USEFUL DATA 



It is well established that vertical members attached to or looped about horizontal 

 members, inclined members secured to horizontal members in such a way as to insure 

 against slip, and the bending of a part of the longitudinal reinforcement at an angle, 

 will increase the strength of a beam against failure by diagonal tension, and that a well- 

 designed and well-distributed web reinforcement may under the best conditions 

 increase the total vertical shear carried to a value as much as three times that obtained 

 when the bars are all horizontal and no web reinforcement is used. 



When web reinforcement comes into action as the principal tension web resistance, 

 the bond stresses between the longitudinal bars and the concrete are not distributed 

 as uniformly along the bars as they otherwise would be, but tend to be concentrated 

 at and near stirrups, and at and near the points where bars are bent up. When stirrups 

 are not rigidly attached to the longitudinal bars, and the proportioning of bars and 

 stirrups spacing is such that local slip of bars occurs at stirrups, the effectiveness of 

 the stirrups is impaired, though the presence of stirrups still gives an element of 

 toughness against diagonal tension failure. 



Sufficient bond resistance between the concrete and the stirrups or diagonals must 

 be provided in the compression area of the beam. 



The longitudinal spacing of vertical stirrups should not exceed one-half the depth of 

 beam, and that of inclined members should not exceed three-fourths of the depth of beam. 



Bending of longitudinal reinforcing bars at an angle across the web of the beam 

 may be considered as adding to diagonal tension resistance for a horizontal distance 

 from the point of bending equal to three-fourths of the depth of beam. Where the 

 bending is made at two or more points, the distance between points of bending should 

 not exceed three-fourths of the depth of the beam. In the case of a restrained beam 

 the effect of bending up a bar at the bottom of the beam in resisting diagonal tension 

 may not be taken as extending beyond a section at the point of inflection, and the 

 effect of bending down a bar in the region of negative moment may be taken as extend- 

 ing from the point of bending down of bar nearest the support to a section not more 

 than three-fom-ths of the depth of beam beyond the point of bending down of bar 

 farthest from the support but not beyond the point of inflection. In case stirrups are 

 used in the beam away from the region in which the bent bars are condsidered effective, 

 a stirrup should be placed not farther than a distance equal to one-foiu-th the depth 

 of beam from the limiting sections defined above. In case the web resistance required 

 through the region of bent bars is greater than that furnished by the bent bars, suflS- 

 cient additional web reinforcement in the form of stirrups or attached diagonals 

 should be provided. The higher resistance to diagonal tension stresses given by unit 

 frames having the stirrups and bent-up bars securely connected together both longi- 

 tudinally and laterally is worthy of recognition. It is necessary that a limit be placed 

 on the amount of shear which may be allowed in a beam; for when web reinforcement 

 sufficiently efficient to give very high web resistance is used, at the higher stresses the 

 concrete in the beam becomes checked and cracked in such a way as to endanger its 

 durability as well as its strength. 



The section to be taken as the critical section in the calculation of shearing stresses 

 will generally be the one having the maximum vertical shear, though experiments 

 show that the section at which diagonal tension failures occur is not just at a support 

 even though the shear at the latter point be much greater. 



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