Vienna Congress on Recent Advances in Earthquake Engineering and Structural Dynamics 2013 (VEESD 2013)

C. Adam, R. Heuer, W. Lenhardt & C. Schranz (eds) 28-30 August 2013, Vienna, Austria

Paper No. 479

Abstract. In seismic areas, many existing reinforced concrete (RC) buildings are vulnerable to dynamic actions

and they need to be retrofitted for resisting seismic loads. In order to enhance earthquake resistance, shear walls

are generally used in reinforced concrete (RC) framed buildings, and steel bracing is the most used in steel

buildings. In the past two decades, a number of reports have also indicated the effective use of steel bracing in

RC frames. Therefore, steel bracing systems can be used as a strengthening or retrofitting technique in both RC

and steel buildings. Steel bracing of RC buildings started as a retrofitting measure to increase the lateral load

resisting capacity of existing buildings. In order to explore the lateral load resisting capacity of such a frame,

nonlinear static analysis is needed. Nonlinear static analysis (pushover analysis) under constant gravity loads and

monotonically increasing lateral forces during an earthquake until a target displacement is reached is generally

carried out as an effective tool for performance based design. The major outcome of a pushover analysis is the

capacity curve which shows the base shear vs. the roof displacement relationship and represents the overall

performance of the building. The use of steel bracing for the strengthening and or stiffening, steel braces are

anchored firmly to boundary beams and columns and modeled as truss elements and increase earthquake

resistance. In this paper, nonlinear static analysis of an existing RC building having 3-story with 7 bays in

X-direction, 3 bays in Y-direction retrofitted with buckling restraint braces has been performed using SAP 2000.

The performance of the retrofitted building is studied in terms of base shear forces and energy dissipations. Inter-

story drift demands of the bare frame and strengthened frame with steel braces have also been discussed. Results

indicate that, steel bracing system can significantly increase the structural stiffness and reduce the maximum

inter-story drift of the frame.

Keywords: Nonlinear static analysis; Strengthening; RC structures; Buckling restrained braced frames

1 INTRODUCTION

An extensive number of existing RC buildings in seismically prone regions of Turkey are considered

to be inadequate by current seismic code requirements. These structures would have deficiencies in

lateral strength and/or ductility. Several strengthening techniques of those structures have been

investigated both experimentally and analytically for enhancing the overall lateral stiffness against

seismic motions. Use of steel bracing for strengthening RC frames has some major advantages and

drawbacks. Employing concentric bracing to the RC frames increase overall lateral stiffness and

decrease lateral drift. However, the bending moments and shear forces in columns to which they are

connected decrease, whereas the axial compression increases. In such cases, reinforced concrete

columns are expected to be stronger enough in compression. Eccentric bracing also reduces the lateral

stiffness of the system and improve the energy dissipation capacity (Viswanath et al., 2010). Goel and

Lee (1990) conducted an experimental research on seismic rehabilitation of RC frames using steel

systems. They indicated that proposed technique significantly improved the strength and the stiffness

of RC frames. Maheri and Sahebi (1995) investigated the use of steel bracing in RC frames. They

concluded that with the proper connection between the brace and the frame, the steel bracing could be

alternative to shear walls in concrete framed buildings. Pincheira and Jirsa (1995), Nateghi (1995),

Ghobarah and Abouelfath (2001) studied seismic rehabilitation of RC frames analytically employing

Non-linear static analysis of strengthened existing RC frame building using

steel braces

B. Doran1, B. Akbaş

2, E. Şenol

1, O. Şeker

3

1 Department of Civil Engineering, Yıldız Technical University, 34220 Esenler, Istanbul, Turkey 2 Department of Earthquake and Structural Engineering, Gebze Institute of Technology, Kocaeli, Turkey 3 Illinois Institute of Technology, Department of Civil, Architectural and Environmental Engineering, Chicago,

IL, USA

B. Doran, B. Akbaş., E. Şenol, O. Şeker / VEESD 2013 2

steel bracing. Youssef et al. (2005) investigated the efficiency of using braces in RC frames

experimentally. They conducted two cyclic loading tests on a moment frame and a braced frame. Test

results showed that the braced frame resisted higher lateral loads than the moment frame and provided

adequate ductility.

Buckling restrained braced frames (BRBFs) in which members yield under both tension and

compression without significant buckling have been used in recent years in order to ensure the desired

seismic performance of special concentrically braced frames. Kim and Choi (2004) performed non-

linear static analysis of steel frames with BRBs and investigated the energy dissipation capacity and

the earthquake response. In their study, they observed that as the stiffness of the brace increased, the

maximum displacements of structures generally decreased. Qiang (2005) investigated various types of

BRBFs with different brace configurations in Asia. He indicated that BRBs show the same load

deformation behavior in both compression and tension and higher energy absorption capacity with

easily adjustable both stiffness and strength. Chang and Chiu (2011) evaluated the seismic

performance of a 6-story steel office building with BRBs considering the capacity and demands of the

BRBs using the test data and response analysis. Their result showed that the BRBs could provide a

high level of confidence, ensuring the building to achieve the performance objectives of immediate

occupancy and life safety. Sarno and Manfredi (2010) studied the seismic performance of a typical

reinforced concrete (RC) existing framed structure designed for gravity loads and then retrofitted this

structure with BRBs. Nonlinear static and dynamic analyses were carried out for both the as-built and

retrofitted structures to investigate the efficiency of the adopted intervention strategy. Almansa et al.

