PEMB Portal Frame Braces
PEMB Portal Frame Braces
(OP)
As part of a feasibility study I was asked to review some owner-directed modifications to some kicker braces in a PEMB. (see image for bracing in question)
I was not able to confirm that the braces as installed meet the stiffness requirements for nodal bracing of a beam AISC-360-10 Appendix 6.3.1b eq A-6-8. In my analysis, the zee girt acted as a fuse to limit the beta of the bracing.
Can anyone provide insight into how the PEMB industry addresses the stiffness issue?
Am I overthinking or incorrectly applying the stability requirements to this installation?
Any assistance is appreciated.
I was not able to confirm that the braces as installed meet the stiffness requirements for nodal bracing of a beam AISC-360-10 Appendix 6.3.1b eq A-6-8. In my analysis, the zee girt acted as a fuse to limit the beta of the bracing.
Can anyone provide insight into how the PEMB industry addresses the stiffness issue?
Am I overthinking or incorrectly applying the stability requirements to this installation?
Any assistance is appreciated.






RE: PEMB Portal Frame Braces
If you asked us before you took this task on, I would of told you to run away, far and fast. The more you look the more you'll find that makes you uncomfortable.
Your best chance is to call the original PEMB designer or the company, be your charming best and ask what the intent and design method for those braces. I can tell you it's unlikely you'll be able to track this down, but it's worth a shot.
RE: PEMB Portal Frame Braces
Dave
Thaidavid
RE: PEMB Portal Frame Braces
2) Depending on the age of the building, the designer may well have been using standard rules of thumb rather than the AISC appendix stuff. 2% flange force etc.
3) I've no doubt that the bracing is fine in its current state. It's the once without the fly bracing that you really have to worry about.
4) I don't feel that this is straight up nodal bracing of the A-6-8 variety. Rather, I believe it to be nodal torsional bracing of the A-6-10 variety. The kicker matters but is, in essence, merely part of the torsion/moment connection between the frame and the Z-girts.
3) Are there fly braces on both sides of the frame such that you could mobilize the stiffness of the Z-girts on both sides of the frames? Even with one sided fly bracing, you might be able to mobilize both sides if the Z-girts are continuous over the frames.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: PEMB Portal Frame Braces
RE: PEMB Portal Frame Braces
I did use the good-ole 2% rule and backed into an upper-bound stress based on the lower-bound strength of the brace. For tension only brace the stress was 10ksi. Double that for both. So this is mostly likely what they did.
I did in-fact run away from this. I told the owner I would not recommend any changes to the bracing system.
RE: PEMB Portal Frame Braces
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: PEMB Portal Frame Braces
RE: PEMB Portal Frame Braces
I looked at using an L shaped assembly to replace the kicker. I was not happy with the stiffness of the L bracket and have abandoned the concept. However, I'm still not sure the existing condition has the stiffness required to meet the code requirement.
RE: PEMB Portal Frame Braces
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: PEMB Portal Frame Braces
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: PEMB Portal Frame Braces
I've told him in my opinion its not feasible. As part of his renovation, he's also changing the R value of the roof. I'm uncomfortable with the risk exposure of being involved with this project. I have politely excused myself from the project.
I'm still curious as to how the PEMB bracing works. It obviously does, it just does not intuitively meet the AISC bracing requirements. Something else is happening that my analysis is missing.
KookK: Thanks for sharing your ideas.
RE: PEMB Portal Frame Braces
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: PEMB Portal Frame Braces
RE: PEMB Portal Frame Braces
I'd tell your client that they saved a lot of money by going with a PEMB and they should use that money to build a different building.