AISC Standard vs. Oversized Holes - Bearing Connections 2

[Nor Cal SE]

I've often had confusion over an issue, and could likely contact AISC directly, but would be interested to get input from the experience of those on this forum. This regards steel bolt hole size. I've had many projects where the contractor needs to fasten a steel plate to an existing concrete surface with expansion or adhesive anchors, and they constantly request "somewhat large" hole sizes so they can use the steel plate as a template and drill into the concrete without binding on the steel.

I am familiar with AISC Table J3.3's hole size provisions. My confusion lies in two adjacent statements. The first says standard or short-slotted holes shall be provided unless long-slotted or oversized holes are approved by EOR. In the very next paragraph the spec says oversized holes are permitted in slip-critical but not bearing type connections. In my view these paragraphs seem to contradict, as the first paragraph seems to give the EOR the judgment call for oversize, or is it only giving EOR the judgment call for slip-critical only? (i.e. strict prohibition on oversize for bearing-type?)

I've also read Section 3.3 of RCSC and its commentaries. The verbiage seems mostly consistent with the AISC spec but is more expansive. The result is that I'm still unclear on how much is left to the EOR's judgment and approval without violating AISC/RCSC.

 

[TehMightyEngineer]

It's saying you need to design the connections as slip critical to use oversize holes. Slip critical is VERY difficult to realistically achieve with post-installed anchor bolts in my opinion.

A better way to do it is provide oversized holes and then weld a plate washer of sufficient thickness and material strength to provide a bearing surface once the bolts are located (doubly provides the washer needed to bridge the oversized holes).

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[WinelandV]

A better way to do it is provide oversized holes and then weld a plate washer of sufficient thickness and material strength to provide a bearing surface once the bolts are located (doubly provides the washer needed to bridge the oversized holes).

You should make this a standard in your office. There's just no other REASONABLE way to do it. I can't imagine trying to preload expansion and epoxy anchors enough for a slip critical connection.

 

[sandman21]

Pretension loads are to high to get post-installed to work, not to mention other issues. Why weld on the plate when its easier and cheaper for the contractor to provide a a template or just use the plate and mark the holes.

 

[wannabeSE]

EOR's judgement can trump hole size in specifications. See Steel Construction Manual Table 14-2 for maximum recommended hole sizes in typical column baseplates. These hole size are significantly larger than the specification allows.

When the EOR goes outside the specification, they should have some reasoning and justification. If the bolt group has significant force in shear, it can be hard to justify oversized holes without weld washers (or similar).

 

[nutte]

AISC Specification section J3 deals with high-strength bolted connections between two steel members. It does not apply to connections between steel and post-installed or cast-in-place anchor rods. That's why AISC calls them anchor "rods" and not anchor "bolts." Table 14-2 doesn't violate the specification - the specification does not apply to those holes.

 

[TehMightyEngineer]

If OP is not aware AISC Design Guide 1 is a fantastic resource for baseplate design.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[cliff234]

With regard to post-installed anchors loaded in shear, I prefer using expansion bolts over adhesive anchors, because with expansion bolts you can use a drill bit size equal to the anchor diameter, and you can then use the plate as a template for drilling the holes. There’s no need for oversized holes with expansion bolts. Adhesive anchors however require a drill bit larger than the bolt diameter – and this requires oversized holes in the plate if you want to use it as a template – which creates problems if you are loading the bolts in shear. The issue of oversized holes and their use with high strength bolts in structural steel connections is completely different. High strength bolted connections with oversized holes loaded in shear must use pre-tensioned high strength bolts, and the connections must have faying surfaces prepared such that they will resist slip (Class "A" or "B" faying surfaces as defined in the AISC Specification). You will not be able to pre-tension post-installed anchors enough to replicate the slip resistance provided by a pre-tensioned A325 bolt. ¾” A325 bolts require a pretension of 28 kips to provide slip resistance. You will not be able to pretension ¾” expansion bolts or adhesive anchors to 28k.

 

[klaus.]

Oversized holes is a never ending story
Oversized holes are often needed to accommodate tolerances.
Welding the washer is one option....but everybody that knows the cost for site welding in USA knows that this can get expensive
It should be up to the engineer to proof the connection and not up to the code ...but often the ERO wants the code to be followed

 

[Nor Cal SE]

Nutte, does your opinion remain the same for a non-gravity-column situation? For example, a steel plate with 4 horizontal adhesive anchors into a concrete wall, and the plate supports a steel gravity beam for a platform/similar. Therefore, the shear loads on the anchors are constant, albeit moderate.

WannabeSE, I understand the thought to come up with some justification, but here is a common scenario: a mechanical/electrical equipment with pre-set holes in its base. Let's say the shear and tension loads due to wind/seismic are pretty low (in the hundreds, not the thousands), and the preset holes are 5/8"-diameter. The natural anchor choice is post-installed 1/2"-diameter. Almost all the time the steel anchor failure mode is not close to capacity with the concrete breakout or pullout invariably governing. But according to J3.3, 5/8"-diameter hole would be oversized for 1/2"-diameter anchor. Does this mean the anchor choice is not allowed per AISC without the use of welded washers to provide fixity? Every general contractor will gripe non-stop to the owner until we, as EOR, are deemed too conservative, rigid, and ultimately not worth using. Of course, our job is to provide structural-sound designs, not make contractors happy.

 

[nutte]

Nor Cal SE, the AISC Specification does not apply to either of the connections you describe. Good engineering judgment and sound connections must be provided, of course. The AISC provisions may indeed be well and good, but they would be recommendations, not requirements, for these cases. "Not allowed per AISC" does not come into play here.