This section is from the book "A Treatise On Architecture And Building Construction Vol2: Masonry. Carpentry. Joinery", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
Roofing. The roof of such a building as that shown in Fig. 90 should be as nearly flat as is consistent with a proper watershed, and built of 2-inch plank laid on roof beams corresponding in position and spacing with floor-beams below. The roof beams need not be the same size and strength as the floorbeams, but should, under any circumstances, be of such proportions of breadth to depth as will insure their slow combustion should a fire get any headway beneath them.
225. This description of the essential features of mill construction applies to such buildings as are built entirely of wood, no important member being of iron or steel, and nothing save the foundation being of brick or stone.
Modifications of the details are, therefore, necessary, where the walls are of brick or stone; where the columns are of steel or cast iron; or, where the girders are composed of rolled-iron or steel beams.
226. Wooden beams, or girders, resting one or both ends on a brick wall, should have an air space all around the top sides and end of the beam to give ventilation and prevent dry rot. This may be done by keeping the brickwork 1/2 inch away from the sides of the beam when it is built in the wall, but is much more satisfactorily accomplished by use of the Goetz-Mitchell anchor, shown in Fig. 93. This consists of an iron box, cast to fit the end of the beam, and possessing two projecting flanges a, which are enclosed in the brickwork of the wall, rendering the anchor a fixture. The box is cast about 11/2 inches wider inside than the wooden beam is broad, and about 1/2 inch higher than the beam is deep.
Along the front edge of the casting are square cheeks b projecting about 3/4 inch from the general surface of the interior. When the end of the wooden beam A is inserted in this anchor it is notched on its under side to fit over a raised fillet c, and the spaces f between the cheeks b give ventilation all around the beam inside the box-like casting. The fillet c is generally 1 in. X 1 in., and the notch d in the beam, fitting tightly over it, prevents the timber from pulling out of the wall. The top of the beam is rounded off at e, so that should the beam become burned through at its center, it can fall down and out of the wall without disturbing any of the brickwork and thereby endangering the stability of the wall. The advantage of this is illustrated in Fig. 94, where (a)
shows the destructive effect of a beam falling from a wall to which it has been anchored with the ordinary strap anchor, shown at k in Fig. 32, while at (b) the beam is seated in the cast-iron anchor above described, from which it can fall without damaging the wall in any way, and leaves the anchor intact to receive a new beam when repairs are made.
227. When this Goetz-Mitchell anchor is used in the masonry of a building, it is customary to replace the hardwood bolster shown at c, in Fig. 90, with the cast-iron post cap shown in Fig. 95. This cap is cast with a socket 4 inches or 5 inches deep on its under side to fit the top of the wood posts, while flanges, shown at d, are left on its upper side to embrace the ends of the girders resting upon it. Holes are cored through the metal at a to ventilate the top of the post and ends of the girders, and thereby lessen the danger of dry rot, while a large hole b in the seat of the cap permits the insertion of a dowel in the top of the column below, to which a superimposed post may be fitted. When used with this cap the girders are notched over the fillet c in the same manner as shown in Fig. 93, and a space of about \ inch is left between the ends of the girders and the sides of the post which is set between the upper flanges.
These anchors and caps are protected by United States patents, but may be cast in any foundry on the payment of one-eighth of a cent per pound to the Goetz-Mitchell Company, of New Albany, Indiana.
228. Cast-iron columns are frequently used in this class of construction on account of the ease of securing rigid connections between the columns and beams, and also between the columns themselves in different tiers. Brackets are left on such columns to which the carpenter is to seat and anchor the ends of his beams and girders; when these brackets are cast with a fillet, as shown at d, Fig. 96, the carpenter simply notches the under side of his beam, as for the caps and anchors above described. If a rolled-iron or steel I beam is used instead of a heavy wood girder, the carpenter will be called upon to sheath the beam in wood to protect it, and unless iron brackets are provided to seat the floorbeams on, he will also be required to provide a wooden ledge, as shown at e, Fig. 96, on which the beams are set and secured.
229. The methods of sheathing iron girders with wood and of securing floorbeams to them are numerous, and depend entirely on the purpose of the building and the economy of its construction. Under most circumstances, an exposed iron beam, such as is shown at c, will not stand the heat of a fierce fire as long as a wooden girder of equal strength, such as is shown at b, in Fig. 90. The iron beam absorbs the heat and soon becomes so hot as to bend in the middle, thereby throwing walls and columns out of plumb, and finally causing a collapse of that entire part of the building in which it is located.
A wood girder does not absorb the heat, and when exposed to flames takes fire very slowly, commencing with the lower corners, or edges, and gradually burning up towards the center until it breaks through and precipitates its superimposed load to the floor, or cellar beneath.
If its ends are supported on a cap, such as shown in Fig. 95, or upon a bracket, as at d, in Fig. 96, it will probably release itself as it falls, and in no way endanger the floor above, as the burning material has been carried down with the girder and is farther away from the next tier above than before the floor fell. When, therefore, iron or steel beams are used in a building with wooden floorbeams they should be protected by wood; and the best method of accomplishingthis protection is shown in Fig. 97, where a is a 15-inch I beam supporting, not only the floorbeams b, but also the partition c. Each side of the beam is filled and enclosed by two pieces of timber g and g', which are worked to fit their positions from 4"x8" stock and are held in place by 3/4-inch to 7/8-inch bolts i and j, spaced throughout the length of the beam a at about half the interval of the floorbeams b. The upper pieces of timber g are notched out where the floorbeams rest, as shown by the dotted line def. The end of the floor-beam is cut to fit this notch and seats itself on the lower timberg', at ef. The bolt j passes not only through the two timbers g', but also through the wooden ledges h, which are provided to give additional support to the floorbeams b. Another piece of timber worked from 3"x8" stock, is secured to the bottom of the girder, thereby enclosing the entire iron beam.
230. Where a partition is to be supported by such a girder, the 2-inch or 3-inch plank forming the body of the partition stands directly on the iron beam, as shown at c, Fig. 97, and the 3-inch splined plank of the under floor is laid up to it on each side.
The mortar filling is then spread on the rough floor, and if the side wall, or partition, is to be plastered, the mortar is then carried up the side wall, on wire lath, before the finished floor is laid. The finished flooring is then laid on the nailing strips between the mortar filling and securely nailed through to the 3-inch plank. If a base is to be carried around the room, grounds are set for it, against which the plaster stops, as in ordinary balloon or braced frame.