Document Type

Thesis - Open Access

Award Date


Degree Name

Master of Science (MS)

Department / School

Agricultural Engineering


Tri-plate beams combine versatility, economy, and strength in steel design. These beams have many applications which indicate increased efficiency and economy in long-span plate girder construction. Their structural advantages are almost unlimited. Tri-plate beams are fabricated by welding three different types of steel to form an I-section. Each flange and the web of the section is fabricated out a different yield strength steel. Because the flanges of a beam contribute most of its movement resistance, the use of high strength steel of in the flanges is desirable. However, in tri-plate beams, the tension flange consists of a higher strength steel than does the compression flange because of the buckling liability of the compression flange. The utilization of a higher strength steel in the tension flange also makes possible a reduction in the tension flange cross sectional area. Because of its low moment carrying capacity, the web is fabricated out of a lower strength steel. The idea of tri-plate beams is not totally new, having previously been used to a very limited extent in plate girder construction is analogous to composite design, where the reinforced concrete slab is used as a large compression flange, the web is of a low strength steel, and the tension flange is composed of a high-strength steel. High strength steel beams have indicated increased economy in steel design. Tri-plate beams will offer still more economy while, at the same time, provide new possibilities in construction with structural steel. Furthermore, these beams represent savings in material and the availability of lighter sections that can meet the competition presented by prestressed concrete.

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South Dakota State University