Bracing both ways will, of course, reduce the length of a long strut.
If the concrete floor is comparatively green, the load must be distributed by blocking, prefer ably of hardwood. At the top of the strut, pro vision must be made against crushing of the wood of the plank or cross-piece. Ordinary soft wood will stand without crushing only about 700 lbs. per sq. in. across the grain; so, if the compression approaches this figure, brackets must be inserted or hardwood cleats used.
Kinds of Lumber to Use. The selection of the lumber must be governed by the character of the work and the local market. White pine is best for fine facework, and quite essential for ornamental construction cast in wooden forms. For ordinary work, however, even for the panels, white pine is apt to be too expensive; and spruce, fir, Norway pine, or the softer qualities of South ern pine must be substituted for it. Some of these woods are more liable to warp than white pine, but they are generally stiffer, and thus better adapted for struts and braces.
Kiln-dried lumber is not suitable for form construction, because of its tendency to swell when the wet concrete touches it. Very green lumber, on the other hand—especially Southern pine, which does not close up quickly when wet —may give trouble by joints opening. There fore the middle ground—or, in other words, par tially dry stuff—is usually best.
Finish and Thickness of Lumber. Either tongued-and-grooved or bevel-edged stuff will give good results for floor and wall panel forms, and is preferable to square-edged stuff. A smoother surface may be attained at first with the tongued-and-grooved stock than with square or bevel-edge, and there is less trouble with opening joints, but it is more expensive because of the waste in dressing; and if the forms are used many times, there is greater tendency to wear at the joints. Even for rough forms, plank planed one side may be economical to cheapen the cost of cleaning. Studs should always be planed one side to bring to size.
The thickness of lumber varies with differ ent contractors, some using 1-in., others while a few employ 2-in. stuff even for panels, these being commercial thicknesses measured before planing. For ordinary walls, stuff
is good; although, for heavy construction where derricks are used, 2-in. is preferable; while for small panels, 1-in. boards are lighter and easier to handle. For floor panels, 1-in. boards are most common; although, if the building is eight stories high or over, 1-in. stuff of soft wood is likely to be pretty well worn out before the top of the building is reached, and the under surface of the concrete will show the wear badly. For sides of girders, either 1-in. or is sufficient, while 2-in. is preferable for the bottoms of girders. Column forms are generally made of 2-in. plank.
In building forms and centering, remember that the cost of timber is a large item of the total cost. With a little ingenuity and forethought, this work can be most always arranged so that repeated use is made of each piece of timber.
Alignment of Forms. Alignment is another item of importance, since it is here that a great deal of time may be wasted by inexperienced or incompetent carpenters. Such workmen may err either on the side of poor alignment or more careful alignment than the structure requires. Mr. W. J. Douglas suggests as a general rule the allowance of %-inch departure from established lines on finished work, and 2 inches on unfinished work.
In removing forms, the green concrete must not be disturbed by prying against it. This seems so obvious as to need no emphasis; but I have known first-class carpenters to attempt to straighten a wall which was an inch out of line, the day after the concrete was laid, by prying the forms over. The wall was straightened, but by a different process from that proposed by the carpenter—the concrete was relaid.
Forms for facework should be tightly put to gether, it being advisable in some cases to close the joints and holes by mortar, putty, plaster of Paris, sheathing paper, or thin metal. This is not, as is commonly supposed, to prevent loss of strength by the cement which flows out with the water, but rather to prevent the formation of voids or stone pockets in the finished surface.