Economical Plate Selection for Bridges

Introduction:
This Technical Bulletin is intended to provide fabricators and engineers with a consolidated reference for use in selecting and ordering the optimum plates for their respective bridge projects. It reflects the data on plates produced by Bethlehem-the widest range available in the U.S.

Specifications:
Both ASTM and AASHTO have published specifications intended to stand alone as the preferred specification designation for bridge steels. ASTM A709 and AASHTO M270-both titled "Structural Steel for Bridges"- consolidate all prequalified bridge steels in one document. These specifications now list complete chemical compositions and mechanical properties. They also include easy-to-use tables for specifying Charpy V-notch toughness requirements for both fracture critical and non-fracture critical members for all grades in all temperature zones. Bethlehem Technical Bulletin TB-311 covers these specifications in greater detail. These prequalified steels are limited to a thickness of 4". Thicker plates are available but must be approved by the bridge owner prior to their use.

The following table (Table A) illustrates the relationship between A709/M270 and the traditional ASTM and AASHTO M numbers:

Grade Price Comparisons:
In general, the unit price of plates increases as their strength increases. This is due to the addition of alloys or to more demanding rolling procedures or to heat treatment requirements. These costs are reflected in the specification extras which are added to the base price. Keep in mind that these costs are more than offset by the savings in weight that result from the use of higher strength grades. The following table shows the relative price differential between those grades prequalified for use in bridges:

Plate Availabilities:
A steel plate is a flat, hot-rolled steel product which is sold in cut lengths. The longitudinal edges can be mechanically sheared or thermally cut by the mill to the ordered width prior to shipment. The key word here is "ordered". Bethlehem will provide plates meeting your exact order requirements. Contact your local sales office for details.

Plate Thickness:
The range of available thicknesses for a plate grade is part of its specification as shown in Table A. the mill will produce to any decimal thickness, plus or minus ASTM A6 tolerances.

However, the standard gage increments are :

For example, for web thicknesses up to _", the designer can investigate increments of 1/32" in order to select the most economical plate. For webs over _" thick, this increment changes to 1/16". The designer should select the exact thickness that meets the design requirements and nothing more-within the standard gages shown. Anything else is wasteful.

Plate Width and Length:
The available width and length of a plate grade are determined by the capability of the producer-not by the specification. Each steel plate producer has individual limitations of width and length for a given thickness. Bethlehem has charts available that reflect our length capabilities for our full range of widths and thicknesses. Contact Construction Marketing for details.

Design Considerations:

1. Corrosion Resistance
In general, the slight variations in steel compositions are relatively unimportant with respect to atmospheric corrosion. One exception is a variation in copper content. The presence of copper increases the atmospheric corrosion resistance of steel by its ability to form an adherent rust layer that acts as a self-healing barrier against on-going corrosion. Nickel, chromium, phosphorus and silicon, in the presence of copper, further enhance this corrosion resistance. This combination constitutes the basic composition for A588 steel.

The superior corrosion resistance of these weathering steels has been clearly documented. The FHWA has published a technical advisory entitled "Uncoated Weathering Steel Structure" addressing the application of weathering steel for highway and bridge structures. With FHWA permission, Bethlehem has reprinted these guidelines as a Technical Bulletin, TB-307. The proper application of unpainted weathering steel can provide substantial savings in both the first cost and the life cycle costs of bridges since little painting or repainting is required. An addition to the weathering steels is A709 Grade 70W (M270 Grade 70W) which is a quenched and tempered steel with a yield strength of 70 ksi.

2. Fatigue Properties
Metal fatigue is the process by which portions of a structure may degrade or fail due to cycle loading. In general, fatigue involves 2 stages: 1) initiation of a crack, and 2) its subsequent growth to failure. The number of loading cycles required for crack initiation depends on the material strength, the load range and stress concentration factor of the notch.

Since crack growth is independent of strength, it is seldom possible to achieve greater fatigue life through material selection alone. Proper methods of design, detailing and fabrication are also necessary in order to maximize fatigue life. Well designed and fabricated details are critical to maximize fatigue life. Well designed and fabricated details are critical to maximizing the fatigue efficiency of a bridge. If current AASHTO details are followed, fatigue is not likely to be a problem.

3. Costs
Any good design must consider the finished cost of the girder. Variables such as stiffeners versus web thickness, flange splices versus common width and thickness of flanges, and ability to erect the girder must all be evaluated. In many cases, simpler fabrication and erection will offset added material costs. The input of a fabricator and contractor during design is encouraged.

Girder Optimization Study:
The selection of the grade of material to be used in bridge girders is one of the most important decisions to be made affecting its economics. The following is an attempt to summarize the effects of grade selection on the economies of a girder design. This study used Bethlehem's preliminary bridge girder optimization program. This program is unique because it optimizes bridge girders on the basis of least cost, not least weight. Both material and fabrication costs are built into the program allowing comparisons to be made on a cost index basis.

WSD

LFD

The above results are typical for any girder comparisons we have made. The most cost effective choice of steel grade is always unpainted A709/M270 Gr 50W weathering steel. Its cost advantage compared to painted high-strength steel designs can range up to 15-16% for the main girders. And this is considering first cost only. When future repainting costs are included, the cost advantage is even larger.

In environments which are inappropriate for unpainted weathering steel, the most economical painted designs use high-strength steel A709/M270 Gr 50. They are always less expensive than Gr 36 designs, generally by 6 to 10%.

Summary:

• Order according to the established specifications.
• Note length, width and thickness availabilities, in particular the graduated increments of thickness available.
• Plates are custom rolled and cut to any ordered size within the availability ranges.
• Use weathering steels whenever applicable; if not, use high-strength steel-A709 Gr 50 or M270 Gr 50.
• Select details that are less fatigue sensitive.
• Fabricator and contractor input is encouraged for economical bridge designs.

For More Information:

Call Construction Marketing, Bethlehem, Pa (610) 694-5906

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