Simplified Load Distribution Factor for Use in LRFD Design

Abstract

Prepared in cooperation with the Indiana Department of Transportation and Federal Highway Administration. AbstractThe "S-over" equation for the load distribution factor (LDF) was first introduced in the 1930s in the AASHTO Standard. Finite element studies, however, have shown it to be unsafe in some cases and too conservative in others. AASHTO LRFD 1994 introduced a new LDF equation as a result of the NCHRP 12-26 project. This equation is based on parametric studies and finite element analyses (FEA). It is considered to be a good representation of bridge behavior. However, this equation involves a longitudinal stiffness parameter, which is not initially known in design. Thus, an iterative procedure is required to correctly determine the LDF value. This need for an iterative design procedure is perceived by practicing engineers as the major impediment to widespread acceptance of the AASHTO LRFD equation. In this study, a new simplified equation that is based on the AASHTO LRFD formula and does not require an iterative procedure is developed. A total of 43 steel girder bridges and 17 prestressed concrete girder bridges in the state of Indiana are selected and analyzed using a sophisticated finite element model. The new simplified equation produces LDF values that are always conservative when compared to those obtained from the finite element analyses and are generally greater than the LDF obtained using AASHTO LRFD specification. Therefore, the simplified equation provides a simple yet safe specification for LDF calculation.This study also investigates the effects of secondary elements and bridge deck cracking on the LDF of bridges. The AASHTO LRFD LDF equation was developed based on elastic finite element analysis considering only primary members, i.e., the effects of secondary elements such as lateral bracing and parapets were not considered. Meanwhile, many bridges have been identified as having significant cracking in the concrete deck. Even though deck cracking is a well-known phenomenon, the significance of pre-existing cracks on the live load distribution has not yet been assessed in the literature. First, secondary elements such as diaphragms and parapet were modeled using the finite element method, and the calculated load distribution factors were compared with the code-specified values. Second, the effects of typical deck cracking and crack types that have a major effect on load distribution were identified through a number of nonlinear finite element analyses. It was found that the presence of secondary elements can result in a load distribution factor up to 40 % lower than the AASHTO LRFD value. Longitudinal cracking was found to increase the load distribution factor; the resulting load distribution factor can be up to 17 % higher than the LRFD value. Transverse cracking was found to not significantly influence the transverse distribution of moment. Finally, for one of the selected bridges, both concrete cracking and secondary elements are considered to invesitigate their combined effect on lateral loa...

Long-Term Effects of Super Heavy-Weight Vehicles on Bridges

“…found to be conservative for many bridges (Sotelino et al, 2004). Furthermore, the complicated GDF equations require computation of several parameters.…”

“…The girder distribution factors (GDFs) computed with respect to the AASHTO (2004) were found to be higher compared to those obtained from detailed analyses and field tests (Sotelino et al, 2004;Mabsout et al, 1997;Eom and Nowak, 2001). Therefore, inaccuracies in the structural analysis of bridges may result in lower bridge ratings, and a permit application for a superload truck may be refused conservatively.…”

“….15 Properties of steel bridges with no skew (Sotelino et al, 2004 Table 5.17 Properties of steel bridges out of typical range (Sotelino et al, 2004) .........393 Table 5.18 GDFs for steel bridges with no skew (analyzed by Sotelino et al, 2004) ....394 Table 5.19 GDFs for steel bridges with skew (analyzed by Sotelino et al, 2004) .........395 Table 5.20 GDFs for steel bridges out of typical range (analyzed by Sotelino et al, 2004 Table 5.21 Properties of prestressed concrete bridges (Sotelino et al, 2004 (Fu and Hag-Elsafi, 2000). Analysis of the recent (between 1989 and 2000) bridge failures in the United States reveals that 8.8% of 503 reported failure cases were due to overloads and 8.6% were due to deterioration (Wardhana and Hadipriono, 2003).…”

Performance Evaluation of Damage-Integrated Composite Steel Girder Highway Bridges

“…These research efforts have included further investigations of the variables and new material 15 properties that influence lateral load distribution, in order to develop new equations for prediction (Cai 2005, Sotelino et al 2004). The work presented herein follows suit, but differs by presenting a basic analytical approach for determining lateral load distribution characteristics using classical plate theory, with modification for a stiffened plate, that can be applied to a wide variety of scenarios.…”

“…Common concerns with the empirical method in the AASHTO LRFD (2012) include the iteration required for the calculation of stiffness parameter ( g K ), limits of applicability, and the uncertainty in validity for alternative bridge systems such as composites, and implications for overload vehicles. In response, researchers have explored alternative approaches to predict the lateral load distribution characteristics, capable of providing significant accuracy without unnecessary complexity inherent to the current AASHTO LRFD method.These research efforts have included further investigations of the variables and new material 15 properties that influence lateral load distribution, in order to develop new equations for prediction (Cai 2005, Sotelino et al 2004). The work presented herein follows suit, but differs by presenting a basic analytical approach for determining lateral load distribution characteristics using classical plate theory, with modification for a stiffened plate, that can be applied to a wide variety of scenarios.…”