USEFUL DATA 



— may be taken; for small beams running into heavy columns this should be increased, 



wl-. 

 but not to exceed -r^ 



For spans of unusual length, or for spans of materially unequal length, more exact 

 calculations should be made. Special consideration is also required in the case of con- 

 centrated loads. 



Even if the center of the span is designed for a greater bending moment than is 

 called for by (a) or (b), the negative moment at the support should not be taken as 

 less than the values there given. 



Wliere beams are reinforced on the compression side, the steel may be assumed to 

 carry its proportion of stress in accordance with the ratio of moduli of elasticity, as 

 given in the section on "Working Stresses." Reinforcing bars for compression in beams 

 should be straight and should be two diameters in the clear from the surface of the 

 concrete. For the positive bending moment, such reinforcement should not exceed 

 one per cent of the area of the concrete. In the case of cantilever and continuous 

 beams, tensile and compressive reinforcement over supports should extend sufficiently 

 beyond the support and beyond the point of inflection to develop the requisite bond 

 strength. 



In construction made continuous over supports it is important that ample founda- 

 tions should be provided; for unequal settlements are liable to produce unsightly, if 

 not dangerous cracks. This effect is more likely to occiu* in low structures. 



Girders, such as wall girders, which have beams framed into one side only, should 

 be designed to resist torsional moment arising from the negative moment at the end 

 of the beam. 



Bond Strength and Spacing of Reinforcement. Adequate bond strength 

 should be provided. The formula hereinafter given for bond stresses in beams is for 

 straight longitudinal bars. In beams in which a portion of the reinforcement is bent 

 up near the end, the bond stress at places, in both the straight bars and the bent bars, 

 will be considerably greater than for all the bars straight, and the stress at some point 

 may be several times as much as that found by considering the stress to be uniformly 

 distributed along the bar. In restrained and cantilever beams full tensile stress exists 

 in the reinforcing bars at the point of support and the bars should be anchored in the 

 support sufficiently to develop this stress. 



In case of anchorage of bars, an additional length of bar should be provided beyond 

 that found on the assumption of uniform bond stress, for the reason that before the 

 bond resistance at the end of the bar can be developed the bar may have begun to slip 

 at another point and "running" resistance is less than the resistance before slip begins. 



Where high bond resistance is required, the deformed bar is a suitable means of 

 supplying the necessary strength. But it should be recognized that even with a de- 

 formed bar initial slip occurs at early loads, and that the ultimate loads obtained in 

 the usual tests for bond resistance may be misleading. Adequate bond strength 

 throughout the length of a bar is preferable to end anchorage, but, as an additional 

 safeguard, such anchorage may properly be used in special cases. Anchorage furnished 

 by short bends at a right angle is less effective than by hooks consisting of turns 

 through 180 degrees. 



197 



