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1950-51 Theatre Catalog, 9th Edition, Page 116 (96)

1950-51 Theatre Catalog, 9th Edition
1950-51 Theatre Catalog
1950-51 Theatre Catalog, 9th Edition, Page 116
Page 116

1950-51 Theatre Catalog, 9th Edition, Page 116

and "downil stage line of the proscenium opening. These blocks may be of the "spot" type and may rest on the gridiron door with the cable passing down between the 3" channels of the floor. This point is mentioned here because in many cases a gridiron is designed with very flimsy supports between the outside loft blocks several feet back of the flupil stage. When dealing with exceptionally large stages, it is always desirable to get at least one carrying beam spaced 6' to 8' on each side of the outside loftblock well. The iispoti' block will, then rest over this beam, and the condition will have been met properly.

Spacing of Head-Block Beams

In determining the spacing of the head-block beams, reference is again made to the stage plan (Fig. 2) and the maximum load carried by any one set, not including the asbestos curtain. If modern sound horn equipment is again taken as an example, it is good practice to use at least a 16" diameter head block as the governing factor. The operating line passes up and over the sheave and back down between the head-block beams. Thus, the spacing of the beams must be somewhat greater than the full diameter of the head-block sheave.

It is advisable to allow 4" clearance between the beam flanges on each side of the head-block sheave, when in position, in order to give proper clearance for the lines and, at the same time, provide room for clamping the head blocks to the head-block beams. In determining the elevation of the headblock beams, it is recommended that the top flanges of these beams be placed not less than 1' 6" above the level of the

gridiron floor. At least 1" clearance should be provided above the top of the largest head block used.

Distribution of Loads

A typical distribution of gridiron loads is shown in (Fig. 3). This drawing should be helpful in laying out the steel, but one should bear in mind that as the stage increases in length (this is based on a 40' proscenium), the load accelerates rapidly. In the case of the gridiron proper, this load increase is not too serious, since, as the width of the proscenium opening increases, the loft-block wells increase in number, and the load is divided equally among the loft-block wells. However, as far as the head-block beams go, the carrying of this accelerated load must be considered, In determining all of the carrying beams, it must be realized that stage equipment is a static load.

Regulations require that the asbestos curtain head block be independent of the regular head-block battery, and it is, accordingly, set on the gridiron with some reinforcing steel (usually by the rigging contractor) at a point closely adjacent to the proscenium opening. The counterweights are guided by a special counterweight track attached to the proscenium wall. Therefore, in calculating the load of the head-block beams, the weight of the asbestos curtain may be deducted. The loads figured (Fig. 3), are intended to reilect the weight of the curtains with the stage equipment included.

Heretofore, in referring to asbestos curtains, only the flexible type has been considered. In many cities steel frame curtains are demanded, especially in proscenium openings over 60 in Width. Steel


frame curtains vary in weight from 8 to 20 .tons. Some additional provision must, therefore, be made for handling this load, and the method of rigging is somewhat diiferent than that used for the regular flexible type curtain.

In the case of the steel frame curtain, it is customary for the rigging contractor to provide two longitudinal channels which set on the gridiron and run all the way across stage from wall to wall. In some instances they are not extended from wall to wall, but only far enough to accommodate the curtain rigging equipment; in such cases, however, they should be securely braced longitudinally. Since these channels are set as close as possible to the proscenium wall with a spacing of not over 10" between the channels, this load is quite conveniently carried without any additional reinforcement. It is customary, in the case of steel frame curtains, to use a double counterweight set with counterweights at both ends of the curtain. The lead lines, running over the loft blocks and down to the curtain, are alternated over the top of the curtain in such a manner as to equalize the longitudinal pull. If it is felt that the regular gridiron carrying beams are not sufficient to carry this load, it is sometimes practical to carry part of this load on the roof beams by means of hangers extending down to the gridiron floor.

The moment of force of the head-block beams (also illustrated in Fig. 3), indicates that it is at an angle of approximately 450 with the center of the sheave, thus imposing the greater part of the load on the on-stage head-block beam. If desired, a lighter beam can be safely used on the wall side or the off-stage beam. There is a tendency for the head

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1950-51 Theatre Catalog, 9th Edition, Page 116