MPC- 421 January 1, 2013- December 31, 2013  

Project Title: Seismic Rehabilitation of Skewed and Curved Bridges Using a New Generation of Bulking Restrained Braces University: University of Utah Principal Investigators: Chris P. Pantelides, Professor, c.pantelides@utah.edu, (801) 585-3991 Luis Ibarra, Assistant Professor, luis.ibarra@utah.edu, (801) 585-9307 Research Need: Buckling restrained braces (BRBs) can be used as structural fuses by dissipating the seismic input energy to a structure while the main structural components remain undamaged. In the case of a bridge, this implies that in an earthquake the BRBs act as dampers and could be damaged; however, the building structural components should be protected to a large degree. BRBs have been used in buildings in Japan since the 1995 Kobe earthquake (Reina and Normile, 1997) and in the US after the Northridge earthquake (Clark et al., 1999). Almost all applications have been limited to buildings, either as components of new buildings as shown in Fig. 1, or as energy dissipation elements in seismic rehabilitation.

Figure 1. Application of BRB elements in new buildings.

Recently, researchers proposed implementing BRBs as structural fuses in steel bridges. Kanaji et al. (2003) used BRBs for retrofit of a truss bridge (Minato Bridge) which is one of the longest bridges in the world. El-Bahey and Bruneau (2010, 2011) introduced the concept of using BRBs

for making ductile end diaphragms or bracing bridge bents in concrete slab on steel girder bridges. This concept has been experimentally verified using specially designed ductile end diaphragms. Cardeni et al. (2004) conducted shake table tests on a 2/5 scale slab-on-girder bridge which had short length BRBs at its end diaphragm. In all previous research BRBs were used in the transverse direction in order to retrofit bridges against seismic excitations; however, Celik and Bruneau (2009) introduced the idea of using BRBs in two directions in steel bridges. BRBs have a very stable hysteretic behavior as shown in Fig. 2 (hysteresis curve of BRB tested at the University of Utah; Raddon et al. 2009); as a result, they are widely accepted. There is a research need for investigating the feasibility of using BRBs in seismic rehabilitation or repair of existing steel and reinforced concrete bridges. In this project, the use of a new generation of BRBs will be investigated for the seismic rehabilitation of skewed bridges. The new generation of BRBs is currently being developed in an accompanying project for another sponsor. The effectiveness of using BRBs for seismic rehabilitation of existing bridges will also be verified. For this purpose, numerical simulations will be carried out to verify the effectiveness of different configurations of BRBs. Finally, a closed-form solution will be derived for design.

Figure 2. Hysteresis curve by full scale subassemblage testing of buckling-restrained braces.

Most modern curved box girder bridges are constructed using pre-stressed concrete techniques, which are more common on the West coast, as well as one cell box girder bridges that are more common on the East coast (National Research Council 2009). The elevation and plan of a two-span three box girder curved bridge are shown in Figs. 3-5, and those of a typical single-span skewed bridge in Figs. 6-8. The response of skewed and curved bridges in large earthquakes is of interest since AASHTO (2011) provides no guidance on how to implement ductile diaphragms in this case. The present proposal includes seismic rehabilitation of both reinforced concrete and steel bridges.

Figure 3. Plan of a three box girder horizontally curved two-span bridge.

Figure 4. Cross-section of a three box girder horizontally curved two-span bridge.

Figure 5. Elevation of a three box girder horizontally curved two-span bridge.

Figure 6. Plan of a RC skewed single-span girder bridge.

Figure 7. Section of a RC skewed single-span girder bridge.

Figure 8. Elevation of a RC skewed single-span girder bridge.

