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Comparison of Retrofitting Techniques for Existing Steel Moment Resisting Frames

Dimitrios G. Lignos1, Carlos Molina-Hunt2, Andrew D. Krebs3, Sarah L. Billington4

ABSTRACT

Seismic protection of critical facilities such as hospitals, emergency response centers, and schools is crucial as these buildings must remain in full service after major earthquakes. A new modular, multiple-panel infill system made of a ductile, high performance fiber reinforced concrete (HPFRC) is currently under investigation with a focus on protecting such critical facilities wherein the primary structural system is a steel, moment-resisting frame. The primary objective of this paper is to compare the proposed HPFRC infill panel retrofit system with the alternative retrofit techniques of buckling-restrained braces (BRBs) and viscous dampers for seismic protection of existing steel moment resisting frames. The effectiveness of the three seismic retrofit techniques is evaluated based on analytical simulations after retrofitting the SAC 3-story structure. It is shown that the HPFRC infill panel system when installed in an existing steel moment resisting frame designed prior to major earthquake events (Northridge, 1994, Kobe, 1995) limits residual story drifts as well as frame damage at a design and a maximum considered event. Its seismic response is very comparable with the state-of-the art BRB and viscous damper retrofitting techniques.

Keywords: retrofit, buckling-restrained braces, viscous dampers, high-performance fiber-reinforced concrete, infill panel, steel moment frames

1. Introduction Followed the Northridge 1994 and Kobe 1995 earthquakes it was proven that welded beam-to-column connections in steel moment resisting frames were likely to fail in a brittle fracture mode (e.g. fracture at fused zone or column flange divot zone) in relatively early inelastic building response. The damaged buildings had heights ranging from one to 26 stories (FEMA-351) and it was found that many fractures

1 Post doctoral Fellow, Stanford University, Stanford, CA, 94305, e-mail: dlignos@stanford.edu 2 Former Masters student, Stanford University, Stanford, CA, 94305, e-mail: carlosm1@stanfordalumni.org 3 Former Masters student, Stanford University, Stanford, CA, 94305, e-mail: adkrebs@stanfordalumni.org 4 Associate Professor, Stanford University, Stanford, CA, 94305, e-mail: billington@stanford.edu

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were distributed in areas that ground shaking was moderate (below design level). The indirect economic losses relating to the temporary or long-term loss of use of space within these buildings was significant. The 1994 Northridge earthquake caused 23 hospitals to suspend some or all of their services and resulted in more than $3 billion in hospital-related damages (FEMA-351).

Over the years many different retrofit techniques were developed for existing steel moment resisting frames. In particular, due to poor seismic performance of welded beam-to-column connections in steel moment resisting frames, Gross et al. (1999), Uang et al. (2000) and many others investigated how an existing welded steel moment frame connection may be modified for adequate seismic resistance.

Leelataviwat et al. (1998) investigated experimentally and analytically a ductile fuse element in shear at mid-span of beams that moves plastic deformation away from the fracture critical regions of existing steel moment resisting frames. Christopoulos et al. (2002) proposed a self-centering structural system as an alternative for seismic retrofit of existing steel moment frames, which acts as a post-tensioned energy dissipating steel frame. Bruneau (2005) summarized analytical and experimental investigations conducted on reduced steel plate thickness shear walls as an alternative retrofit technique by allowing shear buckling at the steel plate providing an energy dissipation mechanism.

Buckling-restrained braces (BRBs) are considered as one of the state-of-the art options for retrofitting existing moment resisting frames (FEMA-273, 351) since they yield inelastically both in tension and compression at their adjusted strengths (Clark et al. 1999). Several experimental studies (Wada et al. 1998, Lopez et al. 2002, Uang and Kiggins, 2003, Kasai et al. 2008) have demonstrated that BRBs have fully balanced hysteretic loops for both tension and compression behavior even after significant inelastic deformations.

