SELECTION OF SYSTEM Not any single system, pfobably, would be used in all cases even if the designer were to choose without any conditions affect ing his selection. Some systems are naturally better adapted than others to certain conditions. Practically there are always a num ber of considerations affecting the choice. No attempt will be made here to specify to what conditions certain systems are better adapted than others, as this is largely a matter of judgment at the present time The considerations in general, however, are as follows: Light or heavy live loads ; dead weight of construction, and con sequent spacing of beams and span of arches ; necessity of lateral stiffness in floor system ; possibility of using paneled ceiling, and consequent increase of clear height story between beams ; necessity of flush ceiling, and comparative advantage of solid floor system and furred-clown ceiling; protection afforded webs and flanges of beams and girders by different systems ; possibility of omitting tie rods and a certain amount of steel in some systems ; corrosive effects on steel under certain conditions ; rapidity of construction, and allowance for final setting of concrete under certain conditions of weather and of heavy loadings ; and comparative cost of differ ent systems.
The weights of hollow-tile floor arches and fireproof materials, in pounds per square foot, are given in the following table : The following table shows the safe loads in pounds per square foot uniformly distributed for hollow-tile floor arches.
Net loads in pounds per square foot, i. e.. excluding weight of arch. Safety factor, 6.
The formula for safe load used in computing the above table is as follows : In the following table are given, in pounds per square foot, the weights of various materials used in floor and roof construction Partitions are of terra cotta, wire lath and plaster, and plaster board.
Illustrations of each are given by Plate VI, Figs. 71 to 77. The element of strength does not form a specially important con sideration here, as the standard forms are all suitable. The higher the partition the thicker should be the blocks or the heavier the metal frame of the partition. Some of the forms are more sound
proof than others and probably more fireproof, but the use of any one is generally determined by architectural conditions. The terra cotta blocks come in standard sizes given by the table below, which also give3 the dead weight per square foot. The constructions around openings in partitions, for the different types of partition, are also shown by the above-mentioned cuts.
Partitions are never as fireproof as the floor system in a build ing. If a form of construction could be used which would prevent the spread of fire through partitions, the modern office building would probably be in truth absolutely, instead of merely in name:, fireproof. The great cause of the weakness of fire resistance lies not in the partitions themselves so much as in the fact that open ings for doors, windows, flues, etc., have to be made in them. The arrangement in a great many buildings makes it necessary, in order to give light in the corridors, to have a line of windows in the partitions between them and the offices. In addition there are the doors into the corridors, and the doors and sometimes windows in partitions between offices.
As stated under "Building Laws and Specifications," some cities require in buildings of a certain height the use of metal or of fireproof wood for all inside casings and finish, but in the majority of buildings these are not used. Sometimes, also, where plaster and wire lath partitions are used, the plaster does not extend to the floor, and the baseboard has therefore no fireproof protection back of it.
All these features indicate the real elements of weakness in a fireproof partition,' and on the extent to which they can be elim inated depends the utility of the partition as a fire barrier. As will be shown later under the paragraphs on tests, there are a number of forms of partition that can be used, which, if without openings and the other features mentioned above, will form effec tual barriers. The extent to which fireproof wood and metal over come the difficulties will be discussed farther on.