Joist spacing

[JT101]

Hi

Our structural engineer has specified double 50 x 175mm joists at 400cc for our span of 4.5m for a small portion of the floor where the floor is unsupported from below.

The problem is that the width between the remaining single joists which either span over a block wall or are short enough in length, is 1100mm. So centres would be 370mm.

This seems really overkill, even though it probably meets building regs.

I'm trying to find some tables for double joists, or a large 100 x 175mm joist, spans and centres.

2 * double joists instead of 3 looks adequate, but obviously gives us 550mm centres.

Perhaps some tables will reveal otherwise.

Thanks

 

[tony1851]

Hi

Our structural engineer has specified double 50 x 175mm joists at 400cc for our span of 4.5m for a small portion of the floor where the floor is unsupported from below.

The problem is that the width between the remaining single joists which either span over a block wall or are short enough in length, is 1100mm. So centres would be 370mm.

Can't quite follow this - sketch plan would help.

 

[freddiemercurystwin]

Why don't people just speak with their SE's?

 

[tony1851]

Why don't people just speak with their SE's?

Possibly because once the SE has come up with a solution, he/she would look daft if the client suggested something that was equally workable and cheaper.

 

[ree]

Your 4.5m (15ft) clear span joist calculation is wrong - perhaps check Bldg Regs table?

 

[tony1851]

If it works out at double 50x175 at 370 centres, it's not overkill.

Deflection of the joists will be the determining factor here and you will be at or possibly a little over the recommended limit.

(that's assuming the joists are the usual C16 grade and are in fact 50x175, and not planed down or easi-edge).

 

[ree]

I've got it wrong or i'm confused:

The clear span of 4.5m @ 400mm centres requires a heavier section than the one posted.