Passive energy dissipation devices including viscous or viscoelastic dampers have been used for seismic retrofit and new design of critical facilities. Soong and Spencer (2002) provide a qualitative description and comparison of passive, active and semi-active control systems for seismic rehabilitation and new design of buildings. Uriz and Whittaker (2001) studied the use of linear fluid viscous dampers for the seismic retrofit of a three story pre-Northridge steel moment frame and concluded that base shear and column axial forces increase substantially with the use of the dampers. Constantinou and Symans (1992) and Makris et al. (1993) investigated experimentally and analytically the use of fluid viscous dampers for the seismic retrofit of structures. Kasai et al. (2008) conducted a series of three-dimensional earthquake simulator tests of a full scale 5-story value-added building with various types of dampers tested at the E-Defense facility. During the tests it was demonstrated that the performance objectives for the value-added building were met with the use of passive devices.

Recent work by Kesner and Billington (2005) demonstrated that innovative materials such as High-Performance Fiber-Reinforced Cementitious Composites (HPFRCC) can be used to add stiffness, strength and dissipate energy to deficient existing steel moment frames. This paper discusses the use of a replaceable, fuse-type infill panel system made with ductile, high performance fiber-reinforced concrete (HPFRC) for seismic retrofit of existing steel moment frames (Olsen & Billington,

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2009). In order to evaluate the effectiveness of the new infill panel system as seismic retrofit compared to state-of-the art retrofitting techniques with BRBs and viscous dampers, a comprehensive analytical study was conducted based on a retrofitted SAC 3-story building (Gupta and Krawinkler, 1999) with the three aforementioned options. 2. Description of High Performance Reinforced Concrete Infill Panel System High performance fiber reinforced concrete (HPRFC) is a quasi-brittle material that exhibits tensile hardening, followed by tensile softening deformation behavior. The primary characteristic of HPFRC is that its ultimate tensile strength is higher than its first cracking strength and during the inelastic deformation of HPFRC components, multiple, fine cracking is evident (Naaman, 2003). Parra-Montesinos et al. (2005) summarizes several examples of HPFRC that can be used for seismic applications. The proposed infill panel system for strengthening and stiffening criticaluse existing steel frame buildings consists of two panels made of an HPFRC mixture that has self-consolidating properties. The panels utilize welded wire fabric (WWF) to aid in crack distribution. The primary reinforcement is standard deformed mild steel Grade 60. The two panels are precast and are bolted into place together and to the existing steel moment frame to act as a fixed-fixed flexural component (see Figure 1). The two panels are connected at the story midheight with a slotted steel connection that forces the point of inflection of the double panel to be at midheight, allowing for equal moments on the panels to facilitate multiple cracking throughout both panels, and preventing any axial load buildup (and potential out-of-plane movement) from lateral loading and vertical live loads. One of the advantages of the infill panel system is that it is modular and partially filled bays may be utilized in case of openings or architectural constraints.

Figure 1. Schematic representation of the HPFRC infill panel retrofit system (left) and double

infill panel specimen after cyclic testing (right) (Hanson and Billington, 2009)

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In order to investigate the effect of reinforcement ratio and layout, panel geometry (rectangular versus tapered), and concrete material on the cyclic behavior of the infill panel system, Olsen and Billington, (2009) conducted a testing and analytical program on various single infill panels in 2/3 scale. Hanson and Billington (2009) extended this work by testing 4 different double panel configurations (see Figure 1). The double-panel testing illustrated that the hysteretic response of the double panel system is ductile at least up to 6% story drift ratios (further discussed later), with no axial load build up or significant out-of-plane movement. A 2/3-scale, 2-story steel moment frame designed in 1985 and retrofitted with the HPFRC infill panel system is scheduled to be tested at the NEES facility at University of California at Berkeley during fall 2009 by the authors using the state-of-the art hybrid testing method. The goal of the testing program is to illustrate experimentally that the new retrofit system meets the retrofit objectives for existing steel moment frames under dynamic loading and that the dynamic response of these frames is comparable with the same steel moment frames retrofitted with commonly used techniques, such as bucking restrained braces (BRBs) and viscous dampers (VD). 3. Description and analytical modeling of a base case, steel moment resisting

frame In order to evaluate the effectiveness of the proposed HPFRC infill panel system compared to other retrofitting techniques in this case BRBs or VD, an existing 3-story steel moment frame designed as part of the SAC project is retrofitted with these three alternative retrofits. The building has 4 by 6 30 ft. bays and each story has a height of 13 ft.. The moment resisting frame of the building together with basic steel section information is shown in Figure 2. The building is known as the SAC-LA-3 story building (REFERENCE?). The predominant period the bare frame is 1.05sec with a base shear coefficient y yn V W =0.26 (where, yV =yield base shear, W =seismic weight).

