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.
In cases where the roof truss is placed on steel columns and is connected with the column by a knee-brace at the first joint (see Fig. 39), stresses caused by the overturning action of the wind take place in those members shown by heavy lines. In this case the stresses caused by 40 pounds per square foot of horizontal projection are not large enough; but the truss will be safe enough if the stresses as determined by the 40 pounds are increased by the amounts indicated in Fig. 39.
Fig. 39. Allowance for Stresses Due to Wind.
For example, let the truss of Fig. 24 be supported by steel columns and knee-bracing. Let the span be 60 feet, and the distance between trusses 16 feet; and let it be required to compute the stresses in L1 U2 and L3 U4. Here P= (40 X 16 X 60) ÷ 8 = 4 800, and the stresses will be:
Ll U2 (0.87 X 4 800) X 2.10 = + 8 350 pounds.
L3 U4 (2 .60 X 4 800) X 1.50 = +18 700 pounds.
In addition to the above conditions, shafting, heating apparatus, small cranes, and electric wiring and other conductors are often attached to the lower chord of the truss. These cause additional stresses. The case is that of a concentrated load or loads at the lower chord, and the stresses may be computed by the methods given in "Statics."
For example, let a 5-ton hoist be connected as shown in Fig. 40. This hoist runs longitudinally of the shop, or perpendicularly to the plane of the roof truss. The maximum stress in the truss due to this cause will occur when the hoist is directly beneath the truss. The stresses will be those caused by a load of 10 000 pounds at the second panel point of the lower chord. Fig. 40 gives the stress diagram for
Fig. 40. Fink Truss Loaded with 5-Ton Hoist; also Stress Diagram of Same.
+ 13 700
+ 6 100
- 6 900
this condition, and Table VI gives the stress record. From this it is seen that the hoist does not affect all members of the truss. The stresses due to the hoist should be added to those caused by the 40 pounds per square foot of horizontal projection, and the member designed accordingly. Of course, if the stress caused by the hoist decreases the stress caused by the 40 pounds, the member must be designed for the stress due to the 40 pounds.
Note that concentrated loads, as in the case of the hoist, cause different stresses in symmetrical members on the two sides of the truss. In the final design, the members are made the same, being designed for the greatest stress. This is done for the sake of economy in manufacture; and besides, it might be desirable to change the hoist to the other side of the truss.
For Fink trusses with pitches of from 1/5 to 1/3, and spans of less than 100 feet, very light angles are usually required for the members, unless heavy, concentrated loads are placed on the lower chord. The thickness of the connection plates is seldom more than 3/8 inch, the top chord angle seldom greater than 5 by 3½-inch, the lower chord angle seldom greater than 3 by 3-inch; and the web members are usually composed of angles either 2 by 2-inch or 2½ by 2½ inch. It appears to be the rule, in present practice, to make the sizes such that the thickness shall be ¼ or 5/16 inch. Connection plates for spans up to 60 or 70 feet are usually 1/4 inch thick, except in the case of that at point L0.
The stresses in knee-braces depend upon the height and also the width of the building. The stresses may be computed according to the methods of the next article, and the knee-bracing should be designed accordingly. The inspection of a number of plans seems to indicate that the sizes of knee-braces vary from two angles 2½ by 2½ by ¼-inch for spans of 30 feet and a height of building of 35 feet, to two angles 4 by 3 by 5/16 -inch for a span of 70 feet and a height of building to the top of the truss of 75 feet.
In case of roof trusses with the chords nearly parallel (see Fig. 3, p. 2), the stresses, on account of the small depth, are usually quite large, and much heavier members than above mentioned are required. In some cases, 6 by 6-inch angles with 8-inch plates are used, and connection plates of 3/8 to ½ inch are common.
In cases where the trusses are subjected to the action of corrosive gases, the thickness of the members should be made greater than that required by the design alone, since corrosion will decrease the section considerably, and this should be allowed for.
Fig. 41. Bending Tendency, Ends Free.
Fig. 42. Bending Tendency, Ends Fixed.
10. The Steel Truss-Bent. When a truss is connected to steel columns at its ends and by means of knee-bracing (see Fig. 39), it forms what is called a steel truss-bent. The stresses in the truss due to the roof covering and snow loads are the same as when it is supported by a masonry wall; but the wind stresses are different. The wind blowing on the roof and also on the sides of the building, causes stresses in the truss. The wind on the building is transferred to the columns, which, by means of the knee-braces, cause stresses in the truss. The whole bent tends to bend as shown in Fig. 41 if the ends of the columns rest on masonry pedestals. If the ends of the columns are securely bolted to heavy masonry pedestals so that the ends of the post will remain vertical, they will tend to bend as shown in Fig. 42. In the first case, the overturning is resisted by the bending of the post as shown at b and b' (Fig. 41); in the second case, by bending as at 6, c, b', and c' (Fig. 42). Since the post is the same size throughout, and the bending caused by the wind the same in both cases, the bending moment in the post at b and b' (Fig. 41) is less than what it is at b and b' (Fig. 42), as in the first case there are only one-half the number of points to withstand the total bending that there are in the second case.