For determining the stresses due to wind pressure alone the force of the wind is usually assumed to act in a direction normal, i. e., at right angles to the slope of the roof. This force is commonly based on a horizontal wind pressure of 30 lbs. per square foot, although quite often it is taken at 40 lbs. per square foot, depending somewhat upon the exposure and the shape or construction of the roof and truss.

Table XL - Normal And Horizontal Wind Pressure On Roofs For 30 Pounds Horizontal Pressure Against A-Vertical Surface

Inclination.

Norm.

Hor.

lbs.

lbs.

5o..................

3.9

0.3

10o ...................

7.2

1.2

15o.....................

10.2

18° - 26'(1/6 pitch)

13.0

4.0

20°...............

13.7

4.5

21° - 48' (1/5 pitch)

15.0

6.0

25°..................

16.9

26° - 34' (1/4 pitch).........

18.0

8.0

Inclination.

Norm.

Hor.

lbs.

lbs.

30°....................

19.9

10.0

33° - 41'(1/3 pitch)

22.0

12.0

35°.................

22.6

40°..................

25.1

15.9

45°(1/2 pitch)......

27.1

19.0

50°...................

28.6

21.9

55°..............

29.7

60°....................

30.0

25.5

The normal and horizontal pressure per square foot of roof surface corresponding to a horizontal pressure of 30 lbs. against a vertical surface is given in Table XI.

For a horizontal wind pressure of 40 lbs. per square foot the pressure given in the table should be increased one-third.

III. ROOF AND CEILING AREAS SUPPORTED BY THE TRUSS JOINTS. - Calculations for the stresses in a truss are always based on the assumption that the loads are transferred to the joints, and that the various members of the truss are free to move at the joints, as though connected by a pin. Before anything can be done towards determining the stresses, therefore, it is necessary to compute the load which is supported, theoretically at least, at each joint. This load is obtained by multiplying the roof or ceiling area contributory to that joint by the weight per square foot previously determined on. If the rafters are supported by purlins and these are supported at the joints of the truss, one half of the load on each purlin will be transmitted to the joint. If the purlins are supported at some distance from the joints, then part of the load from the purlins will come at one joint and part at another, as will be explained later. When there are no purlins, and the rafters bear directly on the top chord of the truss as in Figs. 41 and 113, then the different sections of the chord act as beams, and one half of the load on each section will be transferred to the joint between. As a rule, the joint loads will be the same in either case, unless the purlins are located away from the joints.

The following examples will show how to determine the roof or ceiling areas contributory to a joint: