Do I need a mid span restrain on beams

[snapfish]

ive just had my SE calc come through for taking out a cavity wall for an extension.
I’ve noticed on the internal UB, he put that I should put wood packers and joist hangers for the upstairs joists. But this is not the case as the joists run parallel with the beam.
My question is, the calcs say I need a mid span restraint between the two beams, is this the case if no joist connect onto the beam causing the beam to rotate?
The beams length is around 4.5m and the beams are 254x102Ub 22 and 28.

 

[^woody^]

Isn't the restraint for the wall and floor not specifically the beam?

Otherwise I don't have a Scooby what you mean.

 

[snapfish]

This is the connection mid span of the beams.
Is it there because he think s the joists connect to the inner UB

 

[tony1851]

The problem here is what is known as 'lateral torsional buckling' (LTB); steel beams can fail in a number of ways and slender beams (ie tall beams with narrow flanges) can fail by buckling sideways long before they reach their full load-bearing capacity.

This can be prevented if the beam is firmly held along its length, eg by floor joists running directly into it, whereby the floor acts as a rigid plate and so stops the beam buckling; in your case, this doesn't apply because the joists run parallel and so give no lateral restraint.

There is a mistaken idea among the building fraternity (and some SEs as well), that bolting the two beams together at intervals is sufficient to provide lateral restraint. If both beams are slender - as yours are - this could be likened to two drunks trying to prop each other up, and the BS 5950 code for steelwork specifcally prohibits this practice.

Personally, I think for that span the two beam specified are a little too slender, and I would have gone for the 254 x 146 beams, which are better at resisting buckling on that span. But you are where you are..... In practice, the factors of safety required by design codes are quite high and it is unlikely that your beams will be called upon to support the loads the SE will have computed. Nevertheless, I should be inclined to raise this issue with your SE and ask him to reassure you that LTB will not be an issue. Failing that, he needs to give you some indication of how the beams could be adequately restrained.

 

[^woody^]

That engineer is a visionary or clueless.

I can't see what the restraint is restraining.

You need something like scaffold tube spacers where the bolts are that are bolting them together, and that's it normally.

 

[snapfish]

Thanks guys.
I will phone him tomorrow and ask.
If I was to mention about putting in two 254 x 147 would this prevent me having to put in this restraint?

 

[tony1851]

PS. Just noticed your drawing after my post; your SE has made an effort, so I would give him 6/10, but he is still in effect trying to prop two slender beams against each other. In this case, at least one of the beams should be effectively tied to some rigid partof the structure - see attached from the BS 5950 design code for steel.

 

[catlad]

Woody I think the SE is asking for the beams to be bolted together using off cuts of a smaller section UB sideways!
acting as spacers.

 

[^woody^]

Woody I think the SE is asking for the beams to be bolted together using off cuts of a smaller section UB sideways!
acting as spacers.

Yeah it seems that way.

I'm confused by the engineer calling this a restraint, when it clearly isn't and I've never heard of an SE calling this arrangement a restraint.

It also does not seem to be some sort of contrived method of preventing buckling, as that is designed into the beam capacity not tacked on.

Joining one beam to another like this always tends to be for a load sharing arrangement, and if that's the case here, it's rather extravagant.

 

[Leofric]

Let us know what the SE said ,presume he now knows which way the first floor joists run , and how lateral restraint is provided to the wall.

 

[snapfish]

I’m after a bit more info before I phone the SE.
I will be installing 3 beams in total, 2 in the cavity wall mentioned in this thread and also another. This one is a 254x146UB31, the plan was to put in at joist level so we can have a flush ceiling below. I’ve just realised the joists are 175mm deep and the wall it will be supporting has a doorway in it. So does this mean I can’t put in above the ceiling due to the top of the beam will be poking above to floorboards in the doorway.
Is there away around this, ie a difference type of beam? The span will be around 4.5m again

 

[Leofric]

Won't there need to be a downstand below the ceiling ? How wide is the wall above ,more than 146mm ?

 

[snapfish]

I didn’t want the beam sticking through the ceiling, if that what a down stand is. The SE knew this.
The wall above is single skin block, which makes this beam even worse.
If I was to mention about a 152x152x 37or44 ub as that’s the beam we put in at my previous house which was slightly smaller span. Are these two similar in weight load? The 152 would fit nicely between the joists.

 

[Leofric]

Well the SE doesn't seem to have done what you asked with a beam sticking up above the floor and wider than the wall above. You will have to ask SE if they can design something that fits within the floor. A 152 x 152 is unlikely to be adequate as it is only 6mm wider than a 254 x 146 and is 102mm less in depth !

 

[tony1851]

Probably on that span, deflection of the beam may be an issue. If that's the case, there aren't many alternative sections available which would give a similar stiffness. A 203 x 203 x 46 would do, but is an unwieldy beast and would still show a little below the ceiling.
There is a smaller size of beam - 152 x 152 x 37, which might do if it had a thick plate welded on to the top flange to give extra stiffness, but you would be adding to the cost, and might still be marginal if deflection was critical.
You may have to go with the beam specified and accept a small downstand in the ceiling.