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T Beams Reinforced concrete floor systems normally consist of slabs and beams that are placed monolithically. As a result, the two parts act together to resist loads. In effect, the beams have extra widths at their tops, called flanges, and the resulting T-shaped beams are called T beams. The part of a T beam below the slab is referred to as the web or stem. (The beams may be L shaped if the stem is at the end of a slab.) The stirrups in the webs extend up into the slabs, as perhaps do bent-up bars, with the result that they further make the beams and slabs act together. There is a problem involved in estimating how much of the slab acts as part of the beam. Should the flanges of a T beam be rather stocky and compact in cross section, bending stresses will be fairly uniformly distributed across the compression zone.
T Beams If, however, the flanges are wide and thin, bending stresses will vary quite a bit across the flange due to shear deformations. The farther a particular part of the slab or flange is away from the stem, the smaller will be its bending stress. Instead of considering a varying stress distribution across the full width of the flange, the ACI Code (8.12.2) calls for a smaller width with an assumed uniform stress distribution for design purposes. The objective is to have the same total compression force in the reduced width that actually occurs in the full width with its varying stresses. The hatched area in Figure 5.1 shows the effective size of a T beam. For T beams with flanges on both sides of the web, the code states that the effective flange width may not exceed one-fourth of the beam span, and the overhanging width on each side may not exceed eight times
T Beams the slab thickness or one-half the clear distance to the next web. An isolated T beam must have a flange thickness no less than one-half the web width, and its effective flange width may not be larger than four times the web width (ACI 8.12.4). If there is a flange on only one side of the web, the width of the overhanging flange cannot exceed one-twelfth the span, 6hf, or half the clear distance to the next web (ACI 8.12.3). The analysis of T beams is quite similar to the analysis of rectangular beams in that the specifications relating to the strains in the reinforcing are identical. To repeat briefly, it is desirable to have ϵtvalues ≥ 0.005, and they may not be less than 0.004 unless the member is subjected to an axial load ≥ 0.10f’cAg. You will learn that ϵtvalues are almost always much larger than 0.005 in T beams because of their very large compression flanges.
T Beams For such members, the values of c are normally very small, and calculated ϵtvalues very large. The neutral axis (N.A.) for T beams can fall either in the flange or in the stem, depending on the proportions of the slabs and stems. If it falls in the flange, and it almost always does for positive moments, the rectangular beam formulas apply, as can be seen in Figure 5.2(a). The concrete below the neutral axis is assumed to be cracked, and its shape has no effect on the flexure calculations (other than weight). The section above the neutral axis is rectangular. If the neutral axis is below the flange, however, as shown for the beam of Figure 5.2(b), the compression concrete above the neutral axis no longer consists of a single rectangle, and thus the normal rectangular beam expressions do not apply.
T Beams If the neutral axis is assumed to fall within the flange, the value of a can be computed as it was for rectangular beams: The distance to the neutral axis, c, equals a/β1. If the computed value of a is equal to or less than the flange thickness, the section for all practical purposes can be assumed to be rectangular, even though the computed value of c is actually greater than the flange thickness. A beam does not really have to look like a T beam to be one. This fact is shown by the beam cross sections shown in Figure 5.3. For these cases the compression concrete is T shaped, and the shape or size of the concrete on the tension side, which is assumed to be cracked, has no effect on the theoretical resisting moments.
T Beams It is true, however, that the shapes, sizes, and weights of the tensile concrete do affect the deflections that occur (as is described in Chapter 6), and their dead weights affect the magnitudes of the moments to be resisted.
Analysis of T Beams The calculation of the design strengths of T beams is illustrated in next two examples. In the first of these problems, the neutral axis falls in the flange, while for the second, it is in the web. The procedure used for both examples involves the following steps: 1. Check As min as per ACI Section 10.5.1 using bwas the web width. 2. Compute T = Asfy. 3. Determine the area of the concrete in compression (Ac) stressed to 0.85f’c. 4. Calculate a, c, and ϵr. 5. Calculate φMn.
Analysis of T Beams In the example, where the neutral axis falls in the flange, it would be logical to apply the normal rectangular equations, a couple of method as a background for the solution of next example are used, where the neutral axis falls in the web. This same procedure can be used for determining the design strengths of tensilely reinforced concrete beams of any shape (Т, Г, П, triangular, circular, etc.).
Analysis of T Beams Example 5.1 Determine the design strength of the T beam shown in Figure 5.4, with f’c= 4000 psi and fy= 60,000 psi. The beam has a 30-ft span and is cast integrally with a floor slab that is 4 in. thick. The clear distance between webs is 50 in. Solution
Analysis of T Beams Example 5.2 Compute the design strength for the T beam shown in Figure, in which f’c= 4000 psi and fy= 60,000 psi. Solution
Another Method for Analyzing T Beams The preceding section presented an important method of analyzing reinforced concrete beams. It is a general method that is applicable to tensilely reinforced beams of any cross section, including T beams. T beams are so very common, however, that many designers prefer another method that is specifically designed for T beams. First, the value of a is determined as previously described in this chapter. Should it be less than the flange thickness, hf, we will have a rectangular beam and the rectangular beam formulas will apply. Should it be greater than the flange thickness, hf(as was the case for previous example), the special method to be described here will be very useful.
