Example 7

What joint loads should be used for the truss shown by Fig. 252, assuming that the sloping roof is to be covered with slate and the deck roof with tin, and that the building is located in one of the central states?

Ans. - The weight per sq. ft. of the sloping roof will be

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

7 1/4

lbs.

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

3

"

For 2 x 6 rafters.....................

2 1/4

"

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

2

"

For truss, 64' span.....................

4

"

Wind and snow........................

30

"

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

48 1/2

lbs.

For the deck roof:

Tin....................

1

lb.

Sheathing rafters and purlins...................

6 1/2

lbs.

Truss........................

5 1/4

"

Snow....................

30

"

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

42 1/2

lbs.

Multiplying the roof areas supported at the joints, as found in Example 2 by these loads, we have

Load at joint 2. .

=

118

sq. ft.

x

48 1/2

lbs.

=

5,723

lbs.

Load at joint 3. .

=

59

sq. ft.

x

48 1/2

lbs

=

2.862

=5,667 lbs.

66

sq. ft.

x

42 1/2

lbs.

=

2,805

Load at joint 5. .

=

111

sq. ft.

x

42 1/2

lbs.

=

4,718

lbs.

Load at joint 7. .

=

114

sq. ft.

X

42 1/2

lbs.

=

4,845

lbs.

Example 8

What load per sq. ft. should be taken in computing the stresses in a truss of 64-ft. span supporting a flat roof in the New England states, the rafters to be 2" x 8", 16" on centres, resting directly on the trusses, and the roofing to be 5-ply tar and gravel?

Ans. - The weight per square foot should be estimated as follows:

Roofing.......................

6

lbs.

Sheathing....................

3

"

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

3

"

Truss.................

5

"

Snow.........................

40

"

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

57

lbs.