Cost-Effective Plate-Girder Designs

To get cost-effective results from the many different designs of fabricated girders that can satisfy the requirements of specifications, designers should obtain advice from fabricators and contractors whenever possible. Also useful are steel-industry-developed rules-of-thumb intended to help designers. The following recommendations, modified to reflect current trends, should be considered for all designs.
1. Load-and-resistance factor design (LRFD) is the preferred design procedure. Load-factor design (LFD) yields more economical girder designs than does allowable-stress design (ASD).
2. Properly designed for their environment, unpainted weathering-steel bridges are more economical in the long run than those requiring painting. Consider the following grades of weathering steels: ASTM A709 grade 50W, 70W, HPS70W, or 100W. Grade 50W is the most often used.
3. The most economical painted design is that for hybrid girders, using 36-ksi and 50-ksi steels. Painted homogenous girders of 50-ksi steel are a close second. The most economical design with high performance steel (HPS) will also be hybrid, utilizing grade 50W steel for all stiffeners, diaphragm members, and web and flanges, where grade 70W strength is not required. Rolled sections (angles, channels, etc.) are not available in HPS grades.
4. The fewer the girders, the greater the economy. Girder spacing must be compatible with deck design, but sometimes other factors govern selection of girder spacing. For economy, girder spacing should be 10 ft or more.
5. Transverse web stiffeners, except those serving as diaphragm or cross-frame connections, should be placed on only one side of a web.
6. Web depth may be several inches larger or smaller than the optimum without significant cost penalty.
7. A plate girder with a nominally stiffened web 1⁄16 in thinner than an unstiffened web  will be the least costly or very close to it. (Unstiffened webs are generally the most cost-effective for web depths less than 52 in. Nominally stiffened webs are most economical in the 52- to 72-in range. For greater depths, fully stiffened webs may be the most cost-effective.)
8. Web thickness should be changed only where splices occur. (Use standard-platethickness increments of 1⁄16 in for plates up to 2 in thick and 1⁄8-in increments for plates over 2 in thick.)
9. Longitudinal stiffeners should be considered for plate girders only for spans over 300 ft.
10. Not more than three plates should be butt-spliced to form the flanges of field sections up to 130 ft long. In some cases, it is advisable to extend a single flange-plate size the full length of a field section.
11. To justify a welded flange splice, about 700 lb of flange steel would have to be eliminated.
However, quenced-and-tempered plates are limited to 50 ft lengths.
12. A constant flange width should be used between flange field splices. (Flange widths should be selected in 1-in increments.)
13. For most conventional cross sections, haunched girders are not advantageous for spans under 400 ft.
14. Bottom lateral bracing should be omitted where permitted by AASHTO specifications.
Omit intermediate cross frames where permitted by AASHTO (see LRFD Specification Art, 6.7.4) but indicate on the plans where temporary bracing will be required for girder stability during erection and deck placement. Space permanent intermediate cross frames, if required, at the maximum spacing consistent with final loading conditions.
15. Elastomeric bearings are preferable to custom-fabricated steel bearings.
16. Composite construction may be advantageous in negative-moment regions of composite girders.
Designers should bear in mind that such techniques as finite-element analysis, use of high-strength steels, and load-and-resistance-factor design often lead to better designs.
Consideration should be given to use of 40-in-deep and 42-in-deep rolled sections. These may be cost-effective alternatives to welded girders for spans up to 100 ft or longer. Economy with these beams may be improved with end-bolted cover-plate details that allow use of category B stress ranges (Art. 11.10). Contract documents that allow either rolled beams or welded girders ensure cost-effective alternatives for owners.
With fabricated girders, designers should ensure that flanges are wide enough to provide lateral stability for the girders during fabrication and erection. Flange width should be at least 12 in, but possibly even greater for deeper girders. The AISC recommends that, for shipping, handling, and erection, the ratio of length to width of compression flanges should be about 85 or less.
Designers also should avoid specifying thin flanges that make fabrication difficult. A thin flange is subject to excessive warping during welding of a web to the flange. To reduce warping, a flange should be at least 3⁄4 in thick.
To minimize fabrication and deck forming costs when changes in the area of the top flange are required, the width should be held constant and required changes made by thickness transitions.


Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.