PEMB Modifications 1

[hemiv]

Hi all.

I've been asked to provide a design allowing a PEMB owner to remove X bracing for new doors. I've done this before with chevron bracing and new beam, but this time the openings are going to be quite large and I don't have enough room for that.

So I'm going with a new portal frame. They bay in question is 27'7" wide and the eave height is 15'. Overall 80'x80' building. The bracing is visible on the wall I'm reworking, but the other three walls of the building have an interior plywood wall covering and I cannot see the LFRS elements. There are no drawings available for this building. And of course no one wants me to get into to analyzing this whole building, which would cause the business to shut down in order to move a lot of equipment and remove all the interior wall coverings.

So my plan is to proceed with the design with the 10% rule for lateral loads - I'm not designing any additions, so new lateral loads will be induced in the system - while also designing the portal frame to be quite stiff and reduce the amount of load the other elements of the LFRS will take as redistributions.

Just wanted to get some extra thoughts on this approach. I know the existing elements of the LFRS which are perpendicular to the new portal frame will take load if the portal frame is too soft, so I am primarily trying to eliminate this from happening as much as possible.

 

[hemiv]

@bones - I modeled them the same. I'm running that model alongside another model of the portal frame and comparing the stiffness of each. H/60 may the be the spec'd minimum, but no way these X braces are allowing that kind of deflection. I'm proceeding with a 1/2" max. That keeps me a little under H/400, but still seems like a reasonable number based upon what the existing structure is actually doing.

 

[bones206]

That sounds more than adequate to me. Actual deflection is probably even less when you consider base plate stiffness.

 

[KootK]

I guess I'm just antsy about not increasing the unity by more than 10% under this particular load case. i.e. going from 0% to not more than 10%.

I struggle to understand that.

1) If it's a flexible diaphragm load distribution, then it's a non-issue as we discussed. And, if there's rod bracing, it will almost certainly be a flexible load distribution.

2) It's not about a 10% increase for any particular load case. Rather, it's about the new worst case load in an element being 10% greater than the original worst case load in an element.

3) The only existing frame load increase that seems likely to me is that shown below. And that's only if we're doing the rigid diaphragm dance.

 

[canwesteng]

For it to act like a rigid diaphragm, you need to have a full lined braced in both directions, and PEMB don't do that. I wouldn't be anywhere this hesitant to take this on, but make sure you get a good look around the rafters and have a plan to get the load into the portal frame from the bracing in the roof. If you want to analyze it to death, just make your frame as stiff as the rod bracing you are replacing (the feasibility of this depends on the height of the building).

 

[hemiv]

@kootk - I agree, that's the frame that would see an increase, but only in a rigid diaphragm scenario. I'm just being too conservative about the 10% under a particular load case. Thanks for the insight, it's appreciated.

Thanks also everyone else. I've just about got this thing whipped.

 

[straub46]

I used to work for one of the major PEMB players, but work in the consulting world now. I will say that changing PEMB bracing from X-bracing to portal frames, with no other new loading consideration, is one of the least hesitant modifications to a PEMB I take on, especially if it's going into the same braced bay.

The building definitely would have been designed using a flexible diaphragm if there's roof bracing, which I believe I saw mentioned above. If there were other bays of bracing on that wall, I'd be talking to the owner about the need to change all the braced bays from rod bracing to a portal frame to ensure compatible stiffness. Even the PEMB companies don't mix and match LFRS systems on the same braced line.

Go Bucks!