Although it is not necessary for the architect, draughtsman or superintendent to be able to lay out the frame of a building, cut the timber and put it together, it is necessary that he should have a thorough knowledge of the way in which it should be done, and how all the joints or connections should be made, as otherwise he can not be sure that the work is done as it should be.
In the Eastern States it is customary for the architect to show the complete framing of wooden buildings by a separate set of drawings, and to make separate framing plans for the floors and roofs of brick buildings, but in the West the framing is often left to the care of the builder, the sizes of the timber being specified and the direction the floor joists are to run being indicated by dotted lines on the plan Such a practice should be discouraged, as it is much better for all concerned that a complete set of framing plans be furnished by the architect.
The young architect should also bear in mind that the courts invariably hold the architect responsible for the safety of the building, as far as it depends upon the plans and specifications, and hence nothing of importance should be left to the discretion of the builder, unless perchance he is one in which the utmost confidence may be placed.
To design the framing of a building in a practical and economical manner, it is necessary for the architect or draughtsman to be familiar with the kinds of wood used for framing in the locality in which the building is to be built, and their relative cost, and also with the commercial sizes to which lumber is sawn. Information on these points is given in Sections 20, 23 and 38, but it should be supplemented by inquiries of local contractors or lumber merchants.
The various pieces of timber used in framing buildings have distinguishing names with which it is also necessary to be familiar; thus, floor beams are called "joists," the pieces which support the roof boarding are called "rafters" and the uprights in a wooden wall or partition "studding" or "studs."
Beams which support the floor joists between walls or partitions are called " girders," and similar beams under the rafters of a pitch roof are called "purlins." Small timbers, such as 2x4 inches, 2x6 inches, 3x4 inches, 4x4 inches, etc., are sometimes called " scantlings." Pieces 4x6 inches and over in cross section are almost always designated as "timbers."
There are also various other names for timbers used in special positions, which will be mentioned in describing the construction of which they form a part.
Floor joists, rafters and studding are commonly made 2 inches thick, the depth depending upon the span of the joists or rafters or the height of the studding.
As the strength of a rectangular beam increases as the square of the depth and only directly as the breadth, it is more economical of material to use a deep beam rather than a thick one. Thus, for the same span a 2x10-inch joist has the same strength as a 3x8-inch or 5½x6-inch joist, but it contains less lumber. The deeper beam is also much stiffer, the stiffness being in proportion to the cube of the depth, so that a 2x10-inch and a 3 7/8x8-inch beam have the same stiffness (other conditions being equal), although the latter contains 55 per cent, more lumber than the former. When the depth of a joist exceeds 12 inches, however, the thickness should be increased to 2½ or 3 inches, as a 2x14-inch joist is liable to fail by buckling sideways.
Large timbers are generally made more nearly square, for the reason that it is difficult in most woods to get great depth with 6 or 8 inches in thickness; hence, for girders and purlins, 8x10 inches, 10x12 inches and 12x14 inches are common sizes; 16-inch timbers are seldom used, although they can sometimes be obtained. Posts, on the other hand, should be either round or as nearly square as the conditions will permit, the square post being the most economical section for timber.