The effects of foundation size on the seismic performance of buildings considering the soil-foundation-structure interaction

Copyright © 2016 Techno-Press, Ltd. Shallow footings are one of the most common types of foundations used to support mid-rise buildings in high risk seismic zones. Recent findings have revealed that the dynamic interaction between the soil, foundation, and the superstructure can influence the seismic response of the building during earthquakes. Accordingly, the properties of a foundation can alter the dynamic characteristics (natural frequency and damping) of the soil-foundation-structure system. In this paper the influence that shallow foundations have on the seismic response of a mid-rise moment resisting building is investigated. For this purpose, a fifteen storey moment resisting frame sitting on shallow footings with different sizes was simulated numerically using ABAQUS software. By adopting a direct calculation method, the numerical model can perform a fully nonlinear time history dynamic analysis to realistically simulate the dynamic behaviour of soil, foundation, and structure under seismic excitations. This three-dimensional numerical model accounts for the nonlinear behaviour of the soil medium and structural elements. Infinite boundary conditions were assigned to the numerical model to simulate free field boundaries, and appropriate contact elements capable of modelling sliding and separation between the foundation and soil elements are also considered. The influence of foundation size on the natural frequency of the system and structural response spectrum was also studied. The numerical results for cases of soil-foundation-structure systems with different sized foundations and fixed base conditions (excluding soil-foundation-structure interaction) in terms of lateral deformations, inter-storey drifts, rocking, and shear force distribution of the structure were then compared. Due to natural period lengthening, there was a significant reduction in the base shears when the size of the foundation was reduced. It was concluded that the size of a shallow foundation influences the dynamic characteristics and the seismic response of the building due to interaction between the soil, foundation, and structure, and therefore design engineer should carefully consider these parameters in order to ensure a safe and cost effective seismic design

Influence of Foundation Type on Seismic Performance of Buildings Considering Soil-Structure Interaction

© 2016 World Scientific Publishing Company. In selecting the type of foundation best suited for mid-rise buildings in high risk seismic zones, design engineers may consider that a shallow foundation, a pile foundation, or a pile-raft foundation can best carry the static and dynamic loads. However, different types of foundations behave differently during earthquakes, depending on the soil-structure interaction (SSI) where the properties of the in situ soil and type of foundation change the dynamic characteristics (natural frequency and damping) of the soil-foundation-structure system. In order to investigate the different characteristics of SSI and its influence on the seismic response of building frames, a 3D numerical model of a 15-storey full-scale (prototype) structure was simulated with four different types of foundations: (i) A fixed-based structure that excludes the SSI, (ii) a structure supported by a shallow foundation, (iii) a structure supported by a pile-raft foundation in soft soil and (iv) a structure supported by a floating (frictional) pile foundation in soft soil. Finite difference analyzes with FLAC3D were then conducted using real earthquake records that incorporated material (soil and superstructure) and geometric (uplifting, gapping and P-Δ effects) nonlinearities. The 3D numerical modeling procedure had previously been verified against experimental shaking table tests conducted by the authors. The results are then presented and compared in terms of soil amplification, shear force distribution and rocking of the superstructure, including its lateral deformation and drift. The results showed that the type of foundation is a major contributor to the seismic response of buildings with SSI and should therefore be given careful consideration in order to ensure a safe and cost effective design

Influence of size and load-bearing mechanism of piles on seismic performance of buildings considering soil-pile-structure interaction

© 2017 American Society of Civil Engineers. Pile foundations are usually used to transmit foundation loads through soil strata of low bearing capacity to deeper soil or rock strata with a higher bearing capacity and stiffness. The type and size of a pile foundation that supports midrise buildings in high-risk seismic zones can alter the dynamic characteristics of the soil-pile-foundation system during an earthquake due to soil-structure interaction. To investigate these phenomena, a 15-story moment-resisting frame sitting on differently sized end-bearing and floating pile foundations was simulated numerically. The present paper describes a numerical modeling technique for the simulation of complex seismic soil-pile-structure interaction phenomena. By adopting a method of direct calculation, the numerical model can perform a fully nonlinear time history dynamic analysis to realistically simulate the dynamic behavior of soil, pile foundations, and structure under seismic excitations. This three-dimensional (3D) numerical model accounts for the nonlinear behavior of the soil medium, the piles, and the structural elements. Results show that the type and size of the pile elements influence the dynamic characteristics and seismic response of the building due to interaction between the soil, pile foundations, and the structure. The findings of this study can help engineers select the correct size and type of pile foundation while considering the seismic performance of buildings sitting on soft soil and aim at optimizing their design

Influence of Soft Soil Shear Strength on the Seismic Response of Concrete Buildings Considering Soil-Structure Interaction

