Anchor rods to PEMB buildings - post-installed? 1

[Ingenuity]

Last week Saturday there was a sizeable concrete placement close to a project I am working on - more than 700 cy - and being nosy I decided to swing by and take a look.

RC foundation slab with thickened edge beams for a new industrial warehouse lease complex by a local developer, who has many adjacent buildings of the same type (tin-can framing etc).

Anyway, interesting thing I noticed was that there were no anchor rods/bolts cast into the foundation for the steel frames of the PEMB. I went back on Monday and asked one of the workers why they did not install anchor rods, and he informed me that they 'always drill them after, as the frame columns are installed'.

Is this now the typical procedure for installing anchor rods for PEMB's?

 

[dik]

I've done it a couple of times (only twice), due to schedule and not having shop drawings to work from. It's worked well using Hilti adhesive HAS anchors... only 3/4" dia and had to drill a few baseplates on the PEMB frame. Not recent, though... on one project, there was a great deal of difficulty in getting the PEMB building supplier to provide larger holes... not sure why the resistance. If you mod the frame without their OK, you likely void any warranty.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[WinelandV]

Post-install anchors on a PEMB foundation is not my preference. The main reason being, the spacing of the anchors on the pemb drawings is usually smaller than what Hilti (or anyone else for that matter) tests for - meaning you won't have any official capacity in your post-install anchor group.

As Dik mentioned, you can space them for what's needed, and then go back to the supplier and ask for them to modify their baseplates. Most suppliers I've worked with are reluctant to do so - they like to dictate what works well for them and their processes. Someone is (supposed to be) signing off on approval drawings indicating that the anchor locations are acceptable, and if a change comes late, they now need to put a member back into their shop and re-work it.

As for the OP's scenario, maybe the engineer is aware that this is happening and already has accounted for it. I'd give even odds, though, that the engineer has no idea this is how the anchors are being installed.

At any rate, I need to check to see if a concrete sub can epoxy some anchors in (they neglected to install them when they cast the piers). If the spacing's too tight, looks like they'll be demoing enough of the pier to install the anchors correctly.

 

[XR250]

Would it be possible to bolt a larger, thicker base plate down and then weld the PEMB plate to it?

 

[Ron247]

As long as the anchors meet the loading requirements based on spacing and depth, I do not see why they would not work. Problem is, who checked the new anchor design? As noted in winelandy's post, bolt spacing is generally tight on PEMB baseplates. As far as the PEMB giving a custom bolt pattern (diameter and spacing), they only like to use the already created baseplates. Most suppliers have a set of standard baseplates they choose from.

Also, the PEMB designer only specifies diameter, quantity and spacing. The don't worry about concrete requirements. That is why the columns that not inset for girts get harder to secure even with anchor bolts. When they place their bolts near the edge of concrete, it makes clear cover + stirrup diameter tight.

 

[driftLimiter]

This is a bit sketchy to me. We usually use anchorage reinforcement on PEMB anchor bolts, because they are placed so close to the edge there is no good mechanism to transfer the shear from the BP to the concrete. So first question, if their was anchor reinforcement on the plan, it is usually shown tight to anchor bolts, Did they install the bars?

Next, ACI anchorage loads in seismic area from (4) post installed rebars spaced about 4" apart presents a slough of issues.

Finally if you were to use a bigger plate and weld down, would that void the pemb warranty? Would you miss the hairpins (if present)?


I would frankly be livid if I showed all the anchors as cast in and came to the field to find this, especially on a PEMB.

 

[Brad805]

I am curious if there is a mid column. If not, the loads seem problematic for the typical 4x4 or 5x5 AB pattern we see. I would never allow post installed, but we are in snow country. I suspect your gravity loads in Hawaii are a bit less.

 

[dik]

I'm in snow country... and they work, with a few cautions. If a mid-column, you have to have a slab thickening at mid-span or whatever, and 'same thing all over again'. I've not had that experience. The two structures I did were relatively short span, something in the order of 50'-70', if memory serves. I've heard of no issues.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[dik]

There is a bit of a problem with this... you likely need temporary bolts to hold the frame in position until you can weld it. Also with post-installed anchors, we specified minimum bolt centres so post-installed anchors would be developed.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Brad805]

Dik, I did a clear span 90'-0" pre-eng with 60kip shear last year, not on your life would I allow post installed anchors for that.

 

[dik]

I'd have to look carefully into those loads... That's a pile of force...

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[SandwichEngine]

I'm a PEMB guy and I've never heard of doing all post-installed anchors. Some companies call out post-installed anchors for door jambs but never for columns. The only post-installed anchors that get used are for repairs when the concrete contractor messes up the location or projection of the anchor bolts.

I don't know how you would be able to effectively do all post-installed either. The way a normal PEMB frame is erected is that you stand the columns and bolt them down to the anchor bolts. The pinned baseplates are stiff enough to keep the columns in place until the rafters are installed. To install the anchors after would mean assembling an entire rigid frame, crane it into precise position, mark the bolt locations, put the rigid frame down somewhere else, drill and set the epoxy anchor, then when they're cured, pick the entire rigid frame back up and install onto the anchor bolts. You typically can't just drill through the baseplate holes because you have to drill a hole that's larger than the anchor for there to be room for epoxy and PEMB baseplate holes are usually only oversized by 3/16", which isn't enough room for the correct sized drill bit. I bet these guys aren't doing it how I said and are drilling holes through the baseplate that aren't large enough per code.

