PEMB that experienced maximum design snow load

[RHTPE]

Okay folks, I'm looking for your thoughts on this one.

Related thread:

http://www.eng-tips.com/viewthread.cfm?qid=290915

I have a single slope PEMB frame that experienced extreme snow load this past winter. Both columns bear on a 10" wall extending 48" about the floor (we all know that's not smart, so don't go there). No pilaster under the columns, only 2 out of the 4 anchor bolts were installed. I would bet there's no reinforcing in the wall. The horizontal load at the base of the taller column cracked the 3'-3" wide, 10" thick wall at the floor line and temporarily pushed it out about 2". This wall is between 2 overhead doors. After the roof was cleared and the the weather warmed up, the wall almost returned to its original position.

My question for those with more experience with PEMBs than me: Analysis shows that if the bottom of either column is allowed to move in the horizontal direction, the maximum positive moment in the roof beam increases to about 140% of the fixed column base condition. What kind of weakening of this frame could have occurred? There was no collapse, just damage to the foundation wall at one end.

I will be examining all bolted connections this week (haven't done so yet because of the height) to look for distortions around the bolt holes. I have also posed the same question to the PEMB manufacturer. This has been lingering while we wait for the owner's insurance company to determine what they want to cover. I am advocating rebuilding the wall and foundation at each end of the frame to accommodate all design forces applied by the columns. I sense that the insurance company only wants to cover the failed wall and nothing else.

I will save the editorial comments about residential foundation contractors (who built the foundation) and semi-retired builder inspectors (who asked for a foundation design by a P.E. but didn't follow through).


Ralph
Structures Consulting
Northeast USA

 

[msquared48]

In light of your last sentence, is there any documentation that the metal portion of the PEMB was properly inspected prior to certification? Without that, any assertion of damage to the PEMB could be a job to prove.

An additional reason that I bring this up is that there was another thread here recently where the purlin bottom flanges of a PEMB had been damaged by fire sprinkler-line installers and it appeared initially as web crippling due to an overload condition... It wasn't, but it confused the issue nevertheless.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[RHTPE]

Mike,

By "certification" are you referring to what we know here as a "Certificate of Occupancy" (CO) issued by the Building Inspector after all codes and zoning matters have been determined to be in compliance?

I'm wary that the insurance company will deny coverage for anything since the addition was not constructed to prevailing "standard of practice" for a PEMB. But then, if a CO was issued by the local inspector, and if the insurance company did include the addition under the policy, have they set themselves up to be obligated to cover the loss no matter what?

Ralph
Structures Consulting
Northeast USA

 

[hokie66]

As the frame has returned to its original position, I doubt it has experienced much damage, but you can look. The owner's insurance company is not going to pay to upgrade the building to the standard which should have applied. If there were defects in the original design/construction, then somebody else's insurance company may have to pay. Sounds like the owner needs a lawyer.

 

[Ron]

Ralph,
I agree that it is unlikely the actual material has been compromised. I would concentrate on the connections and do an inspect of the members for obvious plastic deformation. Look for paint failures that do not appear to have other relevance.

I believe Mike is referring to a "certification" that would have been done as an in-process inspection prior to the issuance of a certificate of occupancy. From a quality standpoint, the C.O. means little. In my area, some counties require specific inspection and sign-off by a licensed engineer for all PEMB's.

 

[DaveAtkins]

I will be very interested in what you find out. We all know PEMB's are designed with a pin at each base--I have always wondered what would happen if one of the bases became a roller. This is exactly what happened in your case.

But I agree with the posts above. If no plastic deformation is evident, your proposed fix is the right way to go. Make sure everyone involved knows that just fixing the 10" wall does not solve the problem.

DaveAtkins

 

[RHTPE]

Dave,

I am surprised at the number of owners and contractors who simply cannot (or will not) understand that the bottoms of PEMB columns need to be tied across the width of the building in order to be consisent with the design methodology for these frames.

In this particular case I would chalk it up to pure ignorance on the part of the GC, the foundation contractor, and the former building inspector. Unfortunately, the building's owner will suffer the consequences of everyone's ignorance.

Ralph
Structures Consulting
Northeast USA

 

[SteelPE]

While I have used the ties you reference in PEMB foundation before, I have had instances before where I have not used ties. Typically, this is for very small buildings.

I have seen foundation designs by other engineers (one of whom was the chief engineer at a well know PEMB mfr) that have not used ties in the foundation was well. Again, these were for very small buildings.