Another Method for Analyzing T Beams The beam is divided into a set of rectangular parts consisting of the overhanging parts of the flange and the compression part of the web (as in next slide). The total compression, Cw, in the web rectangle, and the total compression in the overhanging flange, Cf, are computed: Then the nominal moment, Mn, is determined by multiplying Cwand Cfby their respective lever arms from their centroids to the centroid of the steel:
Another Method for Analyzing T Beams This procedure is illustrated in next example. Although it seems to offer little advantage in computing Mn, we will learn that it does simplify the design of T beams when a > hfbecause it permits a direct solution of an otherwise trial-and-error problem.
Another Method for Analyzing T Beams Example 5.3 Repeat previous example using the value of a (8.19 in.) previously obtained and the alternate formulas just developed. Reference is made to Figure 5.8, the dimensions of which were taken from Figure 5.5. Solution
Design of T Beams For the design of T beams, the flange has normally already been selected in the slab design, as it is for the slab. The size of the web is normally not selected on the basis of moment requirements but probably is given an area based on shear requirements; that is, a sufficient area is used so as to provide a certain minimum shear capacity. It is also possible that the width of the web may be selected on the basis of the width estimated to be needed to put in the reinforcing bars. Sizes may also have been preselected, as previously described, to simplify formwork for architectural requirements or for deflection reasons. For the examples that follow the values of hf, d, and bware given.
Design of T Beams The flanges of most T beams are usually so large that the neutral axis probably falls within the flange, and thus the rectangular beam formulas apply. Should the neutral axis fall within the web, a trial-and-error process is often used for the design. In this process, a lever arm from the center of gravity of the compression block to the center of gravity of the steel is estimated to equal the larger of 0.9d or d − (hf/2), and from this value, called z, a trial steel area is calculated (As= Mn/fyz). Then by the process as used earlier, the value of the estimated lever arm is checked. If there is much difference, the estimated value of z is revised and a new Asdetermined. This process is continued until the change in As is quite small.
Design of T Beams Often a T beam is part of a continuous beam that spans over interior supports, such as columns. The bending moment over the support is negative, so the flange is in tension. Also, the magnitude of the negative moment is usually larger than that of the positive moment near midspan. This situation will control the design of the T beam because the depth and web width will be determined for this case. Then, when the beam is designed for positive moment at midspan, the width and depth are already known. Example to followpresents a more direct approach for the case where a > hf. This is the case where the beam is assumed to be divided into its rectangular parts.
Design of T Beams Example 5.4 Solution
Design of T Beams Example 5.5 Solution
Design of T Beams Our procedure for designing T beams has been to assume a value of z, compute a trial steel area of As, determine a for that steel area assuming a rectangular section, and so on. Should a > hf, we will have a real T beamand atrial-and-error process was used. It is easily possible, however, to determine As directly using the method of Section 5.3, where the member was broken down into its rectangular components. The compression force provided by the overhanging flange rectangles must be balanced by the tensile force in part of the tensile steel, Asf, while the compression force in the web is balanced by the tensile force in the remaining tensile steel, Asw.
Design of T Beams For the overhanging flange, we have. from which the required area of steel, Asf, equals The design strength of these overhanging flanges is The remaining moment to be resisted by the web of the T beam and the steel required to balance that value are determined next.
Design of T Beams The steel required to balance the moment in the rectangular web is obtained by the usual rectangular beam expression. The value Muw/φbwd²is computed, and ρwis determined from the appropriate Appendix table or the expression for ρwpreviously given in Section 3.4 of this book. Think of ρwas the reinforcement ratio for the beam shown in Figure 5.7(b). Then
Design of T Beams Example 5.6 Rework Example 5.5 using the rectangular component method just described. Solution First assume a ≤ hf(which is very often the case). Then the design would proceed like that of a rectangular beam with a width equal to the effective width of the T-beam flange. The beam acts like a T beam, not a rectangular beam, and the values for ρ and a above are not correct. If the value of a had been ≤ hf, the value of Aswould have been simply ρbd= 0.0072(54 in.) (24 in.) = 9.33 in.². Now break the beam up into two parts (Figure 5.7) and design it as a T beam.
Design of T Beams for Negative Moments When T beams are resisting negative moments, their flanges will be in tension and the bottom of their stems will be in compression, as shown in Figure 5.12. Obviously, for such situations, the rectangular beam design formulas will be used. Section 10.6.6 of the ACI Code requires that part of the flexural steel in the top of the beam in the negative-moment region be distributed over the effective width of the flange or over a width equal to one-tenth of the beam span, whichever is smaller. Should the effective width be greater than one-tenth of the span length, the code requires that some additional longitudinal steel be placed in the outer portions of the flange. The intention of this part of the code is to minimize the sizes of the flexural cracks that will occur in the top surface of the flange perpendicular to the stem of a T beam subject to negative moments.
