These devices have a sim-pler design and are applicable to chevron bracing. Like the previous device, they are cutfrom a thick steel plate and work in the plane of the plate, being always in a plane stresscondition. Their geometry is such that it guarantees an almost uniform plastification, atleast for small displacements. Figure 33 illustrates the incorporation of the deviceswithin a bracing system. In this case the braces are designed to react elastically both intension and compression, while the devices deform plastically. E-shaped and C-shapeddevices have also found applications in bridges, as dissipative connections between thebridge deck and piers and/or abutments.

Further research by Ciampi and his coworkers (Ciampi et al. 1993a, Ciampi 1993)has focused on the characterization of the low-cycle fatigue life of yield devices. Be-cause of the crucial role that the yield devices play in protecting the structures in whichthey are used, there is an important concern about their safety against premature failure.


Under random displacement history, the maximum ductility is only one indicator of acomplex behavioral relationship and, hence, can only provide limited information on thedamage in the devices. An alternative measure of damage is the total dissipated hyster-etic energy. This parameter accounts more satisfactorily for the history of deformationand considers dependence of the structural behavior on important factors such as fre-quency and duration of excitation. However, the definition of damage as a function ofabsorbed hysteretic energy presents some drawbacks. In comparing only total dissipatedenergies, there is no account for the fact that cycles are not identical. A more satisfactoryapproach should permit differentiation of the damage associated with a smaller numberof large yield excursions as contrasted with a large number of smaller excursions. Adamage functional DF was proposed in order to define an effective damage index for thedeveloped yield devices. This functional is given as

DF5EFpudpuddp (7)

where Fp is the plastic strength and udpu is the absolute value of the cyclic plastic defor-mation measured from the reversal point. As long as a cycle is not completed, this pointis taken as the latest reversal point.

In the above damage functional, the incremental contributions to energy dissipationare weighted with a measure of the actual amplitude of the plastic deformation. The pro-posed damage functional has been validated using the results of a large number of tests,including some standard constant amplitude tests up to failure, using a range of maxi-mum displacement ductilities up to 24. As indicated in Figure 34, compared with thedissipated hysteretic energy, the damage functional describes a more consistent measureof damage in the devices, for general cyclic response up to collapse.

The above study on the characterization of damage in yield devices appears to be apromising rational criterion to assess the safety of yield devices and on which to basedesign recommendations and acceptance requirements.

Figure 33. E-shaped and C-shaped yield devices (Ciampi et al. 1993b).


CONCLUDING REMARKS

As pointed out by Skinner et al. (1975), when practical hysteretic dampers are avail-able, special structural forms may be adopted to provide earthquake resistance. The ex-perience gained over more than two decades on the design, construction, testing, andmodeling of hysteretic devices is so vast, that the engineering community is now awareof the potential of this novel approach to design or redesign earthquake-resistant struc-tures. In general, desirable characteristics for energy dissipation devices of hysteretictype may be summarized as

• optimum hysteretic behavior (i.e., virtually elastoplastic)

• economic (avoidance of exotic or sophisticated materials is advantageous)

• easy to install

• easy to repair, replace or recalibrate

• adequate long term behavior

• existence of calibration and design procedures/guidelines

A large number of experimental programs have shown the ability of hysteretic de-vices to dissipate large amounts of energy. In parallel with this, analytical models to re-produce device behavior have been proposed and successfully validated. In consequence,nonlinear inelastic analysis can be considered as an adequate research tool to estimatethe response of structures with hysteretic devices. In fact, the adaptation of existing de-vices and the proposal of new device configurations are active research areas that arebeing explored following, in principle, analytical approaches.

The enhanced elastoplastic behavior observed in properly designed hysteretic de-vices has led to a unified approach to model either friction or yield devices. This is par-ticularly advantageous because the adoption of either a yielding or a frictional approachwill depend primarily on economic considerations and the availability of the requiredtechnology.

It is believed that as more buildings are designed incorporating energy dissipationdevices, design codes will eventually address explicitly these new structural forms and

Figure 34. Comparison of different damage indices for yield devices (Ciampi 1995).


design guidelines for the devices will emerge. This will make the design of friction-damped and yield-damped structures simpler and available to a broader part of the en-gineering community.

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(Received 18 December 2002; accepted 19 February 2002)

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On the Evolution of Energy Dissipation Devices for Seismic Design

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