is my structural engineer overly cautious?

[biryani_boy]

for reasons I'd rather not discuss on this forum, we need to make some slight alterations to our plans. long story short, 3.9m single storey rear extension, flat roof with 2 skylights. we want to open up as much of the existing rear, load-bearing wall as possible.

the initial plan involved a boxframe, which could hold the weight of the world.

we want to revert to a simpler plan instead, namely a single steel beam to hold as much of the first floor as possible. width of the house, end to end, is 6.65m, all walls are cavity, 300mm wide, brick in/outside.

structural engineer claims we can only open up 3m as we need at least 1.5m each side due to the existing foundations (strip, ~30cm deep concrete + 6 courses engineering brick) not being strong enough. builder disagrees and says 600mm each side would be more than sufficient.

before I approach another structural engineer, may I just check that it should be possible to open up more than less than half the width of the wall and that this engineer sounds overly cautious?

thanks

 

[RonnyRaygun]

The SE is probably also concerned about lateral stability and doesn't want to remove too much of the masonry as the stability of the building will be compromised.
It's just a beam, right, no columns? If so your SE may be correct, as the load from the beam is spread over a reduced area so the bearing pressure is increased. They'll be trying to limit the bearing pressure to 100kPA or thereabouts.

600mm either end is unlikely to be sufficient for either lateral stability or load spread. Then there's the question of whether the SE would even be able to get the masonry to work without rebuilding...

 

[catlad]

Hmm.. Is the builder going to guarantee it!

 

[biryani_boy]

I guess colums each side would help the lateral stability, but they’re anchored in the same foundation, right?

How can the foundation not carry less than the initial weight if the load is apread across 550mm (665mm incl. external wall) each side, but can carry more weight that’s compressed in a column with a tiny footprint?

Would the minimum returns table not apply in my case? There’s something about a 665mm return as a minimum. Would that do?

 

[RonnyRaygun]

If the engineer designed columns they would also design pad foundations, or alternatively a box frame where the base beam essentially spreads the column load back along the existing foundation. No base beam, no load spread.

And no, the 665mm minimum is dependant on a whole host of factors - load carried, lateral load, masonry strength etc etc. It’s a minimum given in Approved Document A to be used with a number of caveats. It can even be ignored if an engineer can design it out, but that isn’t going to be possible in your case if you are trying to support the back of your house.

 

[Bouy]

My neighbours work at the moment, may be posting a picture of a collapsed house next week, but it does beg the question.

 

[biryani_boy]

Sure. One more question:

What if I make a smaller opening, let’s say 3.5m, but towards the left? Would a 665mm return (incl. extrnal wall) be enlugh to hold the left side of the beam if the other side is 2m plus?

 

[RonnyRaygun]

Maybe. All depends on the loads and what is carrying them. Old or new masonry, solid or cavity wall.
You’ll need to ask your structural engineer.

Just remember your SE has the training, knows the regs, and has done the calcs. They’ll most likely be happy to explain their rationale if you ask them.

 

[Bouy]

Doesn't really matter what the other side is, half the load on the beam goes to the left and half to the right. Its all back to your SE on why he feels the need to have a wide spread of the loads to your supporting foundations which will be as said earlier what the existing founds can take and obviously what load the beam is actually taking

 

[RonnyRaygun]

Doesn't really matter what the other side is, half the load on the beam goes to the left and half to the right. Its all back to your SE on why he feels the need to have a wide spread of the loads to your supporting foundations which will be as said earlier what the existing founds can take and obviously what load the beam is actually taking

But it’s a shorter beam (3.5m against the near 6m the builder was suggesting), so less load to the 665mm section of wall, plus the long section of masonry to the far side should deal with the lateral loads.
So more likely to work than the builder’s proposal, but still not guaranteed without some more design checks.
Of course, there is still the issue of whether the foundation can take the load, which will be around the same as the structural engineer’s original proposal. So if they need 1.5m, it still won’t work.

 

[thomp1983]

Bottom line is a trained professional has told you what you need so if that's the opening you want then you'll have to pay for it end of, the future stability and safety of the rear of your house is not somewhere to try and cut corners to save a few quid.

You've been complaining about this for months and the answer has been the same from pretty much everyone that's replied.

 

[Koolpc]

Follow the advice of your SE.