Fitch beam

[darren_forward]

Having the loft converted,

The joiner has specified 2 fitch beams made up of

2 * 50mm * 200mm C16 wood beams, With a 8mm * 175 mm steel plate bolted between with 8mm bolts. This is to span 4.5 meters .

Will this beam be man enough for the job

regards

Darren

[Chris Bacon]

If the meters are succurely blotted to the worl.

[John Rumm]

BTDTGTTS

You neglected to mention the one vital bit of information! i.e. the load on the beam.

If we assume you are talking about just an ordinary floor joist with

0.8kN/m loading on it, and it is in a load sharing system with other beams, then the answer is "yes - plenty strong enough". However since you are only talking about a pair of them I expect that you are planning to string other beams off the flitch beams. This obviously may make a drammatic difference to the answer!

The demo of superbeam is your friend:

may find using 12mm bolts simpler since most other bits like plate washers and timber connectors are readily available in 12mm but not always as easy to get in 8mm).

[darren_forward]

Thanks john for the information, 12mm will be easir to get hold of thanks for that.

Your quite right there will be floor joists between the flitch beams, joist span is 15ft we are using 8*2 for these at 600 centers.

My loft conversion is structural, so i need only the BCO approvial, as where using it for storage, The BCO tells me i can get a way with a loft ladder.

John sorry i have no idea of loading when the wife starts filling the space up, do you think the flitch still be man enough for the job. Please make any recomendations or criticisim as needed.

Many thanks for the help

Darren

[John Rumm]

So we are talking about:

||| ||| |||===================||| 0.5 ||| ||| |||===================||| 1.1 ||| ||| |||===================||| 1.7 ||| ||| |||===================||| 2.3 ||| ||| |||===================||| 2.9 ||| J ||| |||===================||| 3.5 ||| ||| |||===================||| 4.1 ||| ||| F F

Flitch beams at F, ordinary beams at J. Each flitch is 4.5m and the Js are about 3m then? Floor boards running perpendicular to J.

Unless you know that you will be storing particularly heavy items up there then you can assume a standard "floor load". The important point is that there are no additional loads to take into account like partition walls (especially load bearing ones) etc.

So if we model two joists then - the flitch and the ordinary floor joist, then add the downforce from the end of one of the floor joists (about 2.4kN) at seven places along the flitch, we should work out if the flitch is up to it.

My quick model suggests that it is still ok, and you get a maximum deflection of a bit over over 11mm in the centre of the span (which is getting close to the limit). If you wanted it stiffer, then upping the flitch plate depth to the same height as the wood would reduce the deflection to 8mm.

[darren_forward]

Thanks John your a star, Your website on your loft conversion is excellent.

Many thanks for the help and guidance

Regards

Darren

[John Rumm]

Needless to say that you BCO will probably want to see a quick drawing and a full set of calculations of the type knocked up by superbeam... so you may need to cough up for the full version (still cheaper than getting an architect to do them for you though probably)

[Weatherlawyer]

Or you could always ask the BCO if the joiner knows what he's talking about. I wouldn't fall too far from the tree without talking to the woodman if I were you. You might need his help getting back up.

It would really piss me off if some berk I was working for was going over my head for second sight and getting it from amateurs on the internet. Note: I am not saying the advice isn't good advice. But consider:

You have decided to go over the joiner's head about the bolts before checking on the availability of the 8mm stuff AND with him. But you wouldn't just buy the bolts without all the nuts and washers in the first place, would you?

So that advice was a little specious.

And for no real reason, you now expect the joiner to drill effing big holes when he was expecting to only have to drill ones that were effing big enough.

My customers wouldn't do that to me twice. Not even for the extra they were going to have to pay for that little item. Not even if they volunteered to do that job themselves.

[John Rumm]

Assuming you are doing this on a building notice and not proposing to submit full plans, then its unlikely you will cause any offence to the joiner if you ask the BCO if he needs to see calcs. If he trusts the joiner then he may decide he does not. If he does want some, then first port of call would be to ask the joiner if he can furnish them.

Either way there is no harm in knowing if the joiners recommendation is sound before you start.

Not really - check out most suppliers of plate washers, bolts, nuts, timber connectors etc, and you will find them all more readily available in 12mm than the other sizes.

architects original design of the flitch beams in my loft called up

16mm bolts. The steel fabricators suggested using 12mm if possible since they would otherwise have to drill out all the washers and connectors to 16mm at significant extra cost. So I redid the calcs with 12mm which were still plenty adequate.

How much more difficult is it drilling a 12mm hole in a bit of wood than an 8mm one?

The steel fabricators will pre drill the flitch plates for you before pasivating the steel - this is preferable to drilling them after on site.

Get much repeat business do you? ;-)

[Simon Stroud]

Or just do what my "specialist" loft converters did when they realised they had f**ked up the design of the joists and had to convert a double wooden joist to a flitch (note spelling) beam by adding the steel plate in between.

Said steel arrived on site pre-drilled and was slipped in between the two wooden joists, which they suitably drilled. Bolt heads visible on one side but rather suspiciously no nuts on the other side - took a lot of feeling around to discover this as the "other side" of these joists were hidden by the new floor. So basically they just bashed in the bolts to make it look like they were doing something! They actually had to cut the ends off the bolts to get them in far enough - I still have the "ends" that I found for evidence, FWIW. Similarly elsewhere, where there were double joists, supposedly bolted together including the round serrated "timber connectors" in between. They didn't actually bother fitting the timber connectors and because the screwy things were BOLTS rather than screws, they only had a certain amount of thread on them and the nuts got to the end of the thread before they went tight so they ended up with a huge pile of washers. You could still undo them with your fingers.

BCO didn't seem that bothered fortunately (yes these things were mostly left like as I've described in the end - don't you just love getting the "professionals" in to do this kind of job!) and it all hasn't fallen down yet, 5 yrs later, but not ideal.

Anyway, I'm sure if your flitch beam is specified from the start, and bolted together properly before fitting, then it should be fine!

Good luck.

Regards, Simon.

[John Rumm]

It always supprises me the risks some of these people are prepared to take with someone elses property...

Yup, I had the satisfaction of doing up each bolt myself!