Abstract
In this study dynamic analysis of Soil Structure Interaction (SSI) effect on multi story reinforced concrete (RC) frame founded on soft soil (flexible base) is made and compared with fixed base. Two model 2D RC frames with 7 and 12 story are selected for analysis. Winkler Spring and half space direct method models are used for flexible base for the frames founded on two types of soft soils with shear velocity Vs < 150 m/s Asper Seismic Codes of Chinese GB50011-2010 Soil IV and Ethiopian ES8-2015 soil D. The frames are subjected to strong ground motion matched to response spectrums of soft soil of Chinese GB50011-2010 and Ethiopian ES8-2015 for linear time history analysis. The dynamic analysis result shows Spring and Fixed base mass participation 90% reaches in 2 or 3 modes but in direct method 11 to 30 modes for story 12 and 7 respectively. However, both flexible base models have bigger fundamental period of vibration and inter story drift but smaller base shear than fixed base. In addition, within the flexible base models the inter-story drift, second order effect (P-Δ) and Story shear distribution are different along the height of frames. The spring model shows larger Story drift and second order effect (P-Δ) at the bottom of Story for both soft soils types. On the other hand, half space direct method model indicates value reverse to spring model; it gives bigger Story drift and P-Δ effect in the top stories than fixed base. Finally, this study concludes that base shear reduction due to SSI may not be always beneficial. Because the gravity load is constant in both fixed and flexible bases that cause bigger P-Δ effect at the bottom stories due to increase, inter story drift and decrease story shear in flexible base.
Highlights
In dynamic analysis of a building structure, the base support condition is very essential for calculating its dynamic behavior useful in estimating structural responses and distribution within structural members
All response comparisons are made with reference to fixed base for both types of flexible base models presented in subsequent sections
Soil IV it is 659% and 337% greater than fixed base, bottom and top Story respectively. This shows both spring as well as half space models have their own advantage and disadvantage in Soil Structure Interaction (SSI) modelling. This study shows both flexible base models give bigger fundamental period of vibration and drift with reduced base shear than fixed base, good agreement with seismic codes provisions [10] [11] [12] and previous studies [5] [22] [27]
Summary
In dynamic analysis of a building structure, the base support condition is very essential for calculating its dynamic behavior useful in estimating structural responses and distribution within structural members. The building base condition will be different depending on the type of supporting ground. Fixed base foundation could be assumed on stiff soil and flexible base foundation on soft soil. Flexibility of base causes decrease in structural stiffness and increase period of vibration during earthquake ground motion. The building structural responses such as displacement drift, Story shear, and P-∆ effects will be different from fixed base that could beneficial or detrimental. In the past the dynamic analysis building on soft soil has gained serious attention in seismic active areas
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