15 th International Brick and Block Masonry Conference Florianópolis – Brazil – 2012 HYBRID MASONRY SEISMIC SYSTEMS Abrams, Daniel 1 ; Biggs, David 2 1 Willett Professor of Engineering, University of Illinois at Urbana-Champaign, USA, [email protected]2 Principal, Biggs Consulting Engineering PLLC, Troy, New York, USA, [email protected]Hybrid masonry is a new structural concept for buildings that incorporates the in-plane strength and stiffness of reinforced concrete masonry with the ease of erecting conventional steel framing. Since the masonry structural panels can also serve as architectural elements hybrid masonry has the promise to be highly competitive with conventional lateral force resisting systems including reinforced masonry or concrete shear walls, steel braced frames or concrete or steel moment-resisting frames. This new method has been introduced in the eastern United States for low-rise office and commercial buildings, and has been found to result in reduced cost and expedited construction sequencing since structural and architectural components can be integrated. Coordinated research is currently underway at the University of Illinois at Urbana-Champaign, Rice University and the University of Hawaii at Manoa to investigate seismic behaviour and response of this construction type. This paper provides an introduction to this innovative seismic structural system, and presents an overview of research results done to date. Keywords: dynamic response, earthquake, seismic, hybrid masonry, research, structural design INTRODUCTION One of the most popular building systems worldwide is concrete-frame construction with unreinforced masonry infills. There are regional differences in the type of infill masonry and in the detailing of the concrete structure, though the basic concepts are generally similar. Under moderate intensity earthquakes, many buildings of this type have performed well with minimal damage. However in high intensity earthquakes, common loss of performance is attributed to out-of-plane collapse of the walls, distress of the frame, or in-plane shear failure of the infill. In the United States, steel-framed buildings were introduced in the late 19th century as the demand for taller buildings increased. Using familiar masonry construction methods, it was common to encase the exterior steel frame with masonry for fire protection and to enclose the building aesthetically (Walkowicz 2010). These buildings later were called transitional buildings as they transitioned building construction from loadbearing masonry to frame and curtain wall construction. In the 1940’s, transitional buildings evolved fully into the modern steel-framed structures with cavity wall constriction that we know today. Coincidentally, seismic design of buildings became a major concern in California following the Long Beach earthquake of 1933. In subsequent years, seismic requirements spread throughout the western United States. Over a period of nearly eighty years, seismic design has become common throughout the entire country.
Transcript
15th International Brick and Block Masonry Conference
Florianópolis – Brazil – 2012
HYBRID MASONRY SEISMIC SYSTEMS
Abrams, Daniel1; Biggs, David2 1 Willett Professor of Engineering, University of Illinois at Urbana-Champaign, USA, [email protected]
2 Principal, Biggs Consulting Engineering PLLC, Troy, New York, USA, [email protected]
Hybrid masonry is a new structural concept for buildings that incorporates the in-plane strength and stiffness of reinforced concrete masonry with the ease of erecting conventional steel framing. Since the masonry structural panels can also serve as architectural elements hybrid masonry has the promise to be highly competitive with conventional lateral force resisting systems including reinforced masonry or concrete shear walls, steel braced frames or concrete or steel moment-resisting frames. This new method has been introduced in the eastern United States for low-rise office and commercial buildings, and has been found to result in reduced cost and expedited construction sequencing since structural and architectural components can be integrated. Coordinated research is currently underway at the University of Illinois at Urbana-Champaign, Rice University and the University of Hawaii at Manoa to investigate seismic behaviour and response of this construction type. This paper provides an introduction to this innovative seismic structural system, and presents an overview of research results done to date.
