T-beam

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Diagram of two T-beams Diagram of two T-beams.svg
Diagram of two T-beams

A T-beam (or tee beam), used in construction, is a load-bearing structure of reinforced concrete, wood or metal, with a capital '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 (vertical section) of the beam below the compression flange serves to resist shear stress. When used for highway bridges [1] the beam incorporates reinforcing bars in the bottom of the beam to resist the tensile stresses which occur during bending. [2]

Contents

The T-beam has a big disadvantage compared to an I-beam (with 'Ɪ' shape) because it has no bottom flange with which to deal with tensile forces, applicable for steel section. 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.

History

A T-beam is a structural element able to withstand large loads by resistance in the beam or by internal reinforcements. 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. [3] The upright portion carrying the tension of the beam is termed a web or stem, and the horizontal part that carries the compression is termed a flange. However, the materials used have changed over the years but the basic structure is the same. T-beams structures such as highway overpasses, buildings and parking garages, have extra material added on the underside where the web joins the flange to reduce the T-beam’s vulnerability to shear stress. [4] However, when one investigates more deeply into the design of T-beams, some distinctions appear.

Designs

Unlike an I-beam, a T-beam lacks a bottom flange, which carries savings in terms of materials, but at the loss of resistance to tensile forces. [5] T- beam designs come in many sizes, lengths and widths to suit where they are to be used (eg highway bridge, underground parking garage) and how they have to resist the tension, compression and shear stresses associated with beam bending in their particular application. However, the simplicity of the T-beam is in question by some who investigate more complex beam structures; for example, a group of researchers tested pretension inverted T-beams with circular web openings, [6] with mixed but generally favorable results. The extra time and effort invested in creating a more complex structure may prove worthwhile if it is subsequently used in construction. The most suitable materials also have to be selected for a particular T-beam application.

Materials

Steel 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. Most T-beam construction is not with steel or concrete alone, but rather with the composite of the two, namely, reinforced concrete. [7] 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. 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. A problem of shear stress can lead to failures of flanges detaching from webs when under load. [8] 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”. [9] 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., 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”. [4] 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.

Issues

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.[ citation needed ]

Double-T beams

A double-T beam or double tee beam is a load-bearing structure that resemble two T-beams connected to each other. Double tees are manufactured from prestressed concrete using pretensioning beds of about 200-foot (61 m) to 500-foot (150 m) long. The strong bond of the flange (horizontal section) and the two webs (vertical members) creates a structure that is capable of withstanding high loads while having a long span. The typical sizes of double tees are up to 15 feet (4.6 m) for flange width, up to 5 feet (1.5 m) for web depth and up to 80 feet (24 m) or more for span length. [10]

Related Research Articles

<span class="mw-page-title-main">Reinforced concrete</span> Concrete with rebar

Reinforced concrete, also called ferroconcrete, is a composite material in which concrete's relatively low tensile strength and ductility are compensated for by the inclusion of reinforcement having higher tensile strength or ductility. The reinforcement is usually, though not necessarily, steel bars (rebar) and is usually embedded passively in the concrete before the concrete sets. However, post-tensioning is also employed as a technique to reinforce the concrete. In terms of volume used annually, it is one of the most common engineering materials. In corrosion engineering terms, when designed correctly, the alkalinity of the concrete protects the steel rebar from corrosion.

<span class="mw-page-title-main">Rebar</span> Steel reinforcement

Rebar, known when massed as reinforcing steel or steel reinforcement, is a steel bar used as a tension device in reinforced concrete and reinforced masonry structures to strengthen and aid the concrete under tension. Concrete is strong under compression, but has low tensile strength. Rebar significantly increases the tensile strength of the structure. Rebar's surface features a continuous series of ribs, lugs or indentations to promote a better bond with the concrete and reduce the risk of slippage.

<span class="mw-page-title-main">Beam (structure)</span> Structural element capable of withstanding loads by resisting bending

A beam is a structural element that primarily resists loads applied laterally across the beam's axis. Its mode of deflection is primarily by bending, as loads produce reaction forces at the beam's support points and internal bending moments, shear, stresses, strains, and deflections. Beams are characterized by their manner of support, profile, equilibrium conditions, length, and material.

