In planning large stables and buildings containing assembly rooms, it is very often necessary to provide for rooms in intermediate stories, 40 or 50 feet square, without posts or other vertical supports. If such rooms are not more than 50 feet wide the floor above can be supported by riveted or trussed girders, but as the former are quite expensive and either must drop considerably below the ceiling, they are not generally practicable unless the story is a very high one. The usual method of supporting the upper stories in such buildings is by means of trusses and suspension rods, as illustrated by Fig. 480*, which represents a section through the American Express Company's stable on East Forty-eighth Street, New York City.
In this building the first and fourth stories are devoted to the storage of wagons, and are entirely free from posts or rods. The third and fourth floors are supported directly on the bottom and top chords of ordinary Howe trusses, and the second floor is hung from the trusses by rods. The roof over the upper story is supported by an independent truss.
The truss being placed in the story used for the storing of fodder, interferes little with the convenient use of the space. The rods in the story below pass through the stall posts and are thus entirely out of the way.
* From the Engineering Record, Vol. 31, No. 1, by permission.
If only one floor above the open story is to be supported it may be hung directly from the roof truss, as shown in Fig. 481, which shows the method of supporting the floor over the auditorium in the Museum of Fine Arts, St. Louis, Mo. In this case the rods A, A, A are concealed in partitions. Where the arrangement of the stories will permit, it is undoubtedly better construction to support the second and third floors above the open story by means of a Howe truss, as in Fig. 482, and if there are additional floors above, these may be supported by posts, as shown in the same figure. Placing the truss near the bottom of the building lessens the weight borne by the walls above and also increases the stability of the building, as the truss acts also as a brace to stiffen the building against swaying sideways. Where trusses support several stories by means of posts they should always be of steel. When supporting floors by means of trusses it must be remembered that all the weight borne by them must be transferred to the walls, and the higher the truss is placed the more "top-heavy" the building will be and the greater will be the quantity of material required in the piers supporting the trusses.
It is always better, when the purpose of the building will permit, to support the floors on columns running through continuously from foundation to roof, and when an open story must be provided it should be placed as near the top of the building as the conditions will allow. There are numerous buildings, however, in which many stories are supported by trusses after the manner shown in Fig. 482, and several of the tallest buildings in New York have the wall columns for the entire height of the building supported on cantilever trusses resting on the foundation piers.
In the Crocker Building, San Francisco, seven stories are supported by trusses in a manner very similiar to that shown in
The Schiller Building, Chicago, has seven stories supported by trusses placed just over the theatre and carrying lines of columns.
As a rule, not more than one story should be hung by rods below the truss.
The manner of securing the suspending rod is also a matter of importance. The method shown in Fig. 483 is probably as good as any, and is comparatively simple. The truss rods are extended some 6 inches or more below the washer under the tie-beam, with a thread turned the full length. A nut is then screwed on and turned up until the truss timbers are brought tightly together, and then a turn-buckle is screwed on as shown. The turnbuckle can be made larger at the upper end than at the lower, to allow for the difference in size of the two rods. The truss rod, having to sustain the entire weight from the suspension rod, and also an additional strain from the truss timbers, should consequently be larger than the suspension rod.
If turnbuckles cannot be readily obtained, or the strain requires a very large truss rod, two rods may be substituted for the latter, coming down each side of the tie-beam as shown in Fig. 484, and the suspension rod passed through the tie-beam and washer, with a head or nut on the upper end as shown.
This method has the objection that the lower washer or plate must project considerably beyond the tie-beam, and the full load from the suspension rod and from the truss is also brought upon it.
Galleries or balconies are often hung from trusses by suspension rods in the manner described above, thus avoiding posts in the room below. Posts, however, are better, as they carry the load directly to the foundation, and by giving a rigid support to the gallery the latter also strengthens the wall. With rods, on the other hand, the weight of the gallery is transferred to the wall at a considerable distance above the floor, and the whole tendency of the construction is to spring the wall. Only those forms of trusses which have horizontal tie-beams should be used for suspending floors or galleries.
Fig. 485, which shows the manner in which the first floor, and also the sidewalk, is supported between the stone piers of the street front of the Youth's Companion Building, Boston, is interesting as a detail of this class of framing. The rod is suspended from a box girder at the second floor level, and terminates in the nut beneath the steel beam from which the sidewalk beam is hung. A short cast iron column is slipped over the rod, with its base resting on top of the ate
I-beam,-and supports the end of the first story trimmer or girder, the whole being borne by the nut on the end of the rod.