Research Objectives: The objectives of this project are to: (1) find effective configurations for using BRBs in the seismic rehabilitation and repair of both skewed single-span bridges and curved bridges; the verification will be performed by numerical simulation of the candidate bridges; and (2) develop an analytical model which simplifies the behavior of reinforced concrete and steel bridges when BRBs are implemented for control of longitudinal and lateral seismic forces. Research Methods: The proposed research will evaluate the performance of both skewed and curved bridges involving BRBs as energy dissipation elements for seismic rehabilitation. The research will be performed by conducting linear and nonlinear analyses considering different configurations of BRBs. In addition, an analytical model will be developed including design recommendations. Expected Outcomes: The proposed research will explore the best configuration of BRBs in skewed or curved girder bridges for seismic rehabilitation of such bridges. Design recommendations for the seismic application of BRBs as structural fuses in bridges will be developed including a closed form solution. Relevance to Strategic Goals: The project and its outcomes are related to environmental sustainability, and livable communities. With the use of seismic resilient bridges, the safety and livability of the community will be improved through enhanced recovery efforts after a large earthquake event. The State of Utah is implementing Accelerated Bridge Construction (ABC) construction practices for bridges extensively. Successful completion of the proposed project will ensure that there are methods to seismically rehabilitate such skewed and curved bridges for large earthquakes. Educational Benefits: At least two university students will be involved in the project. One PhD student and one undergraduate student are involved in the experimental portion of the work funded through an existing project. One MSc student, who will be funded from this project, will accomplish the research objectives outlined in the present proposal. At the local level, the technology transfer activity will involve high school students through an Annual Exploring Engineering Camp, during which small-scale models will be built to illustrate the details of Buckling Restrained Braces and how they will be attached to bridges. Work Plan: The proposed research will require the execution of the following tasks: Task 1. Rehabilitate a multi-girder curved and skewed bridge which includes BRBs as structural fuses: This task requires design recommendations for the seismic rehabilitation of existing curved and skewed bridges and provisions for designing BRB structures (e.g., AASHTO, 2011). Designing BRBs involves determining structural displacements and the BRBs material characteristics. The design will also utilize anticipated structural failure modes, as mentioned by El-Bahey and Bruneau (2011).

Task 2. Perform static pushover analyses to find the best position of the BRBs: Pushover analyses are necessary for finding the optimal locations of the BRBs. This can be obtained using static pushover analyses to determine the effectiveness of possible locations. The analyses will be carried out for skewed and curved steel and reinforced concrete bridges. One possible scheme for the location of BRBs at the end diaphragm and pier of the curved bridge is shown in Figs. 9 and 10. A scheme for the location of BRBs at the end diaphragm of skewed bridges is shown in Fig. 11.

Figure 9. Location of BRBs at the end diaphragm of a curved two-span bridge.

Figure 10. Location of BRBs at the bridge pier of a curved two-span bridge.

Figure 11. Location of BRBs in a skewed single-span girder bridge.

Task 3. Create a Finite Element Model for nonlinear analysis: Nonlinear time history analyses (THAs) are required to predict the seismic nonlinear performance of the bridges with BRBs. The THAs will be carried out in the software platform OpenSees (2010). First, the bridge models with BRBs will be subjected to quasi-static loading consistent with the experiments. Once the models are calibrated with experimental results, the frames will be subjected to THAs. Two sets of ground motions will be used for this purpose. The first set will consist of ordinary ground motions (Medina 2003) that do not include pulse characteristics due to forward directivity effects. Additionally, a set of near-fault ground motions will be included because the forward directivity effects may have a significant effect on the residual drifts of BRBs (Alavi and Krawinkler, 2003). Moreover, both horizontal and vertical ground motions will be considered in the simulations. For THA evaluations, the relatively low BRB post-yield stiffness and the combined isotropic-kinematic bilinear behavior will require special considerations for the nonlinear performance assessment. To obtain the bridge’s ultimate capacity, recent developments on collapse capacity evaluation will be implemented. BRBs exhibit degrading characteristics under cyclic loading, which have not been systematically addressed up to failure. Data is scarce because most experimental tests on BRBs do not reach collapse. Moreover, several experiments have reported failure of the connections, instead of inelastic deformation of the BRB components. Thus, the controlling failure mechanisms are not well understood. Some prior studies included isotropic and kinematic strain hardening to represent the BRB inelastic behavior, but material degradation was not considered. Also, Ariyaratana and Fahnestock (2011) used the Menegotto-Pinto material model to evaluate the reserve strength of

frames with BRBs, but connection-related failures were not considered; they used Incremental Dynamic Analysis (IDAs) and approximated collapse as the seismic level at which the slope of the IDA curve is less than or equal to 20% of the elastic IDA slope (Vamvatsikos and Cornell 2002). The models implemented in this study will use IDAs to predict collapse, but limit state detection will be based on structural instability caused by strength and stiffness deterioration. To assess the performance of gusset plates and BRB connections, detailed FEMs will be developed that include appropriate material simulation, load modeling and boundary conditions. Shell elements can consider transverse shear flexibility and membrane strains. For nonlinear static analysis of the BRB components, the concrete can be modeled as described by Yu et al. (2011). Selecting proper boundary conditions is an important factor in describing an accurate response of the structure. The boundary conditions can be modeled using proper linear or nonlinear spring/dampers. The results will indicate the required modifications for gusset plates and connections when BRBs are implemented in bridges. A satisfactory performance of these components is necessary to minimize residual story drifts under seismic solicitations. An incident showing the importance of this issue is the collapse of a curved box girder bridge in San Fernando, California 1971 (Barker and Pucket 2006). There are other issues associated with prestressed concrete box girder bridges, such as the critical position of live load and tendon breakout that need to be contemplated in the model (National Research Council 2009). Task 4. Introduce a closed-form solution: This task involves simplifying the behavior of bridges including BRBs. The model will be developed into an analytical methodology for design purposes. Furthermore, the accuracy of the analytical model would be determined by comparisons to the Finite Element Model. Project Cost: Total Project Costs: $339,745 MPC Funds Requested: $162,490 Matching Funds: $ 139,756 (Additional $ 37,500 in in-kind – 15 specimens @ $2,500 each) Source of Matching Funds: Starseismic LLC TRB Keywords: Accelerated Bridge Construction; Bridge Diaphragms; Seismic Rehabilitation; Damping Devices.

