Finley A. Charney

Response-History Analysis for the Design of New Buildings in the NEHRP Provisions and ASCE/SEI 7 Standard: Part IV - A Study of Assumptions

The accuracy of several assumptions made when developing the methodology for nonlinear response history analysis of Chapter 16 of ASCE/SEI 7 is investigated. The major findings of this work include: (1) Modeling the gravity systems lateral influence can have a significant effect on system behavior, but it is generally conservative to neglect its contribution. (2) Exclusion of a residual drift check is acceptable when collapse prevention is the primary objective. (3) Spectrally matched ground motions should cautiously be used for near-field sites. (4) The effects of nonlinear accidental torsion can be influential, and should be considered for the analysis of torsionally irregular buildings. (5) When analyzed using FEMA P-695, a structure designed per ASCE/SEI 7 Chapter 16 may have a probability of collapse that is different than the target value associated with the systems risk category.

The Influence of Accidental Torsion on the Inelastic Dynamic Response of Buildings During Earthquakes

Nonlinear dynamic analysis is becoming an accepted procedure to assess the performance of building structures during earthquakes. Several documents have emerged to provide guidance in terms of mathematical modeling, ground motion selection and scaling, and acceptability of results. While there are some significant differences in these documents, one feature in common is that explicit inclusion of accidental torsion in the nonlinear response history analysis is generally not required. One notable exception is ASCE/SEI 7-16 (2017), which requires performing nonlinear analyses, including accidental torsion when the building has a torsional irregularity. The analysis presented in this paper supports this requirement, but also shows that neglecting accidental torsion in the analysis of torsionally regular buildings can be problematic. Failure to include accidental torsion in nonlinear analysis of torsionally irregular buildings may indicate stable response instead of dynamic instability, or may significantly underpredict deformations and thereby falsely indicate that deformation-based acceptance criteria have been met. Additionally, it is shown that simultaneous application of ground shaking in orthogonal directions is essential, and that the spatial distribution of geometric nonlinearities related to global torsional response must be included directly in the analysis.

Dynamic Response of Steel Moment-Frame Structures with Hybrid Passive Control Systems

The concept of the hybrid passive control system is studied analytically by investigating the seismic response of steel moment-frame structures. A hybrid passive system combines a rate-dependent damping device with a rate-independent energy dissipation device. The innovative configuration exploits individual element strengths and offsets their weaknesses through multi-phased behavior. A 9-story, 5-bay steel moment-frame was used for the analysis. Six different seismic resisting systems were analyzed and compared. The conventional systems included a plain special momentresisting frame (SMRF) and a dual SMRF-buckling-restrained brace (BRB) system. The four hybrid configurations utilize a BRB and either a high-damping rubber damper or viscous fluid damper. The results demonstrate the capabilities of hybrid passive control systems to improve structural response compared to conventional lateral systems and to be effective in performance-based earthquake engineering. Each hybrid configuration improved some aspect of structural response with some providing benefits for multiple damage measures. The multi-phased nature of the device improves performance for small, moderate and severe seismic events.

Retrofit of Steel-Frame Buildings Using Enhanced Gravity-Frame Connections

This paper presents a new retrofit method that employs gravity-frame connections to improve the seismic performance of buildings by enhancing the lateral-force resisting capacity of conventional shear-tab connections. In the retrofit method, proprietary Tstub flange connectors used in the Simpson Strong-Tie ® Strong Frame ® Special Moment Frame are installed adjacent to the shear tab connections to create a ductile partially restrained beam-to-column connection. Beam flanges and buckling restraint plates are used to force the reduced-width segment of the T-stub stem into high-mode buckling and to act as a “structural fuse” in an earthquake. The collapse safety and seismic serviceability performance was predicted for a set of archetypical office buildings with ductile or non-ductile (pre-Northridge) moment connections by employing the FEMA P-695 and FEMA P-58 methodologies. The results indicated that the seismic collapse safety and serviceability performance of retrofitted buildings was generally superior compared to conventional buildings. The retrofit method decreased interstory drifts and reduced repair cost and downtime, although retrofitted buildings were stiffer and experienced higher roof accelerations in some scenarios.

Use of Nonlinear Analysis in the Context of the ASCE 7 Seismic Load Provisions

This paper describes the use of nonlinear analysis within the context of the ASCE 7 seismic load provisions. The methods of analysis that are discussed include P-Delta analysis, nonlinear static analysis, and nonlinear dynamic analysis. All methods of analysis are discussed at the systems (entire structure) level. The use of nonlinear static analysis to perform sub-system evaluation is also described. Also provided in the paper is an assessment of the adequacy of the current analysis techniques in the context of advances that have been made in computational power and analysis capabilities over the last 40 years. Recommendations for improvements in the ASCE 7 analysis procedures are provided at the end of the paper.