Stainless Steel/Carbon Steel Bolt Tension Capacity Inconsistency

[tslewis]

Does any ome know why the calculations for the LRFD capaacity of a stainless steel bolt (ASCE 8-02) and carbon steel bolt (AISC 2005 Steel Building Specifications) is identical. But the calculations for the ASD capacity is significantly different from the ASCE and AISC Codes. The ASD factor fo safety os 3 in s.s. ASCE codes and equal to 2 in the AISC codes.

LRFD

Carbon (AISC 2005 Specificaitons - Chpater J3.6)
Ab - nominal area of bolt
Fn - Nomonial tensile strength = 0.75*Fu*Ab (Table J3.2 and equation J3-1)
phi - LRFD Reduction factor = 0.75
LRFD Strength - phi*Fn = 0.75*0.75*Fu*Ab

Stainless Steel (ASCE 8-02 Specificaitons - Chpater 5.3.4)
Ab - nominal area of bolt
Fn - Nomonial tensile strength = 0.75*Fu*Ab (Equation 5.3.4-1 and Commentary to Section 5.3.4)
phi - LRFD Reduction factor = 0.75 (Table 6)
LRFD Strength - phi*Fn = 0.75*0.75*Fu*Ab

IDENTICAL TO CARBON STEEL

ASD

Ab - nominal area of bolt
Fn - Nomonial tensile strength = 0.75*Fu*Ab (Table J3.2 and equation J3-1)
FS - ASD Factor of Safety = 2
ASD Strength - Fn/FS = 0.75*Fu*Ab/2

Stainless Steel (ASCE 8-02 Specificaitons - Chpater 5.3.4)
Ab - nominal area of bolt
Fn - Nomonial tensile strength = 0.75*Fu*Ab (Equation 5.3.4-1 and Commentary to Section 5.3.4
FS - ASD Factor of Safety = 3 (Table D)
ASD Strength - Fn/FS = 0.75*Fu*Ab/3

NOT IDENTICAL TO CARBON STEEL

I dont understand why the smaller FS for the stainless steel ASD design if the LRFD reduciton factor and nominal strength of carbon and stainless steel is identical.

 

[JStephen]

Just a guess- although both sets of data are based on ultimate strength, perhaps the ASD is trying to avoid yielding and the LRFD is not. SS typically has lower yield/tensile than carbon steel.

 

[tslewis]

Not sure. The equaitons for ASD and LRFD are both based on teh ultimate strength of the material. There are additional ASD and LRFD equations for Yielding in teh gross area of a tension memnber.

 

[UcfSE]

You should look in the references section of both the AISC and the ASCE publication to which you refer. Try to find and read the research upon which those equations and safety factors were derived. That may explain help to explain why the safety factors are different.