PEMB Portal Frame Braces

[IronCityEngineer]

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.

 

[JedClampett]

Welcome to the wonderful world of PEMB modifications!
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.

 

[thaidavid40]

I believe that most PEMBs from manufacturers are designed using plastic-design limit states. Are AISC nodal bracing requirements applicable in these types of designs? I'd be interested in hearing from an AISC expert, or a PEMB manufacturer on this one.
Dave

Thaidavid

 

[KootK]

1) You're not over thinking this. It's important and your observation regarding the z-girts is astute.

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.

 

[ajh1]

Generally the 2% flange force rule is used. This level of load in the braces has been confirmed by Dr. Don White at Georgia Tech recently and appears to be adequate for most braces. All of the points raised above are certainly valid and double sided braces are certainly the preference.

 

[IronCityEngineer]

Thanks for the replies. This is what I expected to hear.

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.

 

[KootK]

Can we know what changes the owner wished to make and what his motivations were?

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.

 

[XR250]

Are the Z-girts ever checked by the PEMB manufacturer for the additional demand of the braces?

 

[IronCityEngineer]

The owner wanted to eliminate the kickers to create more headroom around between the portals.

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.

 

[KootK]

Thanks. I was going to suggest something like the L-brackets. It's a potty it doesn't calc out.

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.

 

[KootK]

You might be able to weld angles into the corners of the bottom flange in order to turn the flange into an HSS strut capable of spanning to the next available brace which would, itself, need to be evaluated.

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.

 

[IronCityEngineer]

The owner wants to eliminate ALL of the kickers.

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.

 

[KootK]

Have you looked at the bracing as torsional rather than discrete nodal? There may be some gain there.

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.

 

[CANPRO]

rather than replacing the bracing, you could also look into reinforcing the frame with flange plates so that the bracing is no longer needed. Considerably more work for you, but I don't think it would be terribly expensive to install the plates (depending on what is required). Warning, this will take you further down the rabbit hole of PEMB design.

 

[JedClampett]

If you change anything on the building and a disaster happens, you can be sure that the original PEMB supplier (you know, the one you can't find hide nor hair of) will tell you and your lawyers that that is the reason the building fell down. I've heard (not substantiated) of cases where the original PEMB supplier blamed a building falling down due to wind on new buildings being added close by. You can be sure that those braces were optimized and scrutinized and that they meet code with ounces to spare.
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.