Advice on proposed blockwork structure please...

[LiamPope]

Hi there,

My company want to put up a blockwork structure to carry out some noise tests. It needs to be 5x5x4m (l,w,h) and of a fairly heavy construction for acoustic properties. We are currently thinking of single skin dense concrete (7n) blockwork with steel beams supporting a felt/bitumened stirling board roof with concrete paving slabs layed on top.

Been trying to get my head round Part A to see if there are any issues and have to admit i'm still in some confusion regarding the permitted building height and required wall thickness! Wonder if anybody could draw on any experience to advise if this proposal is acceptable.

1. Based on a calculation on the windspeed diagram and associated tables of factors, it seems 4m height is fine. But then there are other diagrams/tables stressing a 3m height blanket maximum. Is that just a worst case scenario? Presumably you can go higher than 3m, but then do the suggested wall thicknesses not apply etc etc? Dont have to start reading British Standard documents do I? Noooooo...

2. There seems to be different wall-thickness requirements for residential dwellings and single story non-residential buildings (which I assume this falls under), but it's generally quite unclear which requirements apply to which structures. The paragraph about single story non-residential buildings suggests 90mm - so a single course of blocks should be ok, probably requiring a central pier in each 5m long wall. Can anyone confirm if that's OK.

3. I calculated about 5 tonnes for the proposed roof giving a roof loading of 2KN/sq.m. This is what's suggested for floors in the Part A doc but greater than a roof (max 1.5KN/sq.m for a <6m span). Is this a factor that'll put up the required wall thickness?


Aaarggh it's a minefield for me, so if any experienced builders out there could offer any advice it would be appreciated. At least is this proposal there or thereabouts, or bonkers and liable to collapse around us? Also any idea what I should be paying a small building firm to put up this structure. Oooh I forgot to mention it's going on an existing thick concrete slab so no foundations required.

Lastly, should whomever we engage to do the work be checking the structure meets regs, or will a builder essentially build anything you tell them too (within reason) that may not meet regs? Or will this need planning permission, and the suitability, or not, of the proposed structure come out at that stage?

Any help appreciated. Cheers,

Liam

 

[^woody^]

First you need to decide if the proposal requires planning permission - it probably will.

Likewise does this structure actually need Building Regulation approval?

Then you need this designing for the intended use.

You seem to be confusing the Approved Documents with the actual building regulations - the Approved documents are just an example of how the building regulations can be met, and in any case, most of the Approved Documents deal with domestic situations.

You need to define and specify how the structure needs to perform for these tests. Then, the structure is built for the purpose

So the person who designs this should have expertise in knowing your requirements, and how to design a structure, and what regulations it should conform to and how to apply for them if need be

 

[LiamPope]

Cheers Woody,

I'm well aware the Approved Docs are just a guide to meeting the regulations, but you're dead right I've gotten a bit hung up on them when they may not apply at all! I guess it all comes from having to get my head round so many of the Approved Docs related to what I'm doing privately with the house I've just bought.

A quick glance at planning portal suggests permission is not normally required for extensions to industrial buildings (including errecting new buildings within the curtilage) meeting certain requirements. Need to confirm that but good news.

Also a quick look at the regs and I think this would fall under exempt work Class VI of Schedule 2 - small detached buildings, as it has a floor area less than 30sq.m, does not lay within 1m of the boundary of its curtilage, and is substantially constructed of non combustible materials. Super duper.

I still need to be satisfied the proposed structure is safe with that roof loading, but I guess that's a question for a structural engineer.

Cheers for at least making me stop hurting my head looking at Approved Docs. Might actually get some sleep tonight then. Cheers.

Liam

 

[Shytalkz]

You've got several issues to consider: vertical load-bearing capacity (7MN blocks should be fine with the loads you're suggesting); lateral ie wind; slenderness of the wall construction; position of openings (if any); intermediate support positions (if any); type of DPC for edge fixity; overall panel size and limiting horizontal and vertical dimensions; yadayada...