<span class="mw-page-title-main">Seismic retrofit</span> Modification of existing structures to make them more resistant to seismic activity

Seismic retrofitting is the modification of existing structures to make them more resistant to seismic activity, ground motion, or soil failure due to earthquakes. With better understanding of seismic demand on structures and with recent experiences with large earthquakes near urban centers, the need of seismic retrofitting is well acknowledged. Prior to the introduction of modern seismic codes in the late 1960s for developed countries and late 1970s for many other parts of the world, many structures were designed without adequate detailing and reinforcement for seismic protection. In view of the imminent problem, various research work has been carried out. State-of-the-art technical guidelines for seismic assessment, retrofit and rehabilitation have been published around the world – such as the ASCE-SEI 41 and the New Zealand Society for Earthquake Engineering (NZSEE)'s guidelines. These codes must be regularly updated; the 1994 Northridge earthquake brought to light the brittleness of welded steel frames, for example.

Fibre-reinforced plastic is a composite material made of a polymer matrix reinforced with fibres. The fibres are usually glass, carbon, aramid, or basalt. Rarely, other fibres such as paper, wood, boron, or asbestos have been used. The polymer is usually an epoxy, vinyl ester, or polyester thermosetting plastic, though phenol formaldehyde resins are still in use.

<span class="mw-page-title-main">Shear wall</span> A wall intended to withstand the lateral load

A shear wall is an element of a structurally engineered system that is designed to resist in-plane lateral forces, typically wind and seismic loads.

<span class="mw-page-title-main">Plate girder bridge</span> Type of bridge

A plate girder bridge is a bridge supported by two or more plate girders.

<span class="mw-page-title-main">I-beam</span> Construction element

An I-beam is any of various structural members with an Ɪ- or H-shaped cross-section. Technical terms for similar items include H-beam, I-profile, universal column (UC), w-beam, universal beam (UB), rolled steel joist (RSJ), or double-T. I-beams are typically made of structural steel and serve a wide variety of construction uses.

<span class="mw-page-title-main">Delamination</span> Mode of failure for which a material fractures into layers

Delamination is a mode of failure where a material fractures into layers. A variety of materials, including laminate composites and concrete, can fail by delamination. Processing can create layers in materials, such as steel formed by rolling and plastics and metals from 3D printing which can fail from layer separation. Also, surface coatings, such as paints and films, can delaminate from the coated substrate.

<span class="mw-page-title-main">Formwork</span> Molds for cast

Formwork is molds into which concrete or similar materials are either precast or cast-in-place. In the context of concrete construction, the falsework supports the shuttering molds. In specialty applications formwork may be permanently incorporated into the final structure, adding insulation or helping reinforce the finished structure.

<span class="mw-page-title-main">Girder</span> Support beam used in construction

A girder is a beam used in construction. It is the main horizontal support of a structure which supports smaller beams. Girders often have an I-beam cross section composed of two load-bearing flanges separated by a stabilizing web, but may also have a box shape, Z shape, or other forms. Girders are commonly used to build bridges.

<span class="mw-page-title-main">Structural steel</span> Type of steel used in construction

Structural steel is a category of steel used for making construction materials in a variety of shapes. Many structural steel shapes take the form of an elongated beam having a profile of a specific cross section. Structural steel shapes, sizes, chemical composition, mechanical properties such as strengths, storage practices, etc., are regulated by standards in most industrialized countries.

<span class="mw-page-title-main">Girder bridge</span> Bridge built of girders placed on bridge abutments and foundation piers

A girder bridge is a bridge that uses girders as the means of supporting its deck. The two most common types of modern steel girder bridges are plate and box.

This is an alphabetical list of articles pertaining specifically to structural engineering. For a broad overview of engineering, please see List of engineering topics. For biographies please see List of engineers.

Glass fiber reinforced concrete (GFRC) is a type of fiber-reinforced concrete. The product is also known as glassfibre reinforced concrete or GRC in British English. Glass fiber concretes are mainly used in exterior building façade panels and as architectural precast concrete. Somewhat similar materials are fiber cement siding and cement boards.

