RSJ questions / reassurance

[alibrown18]

Hello there, long time lurker first time poster. But I've found these forums really useful as we embark on our extension project.

We're taking out a 5.3m span of external wall (brick and block), bearing the first storey above and roof, and 2.7m internal wall carrying the joists (which run across the house) to make for an open plan with a new rear extension, as you can see in the pic below. I was just looking for some reassurance or pointers in relation to the SE's design and calcs for the UBs. I'm going to phone them for my peace of mind but wondered what sorts of questions you might be asking in a situation like this?

My thoughts were:

- I understand the standard bearing on a padstone is 150mm so why would they have gone for 140mm (even if the calcs say this is okay)?
- I've seen sometimes people say that an SE has said foundations require reinforcement, but often on this forum people post that's unnecessary. There is about 1.2m brickwork below ground, on top of a raft foundation (not sure how deep as with BC agreement the builder stopped digging the new founds at 1.4m). Given the point load spreads out is it reasonable to assume that by the time it reaches the foundation this will be spread?
- The SE hasn't specified rebuilding the remaining walls in engineering brick. It's a 1970s house, facing brick some sort of Tudor London with internal block. Is it reasonable to assume that it can take the compression?

Is there anything else that I should be concerned/questioning about? We have a competent builder who has seen and done it all before and tells me to trust the process, but understands I'd like peace of mind.

 

[cdbe]

Building control will have their own engineer to check all your engineers calculations. You could employ a third engineer to check again, or a fourth.

Most steel issues on here seem to centre around crap builders - the quality of the masonry below the pads and above the steel - primarily the propping and packing. Your long steel will bend in the middle (hopefully within the designed limits) and the masonry above will settle and you'll want to minimise the effects of this.

 

[alibrown18]

Thanks I appreciate the reminder there's the BC engineer as a backstop.

Our builder is experienced and seems diligent about following plans and checking in with building control, so I'm hoping that won't be the issue. It's more what is outside his control, like the loading on the existing masonry and foundations, that bothers me.

On the big span, the max deflection assumed by the SE is 17mm (tolerance is 18mm). If it does do that, I guess that could muck with some of the plastering above, but as we're going to need needles and make a mess of that anyway, this is less of a major concern.

 

[George_engineer]

What is the plan to ensure lateral stability? Normally masonry returns of 665mm are kept, else a 'goalpost' or 'box' frame are introduced.

This is the sort of thing building control should pick up on but could have a significant effect on the design so sooner it is considered the better.

I'd also like them to use needles to install the beams rather than strong boys.

- I understand the standard bearing on a padstone is 150mm so why would they have gone for 140mm (even if the calcs say this is okay)?

That's for lintels which are designed to be partly composite with masonry. A steel beam can be much shorter bearing lengths and the bearing lengths are controlled by the masonry bearing stresses. If it's been checked then I wouldn't be concerned

- I've seen sometimes people say that an SE has said foundations require reinforcement, but often on this forum people post that's unnecessary. There is about 1.2m brickwork below ground, on top of a raft foundation (not sure how deep as with BC agreement the builder stopped digging the new founds at 1.4m). Given the point load spreads out is it reasonable to assume that by the time it reaches the foundation this will be spread?

Yes. Foundation reinforcement typically is widening or deepening but for loading at ceiling level there is only a small increase in soil stresses by the time you reach foundation formation level, unless it is an isolated pier/column.

- The SE hasn't specified rebuilding the remaining walls in engineering brick. It's a 1970s house, facing brick some sort of Tudor London with internal block. Is it reasonable to assume that it can take the compression?

Again, so long as the loads have been checked I wouldn't worry. A padstone is usually used to resduce stresses down to something the existing masonry can take. The builder should also inspect the masonry to ensure it is sound and if it isn't, rebuild as needed.

 

[alibrown18]

This is a superbly helpful reply, George, and just the sort of things I needed to hear or think about. Especially on the foundations and bearing length.

On lateral stability, I'd assumed this wouldn't be an issue because we have the remaining 1/3 of the house continuing to the left of the beam, and the wall of the adjoining house continuous to the right (which doesn't show on the bit of the plan I included). But I will certainly now include this as one of my questions.

Builder is going to check the blockwork today - the external brick and mortar seems in good condition. Says the 1970s bricks are generally better than today's version.

