Anchoring Steel to Concrete Wall - Steel Hole Diameter

[Jerehmy]

If I'm anchoring to steel to a concrete wall in a bearing type connection, as in, I'm using adhesive anchors into concrete and hanging steel from it, must I abide by Table J3.3 and Section 2 in the AISC which states that oversized holes cannot be used in bearing type connections. It seems this is for steel to steel connections, not steel to concrete.

I'm reviewing shop drawings and the contractor wants to use 7/8" diameter holes in the steel and 3/4" diameter adhesive anchors. I feel installation will be a PITA using 13/16" diameter holes.

AISC Section J9 and Design Guide 1 allow oversized holes, even when there are horizontal forces to be resisted. This is of course for column base plates, not hanging steel off a concrete wall.

Is there any guidance on this situation? Thanks!

 

[MrHershey]

You're allowed to transfer shear through anchor rods in column base plates, but have to be careful. DG1 notes that 'considerable slip may occur before the base plate bears against the anchor rods'.

In a similar situation to your questions, we run into this question a decent amount with screw anchors that actually cannot be installed in standard holes because the screw threads generally won't fit. Based on our internal debates and discussions with engineers from at least one of the major anchor manufacturers, we've generally gone ahead and used screw anchors in slightly oversized holes for gravity loads where a healthy portion of the load is dead load (so it's permanent) and we know for sure the anchor will only ever be loaded in one direction. That way even though we're not in a standard hole, it becomes a bearing connection during construction or shortly thereafter as you load it up and things shift slightly to bear on the anchor and then remains that way for the life of the structure.

For lateral loads, we generally try not to rely on screw anchors to resist the load because of the same issue DG1 notes above. So we would try to use either an epoxy or wedge anchor for situations where we'll be dragging wind/seismic load into a wall or may have net uplift due to wind.

I personally also try and use wedge anchors where I can. Based on a bunch of feedback from contractors and erectors, they're generally easier to install and have fewer steps than epoxy. Sometimes need the extra strength you get from epoxy and can't get away from it though.

 

[Shu Jiang PEng SE]

Normally, prepare oversize hole diameter of 1-9/16" in base plate for 7/8" diameter bolt, and field weld 2.5x2.5x5/16" structural washer with standard hole for the bolt to base plate.

 

[Jerehmy]

MrH - They must be adhesive per the bid drawings. It's external cooling towers. They're supported by wide flange sections. The wide flange sections are support on channels. The flat face of the channels are anchored to a concrete wall. So their definitely can be reversal of forces due to wind.

Shu - I'm familiar with AISC Design Gudie 1, which has those listed in the table. Although these fasteners are anchors, they aren't base plate anchors so I don't think that table applies.

 

[JLNJ]

I would not fret too much over the 7/8" holes slipping into bearing unless you are really up against it capacity-wise.

If 50% of the holes are engaged, does the connection still work? Does your cooling tower care if it experiences another 1/16" of deflection?

 

[hokie66]

Agree with JLNJ. It won't make any difference. I imagine your contractor will want to drill at least some of the holes through the channel, using the channel as a template. Make sure that the required bit size will go through the holes in the channel.