Figure 2. Moment-resisting frame of LA SAC 3-story building (Gupta and Krawinkler, 1999)

The east-west steel moment resisting frame of the SAC 3-story building is modeled in Open System for Earthquake Engineering Simulation (OpenSees, 2007) analysis platform. In order to account for P-delta effects the gravity load is assigned

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on a leaning column. Beams and columns are modeled as elastic elements with concentrated plasticity springs at their ends. The springs follow a bilinear hysteretic rule and the analytical model used to simulate the component behavior is the modified Ibarra-Krawinkler deterioration model (Lignos and Krawinkler, 2009). Deterioration modeling parameters are utilized based on information collected in a steel database assembled by Lignos and Krawinkler (2009). Since existing moment frames were designed with weak panel zones, panel zone shear distortion is taken into account with eight rigid elements connected with hinges at three corners and with two bilinear rotation springs at the fourth corner. Details about this model can be found in Gupta and Krawinkler (1999). 4. Retrofit Objectives and Retrofitted Frames In order to retrofit the SAC 3-story steel moment frame with buckling-restrained braces and viscous dampers, ASCE 31/03, FEMA 356 and FEMA 351 guidelines for rehabilitation of structures were followed. At this point, since there are no retrofit guidelines in the FEMA documents for designing the proposed retrofit system with HPFRC infill panels we treat these elements similarly to buckling-restrained braces (elements that add stiffness and strength to the steel frame) but also as a degrading system per ASCE 41 provisions. The overall objective for the seismic retrofit of the SAC 3-story steel moment frame with the various alternate retrofitting methods is to meet life safety performance goals in a design level earthquake (DLE). Story drift demands need to be reduced to significantly decrease the likelihood of having residual deformations (so as to keep the facility operational), as well as to reduce the inelastic rotation demands on the pre-Northridge steel moment resisting frame connections. 4.1. Seismic retrofit with buckling-restrained braces Buckling-restrained braces (BRBs) are treated as yielding steel elements that add stiffness and strength to the lateral support system of the retrofitted frame. In order to design the BRBs, a nonlinear static analysis procedure was performed based on FEMA 273 guidelines. The BRB elements are designed to yield at about 0.4% story drift ratios and are inserted in the mid-bay. The retrofitted frame is shown in Figure 3 together with BRBs. All the properties of the BRBs used for the nonlinear static and dynamic analysis discussed later in this paper are summarized in Table 1. The BRB elements are modeled with the standard Giuffre-Menegotto-Pinto steel material model (noted as Steel02 material in OpenSees, 2007). The associated parameters used to model the BRBs with the Steel02 model are calibrated based on available experimental data by Kasai et al. (2008). The predominant period of the retrofitted steel frame with BRBs is 1T =0.77sec and the base shear coefficient yn 0.33, indicating that after inserting BRBs to the bare frame both strength and stiffness of the steel frame increase.

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Figure 3. SAC 3-Story steel moment frame retrofitted with buckling-restrained braces

Table 1. Properties of buckling-restrained braces modeled

Length L0 (in)

Area A0 (in2)

Length L1 (in)

Area A1 (in2)

3 360 102 3.33 258 112 360 96 4.45 264 191 360 130 4.45 230 19

FloorTotal

Length (in)

Plastic Portion Elastic Portion

4.2. Seismic retrofit with viscous dampers Using energy dissipating elements, such as viscous dampers (VD) as a retrofit technique for the SAC 3-Story steel frame, add supplemental damping and stiffness to the lateral load resisting frame. The VDs used in this case are designed based on FEMA 351 guidelines using the nonlinear static analysis procedure and nonlinear dynamic analysis procedure with three ground motions representing the seismic hazard of the Los Angeles area in which the building is located. The retrofitted steel frame with VDs has the same geometry as the one presented in Section 4.1 (see Figure 3). The VD is modeled as a nonlinear dashpot. The viscous force dF t is

proportional to the fractional power a of the velocity .

cu t , i.e.,

. .a

c cd dF t C u t sign u t

(1)

where dC and a are the viscous damper design parameters. In addition, the effect of elastic deformation ku of the viscous damper on the viscous force is considered, i.e.,

d d kF t K u t (2) where dK value is calibrated based on available experimental results (Kasai et al., 2008). The total damper deformation history du t is given by,

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d c ku t u t u t (3) This model is also known as the nonlinear Maxwell model and is illustrated in Figure 4 together with damper deformation definitions. The VD properties used for retrofitting the SAC 3-story steel moment frame are summarized in Table 2. The predominant period of the 3-story steel moment frame with VDs is 1T =0.96sec and

yn 0.29.