Keywords: dynamic response, earthquake, seismic, hybrid masonry, research, structural design INTRODUCTION One of the most popular building systems worldwide is concrete-frame construction with unreinforced masonry infills. There are regional differences in the type of infill masonry and in the detailing of the concrete structure, though the basic concepts are generally similar. Under moderate intensity earthquakes, many buildings of this type have performed well with minimal damage. However in high intensity earthquakes, common loss of performance is attributed to out-of-plane collapse of the walls, distress of the frame, or in-plane shear failure of the infill. In the United States, steel-framed buildings were introduced in the late 19th century as the demand for taller buildings increased. Using familiar masonry construction methods, it was common to encase the exterior steel frame with masonry for fire protection and to enclose the building aesthetically (Walkowicz 2010). These buildings later were called transitional buildings as they transitioned building construction from loadbearing masonry to frame and curtain wall construction. In the 1940’s, transitional buildings evolved fully into the modern steel-framed structures with cavity wall constriction that we know today. Coincidentally, seismic design of buildings became a major concern in California following the Long Beach earthquake of 1933. In subsequent years, seismic requirements spread throughout the western United States. Over a period of nearly eighty years, seismic design has become common throughout the entire country.
15th International Brick and Block Masonry Conference
Florianópolis – Brazil – 2012
Essentially, none of the transitional buildings were designed for seismic loadings. However, some of the finest remaining US buildings from the early 1900’s are transitional buildings. Transitional buildings have maintenance problems that are rather unique. These include corrosion of the exterior frame, masonry cracking due to a lack of vertical control joints and horizontal soft joints, and cracking from differential thermal effects on materials. Despite these problems, transitional buildings have survived some extreme events (Biggs 2005) and with that recognition, the concept of hybrid masonry was developed (Biggs 2007). To accommodate modern seismic requirements, the hybrid masonry system relies on the structural strength of reinforced masonry panels that are well connected to the surrounding steel frame. As a new structural concept, hybrid masonry addresses the problems of transitional buildings and offers new opportunities for seismic bracing of steel frames by incorporating the in-plane strength and stiffness of reinforced concrete masonry wall panels with the ease of erecting conventional steel framing. The masonry serves the dual purpose of supporting both out-of-plane and in-plane loadings. This results in a highly competitive system to steel braced frames, shear walls, or moment-resisting frames since masonry structural panels can also serve as architectural elements. This new method has been introduced in the eastern United States for low-rise office and commercial buildings, and has been found to result in reduced cost and expedited construction sequencing since structural and architectural components can be integrated. Large-scale experimental research is currently underway at the University of Illinois at Urbana-Champaign to investigate seismic behaviour of a series of hybrid masonry test structures. The purpose of this research is to extend the use of hybrid masonry to construction markets in areas of moderate and high seismicity. With the development of parallel computational models at Rice University, and associated tests of steel connectors and anchorages in masonry at the University of Hawaii at Manoa, this program of coordinated research will help to demonstrate the seismic effectiveness of hybrid masonry. Laboratory test data, consisting of force, strain, and displacement fields, will be correlated with results of finite element models, which once calibrated, will be used to extrapolate test data to a wide range of structural configurations. As an outcome of this research, new criteria for seismic design of hybrid masonry will be developed, which will be used as the basis for proposed updates to current building codes. HYBRID TYPES Lateral strength of hybrid masonry is provided by reinforced concrete masonry panels, which are engineered and detailed per current codes. There are three types based upon the connectivity of the masonry with the steel frame and their related interaction under in-plane loadings. These types are categorized as Type I, II or III. In concept, Type I is a system of panels at each story that behave as non-loadbearing cantilever shear walls, while panels of Type II are load bearing. Type III panels are anchored to a steel frame along both the beam above and to the steel columns on each side of a panel, and thus resemble a basic infill though the panel is reinforced. Figure 1 shows these types.
