Assessment of Accidental Torsion in Building Structures Using Static and Dynamic Analysis Procedures

Osama Ahmed Mohamed,Mohamed Sherif Mehana

doi:10.3390/app10165509

Abstract

This article presents the findings of a study on assessment of the increase in building’s response due to accidental torsion when subjected to seismic forces. Critical stiffness and geometrical parameters that define buildings torsional response are examined including: (1) the ratio, Ω, between uncoupled torsional frequency ωθ to uncoupled translation frequencies in the direction of ground motion ωx or ωy, (2) floor plan aspect ratio, b/r, which is a function of the floor dimension and radius of gyration. The increased response is assessed on symmetric multi-storey buildings using both static and dynamic analysis methods specified by ASCE-7 and considering parameters affecting the torsional response. It was concluded that static and dynamic analysis procedures predict different accidental torsion responses. Static analysis based on the Equivalent Lateral Force (ELF) method predicts more conservative accidental torsions responses for flexible structures with Ω < 0.7~0.80, while the responses are less conservative for stiffer buildings. The conservativism in static analysis method is attributed to the response amplification factor, Ax. Floor plans and their lateral support system having frequency ratio Ω = 1 will also have a torsional radius equal to radius of gyration, and will experience drop in torsional response relative to more torsionally flexible buildings. This article presents a procedure to overcome the shortcomings of static and dynamic analysis procedures in terms of estimating accidental torsion response of symmetric building structures.

Highlights

Building codes and design standards typically require that torsional effects be accounted for when designing structural systems for seismic forces
Inherent torsion is dominant in irregular structures, torsional response in general has been observed in structures that are symmetric in floor plan as well as in layout of seismic force-resisting been observed in structures that are symmetric in floor plan as well as in layout of seismic forcesystems [10,11]
Dynamic analysis indicates that buildings with uncoupled torsional to translational uncoupled frequency ratio Ω > 1.0 are torsionally stiff while buildings with uncoupled torsional to translational uncoupled frequency ratio Ω < 1 are torsionally flexible

Summary

Introduction

Building codes and design standards typically require that torsional effects be accounted for when designing structural systems for seismic forces. Mt, is caused by the eccentricity between the locations of the center of mass (CM) and the center of rigidity, while accidental torsion moment, Mta, accounts for uncertainties in mass or strength distribution assumed in the analysis, and any torsional vibrations that might be induced by base rotational motion. Some standards such as ASCE 7-16 [1] and codes such as Mexico Federal District Code [2] and National Building Code of Canada [3] provide some form of the Equation (1) for design eccentricity, ed , that accounts for both inherent and accidental torsions

Methods

Results

Discussion

Conclusion