Looking for peoples experience and thoughts on block and beam (B&B) bounce please…

 

I’m middle of Self building single storey K/D extension and due to ground levels using block and beam (B&B) for the extension itself and the patio which leads on to back garden. Extension floor slab B&B to be set off internal block skin and external patio and side walkway B&B to be build off external skin. Extension above ground cavity block work to then continue up off of these.

 

The patio span is 5.2M (by 7M wide) and currently have extension subfloor walls built, and garden supporting patio B&B foundation poured. 
 

Plan back from B&B manufacture is attached (FYI price on beam only is £1.4kExVaT) Their response to my follow up enquiry asking about bounce the manuf. said:

 

‘The design currently has a natural frequency of 4.35 Hz. We could improve to 5.28 Hz by reducing all block spacing to 215, or 6.09 Hz by using 150 wide beams only & 215 spacing.

 

The only way to improve on this would be to introduce a spine wall and use shorter beams.’

 

First off I’ve no concept of bounce from the plan or the improved levels from the proposed alternatives - do folks think the real life experience will be bad/annoying? (very subjective I know but don’t want to be stuck with this and it grate on me at family bbqs or when have friends over entertaining as I guess all will experience the bounce not just person moving. 
 

Secondly, do the reduced figures significantly improve things?! These would further increase cost and 

 

Seriously considering getting the plant back and digging and pouring another foundation to split the 5.2M span down in to a 3M&2M to reduce the bounce. Downside is this is costly as wet conc (and additional pallets of blocks for wall) has to be moved in telehandler bucket driven backwards down a 400m very narrow lane. (Ahh joys of rural living!)

Lastly do people advise on screed topping to further bind the ‘slab’ beyond that of just grouting as a way to reduce bounce. From my reading it seems common occurrence but this does seem in an internal setting. (I’m using 100m concr finish for the internal floor slab but thinking/hoping don’t need to for patio outside. 

 

Thanks for any input.

ed

(note photo shows a partial foundation already but this was added just for a small upstand wall to retain the earth)

 

 

 

 

 

 

You’ve overthinking it With the spans you have there will be no bounce 

Difficult to distinguish between BB and concrete

4 hours ago, nod said:

You’ve overthinking it With the spans you have there will be no bounce 

Difficult to distinguish between BB and concrete

+1. Nothing to worry about.

Is it ok to use beam and block outside?   I thought the metal reinforcement in the beams might blow?

any bounce at the raw stage will disappear when more concrete or screed is put on top.

The thickness adds to the overall structural thickness, eg 150 beam is now 250mm overall, and the effect is squared.

 

Outside? make sure the top surface slopes away and is waterproof.  ventilate any voids.

Crumbs, the things I don’t know!  Thank you.  

you can definately feel it on large spans. Our local tesco express had large spans,6m or so they doubled up the beams. You can see stuff in the fridge moving when a lot of people are trundling about.

 

Drop in a sleeper wall if its likely to irritate.

Thank you all for your comments. After stumbling across another thread on here I’ve decided to put in an intermediate wall and split the 5M in to 3M & 2M. Mixed myself with bell mixer and at just short of a cube it was a long day! I already had the materials on-site so apart from cement it wasn’t that expensive. The 3+2 spans are likely to mean I drop the double beams saving 14 lengths of 5M equating to about £500. More importantly though, any chance of bounce should now be gone. For any that follow behind me on the issue of bounce on 5M spans have a look for the thread titled ‘Spans - Precast Flooring or intermediate support’ started by moggaman 

 

Edited September 15, 20242 yr by EdSt
Typo

hope you put some footing under it as well! Doesnt have to be mega deep but id want at least 400mm below ground.

Anecdotally, I am very pleased with my Beam & Block floor, which has been in place for a year now, but still without further layers (except for grout). I cannot detect any movement.

 

With that said, I did go for a fairly hefty spec. I have 175mm deep Rackham Housefloors beams and ALL blocks are installed in their 215 orientation. My spans range from 2.8m to something like 4.5m.