Drainage lintel strength and bearing

[MortarThePoint]

I've a run of subfloor blockwork that is specified as 190mm 10.4N blocks. The drainage pipes pass through this wall and I wanted to make sure the lintels are strong enough and have the correct bearing. They've only fitted the one in the outer leaf in the photo below but are looking to use 2 of the same type of lintel bearing directly on to the Stranlite 10.4N blocks for the inner leaf. I should have measured the span but I would guess it is 1000mm.

 

Looking at the Structual Engineering calculations there looks to be about 6000 kg per metre length of that inner leaf so that seems like a lot of load to put on those lintels and the 150mm bearing at each end. On paper the pressure would be 60kN/(2 * 150mm * 200mm) = 1N/mm2 which is obviously much less than the blocks 10.4N/mm2 spec but perhaps that's missing something.

 

[PeterW]

Either is fine but personally would have used a 215 concrete naylor on flat like the blocks and just carried through

[MortarThePoint]

52 minutes ago, PeterW said:

Either is fine but personally would have used a 215 concrete naylor on flat like the blocks and just carried through

 

But you'd be happy with it just bearing on the blockwork or coursing blocks?

[Carrerahill]

1 hour ago, MortarThePoint said:

I've a run of subfloor blockwork that is specified as 190mm 10.4N blocks. The drainage pipes pass through this wall and I wanted to make sure the lintels are strong enough and have the correct bearing. They've only fitted the one in the outer leaf in the photo below but are looking to use 2 of the same type of lintel bearing directly on to the Stranlite 10.4N blocks for the inner leaf. I should have measured the span but I would guess it is 1000mm.

 

Looking at the Structual Engineering calculations there looks to be about 6000 kg per metre length of that inner leaf so that seems like a lot of load to put on those lintels and the 150mm bearing at each end. On paper the pressure would be 60kN/(2 * 150mm * 200mm) = 1N/mm2 which is obviously much less than the blocks 10.4N/mm2 spec but perhaps that's missing something.

 

What is that? Type A? 100x70? What is the clear span? about 800mm? 

 

Permissible load assuming at least 5 layers of brick would be around 3000Kg per meter...

 

 

 

 

[Carrerahill]

1 hour ago, PeterW said:

Either is fine but personally would have used a 215 concrete naylor on flat like the blocks and just carried through

I second this - that would have given @MortarThePoint more like the 6000Kg per meter he needs.

[MortarThePoint]

Clear span is about 1000mm. On the 215 blockwork they are planning to use two lintels (side by side). I don't know what Type but they look to be 100x70 with a single metal bar about 6mm in diameter near the bottom edge.

Edited May 19, 2020 by MortarThePoint

[bassanclan]

Are you having a block and beam floor? If so how many courses above the lintel to the beam and will a beam be resting on the middle of a lintel?

[Carrerahill]

58 minutes ago, MortarThePoint said:

Clear span is about 1000mm. On the 215 blockwork they are planning to use two lintels (side by side). I don't know what Type but they look to be 100x70 with a single metal bar about 6mm in diameter near the bottom edge.

That is even worse... 

 

The best a type A can achieve on a 600mm span with 5 courses is about 3000Kg... Who spec'ed the lintels?

 

For the sake of £50 I think I'd pop those out, remove the coursing brick, (maybe use pieces of the old lintel as pad stones) and get a G8 in there, which on it's own without any other courses is just shy of 600Kg/m.

 

If you have a few courses then this could drop to a type F or K9. 

 

All advice above is without having full facts and details, but at a basic level is accurate. 

 

 

Edited May 19, 2020 by Carrerahill

[MortarThePoint]

1 hour ago, bassanclan said:

Are you having a block and beam floor? If so how many courses above the lintel to the beam and will a beam be resting on the middle of a lintel?

 

It's not block and beam but a slab based flooring. It would be bearing directly on the lintel of the inner leaf and chances are only one of the two.

Edited May 19, 2020 by MortarThePoint

[Gus Potter]

Hope this helps.

Lintel sizing.

Engineering units an explanation  and how to get a rough gut feel  for how heavy / how much load a simple wall for example imposes or a beam / lintel.

Keeping it simple if you weigh 15 stones this is roughly 100 kilograms. An old Volvo was commonly referred to (reputably) as weighing 1 Tonne = 1000 kg. You may wish to take this as an Imperial Ton but let’s not split hairs!

UK Engineers tend to work in units of load called Newtons (N). Often seen in structural calculations is the abbreviation  kN  = kilo Newtons  thus 1 kN = 1000 Newtons (N).

To convert kilograms to Newtons you multiply kg by gravitational acceleration which can be rounded for most practical purposes to 9.81m/s^2  (metres per second squared) when producing the calculations. To get a feel  for things just say this is 10 m/s^2.

Now 100 kg x 10 m/s^2 = 1000 Newtons (N) = 1.0 kN.

