WIND-LOAD EFFECTS 50. Top Lateral System Through=Bridges. The unit-loads for this system are given in Article 26. Common practice is to take 150 pounds per linear foot of top chord, the end-post being con sidered part of the top chord in this computation.
In many of the longer-span modern bridges, the diagonals of this system are designed to take either tension or compression; but in the majority of the shorter spans, 200 feet and under, while generally consisting of angles or other stiff shapes, they are designed to take tension only. The verticals or top lateral struts take compression. This combination of tension diagonals and compression verticals makes the so-ealled Pratt system of webbing; and indeed the lateral systems, both top and bottom, are Pratt trusses in a horizontal posi tion. Fig. 95 shows the side elevation of the truss of Article 47, and also the top and bottom laterals. The diagonals shown in full lines act when the wind is right, and those shown by dotted lines act when the wind is left. Wind right indicates that the wind is blow ing from the right hand when a person stands facing the righ` end of the bridge. Wind left indicates that the wind blows from a person's left when standing as above described.
The wind load of 150 pounds is di vided between the two trusses, this being exact enough for practical pur poses; for, by actual experiment, the difference between the readings of wind-pressure gauges placed at points opposite each other in the top chords of a through-bridge was only from 8 to 10 per cent.
The problem, then, is one of a deck Pratt truss with a dead panel load of 150 X 20 = 3.0 divided between the two chords. Fig. 96 shows the distri bution of loads and the reaction, it being considered that the portal brac ings and the end-posts (see Fig. 95) are stiff enough to distribute the reaction equally between the bearing points Each panel load is indicated by an arrow, and is equal to 3.0 _ 2 = 1.5. The re
action at each of the points and L is 10 X 1.5 _ 4 = 3.75. The truss being symmetrical, the stresses in like members on each side of the center will be the same. The shears in the top system are: 1', = +2 X 3.75 - 2 X1.5= +4.5 +2X3.75-3 X 1.5 = +3.0 V, _ +2 X3.75-4X1.5=+1.5 and the secant is + - 17 = 1.544. The stresses in the diag onals are: U,'U, _ +1.544 X 4.5 = +6.95 + 1.544 X 1.5 = + 2.32.
The vertical U; U, = 3.0; and by passing a section b b around U,', the stress in is found to be 1.5.
In obtaining the chord stresses in this system, the case is the same as if the reactions were applied at U; and as the portal and end-posts are not in the same plane as the lateral system. The tan gent method is the simplest to use in this The tangent is 20 ± 17 = 1.176, and the stresses (see Fig. 95) are: _ 4.5 X 1.176 = 5.29 U; U; = (4.5 + 1.5) X 1.176 = 7.06 = 0 = U,'U; = +5.29 Fig. 97 is a diagram with the stresses caused by wind right and wind left indicated thereon. The stresses for wind left can easily be written by inspection.
51. Bottom Lateral Bracing, Through-Bridges. Fig. 95 shows the lower lateral system with the panel points loaded with the fixed or dead wind load. In this case it is all taken as acting on one side, it being assumed that the floor system protects the leeward truss. The problem then becomes that of determining the stresses in a deck Pratt truss of 6 panels of 20 feet each, the height being 17 feet. When wind is right, the members shown by broken lines in Fig. 95 do not act.
The fixed wind load (Article 26) is 150 pounds per linear foot of chord. The panel load will be the same as before, 3.0, but all will be on one chord. The shears are: The secant being 1.544, as previously computed, the web stresses are: Lo L, = +7.5 X 1.544 = + 11.60 L,'L, = -7.5