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How do I calculate what size beam to span a large gap?
smart51 - 24/9/15 at 06:03 PM

We're in the process of moving house and the one we're hoping to buy has a ramshackle set of extensions to the rear. I want to knock them all through and put bi-fold doors along the whole 7.5m length. That will mean spanning the gap between the end walls and holding up the roof, which is pitched off the back wall of the house. The extensions are about 2m deep from the back of the house.

I guess we could put triangle sections on the back wall of the house so that the extension roof is cantilevered off the back wall, but I don't really want the load pulling on the wall of the house. But 7.5m is a big span for a beam. How would I calculate the beam needed to span the gap and support the roof?

zilspeed - 24/9/15 at 06:40 PM

You ask an engineer, is the only answer I'd give here.

Professional liability absolutely prevents the ability to comment on such a thing without knowing every single little detail.


(In Scotland this would certainly need a building warrant, complete with engineer's calculations. I know it's different down there.)

If you pay the engineer and it all goes wrong, you get to sue the nutsack off him. That's not an option you get with free info from a bloke off the internet.

Ben_Copeland - 24/9/15 at 06:53 PM

You will probably need planning permission and building approval.

This will mean you will need a structural engineer to spec the beam for you.

There are calculators available on the internet but they won't be enough for the council

JoelP - 24/9/15 at 06:56 PM

Ditto the above. You won't even get an estimate. Now if you called a structural engineer, then he might well be able to give you an idea over the phone if what's involved, but you'd need a building notice to remove a structural wall, and written calculations.


It's going to be a serious rsj, whichever way you look at it!

smart51 - 24/9/15 at 06:59 PM

Yes, The building inspectors will want to see calculations and we'll pay a structural engineer to do the sums. What I want to do is find out that it's possible and practical before I pay someone. I don't want them to tell me it can't be done, just what needs to be done.

cliftyhanger - 24/9/15 at 07:49 PM

It will be a big old beam, you will certainly need a few mates round to lift it in position!

NigeEss - 24/9/15 at 08:10 PM

There is an Excel spreadsheet called beam.xls which you can use, comes up on google.

mcerd1 - 25/9/15 at 12:41 AM

quote:
Originally posted by smart51
Yes, The building inspectors will want to see calculations and we'll pay a structural engineer to do the sums. What I want to do is find out that it's possible and practical before I pay someone. I don't want them to tell me it can't be done, just what needs to be done.


possible yes - spans over 60m are common on commercial buildings these days so 7.5m is no problem

practical - that will depend on the space, access and budget you have
you may have to consider having some steel columns to support it if the wall isn't up to it (effectively making it steel framed)

as a bonus steel columns are much skinnier than brick / concrete ones, so even if you put one in the middle to reduce the size of beam needed it wouldn't necessary be a disaster


doing the calcs for the size would be simple if it wasn't for a house, I know you've got different rules down there but there should be some data available for the strengths required by the building codes.
even if the date that's available is for timber - you can always just look up the equivalent steel

[Edited on 25/9/2015 by mcerd1]

smart51 - 25/9/15 at 07:58 AM

A bit more time on google and I've found some span tables for different domestic applications that cover the lintel I need.

A 200UB25 203 x 133mm I beam will do the job. At 200kg, I wonder whether we'll need a crane, rather than a dozen burley blokes to lif it but it is doable.

cliftyhanger - 25/9/15 at 08:10 AM

4 or 5 should do it.

I was lucky, managed to borrow an easi lift. Made beam fitting very easy! Especially where it was a pair of beams (bloody engineer didn't mention it could hav been done with a single beam with plates welded to the top, flipside I ran a 4" pipe between them for an extractor....)

Mr Whippy - 25/9/15 at 11:56 AM

I'd be looking at the total deflection in the middle fully loaded, it may hold the load but if it deflects 20mm doing so then you may have issue getting the doors to fit! I'd also make sure the wall holding the ends of the beam are very well reinforced as well as the foundations which will only have been made to suit the orginal wall loadings. Personally I'd dig them out and start again.

jossey - 25/9/15 at 12:24 PM

This post worries me lol

MP3C - 25/9/15 at 03:23 PM

I wouldnt like to post anything in here and im a building surveyor. I personally would get a structural engineer to spec something like that and also will help with building control when they ask for calculations.

