Steel RSJs advice please

[Jude3003]

Hello.

We are currently self managing our 2 storey house extension  (1930s semi).  We have had calculations done for the steel by a structural engineer but the builder thinks that the calculations are wrong. To me they are a bunch of numbers and I really don't have a clue what it means. Basically we are knocking down 2 internal walls. 1 is in the same direction of the joists and has no wall directly above it. 1 is load bearing. Then we have the external wall where the SE has said we only need 1 steel beam butting in even though it's a double skim wall. Both architect and builder say we need 2 on this wall. I feel as though we've paid £250 for SE and he's a bit offended that we've questioned it but I don't know who to trust? Any advice welcome please. 

[Ian]

I suspect that the concern from your architect and builder is simply that the two masonry leafs of your external cavity wall each need a support and the single beam that your engineer has specified is not wide enough on the top flange to provide a support to each of the two leafs.

 

Typically each leaf of the external wall will be 100mm thick and they will be spaced apart by the width of the cavity with steel cavity ties so that the wall acts as a single structural unit. Typically these cavity walls are between 250mm and 300mm wide.

 

If your engineer has specified a single I beam it would mean that the top flange of your steel beam would need to be 250-300mm wide to match the width of your cavity wall. I think your architect and builder are suggesting that two separate beams would be a more practical solution  - ie two beams, each 100mm wide for the 2 leafs of the cavity wall.

Edited October 24, 2017 by Ian

[Ian]

Try looking on the Catnic website. Here's one of their standard cavity wall lintels. It's designed for a 250mm cavity wall (you need to check the width of your own wall) and is in the "extreme duty" category (you may not need something this heavy duty).

 

https://catnic.com/products/cxl240

 

Edited October 24, 2017 by Ian

[Jude3003]

Thank you Ian that does make sense now. Do you have any advice as to whether we need to have steel put in the wall which runs the same way as the joists and not load bearing? Again SE says yes but builder is adamant no. 

[PeterW]

1 hour ago, Jude3003 said:

Thank you Ian that does make sense now. Do you have any advice as to weather we need to have steel put in the wall which runs the same way as the joists and not load bearing? Again SE says yes but builder is adamant no. 

 

Is it supporting anything...? If not, you don't need a steel....

[Jude3003]

23 minutes ago, PeterW said:

 

Is it supporting anything...? If not, you don't need a steel....

No, directly above it is floor only then about a metre away there is a stud wall, but the SE has calculated for a steel? This is why I'm confused - I would hope the SE knows what he was talking about but I'm beginning to think otherwise.

[Ian]

16 hours ago, Jude3003 said:

No, directly above it is floor only then about a metre away there is a stud wall, but the SE has calculated for a steel? This is why I'm confused - I would hope the SE knows what he was talking about but I'm beginning to think otherwise.

I agree with Peter.

In the situation you described with the floor joists running parallel to the wall  you would normally only need a steel beam if the wall carried on up through the first floor.

 

Possible exceptions - the existing ground floor wall will be providing restraint (extra stiffness) to the other ground floor walls that intersect with it. When removing a wall like that sometimes its necessary to provide something to replace the restraint that you lose when removing the wall. Usually its done by leaving a small wall stub at the intersections but occasionally I've also seen engineers provide this restraint via the use of extra steel.

Edited October 25, 2017 by Ian