28-11-2012, 02:52 PM
TRIAGE EVALUATION OF GUSSET PLATES IN STEEL TRUSS BRIDGES
TRIAGE EVALUATION OF GUSSET.pdf (Size: 3.14 MB / Downloads: 138)
Introduction
Problem Statement
A Federal Highway Administration (FHWA) report (Ocel and Wright 2008) has demonstrated that several of the gusset plates in the I-35 Mississippi River Bridge in Minneapolis were significantly overstressed and has identified the inelastic buckling of one of the gusset plates as a likely initiator of the bridge collapse. Thus, there is an urgent need to evaluate the safety of gusset plates on such bridges across the county. In response, FHWA has released Load Rating Guidance and Examples for Bolted and Riveted Gusset Plates in Truss Bridges (FHWA Guide, FHWA 2009), which provides Departments of Transportation (DOTs) with guidance for gusset plate evaluation. In addition to checking the resistance of fasteners, the recommended approach includes four plate checks: compressive buckling, tension, shear and block shear. The shear check requires point-in-time truss element loads (denoted as concurrent loads) for consistent estimation of the shear stress, rather than envelope loads. This requirement can make the check cumbersome and time consuming. While it is important that gusset plates be evaluated for safety, the number of truss bridges that have failed relative to the total number of truss bridges in service suggests that the number of overstressed gusset plates on steel truss bridges throughout the U.S. is small. Thus, a rapid evaluation procedure that is appropriately conservative but can be easily and cost-effectively applied is needed. The procedure should identify gusset plates that may be overstressed and that warrant more detailed investigation, while permitting identification of the many others that clearly do not have a safety concern.
Objectives
The primary objective of this study is to develop a procedure for the safe, consistent and rapid evaluation of gusset plate connections in steel truss bridges. The method will be broadly applicable and utilize member envelope loads rather than concurrent loads to minimize the number of load cases that must be considered for each joint. The procedure will also be demonstrated to be conservative relative to those in the FHWA Guide such that it may be employed in lieu of those methods.
Scope of Work
To achieve the objective above the following tasks have been executed:
1. Review of FHWA methods for evaluating steel truss bridge gusset plate connections and other pertinent previous research and on-going studies.
2. Select joints from WSDOT bridges, in collaboration with WSDOT engineers, to study in detail for the development of the rapid joint evaluation procedure.
3. Develop detailed finite element models of the selected joints to study the general joint behavior including the onset of gusset plate yielding and buckling. Consider the effects of parameters such as joint geometry, gusset plate thickness and distribution of connecting member loads explicitly in a parametric study using the developed finite element models.
4. Develop a rapid evaluation procedure, denoted the triage evaluation procedure (TEP) based on simple mechanics and observations from the simulations. Both checks for gusset plate yielding and gusset plate buckling are included.
5. Use the finite element models to compare the ability of the TEP and the methods in the FHWA Guide to predict the onset of gusset plate yielding and buckling.
6. Apply the TEP to load rate three WSDOT bridges to ensure it is conservative relative to the FHWA Guide procedures and to ensure it is not overly conservative.
7. Load rate the same bridges considering the rivet strength limit state to investigate the conservativeness of the rivet strengths given in the FHWA Guide.
8. Review rivet test data from the literature on rivet yield and ultimate strengths and compare them with the FHWA Guide recommendations.
9. Develop a spreadsheet for implementation of the TEP and provide it to the WSDOT Bridge Preservation Office.
10. Formulate conclusions and recommendations for future research.
Section 2 Review of Previous Research and Recommendations
Previous Gusset Plate Research
The strength and behavior gusset plate connections in both steel truss bridges and braced frames in steel buildings have been studied, with the latter being the focus of the majority studies. Bridge gusset plates differ from those in buildings because: (i) they typically have multiple diagonal members connected, (ii) they often serve as chord splices, (iii) are subjected to fatigue (iv) are used in gusset pairs rather than single plates, and (v) are expected remain essentially elastic (in a building, braced frame gusset plates designed for seismic loading are expected to withstand significant inelastic deformation). Whitmore (1952) proposed that the maximum uniaxial stress in a gusset plate at the end of a connected axially loaded member can be approximated by assuming a uniform distribution over a defined width, known as the Whitmore effective width. A 30° dispersion angle is assumed to calculate the Whitmore effective width as shown in Figure 2-1a. The connection length in the longitudinal direction of the member is taken as the distance from the first connector (e.g. bolt, rivet, or initiation of weld) to the end of the connection or last connector. The predicted maximum uniaxial stress is the member axial load divided by the Whitmore width times the gusset plate thickness. In his experiments, Whitmore demonstrated that this assumption provided a conservative estimate of the maximum uniaxial gusset plate stresses at the ends of members. Bjorhovde and Chakrabarti (1985) demonstrated that the Whitmore width concept was valid for gusset plates in braced frames and also demonstrated the method was appropriate for predicting net section fracture. Hardash and Bjorhovde (1985) studied braced frame gusset plate connections in tension and developed a block shear model based on a combination of shear and tension net section fracture. Other recent studies such as Yam and Chang (2002), Sheng et al (2002), and Yoo et al. (2008) have investigated the seismic performance of gusset plate connections in braced frames under inelastic tension, compression, and/or cyclic loading.
Several models for estimating the buckling strength of gusset plates have been proposed. Thornton (1984) suggested the use of the Whitmore width and an unbraced gusset plate length that is the average of the three lengths, as shown in Figure 2-1b, for use in standard buckling equations. Yam (1994) developed the Modified Thornton Method for estimating the buckling capacity, which accounts for load redistribution caused by yielding in the gusset plates prior to stability failure. The Modified Thornton Method uses a stress dispersion angle of 45° and an unbraced length in the longitudinal direction of the brace that extends from the centroid of the brace at the last row of fasteners to the first intersection with gusset plate support as shown in
Brown (1988) and Astaneh-Asl (1989) both proposed gusset plate buckling models that are functions of the unsupported free edge length, again based on testing of typical gusset plate configurations for braced frames in buildings. Roeder et al. (2005) and Yoo (2006) collected gusset plate buckling data from the literature and used it to compare the various methods of predicting gusset buckling. A hybrid of the Thornton and Modified Thornton Methods that uses a 45° dispersion angle and the average of the three unsupported lengths, as shown in Figure 2-1d, was recommended (denoted as the Yoo method herein).