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A T-beam, used in construction, is a load-bearing structure of reinforced concrete, wood or metal, with a t-shaped cross section. The top of the t-shaped cross section serves as a flange or compression member in resisting compressive stresses. The web of the beam below the compression flange serves to resist shear stress and to provide greater separation for the coupled forces of bending.
The T-beam has a big disadvantage compared to an I-beam because it has no bottom flange with which to deal with tensile forces. One way to make a T-beam more efficient structurally is to use an inverted T-beam with a floor slab or bridge deck joining the tops of the beams. Done properly, the slab acts as the compression flange.
Overview and History of T-Beams
Design of T-Beams
Steel T-beams manufacturing process includes: hot rolling, extrusion, plate welding and pressure fitting. A process of large rollers connecting two steel plates by pinching them together called pressure fitting is a common process for non-load bearing beams. 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 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 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.
Reinforced Concrete T-Beams
Concrete alone is brittle and thus overly subject to the shear stresses a T-beam faces where the web and flange meet. This is the reason that steel is 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 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. 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 are made only if 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 say 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 ultimate strength of 60 to 150 percent were achieved” (p. 295). 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.
An issue with the T-beam compared to the I-beam is the lack of the bottom flange. In addition, this makes the beam not as versatile because of the weaker side not having the flange making it have less tensile strength.
Concrete beams are often poured integrally with the slab, forming a much stronger “T” – shaped beam. These beams are very efficient because the slab portion carries the compressive loads and the reinforcing bars placed at the bottom of the stem carry the tension. A T-beam typically has a narrower stem than an ordinary rectangular beam. These stems are typically spaced from 4’-0” apart to more than 12’-0”. The slab portion above the stem is designed as a one-way slab spanning between stems (see Lecture 6).
- Diagram of a tapered T-beam
- Diagram of a square T-beam
- BS-4 Standard steel T-section beam tables, 1993
- BS-4 Standard steel T-section beam tables, 1971
- 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.
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