The elements of the floor are:

1. The arch, which receives the load directly. .

2. The beams, between which the arch springs.

3. The girders, carrying the beams.

4. The ceiling.

Arches. There are several different kinds of floor arch. In general, as to material of construction, they may be said to comprise the following types: brick, corrugated iron, porous terra cotta, hard tile, concrete, and concrete steel.

In office buildings and nearly all structures with a finished interior, some form of fla.arch is used almost exclusively in order to avoid the necessity of furring down for a flat ceiling. In warehouses, stores, and other buildings carrying heavy loads, segmental arch construction is more frequent. All segmental arch constructions require tie rods passing through the webs of the beams at intervals of about five feet, to take the thrust of the arches. Tie rods are also required in flat arch construction, where the arch is made of separate blocks, but they are not generally used for flat arches of concrete slabs.

The subject of arches will be considered in detail under "Fireproofing."

Beams and Girders. All of the horizontal members that form the framing of the floor come under one or the other of these heads.

A beam carries no other element of the framework, and receives strictly the load of the arch or the partition or other portion of the structure which it is designed to carry.

A girder carries the end of one or more beams. It may at the same time receive direct load from the arch or partitions ,• but if it carries other elements of the framework it should be referred to as a beam.

Other uses of the terms "beam" and "girder" will be considered later.