Reinforced Concrete Stairs

REINFORCED CONCRETE STAIRS. Concrete for stairs is rapidly taking the place of other ma terials in buildings of every kind. A few simple instructions as to the stair load and some remarks concerning the weak points of stairs built several years ago, together with a descrip tion of the reinforced concrete stairway as it is built at present, will be useful.

In Fig. 116 is shown a stairway with plat form and return, that being the type mostly used in public buildings and apartments; but this stairway is equally well adapted to the long single flight from floor to floor, so common in dwellings or business blocks.

This design differs from the earlier form in the use of double reinforcing rods at the upper and lower ends of the flight, one set to tie the stair slab to the floor slab through the beam, the other extending the full length of the stair slab to tie it to the beam rods. This double reinforc ing requires only a few additional rods, and adds much strength to the weakest spot of the stair slab.

The thickness of the stair at its thinnest place, A A, must always be taken as the thick ness of the stair slab in calculating its strength. The treads have no strength, but are dead load; therefore it is well to make the treads hollow if possible.

For single-flight stairs less than sixteen feet long and less than five feet wide, it is well to use a four-inch slab, unless for very heavy loads, and support it so that no two supports are more than ten feet apart.

This slab is reinforced with half-inch square twisted or five-eighths round rods, spaced as fol lows—for dwellings, ten inch centers; for busi ness blocks, six to eight inch centers; and for public halls and factories, two to four inch cen ters. These rods have one inch of concrete

under them. At every tread is a cross-rod of same weight tied to each with No. 10 wire.

The full-length rods all pass over or hook onto the rods in the cross-beams; and the double reinforcing consists of rods about five feet long, passing over the beam, with the ends projecting equally into the stair and floor slabs.

An examination of concrete stairs built with out this additional reinforcing, has in several instances revealed the fact that cracks, if any, appear near the ends of the slab; in fact, within ten inches of the supporting beams, most fre quently at the point marked B, but occasionally at C.

Beams of the size shown, for spans less than ten feet between supports, are amply strong if reinforced with two rods of one-inch diameter placed on two inches of concrete; however, for long flights, four rods should be used, as at D, the rods being looped with strap iron every two to four feet lengthwise of the beam.

The composition of the concrete should not be weaker than one part cement, two sand, and four aggregates. When treads are to be cement finish, the finishing should be done the same as in the case of sidewalks.

When treads are covered with marble or slate, a cinder or light-weight concrete is pre ferable for the treads only; and when covered with wood, locomotive cinder concrete should be used, as it can be penetrated readily with finish ing nails. Great care must be used in mixing and tamping concrete for stair work.