Figure 4. Nonlinear Maxwell model and viscous damper deformation definitions

Table 2. Properties of viscous dampers modeled

Area Ad (in2)

C d aK d

(kips/in)3 360 13.2 111 0.5 11002 360 21.7 166 0.5 22301 360 21.7 166 0.5 2230

FloorTotal

Length (mm)

Damper Information

4.3. Seismic retrofit with HPFRC infill panel system For the retrofit of the SAC 3-story building using the HPFRC infill panels, seven panels were placed in the center bay at each of the three stories (see Figure 5). As mentioned earlier the HPFRC infill panels increase the stiffness, strength and energy dissipation capacity of the steel frame. Thus, they are treated as stiffness and deteriorating elements per FEMA 351 and ASCE 41 provisions.

Figure 5. SAC 3-story steel moment frame retrofitted with HPFRC infill panels

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The analytical model developed to capture the hysteretic response of the HPFRC infill panels is shown in Figure 6a. The model consists of 2 rigid links that are connected together with a hinge connection that allows vertical movement of one panel with respect to the other but forces them to connect together horizontally. Each panel at both ends has a concentrated plasticity spring that utilizes the modified Ibarra-Krawinkler deterioration model with peak-oriented hysteretic behavior. The parameters used to calibrate the panel model are based on experimental data by Hanson and Billington (2009). A typical calibration of infill panel tests using the analytical model described earlier infill panel is shown in Figure 6b.

-0.06 -0.04 -0.02 0 0.02 0.04 0.06

-500

-400

-300

-200

-100

0

100

200

300

400

500Ke = 76000My

+ = 450My

- = -450p

+ = 0.007p

- = 0.007pc

+ = 0.020pc

- = 0.020s = 0.2c = 0.2a = 0.2k = 0.2Mc/My

+ = 1.03Mc/My

- = 1.03k = 0.55

Chord Rotation (rads)

Mom

ent (

k-in

)

Test4 - Single Panel-Moment Rotation

(a) (b) Figure 6. (a) Analytical model for HPFRC infill panel, and (b) typical calibration of infill panel

model with experimental data (data from Hanson and Billington, 2009)

4. Seismic Performance Evaluation In order to compare the effectiveness of the alternate retrofit techniques for the steel moment resisting frame herein, nonlinear time history analysis is conducted. All frames (bare and retrofitted ones) are subjected to a set of 40 ground motions with magnitude wM and distance R from the rupture zone 6.5 wM 7.0 and 13km

R 40km, respectively. The set of ground motions is scaled based on Incremental Dynamic Analysis (IDA) (Vamvatsikos and Cornell, 2002) using the first mode spectral acceleration of each building as an intensity measure (IM) in order to compare statistically the median response of the retrofitted frames. The focus is on two levels of intensity; design level earthquake (DLE) and maximum considered earthquake (MCE) that relate to the retrofit objectives based on FEMA 351. Figure 7 shows the story drift ratio (SDR) profiles of all four frames for the set of 40 ground motions at a DLE of intensity ~0.60g, which is specified from an equal hazard spectrum for 10% probability of exceedence in 50 years. In red is the median response. The general trend and reductions for the retrofit with BRBs and VDs (see Figures 7b, 7c, respectively) is very similar to that of the retrofit with HPFRC infill

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panels (Figure 7d). The median maximum SDR in all cases does not exceed 1%, which satisfies the retrofit objective for immediate occupancy.

