International Science Index


Seismic Fragility Assessment of Strongback Steel Braced Frames Subjected to Near-Field Earthquakes


In this paper, seismic fragility assessment of a recently developed hybrid structural system, known as the strongback system (SBS) is investigated. In this system, to mitigate the occurrence of the soft-story mechanism and improve the distribution of story drifts over the height of the structure, an elastic vertical truss is formed. The strengthened members of the braced span are designed to remain substantially elastic during levels of excitation where soft-story mechanisms are likely to occur and impose a nearly uniform story drift distribution. Due to the distinctive characteristics of near-field ground motions, it seems to be necessary to study the effect of these records on seismic performance of the SBS. To this end, a set of 56 near-field ground motion records suggested by FEMA P695 methodology is used. For fragility assessment, nonlinear dynamic analyses are carried out in OpenSEES based on the recommended procedure in HAZUS technical manual. Four damage states including slight, moderate, extensive, and complete damage (collapse) are considered. To evaluate each damage state, inter-story drift ratio and floor acceleration are implemented as engineering demand parameters. Further, to extend the evaluation of the collapse state of the system, a different collapse criterion suggested in FEMA P695 is applied. It is concluded that SBS can significantly increase the collapse capacity and consequently decrease the collapse risk of the structure during its life time. Comparing the observing mean annual frequency (MAF) of exceedance of each damage state against the allowable values presented in performance-based design methods, it is found that using the elastic vertical truss, improves the structural response effectively.

[1] Lai, J.W. and Mahin, S.A.; “Strongback system: A way to reduce damage concentration in steel-braced frames”, Journal of Structural Engineering, 141(9): 04014223, 2015.
[2] Simpson, B.G. and Mahin, S.A.; “Experimental and numerical investigation of strongback braced frame system to mitigate weak story behaviour”, Journal of Structural Engineering, 144(2): 04017211, 2018.
[3] FEMA-NIBS; “Earthquake Loss Estimation Methodology, HAZUS-MH MR4, Technical Manual”, Federal Emergency Management Agency and National Institute of Building Sciences: Washington, U.S.A., 2003.
[4] FEMA P695; “Quantification of Building Seismic Performance Factors”, Federal Emergency Management Agency, Washington, U.S.A., 2009.
[5] Bozorgnia Y. and Bertero, V.V.; “Earthquake Engineering: form Engineering Seismology to Performance-Based Engineering”, CRC Press, U.S.A., 2004.
[6] Alavi, B. and Krawinkler, H.; “Behavior of moment‐resisting frame structures subjected to near‐fault ground motions”, Earthquake engineering and structural dynamics, 33(6): 687-706, 2004.
[7] Baker, J.W.; “Quantitative classification of near-fault ground motions using wavelet analysis”, Bulletin of the Seismological Society of America, 97(5):1486-1501, 2007.
[8] ASCE/SEI 7‐16; “Minimum design loads and associated criteria for buildings and other structures”, American Society of Civil Engineers, Reston, Virginia, U.S.A., 2017.
[9] AISC 360-16, “Specification for Structural Steel Buildings”, American Institute of Steel Construction, U.S.A., 2016.
[10] Uriz, P. and Mahin, S. A.; “Toward Earthquake-Resistant Design of Concentrically Braced Steel-Frame Structures”, Pacific Earthquake Engineering Research Center, University of California, Berkeley, C. A., U.S.A., 2008.
[11] Mazzoni, S., McKenna, F., Scott, M.H. and Fenves, G.L.; “OpenSees user’s manual”, Pacific Earthquake Engineering Research Center, University of California, Berkeley, C. A., U.S.A., 18: 56-57, 2004.
[12] Vamvatsikos, D. and Cornell, C. A.; “Incremental Dynamic Analysis”, Earthquake Engineering and Structural Dynamics, 31(3): 491–514, 2002.
[13] Cornell, C. A., Jalayer, F., Hamburger, R. O. and Foutch, D. A.; “The Probabilistic Basis for the 2000 SAC/FEMA Steel Moment Frame Guidelines”, Journal of Structural Engineering, 128(4): 526-533, 2002.
[14] Vamvatsikos, D.; “Accurate Application and Second-Order Improvement of the SAC/FEMA Probabilistic Formats for Seismic Performance Assessment”, Journal of Structural Engineering, 140(2): 04013058, 2014.
[15] Vamvatsikos, D. and Aschheim, M.; “Yield Frequency Spectra and Seismic Design of Code-Compatible RC Structures: An Illustrative Example”, Earthquake Engineering and Structural Dynamics, 35(11): 1759-1778, 2016.
[16] Vamvatsikos, D. and Fragiadakis, M.; “Incremental Dynamic Analysis for Estimating Seismic Performance Sensitivity and Uncertainty”, Earthquake Spectra, 39: 141–163, 2010.
[17] FEMA 356; “Prestandard and Commentary for the Seismic Rehabilitation of Buildings”, Federal Emergency Management Agency, Washington, U.S.A., 2000.