(2012) developed a numerical model of the cyclic structural behavior of dissipative buckling-

restrained braces, commonly used as an alternative to classical concentric braces for seismic

protection of building frames and other structures. They claimed that the accuracy of the proposed

model checked by comparison with experimental results showed a satisfactory agreement. Hoveidae

and Rafezy (2012) gave a detailed presentation on the finite element analysis results of the proposed

all-steel buckling restrained braces in order to investigate the behavior of BRBs with different

amounts of gap and initial imperfections. The results showed that BRB flexural stiffness could

significantly affect the global buckling behavior of a brace, regardless of the size of the gap.

The above-mentioned recent studies indicates that the energy dissipation or damage prevention

capacity of a steel moment frame could be greatly enhanced by employing BRBs. Unfortunately, the

effects of BRB on RC frames are not investigated adequately. This paper assesses the effects of BRB

on the overall behavior of the structure and aims to gain a better understanding about the global and

local performance of RC frames retrofitted with BRBs. In order to explore these effects, nonlinear

static analysis (pushover analysis) under constant gravity loads and monotonically increasing lateral

forces is carried out.

2 TYPICAL BRB CONFIGURATION

In a typical BRB configuration, a diagonal brace is used to transfer axial force through primarily

axially loaded members. A typical BRB's steel core can be divided into three sections; the yielding

zone, transition zone and the connection zone (Figure 1).

Figure 1. A typical BRB (Fuqua, 2009)

B. Doran, B. Akbaş., E. Şenol, O. Şeker / VEESD 2013 3

Yielding zones have a reduced cross section to assure that yielding occurs uniformly (Fuqua, 2009).

The transition zones which have larger cross sectional area than the yielding zone are the sections of

the brace directly on each side of the yielding zone. The connection zone which is used to connect the

brace to the frame is the portion of the brace that extends beyond the restraining components.

BRB resist the local and overall buckling of the steel core during compression loading. By restraining

the buckling in compression, the brace gives balanced, stable and predictable hysteretic behavior by

attaining both tensile and compressive yielding (Fuqua, 2009). The hysteresis behavior of a BRB and a

typical brace is depicted in Figure 2.

Figure 2. Comparative illustrations of hysteretic behavior (AISC 2006a)

BRBs are simply connected to the surrounding beams and columns similar to the BRB connections in

a typical BRBF. BRBs will transfer large axial forces to the connection and the columns which should

have sufficient axial force resisting capacity.

3 RETROFITTING OF AN EXISTING RC BUILDING WITH BRBS

A typical 3-story moment resisting RC frame (Figure 3) is selected for this study (Ozkaynak, 2010).

The frame was designed based on the seismic design requirements of TSC-1997 (1997) and evaluated

for the requirements of TSC-2007 (2007). The building has plan dimensions of 42.0m x 18.0 with

seven 6.00m bays and three 6.00 bays in the two orthogonal directions, and a typical story height of

4.2 m. The structural system for the building consists of RC moment resisting frames. Seismic

evaluation of the building with respect to TSC-2007 indicated that the building needs to be retrofitted

in the X direction.

Figure 3. Typical floor plan

B. Doran, B. Akbaş., E. Şenol, O. Şeker / VEESD 2013 4

Cross-section dimensions and reinforcement details are given in Figure 3 and Table 1, respectively.

Concrete and reinforcement grades used for structural members were assumed to be 18 MPa and

S220, respectively. This corresponds to a modulus of elasticity of 2.779x107 kN/m2 for the concrete

and 2x108 kN/m

2 for the steel (TS 500, 2005).

Table 1. Reinforcement details

Structural elements Reinforcement distributions

Column (30/60) Longitudinal Rebars: 8

Confinement: 8/25

Beam (30/50-30/80)

Top Reinforcement :

416+212

Bottom Reinforcement : 316

Confinement:8/25

Structural model of the existing building is given in Figure 4.

Figure 4. Structural model of the existing building

In structural modeling of the existing building, the nonlinear properties for columns and beams are

assumed to be a plastic P-M-M hinge and one component plastic moment hinge, respectively. The

plastic hinges are defined according FEMA 356 with the given rebar distribution in Table 1. The axial

force for columns, and shear force for beams are due to the dead load. Seismic loads were determined

with respect to TSC-2007 (2007). TSC-2007 (2007) defines the response spectrum in terms of peak

ground acceleration which is assumed to be 0.4g. Fundamental periods of vibrations for the first two

modes for the existing building are shown in Figure 5.

