is block and beam strong enough to temporarily support strongboys?

[johnhenstock83]

afternoon all,

 

I was wondering if someone can provide a bit of advice, only looking for a generic yes or no.

 

we have an extension shell (block and beam, no screed, walls + flat roof). we're looking to open up a section of the external wall between extension and existing house, roughly 3.6m. structural calculations done, signed off by building control.

 

the builder proposed support the first floor on the inside, using acrows, and supporting the external wall from the outside, using strongboys. the strongboys would basically rest directly on the block and beam. are these usually strong enough to support an external wall (cavity brick/block, 300mm, 2.3m internal height)? worth mentioning that there's a 700mm return on one side and a 2m return on the other.

 

thanks

[George]

It depends what load each prop is carrying but probably not, the blocks especially would be vulnerable to a point load.

 

Your builder should be able to make an assessment of the load, with the effect of the weight of any floors or the roof needs being considered. Note that Strongboys should be limited to 340kg (acrows are much stronger but due to the eccentric load on the acrow prop it is reduced a lot), which is around 150 bricks or so.

 

Easily solved though, either remove a few blocks and prop off the ground or use thick timber sleepers to spread the load. Be cautious as a prop failure (at either end) would be Very Bad. 

 

 

[Mr Punter]

It is temporary works so often the contractor would be responsible for the design.  Don't take the liability for this yourself.  If the beams run perpendicular to the wall you could spread the load over several beams with scaffold boards.  If they are parallel it is more tricky as all the load will be taken by the beam nearest the wall.

[Canski]

There’s a product called brick brace that I have used before it works fine. Not cheap though. 

[johnhenstock83]

thanks all, a couple of points:

 

1. the beams do run parallel, so the load can be spread. alternatively, we can remove some blocks and somehow support the strongboys on the ground, that might be an option too.

 

2. the builders is taking responsibility for any mistakes he makes, unfortunately the block/beam was laid by another builder we lost trust in. since he doesn't know what's underneath and how it was done, he can't guarantee someone else's work, so should the block/beam fail, he won't cover that.

 

3. for a 3.65m opening, I believe the "guidance" dictates 4 strongboys (one every 900mm). If I go with 5, would that be enough to hold the external wall? it's not just the outer wall (bricks, but also the blocks inside, though I'm guessing propping the ceiling will help with that as well.

 

would it be possible to prop the wall from the inside, so that the existing screed takes all the weight?

[Dave Jones]

if the acro are sitting over the wall the b and b is bearing on then no problem, if the acro is on the span take up some blocks from he b and b and let the acro sit on the floor underneath.

[johnhenstock83]

3 minutes ago, Dave Jones said:

if the acro are sitting over the wall the b and b is bearing on then no problem, if the acro is on the span take up some blocks from he b and b and let the acro sit on the floor underneath.

 

hi Dave,

 

thanks. there's one course of bricks on the existing house foundation on which the beams rest, they were not inserted in the original wall, so I guess that might not be strong/wide enough. in this case, probably best to remove blocks (I'll have to anyway) and build a support on the ground, using paving stones/bricks. I've seen that done before. I'm guessing undisturbed ground is strong enough to hold the floor for 1-2 days, until the beam is installed, right?

[Dave Jones]

slide a couple scaff planks under the b and b to sit the acro's on. will be fine.

[Gus Potter]

On 06/03/2023 at 13:45, George said:

Note that Strongboys should be limited to 340kg

BIG POINT from George.. Use strong boys with great caution!

 

The below is not how it should be done.. I stopped this job.. and went for emergency dead propping just to stop it falling down.

 

The props and strongboys you see are not just holding up the outer skin of masonry but the inner leaf which supports the roof and also the first floor joists were spanning onto the inner leaf. The bottom of the props are sitting on the patio slabs..

 

I had investigated the ground and the soil below the slabs was as soft as putty. Designed a full propping / wind bracing scheme for the slapping including diagonal wind bracing and the internal propping to hold up the inner floor. Client engaged builder off their own back as they were the cheepest. Builder ignored it all (my proping design) and went for the above.

 

I was actually surprised that the building had not fallen down / cracket to bits by the time I got there. Note the "juanty prop" second from the right.. and the STRONG BOYS rated at 340 kg as @George 

 

When I challenged the tosser builder he said.. I'm insured!

 

Very glad to say no one got hurt.

 

 

 

 

 

 

 

 

[johnhenstock83]

thank you all for advice, builder reassured me he'll investigate properly before commencing and come up with a solution.

 

just for clarity, it's not just strongboys they'll use, they confirmed the entire floor will be supported from the inside using acrows. assuming roof joists slot into the inner skin of the external wall, propping it from the inside will surely take some of the load of the wall. the load is then, I think, correct me if I'm wrong, spread on both sides, so that the strongboys don't have to take the entire load.

 

I don't know why, but what I think of a cavity wall, I always imagine tons of weight per square metre, I never understood what a strongboy able to carry 340kg does in terms of support, in my mind this doesn't compute.

 

thanks all

 

PS: if it makes any difference, padstones go in first, without removing the wall. the intention is to first install the beams and THEN remove the wall, so the supporting should only hold for a few hours, until the beams go in.

