Removing X-bracing in existing PEMB 2

[Ben29]

I feel terrible. I had a miscommunication with a very good client. He has a large PEMB and I was contracted to design an addition to the building. So we are essentially doubling the building footprint. The idea was that he was going to remove the exterior wall (purlins and metal panel) of the PEMB so that he can have a passage into the new addition. I did not do a site visit prior to designing the addition. I (wrongfully) assumed that the owner or architect would notify me that there was X-bracing that needed to be removed. I assumed that any existing X-bracing would stay.

So there is X-bracing in (2) bays and the owner wants us to remove. Being that this is a 20 year old PEMB (and we do not have any drawings aside from the foundation drawings) - my company does not want to get involved with modifying the X-bracing. So I am trying to harness the powers of Dale Carnegie to try and smooth things over with my client.

Please send advice. It would be a disastrous idea to try and modify the existing X-bracing in this building, right? There are 9 bays without X-bracing that the client can freely pass through.

We are installing an expansion joint between the addition and the existing PEMB. And we will not have snow drift on the existing PEMB.

 

[JAE]

I see an X-brace on the second from the last bay in your photo. That brace most likely aligns with X-braces in the roof (roof diaphragm) so relocating it to another bay would be problematic.

The possible options you have:
1. Leave the braces alone dummy...they are important
2. Replace the braces with some kind of moment frame between the adjoining main columns. PEMB manufacturers do this all the time.
3. Design a more significant brace in the two adjacent, new bays and somehow reach over and grab the original columns/bents while still allowing movement in other directions (difficult)
4. Replace the braces with chevron braces (upside down "V") and a new cross beam at the eave line between columns. This gets you more passageway space, if not all the bay at least some. The cross beam will not be braced by the roof since it will be below the eave, so it will have to be very stout. Also - the existing columns will have a bit of cantilevering at their tops. Might need plating, etc. there.
5. Create an exterior set of buttresses on each end of your existing building - just to make your owner even more mad at you.
6. Take the braces out, many others do the same and they reply: "ain't nothin' fallen down yet, dude"



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[POPEYES]

Can you replace the bay with an equivalent strength eccentrically braced frame that will maintain load path and still allow pass through?

**edit- I see JAE has recommended something similar.

 

[Ben29]

If we modify the lateral system, won't we have to update the entire building to today's codes? Is it good enough to simply replace the X-brace with a moment frame of equivalent strength?

 

[CANPRO]

Are you tying the two buildings together structurally? If you can transfer the lateral load from the existing building to the adjacent new structure, you can remove the x-brace and support the existing building laterally with the addition.

 

[POPEYES]

Did you not have to re-evaluate the existing LFRS with your addition? Is your addition adding loads to the existing lateral bracing?

 

[JAE]

If we modify the lateral system, won't we have to update the entire building to today's codes? Is it good enough to simply replace the X-brace with a moment frame of equivalent strength?

Not necessarily - depends on how much work you are doing in the original building and how the local building department views these things.
In many areas - if your renovation work (in the original portion of the building) is less than 20%, or something like that, you don't have to bring the whole thing up to current code.

CANPRO - my thoughts exactly - see my point 3 above.

POPYEYES - he stated above that the two buildings were separate.

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[POPEYES]

So if you use the new bays for lateral bracing you’ll have to tie the structures together and then you’ll need to evaluate to make sure you aren’t adding significant loads to the original structure? I guess you should not be it the new bracing is sufficient for the addition +excess margin to make up for existing?

 

[SteelPE]

I’m surpriesd no one addresses the 100 lb elephant in the room. How are your columns going to be braced now that your girts are gone? As far as I know the girts will brace the column for weak axis buckling, and LTB as well.

 

[SteelPE]

I may be mistaken as well, but on the dark side of your picture there appears to be flange bracing at the change in column section (from parallel flange to sloping flange). On the left side of the picture you can see partial depth stiffener plates. I wonder if they might have been used to attach torsion bracing for the column? Did you find any torsion bracing on the wall where you plan on taking the girts and panels down?

 

[JAE]

Good point on the column bracing issue SteelPE

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[bob33]

Would it be feasible to extend the new eave struts through the expansion joint and connect with slotted holes to the existing eave struts? Some strengthening of the eave struts and their connections would be necessary, but the lateral loads are not large because the cross-bracing is only bracing half the last bay. Of course the PEMB manufacturers (old and new buildings) would have to agree to this modification of their structural designs.

