SLABS, CROSS-BEAMS, AND GIRDERS 143 



omitted. The amount of concrete will be more, but the amount 

 of steel required will be less and little saving, if any, is effected 

 by using cross-beams when their use is in doubt. Heavy loads 

 and low stresses, however, call for large weights of concrete, and 

 cross-beams will undoubtedly be needed under such conditions, 

 as the deeper a beam, the greater its moment of resistance for a 

 given volume. The economical spacing of cross-beams will 

 vary between 4 or 5 ft. to 10 or 12 ft. Architectural considera- 

 tions will generally play an important part, but frequently 

 designs must conform to building regulations relative to ratio of 

 span to depth. 



59. Design of Beams and Girders. T -Beams. When a slab 

 and beam (or girder) are built at the same time and thoroughly 

 tied together, a part of the slab may be considered to act with 

 the upper part of the beam in compression. Cross-beam A and 

 a portion of the slab which may be considered to aid in resisting 

 stress (Fig. 66) is shown cross-hatched; a like representation is 

 given for girder C. This form of beam is called a T-beam, and 

 the extra amount of concrete in the compressive part of such a 

 beam makes possible a considerable saving over the rectangular 

 form. The thickness of the flange is fixed by the thickness of 

 slab required to support its load, but the width of slab which 

 can be taken as effective flange width must be estimated. 



This width should not be too great in proportion to the slab 

 thickness, otherwise the shearing stresses on the vertical sections 

 through the slab at the edge of beam will be excessive and greater 

 than those on the horizontal section between stem and flange. 

 Experiments show that it would be difficult to crush a flange with 

 a width four times the width of web without failure taking place 

 by the excessive shearing stresses in the web. 



The arbitrary rules which have been adopted in the assumption 

 of the flange width may be divided into two classes. The first 

 class makes the flange width a factor of the width of stem or 

 thickness of slab, or else of the distance between adjacent beams. 

 The second class makes the flange width a factor of the length of 

 span of the beam itself. The Joint Committee, however, com- 

 bines the two and has recommended a width not exceeding one- 

 fourth the span length of the beams and, in addition, limits the 

 width to use on either side of the web to four times the thickness 

 of the slab. In no case should the breadth of the flange be taken 

 greater than the distance between beams. 