Builder already advised that we'd need needles for the main beam install, so your confirmation these would be required offer futher reassurance he knows his stuff.

 

[alibrown18]

Update that building control have queried the calcs. They want to be assured the different loading for the inner beam (which takes the original roof) is sufficient. SE says that as a diaphragm is being used to connect the beams together they can effectively be treated as one. Maybe fair enough.

But a bigger concern to me is BC also queried 'a design check should be submitted for the supporting masonry piers'. The SE replied saying this is already done in the attached page. I am of course a novice but as the SE hasn't done a site visit to check the masonry (the external leaf is London Tudor and internal seems to be 3.6N block) is this sufficient - in general, not just to get past BC?

And also a concern the internal wall remaining on those piers to left and right is only going to be about 30-35 cm. Albeit we don't have an issue with wind and lateral load as the house continues to the left and is joined to neighbour on the right this feels to me a very thin bit of wall on which to rest half a house!

 

[Dereekoo]

Update that building control have queried the calcs. They want to be assured the different loading for the inner beam (which takes the original roof) is sufficient. SE says that as a diaphragm is being used to connect the beams together they can effectively be treated as one. Maybe fair enough.

What does he mean by a diaphragm?, if he means some tubes between the the webs with a threaded rod through the middle which is tightened up to provide some frictional resistance that will magically transfer bending moments between one beam and the other well good luck with that hypothesis

And as an aside where does the 3.01 FoS come from in the padstone calce?

 

[alibrown18]

Thanks here's the diaphragm design. No idea if this is better or worse than you describe. I'm just the client here wanting to double check. I'll be getting back to building control and the SE on Monday.

I can't see any reference to 3.01 elsewhere in the document so no idea what that number is, but 1.5 and 1.3 factor of safety seem to have been applied in the loading calcs.

 

[Dereekoo]

Thanks here's the diaphragm design. No idea if this is better or worse than you describe. I'm just the client here wanting to double check. I'll be getting back to building control and the SE on Monday.

I can't see any reference to 3.01 elsewhere in the document so no idea what that number is, but 1.5 and 1.3 factor of safety seem to have been applied in the loading calcs.

That is a diaphragm!!! can't fault the SEs logic . 1.3 and 1.5 standard stuff, 3.01may be latest standard but .01?????

 

[^woody^]

If the engineer is a proper engineer, then you can rely on his design. Or rely on his insurance.

The only issue with some engineers is that they over design or don't design economically, but there is typically not an issue with the competency.

Avoid trying to second guess or question an engineer unless you have the knowledge to do so.

 

[alibrown18]

You're quite right in principle I should trust the experts - just quite hard in practice when half your house is on the line.

Especially walking into the rooms each day and imagining it perched on 30cm wall each side...

 

[George_engineer]

Update that building control have queried the calcs. They want to be assured the different loading for the inner beam (which takes the original roof) is sufficient. SE says that as a diaphragm is being used to connect the beams together they can effectively be treated as one. Maybe fair enough.

But a bigger concern to me is BC also queried 'a design check should be submitted for the supporting masonry piers'. The SE replied saying this is already done in the attached page. I am of course a novice but as the SE hasn't done a site visit to check the masonry (the external leaf is London Tudor and internal seems to be 3.6N block) is this sufficient - in general, not just to get past BC?

And also a concern the internal wall remaining on those piers to left and right is only going to be about 30-35 cm. Albeit we don't have an issue with wind and lateral load as the house continues to the left and is joined to neighbour on the right this feels to me a very thin bit of wall on which to rest half a house!

They've checked the masonry bearing but not the supporting wall panel itself. It is usually a pointless check as you don't need much wall to support vertical loads so it rarely controls the design, but that is what they're referring to. I would normally include as part of the bearing design but that's just how my software works.

The alternative meaning is going back to lateral stability. I didn't reply before but you can't rely on neighbouring properties for lateral stability because you've no control over their house and they may do similar work in future. I'd expect to include a statement on how stability is maintained, but I take it the engineer is satisfied with the wall off to the left.

 

[alibrown18]

I didn't manage to get through today. Understand the point on lateral stability. I wonder if BC have taken a commonsense view that because the fence line angles 30 degrees away from our house into their garden, it wouldn't be realistic to expect next door to do a similar project as they'd need to start it around c. 3m from the boundary to make it feasible in the first place. Or perhaps simply an oversight...