<span class="mw-page-title-main">Structural material</span>

Structural engineering depends on the knowledge of materials and their properties, in order to understand how different materials resist and support loads.

Concrete has relatively high compressive strength, but significantly lower tensile strength. The compressive strength is typically controlled with the ratio of water to cement when forming the concrete, and tensile strength is increased by additives, typically steel, to create reinforced concrete. In other words we can say concrete is made up of sand, ballast, cement and water.

<span class="mw-page-title-main">Arching or compressive membrane action in reinforced concrete slabs</span>

Arching or compressive membrane action (CMA) in reinforced concrete slabs occurs as a result of the great difference between the tensile and compressive strength of concrete. Cracking of the concrete causes a migration of the neutral axis which is accompanied by in-plane expansion of the slab at its boundaries. If this natural tendency to expand is restrained, the development of arching action enhances the strength of the slab. The term arching action is normally used to describe the arching phenomenon in one-way spanning slabs and compressive membrane action is normally used to describe the arching phenomenon in two-way spanning slabs.

<span class="mw-page-title-main">Double tee</span> Type of load-bearing structure

A double tee or double-T beam is a load-bearing structure that resembles two T-beams connected to each other side by side. The strong bond of the flange and the two webs creates a structure that is capable of withstanding high loads while having a long span. The typical sizes of double tees are up to 15 feet (4.6 m) for flange width, up to 5 feet (1.5 m) for web depth, and up to 80 feet (24 m) or more for span length. Double tees are pre-manufactured from prestressed concrete which allows construction time to be shortened.

This glossary of structural engineering terms pertains specifically to structural engineering and its sub-disciplines. Please see glossary of engineering for a broad overview of the major concepts of engineering.

References

  1. ↑ "NCDOT: Reinforced Concrete Tee Beam Bridges".
  2. ↑ Ching, Francis D.K. (1995). A Visual Dictionary of Architecture. New York: John Wiley and Sons. p. 203. ISBN   978-0-471-28451-2.
  3. ↑ Ambrose, James; Tripeny, Patrick (2007). Simplified design of concrete structures (8th ed.). Chichester: Wiley. p. 104. ISBN   978-0-470-04414-8 . Retrieved 26 April 2015.
  4. 1 2 Chajes, Michael J.; Januszka, Ted F.; Mertz, Dennis R.; Thomson, Theodore A. Jr.; Finch, William W. Jr. (1 May 1995). "Shear Strengthening of Reinforced Concrete Beams Using Externally Applied Composite Fabrics". ACI Structural Journal. 92 (3). doi:10.14359/1130 . Retrieved 26 April 2015.
  5. ↑ Furlong, Richard W.; Ferguson, Phil M.; Ma, John S. (July 1971). "Shear and Anchorage Study of Reinforcement in Inverted T-Beam Bend Cap Girders" (PDF). Research Report No. 113-4. Retrieved 26 April 2015.
  6. ↑ Cheng, Hock Tian; Mohammed, Bashar S.; Mustapha, Kamal Nasharuddin (3 March 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. doi:10.1007/s10999-009-9096-4. S2CID   136040255.
  7. ↑ University, Jack C. McCormac, Clemson University, Russell H. Brown, Clemson (2014). Design of reinforced concrete (Ninth edition, ACI 318-11 Code ed.). Hoboken, NJ: Wiley. ISBN   978-1-118-12984-5 . Retrieved 26 April 2015.{{cite book}}: CS1 maint: multiple names: authors list (link)
  8. ↑ Paramasivam, P.; Lee, S. L.; Lim, T. Y. (9 January 1987). "Shear and moment capacity of reinforced steel-fibre-concrete beams". Magazine of Concrete Research. 39 (140): 148–160. doi:10.1680/macr.1987.39.140.148.
  9. ↑ Chou, Takashi (August 1977). "Optimum Reinforced Concrete T-Beam Sections". Journal of the Structural Division. 103 (8): 1605–1617. doi:10.1061/JSDEAG.0004697 . Retrieved 26 April 2015.
  10. ↑ Gurley, Evan; Hanson, Kayla (13 October 2014). "Strength to a Double Tee". Precast Solutions Magazine. Retrieved 26 April 2015.
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