High Performance Steel Designers' Guide
3.0 Design Features
High performance steels give the designers another option to achieve durable and cost effective steel bridges . HPS design follows the same design criteria and good practice as provided in Section 6 Steel Structures of the AASHTO LRFD Bridge Design Specifications.
Use of HPS 70W generally results in smaller members and lighter structures. The designers should pay attention to deformations, global buckling of members, and local buckling of components. The Service Limit State should be checked for deflection, handling, shipping and construction procedures and sequences.
The live load deflection criteria is considered optional as stated in Section 2, Article 18.104.22.168.2 of the AASHTO LRFD. The reason for this is that past experience with bridges designed under the previous editions of the AASHTO Standard Specifications has not shown any need to compute and control live load deflections based on the heavier live load required by AASHTO LRFD. However, if the designers choose to invoke the optional live load deflection criteria specified in Article 22.214.171.124.2, the live load deflection should be computed as provided in Section 3, Article 126.96.36.199.2 of the AASHTO LRFD. It may be expected that HPS 70W steel designs would exceed the deflection limit of L/800. The designers have the discretion to exceed this limit or to adjust the sections by optimizing the web depth and/or increasing the bottom flange thickness in the positive moment region to keep the deflection within limit.
The AASHTO HPS Guide encourages the use of hybrid girders, i.e. combining the use of HPS 70W and Grade 50W steels. A hybrid combination of HPS 70W in the negative moment regions and Grade 50W or HPS 50W in other areas results in the optimum use of HPS and attain the most economy.
Many State Departments of Transportation and other agencies have designed and constructed HPS bridges. Several organizations have done comparative designs to optimize the use of HPS in combination with other grades of steels. Brief descriptions of the design experience and cost studies of some of the State DOTs and organizations are given in the following sub-sections.
Photo 3.1.1 Snyder Bridge
Nebraska DOT was the first to use HPS 70W in the design and construction of the Snyder Bridge - a welded plate girder steel bridge (See Photo 3.1.1).
The bridge was opened to traffic in October 1997. It is a 150-foot simple span bridge with 5 lines of plate girders of 4' 6" deep. The original design utilized conventional grade 50W steel.
When HPS first became available, Nebraska DOT replaced the grade 50W steel with HPS 70W steel of equal size. The intent was to use this first HPS 70W bridge to gain experience on the HPS fabrication process. The fabricators concluded that there were no significant changes needed in the HPS fabrication process.
The Nebraska DOT in cooperation with the National Bridge and Research Organization commissioned J. Muller International to develop an innovative concept optimizing the use of HPS. The result of this initiative is a two-box girder bridge with full depth composite deck system. The cross section of the system is shown in Figure 3.1.2. This system has two spans of 120 feet each. It is designed for two lanes of traffic with wide shoulder, measuring 44' curb to curb. The system can be used for new bridges and to replace many existing grade separation structures.
Fig. 3.1.2 Nebraska HPS Twin-Box
A two-box girder system was selected because of its simplicity, small size, and efficiency of load distribution, stability for handling and erection, and robustness against vehicle impact. The substructure might be semi-integral or fully integral abutment to eliminate joints and bearings. The superstructure might be semi-continuous or continuous for live load only, or fully continuous for dead load and live load.
HDR Engineering, Inc. in association with the University of Nebraska-Lincoln performed a study to compare the cost differences between bridge designs using HPS 70W, conventional grade 50W and a combination of the two grades of steels. A total of 42 different girder designs were made using the AASHTO LRFD Bridge Design Specifications - HL-93 Live Load. The girder designs had 2-span continuous layout, covering a span range of 150', 200' and 250', variable girder spacing of 9' and 12', and designs in grade 50W, HPS 70W and a variety of hybrid combinations.
The following unit cost data was obtained from the fabricators and used in the relative cost comparison of the various designs:
|Material Cost*||Fabricated Cost*|
|HPS 70W Q&T||$0.54/lb.||$0.75/lb.|
|HPS 70W TMCP||$0.51/lb.||-|
* These costs are subject to change. Contact NSBA for the most current information.
The study concludes that:
- HPS 70W results in weight and depth savings for all span lengths and girder spacing.
- Hybrid designs are more economical for all of the spans and girder spacing. The most economical hybrid combination is grade 50 for all webs and positive moment top flanges, with HPS 70W for negative moment top flanges and all bottom flanges.
- LRFD treats deflection as an optional criteria with different live load configurations. If a deflection limit of L/800 is imposed, deflection may control HPS 70W designs for shallow web depth.
Photo 3.1.4 Martin Creek Bridge
In 1996, Tennessee Department of Transportation (TNDOT) was completing the design of the SR53 Bridge over the Martin Creek using ASTM A709 Grade 50W steel (See Photo 3.1.4). It was TNDOT's first steel bridge design utilizing the new AASHTO LRFD Bridge Design Specifications. The bridge consisted of two 235.5-foot spans, carrying a 28-foot roadway on three continuous welded plate girders spaced at 10' 6" on centers.
