Enhancing displacement coefficient method for multi degree of freedom buildings (MDOF) considering nonlinear soil structure interaction

• Published in: Bulletin of earthquake engineering Vol. 20; no. 15; pp. 8217 - 8252
• Main Authors: Khanmohammadi, Mohammad, Sayadi, Sina
• Format: Journal Article
• Language: English
• Published: Dordrecht Springer Netherlands 01-12-2022; Springer Nature B.V
• Subjects: Buildings; Civil Engineering; Coefficients; Degrees of freedom; Displacement; Earth and Environmental Science; Earth Sciences; Earthquakes; Environmental Engineering/Biotechnology; Far fields; Geophysics/Geodesy; Geotechnical Engineering & Applied Earth Sciences; Hydrogeology; Methods; Nonlinear systems; Nonlinearity; Original Article; Parameters; Plastic properties; Plasticity; Safety factors; Seismic activity; Shear walls; Soil structure; Soil-structure interaction; Stiffness; Structural Geology; System effectiveness; Parallel processing; Nonlinear soil–structure system; Soil–structure-interaction; Rocking motion; Displacement demand
• ISSN: 1570-761X; 1573-1456
• DOI: 10.1007/s10518-022-01513-w

One of the main challenges in seismic assessment of existing structures is estimating the displacement demand under earthquake motions. Displacement coefficient method introduced in current code and instructors correlates displacement of an equivalent single degree of Freedom (ESDOF) system to roof or any story of corresponding MDOF. An important coefficient in this method,
C
1
, defines the ratio of inelastic to elastic displacement of ESDOF. The effects of soil structure interaction (SSI) on parameter
C
1
for SDOF has been investigated by many researchers, however, this parameter on MDOF system, itself, has not been properly investigated. This is a challenging issue since many influential behaviors cannot properly be addressed in ESDOF systems such as: P–delta effects, higher mode effects, forming of plastic hinges and their sequences considering strength and stiffness deterioration, and nonlinear SSI. In this study, to investigate this approach, seven buildings representing a reasonable range of effective period as MDOF systems were selected (three moment resisting frames and four shear wall buildings) and designed with different strength reduction factors (R = 3, 4, 5 and 7). To investigate the effect of SSI on responses, the foundations were designed with (1.5, 3, 4 and 5) factor of safety vertical (FSV) to cover all probable rocking and uplifting behaviors. All designed buildings were analyzed for far-field Design Basis Earthquake (DBE), Maximum Considered Earthquake (MCE), and near field pulse-like earthquake records. The responses were investigated for the effects of SSI on global response of buildings considering both R-factor and FSV as well as MDOF displacement inelastic ratio (
C
1
MDOF
) and modified amplification factor coefficient (
C
m
). The results showed that the method suggested by ASCE-41-17 for prediction of inelastic displacement ratio underestimates the responses in shorter period buildings and overestimate for longer period buildings for all values of R-factors and FSV. The results showed that the coefficient of
C
0
introduced in ASCE-41-17 should include the effects of structural and SSI nonlinearity instead of elastic mode participation factors. Based on results, two practical equations and methods were proposed to enhancing displacement coefficient method considering SSI effects on MDOF systems.