Basic wall slenderness ratio <= 27 ie 2.7m high for 100mm blocks; can be stretched by combining as a cavity wall, but wouldn't advise it. Taller unrestrained walls can be diaphragm or fin construction; or intermediate restraints allowing either 2-way spanning or horizontally spanning panels. Spanning arrangement for panels depends on opening size and location.

If you're wanting a 4m high single skin wall, I would use 150mm wide blocks, with a central block pier on each elevation (assuming this arrangement doesn't foul openings) and a hi-load dpc. You could use a pot and beam arrangement for the roof structure. 7MN blocks will adequately cope with that for the dims you need, although this whole set up might then put too much load on the existing slab.

 

[Deluks]

Do you need this room to be soundproof to the outside for loud equipment/machinery testing within, or is it the room itself being tested?

If you require a soundproof room then you require a room-in-a-room. A heavy single skin structure will have much less soundproof qualities than 2 walls/ceilings of much lighter construction.
Your standard concrete blocks as you propose will be fine for an outer room, the inner can simply be made from timber studs with several layers of plasterboard. An 8" gap between walls and filled with rockwool, job sorted.

Likewise, you can do away with expensive and heavy steels, and simply use plasterboard under deep joists for the inner ceiling, and a couple of layers of osb/ply for the outer roof.

For getting in and out, seperate heavy duty doors (one per wall layer) fire doors are fine for this but will require a tight fit and seals all round.

There are also ventilation considerations when you build as tight as it needs to be. Roof vents are a no-no for example.

 

[noseall]

it is rather appropriate Liam that your heavy weight building, has generated responses from our 'heavyweight' forum members.

Softus, would you care to add....

 

[LiamPope]

Lol. Isn't Softus a plumber?

Yeah - I've only been here a little while, but immediately recognized the replies I've got here as coming from veterans who obviously know their stuff. Right?

Cheers for the advice so far. The box needs to keep noise out so that we can test very quiet valves in a suitably low background noise environment. Ultimately the only way to attenuate noise is mass (given a fixed distance from any sources). Sure it is nice if you can split that mass into isolated layers to provide decoupling, but it's a small advantage compared to the mass itself. So it's heavy walls and a heavy ceiling required regardless of how that is made up. The heavier the better. Unfortunately anything more complex than a single wall construction will, I think, kill the budget (which is, of course, too small)!

From what I could understand from Part A, I was coming to the conclusion myself that a central pier on each wall, and possibly upping the thickness from the basic 100mm blocks might be the way to go.

Not really sure what pot and beam is - could you possibly elaborate?

What I really need now is to find someone to do it! Or getting quotes would be a start. The smallish contractor I used to work for suggested I ought to be able to get a small builder to put this up for 10-12k. Hope so. Any Bedford area builders here looking for something to do? Generally is this forum good for actually finding people to do work, or what avenues other than choosing at random from the yellow pages would you guys reccommend?

Again all help much appreciated. Cheers,

Liam

 

[Deluks]

What is so noisy outside that you have to insulate from? Is it a particularly loud industrial area?

Whilst you are correct about mass playing a large part in sound insulation, the importance of decoupling should not be underestimated. And the cost of two seperate walls will not be any more expensive than one overly heavy wall as you will be able to build them 'lighter' and of standard construction. 100mm blocks are dirt cheap, and multiple layers of plasterboard is a cheap and highly effective way to add mass to an internal wall.



Better to wear a belt and braces than just a massive belt. It's all something to do with sound waves and the laws of physics.

 

[Shytalkz]

Heavyweight?! I'll have you know I'm strictly a tinned tomatoes on unbuttered bread man

Pot and beam is precast concrete inverted tee beams with infill blocks of your choosing. Example; wide beam and hollow core are variations on a theme and would give you density, same site has examples.

Delux: the thickness depends very much on the height he wants to go, cavity or solid and I wouldn't want to be doing 100mm thick at 4m height.

