Signs of Distress to PEMB??? 1

[4inch_orange_doorhinge]

Hi!

In the wake of strong hurricane winds, I've seen photos of damage to PEMB cladding and roof decks. How do you in forensics engineering (or as a structural engineer) determine distress to the main structural members of a PEMB?

I would love to know your thoughts! There is potential for me to have an opportunity to go check some out in the field and I wanted to get a headstart on figuring out what I should be looking for!

Thanks in advance!

 

[JedClampett]

I've done this (Irma, Michael), although I don't consider my myself an expert. Maybe two hurricanes makes me one.
[ol 1]
[li]Most of the time, the cladding fails (maybe with the purlins and girts) first. Skylights are notoriously bad. [/li]
[li]This relieves the internal pressures in the PEMB.[/li]
[li]The main frames stay in pretty good shape. And if they're not, the damage is pretty obvious. I took some pictures driving around (there was no time to stop, plus the residents didn't really appreciate hurricane tourists) and here was a frame "issue." I suspect the wall panels failed, wrapped themselves around the frames and took them down. [/li]
[/ol]

 

[dik]

As Jed notes, unless the frame is visually damaged (more than a little deformed), the frame is likely OK. Some added stuff, ICYC, PEMBs are designed to 'reflect' the elastic moment diagram. If a structure exactly mimics the elastic diagram, it goes plastic at once and there is no redistribution. The collapse is immediate (subject to second order issues). That it's still standing is a good sign.

There is an additional issue to that (actually a couple, but this is the main one). The residual stresses can promote a 'shakedown' type of failure. This is a rare condition, but a possibility. I think that would be my only concern.

I should have noted that when a steel structure is overloaded (not necessarily to failure) and the load is reduced, there are internal stresses within the steel structure caused by the overload condition. These internal stresses can result in overloading with a different application of load. This is generally considered as 'shakedown' loading... something like that simply put... If I confused anything, please ignore this edit.

The loss of cladding, in addition to reducing the design pressures, also reduces the tributary area . One issue is that if the frame is taken subjected to those loads again, it will behave elastically .

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[SandwichEngine]

There is basically no way for you to check a PEMB rigid frame without using the exact software and configuration that was used to design it. About half of the PEMB companies use proprietary software. The other half use a program called MBS (metal building software) but it has something like 70,000 different options for configuring it. There are tons of examples of things that the code doesn't fully address that plays into PEMB systems and different engineers have different opinions on the "right" way to engineer things. If you wanted to really get up to speed on how this works, you'd need to hire an experienced PEMB engineer to walk you through it. Like any other profession, there's a lot to it.

But I'll mostly agree with earlier posters. Typically damage is limited to the cladding, purlins and girts. You also want to keep an eye out for damaged flange braces. These are angles that come off the main rigid frames to brace the inside flanges from buckling in LTB or compression. They are very critical to a metal building system.

I have seen where PEMB structures totally or partially collapsed in a hurricane. I saw this in a presentation a professor gave on a damage assessment report after hurricane Harvey. It was typical of a storm like that where buildings along a narrow line all failed while buildings a few yards away looked completely undamaged. In some cases the cause of the failure was obvious such as where an airplane hangar door was left open and the wind blew directly in and basically popped the building like a balloon.

In some cases, a hurricane can also result in total collapse. In any case, a rigid frame that's damaged is trash.

 

[JLNJ]

We got caught up in a PEMB collapse lawsuit where the compression-side girder bracing to the joist extended end was too flexible to brace the girder under load. The PEMB guys don’t do it like that any more, but tons of those building are still around, living on the ragged edge and hoping never to see design-level wind and snow.