 

[JAE]

Of course the PEMB manufacturers (old and new buildings) would have to agree to this modification of their structural designs.

I don’t think they would ever agree to any modification to their structure.

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[Ben29]

SteelPE: I have been asking my boss that exact question!! He said that the purlins just provide the support for the facade and aren't required structurally. Can you point me to documentation that says that the purlins are required for structural reasons? Getting nervous...

 

[Ben29]

And when I say "purlins", I mean "wall girts"... but I think you all understand what I mean.

 

[WinelandV]

Ben29,

Unfortunately, you are modifying the building, so you get to determine what is required for strength/stability. Removing the column girts may very well reduce your weak axis buckling capacity. Remember, PEMB are made to fit whatever purpose the owner gives them at the time of the order. They DO NOT build in future expansion unless explicitly told to.

Having analyzed some existing PEMB's in my past, I can tell you that I absolutely had to count on the bracing abilities of the girts on the column faces. Maybe PEMB's are designed differently wherever you are located. But I'd at least make the check.

 

[SteelPE]

Ben29,

Sorry, I can't point to a specific document that states the building was designed that way. I can tell you that when designing a conventional building we assume the siding will stabilize the columns in the weak direction at intervals equal to the girt spacing. This is something that I was taught in the very beginning (first month) 20 years ago. It is possible that this end wall was designed to be expandable and reinforcing may have not been required.... but then again, owners seem to remember things with rose colored glasses so YMMV. The only way to prove that it works is to measure it and check it.

I can also tell you that one of my contacts was the chief engineer at a large well know metal building supplier in the 80's and 90's. I often ask him questions about about projects I have with metal buildings (it's a two way street). I recently had a project where the client wanted to remove the entire side wall of a building. We told the client at the very beginning that reinforcing would be required. We went back and forth with the client about the reinforcing requirements. In the end, we allowed them limited access from one building into the next such that the girts and metal siding were left on the original building in every other bay. This eliminate our issues and satisfied the client (who wasn't overly thrilled).

FYI, the proposed reinforcing consisted of welding a continuous channel to one of the flanges from the base up to the roof to cut down on the KL/r. He has told me over an over again to design that channel for 2% of the flange force of the existing column. I think he is a little liberal with that assumption, but he is the absolute best that I know when it comes to the design and modification of metal buildings.

 

[Ron247]

Wall girts generally are used to reduce the unbraced length of the column. The tab welded to the column where it changes from tapered to straight inside flange is probably for an interior flange lateral brace that may not have ever been installed but should have or it may just be there because of the change in flange direction. Sometimes a stiffener is added at a change in flange direction. Does the tab have a hole in it for a bolt? Are you wanting to open up the high-side wall or the endwall?

You will have to do an analysis of the frames and bracing if you want to modify this in any fashion if you are not sure of the original design parameters. As already stated, removing the wall girts may be a big deal. It is common to find contractors leaving out flange braces when they are actually needed. Unless someone specifically ordered the original building to have the high-side wall opened up in the future, it is doubtful the high-side columns were designed to be free-standing.

If the flange braces on the rafters are welded to the rafter without a tab, that may have been what was designed for the columns. In that case, you have no way of knowing if flange braced were designed for the columns but not installed.

If the rods are in the way, how were you going to brace your new addition?

 

[DaveAtkins]

I would try to add a moment frame at each bay where X-bracing is removed. As JAE stated, PEMB manufacturers do this all the time. Generally, the moment frame does not even need to have its own base plates--it transfers the base force into the main building columns. I expect your client will have no problem with this.

Regarding buckling of the columns, the main frames will be resisting less wind load than they were designed for, since an entire side of the building is being eliminated (because it is now becoming part of the interior space). Perhaps this will be enough reduction in force to allow them to work unbraced. If not, then additional flange plating could be used to make the columns work.

DaveAtkins

 

[JAE]

Wall girts generally are used to reduce the unbraced length of the column.

I'd second Ron247's assertion - that is very typical of girts and your fellow engineer is wrong...unless you can show that the column works otherwise by your independent analysis/check.

Assuming that they ONLY serve to allow wall panel fastening is a very wrong-headed and unconservative assumption that could bite you in the a$$.

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