 

[Deluks]

Heavyweight?! I'll have you know I'm strictly a tinned tomatoes on unbuttered bread man

These aren't just tinned tomatoes on unbuttered bread..... they're Shytalkz M'n'S tinned tomatoes on unbuttered bread.

(Thanks for the inspiration for my lunch today!)

Delux: the thickness depends very much on the height he wants to go, cavity or solid and I wouldn't want to be doing 100mm thick at 4m height.

My point was strictly about the sound insulation qualities, but I see what you mean.

IMO the price of concrete/steel ceiling beams vs timber, taking into account the extra logistical costs involved in installation and construction, would itself cost more than adding an internal timber/plasterboard room, which (if using 100mm blocks combined with the internal stud walls) would provide superior soundproofing over even a single 200mm solid concrete block wall.

Liam, how big are these valves, do you actually need 4m internal height, or were you just going as high as you can, because you can?

 

[^woody^]

You can construct this noise limiting structure with standard blocks and a timber roof, and line it with suitable boards, insulation and coating/decoration to get a better db reduction than some massive concrete structure.

You will also need to pay attention to the door and window openings

 

[noseall]

if yer gonna do it do it right.

line it with inch thick lead.

every sound you make, every rustle, every breath, even the blinking of your eyes will be contained and heard within the space.

a sound proof room is a weird place to be.

 

[LiamPope]

Thanks for the input guys

The dimensions are determined by the standard we're testing to. Made up mainly of required distance from the valve to the microphones plus required distance from the microphones to any potentially reflecting surface - i.e. a wall.

We've been thrashing out what's required internally for too long already, and I've been brought on board to actually nail it on the head and get it built! In the end we will defer to the experience and knowlege of the consultant who's advising us and will be supervising the testing cos that's what we pay him for and the responsibility he assumes.

It really does come down to mass though (and ideally non-rigid mass, but we can't build it out of dense rubber) to effectively attenuate sound - particularly low frequencies. A lightweight roof however many layers of insulation/foam/plasterboard it comprises will not cut out as much sound, over all frequencies, as a dense layer. We're looking at a few hundred kg/sq.m minimum. Stuff like fibreglass insulation, the wedges/cones you see in anechoic chambers, hanging up rugs/matresses etc etc are to kill internal reverberation of sound. None of that does anything against transmission of sound though - soundproofing is a separate matter to sound deadening. Of course decoupling, i.e. room-within-a-room is all worthwhile but beyond the budget for this project. We've got to do as much as possible with the basic structure alone. With a heavy structure we've then got the option of hanging dense acoustic foam etc etc inside afterwards if budget allows. Straw bales minimum!

Er, excuse me while I ramble on and justify decisions to myself . Again all the comments appreciated. Actually got a couple of builders coming round to chat about it so at least things are moving.

Cheers,

Liam

 

[Deluks]

I hate to bang on... but several hundred kg per sqm is overkill, unless you are building next to an airport runway. What you are veering towards building is a bunker, very handy if you wish to keep out bullets and shrapnel, but sound isn't actually that hard to keep in/out.

Sure it's true that retrofitting soundproofing into an existing structure is very expensive, but as you are building from scratch then it needn't be so, providing it's designed properly. Plasterboard/osb/plywood and even MDF sheets are heavy enough to keep out sound, if several staggered layers are used. You also have the luxury of being able to add more layers internally, until it meets requirements. A couple of pallets of standard 12mm plasterboard costs just a few hundred quid, and represents an awful lot of mass.

Also be aware that builders can give varying advice when it comes to soundproofing. If any of them suggest standard cavity walls or celotex then politely show them the door.

 

[LiamPope]

Funny you should mention that! We were actually hoping to use an old wartime fuel dump on the adjacent airfield which essentially is a concrete bunker . That would have been ideal but unfortunately was not high enough inside.

The valve is very quiet and to satisfy the standard the background noise level during testing must be very very quiet!

Liam