Slip Critical w/o Bearing? 6

[KootK]

The usual process with SC bolts is to design such that the service loads don't slip and the factored loads can be resisted in common bearing if necessary.

If the bearing mechanism is simply unavailable to you for some reason, do you have the option of designing your connection as slip critical for the factored loads? My gut tells me no but I figured I'd ask around before I abandon the idea...

 

[slickdeals]

The AISC tables provide you design values where slip is either a serviceability function or a strength function. I am unaware though of a way out of not designing for bearing.
Look in AISC 7-24 through 7-27

 

[kelowna]

Interesting question. I guess that as long as the slip resistance is less than the factored ULS load you should be OK, although you might want to increase the level of inspection on the bolt tensioning.

I had a quick look at the S6 code and it is silent on that, but I seem to remember that the Eurocode gave you the posibility to design connections that way.

 

[nutte]

Like Slickdeals said, slip critical connections can be designed such that slip is a strength consideration, and thus subject to factored loads. As you would expect, the capacity of the bolt is less for strength than for serviceability.

I'm curious, how can you have a connection where the bearing mechanism is unavailable?

 

[KootK]

Thanks guys. As Kelowna mentioned, the Canadian code is silent on ULS slip critical design.

I have a galvanized, existing HSS post that needs to have a fall arrest bracket fastened to it. The fabricator has detailed two 10 mm plates sandwiching the post. Four bolts connect the two plates. Unfortunately, the bolts are located to the sides of the post rather than passing through it.

I was hoping to justify the connection through SC friction. I'm nervous though. Some poor bastard WILL be hanging off this thing to wash windows etc.

Interestingly, the coefficient of friction is better for a prepped galvanized surface than it is for bare steel.

 

[msquared48]

Would it be possible to install a single knife plate through the column and weld it off instead, then you avoid the problem.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[KootK]

That would work just fine MS, were it not for the fact that the post is already in place.

 

[Lion06]

If you have access to the 13th edition manual from AISC, the values against slip at strength level loads are in there. If you don't have the manual, but have access to the spec you should be able to determine the values.

Can you use AISC for this one design issue? Or can you just design the service level slip resistance (from your code) to be greater than the factored loads (and maybe provide an additional safety factor on it)?

 

[paddingtongreen]

KootenayKid, I'm confused, this has nothing to do with the rules for slip critical connections, it is about clamping two plates to an HSS. Slip critical is about having the plies in contact and uses specific high strength bolts (A325 and A490 here in the States) that are not, at least I think they are not, candidates for your usage. You will have to use another high strength galvanizable material, I don't remember if there is a version of A193 that can be galvanized.

You have two plates being clamped to an HSS with a gap between the bolted plies, I don't doubt that it will work, the zinc is soft and the steel will bite for a mechanical bond. The plates will bend a little, depending on the distance between the side of the HSS and the bolt, so most of the contact force will be at the corners of the HSS. Is there any chance it will crush the HSS?

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[Lion06]

Can you post a sketch? I'm having a hard time seeing what the issue is.

 

[Stillerz]

Can you post a sketch of this?
If I understand this correctly, then I am with Paddington....not your typical SC connection.

 

[iponom]

The way I see it is that, when you design for slip you use bolt values which have nothing to do with bearing. That is you are designing for "friction". If the bolt was to slip, it would need something to bear on (e.g. a plate, angle, web) So the value of the bolt in bearing would be higher, and therefore your calculations would show a different bolt value for slip and bearing.

 

[msquared48]

The kerf plate can be installed on site with the column in place.

But another solution might be to weld two bearing plates to the side of the column. Heck if you do that, why not weld the two plates to the column in the first place?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[KootK]

StructuralEIT / Stillerz: the details are attached to this post.

StructuralEIT: I do have the AISC 13th. I'm comfortable with the process of designing a SC connection for ULS loads. I'm just not sure if the intent of the CAN code is to preclude solutions that rely soley on friction mechanisms, as a philosophical matter. I take it that you believe that it would be okay using AISC design philosophy?

Paddington: I agree, this is not typical SC. However, many of the same principles apply. The HSS may crush AND the plates may fail in bending. Both will have to be checked. On the plus side, the loads are very small. I hadn't thought of the bolt concerns that you mentioned. Can A325 / A490 not be galvanized? Could I go with SS bolts perhaps?

MSquared: To install a through plate, they would have to flame cut holes for it through the HSS. Wouldn't that be ticky on site? I'm leaning towards a welded solution, despite the associated galvanizing headaches. The fabricator galvanized the posts ahead of the brackets. They then didn't want to compromise the post galvanizing with the welding. That's why it wasn't welded in the first place.

 

[msquared48]

There are procedures to galvanize after welding. I would seriously consider welding and shoring temporarily as required to accomplish that.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[paddingtongreen]

KootenayKid, the bolts shown on the detail are A307, a low strength bolt. You can get galvanized A325 bolts but I forget the designation, I don't know if you can get them as long as you need, their purpose is to connect plies in contact, not to bridge gaps. Check the sizes in the specification.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[BAretired]

You cannot have a slip-critical connection with the detail shown. You must have solid steel between bolt head and nut in order to develop the necessary tension in the bolt.

Either use welding as suggested above or use bolts in bearing. This can be done by drilling holes on site and reaming if necessary.

BA

 

[ToadJones]

There is no reason they would have to flame cut the slots.
They could use a mag-drill and cut-off wheels to make the slots. Cold galvanizing could be used after welding. There are procedures for welding galvanizing safely.

 

[paddingtongreen]

While welding, slotting and through bolting are much better details, the idea of clamping to the post is not off the wall, we successfully use pipe clamps all the time, same principle here. It will surely work on galvanized material, if the clamp tries to move, the zinc will gall; zinc galling is what makes it is so important to lubricate galvanized threads.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[KootK]

Toad: cold galvanizing = zinc rich paint or metalizing?

BA: I agree, this is not textbook SC. Similar principles still apply however. There are more things to check here of course. Most importantly, the bolt tension must be verifiable as turn-of-the-nut will not apply here.

Paddy: what is zinc galling and why dooes it necessitate lubricating galvanized threads?

As a general question: When a bolt is galvanized, what measures must be taken to ensure good fit with the nut?