Hlp needed timber span tables and Superbeam calcs to add up

[champagnecharly]

Hi,

From what I gather
intermediate floor domestic


Live load = 1.5kN/m2
Dead = 0.7-1kN/m2


On 400 centres and using 0.7kN for Dead load... if I am not mistaken

my loads/ linear meter of timber are..
D= 0.7 x 0.4 = 0.28
Live=1.5 x 0.4 =0.6

I chose Timber c24 49 x 195
Aprox deadload from 1m 0.049x0.195*370kg = 3.5Kg = 35N =0.035Kn

Span of timber I select at 4.36M which according to tables is max span for c24 49x195.

Results from SuperBeam fail on deflection!! With TOTAL deflection at 19.02mm

I must have gone wrong somewhere but where? Can anybody help?

 

[tony1851]

Panic not!
It could be that you have input the minimum 'E' value (modulus of elasticity) for the timber, which you only do for isolated timber beams.
(or the program may have assumed you are only calculating an isolated beam).
In a floor, you have a load-sharing situation and you can use a higher value of E. This would bring your deflection down by a factor of around 1.5 (assuming the program is to the BS 5268 code).
So your 19mm deflection would come down to around 12.6mm, which is probably OK on that span.

 

[champagnecharly]

Thanks.. no panic just curious.
E= 10800N/mm2 which (from what I read) is Mean value for c24.
source: http://www.roymech.co.uk/Useful_Tables/Timber/Timber_design.html

Is that correct?
Settings are Based on load sharing system but god know what i'm doing to make it disagree with standard tables..


Conformity of calcs are to:
Steel beam design and checking to BS449 Part 2
Timber and flitch beam design and checking to BS5268 Part 2
Column design and checking to BS449, with simple base plate sizing
Checking of timber posts and studs to BS5268 Part 2

You can see software output here: https://www.dropbox.com/s/10alg6nes4frshc/BeamCalcs.jpg
but it may be overly simplified.

*** updated image to make it readable ***

 

[champagnecharly]

Ahh..

I noticed that my initial deflection was incorrect.

I also noticed that when removing the timber's own weight... then deflection fails by just 0.0001xLength. and is 13.61 .

However i noticed that max deflection is set at <=0.003 x Length Where l/320 =0.003125

So what doesn't tally is the deflection limit and the app is a little more conservative than the table/regs.

 

[tony1851]

Don't know if it's relevant but I think the TRADA tables are based on Eurocode 5, and domestic floor joists have to satisfy a vibration limit, which implies a lower deflection limit.

 

[jeds]

Could also be the software is defaulting to NHBC minimum - which is a higher standard than the strict minimum.

 

[champagnecharly]

Thanks. Not sure if eurocodes also but certainly complying with NHBC
which specifies:
"dictated by maximum floor deflection = 0.3% of the span up to a maximum of 12mm."

Thanks guys. puzzle solved.

 

[champagnecharly]

Just found a copy of trada http://www.dfpni.gov.uk/d.pdf and it works in a more precise way specifying by deadload brackets.