Hi All,
In Table 6-9 ‘Allowable Stresses for Casting Alloys’ (Aluminum) in AASHTO's Standard Spec for Structural Supports for Hwy Signs, Luminaires..., Section 6, the allowable stresses for A444 and A356 are given for various tempers. The allowable tensile/compressive stress for bending for A356.0-T61 (permanent mold) is just 10.1 ksi.
Foundries I've seen and one I've talked to list A356's Fy = 30 ksi. Using AASHTO formulas for other non-listed alloys I've calc'd Fb = 1.17 Fy/1.65 and with wind loading I can multiply that by 1.33. So for A356.0-T61, using the formula above: Fb=1.17(30ksi)/1.65 = 21.2 ksi. About twice the value in Table 6-9.
I must be missing something. I've used the calculations above for a casting alloy not listed in Table 6-9 (aluminum 771.0-T71) and no one had a problem with it (DOT, peer review, etc.). But I need to understand this.
Thanks for any suggestions.
m
In Table 6-9 ‘Allowable Stresses for Casting Alloys’ (Aluminum) in AASHTO's Standard Spec for Structural Supports for Hwy Signs, Luminaires..., Section 6, the allowable stresses for A444 and A356 are given for various tempers. The allowable tensile/compressive stress for bending for A356.0-T61 (permanent mold) is just 10.1 ksi.
Foundries I've seen and one I've talked to list A356's Fy = 30 ksi. Using AASHTO formulas for other non-listed alloys I've calc'd Fb = 1.17 Fy/1.65 and with wind loading I can multiply that by 1.33. So for A356.0-T61, using the formula above: Fb=1.17(30ksi)/1.65 = 21.2 ksi. About twice the value in Table 6-9.
I must be missing something. I've used the calculations above for a casting alloy not listed in Table 6-9 (aluminum 771.0-T71) and no one had a problem with it (DOT, peer review, etc.). But I need to understand this.
Thanks for any suggestions.
m