References: AASHTO (2011). “LRFD Seismic Bridge Design.” 2nd Edition. American Association of State

Highway and Transportation Officials. Alavi, B. and Krawinkler, H. (2001). “Effects of near-fault ground motions on frame structures.”

John A. Blume Earthquake Engineering Research Center, Report No. 138, Department of Civil Engineering, Stanford University.

Ariyaratana, C. and Fahnestock, L.A. (2011). “Evaluation of buckling-restrained brace frame seismic performance considering reserve strength.” Engineering Structures, 33, 77-89.

Barth, K. E. and Wu. H. (2006). “Efficient nonlinear finite element modeling of slab on steel stringer bridges.” J. Finite Element in Analysis and Design, 42(14-15), 1304-1313.

California Department of Transportation (2012). “Bridge Design Practice Manual.” Caltrans,

Sacramento, CA. Barker, R.M., and Puckett, J.A. (2007). “Design of Highway Bridges - Based on AASHTO

LRFD Bridge Design Specification.” Wiley and Sons, New York, 2nd Ed., 1009 pgs. Cardeni, L., Itani A., Buckle, I., and Aiken, I. (2004). “Buckling Restrained Braces for ductile

end cross frames in steel plate girder bridges.” 13th World Conference on Earthquake Engineering.

Clark, P., Aiken, I., Kasai, K., Ko, E., and Kimura, I. (1999). “Design procedures for buildings incorporating hysteretic damping devices.” Proc. 68th Annual Conv., SEAOC, Sacramento, CA., 355-371.

Celic, O.C., and Bruneau, M. (2009). “Seismic behavior of bidirectional-resistant ductile end diaphragms with buckling restrained braces in straight steel bridges.” Eng. Struct., 31(2), 380-393.

El-Bahey, S., and Bruneau, M. (2011). “Buckling restrained braces as structural fuses for the seismic retrofit of reinforced concrete bridge bents.” Eng. Struct., 33, 1052-1061.

El-Bahey, S., and Bruneau, M. (2010). “Structural Fuse Concept for Bridges.” Transportation Research Record: J. Transport. Research Board, No. 2202, Transportation Research Board of the National Academies, 167–172, Washington, D.C.

Kanaji, H., Kitazawa, M., and Suzuki, N. (2003). “Seismic retrofit strategy using damage control design concept and the response reduction effect for a long-span truss bridge.” 19th US- Japan Bridge Engineering Workshop.

Lee, J., and Fenves, G.L. (1998). “Plastic-damage model for cyclic loading of concrete structures.” J. Eng. Mech, 124(8), 892-900.

Medina, R.A., and Krawinkler, H. (2003). "Seismic Demands for Nondeteriorating Frame Structures and their Dependence on Ground Motions." JABEE Center Report No. 144, Department of Civil and Environmental Engineering, Stanford University, Stanford, CA.

National Research Council. (2009). “NCHRP Report 620: Development of design specifications and commentary for horizontally curved concrete box-girder bridges.” The National Academies Press, Washington, DC.

OpenSees (2010). “ Open System For Earthquake Engineering Simulation.” Pacific Earthquake Engineering Research Center, http://peer.berkeley.edu/

Reina, P., and Normile D. (1997). “Fully braced for seismic survival.” Engineering News Record, July 21, 34–36. Raddon, B.J., Pantelides, C.P., and Reaveley, L.D. (2009). “Full scale subbassemblage testing of

powercat series buckling restrained brace.” Report CVEEN-09/1, Dept. Civil and Env. Engrg., Univ. of Utah, Salt Lake City, Utah, 24 pg.

Vamvatsikos, D., and Cornell, C.A. (2002). “Incremental dynamic analysis.” Earthq. Eng. and Struct. Dyn., 31(3), 491-514.

Yu, Y.J., Tsai, K.C., Li, C.H., Weng, Y.T., and Tsai, C.Y. (2011). “Analytical simulations for shaking table tests of a full scale buckling restrained braced frame.” J. Procedia Eng., 14, 2941-2948.