Edited March 16, 2023 by johnhenstock83

[jon-lee]

If you know approximately where the point loads are. Mark up the beam and block drawing you should have from the manufacturer. They will do a revised calculation. I'm assuming you will be propping close to the perimeter so applied loads to the beams that are perpendicular will be in shear. Spreading the loads over a few beam should be OK. Be careful on the beams that parallel as there's not much design tolerance so this would be where the beam fails in bending. 

[Dave Jones]

On 08/03/2023 at 00:51, Gus Potter said:

BIG POINT from George.. Use strong boys with great caution!

 

The below is not how it should be done.. I stopped this job.. and went for emergency dead propping just to stop it falling down.

 

The props and strongboys you see are not just holding up the outer skin of masonry but the inner leaf which supports the roof and also the first floor joists were spanning onto the inner leaf. The bottom of the props are sitting on the patio slabs..

 

I had investigated the ground and the soil below the slabs was as soft as putty. Designed a full propping / wind bracing scheme for the slapping including diagonal wind bracing and the internal propping to hold up the inner floor. Client engaged builder off their own back as they were the cheepest. Builder ignored it all (my proping design) and went for the above.

 

I was actually surprised that the building had not fallen down / cracket to bits by the time I got there. Note the "juanty prop" second from the right.. and the STRONG BOYS rated at 340 kg as @George 

 

When I challenged the tosser builder he said.. I'm insured!

 

Very glad to say no one got hurt.

 

 

 

 

 

 

 

 

 

interesting. is there really that much load though ?  Looks like maybe a pack of bricks above the cutout, same in blocks on inside. A share of the roof as well. The corner is still there taking most of the load and assume the floor joists are still sharing the load with the other wall.

 

Fair enough they should have dropped a scaff plank or 2 under the acro's. Doesnt look that bad though ?

 

 

[Gus Potter]

On 13/06/2023 at 21:09, Dave Jones said:

 

interesting. is there really that much load though ?  Looks like maybe a pack of bricks above the cutout, same in blocks on inside. A share of the roof as well. The corner is still there taking most of the load and assume the floor joists are still sharing the load with the other wall.

 

Fair enough they should have dropped a scaff plank or 2 under the acro's. Doesnt look that bad though ?

 

 

There was a fair bit of load. Behind that wall is a big open plan space running gable to gable, there is a fair chunk of roof also. The first floor joists frame into the inner leaf, thus floor load also on the strongboys.

 

The strong boys were supporting both inner, outer skins, the first floor and part of the roof.

 

I had designed and specified props on the inside, steel needled with lacing and diagonal bracing between the props. The temporary works were also designed to resist a bit of temporary wind load on the gables. The props were resting on the edges of the concrete slabs, under the slabs was clay.. like soft butter, no appreciable strength at all. None of the props were in tolerance plumb wise as per manufacture's recommendations.

 

If that was not bad enough they had then gone ahead and knocked out other walls on the inside, the folk were still living in the house.

 

The thing is that to do it right would have cost another say £700.00 quid and an extra day and a bit time wise for a couple of guys. All they had to do was follow my drawing. The lacing and bracing is just done with standard scaffold tubes and couplings.

 

 

[saveasteading]

On 08/03/2023 at 01:51, Gus Potter said:

tosser builder he said.. I'm insured!

Except the insurer would have denied responsibility because the policy will have conditions about taking expert advice.

[Canski]

12 minutes ago, saveasteading said:

Except the insurer would have denied responsibility because the policy will have conditions about taking expert advice.

Mine doesn’t. 

[saveasteading]

30 minutes ago, Canski said:

Mine doesn’t

Perhaps in a different form....."reasonable care and expertise."

 

And then there are the exclusions....I had a roofer who was not allowed to work above 3m and there were more like that.

 

You have probably been more professional than they, who had chosen the cheapest quote from The Sun.

[Canski]

10 hours ago, saveasteading said:

Perhaps in a different form....."reasonable care and expertise."

 

And then there are the exclusions....I had a roofer who was not allowed to work above 3m and there were more like that.

 

You have probably been more professional than they, who had chosen the cheapest quote from The Sun.

Yes maybe. Every year I have a battle with the insurers who by default limit my working height to 4 m. 

[Nickfromwales]

Statistically, falling from a single storey is worse than 2, as you do one revolution in mid air apparently and not land on your head.

Statistically, I’ll attempt to avoid either  

[johnhenstock83]

ah, my old topic

 

just an update, everything went well. the outer skin (which, btw, was tied to the inner skin through very thick, old-school, galvanised ties) was held up by 4 strong boys, which were spread equally and rested on scaffolding boards perpendicular to the block and beam. the builders were extremely relaxed about everything and at times, there was some rubble on the subfloor, so more weight than just the house.

 

the inner skin was propped from the inside with 5 acrows, the floor joists holding everything above as they slotted in. nothing moved, nothing cracked and the beams were in the same day, so aside from several hours during which several courses of bricks were missing, everything was supported at all times. no damage in the house, other than a skirting board that snapped, it got pushed by the strongboy from the outside.

 

so the answer is yes, block and beam is strong enough, usually, but it all depends what's above. our roof rests mainly on the side walls and the inside walls, so not much roof weight was taken by the strongboys. as for the brick wall, there was a very wide upvc window just above, so we're talking less than a few sqm of bricks, that probably doesn't weigh much.

 

thanks all

[Big Jimbo]

Fab @johnhenstock83. @Nickfromwales i will make a note of that.