If you weigh 15 stones in Engineering parlance the load on the ground when you are standing still is ~ 1.0 kN.  An old Volvo weighs about 10.0 kN ~ a tonne.

What about the lintel?

Dense concrete block wall (block compressive strength 7.0 N/mm^2) weighs round about about 20 kN/ cubic metre (density) including the mortar to stick it together, (ref: Structural Engineers Pocket Book. F. Cobb).

If you have a wall 3.0m high you have a load per metre run of wall of 20 (density) x  0.1(thickness) x 3.0(height) = 6.0 kN/m ~ 600 kg per metre run of wall. You often see this referred to in calculations as a “UDL”, a uniformly distributed load.

Key Point!

The load calculated above is a “working load” per metre run of wall (kN/m) also called an unfactored load. This calculated load has to be less than the “safe working load (SWL)” declared by the manufacturer.

Some manufactures give tables based on a “total safe working load” or “total permissible load” thus if you have a lintel of 1.2m long the total load is 1.2m x 6.0 kN/m = 7.2 kN

Some give information based on a safe working load per metre run of lintel (kN/m). Look carefully!

You’ll also see data tables that refer to a “characteristic load” Please do not confuse the two as the SWL / permissible load tables have some (but not all) safety factors built into them. Characteristic loads do not. These loads have a higher value so are not safe to use without safety factors. You still need to make sure the rest is adequate and anything below and to the side.

Not all lintels are the same!

For pre stressed concrete domestic type lintels there are generally two common types. One is called a “composite lintel” the other a “non composite lintel.

A composite lintel works mostly by creating a triangulated “A” frame where the masonry above acts in combination with the steel rod in the lintel which is in tension with the masonry above acting in compression. Essentially, you create a deep composite beam to span the opening. To make this work you need a good few courses of continuous masonry above it, a bit either side too and generally no concentrated loads directly above. This is usually qualified in the manufactures tables.

However, if you have say an offset opening above the lintel which introduces concentrated load or some floor joists perhaps that bear on or near the top of the lintel then you can lose the effect of the composite action. You need then to perhaps consider a non composite lintel.

I have copied a screen shot from Robeslee data table below for a flavour. For a clear span (structural opening, not to be confused with an effective span... best for another day) with 5 layers of brickwork above  and assuming correct and adequate lintel rest at the ends, the lintel will support a UDL (distributed load) of 30 kN/ m run. This looks promising when we look back at a 3.0m wall leaf loading of 6.0 kN/m run. If you have the wall width you possibly can use two side by side and introduce a header course or two of brick to tie them together along their length.

But “without brickwork” to create the “A” frame / composite action” it drops massively to 3.86 kN/m which is ~ 380 kg /m and that is roughly a ninth of the allowable permissible full composite load Now you have overloaded the lintel by a serious amount when we compare this to the load from a 3.0m wall leaf.

However it looks like you are using blockwork not brick. When used to support blockwork the composite strength of these lintels can be a good bit  less than when used with brickwork so check with the manufacturer.

Much also depends on where you put the DPC. Clearly if you are opting for the composite route and you have a slippy piece of DPC inserted in the bricks that are acting compositly it stops working.

Practically you may just want to go for a non composite lintel in case you want to knock a hole in the wall later on!

In summary. Look up, see what is above and what you need to hold up and just as importantly how stop  anything moving sideways (lateral restraint) or twisting (torsion). Remember that the inner and outer leaf of a wall could be carrying different amounts of load. Also remember that it is important to look down too and see what you have below as a support.

There is a saying  “if it doesn’t look or feel right it probably isn’t”! If you have any doubts at all it’s always best to ask the structural designer which you may want to do once you have had another look.

 

[MortarThePoint]

Thanks @Gus Potter, very informative. There is a door immediately above the floor slab that bears on the lintel. The load from that is 17kN/m but there's no scope for composite action. They did swap it to a P215 lintel. There is a helper pillar breaking the span into 2 spans, a 650mm span and a 300mm span.

 

There's concrete packing underneath the lintel but of unknown quality and therefore unknown strength.

[MortarThePoint]

I just looked and a P215 appears to have an unfactored UDL of 27kN/m, but would that be in the A frame scenario?

[Gus Potter]

I think the Naylor P215 lintel is still only 65mm deep. I would run it by your Engineer just to be sure. It should just be a quick phone call.

 

All the best.

[MortarThePoint]

Just now, Gus Potter said:

I think the Naylor P215 lintel is still only 65mm deep. I would run it by your Engineer just to be sure. It should just be a quick phone call.

 

All the best.

 

Yes it is only 65 deep. Thank you for the insight.

[MortarThePoint]

5 minutes ago, Gus Potter said:

I think the Naylor P215 lintel is still only 65mm deep. I would run it by your Engineer just to be sure. It should just be a quick phone call.

 

All the best.

 

Out of interest, do you know if the Naylor P215 is a composite lintel or not?

[Gus Potter]

I would give them a quick call just to check.

[Oz07]

What are you running through the hole a public sewer?!