At about the same time, HPS 70W steel became available. With support from FHWA, TNDOT offered to test the application of HPS 70W in an actual bridge. In order to provide a true comparison, TNDOT optimized the redesign using HPS 70W for the girders and Grade 50W shapes for the cross-frames. The HPS redesign resulted in 24.2% reduction in steel weight and 10.6% savings in cost. The weight and cost savings of the HPS 70W bridge are shown below.
|Conventional Grade 50W||HPS 70W & Grade 50W|
|Steel Weight||675,319 lb.||511,908 lb.|
|In-Place Cost||$1.00/lb. *||$1.18/lb. **|
|Total Steel Cost||$675,319||$604,051|
* Construction cost in Tennessee in 1996
** This unit cost includes change orders for $25,000 for additional shop splices and $10,000 for change in welding flux.
The bridge was opened to traffic in February 1998. Since then, TNDOT has completed two more HPS 70W bridges. The TNDOT is satisfied with the three HPS 70W bridges constructed to date. TNDOT is currently designing a fourth project utilizing HPS 70W TMCP for webs and flanges, and HPS 50W in other members. The use of HPS has become a routine practice in Tennessee.
Some of Tennessee's optimization techniques are:
- Use uncoated HPS steels.
- Use HPS 70W steel for flanges and webs over interior supports, where moments and shears are high.
- Use hybrid girder sections for composite sections in positive bending, where moments are high, but shears are low.
- Use undermatching fillet welds with HPS 70W to reduce cost of consumables.
- Use constant width plates to the greatest extent possible. Consider plate width changes at field splices wherever practical.
- Consider waiving live load deflection limits for lane loads.
- Use TMCP plates to greatest extent possible.
- Use the new AASHTO Guide For Highway Bridge Fabrication With HPS 70W Steel. Recommendations in the Guide should be followed, with no more stringent requirements added.
The Pennsylvania Department of Transportation (PennDOT) has used HPS 70W in the Ford City Bridge, which was opened to traffic in July 2000. PennDOT performed full-scale tension and fatigue testing, extensive material testing and weld testing on this project.
Photo 3.1.5 Ford City Bridge
It is a three-span continuous welded steel plate girder bridge with spans of 320'-416'-320'. The first span is curved horizontally with a radius of 508'. The other two spans are on tangent. There are four lines of girders spaced at 13.5'. HPS 70W is used in the negative moment regions and grade 50W elsewhere. This hybrid combination of steels resulted in 20% reduction in steel weight, and enabled the girder sections to be constant depth instead of haunched. By eliminating the variable web depth, a costly longitudinal bolted web splice was avoided.
PennDOT has several HPS bridges under construction and design. PennDOT is sponsoring research to realize additional benefits from HPS. It intends to construct an HPS demonstration bridge with innovative corrugated web I-girder. PennDOT is optimistic that HPS will reduce bridge construction costs.
Photo 3.1.6-1 Berkshire over Muitzes Kill Bridge
The New York State Thruway Authority (NYSTA) participated in a cooperative agreement with the Federal Highway Administration (FHWA) to evaluate and document the use of HPS 70W on bridges fabricated and constructed at various locations on the limited access highway system. Under this agreement, NTSTA constructed 7 structures.
The first project was the Berkshire Thruway over the Muitzes Kill Bridge using HPS 70W. It is a 200 ft. long simple-span, jointless bridge carrying two lanes of traffic and consisting of six 72 in. deep plate girders spaced at 8 ft. centers. This bridge was originally designed as a two-span structure using conventional Grade 50W steel. The plans were revised to take advantage of the strength of HPS 70W by eliminating an interior pier.
Next, a series of five bridges that carry local traffic over I-90 were constructed of HPS 70W steel. The NYSTA engineers took advantage of the high strength of HPS 70W to design two-span jointless structures to replace the existing deficient four-span structures. A typical two-span structure is similar to that shown in Photo 3.1.6-2, and consists of 5 lines of 29 in. deep plate girders spaced at 7.5 ft. centers. The span lengths are 100 ft. each.
Photo 3.1.6-2 I-90 Exit 54 Interchange Overpass
For all these bridges, weld procedure qualification tests and diffusible hydrogen tests were conducted prior to commencing fabrication. These tests used the submerged arc welding (SAW) process with matching consumables, i.e. Lincoln LA-100 electrodes in combination with Lincoln Mil800H flux. These bridges were welded in accordance with the Guide for Highway Bridge Fabrication with HPS 70W Steel.
NYSTA concludes that the 40% higher yield strength of HPS 70W over Grade 50W gives the engineers liberty to design longer, shallower spans when strength is the controlling limit state. NYSTA has found this to be beneficial when replacing simply- supported, multi-span structures with continuous-span structures. Should piers could be eliminated without increasing the depth of the girders and vertical clearance increased with little or no modification to the bridge approaches.