Research Approach

The approach taken in this research study was to develop and use the IAP, which allowed for direct integration of multilinear soil supports into the commonly used structural analysis software STAAD.Pro for the purpose of investigating the influence of   soil stress history effects on the outcome of a scour susceptibility analy- sis. In the following sections, the development and verification of the IAP are described, and then the details of the specific model used in the study are  presented.

Development and Verification of the  IAP At the outset of creating the IAP, the authors had two goals: (1) to create a program that allowed for consideration of soil stress history effects, and (2) design the software such that the superstructure and substructure analyses could be performed in an integrated manner that did not require manual  iterations.

The IAP was designed to operate by integrating two analyses: the structure model and the soil model. The structure model, con- structed in STAAD.Pro 2007, consists of superstructure (deck, gird- ers, and piers) and pile structural elements. The soil model is only concerned with the behavior of the soils surrounding the piles under lateral loading and is generated in a utility termed the soil spring module (SSM). The SSM was developed by the authors using Microsoft Visual Basic 2010 Express. The SSM includes a suite of soil models derived from p-y curves that can be implemented in STAAD.Pro as nonlinear Winkler springs.  A full  description of the functionality of the SSM, including  detailed  information on the p-y curves, can be found in Lin    (2012).

The soil models created within the SSM utility were sourced from p-y curves available in the literature and were approximated by a series of multilinear lines; each slope of the piecewise linear function represents the stiffness, ki, as shown in Fig. 1. The stiff- ness of each segment of the multilinear spring is the product of ki and the length of the pile element, Li. A series of the springs approximately represents nonlinear Winkler springs and acts as soil supports to the piles in the structure   model.

The IAP achieves the analysis of the soil-structure system by linking STAAD.Pro and the SSM; this was accomplished using the OpenSTAAD function that allows external programs (e.g., the SSM) to access the internal functions of STAAD.Pro. Fig. 2 illustrates the  operation  of  the  IAP  through  the following steps: (1) building the structure model, including the bridge super- structure and foundation, in STAAD.Pro, (2) inputting the soil parameters and,  if desired,  scour depths through the  SSM,    and

(3) assigning the nonlinear soil supports to selected piles in STAAD.Pro via the SSM and performing the desired analyses in STAAD.Pro.

The IAP, therefore, is an instrument to link the SSM utility with STAAD.Pro’s functionality. As such, it harnesses the advantages that STAAD.Pro and the p-y method possess but also carries their limitations. The IAP may be used to analyze a variety of structures (e.g., bridges, buildings, water tanks, offshore structures, and so forth) that can be analyzed in STAAD.Pro and can utilize the powerful design functions and analysis engines inherent within STAAD.Pro. Furthermore, by utilizing the p-y method for model- ing soil behavior, the IAP is a computationally efficient method for modeling an entire bridge and supporting soil system rather than relying on three-dimensional finite-element or finite-difference software packages. However,  a disadvantage  of  the IAP  is  that it is unable to accommodate p-delta analyses when  nonlinear soils are utilized because the p-delta analysis functionality in STAAD.Pro is incompatible with the multilinear soil spring func- tion. In addition, the IAP is unable to analyze the dynamic behavior of structures in soils described by the p-y   curves.