Bolts in tension - can't use slotted holes, right?

[dem0001]

I've always had it in my head that codes don't allow the use of slots (short nor long) with bolts that are subject to tension. However, in a design I'm working on currently, that notion was challenged and I cannot find it specifically stated in RCSC or AISC steel manual. Am I just blind or have I been wrong all of these years? Thinking about it, it stands to reason it would be ok if not code-prohibited as long as a proper washer thickness and diameter is utilized like on a slip-critical connection where bolts are tensioned.

 

[STpipe]

I don't see any reason why they wouldn't be allowed. To achieve a slip-critical connection (required to transfer shear loads in the direction parallel to the slot), you need to apply a significant preload to the bolt which is only achievable by following the washer requirements in the specifications. Further, there is a method to reduce your effective slip resistance when a tensile load is applied, so since there is no provision against slotted holes in tension, then it shouldn't be an issue.

 

[Once20036]

I think that the RCSC prohibits loads parallel to the slot in snug tight connections.
Think about a column base plate that's part of the lateral system. Those bolts will have tension perpendicular to an oversized hole, and there are no problems there.

 

[dem0001]

Thanks for the replies.

 

[drawoh]

dem0001,

A couple of years ago, I did an analysis on a structure I was designing, and one of the things I checked was shearing of the flange under the washer. A slot significantly reduced the area of shear. I wound up removing slots from my design.

--
JHG

 

[slickdeals]

Slots "transverse" to the load direction is what's stated in the code. You can also use slip-critical connections with oversized, short-slotted and long-slotted holes (hole parallel). See J3.2 of AISC manual.

 

[rb1957]

"can't" is a mighty big word. maybe not even "shouldn't", but maybe "be careful", clearly some loss of tensile strength.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[NL4L1F3]

In a normal hole the bolt is supported under the entire bolthead. with a slotted hole this is not the case. You can check the shear force under the bolthead with the red area in picture below as an indication how strong the slotted hole may be:

 

[human909]

Sorry. In advance for my snarkyness.

so since there is no provision against slotted holes in tension, then it shouldn't be an issue.

Umm... I'm not sure that is how engineering works. Just because there isn't a specific provision in the code saying you shouldn't do something then that doesn't mean there "shouldn't be an issue.

Those bolts will have tension perpendicular to an oversized hole, and there are no problems there.

Well there are in fact problems. Without suitable washers (eg suitably sized and thick plate washers) then you have a loss of tensile capacity and rigidity.
This AISC refernce came up in google. I didn't read the full thing though, but I think it presses home the point.

if the anchor rods are not set in the concrete foundation such that they fit in the
holes of the column base plate, those holes should be slotted, and plate washers should be used to
cover the gaps in the hole (AISC, 2006)

In the code I use plate washers on oversize holes are required. (Though I frequently see it not done in practice.)

 

[STpipe]

human909,

I meant it in the sense that there is no indication that there should be any problems based on the fact that there are no clauses forbidding slotted holes in tension, and there is nothing in the commentary or literature indicating that this type of connection would not perform reliably, provided the washer requirements are met.

Logically, if the standard requires slip-critical connections for slotted holes with shear parallel to the length of the slot, and it also provides a method to account for a reduction in slip-resistance due to an externally applied tension load, then it stands to reason that joints with slotted holes are not forbidden when there is an applied tensile force.

 

[ProgrammingPE]

Sections 6.1.2 and 6.2.6 of RCSC's 2020 Specification for Structural Joints Using High-Strength Bolts require you to use washers that completely cover the hole whenever oversized or slotted holes are used, so you would still get bearing in tension the entire way around the hole.

Local stresses in the area around the bolt holes due to tension is not a typical check for steel connections, but I would suspect that the larger perimeter you get from the washers would increase the tensile capacity to be comparable to a bolt without a washer in a standard hole.

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