Bridge Structures

This article discusses issues of structural design revealed by the collapse of I35W Bridge on August 1st, 2007. The article is based on analysis of the original bridge design drawings, a series of detailed finite elements computations, and material evidence disclosed by National Safety Transportation Safety Board (NTSB) official investigation. These issues include (i) redundancy considerations for multi-span bridge; (ii) reason for under-designed bridge elements; (iii) effects of lateral force on gusset plates' load capacity in a steel-truss structure; (iv) criterion of gusset plate's stability and thin-plate theory-based model for load-rating. This analysis concludes that the lessons learned from the I35W Bridge collapse may have certain significance for the safety assessments of similar steel bridges. According to recent surveys, there are scores of this kind of aged bridges that are still in service [38, 41].

Involving shrink fitting, two procedures for assembling steel fulcra of simple-trunnion bascule bridges are quantitatively compared for the likelihood of fracture during assembly. In assembly procedure called AP1, the trunnion is shrink fitted into a hub, followed by shrink fitting the trunnion-hub assembly into the girder of the bridge. In assembly procedure called AP2, the hub is shrink fitted into the girder, followed by shrink-fitting the trunnion in the hub-girder assembly. A formal design of experiments is conducted to find the influence of geometrical parameters such as the radial thickness of the hub, radial interference, and various shrink-fitting methods on the design parameter of critical crack length – a measure of likelihood of fracture. For single-staged shrink-fitting methods, for high and low hub radial thickness to hub inner diameter ratio, assembly procedure AP1 and AP2 are recommended, respectively. For fulcra with low hub radial thickness to hub inner diameter ratio and where staged shrink-fitting methods are used, for AP2, cooling the trunnion in dry-ice/alcohol and heating the girder, and for AP1, cooling the trunnion-hub assembly in dry-ice/alcohol followed by immersion in liquid nitrogen is recommended. For fulcra with high hub radial thickness to hub inner diameter ratio and where staged shrink-fitting methods are used, cooling the components in dry-ice/alcohol and heating the girder is recommended for both AP1 and AP2.

The vulnerability of bridges to blast hazards is an increasing concern for engineers and governments agencies, and the public. Blast hazards on structures can be classified as either accidental hazards or intentional blast attacks and structures should be protected to mitigate these hazards. Current bridge design codes do not account for blast loading and there is a need for guidelines for designing structures such as bridges and other structures to resist blast loads. The individual response of structural elements subjected to blast loads depends mainly on the standoff distance, charge weight, and the distribution of the blast pressure along the length of the loaded member. Structural components such as bridge piers subjected to blast hazards along with their connections should be designed to have considerable ductility and energy absorption capacity. This paper presents a simplified design method for members subjected to unconfined blast loads such as bridge piers. Estimating blast loads on a member due to specific blast scenario is complex. To simplify the analysis, dynamic blast loads were transformed into equivalent static loads and simplified blast load response spectra were developed based on approximate blast pressure distributions. Blast response spectra can be used to analyze and design individual structural components subjected to blast loads in flexure, estimate the required ductility, and estimate the minimum standoff distance for the different probable blast charge weights. For complex critical structures, more rigorous analysis is needed including the global response of the structure.

The obvious, if not “scandalous”, preference of Codes in rectangular sections (regarding the mathematical expressions of their resistances, etc.) compared to other types of sections rules out circular sections; although circular sections possess a significant share in constructions, especially concerning columns and piers. A common deficit is the serviceability cracking check of circular section columns and piers. This check proves to be critical at bridges of large span, mainly due to constrained expansion/contraction of the bridge superstructure. In the present study, an analytical investigation of the problem regarding circular sections takes place. Afterwards, the current work suggests two groups of diagrams, as far as the serviceability cracking check is concerned. One group has to do with the environmentally favourable cases and the other with the unfavourable cases. On one hand, the consultation of those diagrams takes place through the service eccentricities and on the other hand through the longitudinal reinforcement ratio. Both the maximum compression stress of concrete and the tensile stress of reinforcement are defined in this way. The latter is defined either in the centre of gravity of the tensile zone or in the extreme fiber of the tensile zone. The work herein concerns mainly designers and it covers a subject that neither Design Codes nor international bibliography provides sufficient data.