Motion Picture News (Nov-Dec 1922)

3420 Motion Picture New s Structural Engineering Important ( Continued from page 3419) trusses. A slope of twenty degrees beginning at the ridge to a distance of one-quarter the span from either side is frequently used with a greater pitch of not exceeding forty-five degrees for the end quarters of the span. This will usually permit of a good truss depth in both directions, provided the ceiling line is of the usual design with a center dome, not exceeding in width fifty per cent of the span of the house. Two columns are usually employed on either side at the boxes to support one of the main roof trusses. This is economical, as it reduces the span. These columns, as well as all truss compressing members, must be securely braced at all panel points. The lack of proper bracing is a common source of weakness and cannot be too strongly emphasized. More failures are due to lack of bracing than from any other cause. A well balanced design may be entirely deficient, due to the omission of certain braces. Balcony design is another problem. This is the most complex of all and requires a thorough knowledge of balcony construction from every angle in order to obtain best results. First consideration is given to arch construction. The writer has devoted much study to this all-important subject, and has been unable as yet to determine anything more practicable than cinder concrete with certain modifications of design. Foi example, the balcony stepping may be made in several ways — solid riser and treads supported on fireproofed beams is the heaviest and unquestionably the most durable. A modification of this to the extent of exposing the sides and bottom of beams and using partition blocks for risers, is an improvement, due to lightness of construction and facility of erection. In this ease the concrete slab is supported directly on top of steel work. Steel brackets may also be used to build up from the sloping beams, forming a support approximating to the contour of the balcony steppings. In modern construction it is generally required that the entire balcony be carried on concealed supports, leaving a clear, unobstructed view from the rear of the orchestra floor. This necessitates locating the supporting columns in the side walls of the theatre and spanning the distance between by means of cantilever design. In all cantilever design it is necessary to have proper anchor arms and means of anchorage to counterbalance the cantilevers. In the average theatre with its large, spacious mezzanine, it is not possible to locate these anchor arms in the rear of the balcony. This has made it necessary to invent some method of accomplishing the same results by other means. Generally this condition is met by introducing two diagonal trusses or girders, extending from about the quarter point of the main transverse truss between wall columns to a pier or column also in the side walls, forming an angle of approximately forty-five degrees. These diagonal girders then support two major diagonal cantilever trusses, supporting a front balcony truss of exceptionally shallow dimension, due to the limited depth at this point in the balcony layout. See sketch “ A.” This depth is usually about five feet and is dependent upon the slope of the balcony steppings and sight lines below. The two converging to a point just in front of the balcony facia. By using this method the entire space in the rear of the main cross truss is left clear and available for live space. In working out any design along these lines, many minor problems are presented which are not usually easy of solution. For example, there are usually at least two ramps or vomitories leading from the mezzanine to the first cross-over in the balcony. This means that two openings through the main truss must be provided and of sufficient size to comply with the requirements and the law. These openings are generally about five feet wide and eight feet high in the clear. To provide these large clearances through the main truss will not usually be found to be a simple matter. It will require engineering skill to properly work the truss members around these openings so that all conditions of loading will be properly provided for in the most economical manner. The difficulties encountered will depend largely upon the length of span and depth of truss. Sketch “ B ” shows a diagram of a truss supporting an eighty-eight-foot wide house with columns concealed in the side walls. For theatres one hundred feet in width it is usually advisable to locate the columns supporting the main balcony truss about five feet inside of the walls, leaving a space on the orchestra floor between the column and the wall of just sufficient width to clear the side aisles. This arrangement kills about three seats on each side of the Orchestra, but the loss in return from these seats which are in the rear side of the house is insignificant in comparison with the additional cost of construction involved due to the increased span if these columns are placed in the walls. There are many things to be considered in designing the structural work for the balcony of a theatre, and they can only be successfully met by extensive experience in this class of work. Details must be fully considered in connection with the design and the two fully kept in mind at all times hy the designer. The Indiana theatre, I crre Haute, Ind., which was opened during the pa st year lays claim to being one of the most completely finished houses in every detail that has ever been constructed