The roof areas multiplied by the roof load per square foot will give the joint loads.

Example 5

What roof loads should we use in determining the stresses for the trusses Figs. 249 and 250?

Ans. - The dead weight per square foot would be about as follows, assuming that the roof is to be covered with slate, 3-16-in. thick:

For slate......................

7 1/4

lbs.

For sheathing.................

3

"

For rafters....................

3

"

For purlins.................

2

"

For truss (Table VII.)

3 1/4

"

Total dead weight...............

18 1/2

lbs.

For the wind and snow combined, we should allow by Table X. 27 1/2 lbs., the pitch of the roof being a mean between 450 and a 1/3 pitch. Therefore the roof load per square foot should be taken at 46 lbs. The roof areas supported at the joints were found in Example 1, to be as follows:

For truss 1, area at joint 2, = 93 1/2 sq. ft.; and at joint 3, 100 sq. ft. For truss 2, area at joint 2, = 118 sq. ft., and at joint 3, 128 1/2 sq. ft. For truss 3, area at joint 2, = 124 2/3 sq. ft., and at joint 3, 135 2/3 sq. ft. Multiplying these areas by 46, we obtain the following loads:

For truss

1,

at joint

2, 4,301

lbs.;

at joint

3, 4,600

lbs.

For truss

2,

at joint

2, 5428

lbs.;

at joint

3, 5,911

"

For truss

3,

at joint

2, 5,734.

lbs.:

at joint

3. 6.240

"

Example 6

Estimate the joint loads for the truss shown by Fig. 253, the roof being shingled on 7/8-in. sheathing nailed to 2x4 rafters.

Ans. - The dead weight will be made up as follows:

Shingles..........................

2 1/2

lbs.

Sheating.....................

3

"

Rafters.................

1 1/2

"

Purlins...............................

2

"

Truss (56-ft. span, 1/3 pitchTruss (50-ft. span, % pitch)...........................

4

"

Total...........................................

13

lbs.

Add for wind and snow, Table X.................

25

"

Total load per square foot...............

38

lbs.

In Example 3 the roof areas were found to be as below; multiplying: these by 38 lbs. we obtain the respective weights:

Roof areas in sq. ft.

Joint

2, 96;

Joint

3, 57.6;

Joint

4, 115.2;

Joint 5, 190.4

Loads in lbs.......

Joint

2, 3,648;

Joint

3, 2,189;

Joint

4, 4,377:

Joint 5, 7,235