15th International Brick and Block Masonry Conference
Florianópolis – Brazil – 2012
Figure 1: Types I, II and III Hybrid Masonry Frames The structural frame is typically designed with simple connections. However, designers can use moment connections to increase the overall stiffness of the system. Axial loads from the framing are transferred to the masonry in bearing. Shear loads are transferred from the beam or girder to the masonry by connectors or fuses. CONNECTORS AND FUSES The connection of a steel beam to the top of the masonry panels must transfer in-plane shear forces. Vertical forces are not transferred to the masonry for Type I construction because a vertical slotted hole is provided in the connector plate or fuse. For Type II, the masonry panel resists vertical loads through direct bearing of the beam flange on the masonry panel. This same connection is used for Type III construction, however, the steel columns are also attached to the sides of the masonry panel with a similar detail. Research at the University of Hawaii-Manoa has found three viable methods for connecting a steel frame to a masonry panel to transfer in-plane lateral shear forces. These include: plate connectors, fuses, and headed stud connectors. A typical steel connector plate is shown in Figure 2 along with the measured hysteretic force-deflection curve for a pair of them working together. Additional data on testing of connector plates can be found in Johnson, et al 2011. The connector can either be bolted or welded to the steel girder. The test results indicate that the strength of a series of connector plates acting
15th International Brick and Block Masonry Conference
Florianópolis – Brazil – 2012
as a cantilever along their length (160 kN for a pair of plates) is sufficient to develop lateral strength of a reinforced panel of masonry. The ductility of the plates is relatively high as is their energy dissipation, however because the vertical reinforcement in the masonry panel is expected to yield at a smaller lateral force than the strength of these plates, nonlinear deformations of the plates will not occur.
A typical fuse connector is shown in Figure 3 along with its measured force-deflection behaviour again for a pair of connectors. As noted, the strength of this fuse (80.1 kN for the pair) is low enough that the flexural or shear strength of a reinforced masonry panel will not be reached and thus the nonlinear inelastic behaviour of the fuse will be realized. The fuse may be welded to the steel beam; however, bolting as shown in the figure is preferred for easy replacement following an earthquake. These test results indicate that such fuses are quite ductile and thus an excellent energy dissipater. The fuse connector is appropriate for all seismic zones as well as wind-dominated areas when using Type I hybrid. Since the fuses provide the system ductility, masonry wall ductility is not needed, and simpler reinforcement details can be used (e.g. such as for an intermediate reinforced shear wall).
15th International Brick and Block Masonry Conference
Florianópolis – Brazil – 2012
As an alternative to steel plate connectors, steel headed stud connectors can be welded to the bottom of the beam flange as shown in Figure 4. These studs are embedded in grout to provide a firm connection with the masonry. Like the plate connectors described previously, this stud connection should be designed so that the vertical reinforcement in the masonry panel below can yield and dissipate energy. Because there is no gap between the bottom of the beam and the top of the panel, this detail is appropriate for Type II and Type III. A similar detail can be used to connect the steel columns to the side edges of a masonry panel for Type III. With this connection, system ductility is dependent upon the wall design and detailing. Analysis and design aspects of the connections and top of masonry wall are outlined elsewhere (Biggs 2011). MASONRY WALL PANELS For seismic zones, the masonry wall panels are typically designed as either intermediate reinforced masonry shear walls or special reinforced masonry shear walls (TMS 402). Intermediate reinforced masonry shear walls require partial grouting and reinforcement spaced horizontally not to exceed 600mm (48in.). Special reinforced masonry shear walls are solidly grouted with reinforcement not to exceed 600mm (48 in.). The ductility of the system is provided by the vertical reinforcement, which must yield as is typical for any masonry shear wall. FULL-SCALE TESTING The overall research program (Abrams 2011) studies the seismic behavior and performance of the hybrid masonry system. Large-scale hybrid masonry test structures will be constructed and tested at the University of Illinois at Urbana-Champaign in the Multi-Axial Full-Scale Sub-Structured Testing and Simulation (MUST-SIM) facility, which is part of the U.S. National Science Foundation (NSF) George E. Brown, Jr. Network for Earthquake Engineering Simulation (NEES). The test structures will include frames with masonry panel height-to-length aspect ratios of 1.0 and 0.5. Two types of panels (intermediate and special) will be tested along with the three connecting methods. Loading and Boundary Condition Boxes (LBCBs) in the MUST-SIM facility will impose a horizontal displacement at the top of the structure while vertical force is held constant. Loadings will be applied at a static rate but be reversed and cyclic. Instrumentation will include strain gages, displacement transducers, and a non-contact coordinate measurement system to track the displacements and strains to determine the load path. A typical large-scale test structure is shown in Figure 5. Steel W12 columns will run the full height and be connected to steel W16 beams at each of two levels. Reinforced concrete masonry panels at each story will be constructed using standard 200mm (8 in.) concrete masonry units and Type S mortar and typical Grade 60 reinforcing steel.
