what is meant by "clear span"

[SilverSaloon]

Hi

I am looking at a loft conversion.

I've spoken to a structual engineer who says that the 'clear span' for the new joists go between the new vertical walls that go from the new joists and onto the rafters at the sides of the loft... rather than the span between the two walls the joists sit on.

is that correct? From reading 'the internet' things suggest the span is wall to wall.

 

[freddiemercurystwin]

The clear span is the clear gap between supports. I don't quite follow your post tbh.

 

[Nakajo]

I don't understand the question either. But if your engineer is appointed then why are you asking us?

 

[SilverSaloon]

structual engineer says clear span is the green line (between the vertical blue timbers)

i thought it would be the red line (between the two wall plates)

Untitled

• SilverSaloon
• 24 Feb 2015
• 1

asking on here just for peace of mind as the size of timbers specced are based on that span rather than the longer span

 

[freddiemercurystwin]

you seem to be suggesting that your joists are spanning from external wall to external wall (the red bit)?

Typically you'd have a steel under each dwarf wall and the joists would span between those steels (ie the green bit).

 

[tony1851]

Is the SE confusing 'clear span' with 'loaded span', I wonder?

In a loft, the voids each side at the eaves would not be fully loaded to normal domestic loading - the normal load would only be acting over the length of the green line.

On this assumption, you can often have slightly smaller joists than specified in tables, though they would need to be properly calculated.

You would also need to take into account the point loads represented by the dwarf walls.

This approach can be made to work - in some circumstances - but to say that the green line is 'the clear span'. is wrong.

 

[SilverSaloon]

the SE never said clear span, just 'span'. Clear span was my interpritation based on what i have read online - didnt realise there is a difference between a loaded span and a clear span....

there are no steels - the joists run from wall to wall, resting on the wall.

i just wondered if this sort of design was normal as i've done a fair bit on research online and nowhere seemed to allow the span to be between dwarf walls unless there were steels there (which there isnt in my design).

thanks for all the advice, unless someone else comes aslong as says its iffy, i'll let it all carry on

 

[freddiemercurystwin]

I've spoken to a structual engineer who says that the 'clear span'

 

[SilverSaloon]

sorry i didnt realise there is a difference between clear span, span or loaded span

 

[tony1851]

@OP.

Three questions;

1. what is the distance between your brick walls (ie the length of the red line)

2. what is the distance between the loft walls (ie the length of the green line)

3. what is the section size of the joists?

 

[SilverSaloon]

hi;

1. what is the distance between your brick walls (ie the length of the red line)

> 5.8m

2. what is the distance between the loft walls (ie the length of the green line)

> 4m

3. what is the section size of the joists?

> 200 x 55 C24

 

[tony1851]

Even allowing for the fact that the joists will not be loaded over their full span, the joists are over-spanned and you will get excessive deflection and bounce.

Has your SE done calcs for this, and is Building Control involved?

 

[Hangus79]

Even allowing for the fact that the joists will not be loaded over their full span, the joists are over-spanned and you will get excessive deflection and bounce.

Has your SE done calcs for this, and is Building Control involved?

Another question is 'What centres are the joists at'?

 

[SilverSaloon]

450 centres.

My plan is to get the calcs from the SE and then apply to BC.

This is why i am asking; from reading various things on the internet, it suggests 220x75 C24 would be more suitable?

Obviously the SE work is costing money and the application to BC. If it then isnt good enough then its all a waste of money. I cant use steels as the house is very long at 20m end to end for any steels, and I dont want to mess with walls downstairs.

 

[Hangus79]

Based on the span tables from TRADA, your first suggestion of 200x50 C24, at 450 centres will span 4.47m under normal domestic loadings.

The 220x75 C24, at 450 centres will span 5.43m under the same loadings.

Both of these fall short of your 5.8m span and therefore you will have some deflection and bounce (as per other replies above).

You could just let the engineer run the calcs, they may be able to come up with something clever by introducing additional bracing, or incorporating flitch beams.

235x89 C24 @ 450 centres would span 5.93m, or you could look at engineered joists (ply or metal webbed joists).