Steel beam connection question

[several]

When a horizontal steal beam meets a vertical steel post with a bolted connection similar to this:

http://studio-tm.com/constructionbl.../03/steel+beam-to-steel+column-connection.jpg

If there is a couple mm play in the bolt holes, does the horizontal beam have to sit tight down on the bolt?

As if to say, if the bolt is tightened while the horizontal steel it push up, over time/load will the beam work it's way down so it's tight down on the bolt?

I have a scenario where, a bunch of joists have been fixed to a horizontal steal beam while it was bolted to the vertical steel post, NOT tight down on the bolt.

 

[^woody^]

The bolts will have to be loose for the beam to slip. In which case there would be an even bigger problem

In real life, if it did move down 1mm, is this going to be a major problem

Again in real life, how does the steel erector actually centre the bolt to 1mm accuracy when aligning and tightening the bolt?

 

[tony1851]

When the fabricator drills holes for the bolts, he drills them oversize to allow for slight inaccuracies, and to make it easy for the erector to fix. For bolts up to 24mm, the holes are 2mm bigger than the bolt diameter.
If your bolts are ordinary-grade bolts (ie not friction-grip bolts)
the steel beam will slip that small amount no matter how tight the bolts are.
In that case, the steel will be bearing on the bolt, but your SE will have checked the bearing capacity of the bolts, taking into account the load the beam is carrying.
Its not a problem in simple connections such as yours.

 

[RonnyRaygun]

In that case, the steel will be bearing on the bolt, but your SE will have checked the bearing capacity of the bolts, taking into account the load the beam is carrying.

I've often wondered, if there are two bolts in a connection (on the vertical axis), unless all of the holes are drilled to exactly the same dimensions, only one of the bolts will be bearing, not both. How does this work if the connection is highly stressed? In theory it should fail...

My guess is that there is enough friction capacity in the connection that the actual bearing stress is reduced, despite the connection not being designed as a friction grip connection.

 

[several]

I undid the bolts and checked the play, with all bolts slackened(3 of them), the beam was only tight down on 1 bolt.

 

[RonnyRaygun]

I undid the bolts and checked the play, with all bolts slackened(3 of them), the beam was only tight down on 1 bolt.

As I suspected. It's unlikely all bolts will be bearing. However, this is how steelwork connections are designed; they rarely fail and I wouldn't expect yours to.
There are safety factors in the loadings and the materials, it's unlikely the connection is stressed to near its limit, and as I said before, I believe that if the bolts are tightened sufficiently, some of the load will be taken by the friction between the connecting surfaces.

 

[tony1851]

Ronny; an interesting point. FWIW, here's my take on the situation;

Assume you have a 2-bolt connection, and each bolt-joint has a maximum capacity of 100kN. One bolt (A) is a tight fit, with barely enough tolerance, and the other bolt (B) is in a vertical slotted hole. If the pair are loaded to 101kN, then bolt A would fail, and all the load would be on B, which would also fail.
There are 3 ways bolt A could fail; 1. by excessive shear on the bolt; 2. by excessive bearing on the bolt (where the plates can cut through the bolt if the bolt is softer than the plates) and 3, excessive bearing on the plates (where the bolt tears through the plates).
This is of course an extreme case. Bear in mind that most beam/column connections have a minimum 4 bolts, and the tolerances are unlikely to differ that much. The Code states; 'the ductility of steel assists in the distribution of forces generated within a joint'.
The Code therefore allows that all bolts take equal forces (in non-rigid connections). My guess is that you would get some very localized crushing in an over-tight bolt, and the downward movement caused would quickly bring the other bolts into play, thus equalizing the forces. And of course, as you state, there will be a little friction due to the clamping effect, though that can't theoretically be allowed for, except with HSFGs.
I'll have to have a lie down now because my brain hurts

 

[tony1851]

Yes, I know 2-bolt connections are horiizontal - as per sketch attached; I was merely making a theoretical point about the loading.

Untitled

• tony1851
• 25 Nov 2012
• 1

 

[tony1851]

So what do the loading tables say about a vertical slot. Let alone a two bolt horizontal slot.

I wonder if you're missing the point I was trying to get across?

 

[RonnyRaygun]

The tolerance is there for the deformity in the beam.

No, it's there for fabrication and erection inaccuracies.

Personally, I don't believe that friction has anything to do with it. In a different designed connection maybe. But not in this instance.

Friction will definitely play a part. It's just an unknown quantity in a "simple" connection. Friction doesn't disappear just because it hasn't been designed for.

So what do the loading tables say about a vertical slot. Let alone a two bolt horizontal slot.

You should only use slotted holes in High strength friction grip connections. The load capacity is reduced for a vertically slotted hole compared with a standard clearance hole, and must be allowed for in the design.
Although Tony was only trying to show my point that theoretically both bolts wouldn't be bearing, not that a slotted hole should be used in such a connection.

 

[RonnyRaygun]

OK Harger, I don't like to give inaccurate information, so it's important that the OP and any future readers of this thread believe me and not you.
With that in mind, please see below.

Your first quote is inaccurate. But believed in the trade.

Assuming you are referring to my assertion that clearance holes allow for fabrication and erection inaccuracies, read this document if interested

and as to hsfg bolts, I have never come across them. With a slot.

Scoll down to page 674 of the Steel Designers' Manual and it will show that long or short slotted holes are perfectly acceptible in HSFG connections.

 

[tony1851]

2 - 0

 

[RonnyRaygun]

2 - 0

Now shut up!