Design of T Beams for Negative Moments In Section 3.8, it was stated that if a rectangular section had a very small amount of tensile reinforcing, its design-resisting moment, φMn, might very well be less than its cracking moment. If this were the case, the beam might fail without warning when the first crack occurred. The same situation applies to T beams with a very small amount of tensile reinforcing. When the flange of a T beam is in tension, the amount of tensile reinforcing needed to make its ultimate resisting moment equal to its cracking moment is about twice that of a rectangular section or that of a T section with its flange in compression. As a result, ACI Section 10.5.1 states that the minimum amount of reinforcing required equals the larger of the two values that follow:
Design of T Beams for Negative Moments For statically determinate members with their flanges in tension, bwin the above expression is to be replaced with either 2bwor the width of the flange, whichever is smaller.
L-Shaped Beams The author assumes for this discussion that L beams (i.e., edge T beams with a flange on one side only) are not free to bend laterally. Thus they will bend about their horizontal axes and will be handled as symmetrical sections, exactly as with T beams. For L beams, the effective width of the overhanging flange may not be larger than one-twelfth the span length of the beam, six times the slab thickness, or one-half the clear distance to the next web (ACI 8.12.3). If an L beam is assumed to be free to deflect both vertically and horizontally, it will be necessary to analyze it as an unsymmetrical section with bending about both the horizontal and vertical axes.
Compression Steel The steel that is occasionally used on the compression sides of beams is called compression steel, and beams with both tensile and compressive steel are referred to as doubly reinforced beams. Compression steel is not normally required in sections designed by the strength method because use of the full compressive strength of the concrete decidedly decreases the need for such reinforcement, as compared to designs made with the working-stress design method. Occasionally, however, space or aesthetic requirements limit beams to such small sizes that compression steel is needed in addition to tensile steel. To increase the moment capacity of a beam beyond that of a tensilely reinforced beam with the maximum percentage of steel [when (ϵt= 0.005)], it is necessary to introduce another resisting couple in the beam.
Compression Steel This is done by adding steel in both the compression and tensile sides of the beam. Compressive steel increases not only the resisting moments of concrete sections but also the amount of curvature that a member can take before flexural failure. This means that the ductility of such sections will be appreciably increased. Though expensive, compression steel makes beams tough and ductile, enabling them to withstand large moments, deformations, and stress reversals such as might occur during earthquakes. As a result, many building codes for earthquake zones require that certain minimum amounts of compression steel be included in flexural members.
Compression Steel Compression steel is very effective in reducing long-term deflections due to shrinkage and plastic flow. In this regard you should note the effect of compression steel on the long-term deflection expression in Section 9.5.2.5 of the code (to be discussed in Chapter 6 of this text). Continuous compression bars are also helpful for positioning stirrups (by tying them to the compression bars) and keeping them in place during concrete placement and vibration. Tests of doubly reinforced concrete beams have shown that even if the compression concrete crushes, the beam may very well not collapseif the compression steel is enclosed by stirrups. Once the compression concrete reaches its crushing strain, the concrete cover spalls or splits off the bars, much as in columns (see Chapter 9).
Compression Steel If the compression bars are confined by closely spaced stirrups, the bars will not buckle until additional moment is applied. This additional moment cannot be considered in practice because beams are not practically useful after part of their concrete breaks off. (Would you like to use a building after some parts of the concrete beams have fallen on the floor?) Section 7.11.1 of the ACI Code states that compression steel in beams must be enclosed by ties or stirrups or by welded wire fabric of equivalent area. In Section 7.10.5.1, the code states that the ties must be at least #3 in size for longitudinal bars #10 and smaller and at least #4 for larger longitudinal bars and bundled longitudinal bars. The ties may not be spaced farther apart than 16 bar diameters, 48 tie diameters, or the least dimension of the beam cross section(code 7.10.5.2).
Compression Steel For doubly reinforced beams, an initial assumption is made that the compression steel yieldsas well as the tensile steel. (The tensile steel is always assumed to yield because of the ductile requirements of the ACI Code.) If the strain at the extreme fiber of the compression concrete is assumed to equal 0.00300 and the compression steel, A’s, is located two-thirds of the distance from the neutral axis to the extreme concrete fiber, then the strain in the compression steel equals ⅔ × 0.003 = 0.002. If this is greater than the strain in the steel at yield, as say 50,000/(29 × 10⁶) = 0.00172 for 50,000-psi steel, the steel has yielded. It should be noted that actually the creep and shrinkage occurring in the compression concrete help the compression steel to yield.
Compression Steel Sometimes the neutral axis is quite close to the compression steel. As a matter of fact, in some beams with low steel percentages, the neutral axis may be right at the compression steel. For such cases, the addition of compression steel adds little, if any, moment capacity to the beam. It can, however, make the beam more ductile. When compression steel is used, the nominal resisting moment of the beam is assumed to consist of two parts: the part due to the resistance of the compression concrete and the balancing tensile reinforcing, and the part due to the nominal moment capacity of the compression steel and the balancing amount of the additional tensile steel. This situation is illustrated in Figure 5.13. In the expressions developed here, the effect of the concrete in compression, which is replaced by the compressive steel, A’s, is neglected.
