PEMB Spec Help 1

[XR250]

I am helping an Arch. spec a PEMB. It is going to be one story and about 28 ft. tall with brick veneer.
I realize unless told otherwise, it will be designed to bare minimum code. I figure at a minimum I would specify L/360 for girt deflection. What about drift?
What other areas does it make sense to go above minimum without losing the economy of the building?

Thanks

 

[TLHS]

Grab a copy of "Metal Buildings: Design and Specification" by Newman. It's a good reference in general, but it also contains a suggested model specification that he's drafted that's probably a good starting point if you're not that experienced with them.

 

[XR250]

Thanks

 

[phamENG]

about 28 ft. tall with brick veneer....What other areas does it make sense to go above minimum without losing the economy of the building?

Too late. PEMB's are best left to warehouse. Once you start getting fancy, most of them lose their efficiency pretty quickly. Not all - there are some companies doing some neat custom stuff - but I've never actually seen one of those specified. Mostly just trying to shove whatever program they can get into a "standard" box.

As you know, there are no code requirements for serviceability. So PEMB manufacturers will design to H/60 unless you tell them otherwise. And they'll usually do it anyway and see if you catch it in shop drawing review. (There are some PEMB engineers on here that may take offense to that, but it's been my experience more than 50% of the time). I would suggest using AISC Design Guide 3 as the basis for your spec. I believe the MBMA references it in some of their literature. I know DG3 specifically calls out the MBMA:

MBMA in its "Common Industry Practices" states that it is up to the end customer or his agent to identify for the metal building engineer any and all criteria so that the metal building can be designed to be "compatible with other materials used on the construction project."

If this is going to be an occupied building with partitions, etc...H/400 is a good baseline. If a warehouse, you could get away with a lot less but you have to do some fancy detailing in the masonry to avoid cracks in the wrong places. DG3 has a few ideas, though they're geared more toward CMU.

The girts probably need to be more like L/600. And are you going to infill with metal framing for your backup? Unless you're doing an engineered veneer, you need to keep your brick ties at the prescriptive spacing.

 

[XR250]

Thanks Pham. Probably going to do bypass framed metal studs clipped off to each girt so brick ties will be standard spacing. It is an auto repair shop.
Found a used copy of that Newman book for $28 - score!
I have reviewed a building with brick veneer and L/60 girts. Not a crack to be seen. I imagine the brick was bracing the girt!

 

[phamENG]

Ha - you're probably right. I take it back - girts are L/360. L/600 is for vertical support to minimize sagging while the mortar sets up and cures.

I think it's the Brick Industry Association that has a good technical paper on brick veneer back up and required deflection limits. It goes into cracks that open under load momentarily and allow wind driven rain to penetrate the veneer. I think anything lighter than L/360 for the backup and the amount of water seepage can exceed what a lot of moisture barriers behind the backups can handle...or maybe it was weep hole drainage? Something like that.

 

[KootK]

Canada's cold formed steel association promulgates the L/360 for brick backup as a liberalization of what has commonly be done in the past: Link

 

[JoelTXCive]

Have a look at this thread from a few months ago.

I ended using the spec's from the MBMA that I tweaked. Also Nucor has spec's available for free. (I bet that they have items buried in the fine print that only a Nucor building complies with, but that's okay for a starting point)

Link

https://www.eng-tips.com/viewthread.cfm?qid=486394

 

[bridgebuster]

I can see Mike Holmes shaking his head now about minimum code when he has to come in to do a repair.

 

[JLNJ]

You might consider some of the following:

- We structural engineers clutch our pearls over PEMB sway, but I hear few stories of it actually being problematic.
- Require braced endwalls rather than allowing an endwall frame (if you are not looking at expansion). This way the tendency for the veneer to act as a shear wall and the weird veneer corner forces under sway have a lesser effect.
- Make sure you specify the Exposure Factor for wind. PEMBs will try to use B when C is really the correct and appropriate exposure.
- ASCE 7-16 reduced some wind loads from those in ASCE 7-10. Take advantage of those revised contours. I believe the 7-22 is lowering some west coast loads even further and maybe you can take advantage there as well.
- For sway, be specific about the wind return period. If all you want is a 10-year wind, then say so. If you want h/300 or whatever checked against the full Monty, then make sure you spec it that way.
- Inform the client about inherent incompatibility issues with PEMBs and brick veneer.

 

[dik]

I had a bit of an issue with a PEMB structure in Lindsay, Ont. There was a 4' high CMU protection all around the perimeter. This was braced at the top by a horiz girt. Deflection for the horiz girt was L/90... not the best for CMUs.

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

-Dik

 

[Enable]

Do you happen to have any pictures dik? I'd be interested to see how the cracks propagated (I'm assuming there were some of course!)

 

[dik]

No pictures... I caught the 'oversight' before it was fabricated... seem to recall 6" C spanning 20' or 25'; it looked way out of whack.

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

-Dik