Joist Spacing

[russellsmiler]

Mornin All

I am building an extension and wish to try and determine my floor joist options.

Firstly the joist serction size is to 178 x 50mm (these I already have from a demolition job i done some time ago, (they are 6 meteres long and i have plenty of them.)

I have a single span of 4,5 Meteres, the floor above will be a bedroom, there is no other loadiing to consider.
Anyone any idea what centres i should put the joists, I can go down to any C/C to suit, can even put them in doubles, possibly bolted or glued together to make 178/100mm joists.

Any advice, greatly appreciated.

Thanks
Russell

 

[tony1851]

The maximum span for joists of that section is usually around 3.5m.

Without doing figures, you would probably need to space them much closer than the normal 400, probably doubling them as you suggest.

They would never collapse under normal domestic loading, but even doubled you could find that deflection/bounce might be excessive with that depth of joist.

 

[themiddleagedun]

They're not deep enough, need to be 9x2's

 

[russellsmiler]

Unfortunately I have to make them work

What C/C anyone Know.

Rusell

 

[tony1851]

No-one on a forum will tell you that.
Your critical factor will be deflection, which has to be kept within certain limits to satisfy building regs.
(for domestic floor joists, it's normally span x 0.003, so in your case the deflection would be limited to 13.5mm).
To check the centres required to keep deflection under this would require proper structural calcs.

 

[ban-all-sheds]

Unfortunately I have to make them work

No you don't.

 

[maltaron]

If you have the facilities to rip some of them down into 2-3" strips, then glue & screw to 7 x 2's to make 9 or 10 x 2's its possible that building control may accept them if you do a deflection test. Not ideal but may work.

 

[russellsmiler]

May be better to ask the forum what is my maximum span for a joist this section (178 x 50mm) I can reduce spacings to whatever i want if it gives me a better span.

All help much appreciated!!

Russell

 

[ban-all-sheds]

May be better to ask the forum what is my maximum span for a joist this section

The maximum span for joists of that section is usually around 3.5m.

 

[RonnyRaygun]

Unfortunately I have to make them work

What C/C anyone Know.

Rusell

Double them up and put them in at 400mm crs. That will work for domestic loading.

 

[russellsmiler]

Slightly confused now as 1st reply said they would deflect whatever c/c
I see that tables are realtively easy to find, but cannot find my answer!

Russell

 

[tony1851]

May be better to ask the forum what is my maximum span for a joist this section (178 x 50mm) I can reduce spacings to whatever i want if it gives me a better span.



Russell

It doesn't work like that, as there is no linear relationship between span and joist depth/spacing

 

[tony1851]

Unfortunately I have to make them work

What C/C anyone Know.

Rusell

Double them up and put them in at 400mm crs. That will work for domestic loading.

For bending and shear deflection, I reckon it would be just over 14 but not much, so yeah, agree should be OK.

 

[noseall]

Throw in some 1/3rd span solid bracing and them there joists are goin' nowhere.

 

[amwc]

Hi
you don't say what grade these joists are as if they are C24 you can get a bit extra out of em.