I can't really say if I have ever designed a PEMB with the base located above the slab though. I'm sure there is at least one instance but I just can't recall. Maybe the contractors I work with kind of get it (yeah right)!

 

[JedClampett]

Not to get all conspiratorial, but part of this is the PEMB’s fault. They will have nothing to do with assisting the foundation design except giving out the loads, usually after the foundation is designed or built. While it might seem obvious to most structural engineers that lateral column support is critical for the vertical load carrying capacity of a rigid frame, apparently it’s not obvious to all.
You almost wonder if this is intentional, to blur the liability.

 

[a2mfk]

We've beat up PEMB mfrs pretty good several times on this board. I am a fan of the concept of PEMB, they are steel construction after all, and the parts are fabricated in a controlled environment. In many ways they should be an ideal structure, shop fabricated and then assembled on site with almost no waste, eliminating many field errors... And they CAN be that. If the right hand was talking to the left hand that is.

It obviously depends on the size and complexity of the structure, and who the PEMB mfr is, because they are not all the same, not by a long stretch. We have had some good experiences working with the ones that seem to give a crap about the entire building, not just their part of it, and will work with you and coordinate and make modifications to their design.

Some of the fault lies with the owner/GC, who unlike the traditional design-bid-build delivery method, they tend to drive the ship right from the start. And sometimes they will drive right into a storm or a rock before they know what they hit.

The owner/GC often treats the PEMB mfr like a component supplier instead of a design team member. They want the bare bones structure at rock bottom prices and are not willing to pay more for good design consulting and CA support. They want the cheapest structure per SF and are not usually that interested in quality, as long as it gets them a permit and "meets code" as if they had a clue what those mean. They want a price for the shell and then they seem to "figure out the rest" on the fly, after the fact, sometimes after the building has been ordered or even fabrication has started.

And then there are the PEMB mfrs who like this role, they want to just be component suppliers, as if their product is a pre-engineered truss package. Drop a box of parts off on site with some instructions, pick up a check, and then they are off.... This may be fine with smaller garages, simple warehouses, shop buildings, etc., but we all have seen PEMBs morph into office buildings, churches, and other more complex structures. The times have changed and the role of PEMBs have changed, but not everyone has kept pace.

I think some of these failures we have seen recently are related to a failure of architects, engineers, GCs, owners and PEMBs to not keep pace with how PEMBs are used, and to not properly delegate responsibility and do proper coordination. PEMBs are not just simple components anymore, and the PEMB mfr should play the role of a member of the design team and be compensated for this role, including CA and coordination with the rest of the design team. Owners need to be educated by the architects of how this process should work, and it should start early on before owners thing that a PEMB is their best option and so they understand what they are getting.

EORs should not be handed a PEMB package that was designed by the owner/GC and the PEMB mfr, and then asked to design foundations and slabs with no further discussion or communication unless it is a pretty simple structure. It is time these things got treated like "real buildings".

The building department plays a role in all of this too somehow, but lets face it, they are clueless half the time and just want to see a stamped drawing of some sort....

 

[audeuce02]

I agree completely with a2mfk. The problem is that owners and contractors are not willing to absorb the expense associated with coordination. I have 15 years design experience with multiple PEMB companies and have now moved to the sales side. It is appalling the lack of knowledge some of the decision makers have. As an Engineering and construction industry we have to get away from the lowest price mentality. There are many ways for me to provide the lowest cost PEMB, and if the contractor does not coordinate with all other trades not only does the owner receive a low cost building, but a "cheap" building that is not going to perform as needed.

 

[RHTPE]

Gentlemen,

While I understand the aggravation, frustration, and angst that many of us have regarding the low price mentality of the construction industry (owners, contractors & suppliers), the purpose of my post was not to provide another forum to vent. As I had asked:

My question for those with more experience with PEMBs than me: Analysis shows that if the bottom of either column is allowed to move in the horizontal direction, the maximum positive moment in the roof beam increases to about 140% of the fixed column base condition. What kind of weakening of this frame could have occurred?

A follow-up visit this morning found no visible sign of distress in the paint on the PEMB frame. Nor was there any distortion in the splice plates on the frame. However, I did find another crack in the foundation wall (inside face) at the other end of the frame. This will serve to support my position that the foundation walls at both end of the frame should be reconstructed. Perhaps now the insurance company will concur.



Ralph
Structures Consulting
Northeast USA

 

[hokie66]

Moments of 140% of the fixed column base condition represent a much greater overload when compared to the design condition, which would have been with the column bases pinned. I suspect the loading was not as severe as you have assumed.

