Although the larger stores and warehouses, particularly those in large cities, are now generally constructed with iron or steel posts and joists, yet the larger proportion of two and three-story business buildings, lodging houses, etc., are still built with wooden floors and partitions, and will probably continue to be for many years to come. Steel construction is undoubtedly preferable to wood in many ways, and when it can be employed and allow a fair return on the investment, the architect should certainly recommend it to the owner.
There are many ways, however, in which the ordinary wood construction, as found in the smaller cities and in country towns, may be greatly improved and made to answer its purposes nearly as well as steel construction.
One of the greatest defects in the ordinary construction of business blocks, small office buildings, etc., is supporting the floors on woodenpartitions. No building other than a dwelling should have the floors supported in that way.
If the distance between the walls is not more than 24 feet the floor joists should be made of sufficient size to span from wall to wall without assistance from partitions. If the distance between the walls is greater than 25 feet then brick partition walls, or posts and girders should be used for intermediate supports. For large rooms single spans of 28 feet may be used, but this is about the maximum span for which wooden joists can be used with economy.
The principal objections to the use of partitions for supporting the floor joists in this class of buildings are:
1 st. Inconvenience in changing partitions. Business blocks in particular often require alterations in the partitions to suit the convenience of tenants, and when the partitions support the floors they cannot well be moved.
2d. The weight of the joists on the partitions has a tendency to spring the studding and loosen the plastering. The partitions themselves often have insufficient supports, and the ordinary 2-inch cap is too thin to support heavy floors. It will generally be found that where floors are supported on partitions that the plastering on the partitions is badly cracked and the ceiling sags at the centre of the partitition.
3d. Less security in case of fire. Stud partitions, being constructed of small timbers, are quickly consumed by fire, and but a few moments are required to weaken the studding sufficiently to cause the floors to fall. Girders, on the contrary, being large, solid timbers, do not burn readily, and will often stand until the fire is extinguished.
Posts and Girders. - These should be arranged so that the span of the joists will not exceed 24 feet, and 16 feet gives greater economy. The girders may be of wood or steel; the former will generally be used except where girders of considerable length are required. With a joist span of 16 feet it is not good practice to have a greater span than 14 feet for the girders, and 12 feet is the maximum span permitted in several cities for wooden girders.
The posts may be either of wood, cast iron or steel. In buildings of not more than three stories, having wooden floor beams and girders, iron columns offer no particular advantage over wood, except that they may be made a little smaller. Metal posts, unprotected, will not stand as long in a fire as heavy wooden posts.
For the comparative advantages of cast iron and steel posts, and rules for determining their strength, the reader is referred to Chapter XI (Lathing And Plastering). of the Architects* and Builders' Pocket Book.