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Figure 4: Typical Steel Stud Connector
15th International Brick and Block Masonry Conference
Florianópolis – Brazil – 2012
The steel frame is designed to remain essentially elastic during testing so that it can be reused throughout the testing program. The masonry and connector plates are designed to develop significant inelastic action and dissipate energy imparted by seismic demands. The steel frame consists of W12x58 columns with W16x57 and W16x77 beams for the tests with panels of aspect ratios of 1.0 and 0.5, respectively.
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Figure 5: Large-Scale Hybrid Masonry Test Structure
All frame members are ASTM A992 steel. The beam-to-column connections are simple shear double angle connections. ASTM A36 steel 2L4x4x1/2 elements were chosen to provide adequate strength while also allowing beam end rotations that roughly realize the engineering approximation of pinned connections. The columns are bolted to anchorages set in the concrete base beam to allow simple replacement of the columns if needed. These elements
15th International Brick and Block Masonry Conference
Florianópolis – Brazil – 2012
can be designed as connectors, which transfer enough force to the masonry panel so that it can reach the ultimate strength level, or fuses that yield and dissipate energy prior to significant inelastic response in the masonry panel. Lateral loading will be done in a cyclic reversed manner with progressively increasing amplitudes of lateral displacement. At the start, masonry panels will be connected to the frame with fuse connectors to provide evidence that global system behavior is indeed controlled by the nonlinearly behaving fuses. Then, these fuses will be replaced with connector plates with sufficient strength to transfer lateral forces to the base-story masonry panel such that vertical reinforcement will yield. Once a masonry panel is damaged extensively, it will be replaced with another masonry panel. The story height for the second story is explicitly made a course of masonry shorter than the height of the first story panel, and shear strength is larger for the second story (more horizontal reinforcement), so that damage will be confined to the first story. Behavior of systems with panels reinforced with different schemes (ordinary, intermediate and specially reinforced) will be explored, as well as behavior of systems with different connection types (Types I, II and III). REDUCED-SCALE TESTING Prior to the first large-scale test, a reduced-scale model of the test structure was tested using the test setup shown in Figure 6. The primary purpose of this test was to confirm loading control algorithms to be used for the large-scale test. However, this reduced-scale test also provided some insight into the behaviour of the large-scale structures since materials and configuration matched the larger counterpart. Technically, this was a one-sixth scale model, however, one-fourth scale concrete block were used because of their availability. Since the blocks were fully grouted, it was felt that this distortion of scale was admissible. Model reinforcement was threaded steel rods, which were anchored to a base plate and to the first-
level steel beam with nuts. Steel beams were connected to steel columns with a simple bolted connection so that the test structure could be easily dismantled and rebuilt. Construction of the first large-scale test structure was delayed until results from this reduced-scale test were available. This scale testing was also quite helpful for educating students on
instrumentation, photography, and data collection and reduction for the series of large-scale tests.
