Alternative Method to Estimate the Seismic Demand of Wharves by Using 3D-Modal Pushover Analysis

Abstract: Displacement-based design has become a common, accepted procedure for evaluation of inelastic performance of pile-supported wharves under seismic excitations. ASCE/CORPI 61-14 “Seismic Design of Piers and Wharves” was developed specifically for piled deck structures not occupied by the public. This document presents nonlinear static procedure as a standard method to estimate the displacement demand and capacity of piled deck under seismic levels. There are several methods recommended for assessing the displacement demand, including three-dimensional linear elastic response spectrum analysis, the substitute structure method and nonlinear time history analysis.

The modal response spectrum analysis is a linear elastic method and considered appropriate for structures that response elastic under seismic excitations and suitable for preliminary design. Nonlinear time history analysis method is the most accurate for determining the displacement demand; however, the applicability of this method in design practice is quite limited because of complexity in selection and scaling ground motion records, in determination of appropriate hysteretic models for structural elements and soil-pile structure interaction. The most common substitute structure method is normally used for designs, as this is a single-pure transverse analysis, the final displacement demand will be modified by a dynamic magnification factor to account for the effects of simultaneous seismic excitation in orthogonal horizontal directions. However, this method is only applicable for regular wharves, in which the dynamic behavior of structure is governed by the fundamental mode.

In practice, the irregularity occurs when wharves have unsymmetrically pilling layout, variation of geotechnical profile in longitudinal direction, or eccentricity of loading regarding to the position of quay crane or cargo on the deck. As a result, sufficient modes shall be included in the three-dimensional analysis so that participating mass is captured in each horizontal direction. Application of substitute structure method for irregular wharves involving three-dimensional response may lead to unrealistic outcome. Therefore, an alternative method known as three-dimensional (3D) modal pushover analysis has been proposed in order to take into account the effect of higher modes to the response regular or irregular wharves subjected simultaneously to two components of ground motion.

The modal pushover analysis was firstly developed by Chopra and Goel (2002), then was improved by Reyes and Chopra (2010) for three-dimensional building subjected to two components of ground motion where pushover analyses are performed separately for individual modes under each horizontal component of ground motion. The response quantities are combined using modal combination rule (CQC or SRSS), the multidirectional seismic effects are accounted by using directional combination rules.

This paper presents the study on the application of 3D-modal pushover analysis for wharves. An example of wharf that has been designed according to ASCE/CORPI 61-14 is used to illustrate the applicability of 3D modal pushover analysis and quantifies its accuracy versus the outcome obtained from standard methods.

How does this project / topic contribute to the advancement of the industry and profession?: This topic presents an additional method to determine the displacement demand of structure which is easy to adopt in design practice and improve the result of the analysis.

Does the project / topic implement new and innovative techniques, materials, technologies, and delivery methods?: The modal pushover analysis can be considered as an innovative method to account the 3D effect of structure under seismic excitation especially for irregular wharf structure.

What was the most challenging aspect of your project / topic and how did you handle it to ensure success?: The most challenging in this is to perform nonlinear dynamic time history to get results and compared with ones from modal pushover analysis to investigate the feasibility of this method.

Who is the target audience for this paper?: Maritime engineers will benefit from this method as this will provide a useful method to account the 3D effect of wharf structure under seismic condition.