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RE: AISC Steel Spec Combine forces question

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>Sam Chang wrote:
>I am reading the Charles Page's article in

>MSC Nov. 2005.  His got axial compression

>capacity of 892 kips. Looking up the same column

>for LRFD I found the compression capacity is 844 kips.

>This is a +6% of capacity. I did not know that the

>capacities changes was happening to this code

>change.  Is that the case?

 

Yes, the resistance factor (phi) and safety factor (omega) were changed for columns in the 2005 AISC Specification.

 

In previous LRFD Specifications, phi was equal to 0.85. In previous ASD specifications, the safety factor was approximately 1.76. These values were set based upon a variety of products, including columns that might be fabricated from universal mill plates. In fact, UM plates were the controlling material and dragged the phi down (the safety factor up) all by themselves

 

For the 2005 AISC Specification, we recognized that UM plates are no longer available and eliminated them from the determination of the resistance factor and factor of safety. As a result, phi is 0.9 and omega is 1.67 in the 2005 AISC Specification.

 

Hence the difference in strength you noted.


Charlie