Problem 2.4

 

 

Dead Loads

 

entire roof

 

Component	D (psf)	Reference
roofing, 5-ply felt  (without gravel)	2.5	Appendix B.2
sheathing, 15/32" plywood	3.0/in.	Appendix B.1
subpurlin, 2x4 @ 24" O.C.	0.6	Appendix A.1
purlin, 4x14 @ 8' O.C.	A	NDS Supplement sec. 3.1,  NDS table 11.3.2A,  NDS Supplement table 1A, NDS Supplement table 1C
girder, 6-3/4x33 DF-L glulam @ 20' O.C.	A

 

^A Assume Douglas Fir-Larch with 12% moisture content for both purlin and girder.

specific gravity G = 0.50

density = 62.4 G (1 + mc/100) / (1 + G * 0.009 * mc) = 62.4 * 0.50 (1 + 12/100) / (1 + 0.50 * 0.009 * 12) = 33.15 lb/ft³

distributed self weight = density * cross sectional area / width of tributary area

distributed self weight of purlin = 33.15 lb/ft³ * (3.5 in. * 13.25 in. * (ft / 12 in.)²) / 8 ft = 1.33 psf

distributed self weight of girder = 33.15 lb/ft³ * (6.75 in. * 33 in. * (ft / 12 in.)²) / 20 ft = 2.56 psf

 

D = 2.5 + 15/32 * 3.0 + 0.6 + 1.33 + 2.56 = 8.40 psf  (without re-roofing allowance)

 

tributary to subpurlin:  w_D = (2.5 + 15/32 * 3.0 + 0.6) * 2 = 9.01 lb/ft

tributary to purlin:  w_D = (2.5 + 15/32 * 3.0 + 0.6 + 1.33) * 8 = 46.69 lb/ft

tributary to girder:  w_D = (2.5 + 15/32 * 3.0 + 0.6 + 1.33 + 2.56) * 20 = 167.9 lb/ft

tributary to column:  P_D = (2.5 + 15/32 * 3.0 + 0.6 + 1.33 + 2.56) * 20 * 50 = 8396 lb

 

Live Loads

 

roof

 

Term	Value	Reference
L0	20 psf (at 0:12)	ASCE 7 table 4-1
R1	1 (assume AT ≤ 200 ft2)	ASCE 7 section 4.9.1
R2	1 (F ≤ 4)	ASCE 7 section 4.9.1

 

L_r = 1 * 1 * 20 psf = 20 psf

 

rain

 

Term	Value	Reference
ds	5 in.	ASCE 7 section 8.3 or IBC section 1611.1
dh	0.5 in.

 

R = 5.2 (5 + 0.5) = 28.6 psf