Alexander Rosenthal Design Project 2 Arch 562: Structural Design, Winter 2011 Stephen Duff, Mark Donofrio, Nate Poel
Design and Analysis of a Timber Truss for a 30’ x 50’ Meeting Hall
model photograph
Introduction: The 30’ x 50’ meeting hall is crowned by six heavy timber trusses, supported by two 50’ masonry bearing walls that run longitudinally on each side of the hall. The walls are 15’ tall, and are penetrated by five square openings, centered in each bay of the structure. The shape of the trusses implies a volume of space that is loosely subdivided into a large middle zone and two smaller zones at the long edges of the hall. The top chord of the truss is raised 4’ above the masonry bearing wall, creating space for clerestory windows that bring generous amounts of natural light into the meeting hall. The trusses are joined by 4”x6” #1 Douglas Fir purlins, spaced 30” on center and covered by 2” nominal Douglas Fir tongue and groove boards. Above is 5/8” structural plywood sheathing (providing lateral bracing in the roof plane), rigid insulation, a second layer of 5/8” sheathing, 15# felt and cedar shingles. The members of the truss are joined by a 1/2” steel knife plate, centered in each member, and secured with bolts.
early concept drawings
12 8
20
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’- 4
diagram of truss members, and their naming in this report
top chord king post scissor high fan 4 high scissor low fan 4 low fan 3 fan 2 fan 1
Load calculations: Snow Load: S=pf=.7CeCtIPgCs where: terrain = C Ce = 1.0 (partially exposed) Ct = 1.0 I = 1.0 (occupancy category II) Pg = 35#/ft2 Cs = .93 (8:12 roof pitch, “all other surfaces”) S=pf=.7(1.0)(1.0)(1.0)(35#/ft2)(.93) snow load = 22.785#/ft2 at 8:12 pitch, snow load = 18.96#/ft2 with a 10' bay width, snow load as a uniform distributed load acting on truss = 189.6 pounds/ft
Dead Load: Roofing and sheathing (from IBC 1606): 2” Douglas Fir T&G Boards: 4 psf 5/8” Plywood Sheathing: 2 psf 4” Rigid Insulation: 6 psf 5/8” Plywood Sheathing: 2 psf Felt Underlayment: 2 psf Cedar Shingles: 3 psf total dead load from roofing, insulation and sheathing = 19 psf with a 10' bay width, dead load as a uniform distributed load acting on truss = 190 pounds/ft
Self Weight: Douglas fir trusses and purlins, 32 pounds per cubic foot (Peattie, Donald C. A Natural History of Western Trees. Boston: Houghton Mifflin Co, 1950, 1981.)
Truss self weight: 1236.68 pounds Self weight applied as external load: 30.43 pounds/ft Purlins: Using 17 4”x6” purlins, 30” o.c., the total load of the purlins on the truss = 727.2 pounds, or 17.9 pounds/ft
Total Load: Total load (snow, dead, self) acting on truss = 427.93 pounds/ft
Loading
diagram of applied loads
diagram of analysis model with member sections indicated
Truss Force Diagrams Moment:
Shear:
Axial:
Deflection: maximum deflection: .405�
Top Chord Forces
Maximum Moment: 6.038 kip-ft
Maximum Shear: 2.689 kips
Maximum Axial Force (compresson): 19.079 kips
Scissor Low Forces
Maximum Moment: .528 kip-ft
Maximum Shear: .126 kips
Maximum Axial Force (tension): 13.899 kips
Fan 1 Forces
Maximum Axial Force (tension): .444 kips
Fan 2 Forces
Maximum Axial Force (compression): 4.680 kips
Fan 3 Forces
Maximum Axial Force (compression): 5.916 kips
Fan 4 Low Forces
Maximum Axial Force (tension): 3.345 kips
Fan 4 High Forces Maximum Axial Force (tension): 3.028 kips
Scissor High Forces Maximum Axial Force (compression): 4.465 kips
Kingpost Forces Maximum Axial Force (tension): 9.334 kips
Connections Truss members are joined using bolts through ½” thick steel knife plates, centered in the cross-section, expressing an offset between members at joints.
Connection 3:
Use (4) 5/8” Bolts, 2 1/2” o.c., 4” from the end of member
Connection 2:
Use (3) 1/2” Bolts, 2” o.c., 4” from the end of member
Connection 1: 1/2” steel knife plate centered in all members
Use (4) 3/4” Bolts, 3” o.c., 4” from the end of member
Connection 6:
Connection 5:
Use (3) 1/2” Bolts, 3” o.c.
Use (3) 1/2” Bolts, 2” o.c., 4” from the end of member
Connection 4:
Use (3) 1/2” Bolts, 2” o.c., 4” from the end of member
1/2” steel knife plate centered in all members