Do box lintels require padstones?

[Digmixfill]

I'm about to fit a catnic bsd100 over a doorway, 900mm span, in a wall made of celcon standard 3.9kn blocks.

Do i need to put padstones under it or not? Original plans specified a concrete lintel without padstones.

 

I don't think I need to fit them, but i have a nagging doubt so thought i'd ask the floor.

 

[PeterW]

Load bearing 140mm internal wall ..??

 

Nope, but buy it 1350mm and give yourself a better beating end if possible. 

[Digmixfill]

It's in a 100mm inner leaf. Roughly another 2m of blockwork to go above it.

[PeterW]

Crack on then - but 150mm end bearing is ideal. 

[Digmixfill]

For this particular opening I've got a 1200mm catnic and the span is 866mm.

Regs suggest 100mm bearing minimum for this span, but I thought i'd go 150mm anyhow.

[PeterW]

That’s fine - 150mm is much better as you get some wriggle room. 

 

Is that a 762mm door going in then..?

[Digmixfill]

I don't think we have two openings the same size here. The doors will be whatever size we can have made to fit the hole

 

Edited October 2, 2019 by Digmixfill

[Temp]

The basic principle of a pad stone is used to spread the load over a larger area than would otherwise be the case. I'm not an SE but the calculation goes something like...

 

Find out/calculate the maximum safe pressure (Newton's per square mm) that the blocks can withstand including safety factors.

 

Calculate the load in (Newton's) on each end of the beam.

 

Divide the pressure by the load to get the minimum bearing area  (square mm)

 

Divide the minimum bearing area by the width of the wall to get the minimum length of the bearing (eg how much of the beam must rest on the wall.  Regs say this must be >100mm).

 

If that minimum length is more than the beam allows you need a pad stone to increase it. (Eg if the minimum length calculates out at 240mm and your beam only allows 100 or 150mm then you need a 250mm pad stone.)

 

What I'm not sure about is how to calculate the load or the safety factor required.

 

Pad stones are more commonly required where a beam is at 90 degrees to a wall because the bearing area is fixed by the width of the wall and width of beam. Think cheese wire cutting cheese.

Edited October 3, 2019 by Temp