49. When it is necessary to spread the foundations over 12 or 15 feet in each direction, with a very small height to the footings, as is the case in Chicago, steel beams must be used to furnish the necessary transverse strength. Even when building on solid ground, it is claimed that iron and steel footings for tall buildings, at the present price of steel (1895), are cheaper than masonry footings. The author doubts, however, if steel footings will prove as durable as those of masonry.
The manner of using the beams is shown in Figures 15 to 18.
In preparing the footings, the ground is first carefully leveled and the bottom of the pier located. If the ground is not compact enough to permit of excavating for the concrete bed without the sides of the pit or trench falling in, heavy planks or timbers should be set up and fastened together at the corners, and, if necessary, tied between with rods, to hold the concrete in place and prevent its spreading before it has thoroughly set. A layer of Portland cement concrete, made in the proportion of 1 to 6, and from 6 to 12 inches thick, according to the weight on the footings, should then be filled in between the timbers and well rammed and leveled off. If the concrete is to be 12 inches thick it should be put in in two layers. Upon this concrete the beams should be carefully bedded in 1 to 2 Portland cement mortar, so as to bring them nearly level and in line with each other.
The distance apart of the beams, from centre to centre, may vary from 9 to 20 inches, according to the height of the beams, thickness of concrete, and estimated pressure per square foot. They must not be so far apart that the beams will crush through the concrete (see Section 53), and on the other hand there must be a space of at least 2 inches between edges of flanges to permit the introduction of the concrete filling. As soon as the beams are in place the spaces between them should be filled with 1 to 6 concrete, the stone being broken to pass through a 1½-inch ring, and the concrete well rammed into place, so that no cavities will be left in the centre. The concrete must also be carried at least 3 inches beyond the beams on sides and ends, and kept in place by planks or timbers.
50. If two or more layers of beams are used, the top of each layer should be carefully leveled (after the concrete has been put in place) with 1 to 2 Portland cement mortar, not more than ½ inch thick over the highest beams, and in this the next layer of beams should be bedded, and so on.
After the base plate or stone footing is in place at least 3 inches of concrete should be laid above the beams and at the sides and ends, and when this is set the whole outside of the footings should be plastered with 1 to 2 Portland cement mortar.
Mr. George Hill, Consulting Engineer, recommends that before laying the steel beams two thicknesses of tarred felt laid in hot asphalt ■should be spread over the concrete, and on top of this a layer of rich cement mortar 1½ inches thick, in which the beams should be placed. He also recommends that the whole footing be covered with two coats of hot asphalt.
51. Before the beams are laid they should be thoroughly cleaned with wire brushes, and, while absolutely dry, either painted with iron paint or else heated and coated with two coats of asphalt. Before covering the beams with the concrete every portion of the metal should be carefully examined, and wherever the paint or asphaltum has been scraped off in handling, the iron should be thoroughly dried and the coating renewed.
Every pains should be taken to protect the beams from rusting, for, when unprotected, steel beams rust very quickly, and if once the beams were subjected to the rusting process it would probably not be long before the building commenced to settle.
52. When iron and concrete foundations were first used in Chicago railway rails were employed, on account of their lesser cost, to give the transverse strength.
The footings were built up with five or six layers of rails, placed at right angles to each other, each layer diminishing in number until the upper surface was stepped off small enough not to unduly exceed the proper size of the column base. As each layer of rails was laid, concrete was filled between and around them, and when completed the footing resembled a simple concrete pier.
The footings under the Rand and McNally Building (erected in 1891) were of this character, five layers of rails being used in most of the footings. In some of the footings the upper layer consisted of 12-inch beams.
Building up the footings in successive tiers, however, is not as economical in the use of the steel as when two layers of deep beams are used. The beams being large and smooth, the concrete does not unite with them to form a composite beam, as is the case in the Ransome construction; therefore, no dependence at all can be placed on the concrete for spreading the weight.
It should also be borne in mind that the beams spread the load over the ground only by their transverse strength, and they should, therefore, be used in the same way that they would be were the foundation reversed, the wall or column becoming the support and the ground the load.
53. When several beams are used in the upper course or layer there is a tendency to concentrate the weight on the outer beams of the upper layer, owing to the deflection of the beams below. The author therefore advocates the use of as few beams as practicable in the upper course, and where the conditions will permit either a single built up girder or two heavy beams, and in the lower course the deepest beams consistent with economy. If the beams in the lower course permit of a spacing much greater than their height, a layer of rails should be imbedded in the top of the concrete to prevent the beams from breaking through. The rails, however, would in no way affect the stress or bending action in the beams.
For a further discussion of the use of steel beams in foundations, the reader is referred to an article by the author in Architecture and Building of Aug. 24, 1895.
Method of Determining the Size of the Steel Beams.