Structural calcs on extension

[jesteh]

we are having single story extension 7mx3m built with a large door and window along the long wall which are 2.75m wide each. Architect has designed it with 2 corners of 0.665m (minimum allowed to document A) and a middle pier of 0.785m wide.

Building control have quite rightly failed it on the width of middle pier as it fails Document A (middle pier should not exceed width of the two openings divided by X-factor set to 6) and requested that we expand the middle pier to 0.916m.

At this point I got structural engineer involved to see if we can prove the middle pier. What he came back with is that it fails on wind loading and recommendation to put in a wind post in middle of this pier (an L shaped steel bolted into floor, built into the blockwork and bolted into the roof joist). Both me and the builder are surprised at this and I am also wondering how safe this actually is - roof is made of wood so any expandion in roof timbers would be flexing the wall and causing cracks. I do think the structural engineer knows what he's doing but he's telling me that he can't prove this middle pier at any width - it fails by a factor of 10 so not a small margin. Also he mentioned that to comply to openings in document A we should have used stronger blocks (we used 3.6N/mm2 ones rather than 7N/mm2 but its a single storey extension at the end of the day - builders merchants told me they don't stock 7N ones as they are hardly used).

What should I do at this stage? This wind post sounds a bit extreme. I can make the window narrower (building control have even said they'll let me go down to 0.905m to fit with brick courses) but I've got the structural engineer telling me that its failing his calcs even at that width (0.916m). This shouldn't be so complex!

 

[freddiemercurystwin]

So you employed a structural engineer who must provide calcs to Building Control and has provided you with a solution. You're merchants probably don't supply them as they may be expensive suppliers and the wise use another supplier. Two 2750 openings are pretty big you know, imagine a 100mph wind blowing against that weedy bit of wall, that's supporting the roof! You could barely stand up in it, think about the pressure on a whole slab of wall! Extensions get complicated as soon as you start introducing big openings.

 

[RonnyRaygun]

A wind post doesn't sound that extreme to me. Sounds like a sensible solution. Unreinforced masonry piers have almost no capacity on their weak axis when designing to BS 5628, hence the need for steelwork...although I think he might be exaggerating when he says the pier is failing by a factor of 10...
Quite simple to weld some 100x100 box section or similar in an L shape and bolt to the ground. Will also need a solid bit of concrete to bolt to and some good resin fixings, all of which your engineer should design.

Not sure why he wants it bolted to the roof joist...maybe for lateral restraint, although this could be achieved by shotfiring or welding ties to the post and building into the masonry.

7 Newton blocks aren't that difficult to come by, try calling some other merchants. If they aren't in stock they should be able to get them in. Although I don't think Approved Doc A says you have to use 7N blocks...just that using 7N blocks allows you to take an X factor of 6 in all circumstances.