The apex load for joint (2) is 14 of 12,400, or 0,200 pounds, and for joints (1) and (3), 54 of 12,400, or 3,100 pounds (see Fig. 29).
If the left end of the truss is fastened to its support and the right rests on rollers*, when the wind blows on the left side the right and left reactions equal 3,780 and 9,550 pounds respec tively and act as shown. When the wind blows on the right side, the right and left reactions equal 6,380 and 8,050 pounds and act as shown. The computation of these reactions is shown in Example 1, Page 58.
For the wind on the left side, OA, AB, and BO (Fig. 29b) represent the apex loads at joints (1), (2) and (3) respectively and CE and EO represent the right and left reactions; then the poly gon (clockwise) for the loads and reactions is OABCDPEO. The Point C is also marked D and P because there are no loads at joints (5) and (0).
The polygon for joint (1) is EOAFE, AF and FE represent ing the stresses in of andie respectively. The values are recorded in the adjoining table. The polygon for joint (2) is FABGF, BO and GF representing the stresses in bg and fg. The polygon for joint (3) is GBCIIG, CII and IIG representing the stresses in eh and log respectively. At joint (5) there is no load and two of the members connected there are in the same line; hence there is no wind stress in the third member and the stresses in the other two members are equal. The point II is therefore also marked I to make III equal to zero. The polygon for joint (5) is IICDIII.
At joint (4) there are four forces, all known except the one in ie. EF, FG, and GI-I represent the first three; hence the line
joining I and E must represent the stress in ie. This line, if the drawing has been correctly and accurately made, is parallel to ie.
For wind on right side, BC, CD, and DP Fig. 20(0 represent the loads at joints (3), (5) and (6) respectively and PE and EB the right and left reactions; then BCDPEB is the polygon for the loads and reactions. The point B is also marked A and 0 because there are rio loads at joints (2) and (1).
The polygon for joint (6) is DPEID, EI and ID representing the stresses in ei and id respectively. The polygon for joint (5) is CDIHC, Hi, and HC representing the stresses in ih and he respect. ively. The polygon for joint (3) is BCHGB, HG, and GB repre senting the stresses in hg and gb respectively. The polygon for joint (2) is BGFAB, FA representing the stress in fa, and since GF equals zero there is no stress in gf.
At joint (1) there are three forces, the left reaction, AF and the stress in fe. This third force must close the polygon, so we join F and E and this line represents the stress in fe. If the work has been accurately done, FE will be parallel to fe.
Analyze the truss represented in Fig. 26 for wind pressure, the distance between trusses being 15 feet. (See Ex. 3, Page 27, for apex loads.) Assuming both ends of the truss fastened to the supports, the reactions are both parallel to the wind pressure and the reaction on the windward side equals 6,692.5 pounds and the other equals 3,037.5 pounds.