194 APPLIED MECHAN ICS 
be drawn, since the truss is symmetrical, ai symmetrically loaded. The 
complete stress diagram for the whole truss is, however, shown in Fig. — 
263. The results are tabulated under the heading “dead load” in the — 
table on p. 196. ; 
179. Wind Pressure.—The force exerted by the wind on a plane — 
surface at right angles to the direction of the wind may amount to about 
50 lbs. per square foot of surface. When the surface is inclined at an 
angle @ to the direction of the wind, the normal pressure on the surface 
is usually determined by Hutton’s formula, which is | 
5 (sin 0)1'84 cos @-1 
or log? pril 84 cos 0-1) log sin 8, 
where p is the normal pressure, and P is the pressure On & plane at right 
angles to the direction of the wind. Values of »+P for various values 
of @ are given in the following table :— 
abe 10° 15° 20° 25° 30° 35° 
plP 0°241 0°350 | 0°457 0563 | 0°663 | O°754 
0 40° 45° |. 50° 60° 70° 80° 
p[P 0°834 0°901 0952 1°012 1023 1°010 
When 0=90°, p=P. 
180. Stress Diagrams for Wind Pressure.—It is usual to assume 
' that. the direction of the wind is horizontal, and that its maximum — 
pressure on a plane at right angles to its direction is 50 lbs. per 
square foot. The inclination of a roof being known, the normal pres-— 
sure of the wind on it may be determined by the formula given in 
the preceding Article. It is assumed that the wind acts on one side 
of the structure only at one time. The total load due to the wind 
pressure is divided up into parts, which are placed at the joints, as 
explained in Art. 177. 
Figs. 264 and 265 show the stress diagrams for the wind pressure on 
the roof truss whose dimensions are given in Fig. 263. The truss is 
assumed to be fixed rigidly at the right-hand end and simply supported 
at the other end, so that it may expand and contract freely with changes 
of temperature. The reaction at the left-hand end must therefore be 
vertical, and the line of the reaction at the right-hand end must therefore 
pass through the point where the resultant of the wind pressure cuts the 
line of the reaction at the left-hand end. 
The directions of the reactions having been fixed, the load polygon 
abcde can be drawn, and upon this the stress diagram is built, as in Fig. 
263. which shows the stress diagram for the same truss under the dead 
load. 
The mitens in the various bars of the truss due to the dead load and 
