T-Beams: A Structural Element

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Overview and History of T-Beams

In some respects, the T-beam dates back to the first time a human formed a bridge with a pier and a deck. After all, a T-beam is, in one sense, no more than a pillar with a horizontal bed on top, or, in the case of the inverted T-beam, on the bottom (Ambrose & Tripeny, 2007, p. 104). The upright portion is termed a web, and the horizontal part is called a flange. Though the materials used have changed over the years, this basic structure seems moderately immutable, with the exception that sometimes, as one can see merely from observing real-life structures such as highway overpasses, extra material is added on the underside where web joins flange, presumably to reduce the T-beam’s vulnerability to shear stress. However, when one investigates more deeply the design of T-beams, some nuances appear.

Design of T-Beams

The T-beam, though simple in essence, contains multiple design elements of interest. Unlike an I-beam, a T-beam lacks a bottom flange, which carries savings in terms of materials, but at the loss of some resistance to tensile forces (Mirza & Furlong, 1985, p. 112). When used for bridges, however, it is obvious upon reflection that this lack of a bottom flange on a T-beam actually serves as an advantage in that the narrow bottom end of the T-beam can be more easily inserted into the bed of the body of water in question. However, the simplicity of T-beams has been called into question by some who would rightly test more complex structures; for example, Cheng, Mohammed, and Mustapha (2009) tested pre-tensioned inverted T-beams with circular web openings (p. 203), with mixed but generally favorable results. Thus, tentative conclusions can be drawn that in some cases, the extra time and effort invested in creating a more complex structure proves worthwhile. A simpler matter to consider is that of which material or materials will be used in the planning and construction of T-beams.

Steel T-Beams

Though steel T-beams would be quite strong if used, the reality is that for most roadways and bridges today, it is more practical to bring concrete into the design as well. As pointed out by McCormac and Brown (2007), most T-beam construction is done not with steel or concrete alone, but rather with the composite of the two, namely, reinforced concrete. Though the term could refer to any one of a number of means of reinforcement, generally, the definition is limited to concrete poured around the rebar. MacGregor et al. (1997) describe this material and its use in T-beams at length, concluding that it is invaluable for the types of structures found in modern architecture. This shows that in considering materials available for a task, engineers need to consider the possibility that no one single material is adequate for the job; rather, combining multiple materials together may be the best solution. Thus steel and concrete together can prove ideal.

Concrete T-Beams

Concrete alone can be brittle and thus overly subject to the shear stresses a T-beam faces where web and flange meet, even if the concrete is poured as a unit to strengthen it, and it is for this reason that steel is often combined with concrete in T-beams. Lim, Paramasivam, and Lee (1987) discuss the problem of shear stress leading to failures of flanges detaching from webs when under load. This, of course, could prove catastrophic if allowed to occur in real life, hence the very real need to mitigate that possibility with reinforcement for concrete T-beams. Of course, in such composite structures, many questions arise as to the particulars of the design, including what the ideal distribution of concrete and steel might be: “To evaluate an objective function, a ratio of steel to concrete costs is necessary” (Chou, 1977, p. 1605). This demonstrates that for all aspects of the design of composite T-beams, equations can be derived if only one has adequate information. Still, there are aspects of design that some may not even have considered, such as the possibility of using external fabric-based reinforcement, as described by Chajes et al. (1995), who says of their tested beams, “All the beams failed in shear and those with composite reinforcement displayed excellent bond characteristics. For the beams with external reinforcement, increases in the ultimate strength of 60 to 150 percent were achieved” (p. 295). Clearly, when it comes to resistance to shear forces, external reinforcement is a valid option to consider. Thus, overall, the multiple important aspects of T-beam design impress themselves upon the student of engineering. 

References

Ambrose, J. E., & Tripeny, P. (2007). Simplified design of concrete structures (8th ed.). Hoboken, NJ: John Wiley & Sons.

Chajes, M. J., Januszka, T. F., Mertz, D. R., Thomson Jr., T. A., & Finch Jr, W. W. (1995). Shear strengthening of reinforced concrete beams using externally applied composite fabrics. ACI Structural Journal, 92(3), 295-303.

Cheng H.T., Mohammed B.S., & Mustapha K.N. (2009). Experimental and analytical analysis of pretensioned inverted T-beam with circular web openings. International Journal of Mechanics and Materials in Design, 5(2), 203-215.

Chou, T. (1977). Optimum reinforced concrete T-beam sections. Journal of the Structural Division, 103(8), 1605-1617.

Lim, T. Y., Paramasivam, P., & Lee, S. L. (1987). Shear and moment capacity of reinforced steel-fibre-concrete beams. Magazine of Concrete Research, 39(140), 148-160.

MacGregor, J. G., Wight, J. K., Teng, S., & Irawan, P. (1997). Reinforced concrete: Mechanics and design (Vol. 3). Upper Saddle River, NJ: Prentice Hall.

McCormac, J. C., & Brown, R. H. (2007). Design of reinforced concrete (8th ed.). Hoboken, NJ: John Wiley & Sons.

Mirza S. A., & Furlong R. W. (1985). Design of reinforced and prestressed concrete inverted T- beams for bridge structures. Prestressed Concrete Institute, 30(4), 112-136.