609. While the use of concrete and steel for the walls and floors of buildings is about fifty years old, yet it is only in comparatively recent years that its value has become generally known. It is now applied to all classes of structures, warehouses, factories, residences, station and office buildings, and it is anticipated that in the next twenty years concrete-steel will be as familiar in architecture as steel skeleton, stone, and brick are now.
610. It happens that at present the concrete-steel building industry is largely in the hands of companies who are exploiting some particular form of steel rods or bars applied according to some one of the many "systems" of reinforcement. This condition has both good and bad features. A reputable concern of this kind will have in their employ engineers who should satisfy themselves that each design is a safe one, for the failure of a building will cast disrepute on their particular system. It is this fact that leads the companies to keep the construction entirely, and the design largely, in their own hands. Another advantage is that these concerns are able to perfect methods of construction by experience, and to lessen the expense of one structure by making use of the concrete plant and the molds that have been used on another.
611. In making plans for a building, the owner is usually represented in the first instance by an architect whose business it is to dictate the design. If concrete-steel is considered, the architect may call an engineer in consultation and they may together harmonize the features of utility and appearance with economy and strength, but in letting the contract it is found that the competition is limited to one or two companies using the particular system which the engineer considers the best adapted to the particular conditions in question.
On the other hand, the architect will hesitate to go to the construction company for assistance, since he must first select the system he shall use, a question upon which his ideas may be neither clear nor well grounded, and he is then having the prospective contractor assist in the design. Under these circumstances the architect will usually consider concrete-steel construction as something he wishes to avoid if possible. But this condition will correct itself in time, for owners will demand a consideration of this form of construction, engineers will become familiar with its use and will be employed to design the engineering features, while reliable contractors in every city will obtain permission to build in accordance with any "system" under the supervision of a competent engineer.
While a pitch roof is sometimes built of concrete-steel, this form of construction is particularly adapted to so called flat roofs. The roof is constructed much the same as a floor slab (Art. 65-67), except that expansion joints are sometimes provided, and the roof is covered with tar and gravel, or some of the patent roofings ordinarily used. While the roof loads are usually light, permitting a greater span of slab between beams than for floor construction, it will seldom be economical to introduce these longer spans because of the changes necessary in the molds. In most buildings it is necessary to provide against condensation, and for this purpose a flat ceiling may be suspended at the level of the under side of the beams giving an air space.
The floors may be constructed in conformity with the principles stated in Chapter XIX. The strength of short span arches, such as are used for floors, where the haunches are built up level with the top of crown of arch, is a matter of experiment and cannot be accurately determined theoretically. Empirical formulas may be derived for a certain system based on a sufficient number of tests. The principles underlying the strength of slabs may be considered the same as those applying to beams (Art. 69), although if the length of slabs is not much greater than the span, they are not strictly applicable, but will err on the safe side.
614. A decision must first be made as to the size of bays into which the floor space is to be divided. This will of course depend on the use of the building, the engineering features conforming to requirements of utility. If the bays are not square, the girders should usually take the shorter span between columns. This length is then divided into the number of slab spans that will give maximum economy. The shorter these spans the less the amount of material required in slabs and the greater the number and Cost of floor beams. Computations should be made, therefore, for two or three arrangements to determine this point. As this distribution for maximum economy will vary with the loads to be provided for, it is well, if the floors are not all to carry the same load, to take for this computation a load intermediate between the heaviest and lightest, and use if possible the same arrangement of spans throughout the building.
The strength of slabs for given bending moments may be taken directly from Table 161, after deciding upon the working stress to be allowed in the concrete and the probable modulus of elasticity. The beams and girders, if single reinforcement is used, are taken from the same table or computed by the methods of Art. 69.
615. In some instances it may be found economical to use concrete-steel slabs for floors supported by concrete protected steel beams and girders. One advantage of this system is that the forms for building the protecting concrete and for the floor slabs may be hung from the steel girders and beams. For this method of construction the enveloping concrete should not be less than one and one-half inches thick over the edges of flanges, and wire fabric or metal lath wrapped about lower flanges of beams will insure the concrete remaining in place. This is not properly concrete-steel construction, but simply concrete protected steel, and except in case the concrete extends well above the steel, forming an independent compression flange, no added strength should be computed for the concrete covering.
In the foundations of buildings of moderate height the supporting columns may be built entirely of concrete. Since, however, the pressure on the concrete, even when it is constructed with the greatest care, should not exceed two hundred to three hundred pounds per square inch, the required area of cross-section in the lower stories is usually so great as to preclude the use of columns built entirely of concrete.
A steel column of any of the ordinary styles, built up of steel shapes may be used, and protected from corrosion and fire by filling and covering with concrete. This not only serves as a protection against rust, but materially increases the stiffness and permits the use of a somewhat higher working stress in the steel. The concrete filling should be mixed quite wet in order that it shall work into all angles. The edges of the metal should not approach nearer than one and one-half inches to the exterior of the concrete, and flat surfaces of metal should have a covering of at least two and one-half inches. Where it is necessary to cover large, flat surfaces, they should be first covered with expanded metal or wire fabric, locked on by twisting around the edges of the plate or channel.
Concrete-steel columns differ from the above in that the main dependence is placed on the concrete rather than on the steel. For such columns longitudinal reinforcement has generally been employed. Steel bars extending from end to end of the column are distributed throughout the cross-section, and are tied together at intervals of four to twelve inches by smaller bars forming loops to hold them in place. The splicing of the bars is effected by placing a small tube over the upper end of the lower bar and projecting above it, and then setting the lower end of the upper bar within the tube resting on the lower bar. Where this is done it is essential that the two ends be planed perfectly square, and it is much better to avoid splices in a column between lateral supports. In a building the reinforcing rods project up through the floor above and are spliced into the bars of the columns in the next story.