Composite Construction

[craigory28]

Has anyone heard that LRFD 3rd version reduced the capacity of studs by 25% due to an offset required because of ribs in the metal deck? Someone in our office was talking about this today and I'm wondering where I can read more about this. How does this effect our ASD beams?

Thanks,
Craig

 

[Taro]

Section I3.5b of the 1999 LRFD spec states "Where there is only a single stud placed in a rib oriented perpendicular to the steel beam, the reduction factor of Equation I3-1 shall not exceed 0.75".

The commentary says that recent test data indicates that the equations that have been used for stud strengths may be unconservative when a single stud per rib is used in the configuration noted above. Several potential reasons are given. One of the main reasons is that modern deck profiles include a reinforcing rib at the middle of the bottom flute. This rib forces the stud to be installed to one side of the flute and results in a reduced edge distance. The 0.75 factor is supposed to be a temporary measure until further testing is completed.

Although the 1999 LRFD spec does not specifically apply to ASD design, the same physical phenomenon would exist no matter what design methodology was used. I think it would be prudent to incorporate reduced stud capacity in ASD designs as well.

The 1999 LRFD spec can be downloaded freely from

http://www.aisc.org.

Many other items, including an ASD supplement, are also available.

 

[craigory28]

ASD has also included this in its latest supplement.

Thank you.