Hybrid Comes Standard

Feb. 12, 2007

The future for economical bridge designs using high-performance steel (HPS) is promising. HPS 70W (yield strength of 70 ksi) is a superior steel with higher yield strength, improved weldability, greater levels of toughness and improved weathering resistance that can lead to less costly and more durable bridges than past conventional designs.

The future for economical bridge designs using high-performance steel (HPS) is promising. HPS 70W (yield strength of 70 ksi) is a superior steel with higher yield strength, improved weldability, greater levels of toughness and improved weathering resistance that can lead to less costly and more durable bridges than past conventional designs.

The effort to produce a new HPS started in 1992 with collaboration between the American Iron & Steel Institute, the Federal Highway Administration (FHWA) and the U.S. Navy. HPS was quickly implemented with the first bridges using HPS built in Tennessee and Nebraska in 1997. In 2006, there were approximately 200 bridges using HPS in service in 43 states. The success of HPS can be attributed to better economy with a material that behaves like conventional steel in the design, fabrication and erection processes, all while exhibiting superior qualities.

It has been shown that the most economical use of HPS is in hybrid steel girder bridges, only using the HPS material (with its increased costs) in the highly stressed regions of the girder and conventional steel (50 ksi) in the remaining portions. Savings of up to 28% of the steel weight and 18% of the steel cost have been realized in New York state.

The Missouri Department of Transportation (MoDOT) built the state’s first HPS bridge in 2002 (let in 2001, opened in 2002) as part of the FHWA’s Innovative Bridge Research and Construction (IBRC) program. MoDOT used HPS 70W steel for a 138-ft x 138-ft, two-span, two-lane, five-girder bridge. The girders are hybrid with HPS used for the bottom flange in the composite positive-moment region and for both flanges in the noncomposite negative-moment region. The American Association of State Highway Transportation Officials (AASHTO) Load Factor Design (LFD) provisions show that using HPS hybrid girders led to a superstructure steel weight savings of nearly 17% over a conventional 50-ksi bridge design. At the time of fabrication, HPS 70W was estimated to cost approximately 30% more than the 50-ksi material. Thus, the 17% steel weight savings corresponds to an overall steel cost savings of 11%.

Having the first HPS bridge in Missouri, MoDOT was interested in the deflection serviceability performance of the in-service structure. There was some concern about the live-load deflections with the lighter superstructure. HPS girders result in lighter sections with less steel material corresponding to a reduction in girder stiffness, therefore, live-load deflections increase over conventional 50-ksi bridges.

AASHTO LFD design requirements limit live-load deflection to 1?800 of the span length (L/800) for bridges with minimal pedestrian traffic. L/1,000 is imposed for bridges with significant pedestrian traffic. The AASHTO limits, although controlling deflection, are actually intended to limit the bridge’s acceleration. Accelerations or vibrations can annoy pedestrians and those riding in vehicles. In addition to causing user discomfort, it also has been thought that excessive deflections may contribute to bridge deterioration.

AASHTO LFD procedures specify that side-by-side standard HS20 trucks on the bridge are to be distributed equally to all of the girders. Irrespective of the number of striped lanes, the number of loaded lanes for deflection calculations is assumed to be the number of whole 12-ft lanes across the clear width of the bridge. In addition, a multipresence intensity reduction factor (1.0 for one or two lanes, 0.9 for three lanes and 0.75 for more than three lanes) is applied as the number of lanes increase. Therefore, for the Missouri HPS bridge there are three 12-ft lanes and the appropriate multipresence reduction factor is 0.9. The American Iron & Steel Institute design software SIMON predicts the maximum live-load deflections according to the AASHTO LFD provisions. The maximum live-load deflections predicted were 1.54 in. for an interior girder and 1.57 in. for an exterior girder. Thus, the AASHTO deflection limit of L/800=2.07 in. (138 ft x 12 in./ft/800) was satisfied.

Loading . . .

The University of Missouri-Columbia worked with MoDOT and West Virginia University to instrument, field test, analyze and evaluate the service performance of Missouri’s first HPS bridge. The testing concentrated on the serviceability behavior of the completed structure. Finite-element modeling was employed for verification and parametric studies.

Live-load static-deflection testing of Missouri’s HPS bridge was accomplished by running a calibrated load truck across the bridge at a crawl speed along 12 lateral positions. The lateral positions were chosen so that superposition could be used to add effects of side-by-side truck loading to maximize the individual girder deflections.

For instance, Figure 1 shows that girder three (middle girder) deflection is maximized for the AASHTO deflection design criteria (three 12-ft lanes) by adding measured deflections from truck runs one, five and 10 or truck runs three, eight and 12.

Table 1 shows the equivalent maximum total experimental measured deflections and the controlling load cases for each girder from the serviceability field testing. For the AASHTO criteria, girders one and four are controlled by two-lane loading with an intensity reduction factor of 1.0 instead of three-lane loading and a 0.9 intensity reduction factor. Girders two, three and four are controlled by three-lane loading with an intensity reduction factor of 0.90. The AASHTO design predicted maximum deflection of 1.57 in. matches well (within 1%) with the AASHTO provisions experimental maximum deflection of 1.58 in. In addition, both the interior and exterior girder maximum deflections match well.

More to follow

Missouri’s experience with its first HPS bridge was a success. The state gained confidence in HPS as a bridge material and in the fabrication and erection of HPS bridges. Since that first HPS bridge, Missouri has let 11 projects using HPS materials. A total of 2,080 tons of HPS 70W and 596 tons of HPS 50W (HPS with yield of 50 ksi) were used on the 11 structures let for construction. All of the girders were hybrid plate girders primarily using HPS 70W in both negative-moment region flanges at the interior supports. Two projects also used the HPS 70W in the positive-moment region.

MoDOT will continue to use HPS where it is deemed by the design engineer to be economically beneficial. This would include instances where girder weight and total steel costs can be reduced, deflection limits are satisfied and the location is applicable for the use of weathering steel. The benefits of reduced initial cost, lower life-cycle costs and improved performance over the life of the structure are important considerations when selecting material for structures designed for a life of 75 years or more. HPS has demonstrated that it can and has met those desirable attributes.

About The Author: Barker is a professor of civil and architectural engineering at the University of Wyoming. He can be contacted at 307/766-2916.

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