133mm wide UB on 100mm wall?

[madammim]

Hi, I'm new to the forum and would really appreciate some advice. We are building our extension ourselves, due to cost - please excuse any non technical terms - I am learning as we go!

We have just installed padstones for a UB and have come across an issue.

Our walls are 100mm concrete block, 100mm cavity and 100mm thermalite blocks. We have a large window in the kitchen above which will sit a 203x133 UB to support a larger UB which will sit on it to form a T shape. (The larger UB will support the joists for the second floor). The issue we have is that the 203x133 UB which sits above the window is 33mm wider that the inside skin it sits above. Cosmetically it will protrude into the room but that aside, I am concerned that its not fully supported as the wall it sits on is 100mm not 133mm! I hope that makes sense - I have tried ringing our architect who specified all beams on the drawings but cant get any answer. I shall stress about it all weekend so thought I would come here to see if anyone could offer any advice. Thanks for reading.

 

[tony1851]

Panic not.

It doesn't matter if the flange of the beam overlaps the wall, as long as the overlap is more-or-less the same both sides. It is purely a cosmetic issue, which is easily sorted out at the plastering stage.

The vast majority of the load the beam is carrying is transmitted down to the wall through the vertical web of the beam. The fact that some of the flange is unsupported makes absolutely no difference for domestic loadings. As long as your architect/SE has specified the correct size of padstone for the load, it will be fine.

 

[madammim]

Hi Tony, thanks for the reply. I wont be able to get the overhang equal each side as there is a catnic lintel alongside the UB thereby preventing us from moving the UB over so that it overhangs 16.5mm into the cavity and 16.5mm into the room. Someone has mentioned we could have used a Z lintel with the UB but have googled and cant find out what this is! Can anyone enlighten me?! I am assuming the total overhang of 33m all on one side is not good?

 

[tony1851]

I am assuming the total overhang of 33m all on one side is not good?

It depends.

If all the overhang of the flange is on one side, it follows that the web of the beam will not be over the centreline of the wall, in the vertical plane.

This means that the loading on the inner skin will be off centre, and this can induce a bending moment in the wall, which is not ideal for blockwork. However, if the loads are not too great (which they probably won't be in a domestic situation) it won't be a problem, as blockwork can take a limited amount of bending stress.

Whether or not you would have a problem with this depends onvarious factors, including the loading, the height of the wall, whether it is stiffened by return walls etc, so it's not possible to say on a forum. But if you are concerned about this, have a word with your architect/SE. Rather than 'phone, send an email, then it's on record.

 

[madammim]

Hi Tony, I think the 33mm overhang will be unacceptable. I'm pretty sure the catnic will have to go in order to allow overhang into cavity. I will speak to architect tomorrow. Thanks very much for your help, it is really appreciated.