> > > >

1950-51 Theatre Catalog, 9th Edition, Page 115 (95)

1950-51 Theatre Catalog, 9th Edition
1950-51 Theatre Catalog
1950-51 Theatre Catalog, 9th Edition, Page 115
Page 115

1950-51 Theatre Catalog, 9th Edition, Page 115

sen.. FIGURE l. METHOD NO. l-Connnon type of gridiron used when plenty of height is available to operate the asbestos curtain underneath the gridiron

or where the total height oi the building is not important.

cables to the top of the asbestos curtain, and the attachments occupy some space; consequently, it is advisable to add 3' to these figures. For example, if the proscenium opening is 30' high, the asbestos curtain would be 3'1'6". By doubling this figure we would have 63' and, if we allow the 3' for the tie-off and adjusting purposes, the correct height of the gridiron would be 66' to the bottom side of the gridiron steel. It is not good practice to place the valance in front of the asbestos curtain in order to cut down this height because many states and municipalities do not permit the valance in front of the asbestos curtain.

Where this procedure does not work out conveniently, it is possible to provide a slot in the gridiron through which the asbestos curtain may pass (Method #2, Fig. 1), and, by using brackets on the proscenium wall above the gridiron to support the blocks, it would be possible to cut the foregoing figure down by three or four feet. Since this latter method, however, involves some structural problems and additional installation costs of the asbestos curtain, Method #11 is generally used. The construction of the gridiron is as shown on the schematic drawing (Fig. 1). Regulations prescribe that the gridiron shall be at least 50% open. This is to permit smoke and gases to travel upward and outward through the ventilators in the stage roof in case of fire.

The method of using 3" channels on 6" centers with flanges pointed downward, as shown in the drawing (Fig. 1), has been accepted as standard for many


years, and none of the numerous other types of construction tried has proved satisfactory. The problem of supporting these channels is one that must be solved with each individual job, depending, of course, on the span and the load to be carried.

Spacing of Carrying Beams

The next important step is to determine the correct spacing of the carrying beams or ttwells," as they are generally termed. To do this intelligently, it is necessary to make a layout of the stage in order to determine the number of sets and the total combined loads of these sets (Fig. 2). At this point the width of the proscenium opening plays an important part in the design, since the number of blocks necessary to pick up properly the various pieces of equipment is determined not only by the load, but also by the width of the proscenium.

The operating or "promp" side of the stage and corresponding head-block wells may be on either side of the proscenium opening. The governing factor is the placing of the scenery loading dock on the opposite side of the stage from the operating equipment; this is important from the standpoint of smooth operation.

As an illustration, if the proscenium opening is approximately 40/ wide, the sets will automatically become 5-1ine sets on 10' spacing, and, since with the 40' proscenium the regular stage equipment (border lights, etc.) would be comparatively light, it is in keeping with the principles of good design to set the loft

METHOD No. 2-37 hanging the gridiron iron: the root beams and passing the asbestos curtain up through the slot in the gridiron. Lighter gridiron steel may he used and the total height at the building reduced.

block wells to accommodate 8" diameter sheaves. If it is assumed, however, that a modern set of sound 'horns, weighing from 2,000 lbs. to 2,800 lbs. (cradle included), is required, it is necessary to use 12" diameter loft blocks, and the 12" diameter block then becomes the governing factor in spacing the distance between the channels of the loft-block wells.

All stage equipment is built to standards and it is always desirable to space the steel in the gridiron so as to utilize the standard blocks, since special construction runs into additional money and sometimes additional installation costs. It will be noted that the cables over the loft blocks are single cables. This permits narrower spacing of the loft-block channels than is possible on the headblocks which will be discussed later; but, in any case, the loft block should so set as to distribute the load equally upon both channels. This factor is quite important in the interest of good practice. The two outside wells of the gridiron obviously should be tied in with the columns on each side of the proscenium and need not be located away from these columns, as is sometimes done. This method will work out satisfactorily on all normal

width stages.

In the case, however, of extremely wide prosceniums40' and 100' widthsthe problem of packing these wide curtains away on each side of the stage arises. Due to the bulk of the curtain, it is necessary to get additional supporting loft blocks several feet back of the "up"
1950-51 Theatre Catalog, 9th Edition, Page 115