 

[RHTPE]

Hokie,

I did a rough analysis to compare against the actual amount that the foundation wall was pushed out (perhaps 2"). I do not have the actual fabrication drawings for the PEMB frame so it's hard to get a really close approximation of the members. This structure has numerous design faults that were overlooked by everyone when it was constructed. I suspect the actual loading was greater than what had been designed for and that the owner's guardian angel was looking out for him.

Ralph
Structures Consulting
Northeast USA

 

[msquared48]

Perfect example here of a knuckle joint in a foundation wall and what can happen - this time not from soil lateral load from the outside of the wall, but from the lateral thrust of a mainframe applied to the top of an improperly reinforced wall/pilaster.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[Ron]

Ralph...
Could you post a few photos or can I contact you directly? I have a possible use for a couple of photos.

Ron

 

[RHTPE]

Ron,

You can contact me directly, though I'm not sure how we can exchange contact information through this site, since the posting of contact information is verboten.


Ralph
Structures Consulting
Northeast USA

 

[a2mfk]

You are right Ralph, my apologies. This subject hits close to home with my experiences, did not mean to hijack your thread...

A few more photos would help.

Sorry if you already told us, but I cannot figure out who are you working for and what specifically is your scope? It matters a lot to know where this is all going.

I feel silly asking but how do you do the quotes on this board??

"I'm wary that the insurance company will deny coverage for anything since the addition was not constructed to prevailing "standard of practice" for a PEMB.

I have done forensics for various clients including a lot for insurance companies over the last 7 years, and this is not how they work generally. You should see the crap they cover down here in Florida that wasn't built with a permit much less to standard of practice. Structures that would be best described as glorified tree forts have been covered...

They may also offer coverage and then subrogate against other parties who may be at fault, such as the original contractor, PEMB mfr, A and E team... But that is if the payout is high enough to justify that legal action, which in your case it sounds like the only damage was to the wall.

From my experience, when something requires a repair, strengthening, mitigation, etc., then those must be done per current code requirements, and they do not usually question that. It usually doesn't matter if it wasn't built to a certain code or standard, they have to return the property to its pre-damage condition, and this has to be done correctly. Unless there is a LOT of money involved, they are usually not interested in the technical nitty gritty of it all. It is in insurance companies best interests, within certain dollar amounts, to offer coverage and move on. Otherwise they may have to pay for extensive engineering studies, lawywers, mediation, etc. only to have to pay for the repairs plus loss of use of the building...

Is there accurate LOCAL weather data available in terms of snowfall or snow weight?

If not, if may be difficult to impossible to prove that the weight of the snow did NOT exceed the design snow load as required by code at the time the structure was constructed, then despite poor construction, design, etc., they will likely offer coverage. That has been my experience with wind and other types of damage, but not snow since I work primarily in the Sunshine State..

I am sure that Ron, who has similar but more extensive experience, will have some good input into all that works.

 

[RHTPE]

a2mfk:

As far as the snow load is concerned, this building is in my home town. I did a number of snow load evaluations this past winter (I lost a pole barn from it all), including in my back yard. In every case we were right on, or slightly over, the code load for snow.

As far as photos, I have a bunch. But it will take some time to bind them up into into one .PDF and post them. Most will require some explanation as they focus more on the foundation fractures and less on the big picture.

Quotes are done in a similar format as bold or italics - use [] to turn it on, and [/] to turn it off. Between the [] use the word quote.


Ralph
Structures Consulting
Northeast USA

 

[a2mfk]

quote test [quote/]

You may not want to say who you are working for which I completely understand.

If you are working for the insurance company, then it'd be in your client's interest for you to share your local, direct knowledge of similar failures and that it would seem likely the snow load was exceeded. This will avoid them going down a road that will lead them nowhere but to a fight with someone who can prove this, and then they'll be on the hook for it anyway. They are not scientists or engineers and they don't usually want all the facts, should we pay or not? Will we have a good chance of having to pay at the end, plus damages and attorney and engineer's fees? They calculate losses into their business model, its not like you or I losing 50 grand

Now if you are working for the owner, then that complicates things a bit depending on how the insurance company is treating them... You may have to get your ducks in a row if they have another engineer doing an investigation also, which they usually do.

I know I am not giving you tech advice, but that can end up being the easier part of this kind of problem. Convincing a layperson that what you think or know is correct is going to be the true challenge.