Effect of Dynamic Soil-Structure Interaction on Buildings in Selected Sites of Addis Ababa
No Thumbnail Available
Date
2025-01
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Addis Ababa University
Abstract
Currently, structural engineers typically design structures as fixed-base systems, neglecting
soil-structure interaction (SSI). This practice is primarily due to the complexities associated
with soil modeling and the absence of specific procedures for incorporating SSI effects in many
seismic design codes including the Ethiopian code.
Both the European and Ethiopian building codes recommend dynamic SSI effects for slender
structures or those with significant second-order (P-Δ) effects. However, they mandate the
inclusion of SSI in design without providing explicit guidelines for its calculation. Structures
founded on piles or massive foundations, such as offshore caissons, silos, and bridge piers,
should account for SSI effects in their design (ES EN 1998-5:2015; ES EN 1998-5:2015).
In contrast to the European and Ethiopian building codes, international codes and standards,
such as ASCE/SEI 7-10 (2010), ASCE/SEI 7-16 (2017), and ASCE/SEI 41-17 (2017), provide
procedures for incorporating SSI into structural design.
This research delves into various studies and evidence, emphasizing the need for a nuanced
understanding of SSI effects on building structures. Additionally, it explores the main
approaches employed to evaluate seismic SSI problems, focusing on commonly used modeling
techniques and computational methods. Notably, the thesis adopts the code approach outlined
by NIST GCR 12-917-21 (2012) based on Pais and Kausel (1988), FEMA P-2082-1 (2020)
and ASCE/SEI 7-16 (2017) to address and analyze SSI-related challenges in seismic analysis
of buildings.
To ground the study in real-world scenarios, five specific sites in Addis Ababa were selected
based on recent ground response analyses conducted by Getu (2023). Additionally, one
idealized softer site was included. The selected sites from Getu (2023) correspond to two
ground types, C and D, according to ASCE/SEI 7-16 (2017). The inclusion of the idealized soil
condition was intended to more clearly observe soil flexibility, which is assumed to correspond
to ground type E according to ASCE/SEI 7-16 (2017).
Five earthquake motions selected from the PEER ground motion database were matched to
these ground types, and four building models, ranging from G+5 to G+30, were assumed. The
analysis was conducted using ETABS 21.1.0 software.
The common perspective on Soil structure interaction (SSI) has long regarded it as a beneficial
factor in structural response. However, contemporary research and insights gleaned from past seismic events have revealed instances where consideration of SSI can exhibit detrimental
effects under specific conditions.
In this study, it is observed that some structures with flexible base models show a smaller
response compared to their fixed-base counterparts. However, in certain cases, a significant
increase in response is observed. This increase is dependent on the predominant period of the
earthquake motions and the fundamental period of the structure. Within the scope of the study
conducted, as the number of stories increases, the SSI influence increases, particularly for
storey displacement, storey drift ratio, and storey overturning moment. Additionally, SSI also
leads to reduced base shear for shorter building models in most earthquake motions.