- Azizinamini, A; Kowalski, DT
- This paper describes the design and construction of a new steel bridge system that is being used on a more regular basis in the U.S. The system is designed to force the elements of the bridge to act as simple spans for dead loads and continuous under any superimposed loads, after the concrete has cured. This paper provides highlights of the design and construction of the first bridge in fhe U.S. that utilized this concept in conjunction with the use of box sections. The use of this system has resulted in significant economy and is making steel bridges competitive with alternate concrete bridges. Following Nebraska, Tennessee, Illinois, New York, Colorado and Oregon have recently begun using this new system. A joint effort by the Nebraska Department of Roads (NDOR), Federal Highway Administration (FHWA), National Steel Bridge Alliance (NSBA), National Bridge Research Organization (NaBRO) at the University of Nebraska-Lincoln (UNL) and the steel industry recently led to the design, fabrication and construction of the first steel bridge in the U.S. utilizing 689 MPa (100 ksi) High Performance Steel. The project is part of the FHWA IBRC initiative. The bridge is located near Grand Island, Nebraska where highway N-2 passes over interstate highway I-80. The N-2 over I-80 bridge is a two-span steel box-girder bridge, with each span being 42.4 m (139 ft) long. Since the bridge crosses over the 1-80 interstate highway, minimizing the interruption to traffic was a major design consideration. The bridge was designed by NDOR and NaBRO at UNL. It incorporates several features that facilitated both fabrication and construction of the bridge and it incorporates unique and innovative design features. Traditionally, in Nebraska, the maximum bridge girder length has been limited to approximately 36.6 m (120 ft). This limitation reflected the crane capacity of the fabricators, rather than the shipping limitations. The maximum crane capacity for most fabricators in the region is limited to approximately 267 kN (60,000 lbs). Therefore, the maximum weight of each girder had to be kept under this weight limit. The design of the box-girders was based on the assumption that the girders would utilize an hybrid arrangement, with the bottom flanges of the box sections using 483 MPa (70 ksi) High Performance Steel and webs and top flanges utilizing conventional 345 MPa (50 ksi) steel. Based on the parametric study carried out on this hybrid arrangement, it was determined to be the most ideal for the bridge. After the design was completed, conservatively the 689 MPa (100 ksi) HPS was substituted for all webs and flanges. The intent was to demonstrate that there is nothing unusual about 689 MPa (100 ksi) HPS and that it can be fabricated and constructed similar to other steel types. The use of HPS in a hybrid arrangement limited the maximum weight of each girder to 267 kN (60,000 lbs) - the maximum crane capacity of the local fabricators, while increasing the girder lengths from the traditional 36.6 m (120 ft) to 42.4 m (139 ft). The use of HPS plates allowed the use of thinner material for the bottom flanges and reduced the web depth, while meeting all design limitations. The webs of the girder were perpendicular to the bottom flange and were not sloped. This significantly reduced the fabrication time and cost. This allowed the fabricator to use equipment and procedures commonly used for fabricating I-shapes. A unique feature utilized in the bridge was a new system developed at NaBRO. This system is referred to as simple for dead load, continuous for live load. In this system, each individual girder is placed over the supports, in this case over the abutment and middle pier. Prior to hardening of the deck slab, each girder behaves as a simple beam, with maximum moment being at the middle of the span. The detailing is such that the bridge behaves as continuous for any loads applied after the concrete deck has hardened. This includes the live loads due to traffic and dead loads that are applied after the concrete has hardened, such as the weight of the barrier and overlays. The continuity for live and superimposed dead loads is provided by placing additional reinforcement in the slab over the pier, similar to what has been practiced in prestressed concrete bridges for years. In the simple for dead load, continuous for live load bridge system, the need for bolted splices is eliminated, significantly reducing costs. In the new system, the girders are joined together over the pier by casting a concrete diaphragm. The challenge is to connect the girders over the pier in such a manner that would allow them to act as a simple beam during casting the slab. Under the live loads, the bottom flanges of the girders near the pier are subjected to relatively large compressive forces. This compressive force has to be transferred to the concrete diaphragm, joining the girders over the pier. Therefore, if the girders were simply embedded in the concrete diaphragm, there would be a possibility of crushing the concrete in the diaphragm near the bottom flanges of the girders. The same possibility also exists for prestressed girders. However, the difference between steel and concrete girders is that in the case of prestressed girders, bottom flanges are larger and the compressive forces are transferred to a larger area, reducing the compressive stress applied to concrete diaphragm near the bottom flanges. Further, the modulus of elasticity of steel is several times larger than concrete. Full scale tests carried out at NaBRO confirm the fact that under even service loads, there would be a possibility of crushing the concrete in the diaphragm near the bottom flanges if the steel girders are directly embedded in the concrete diaphragm. For the case of I-girders, preliminary research has been conducted at NaBRO to come up with a possible solution. Test data indicates that the proposed detail passes both fatigue and ultimate load tests. The use of the new steel bridge system, namely simple for dead load and continuous for live load, significantly reduced the time required to place the girders over the abutment and middle pier. The I-80 interstate highway had to be closed for only 90 minutes for placing the three girders for each span. This significantly accelerated the construction time and reduced the interruption to traffic. The N-2 bridge over I-80, by utilizing several unique design features and HPS, resulted in a steel bridge system that demonstrated faster construction and reduced cost. The use of HPS allowed the increase of the span length of each girder to beyond the traditional 36.6 m (120 ft), while keeping the total weight of each girder below the crane capacity of local fabricators. The use of the new steel bridge system also allowed the elimination of bolted splices. The girders were connected over the pier with a concrete diaphragm, using a detail recommended and developed by NaBRO. © 2006 Taylor & Francis Group.
- January 1, 2006
Digital Object Identifier (DOI)
International Standard Book Number (ISBN) 10
International Standard Book Number (ISBN) 13
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