© ASCE. Influences of undrained shear strength on seismic response of moment resisting concrete building considering soil-structure interaction (SSI) have been studied. A 15-storey building model resting on class Ee soil with different values of undrained shear strength has been simulated through FLAC3D. Fully nonlinear dynamic analysis under four different earthquakes including two far-field and two near-field recordings has been conducted by direct method and results in terms of base shear, maximum lateral displacement, inter-storey drift and spectral acceleration have been compared and discussed. Results indicate that by increasing the undrained shear strength of the subsoil, the superstructure experiences extra base shear under earthquake excitations due to SSI. Furthermore, the maximum lateral displacements and inter-storey drifts of the superstructure increase by adopting higher values for the undraied shear strength of the subsoil. It is concluded that practicing engineers should treat soil properties gained from field or laboratory tests with extreme care when dealing with numerical based seismic design of the soil-structure systems

Full scale lateral behaviour of monopiles in granular marine soils

Monopiles are used in piers as mooring or berthing dolphins. This article reports the results of full-scale lateral loading tests on monopiles constructed as dolphins in the Pars Special Economic Energy Zone in southern Iran. The length and diameter of the monopiles were approximately 40 m and 2 m, respectively. Lateral loading tests of such large monopiles are not commonly undertaken, thus there is limited data available. This research developed a lateral analysis of piles computer code to examine analytical methods for pile analysis. Appropriate models were introduced resulting in accurate predictions in the analysis of lateral loaded piles. The results showed that traditional p-y curves and strain wedge models calculate larger pile head deflection in comparison to the field test data and therefore local calibration is essential. © 2012 Elsevier Ltd

Seismic Strain Wedge Model for analysis of single piles under lateral seismic loading

One of the most effective methods of analysing a single pile and pile groups under lateral loading is Strain Wedge Model (SWM). SWM has a number of advantages in comparison with traditional p-y curves, but this model could traditionally only be used to analyse piles under monotonie loads. In the present paper, SWM has been modified to consider dynamic lateral loading. Based on this new method, called Seismic Strain Wedge Model (SSWM), a computer code has been developed for lateral analysis of piles. Using this computer code, some case studies have been analysed and the results show good agreement with test data. This paper introduces SSWM as a simple and powerful solution to analyse piles under lateral seismic loading

Recording inter-storey drifts of structures in time-history approach for seismic design of building frames

The growing trend in the application of direct displacement-based or performancebased design, lays more emphasis on the precise prediction of design parameters such as the inter-storey drift controlling the performance level of the structure. Practising engineers employ different methods to record the inter-storey drifts in time-history approach mainly based on the maximum lateral deformation of the structure. In this study, a 15-storey concrete moment resisting building is designed using time-history analysis. Then reliability and accuracy of each method in predicting the maximum inter-storey drifts under the infl uence of three earthquake records, namely 1995 Kobe, 1994 Northridge and 1940 El Centro earthquakes, are investigated. Results clearly indicate that to choose the most critical drift to evaluate the performance level of structures, the absolute maximum drift over time should be calculated. Other methods based on the maximum storey deflection may result in unconservative design. © Institution of Engineers Australia, 2012

Assessment of soil-pile-structure interaction influencing seismic response of mid-rise buildings sitting on floating pile foundations

The role of the seismic soil-pile-structure interaction (SSPSI) is usually considered beneficial to the structural system under seismic loading since it lengthens the lateral fundamental period and leads to higher damping of the system in comparison with the fixed-base assumption. Lessons learned from recent earthquakes show that fixed-base assumption could be misleading, and neglecting the influence of SSPSI could lead to unsafe design particularly for structures founded on soft soils. In this study, in order to better understand the SSPSI phenomena, a series of shaking table tests have been conducted for three different cases, namely: (i) fixed-base structure representing the situation excluding the soil-structure interaction; (ii) structure supported by shallow foundation on soft soil; and (iii) structure supported by floating (frictional) pile foundation in soft soil. A laminar soil container has been designed and constructed to simulate the free field soil response by minimising boundary effects during shaking table tests. In addition, a fully nonlinear three dimensional numerical model employing FLAC3D has been adopted to perform time-history analysis on the mentioned three cases. The numerical model adopts hysteretic damping algorithm representing the variation of the shear modulus and damping ratio of the soil with the cyclic shear strain capturing the energy absorbing characteristics of the soil. Results are presented in terms of the structural response parameters most significant for the damage such as foundation rocking, base shear, floor deformation, and inter-storey drifts. Comparison of the numerical predictions and the experimental data shows a good agreement confirming the reliability of the numerical model. Both experimental and numerical results indicate that soil-structure interaction amplifies the lateral deflections and inter-storey drifts of the structures supported by floating pile foundations in comparison to the fixed base structures. However, the floating pile foundations contribute to the reduction in the lateral displacements in comparison to the shallow foundation case, due to the reduced rocking components. © 2013 Elsevier Ltd