Burp holes 1

[JT-1995]

Good morning. We are finalizing a few foundation details, I was curious what folks do to protect against concrete voids under embed plates in flat concrete. As the concrete is placed and the concrete is vibrated are there any measures anyone takes to eliminate poor concrete/steel contact under an embedded flat plate? I have used burp holes through the plate but I have never seen a rule of thumb about size of plate versus need of burp hole. Yesterday I asked a friend experienced with placing concrete and he thought something 2ftsq and over could use a burp hole. In the case I have I would rather skip the burp hole because they tend to make a mess and hinder the column sitting down on the plate. In our case we have a 6" WF welded to the bottom side for the embed anchor, so now I am thinking the WF may increase the necessity of the Burp hole(s). I would appreciate any thoughts. Also the load is not significant, the 6" WF anchor is client preference due to long term fatigue (small vibrations forever) and they have seen nelson studs and welded anchor bars rust and otherwise loose contact with the plate.

 

[GeorgeTheCivilEngineer]

Here we would normally grout the underside of the baseplate. Embedded plates would typically have welded studs and so aren't entirely reliant on direct bearing.

In terms of size of 'burp hole' (love it), then using the minimum spacing between reinforcement depending on size of aggregate would seem like a good rule of thumb. From memory... 25mm for 20mm aggregate concrete.

 

[JT-1995]

Thank you for your thoughts George. That is a good point about the load transfer from the plate into the concrete through the embedded anchors, particularly for low load conditions. I think the client has had bad experience loosing the connection to the anchors, probably from rust generated by water seeping around and under the plate. I think the rather substantial 6" WF will help fight against total loss of connection even if water does get under the plate.
Grout...I will give that some thought too. In our case, anchor bolts thru the plate are frowned on (again small vibrations and loosening bolts over time...loctite has been suggested but denied) so I never considered keeping the 6" WF embed anchor and just letting the plate be above the concrete for the footing pour and then returning to pack 5 star under the base plate. That may be a bit of further overkill but would certainly guarantee solid steel contact with the footing.

I appreciate your help. Cheers.

 

[TLHS]

The base plate design guide from AISC says you should have a grout hole when the shorter size is longer than 2 ft and recommends a 2-3" diameter hole.

The intent is to allow air bubbles and to let you visually confirm that there's contact away from the edges.

I've never thought about it, but the logic holds for embedded plates as well. That dimensioning should still be okay.

 

[JT-1995]

Thanks TLHS, your post helps give some code reference for a decision. I appreciate it.