Per the prescriptive tables found in Chapter 8 of the International Residential Code (IRC), the standard roof rafter spacing used is 12, 16, 19.2, or 24 inches on center. Design member DJ, AB using the same kind of wood. The spacing of roof rafters is just one of the components used to determine the minimum size of the roof rafter. Typical rafter spacing ranges between 12 inches to 24 inches on center. Determine the section that will meet the deflection and strength criteria. Rafter spacing how much space separates each rafter course on the center. In EXAMPLE 2, the rafter failed in deflection. Span Options Calculator for Wood Joists and Rafters. ridge board rafters top plate L = 770 Pa W=1055 Pa 9 D = 280 Pa 4.40m 39.75 3.66m Solution: Evaluate Load Combination Applicable Load Combinations D+L D + 0.6W Load Duration Factor 1.0 1.60ĭetermine Relevant Properties of 80% Stress Grade Apitong Fb = 16.5 MPa Fv = 1.73 MPa E = 7310 MPa Determine the adjustments to the property values Cp = 1.60 (for wind load combinations) Co = 1.00 (for live load combinations) CL = 1.00 (compression edge is laterally supported by the purlins) Fo' = Fox Cox Cu = 16.5 x 1.60 x 1.0 = 26.4 MPa (for wind load combinations) Fo' = Fox Cox Cu = 16.5 x 1.0 x 1.0 = 16.50 MPa (for live load combinations) Fv' = Fy x Cp = 1.73 x 1.60 = 2.77 MPa (for wind load combinations) Fy' = FvxCp = 1.73 x 1.0 = 1.73 MPa (for live load combinations) E' = E = 7310 MPa (for both combinations) Resolve Vertical Loads into Perpendicular Loads to the Rafters. EXAMPLE 2: DESIGN ANALYSIS OF RAFTERS Rafter spacing = 2.0 mon center Rafter horizontal span is 3.66 m (actual sloped span is 4.40 m) Inclination of rafter, e = 39.75" Allowable Deflection due to Gravity Loads = L/180 = 4400 mm/180 = 24.44 mm Design Loads: Dead load = 280 Pa (Weight of roofing + purlins) Live load = 770 Pa Wind load = 1055 Pa Determine if a 37.5mm x 181.25mm dressed size of 80% Stress Grade Apitong is able to resist the given loads.
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