Figure 7-24.—Steel in place in a floor slab.

made to a depth of one half of the bar diameter and for a length eight times the bar diameter. The minimum clear distance between parallel bars in beams, footings, walls, and floor slabs should either be 1 inch (25.4 mm) or 1 1/3 times the largest size aggregate particle in the concrete, whichever distance is greater. In columns, the clear distance between parallel bars should be not less than 1 1/2 times the bar diameter or 1 1/2 times the maximum size of the coarse aggregate. Always use the larger of the two. The support for reinforcing steel in floor slabs is shown in figure 7-24. The height of the slab bolster is determined by the required concrete protective cover. Concrete blocks made of sand-cement mortar can be used in place of the slab bolster. Wood blocks should never be used for this purpose. Highchairs (fig. 7-18) can be obtained in heights up to 6 inches (15 cm). When a height greater than 6 inches is required, make the chair out of No. 0, soft, annealed iron wire. To hold the bars firmly in position, you should tie the bars together at frequent intervals where they cross with a snapat. Steel for column ties may be assembled with the verticals into cages by laying the vertical bars for one side of the column horizontally across a couple of sawhorses. The proper number of ties are slipped over the bars, the remaining vertical bars are added, and then the ties are spaced out as required by the placing plans. All intersections are wired together to make the assembly rigid so that it may be hoisted and set as a unit. Figure 7-25 shows atypical column tie assembly. After the column is raised, it is tied to the dowels or reinforcing steel carried up from below. This holds it firmly in position at the base. The column form is erected and the reinforcing steel is tied to the column form at 5-foot (4.5-m) intervals, as shown in figure 7-26. The use of metal supports to hold beam reinforcing steel in position is shown in figure 7-8. Note the position of the beam bolster. The stirrups are tied to the main reinforcing steel with a snap tie. Wherever possible you should assemble the stirrups and main reinforcing steel outside the form and then place the assembled unit in position. Precast concrete blocks, as shown in figure 7-27, maybe substituted for metal supports. The horizontal and vertical bars are wired securely to each other at sufficiently frequent intervals to make a rigid mat. Tying is required at every second or third intersection, depending upon the size and spacing of bars, but with not less than three ties to any one bar, and, in any case, not more than 4 to 6 feet apart in either direction. 127.86 Figure 7-24.—Steel in place in a floor slab. 7-17