Structural Behavior of Integral Abutment Bridge Approach Slabs
Advisors: Prof. Larry A. Fahnestock and Prof. James M. LaFave
Location: Zoom (link, Meeting ID: 820 7328 5000, Password: 208639)
Abstract
Integral abutment bridges (IABs) have gained increasing popularity in the United States due to their relatively low cost, simpler construction, greater service life, and better seismic performance – compared to stub abutment bridges. However, elimination of joints in IABs raises concerns about distress of the structural system induced by live load and thermal effects. IAB approach slab cracking has been a frequent example of such distress in Illinois, and in other states, based on a conducted agency survey. Thus, there is a need for thorough study of approach slabs at IABs to elucidate fundamental structural behavior and determine the influence of key parameters on demands that develop in approach slabs due to traffic loading and thermal effects. This research employs field monitoring and numerical simulation of approach slabs to provide a comprehensive assessment of their response and inform future decisions about approach slab design and maintenance.
A four-lane cast-in-place approach slab and a three-lane precast approach slab were instrumented during construction and subsequently monitored for 2.5 years. The field results suggest strong correlation of slab deformation and stress with temperature. Nonlinear relationships between slab stress and temperature were also observed, which can be attributed to boundary condition evolution over time. Field data suggest several locations at the bottom of the slabs with a potential risk of cracking. Static truck load tests were conducted at various traffic lane and shoulder locations on each of the instrumented approach slabs. Finite-element models were developed to simulate slab behavior under controlled live loading and thermal effects. Numerical modeling of slabs subjected to truck loads is used to estimate the modulus of subbase support under the approach slab. A data driven method using Multilayer Perceptron (MLP) networks to help efficiently estimate approach slab support conditions is proposed. A nonlinear thermal gradient profile is used to improve the ability of the finite element models to properly capture slab behavior under thermal effects. Solar radiation is found to introduce peak stresses greater than the live load stresses. It is also observed that simplified structural analysis in practice (neglecting parapets) can significantly underestimate stresses in slab edge regions.
A parametric study is then carried out, which uses validated numerical simulations of IAB approach slabs in Illinois based on typical design and construction practices. From the parametric study, it is suggested that such approach slabs would generally not be prone to cracking from truck live loads, although certain skews and widths can increase the chances of cracking. When an approach slab is subjected to the combined effects of low temperature and solar radiation, the principal stress can reach the concrete modulus of rupture, and its distribution confirms some crack patterns observed in the field, especially for highly skewed approach slabs. Increasing the slab thickness and/or releasing restraint at the approach slab-abutment interface may be able to mitigate structural distress from live load and/or thermal effects for IAB approach slabs in Illinois. In addition, lateral restraint from the abutment can be partially released by introducing new details that allow the approach slab to expand and contract with less restriction. This “breathing room” in the transverse direction is expected to reduce the potential risk of cracking due to thermal loads.
