PEMB anchor bolt holes 1/16" oversized 1

[canwesteng]

Is it normal for holes in PEMB baseplates to be 13/16" for 3/4" anchors? I swear the last PEMB job I had holes 1/2" bigger, but this vendor is telling me they normally do 1/16". How are we supposed to cast the anchors that accurately?

 

[phamENG]

It's not uncommon. To a lot of PEMB suppliers, they are designing a steel building and ONLY a steel building. Therefore, their connections should meet standard steel connection requirements. (I actually go this reasoning from a PEMB guy once, though I don't recall who he worked for.)

And the answer is templates. Really accurate templates. And a surveyor (or two) to verify the templates are dead center where they need to be.

 

[dik]

YOur PEMB building specs should stipulate that PEMBs base plates match the requirements of the AISC oversized baseplate holes... you will likely have to set the current ones with a template... that sort of precision is possible, albeit difficult.

Dik

 

[jimstructures]

This is from an older AISC Manual, 9th Ed., but is the one I have in front of me at home:

Page 4 - 130

Holes size fore anchor bolts
3/4 to 1 in. db - 5/16 in. oversize
1 to 2 in. db - 1/2 in. oversize
Over 2 in. db - 1 in. oversize

Jim

 

[phamENG]

Another issue you run into here is coordinating the shear transfer between the PEMB and the foundation.

Because of the way they're designed, nearly all PEMBs have significant net uplift at every column (depending on the direction of wind), so you can't rely on friction between the plate and the concrete for shear transfer in those load combinations. That leaves you with either bolt shear or shear keys. Shear keys require coordination between the foundation design and steel design - doable but not always easy with a PEMB (especially if you're being called in to do the foundation AFTER the PEMB is already designed and scheduled for fabrication). So that leaves bolt shear. You either make small holes that are really hard to match in anchor bolt placement, or you make the plates large enough to accommodate the larger hole AND a plate washer to be welded after everything's in place. The MO for most PEMBs is to shave material down to slightly less than they should be able to get away with, the choice is clear if they're not otherwise compelled to do otherwise.

 

[SwinnyGG]

You can always drill them after your concrete pour. If your allowables... allow it.

 

[r13]

You can always drill them after your concrete pour. If your allowables... allow it.

It is pretty much the way for most cases. The other difficulty is the small edge distance that causing headaches.

 

[Rabbit12]

We just completed a PEMB a few months ago. We were responsible for foundation design including anchor bolts. The PEMB oversized the holes which is fine except when that happens I think AISC DG#1 recommends only utilizing 2 anchors for your shear calculation. Of course I couldn't make anchors work in shear so I had to require plate washers.

The PEMB manufacturer came back and said plate washers weren't necessary. I told them unless they were designing and stamping the anchor rods they were necessary.

 

[JoelTXCive]

This is Table 14-2 from the 14th Edition AISC Steel Manual

 

[phamENG]

One thing to keep in mind - that table doesn't solve the base shear problem. AISC recommends welding the plate washers as a last resort. If you do weld them, those minimums may not be sufficient.

 

[JoelTXCive]

This is the best book of practical design knowledge that I have come across on the subject. I also attended an ASCE Seminar taught by Mr. Newman. I highly recommend both.

https://www.amazon.com/Foundation-A...ocphy=9027766&hvtargid=pla-421323031323&psc=1

 

[JedClampett]

Try these: Deco

 

[canwesteng]

The 1/16" precision is possible, although an absolute headache, for bolt groups. For anchor groups under a common frame, is the contractor supposed to bend the portal to fit though? I have zero confidence in their ability to be within 1/16" over 20' during concrete placement, as a matter of practice I always assume all survey coordinates are off by at least 1/8". It doesn't help that the anchor group is too tight to fit 3/4" epoxy anchors. Thankfully these are small buildings so I may be able to get 1/2" bolts to work, although this would be a big problem on some of the larger buildings on the job.

 

[XR250]

Getting the holes in a template to 1/16" should not be a problem using water jet, laser or plasma jet.
I can't imagine it would be too difficult to force a small PEMB's frame into place with a come-along if the template is set 1/4" off.

 

[dik]

The templates I've encountered are generally made of 1/2" plywood. Just a normal wood bit...

Dik

 

[Ingenuity]

It is interesting with today’s technology available to contractors.

About 1 year ago I purchased a $40+k robotic total station (RTS) for some small design-build projects I get involved with. Because it is so simple to use I end up using it to set out everything (from CAD and BIM) with C/L Rebar dowels, slab edges, pedestals, anchor rods as layout points with elevations.

With systems accurate to 1 second and judicious control points, it is possible to be set out to 1/16”. I am involved with a 1,500’ long aggregate conveyor system and every anchor rod C/L will be set out using the RTS - on this project to a precision of 1/8”.

Long gone are the tape-and-stringline days of setout, even for simple tasks, in my experience.

 

[dik]

Ingenuity: My experience, too... but, templates are still used for setting anchor rods, only the template is set by instrument.

Dik

 

[canwesteng]

Ingenuity - I have some bad news for you if you're counting on the total station to place all the anchors within 1/8" for 1500' conveyor. I've never used tape and line (maybe a bit younger than some of you), all jobs have been set by total station. I know a competitor attempted to have 1/8" oversize holes on a conveyor job (longer than 1500' but you'll have the same problem) and they had to ream out over half the anchor holes.