Motion Picture News (Nov-Dec 1922)

D e c e m her 30, 1922 3419 NEW THEATRE CONSTRUCTION tS EQUIPMENT projection Department c a m e r a P. M. ABBOTT TECHNICAL EDITOR Structural Engineering Important in Theatre Construction Owners and Architects Should Engage Only Experts to Layout Design By H. Robins Burroughs, Theatre Engineer STRUCTURAL engineering plays a leading part in theatre construction for the simple reason that the structural steel frame is the backbone of the theatre and therefore carries a great load. Some types of construction are heavier than others and each particular type used has a direct bearing on the amount of steel required to support it. Heavy construction such as stone conci’ete always involves heavy tonnages of steel, cinder concrete a less amount and still lighter materials reduce the tonnage still further. Hence the first consideration in laying out a structural steel frame is to determine the type of construction to be used in order that the weight to be carried may be approximately obtained and the decision as to the proper type to use should depend largely on its weight. It should be as light as practicable but consistent with local and standard building law requirements. Difference in Requirements There is a wide difference in municipal requirements in theatre construction, and especially in arch design. That is, balcony or floor arches. In general, there are several kinds of arches usable for fireproof theatre floors, namely, cinder, concrete, stone concrete, gypsum and a precast porous concrete known as Porete. The latter is the lightest material in weight, averaging about seven pounds per square foot, but is only adaptable to short spans of thirty-two inches. Also being precast, it does not lend itself to the curves and carved surfaces usually found in theatre balcony design. Furthermore, it does not possess fireproofing qualities for supporting beams. This fireproofing quality is not always required. Many building departments accept suspended ceilings as fireproof construction and as practically all theatre ceilings are suspended, other forms of fireproofing are not necessary. This is one important feature that should not be overlooked and in many instances should be taken advantage of. The usual type of arch construction used in theatres in New York and vicinity is cinder concrete, as required by building laws. That is, cinder concrete reinforced with suitable wire mesh and poured in place so as to entirely envelope all supporting steel work. It is recommended as first-class construction and consequently may be used with impunity. After the type of floor and roof construction has been carefully determined, the loads are BaLCoN/ \_AyouT SK^Tch R then available for computing the steel design, which should then be studied as to efficiency and economy of layout. Time given to the study of economics in steel design is well spent, provided it is given by the theatre specialist or one experienced in that particular class of work. Owners and architects should engage only experts to do this work for structural designing of even the usual modern theatre is one of the most complex problems known in structural engineering and can be successfully accomplished only by wide and extensive experience in theatre work. Therein frequently lies the fatal error made in numerous cases. The value of specialization is underestimated or overlooked altogether. If a specialist is approached his fee is considered excessive because the results of his services are not visible. Saves Tons of Steel The specialist may and probably would save the owner several tons of steel and still give him a better design, but the owner or even the architect does not realize this and apparently there is no means available to prove it. Therefore the competent engineer must rely on his reputation to convince his clients and especially his new ones. This may be properly done. It then behooves the client to accept reputation as being conclusive. In the majority of cases it is conclusive. However, not always. The writer has known a few cases where reputation was founded wholly on a false premise and yet accepted for its full face value. This is something to guard against. An owner or architect should select his engineer with equal care of that of any other professional person. Let the specialist be selected by and to represent the owner, indirectly or directly through the architect. Let his remuneration come direct or indirect from the owner since the owner is the beneficiary and not the architect. The engineer’s fee should be entirely disconnected from the architect’s, unless the engineers and architects are associated. This is the ideal combination for maximum economy and efficiency and is strongly recommended. Complex Problems In designing the steel work for a theatre, the engineer is confronted with a multiplicity of problems which contain a marked degree of dissimilarity. In point of fact not any two of them are anything near alike. For example, there is the roof design, a problem wholly independent of other parts of the structure. The roof may be designed for several different kinds of construction, namely, cinder concrete, gypsum and Porete, each varying in weight to a high degree. This difference in weight has a marked effect on the tonnage of steel required and should be given due consideration. In cinder concrete work the supporting beams may be spaced, not to exceed eight feet on centers, whereas for gypsum and Porete, the supports are required to be about thirtyinch centers. This increases the amount of steel in the purlins, but due to the lighter weight reduces the amount in the main structure. The writer has used all three types of roof construction and finds that there is some economy in the light weight type. It is more easily erected and presents a workmanlike appearance when completed. Especially for steep roofs the precast light weight slab is particularly practicable. When cinder concrete is used, the roof beams are usually enveloped in concrete and hence fireproofed, whereas with the short span construction this is not obtainable. Therefore the former is better fireproofed and generally considered higher grade work. The writer does not hold this element of additional fireproofing to be essential and considers that the additional cost involved is not justified except for stage roofs. They should be thoroughly fireproofed, and here cinder concrete is strongly recommended as being the most practicable. The formation or contour of the finished roof line is important. Flat roofs are not economical. The ridge of the roof should be raised so as to provide an economical depth of ( Continued on page 3420)