Introduction
When performing a joint can strength analysis, the brace strength capacity does not appear to match the reported brace allowable stress for member unity checks. This is due to changes in safety factors which will be illustrated in this post.
AISC Code Commentary on Allowable Stress Safety Factor
According to AISC, the allowable axial stress for a member in compression which satisfies certain cross-sectional and geometric requirements can be expressed as:
The denominator of this equation is a safety factor according to section E2 commentary of the AISC code.
AISC E2 Commentary
The portion of the allowable stress calculation will be omitted when calculating the brace strength capacity for the joint strength check.
API Code Joint Strength Requirement
Section 4.2.3 of API RP 2A requires that the joint strength should not be less than half of the effective strength of the brace member. The effective strength is the strength of the brace using without safety factors.
API Joint Strength
Allowable Stress Comparison
As an example, see the comparison of the allowable stress and the reported brace strength capacity from the Sample 02 analysis provided with your SACS installation.
Brace Allowable Stress
Brace Strength Capacity
The reported allowable stress of 19.93 does not match the brace strength capacity of 14.00. The difference is due to the removal of the safety factor.
Let's look at the equations used to calculate the allowable stress and the brace strength capacity. I am going to be using the stiffness properties for the brace reported by SACS below:
Brace Stiffness
First we need to calculate :
The allowable stress is calculated using the above equation with the safety factor and an additional allowable stress modifier (AMOD) of 1.333 which has been entered by the user:
The brace strength capacity is half of the effective brace strength (i.e. the allowable brace stress with all safety factors, including AMOD, set to one):
Conclusion
As was shown, the allowable stress reported in the member report for braces does not necessarily match the brace strength capacity due to the changes in safety factor.