Superseded by 48 FR 21409 on 5/12/1983
Alternate Bridge Designs FHWA Technical Advisory T 5140.12
December 4, 1979
- Purpose. To acquaint engineers and administrators with bidding alternate bridge designs and to emphasize the need for alternate designs which provide competition and resultant economy.
- In light of dramatic increases in highway construction costs in recent months, there is a strong Federal effort to take measures to keep contracts within price guidelines. One method to reduce costs is to encourage competition in bidding.
- Currently, in the United States, there is a trend among some State highway agencies, with encouragement from the Federal Highway Administration, to produce alternate designs and to use price adjustment clauses which encourage cost saving changes initiated by the contractor. Because of fluctuating economic conditions, it is felt that on multiple repetitive spans, long spans or major bridges, or where there is an extended period of design from conception of the project to a release for bids, there can be no assurance of price stability for a particular material or construction methodology. With alternate designs, no matter how the economy changes, more designs are available at the time of bidding than are likely to be suited to the prevailing economic conditions. Further, because of the competition reflected in the alternates-between materials (structural steel vs. concrete or prestressing bars vs. strand) or construction procedures (cast-in-place vs. precast segmental or balanced cantilevered vs. incremental launching, etc.)--a cost savings in construction of 10 percent or more may be realized.
- Erection methods are as varied as the ingenuity of the contractor and as dictated by the constraints of the site, equipment available to the contractor, and the economic limitations of the project site.
- To receive the most economical construction between basic structural materials, consistent with geographic, environmental, ecological or other site restrictions, there should be maximum opportunity for competition between structural steel and concrete.
- Within environmental, aesthetic, site constraints, etc., the plans and bid documents should show or otherwise indicate what alternate type structures will be allowed or considered. The contractor should be allowed the option to bid any designated alternate design that is consistent with the contractor's expertise, available equipment, etc.
- Bid documents and the contract plans should clearly indicate the design criteria and what type alternate designs and/or contractor options will be acceptable. Determination of practical and economical alternates and/or contractor options should be developed in the preliminary design.
- Bid documents should be considered as "open" documents in regard to construction method, erection systems, and prestressing systems.
- Consistent design criteria should be used for alternates, i.e., if load factor design is used, it should be used for all alternates.
- Span lengths should be identified on the contract plans. However, other than where pier locations are constrained by physical and geological conditions at the site, consideration should be given to allowing a tolerance in pier location to avoid placing a particular alternate at an economic disadvantage. For example, in a typical three span structure, the side span should be approximately 80 percent of the center span for structural steel, 70 percent for conventional cast-in-place concrete on falsework, and 65 to 70 percent in segmental balanced cantilever construction.
- To avoid an economic disadvantage to a particular superstructure alternate, alternate substructure designs may be required. Limitations on the substructure, such as allowable axial load and moment, should be clearly identified on the contract plans.
- Where specific design requirements are not covered by the American Association of State Highway and Transportation Officials (AASHTO) Bridge Specifications, the contractor should be allowed to use other recognized codes and standards where applicable. However, the alternate design should document where these provisions are to be used, why the AASHTO requirements do not apply, and which articles of the substituted code or standard are to be used. Such provisions should be subject to approval by the engineer and appropriate agencies.
- Prebid conferences are to be encouraged as a means of communication between the engineer, highway agencies and contractors.
- In order to allow a contractor adequate time to investigate the various alternatives and prepare plans, it is recommended that the advertising time be commensurate with the size and complexity of the project with a minimum of 60 days.
- In order to allow adequate review and checking of the low bidder's proposal, award of contract should be extended commensurate with the size of project.
- Prestressed Concrete Alternate
- To increase the competition in post-tensioned concrete construction, it is recommended that plans and other bid documents allow conventional cast-in-place on falsework, precast prestressed span units, and segmental construction or combinations thereof.
- Segmental construction should allow the following at the contractor's option:
- Precast or cast-in-place segmental construction.
- Any of the post-tensioning systems, i.e., strand, wire or bars or combinations thereof.
- Any of the following construction methods: balanced cantilever, span-by-span, progressive planning, incremental launching, or combinations thereof.
- Exterior dimensions of the cross-section should be fixed. At the contractor's option, the thickness of webs and flanges may be varied to accommodate proposed construction and erection methods and post-tensioning systems, providing that any changes in the dead weight, shear, etc., are accommodated in the design.
- The contract plans should indicate the maximum and minimum final prestressing force (Pf) and moment (Pf x e) required, after all losses, for the final condition of the structure; i.e., dead, live, impact and all superimposed loads. Any increase in prestressing force requirements as a result of the method of construction, erection, type of tendon system should be evaluated at the shop drawing stage.
- Changes in eccentricity of prestress should be accompanied with appropriate changes in prestress force to produce the same minimum compressive stress due to prestress.
- The minimum prestress force should be such that under any loading condition, both during and after construction, stresses will be within allowable limits. Consideration should be given to secondary moments due to prestress, redistributed moments due to creep, and stresses resulting from thermal gradient (between the top and bottom of the girder and between the inside and outside of webs).
- Contractor revisions to contract plans, with supporting calculations, should be submitted to the engineer for approval.
- Structural Steel Alternate
- Any changes in member sizes or details resulting from erection procedures or methods should be evaluated at the shop drawing stage.
- To increase the competition in structural steel construction, it is recommended that plans and other bid documents allow the following, where feasible:
- Option in field and shop splice locations.
- In bolted compression splices, the option of transferring 50 percent of the load through milled ends of the members.
- Consideration of options in details to simplify or facilitate fabrication and erection.
- Option of using rolled sections in lieu of fabricated members.
Rex C. Leathers
Director, Office of Engineering