Órgano de Control Institucional

Earthquakes are a major natural hazard that causes economic losses, human casualties, and indirect social impacts. Driven by the potential for heavy losses, there is a growing interest in the earthquake engineering community in evaluating the seismic performance of the built environment for future seismic events. For the past two decades, the development of Performance-based Earthquake Engineering (PBEE) methods has enabled seismic performance assessment in a more rigorous way by integrating seismic hazard, structural response, damage, and various loss estimates to provide a quantitative and probabilistic description of the seismic risk. The end-product of this seismic performance provides a wealth of information that enables comparison of different design methodologies for new structures and identification of existing structures that are vulnerable and pose a threat to their occupants.

Steel concentrically braced frames (CBFs) are a very popular type of lateral force resisting system (LFRS) which provides large elastic stiffness while being economical. Past and current building codes permit CBFs in vast areas of the United States (US), which are in low to moderate seismic zones, such as the mid-Atlantic east coast region of the United States (ECUS), to be designed with a Response Modification Factor R equal to 3 and without seismic detailing to promote ductility. While the design and construction of low-

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ductility CBFs are pervasive in the US, there is very little understanding of their inelastic behavior and seismic performance. Therefore, the seismic performance of this class of CBFs is of concern. On the other hand, the seismic hazard in the ECUS is also not well characterized, compared to the western United States (WUS, west of the Rocky Mountains). The 2011 Mineral, Virginia earthquake increased public awareness of potential seismic hazard in low to moderate seismic zones and motived researchers to investigate the seismic performance of low-ductility CBFs in the ECUS seismic hazard environment. Due to the infrequent nature of seismic events in the ECUS, the performance objectives for low-ductility CBFs is focused on preventing life-threatening collapses (Nelson 2007; Nordenson and Bell 2000). In some cases, however, the non-collapse performance can be of concern for stakeholders for special reasons.

Variability in the collapse capacity of the building inventory, which comes from various sources of uncertainty, is a critical component in a probabilistic seismic performance evaluation. This variability is of special importance to low-ductility CBF systems because the variation in their collapse capacity may be larger than that of special concentrically braced frame (SCBF) which are detailed to have higher ductility. Current design code provisions do not preclude undesirable brittle failure mode (e.g. brace connection fracture) in low-ductility CBFs. Therefore, low-ductility CBFs may have various limit states occurring in various sequences and do not follow a clear hierarchy of limit states like SCBFs, which respond with selected limit states occurring (brace yielding and buckling). The inherent uncertainty of the structural properties, for example, material properties and design variations, may trigger a variation of the governing limit states and create different paths to failure. Furthermore, low-ductility CBFs are reported to have the so called “reserve

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capacity” (Li and Fahnestock 2013) that are not considered in design but may contribute to collapse prevention after the primary LFRS becomes severely damaged. The uncertain nature of the reserve capacity contributes to the total uncertainty in the collapse capacity. Therefore, it is important to evaluate all significant sources of uncertainty in the collapse response and to comprehensively assess the seismic performance of a general inventory of low-ductility CBFs.

FEMA P695 (FEMA 2009) provides a methodology to evaluate the collapse performance of a structural system type considering the effects of uncertainties. This methodology was developed based on ductile reinforced concrete (RC) moment resisting frames (MRFs) in WUS. It is not known whether this methodology applies to evaluating ECUS structures for several reasons. For example, the FEMA ground motion set that is used to capture the uncertainty in seismic demand consists of recorded ground motions mainly from WUS. The spectral shape factor (SSF) to address the spectral shape effect is based on ductile structures. The 𝛽 factors that account for various sources of uncertainty in the collapse capacity are also developed for ductile structures. Therefore, it is necessary to examine the application of FEMA P695 methodology to the evaluation of the collapse capacity of ECUS structures.

Recent large scale tests on low-ductility CBFs (Bradley et al. 2015; Sen 2014; and Sizemore et al. 2015) provide insight and valuable data on the seismic behavior of low- ductility CBF at a fundamental level. However, these tests were limited to certain structural parameters, configurations and designs, and do not address the effect of uncertainty. In addition, the test results (mostly from static cyclic loading) do not provide direct

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information on the collapse capacity. Numerical studies that explore the behavior of low- ductility CBF in broader parametric space and evaluate the collapse performance are necessary.

Motivated by the above factors, this research seeks to develop accurate numerical models facilitated by recent test data and to evaluate the seismic performance of low-ductility CBF in the context of the ECUS seismic hazard environment. Special attention is given to the collapse performance and how it is affected by various sources of uncertainty. The FEMA P695 methodology (FEMA 2009) was used as a baseline reference to evaluate the collapse performance. The application of the FEMA P695 methodology for collapse performance assessment of low-ductility CBFs in the ECUS is examined and modifications in the methodology are proposed.