This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
Determine all the stresses and suitable sizes to use for a truss loaded as shown in Fig. 283, and resting on a brick wall at each end. The load consists of floor joists resting directly on the top chord; and a 6 x 4 x 3/8-inch angle should be provided near every other panel point, punched for lag screws to secure to wood joists for forming a lateral support to truss.
Make a complete shop detail of the above truss.
Trussed Stringers. Figs. 285 and 286 show the two common forms of trussed wooden stringers. These consist of a wooden beam, composed of one or more timbers, stiffened by one or two struts bearing on steel rods, as shown. They are used in timber-framed structures where it is impracticable to obtain timbers sufficiently strong to support the loads.
The trussed stringer is not a true truss, and the stresses cannot be accurately determined by the methods used for trusses, because the stresses in the members depend upon the deflection of the beam as a member of a truss and as a beam also. The exact solution is very complicated. An approximate solution can be made as follows:
In Fig. 285, if a load P is applied over the center strut as shown, then
Stress in ac = P/2 x ac/dc;
Stress in db = P/2 x ad/ac; and
Stress in dc = P.
If the load P is applied uniformly over the whole length of ab, then the stresses are approximately as follows:
Load at d =5/8 P;
Stress in ac = 5/16 P x ac/dc;
Direct stress in ab = 5/16 Px ad/ac;and
Stress in dc = 5/8 P.
The beams ad and db are however subjected to bending stress due to the load acting directly on the beam between the unsupported points a and d and b and d. If I is the moment of inertia of the beam, this bending stress can be found approximately from the formula f =
My/I, in which y = Half the depth of the beam.
The bending moment may be taken as 3/8 P X ab.
The beam must be proportioned so as to provide for the direct stress plus the stress due to bending, without exceeding the allowable fiber stress of the timber.
In Fig. 28G, if a load P is applied over each of the struts, the stresses can be determined approximately as follows:
Stress in ac = P x ac/ec; Stress in ac = P x ae/ac; and Stress in ec = P.
If the load 2 P is applied uniformly over the whole length ab, then the stresses are approximately as follows:
The load at e and f can be taken approximately as 5/6 P; then or Stress in ac = 5/6 P x ac/ec;
Direct stress in ae = 5/6 P x ae/ac; and
Stress in ec = 5/6 P.
The portions ac, ef, and fb are subjected to bending stresses as before; and if I is the moment of inertia of the beam, the bending stress in ac = My/1 in which y == 1/2 Depth of the beam; the bending M may be taken as 1/3 P X ab. The beam must be proportioned so that
Fig. 284 the combined bending and direct stress shall not exceed the safe fiber stress for the timber.
Owing; to the fact that the actual distribution of stress in trussed stringers is uncertain, and the methods of determining these stresses only approximate, a factor of safety of not less than 5 should be used.
The detail of the connection of the rods with the end of the beam is shown in Fig. 2S7. Sometimes a single rod going between a horizontal beam made of two timbers, is used; and sometimes where two rods are used, these are placed outside of the timber. A detail which will avoid boring through the timber is preferable. The plate at the end must be lare;e enough to distribute the stress without exceeding the
Fig. 287 safe compression value of the timber used; for hard pine, this should be 1,000 pounds per square inch. The plate should be thick enough to provide for the shearing stress on the metal, and the bending stress induced by the pull of the rod on the unsupported portion of the plate. It is important to have the center lines of the members intersect at the center of the bearing, as otherwise considerable additional bending stress will be caused, owing to the eccentricity.
THE OLIVER BUILDING, PITTSBURG, SHOWING STEEL FRAMING IN ALL ITS STAGES.