Figure 6: Reduced-Scale Hybrid Masonry Test Structure
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15th International Brick and Block Masonry Conference
Florianópolis – Brazil – 2012
COMPUTATIONAL SIMULATIONS One purpose of the large-scale testing is to provide benchmark data for calibration of numerical simulation models. Once calibrated, these models can be used to extrapolate laboratory findings to a much larger scope of structural configurations through parametric studies. Currently such simulation models are being developed at Rice University (Stanciulescu and Gao 2011). Results are too preliminary to publish at the time of this paper preparation, but comparisons of experimental and simulation models will be given at the conference. In addition to comparisons of load-deflection relations, mesh distortions as calculated with finite element models will be overlaid with non-contact measurements of movements of a series of LED nodes attached to a masonry test panel. This comparison may not be exact because of local effects of cracking in the masonry and load-reversal hysteretic effects, but it will be useful in learning of the precision limits of modeling masonry and understanding overall behavior of the test structure. Experimental data will also be compared with results obtained using commercial software that has recently been developed for design of hybrid masonry structures. CONCLUDING REMARKS Hybrid masonry has a good potential to become an effective structural system for low to mid-rise buildings in areas of all seismicities. Much work has been done to develop the system to date as evidenced by the actual construction of buildings in low seismic zones of the United States. Research outlined in this paper will extend its use in regions of higher seismicity. In particular, design of hybrid masonry systems can fit well within a performance-based design approach where fuse elements can deform inelastically for small or moderate earthquakes, and then be replaced in anticipation of high-intensity shaking. Buildings can be occupied a short time following such earthquakes and be safe against major credible earthquakes of the future. Thus, safety at a relatively low cost with enhanced serviceability is a distinct possibility. As with any innovative construction scheme, adoption by building owners and engineering practitioners is a challenge. However because design of hybrid masonry is governed by existing codes of practice (AISC, TMS, ASCE7, etc.), design professionals can proportion strength proportioning strength to the steel frame members, to the masonry panels and to the connector plates or fuses using familiar requirements. Moreover, they also have analytical tools such as Bentley RAM software for estimating component forces from global loadings in hybrid masonry structures. The large-scale tests underway at the University of Illinois will provide the evidence needed to confirm these modeling and design approaches, and thus help to promote the construction of hybrid masonry not only in the United States, but internationally as well. ACKNOWLEDGEMENTS This research is supported by the National Science Foundation under Grant No. CMMI 0936464 as part of the George E. Brown, Jr. Network for Earthquake Engineering Simulation. Appreciation is extended to team members Larry Fahnestock, Ian Robertson, and Ilinca Stanciulescu, and their graduate students. Partial support from the American Institute of Steel Construction, the National Concrete Masonry Association, and the International Masonry Institute is gratefully acknowledged.
15th International Brick and Block Masonry Conference
Florianópolis – Brazil – 2012
REFERENCES Abrams, D.P., “NSF-NEESR research on hybrid masonry seismic structural systems,” Proceedings of 11th North American masonry Conference, Minneapolis, MN, USA, 2011, paper 2.01-2. Biggs, D., “Using hybrid masonry bracing for steel frames,” Proceedings of 11th North American masonry Conference, Minneapolis, MN, USA, 2011, paper 2.01-1. Gregor, T., Fahnestock, L.A., and Abrams, D.P. “Experimental evaluation of seismic performance for hybrid masonry,” Proceedings of 11th North American masonry Conference, Minneapolis, MN, USA, 2011, paper 2.01-6. Johnson, G., Robertson, I.N., Goodnight, S., Ozaki-Train, R., “Behavior of energy dissipating connectors and fuses,” Proceedings of 11th North American masonry Conference, Minneapolis, MN, USA, 2011, paper 2.01-5. Gao, Z., Stanciulescu, I., “Computational modeling of hybrid masonry systems,” Proceedings of 11th North American masonry Conference, Minneapolis, MN, USA, 2011, paper 2.01-4. Stanciulescu, I., and Gao, Z., “Computational modeling of hybrid masonry systems,” Proceedings of XI International Conference on Computational Plasticity: Fundamentals and Applications, Barcelona, Spain, September 2011. TMS 402, Building Code Requirements for Masonry Structures, TMS 402-11, ACI 530-11, ASCE 5-11, Boulder Colorado, USA, 2011. Walkowicz, S. W., “Steel framing with masonry walls – an historical perspective,” Proceedings of 2010 ASCE Structures Congress, Orlando, FL, USA, 2010.
