Once20036
Structural
- Oct 7, 2008
- 533
I`m doing some work in an existing building constructed circa 1995. I've measured the existing joists, retreived a joist tag, and know that the joist is a 26K7. From there, it's easy to look up the loads.
Because I`m adding new point loads to the joist (New RTU) I wanted to model the members in RAM Elements, to better see how the new point load distributes to existing members.
Under the SJI design loads, the top chord is showing 130% of allowable stress. Clearly this doesn't agree with the SJI tables.
The discrepency is fundamentally because the top chord compression elements are slender. Because of the slenderness, compression is checked via AISC 7-05 section E7.
Section E7 states that for singly symmetric members, you need to check Fe in accordance with equations E3-4 and E4-5 and for single angle members you only need to check it per equation E3-4. When you look into the slenderness reductions factor, Qs, there are various categories including built up members and single angles.
Reviewing the RAM calculations, they calculate Qs as though it was a single angle, then check equation E4-5 as though it was a singly symmetric member (and not a single angle).
So the question is: if we ignore the fact that this is a joist chord (and governed by the SJI specs) is the compression capacity of a double angle calculated as the sum of the capacities for two single angles? Or is the compressive capacity of a double angle calculated for one singly symmetric member?
Because I`m adding new point loads to the joist (New RTU) I wanted to model the members in RAM Elements, to better see how the new point load distributes to existing members.
Under the SJI design loads, the top chord is showing 130% of allowable stress. Clearly this doesn't agree with the SJI tables.
The discrepency is fundamentally because the top chord compression elements are slender. Because of the slenderness, compression is checked via AISC 7-05 section E7.
Section E7 states that for singly symmetric members, you need to check Fe in accordance with equations E3-4 and E4-5 and for single angle members you only need to check it per equation E3-4. When you look into the slenderness reductions factor, Qs, there are various categories including built up members and single angles.
Reviewing the RAM calculations, they calculate Qs as though it was a single angle, then check equation E4-5 as though it was a singly symmetric member (and not a single angle).
So the question is: if we ignore the fact that this is a joist chord (and governed by the SJI specs) is the compression capacity of a double angle calculated as the sum of the capacities for two single angles? Or is the compressive capacity of a double angle calculated for one singly symmetric member?