0 0.02 0.04 0.061

2

3

roof

SDR (rad)

Floo

rBare Frame T1=1.05sec

0 0.02 0.04 0.061

2

3

roof

SDR (rad)

Floo

r

Frame with BRB T1=0.77sec

(a) (b)

0 0.02 0.04 0.061

2

3

roof

SDR (rad)

Floo

r

Frame with VD T1=0.96sec

0 0.02 0.04 0.061

2

3

roof

SDR (rad)

Floo

rFrame,HPFRC Panels T1=0.76sec

(c) (d)

Figure 7. Story drift ratios (SDR) for the bare and retrofitted frames at DLE

Figure 8 shows that the median residual story drift ratios for DLE are almost zero for all of the retrofitted frames compared to the bare frame, which has an average 0.5% residual drift along its height. Based on these analyses, all of the evaluated retrofit systems here have the same efficiency in improving the seismic performance of the bare frame for the DLE. Of particular interest is the response of the retrofitted frames during an earthquake event with 2% probability of exceedence in 50 years (i.e. the maximum considered event, or MCE). In Figure 9a the median SDR of all three retrofitted frames for the set of 40 ground motions are compared with the bare frame. All three retrofitting systems improve the seismic performance of the bare frame and keep the maximum SDR at about 2.5% compared to the roughly 6% that the bare frame exhibits. Figure 9b shows the median residual story drift ratios of all four frames. All three retrofitting systems meet the collapse prevention retrofit objective by keeping residual deformations below 1%. Based on the same figure, residual drifts of the frame retrofitted with HPFRC infill panels are somewhat larger compared to the residual

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drifts of the frame retrofitted with BRBs or VD. The advantage of the HPFRC infill panel system is that since it does not build in any axial load can be easily replaced with minor cost.

0 0.02 0.04 0.061

2

3

roof

Residual SDR (rad)

Floo

r

Bare Frame T1=1.05sec

0 0.02 0.04 0.061

2

3

roof

Residual SDR (rad)

Floo

r

Frame with BRB T1=0.77sec

(a) (b)

0 0.02 0.04 0.061

2

3

roof

Residual SDR (rad)

Floo

r

Frame with VD T1=0.96sec

0 0.02 0.04 0.061

2

3

roof

Residual SDR (rad)

Floo

rFrame,HPFRC Panels T1=0.76sec

(c) (d)

Figure 8. Residual story drift ratios (SDR) for the bare and retrofitted frames at DLE

0 0.025 0.05 0.075 0.11

2

3

roof

SDR (rad)

Floo

r

MCE: max SDR Comparison

Bare FrameBRBVDHPFRC Panels

0 0.02 0.04 0.061

2

3

roof

Residual SDR (rad)

Floo

r

MCE: Residual SDR Comparison

Bare FrameBRBVDHPFRC Panels

(a) (b)

Figure 9. Median seismic response of bare and retrofitted frames for maximum considered event; (a) maximum story drift ratios (SDR) ; (b) residual story drift ratios

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5. Summary and Observations This paper evaluates a new infill panel system made of precast, high performance fiber reinforced concrete for seismic retrofit of existing steel moment frame structures. A comparison between the proposed infill panel system and two alternative retrofitting techniques, buckling-restrained braces and viscous dampers, for retrofitting a 3-story steel moment frame was conducted using a set of 40 ground motions and utilizing incremental dynamic analyses. The analytical results show that the seismic behavior of the baseline steel moment resisting frame is significantly improved for both design level (DLE) and maximum considered earthquake (MCE) events using each of the retrofit techniques. All three systems meet the retrofit objectives on average for a DLE earthquake, that is to reduce peak story drift ratios compared to the bare frame below 1% and minimize residual deformations. For MCE events, peak story drift ratios of all three retrofitted frames diminish to 2.5% compared to 6% of the bare frame. Residual drift ratios for the retrofitted 3-story frame with HPFRC infill panel system are slightly larger compared to the other two retrofitted frames but still below 1% for an MCE event. The ease of replacement for infill panels is considered to be a potential advantage over BRBs and viscous dampers, since braces often carry residual forces after a major earthquake.

Acknowledgements This study is based on work supported by the United States National Science Foundation (NSF) under Grant No. CMS- 0530383 within the George E. Brown, Jr. Network for Earthquake Engineering Simulation Consortium Operations. The financial support of NSF is gratefully acknowledged. Any opinions, findings, and conclusions or recommendations expressed in this paper are those of the authors and do not necessarily reflect the views of NSF. 6. References Bruneau, M. (2005). Seismic retrofit of steel structures, Proceeedings 1st Canadian Conference on

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