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SECTION II. STRUCTURAL MEMBERS

REINFORCEMENT

6-14. Reinforced-concrete structures consist of many types of reinforced structural members.

TYPES OF STRUCTURAL MEMBERS

6-15. Structural members include beams, columns, girders, walls, footings, and slabs. Each of these different structural members interacts with one another to a considerable degree, thus forming a monolithic whole.

BEAM REINFORCEMENT

6-16. Figure 6-4 shows four common shapes of reinforcing steel for beams. The purpose of both the straight and bent-up bar is to resist the bending tension in the bottom of a beam that is over the central portion of the span. A beam requires fewer bars near the ends of the span where the bending moment is small. This is where the inclined portion of a bent-up bar is placed to resist diagonal tension. The reinforcing bars of continuous beams continue across the intermediate supports to resist top tension in the support area. Two types of beam reinforcements are--

Stirrups. Add stirrups or U-shaped bars when the bent-up bars cannot resist all the diagonal tension. Because of the tensile stress on the stirrups, they pass under the bottom bar and are inclined or perpendicular to it to prevent lateral slippage.

Bolsters or high chairs. Devices called bolsters or high chairs (see Figure 6-5) usually support the horizontal reinforcing bars and hold them in place during construction. The purpose of a high chair is to ensure that rebar does not rest on the earth or vapor barrier, but instead is suspended inside the concrete beam where it is more effective and protected from moisture. Stress-carrying reinforcing bars must be placed according to the ACI Code 318-83. Select high chairs with skids for use on vapor barrier or wet sub-base. Select metal highchairs for two or more levels of rebar or when workers will be required to walk on the robber during placement.

 

Figure 6-4. Typical shapes of reinforcing steel for beams

 

Figure 6-5. Supports for horizontal reinforcing steel

COLUMN REINFORCEMENT

6-17. A column is a slender vertical member that carries a superimposed load unless its height is less than three times its least lateral dimension. In that case, the vertical member is called a pier or pedestal. Allowable column loads and minimum column dimensions are governed by ACI Code 318-83. Concrete columns must always have steel reinforcement because they are subject to bending. Figure 6-6 shows two types of column reinforcement. Vertical reinforcement is the main type. Lateral reinforcement consists of individual ties or a continuous spiral that surrounds the column. These reinforcements are discussed below:

Vertical reinforcement. A loaded-concrete column shortens vertically and expands laterally, see view 1 of Figure 6-6. Four vertical reinforcing bars help to carry the direct axial load as the column shortens under load, while another four serves as intermediate vertical reinforcement. All eight bars are located around the column periphery to resist bending.

Lateral reinforcement. This mainly provides intermediate lateral support for the vertical reinforcement. The vertical bars bulge outward in the direction of least opposition. They need lateral reinforcement to hold them securely--at close vertical intervals--and too restrain the expansion. A tied column is when lateral ties (see Tl Figure 6-6) restrain the expansion. A second system of ties (see T2 in the cross section of view 1 in Figure 6-6) confines the four intermediate vertical reinforcing bars to prevent the eight-bar group from tending to form a slightly circular configuration under load. Without the second system of ties, the square concrete shell would bulge and crack, and the column would fail. A round column has obvious advantages in this respect.

Spiral columns. A spiral column is a round column that has a continuous spiral winding of lateral reinforcement (see view 2 of Figure 6-6) encircling its core and vertical reinforcing bars to restrain expansion. A spiral column is generally considered to be stronger than a tied column. The spiral reinforcement is superior to the many imperfect anchorages that are at the ends of the individual lateral ties in a tied column. The pitch of the spiral reinforcement can be reduced to provide more effective lateral support. The design engineer specifies the pitch of the spiral, the tie size, and the number of vertical bars for a spiral column.

Composite and combination columns. A composite column is a structural steel or cast-iron column thoroughly encased in concrete and reinforced with both vertical and spiral reinforcements. The cross-sectional area of the metal core cannot exceed 20 percent of the gross column area. A structural steel column that's encased in concrete at least 2 1/2 inches thick overall and reinforced with welded-wire fabric is called a combination column. Composite and combination columns are often used in constructing large buildings.

 

Figure 6-6. Reinforcing concrete columns

SLAB AND WALL REINFORCEMENT

6-18. Slabs and walls are reinforced both vertically and laterally. The vertical reinforcement counteracts applied loads. The lateral reinforcement in the form of wire mesh resists reduction in concrete volume due to drying, cooling and, in some cases, chemical reaction. Depending on how much disturbance adjacent construction creates, this movement tends to cause tension stress in the concrete. The amount of lateral reinforcement usually provided is approximately one-fifth of 1 percent of the cross-sectional area of the concrete, as required by the specifications.

David L. Heiserman, Editor

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Revised: June 06, 2015