2001 ASD Supplements - unprotected PDF - American Wood Council

GL-78DESIGN EXAMPLES8.1 GeneralGeneral design examples for tension members, compressionmembers, and bending members are available invarious wood engineering handbooks, such as theMcGraw-Hill APA Engineered Wood Handbook, WoodEngineering and Construction Handbook, and AITC TimberConstruction Manual. See Section 2.4 for theassociations that are available for technical assistance.8.2 ExamplesDesign Example 1: Low Slope RoofDesign Using Section CapacitiesGiven:• 24-ft. x 24-ft. column grid layout• Live load = 30 psf (snow); C D = 1.15• Dead load = 10 psf (actual)• Allowable total load deflection = L / 180• Allowable live load deflection = L / 240• Use 24F-1.8E/DF• The compression edge of the beam is supported bysheathing throughout its lengthThen:• Glulam span = 24 ft.• Load w = (30 + 10)(24) = 960 plf• Moment max = w L 2 / 8 = (960)(24) 2 / 8 = 69,120 lbf-ft.• Shear max = wL / 2 = (960)(24) / 2 = 11,520 lbfNote: For preliminary design, all loads within a distancefrom supports equal to the beam depth are not neglected.Design:• From Table 5.1, try 5-1/8 x 21• Beam wt = (35 pcf)(5.125/12 ft.)(21/12 ft.) = 26 plfNote: Beam weight has been assumed to be 35 pcf, whichis typical for glulam beams made of Douglas fir-Larch.See Section 6.2 for more information.• Total load = 960 + 26 = 986 plf• From Table 4.5, volume factor = 0.9330• From Table 5.1, Design Moment Capacity = (75,338)(0.933)(1.15) = 80,834 lbf-ft.Note: Beam stability factor is equal to 1 as the compressionedge of the beam is supported by sheathing throughoutits length.• Moment max = (69,120)(986) / (960) = 70,992 lbf-ft.