Homework Number 4 - Timber Design

Design a three-car garage suitable for Juneau, Alaska. It can have

(a) 3 single garage doors, or

(b) 1 single and 1 double garage doors, or

(c) 1 triple garage doors.

Structurally, choice (a) would be the simplest. Standard garage door widths are 8', 9', 10', 12', 14', 15', 15.5', 16', 18'.

See IBC Section 16.08 for snow loading factors (page 15). See IBC Chapter 23 for wood construction requirements.

Information for Douglas Fir (common in Pacific Northwest):

• Elastoc Modulus: 1,760,000 psi
• Allowable stress, tension or compression: 1900 psi
• Allowable stress, shear: 120 psi
• Ultimate Strength, compression: 7400 psi
• Ultimate Strength, shear: 1500 psi
• Unit weight: 0.018 pci

Requirements: Maximum deflection at any point in the roof structure must be less than L/240 for the total load, in which L is the span (half the width of a symmetrical roof truss).

Recommendations: Use mostly 2X4 for vertical structural members. Use mostly 2X6 for rafter elements. Stud spacing is 16" and rafter spacing can be 24" to 48" (32" is common).

Size Chart

Framing Lumber

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Nominal Size	Actual Size
2 x 2	1 1/2 x 1 1/2
2 x 3	1 1/2 x 2 1/2
2 x 4	1 1/2 x 3 1/2
2 x 6	1 1/2 x 5 1/2
2 x 8	1 1/2 x 7 1/4
2 x 10	1 1/2 x 9 1/4
2 x 12	1 1/2 x 11 1/4
4 x 4	3 1/2 x 3 1/2
4 x 6	3 1/2 x 5 1/2
4 x 10	3 1/2 x 9 1/4
6 x 6	5 1/2 x 5 1/2




Boards

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Nominal Size	Actual Size
1 x 2	3/4 x 1 1/2
1 x 3	3/4 x 2 1/2
1 x 4	3/4 x 3 1/2
1 x 5	3/4 x 4 1/2
1 x 6	3/4 x 5 1/2
1 x 8	3/4 x 7 1/4
1 x 10	3/4 x 9 1/4
1 x 12	3/4 x 11 1/4