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Title:Serviceability Limits and Economical Steel Bridge Design
Authors:Michael G. Barker, Lorehana Gandiaga, and James Staebler
Publication Date:Dec 2008 (Interim Report)
Report #:MPC-08-206
Project #:MPC-270
Keywords:steel bridges, high performance steel, deflection
Type:Research Report – MPC Publications

 

Abstract

High Performance Steel (HPS) is a superior material with higher yield strength, improved weldability, greater levels of toughness, and improved weathering resistance that can lead to more economical bridges than conventional steel bridge designs. AASHTO Load and Resistance Factor Design (LRFD) provisions show that using HPS only in the highly stressed regions (hybrid design) can lead to superstructure steel weight savings of up to 20 percent and a cost saving above 10 percent compared to conventional steel bridges.

However, using HPS results in less steel material and, therefore, live load deflections increase over conventional bridges. This means that sometimes deflection will control and the weight and cost savings high performance steel affords will not be realized. This problem is exacerbated in that many states enforce deflection criteria that are more restrictive than those prescribed in AASHTO. In fact, these conservative states actually require additional steel over conventional designs due to stiffness requirements and not strength design. States are spending more on bridges than prescribed by AASHTO design provisions. Limited studies show that many states put an additional 25 percent steel into bridges to meet their conservative deflection criteria rather than saving 20 percent by using high performance materials. This results in a difference of 40-50 percent in the weight of the steel.

States, counties and local municipalities are replacing state and local bridges at an increasing rate due to design life termination, deterioration and functional and structural deficiencies. High Performance Steel shows great promise for producing more durable and more economical bridges. With current deflection limitations, however, many of these state and local bridges will be negatively impacted by deflection limitations. Success will be measured by state adoption of reasonable deflection criteria (this has happened in New Hampshire and Missouri during past limited studies.)

A group of 6 states will be selected for the serviceability comparisons. The states will represent various levels of conservativeness across the country. The states include representation from the Mountain-Plains Consortium region. A set of 5 High Performance Steel bridges that are in service (one with significant field test data) will be used for the study. The bridges range in length, width, girder spacing and geographical location.

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