THE correct construction of a theatre is a subject for serious reflection. Safety being the chief essential, only reliable fire-resisting materials should be employed. As the additional cost of constructing a building absolutely fireproof is considerably less than ten per cent and usually not more than six per cent above erecting it non-fireproof, the rational procedure is clearly evident. The saving in fire insurance premiums alone will soon compensate an owner for the comparatively trifling addition to the initial expenditure. Ignorance of the relative cost of structural materials and an inborn but mistaken sense of the cheapness of wood have fostered an erroneous impression that rendering a building absolutely fireproof is expensive.
In large or medium sized theatres a steel framework is not only desirable but necessary. With medium steel, the kind used for building construction, a force of 70,000 pounds is needed to break a rod one square inch in cross section, but its elastic limit will not stand a stress of more than 40,000 pounds. In designing steel framework a margin of safety must be allowed that will bring the weight and stress well within the elastic limit.
The services of a competent engineer will be necessary to compute this, but as an economic precaution the author would suggest that after the steel plan has been prepared, it should be checked and revised by an outside engineer on a contingent fee basis; the revising engineer receives as his fee a percentage on any saving effected without sacrificing either the strength or utility of the framework. No risk will be incurred in such a procedure, as the various building laws in this country more than amply provide for safe steel construction.
This steel framework should include uprights and light roof trusses spaced from sixteen feet to twenty feet apart, center to center, with steel floor beams, balcony trusses and supports. Theoretically the shorter the spacing of trusses the less will be the combined weight of truss and purlins per square foot of covered area, but on account of practical limitations in size of materials and the greater cost per pound for manufactured trusses than for beams or purlins the spacing is usually regulated as above stated.
Balcony trusses should be supported by an application of the cantilever principle and not by exposed posts or columns. Where the anchoring distance is not sufficiently long for a direct application of this principle a transverse truss supported at both ends by columns imbedded in the walls should be employed to help support the overhang. A series of small-diametered (2 or 3 inch) steel Lally columns encased in the box partitions often may be used as supports for the same purpose.
Steel Lally columns are the simplest and cheapest forms of steel construction known. Originally they were covered by a patent that has since expired. They consist of various sized steel tubing (pipes) filled at the factory with a strong mixture of concrete to give added strength. Steel Lally columns withstand intense heat even without extra covering, far better than do ordinary fireproof-covered steel columns.
An ingenious application of the cantilever principle is indicated in the diagram illustrated at the conclusion of this chapter showing the trussing and support of the balcony designed for a theatre with the auditorium reversed. Here the anchorage span between the rear wall and the transverse truss was not long enough to counterbalance the unsupported overhang of the balcony. It therefore became necessary to contrive some form of construction that would support the balcony without dependence upon the masonry walls. This could be done by imbedding and anchoring the balcony upright members in heavy foundations extending over an area equal to that of the structure to be supported. These reinforced foundations, arranged in longitudinal sections six feet wide and six feet deep beneath the basement floor, extend from the rear walls forward to a point immediately below the extreme front of the balcony, and are spaced an equal distance from the side walls, and sufficiently far apart to receive the superior upright supports, thereby providing for the entire structure a solid base that could not be toppled over by any load that might be placed upon it. Where intermediate Lally columns are used a transverse foundation connecting the two main foundations should be employed.