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Bolted Connection Design in a Wideflange Steel Column Supporting a Large Free-Standing Sign

Bolted Connection Design in a Wideflange Steel Column Supporting a Large Free-Standing Sign

Bolted Connection Design in a Wideflange Steel Column Supporting a Large Free-Standing Sign

(OP)
I am designing the column splices in the columns supporting a large free-standing sign. See attached sketch. All holes will be standard, not over-sized or slotted. Do the flange bolts need to be designed as slip-critical? If so, why? Thanks in advance.

RE: Bolted Connection Design in a Wideflange Steel Column Supporting a Large Free-Standing Sign

In one direction this is cantilever construction. Since you're presumably exposed to wind in both directions, you're going to get a reasonable amount of load reversal. Plus, with deflection the direction of your p-delta and other effects can switch on you. I'd be concerned.

In Canada, at least, there's no *requirement* that you use slip critical in this situation. My read is that you do at least need to pretension your bolts, but our code doesn't touch this exact situation. The assumption that slip isn't significant is based on conventionally framed structures where the majority of the load is vertical to the structure and you have infrequent load reversals. In that case, your initial slip happens and then likely never happens again for the life of the structure. This isn't the case in your structure.

Generally I'd avoid splicing a vertical cantilever if at all possible. If I couldn't avoid it, I'd weld it or use a slip critical connection.

RE: Bolted Connection Design in a Wideflange Steel Column Supporting a Large Free-Standing Sign

Sekwahrovert:
I agree with what TLHS has suggested. Based on your design needs, you have to determine what type of connection you need. This is a highly dynamic connection and loading condition, so be careful. If you are going to do what you propose, why not place the connection up inside of the sign, where the moment will be smaller? This will also keep it out of the weather and make shipping the sign easier without those stubs of the posts sticking out of the bottom of the sign. You might also consider moment end plates on the cantilevered columns and matching base plates up in the sign structure. Usually you can take the faces off parts of the sign so that you can work inside the sign to make these connections, or work on lighting and electrical, etc.

RE: Bolted Connection Design in a Wideflange Steel Column Supporting a Large Free-Standing Sign

(OP)
I recognize the inherent load reversal, however I am planning to use pre-tensioned bolts, not slip-critical bolts. The closest statement I have found in the code that could be interpreted as requiring slip-critical is this, from the 14th Edition AISC manual, section 4.3 of the RCSC section (page 16.2-30): "Slip-critical joints are required in the following applications involving shear or combined shear and tension: (1) Joints that are subject to fatigue load with reversal of the loading direction..." I don't believe we have a fatigue issue here. Fatigue on bolts would require the faying surface to slip repeatedly over many many cycles. With pre-tensioned bolts, the friction between connected parts will be very significant, but not quantifiable since we aren't taking care to prep the faying surfaces. It would take a significant wind event to cause any slip, so I don't believe I am required to use slip-critical. I understand that I need to be careful, and I get that some of you might want to design as slip-critical, but I feel that it would unnecessarily add to the overall cost of the structure. At least I'm not yet convinced otherwise.

Regarding the other issues the dhengr brought up:
- The connection is placed as high as possible already, given the available length of the WF column section. The columns are actually embedded directly into concrete pier foundations, so that's why the splices may look low in my sketch.
- Mating end plates would be a step in the wrong direction if you ask me. In that case, you would have a fatigue concern because the bolts would constantly be loaded and unloaded as wind comes and goes. The way I've sketched it, the flange bolts will never see any shear at all except maybe during the occasional strong wind storm strong enough to cause slip. And I don't have any concern about the nuts tending to loosen like I would have with the bolts in tension.
- Access to the connection will not be a problem. This is a very large sign with multiple catwalk levels inside for maintenance.

I appreciate your comments and would be open to hearing more from you or others.

RE: Bolted Connection Design in a Wideflange Steel Column Supporting a Large Free-Standing Sign

Just curious - did you consider adding inside plates to put the bolts in double shear?

It seems like an easy way to make the connection considerably more robust, regardless of bolt tension.

RE: Bolted Connection Design in a Wideflange Steel Column Supporting a Large Free-Standing Sign

(OP)
JLNJ, that's not a bad idea and could certainly be investigated. I think the problem in this case is that the columns are actually not the same size. They are the same nominal depth, but they already have to add shim plates to make up a slight difference in the actual depth. Plates on the inside would require more shim plates, and I think that would over-complicate the connection in my client's eyes. I'm more interested in others' opinions regarding slip-critical vs. pre-tensioned bolts, but I appreciate the other feedback as well.

RE: Bolted Connection Design in a Wideflange Steel Column Supporting a Large Free-Standing Sign

Sekwahrovert:
Sounds like a really good way to save 67lbs. of column steel and then spend $800 for shim plates and labor. If you can change this part of the design it might be worth it. You will also save something in the way of cutting and handling costs by ordering 300' of one WF size, as opposed to 200' of one size and 100' of another size. This is penny wise and pound foolish design. Some times we, as engineers, have to explain to our clients why one solution is better than another, even though a first glance might suggest otherwise.

RE: Bolted Connection Design in a Wideflange Steel Column Supporting a Large Free-Standing Sign

(OP)
Dhenger,

This is a very experienced client I'm working for--they know what they are doing and know how to save on overall material and labor costs. They are "in" with fabricators and seem to be able to get whatever size they want without paying for custom sizes. They also happen to employ licensed engineers who actual do a preliminary analysis and design, so they actually chose the member sizes. I have verified that they are adequate. I was hired to review and revise their design where necessary.

RE: Bolted Connection Design in a Wideflange Steel Column Supporting a Large Free-Standing Sign

If it's the same 'class' of wide flange, the distance between the inside of the flanges should be the same - so no extra shims inside. Extra flange thickness is added to the outside if you look at the dimensions. Granted, there are manufacturing tolerances for the wide flanges.

RE: Bolted Connection Design in a Wideflange Steel Column Supporting a Large Free-Standing Sign

(OP)
Thanks for pointing that out, azcats. That fact never occurred to me, but it looks like you're right. Makes sense if any W33 (for example) is made from a web that is always the same width. I could actually eliminate any need for shimming if I place all flange plates on the inside. I'd just have to make sure the narrower plates have adequate capacity to resist tensile rupture (along with all of the other checks).

RE: Bolted Connection Design in a Wideflange Steel Column Supporting a Large Free-Standing Sign

The web thicknesses vary within a series, the inside rollers are further apart when rolling the heavier sections.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

RE: Bolted Connection Design in a Wideflange Steel Column Supporting a Large Free-Standing Sign

(OP)
Back to the original question...

JLNJ, dhengr, azcats, and paddingtongreen, do you guys have any thoughts about the flange bolts as shown in my original sketch?

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