STRENGTH PLAIN CONCRETE COLUMNS. Table 45, page 244, gives the strength of plain concrete columns, and is here included to permit subsequent comparisons. Notice that the richer mixtures give considerably higher strength. Fig. 30 shows the relation be tween the strength and the proportion of cement, the amount of cement being given in terms of the weight of the sand and the stone.* Fig. 30 shows that cement is an effective reinforcing material. For example, a cubic yard of 1 • 2 : 4 concrete will require 0.42 barrel of cement more than a 1 ° 3 : 6 mixture, and will therefore cost 10 of 15 per cent more; but the strength of the 1 : 2 : 1 column is 168 per cent of that of a 1 : 3 : 6 mixture. If only compressive resistance is required, it is more economical to use an increased amount of cement than to employ any form of steel reinforcement; but in practice columns are subjected to bending moments, and plain concrete, owint; to its small tensile strength, is not suitable for use where tensile stresses are to be resisted. Therefore steel reinforcement, which will aid the column in resisting bending mo ments, is advantageous. Further, steel is a more reliable material than concrete; and hence steel re inforcement is particularly advan tageous in small columns, where spots of improperly proportioned or badly mixed concrete are rela tively more serious.
Columns are reinforced in either of two ways: (1) by longitudinal rods extending the full length of the column, and (2) by circumferentially wound metal. In the
first case the steel supports a portion of the load directly; and in the second the steel gives lateral support to the concrete and thereby increases its load-carrying power.
The longitudinal reinforcement may be either plain rods, deformed bars, or Kahn bars; and is occasionally a light lattice column, made heavy enough to carry the forms of the story above.* If only one rod is used, it is placed at the center; and if several are used, they are placed symmetrically about the center and usually about 2 inches from the outside of the column. The circumferential rein forcement is usually either a succession of hoops or a spirally wound wire or round rod or flat bar; but sometimes it is a cylinder of wire net or expanded metal. The circumferential reinforcement is placed outside of the column proper, although in practice about 2 inches of concrete or mortar is placed outside of the steel for fire protection.
Usually the two methods are used together, the longitudinal rods being bound together transversely at intervals, and the hoops or the spiral being held in position by vertical spacing-bars which are often so large as to give considerable longitudinal reinforcement.