It is important that bars be placed and held impositions as shown on the placing drawings. The strength of any concrete member can be affected by the improper positioning of the reinforcing bars. For example, lowering the top bars or raising the bottom bars by one-half of an inch more than specified in a 6 inch deep slab could reduce its load carrying capacity by 20 percent. A tolerance of one-fourth of an inch plus or minus is a generally accepted practice in the positioning of bars above from in the bottom of beams, joists, and slabs. The exact positioning of lengthwise bars is generally not critical to the strength of the structure; therefore, the placing tolerance is not a close as these for cover. The same tolerances apply to truss bars at the hooked end. In long runs of slabs or walls, the exact spacing of bars specified on the plans in not critical. It is satisfactory practice to place the number of bars called for in any given length of panel at approximately the spacing required, but no exact spacing measurement is necessary for each bar. Thus, if the placing drawings called for 11 bars spaced 6 inches center to center the bars should be spread over the 60 inch space at about 6 inch spacing, and , if necessary, a few of the bars could be shifted to avoid an insert. For buildings, ACI specifies the minimum amount of cover over stirrups, ties, and other bars, as well as the minimum clear space between bars. The engineer determines that such spacings can be maintained and will call for bars to be positioned over each other in multiple layers should it be necessary. Side cover should be maintained within a tolerance of one-fourth inch. Stirrups can be placed at an angle across the beam if otherwise they would be too wide.