Soil-foundation-structure interaction considerations for performance-based design of RC wall structures on shallow foundations

Abstract

Modern earthquake engineering appears to be embracing the concepts of performance-based seismic design. This is becoming possible because the past decade has seen the development of a range of tools for performance-based seismic design, including the direct displacement-based design method. Much of the developments made to date, however, have focused on the performance-based design of structures without consideration of soil-foundation structure interaction. In this work a number of SFSI considerations are made for performance-based design of reinforced concrete wall structures on shallow foundations. In particular, possible performance criteria for foundation systems are discussed, existing methods of accounting for SFSI in seismic design are reviewed and a new Direct DBD procedure to account for SFSI in RC wall structures is proposed. The design procedure is applied to a number of case study structures to highlight the impact of SFSI on design requirements, and trends are compared with force-based design solutions. Table 2. Preliminary performance criteria proposed for foundation and retaining structures in Draft Model Code for DBD (Calvi & Sullivan, 2009). _______________________________________________ Performance Level _______________________________ Soil System Level 1 Level 2 Level 3 _______________________________________________ Foundation γ such that γ such that γ such that Structures G/Gmax >0.80 G/Gmax >0.20 G/Gmax >0.20 Retaining γ such that γ such that γ such that Structures Distant G/Gmax >0.40 G/Gmax >0.20 G/Gmax >0.20 from Buildings _______________________________________________ The strain limits proposed in Table 2 are defined as a function of a G/Gmax ratio. This ratio refers to the soil-foundation interface, and does not suppose that engineers can control the soil strains imposed by the passage of seismic waves. The G/Gmax limits should be considered equivalent to limits in the allowable reduction in foundation stiffness, although the relation between G and K is not linear and the direct specification of foundation stiffness reduction limits may be more appropriate. Note that the elastic dynamic rotational stiffness, Kf,0, of shallow rectangular foundations about their lateral axis can be approximated as (Gazetas, 1991):