Matt

coozer - 25/9/15 at 03:31 PM

Its not just the beam, its the walls and pads that need speccing.

Get a structural engineer in. He will have a look at what you propose and give you pages and pages of calculations to keep the building inspector happy.

smart51 - 25/9/15 at 05:40 PM

We will be getting full architects drawings with structural engineers calculations for building control. As stated above, I want to be sure it is practical before I pay people to do the calculations. I've found out enough to know that it is. There's nothing worse than handing over a wedge for the engineer to say, sorry, it can't be done.

We'll be applying for planning permission for an extension over the garage at the same time, rather than sneaking the work at the back in and claiming it was under permitted development. There's no point in bodging something like this.

[Edited on 25-9-2015 by smart51]

drt - 25/9/15 at 08:17 PM

steel, wood or concrete ?

first two easy peasy

mcerd1 - 26/9/15 at 09:43 AM

quote:
Originally posted by smart51
A bit more time on google and I've found some span tables for different domestic applications that cover the lintel I need.

A 200UB25 203 x 133mm I beam will do the job. At 200kg, I wonder whether we'll need a crane, rather than a dozen burley blokes to lif it but it is doable.


^^^ assuming that's the beam your engineer picks for his calcs (doesn't sound too far off depending on the load your roof will put on it of course...)

its actually called a 203x133x25 UB or 203x133x30 UB
that's nominally 203mm deep, 133mm wide and 25 or 30kg per meter length (so 188 or 225kg for 7.5m)
that will probably be in grade S275-JR or S355-JR

to give you an idea we pay around about £750 - 800 per tonne for this sort of steel stock (but we do buy quite a lot of it...)

Universal Beams (aka: UB) are the modern equivalent of the old RSJ's, some builders still insist on calling them RSJ's but they haven't been commonly made for at least 20+ years !

here's the list of available ones in the UK:
http://tsbluebook.steel-sci.org/EN/Browsers/Main.htm
this sites got loads of good info too:
http://www.roymech.co.uk/Useful_Tables/Sections/UBA_dim_prop.html
you'll see that the 203x133UB's are actually the 5th and 6th smallest you can buy...


if you need the same strength with less depth then you may be able to consider the 152x152 UC's - these are now available in extra heavy 44 and 51 kg per meter versions

[Edited on 26/9/2015 by mcerd1]

smart51 - 26/9/15 at 03:36 PM

quote:
Originally posted by mcerd1
quote:
Originally posted by smart51
A bit more time on google and I've found some span tables for different domestic applications that cover the lintel I need.

A 200UB25 203 x 133mm I beam will do the job. At 200kg, I wonder whether we'll need a crane, rather than a dozen burley blokes to lif it but it is doable.


^^^ assuming that's the beam your engineer picks for his calcs (doesn't sound too far off depending on the load your roof will put on it of course...)

its actually called a 203x133x25 UB or 203x133x30 UB
that's nominally 203mm deep, 133mm wide and 25 or 30kg per meter length (so 188 or 225kg for 7.5m)
that will probably be in grade S275-JR or S355-JR

to give you an idea we pay around about £750 - 800 per tonne for this sort of steel stock (but we do buy quite a lot of it...)

Universal Beams (aka: UB) are the modern equivalent of the old RSJ's, some builders still insist on calling them RSJ's but they haven't been commonly made for at least 20+ years !

here's the list of available ones in the UK:
http://tsbluebook.steel-sci.org/EN/Browsers/Main.htm
this sites got loads of good info too:
http://www.roymech.co.uk/Useful_Tables/Sections/UBA_dim_prop.html
you'll see that the 203x133UB's are actually the 5th and 6th smallest you can buy...


if you need the same strength with less depth then you may be able to consider the 152x152 UC's - these are now available in extra heavy 44 and 51 kg per meter versions

[Edited on 26/9/2015 by mcerd1]


Thanks Mcerd. It was the 2nd largest beam on the tables I found. Its good to know there are bigger ones if that's what we need. I cant see the depth being a problem, but that's for the pros to work out.