timber spacings.....

[limerockconstruction]

.....bit of a long story, but a surveyor on a job that we are on has made an incorrect measurement and as such our 8x2 timber span is now well over the max span according to the tables.

The clear span we need to make is 4500mm. UNfortunately the 8x2 is in place at 400 c/c. I wondered if we doubled up the timbers if that would make it pass?

Any thoughts please? (there has been a suggestion of plywood bracing as well?

 

[tony1851]

Deflection will be the critical factor here, and if you double them up, you will halve the load on each 50x200; the deflection will then be within limits.

You could either fix the beams together in pairs, or space them evenly at 200 centres - makes no difference.

 

[limerockconstruction]

Thanks Tony. It's what I'd hoped. We're a bit governed by all of the existing ceiling joists that have been retained so space is at a premium. I guess we'll get them in where we can.

Also, as an afterthought. There was a block spine wall in the middle (ish), founding it amongst all of the other timber I'd going to be tricky, do you think a six inch structural stud wall could offer the same restraint?

 

[catlad]

We had a similar cock up a few years back building some new floors, the builders merchants sent us 8x2s instead of 9x2s and nobody noticed until the floors were almost finished anyway the building inspector let us double every other joist.

 

[AronSearle]

/Not an engineer.

According to some info I have, C24 will be OK, at C16 grade it still fails.

 

[tony1851]

/Not an engineer.

According to some info I have, C24 will be OK, at C16 grade it still fails.

Are you saying that the floor still fails when the 50x200s are doubled up?

 

[AronSearle]

/Not an engineer.

According to some info I have, C24 will be OK, at C16 grade it still fails.

Are you saying that the floor still fails when the 50x200s are doubled up?

Yup.

I agree it seems abit strange, but computer says no.

Tells me it either has to be C24 or increased to 47x220 @ 200c.

This is based on euro code, not BS5268. My vague recollection is that some loading factors in the new codes cause longer spans to be an issue. Probably works under the old code (/not an engineer)

 

[tony1851]

[

I agree it seems abit strange, but computer says no.

Can't understand that. I've done some hand calcs to BS 5268 and stuck them in an album called 'limerocks beams' = don't see any problem at all with spacing them at 200 centres. Bending stress and deflection are within acceptable limits.

I doubt they would fail EC5 calcs either, as their deflection limits are marginally better, even allowing for long-term deflection.

 

[AronSearle]

[

I agree it seems abit strange, but computer says no.

Can't understand that. I've done some hand calcs to BS 5268 and stuck them in an album called 'limerocks beams' = don't see any problem at all with spacing them at 200 centres. Bending stress and deflection are within acceptable limits.

I doubt they would fail EC5 calcs either, as their deflection limits are marginally better, even allowing for long-term deflection.

This is all probably of no interest to the OP, building control cannot insist upon EC5, though they sometimes try, BS5268 can still be used.

However, if you compare EC5 span tables to BS5268 span tables, most of the EC5 spans are about <1-4% less than BS5268. I've had it explained to me that this is due to some higher loading requirement within EC5.

 

[limerockconstruction]

Certainly interesting, essentially what BC have said is that they would be happy with them, as long as a qualified engineer was willing to sign it off.

 

[jeds]

Here’s what I make it: Doubling the timbers to 2x50mm (retaining 350mm overall spacing) works easily at C24 and just scrapes in at C16. Adding an extra timber in between (to make the spacing 200mm) is ok at C24 but fails at C16.

 

[limerockconstruction]

i'm guessing by the difference of opinion here that I need an engineer to look at it and put his name to it.

 

[jeds]

Send me a PM and I'll give you a call. I'm in Bradley Stoke.

 

[tony1851]

but fails at C16.

How did you determine that? I reckon it passes with C16 in stress and deflection.

 

[jeds]

You've probably just used a shade less dead load than I have. Taking it as 0.80kN dead and 1.5kN live load at 200 centres I make deflection 14.48mm. You only have to knock it back to 0.6 and it would pass. Won't make any difference either way in the scheme of things but for a squids worth of timber it's not worth risking somebody moaning about a bouncy floor.