Actual costs of reinforced-concrete columns in place per linear foot per kip of loadcarrying capacity vary widely. The following recommendations based on relative costs are generally applicable:
Formwork. Use of the same size and shape of column cross-section throughout a floor and, for multistory construction, from footing to roof will permit mass production and reuse for economy. Within usual practicable maximum building heights, about 60 stories or 600 ft, increased speed of construction and saving in formwork will save more than the cost of the excess concrete volume over that for smaller column sizes in upper stories.
Concrete Strength. Use of the maximum concrete compressive strength required to support the factored loads with the minimum allowed reinforcing steel area results in the lowest cost. The minimum size of a multistory column is established by the maximum concrete strength reliably available locally and the limit on maximum area of vertical bars. (Concrete with a compressive strength of 17,000 psi Æ’c is commercially available in many areas of the United States.) If the acceptable column size is larger than the minimum possible at the base of the multistory stack, the steel ratio can begin with less than the maximum limit (Art. 9.83). At successive stories above, the steel ratio can be reduced to the minimum, and thereafter, for additional stories, the concrete strength can be reduced. Near the top, as loads reduce further, a further reduction in the steel area to 0.005 times the concrete area may be made (Art. 9.83).
Reinforcing Steel. Comparative cost estimates should be made for combinations of different strengths of concrete and reinforcing bars. For high-rise buildings, using concrete with a high f’c combined with Grade 75 vertical bars should provide the greatest economy. Minimum tie requirements can be achieved with four-bar or fourbundle (up to four bars per bundle) arrangements, or by placing an intermediate bar between tied corners not more than 6 in (clear) from the corner bars. For these arrangements, no interior ties are required; only one tie per set is needed. (See Fig. 9.53 and Art. 9.83.) With no interior ties, low-slump concrete can be placed and consolidated more easily, and the cost and time for assembly of column reinforcement cages are greatly reduced. Note that, for small quantities, the local availability of Nos. 14 and 18 bars should be investigated before they are specified.
Details of Column Reinforcement. Where Nos. 14 and 18 bars are used in compression only, end-bearing mechanical splices usually save money. If the splices are staggered 50%, as with two-story lengths, the tensile capacity of the columns will also be adequate for the usual bending moments encountered. For unusually large bending moments, where tensile splices of No. 10 bars and larger are required, mechanical splices are usually least expensive in place. For smaller bar sizes, lap splices, tensile or compressive, are preferred for economy. Some provision for staggered lap splices for No. 8 bars and larger may be required to avoid Class B tension lap splices (Art. 9.49.7).
Where butt splices are used, it will usually be necessary to assemble the column reinforcement cage in place. Two-piece interior ties or single ties with end hooks for two bars (see Art. 9.84) will facilitate this operation.
Where the vertical bar spacing is restricted and lap splices are used, even with the column size unchanged, offset bending of the bars from below may be required.
However, where space permits, as with low steel ratios, an additional saving in fabrication and erection time will be achieved by use of straight column verticals offset one bar diameter at alternate floors.