B. Doran, B. Akbaş., E. Şenol, O. Şeker / VEESD 2013 5

Mode 1: T=1.09 s Mode 2: T=0.76 s

Figure 5. Mode shapes in plan and fundamental periods of vibrations for the existing building

BRBs are used for seismic retrofitting of the building in X-direction (Figure 6).

Figure 6. Structural model of BRB-retrofitted building in X-direction

The load combinations from TS 500 (2005) are used in order to design BRBs. According to AISC 341

(2010), maximum tensile and compression strengths are

yscPC max (1)

yscPT max (2)

where is the compression strength adjustment factor, is the strain hardening adjustment factor.

The steel core has to be designed to resist the entire axial force, Pysc in the brace. The brace axial

strength, Pysc in tension and compression, in accordance with the limit state of yielding is determined

as follows (AISC 341 (2010)):

yscscysc FAP (3)

B. Doran, B. Akbaş., E. Şenol, O. Şeker / VEESD 2013 6

where scA is the cross-sectional area of the yielding segment of the steel core (mm2) and yscF is the

specified minimum yield stress of the steel core (MPa). In this study, Fysc is chosen as 235 MPa, and

is assumed as 1.1. In order to define the nonlinear properties for BRBs, axial load-displacement

diagram should be defined (Figure 7).

Figure 7. Axial force-displacement diagram

In Figure 7, displacements can be calculated as:

scyscbxbx EALP / (4)

bxbm R (5)

where Lysc is the yield length and can be assumed as “0.7L” (Mollaoglu, 2013), Pbx is the yield force, E

is the elastic modulus of steel and R is the lateral load reduction factor and is assumed as 5 in this

study.

The cross section areas of BRBs and the corresponding maximum tensile and compression strengths

and displacements in tension and compression are given in Table 2.

Table 2. Tensile and compression strengths and displacements

For the nonlinear behavior of BRBs, plastic “axial P hinge” is defined in the structural model of BRB-

retrofitted building using Figure 7 and Table 2. Fundamental periods of vibrations for the first two

modes for the BRB-retrofitted building are given in Figure 8. The fundamental periods of vibration

reduced approximately to 67% in X-direction.

Story Lysc

(m)

Pbx

(kN)

Asc

(cm2)

Δbx

(mm)

Δbm

(mm)

Pysc

(kN) Pysc

(kN)

Tmax

(kN)

Cmax

(kN)

3 5.13 3300 156.03 5.42 27.11 3760 3384.00 3722.4 4094.64

2 5.13 5200 245.86 5.42 27.11 5875 5287.50 5816.25 6397.875

1 5.13 6000 283.69 5.42 27.11 7050 6345.00 6979.5 7677.45

B. Doran, B. Akbaş., E. Şenol, O. Şeker / VEESD 2013 7

Mode 1: T=0.61 s Mode 2: T=0.36 s

Figure 8. Mode shapes in plan and fundamental periods of vibrations for BRB-retrofitted building

Nonlinear static analyses of existing building and BRB-retrofitted building are performed in order to

obtain lateral load carrying capacities and energy dissipations. Pushover analysis is a nonlinear static

procedure in which the magnitude of the lateral structural loading is incrementally increased in

accordance with a certain predefined lateral load pattern. At each increment, the plastic hinges, base

reactions and maximum roof displacements are recorded separately. Acceleration-Displacement

Response Spectrum and Modal capacity diagram for the existing building and BRB-retrofitted

building in X-direction are given in Figure 9 and Figure 10, respectively. Base shear vs. roof

displacements for the two buildings are also plotted in Fig. 11. Target roof displacements were

determined as 0.19 m and 0.051 m for the existing building and BRB-retrofitted building in

X-direction, respectively.

Figure 9. Acceleration-displacement response spectrum vs. modal capacity diagram of the existing building

Figure 10. Acceleration-displacement response spectrum vs. modal capacity diagram of the BRB-retrofitted

building

B. Doran, B. Akbaş., E. Şenol, O. Şeker / VEESD 2013 8

Figure 11. Pushover curves

Story drift ratios in X-direction are also plotted for the existing and BRB-retrofitted buildings (Fig.12).

Story drift ratio decreased about 97%.

Figure 12.Story drift ratios in X-direction

B. Doran, B. Akbaş., E. Şenol, O. Şeker / VEESD 2013 9

4 CONCLUSIONS

The major aim of this paper was to explore the effects of BRBs on the structural behavior of 3-D RC

framed buildings. Based on the limited results for an existing and BRB-retrofitted RC framed

building, the following conclusions can be drawn:

1. Pushover analyses in both orthogonal directions have been performed with a lateral load pattern in

conformance with the first- and second modes of vibration. It is found that BRBs are effective to

enhance the ductility and also energy dissipation capacity of the structure.

2. Target displacement reduces substantially when BRBs are employed in the structural system for

seismic retrofitting, which in turn reduces the story drift ratios.

3. In RC frames, ductility capacity of beam-column connections has a major effect on the efficiency of

BRBs. These connections should be strong enough to transfer axial loads safely between frame and

the braces. Generally, weak connections do not allow the full capacity of the braces to be used and

columns might fail under additional axial loads imposed by the braces.

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