Spacing of Moment Bars in Precast Joints
F. N. Menefee and H. L. Kinnier
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Spacing of moment bars in precast reinforced concrete joists in some cases has been as small as 3/16 in. although it is probable that most maufacturers use 3/8-in. shear or diagonal tension bars with corresponding spacing of moment bars. ACI Building Code places a one in. minimum on spacing. Undoubtedly, the regulations were written with monolithic concrete rather than light precast joists in mind. Spacing has no particular significance if all the requirements for bond are met independently of it. In a properly designed precast joist, theoretical computations show that bond stresses are always less in proportion to the maximum allowed than are any other stresses. The tests herein reported were an attempt to determine whether the artifical reduction of bond area, up to 30 percent, had any effect on the over-all strength of the joist, and give some indication as to whether 3/8-in. spacing of moment bars introduced a controlling weakness. While the results on 14 joists, all of the same dimensions, could hardly produce conclusive evidence, the results, along with the service record of precast joints, indicate that 3/8-in. spacing of moment bars, with 3/8-in. maximum size aggregate and with all other requirements of the Building Code met, will give a joist which will fail otherwise than in bond, from which it appears that the present one in. minimum in the current Building Code should be throughly studied with a view toward modification for precast joists, with well anchored reinforcement - if opinon and test justify such a change.