Also, it puts the onus of precisely locating the anchors on the PEMB erector, not the concrete sub, which is unusual. I'd think that would mean that it would take much more time to erect the PEMB portion.

The fact that someone worked out a system where this is what they do must mean they're cutting corners or they're maybe getting the PEMB manufacturer to provide oversized holes and weld washers.

To answer some other comments from this thread:

I'm not aware of PEMB companies having standard baseplates in a shop that they want to use on everything. Everywhere I've been, every single part and piece of every rigid frame is custom.

Also, this is why I would be reluctant to change the spacing of the anchors; because it would involve adding cost to my portion of the project likely without compensation. Spacing the anchors farther from the web creates more bending in the baseplates for uplift loads and would require thicker baseplates. Same with spacing along the web. It can totally be done but it's not clear cut who should foot that bill. PEMB guy will say that someone else should. GC may feel that accommodating the foundation engineer's requirements should just be part of the job and any increased cost should be borne by the PEMB guy. My easiest way of getting on the same page and hopefully avoiding a change order would be to get the foundation engineer to go with the spacing I've already designed for.

 

[XR250]

You typically can't just drill through the baseplate holes because you have to drill a hole that's larger than the anchor for there to be room for epoxy and PEMB baseplate holes are usually only oversized by 3/16"

Don't most adhesive systems only require 1/8" larger hole?

 

[SandwichEngine]

XR-250. You're correct for most anchors but I can assure you that it is not common practice or even feasible to drill through a PEMB baseplate hole. If the PEMB baseplate hole is oversized by 3/16" and the drill bit is oversized by 1/8", you only have 1/32" around the drill bit. Hand drilling in the field isn't that precise so you would end up ruining a bunch of drill bits trying to do one. Hypothetically possible but not done in the real world.

 

[CrabbyT]

You might want to inform them of this code requirement.

... Adhesive anchors shall be installed in concrete having a minimum age of 21 days at time of anchor installation.

And the associated commentary.

...Bond strength is, in general, not highly sensitive to concrete compressive strength. The design performance of adhesive anchors cannot be ensured by establishing a minimum concrete compressive strength at the time of installation in early-age concrete. Therefore, a minimum concrete age of 21 days at the time of adhesive anchor installation was adopted.

 

[BridgeSmith]

Don't most adhesive systems only require 1/8" larger hole?

The adhesive anchors we spec for bridge bearing anchor bolts state the maximum hole size is 1/8" larger than the anchor rod.

We no longer allow adhesive anchors for tension applications - too many screw-ups causing to many headaches for us. If they don't get the holes cleaned and dried really well, the adhesive doesn't bond to the concrete, or if they don't get the 2 parts of the adhesive mixed well, it doesn't harden. Either way, the first time it sees tension, they just pop right out. On bridge girders that need to be held down until construction is complete, and the girders are locked down by the reinforced concrete, it's a PITA to correct, but at least we know, and it's corrected during construction. On a PEMB, it might go unnoticed until there was a really strong wind, and at that point, it could be catastrophic.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[dold]

The only post-installed anchors that get used are for repairs when the concrete contractor messes up the location or projection of the anchor bolts.

Luckily contractors always nail the location on the second, third, fourth, and fifth go-arounds...(sorry about this picture of a computer screen...coworker showed it to me and I had to squirrel it away for show and tell)

 

[Ingenuity]

Thanks all for your replies. Good information. Seems that post-installed anchor rods are indeed the exception for PEMB.

Seems this owner/developer is also the contractor - and possibly the steel erector too. I am going to drop by the project site occasionally and see how they take care of the frame installation and specifically the anchor rod install.

 

[RWW0002]

@Sandwich - I am not sure most of the PEMB guys around here know whether the anchor rods are being post-installed or not. In my experience, except for cases of mis-set anchors, post-installed anchors are still being set prior to PEMB erection. - so there is little indication that they have been epoxy-set in less you are looking closely.

I do not see contractors wanting to wait until the PEMB is erected, but just want to drill and epoxy into the slab or foundation element for ease of layout (I think the advantage is having the slab surface for reference - they have a bit easier time locating and marking anchor locations with the slab in place). It also keeps them from having to suspend a template and anchors over a monolithic slab pour - which also has its challenges at times.

@ Ingenuity - I do not think that eposy-set anchor rods are the exception around here... I have contractors request it often enough to say that it must be fairly common. I have allowed epoxy installations at times, depending on the load magnitude, but force anchor spacing and concrete age restrictions. Based on the response I get out of contractors I get the idea that other engineers are allowing epoxy-set anchors without exception (but you can't always trust a contractor when they say "but ___ always lets me do this..." Ha!)

 

[SandwichEngine]

RWW0002,

I agree that most PEMB engineers wouldn't know how the anchors are being set, other than referencing the structural foundation drawings. My case is different because my company also erects the PEMB structure so we touch the project until substantial completion, which most PEMB companies don't. We also fabricate and sell the anchor bolts on almost every job and we've never even had the request for those to be post installed. So I consider myself to have good enough knowledge to speak to what's common, at least in my region.

That being said, your idea of setting the epoxy anchors prior to the PEMB erection to have the slab to reference does make sense. That would be a much better work flow than the one I speculated on. Mis-poured anchor bolts are the most common PEMB erection problem and probably impact 20% of structures. I could see this method helping if you could get the anchors installed correctly and if they're actually good for the loading.