Lintel Loading Check

[bontwoody]

I would just like a little reassurance that I am calculating this correctly please. I have two windows at the back of my timber frame extension. The span is 1.5m.

 

There are no complicating loads above the windows so its just the dense concrete blocks. Ive calculated the weight of the Loaded Traingle above the lintel and come out with a total load of 1.37 kN or 0.911 kN/m. My bricklayed thinks I should put a 100mmx150mm lintel over them but looking at the loading table it suggests that a 100mm x 65mm is more than sufficient with a UDL of 2.6 kN/m.  Have I made a mistake somewhere?

 

Thanks in advance.

Edited February 20 by bontwoody
Misread of table

[JohnMo]

Why are you calculating, what's on your structural drawings? If you don't have structural drawings, you need to ask yourself why you don't?

[Russell griffiths]

Why use a 65 when a 150 is really not expensive, don’t be skimping on a couple of quid.  

[bontwoody]

26 minutes ago, Russell griffiths said:

Why use a 65 when a 150 is really not expensive, don’t be skimping on a couple of quid.  

I just wanted to be sure I was calculating correctly really, if it is more than ample why use a bigger one? It not really about the cost, although the smaller one will be a lot lighter.

43 minutes ago, JohnMo said:

Why are you calculating, what's on your structural drawings? If you don't have structural drawings, you need to ask yourself why you don't?

I drew the plans myself so this is me doing the structural calculations.

[JohnMo]

2 minutes ago, bontwoody said:

this is me doing the structural calculations

Sorry I like the Scottish system, either certified structural engineer provides a design certificate or you have to provide all the long calculations, for design approval, prior to getting a building warrant. No warrant no permission to build.

[bontwoody]

1 minute ago, JohnMo said:

Sorry I like the Scottish system, either certified structural engineer provides a design certificate or you have to provide all the long calculations, for design approval, prior to getting a building warrant. No warrant no permission to build.

Fair enough, I like to do as much as I can myself, I enjoy the learning and have noone else to blame if something goes wrong 🙂 The timber frame has all the required plans etc, its just the outer shell. Worse case I just do as Russell suggests and over spec it.

[Russell griffiths]

In England you can build under permitted development for planning

and a building notice for building regs, 3 inspections maybe 4 and your ready for sign off. 🤪

[kandgmitchell]

Just check that lintel doesn't need bonded masonry over to achieve that UDL. If it does make sure it's there and not interferred with by any dpc cavity tray etc.

[bontwoody]

1 hour ago, kandgmitchell said:

Just check that lintel doesn't need bonded masonry over to achieve that UDL. If it does make sure it's there and not interferred with by any dpc cavity tray etc.

Thats a good point, it would need the bonded masonry.

[Iceverge]

The computer super brain thinks you're right. II just dropped your question in there and the image of the span tables. 

 

Me on the other hand have no idea without eyeballing it on site and jumping up and down on it. 

 

 

 

 

EDIT. I notice it just read the span table incorrectly!!! If in doubt always jump harder on member in question. 

Edited February 20 by Iceverge

[bontwoody]

12 hours ago, Iceverge said:

The computer super brain thinks you're right. II just dropped your question in there and the image of the span tables. 

 

Me on the other hand have no idea without eyeballing it on site and jumping up and down on it. 

 

 

 

 

EDIT. I notice it just read the span table incorrectly!!! If in doubt always jump harder on member in question. 

Well I never thought of using ChatGPT like that!

[Canski]

Your bricklayer knows from experience that a 65 mm will deflect over that span especially with a ‘wet’ load. Either prop a 65 mm lintel in the middle while they build over it and then let the first course set before continuing or spend the extra £’s and use a 145 mm lintel with no propping. 

[bontwoody]

3 minutes ago, Canski said:

Your bricklayer knows from experience that a 65 mm will deflect over that span especially with a ‘wet’ load. Either prop a 65 mm lintel in the middle while they build over it and then let the first course set before continuing or spend the extra £’s and use a 145 mm lintel with no propping. 

Thanks. That also makes a lot of sense

[bontwoody]

Having read a bit more about the differences between composite and non-composite lintels, I have a question. Does the use of a cavity tray negate the support effect of the blockwork above a lintel and if it does then should a non-composite lintel always be used in that situation?

 

[Iceverge]

In a masonry build lintels are often misunderstood. 

 

All they do is. 

 

1. Hold the bricks in place until everything sets. 

2. Support the bricks that are below an imaginary "arch" line where everything above it is in compression. 

 

That's why they're often very slender vs what intuition might suggest. 

[Gordo]

The lintel seems over engineered but out of interest when working out any loading you should always include a "factor of safety" I generally allow a FOS of two but think 1.5 is the requirement for beams. I think for bolts and screws the FOS is as high as 7 or so. Where quality of workmanship is potentially suspect a greater FOS should be used. A structural engineer could confirm exact FOS required. 

Edited February 23 by Gordo

[Gus Potter]

On 21/02/2025 at 12:40, bontwoody said:

Having read a bit more about the differences between composite and non-composite lintels, I have a question. Does the use of a cavity tray negate the support effect of the blockwork above a lintel and if it does then should a non-composite lintel always be used in that situation?

Yes as the plastic introduces a slip plane which destroys the composite action between the masonry above and the lintel.  If you bend a beam (also a composite beam.. composite lintel) downwards then there is compression in the top, tension in the bottom. When you balance all the forces within the beam you'll find not least that there are axial shear forces along the length.. the top of the beam is in compression so it shortens.. the bottom is stretching getting longer.. so there must be some axial shear forces in there. If you put a cavity tray in then the longitudinal shear force is not transferred between the brick courses above and the concrete lintel.. add a bit of water to that.. as that is what cavity trays are for and you have a good slip plane. Hope that makes sense?

 

In this case you need a non composite lintel.

On 21/02/2025 at 21:59, Iceverge said:

All they do is. 

 

1. Hold the bricks in place until everything sets. 

2. Support the bricks that are below an imaginary "arch" line where everything above it is in compression. 

This is roughly how a composite lintel works.. but you often need sufficient masonry each side to act as bit of a buttress to the imaginary arch to be on the safe side. If you look at the manufacturer data for composite lintels they give you this information on the number of bonded courses of brick you need over the composite lintel to make it work.

 

1 hour ago, Gordo said:

The lintel seems over engineered but out of interest when working out any loading you should always include a "factor of safety" I generally allow a FOS of two but think 1.5 is the requirement for beams. I think for bolts and screws the FOS is as high as 7 or so. Where quality of workmanship is potentially suspect a greater FOS should be used. A structural engineer could confirm exact FOS required. 

Many manufactures data tables use a safe working load (SWL) which has the factor of safety (FOS) built in. This is fine for normal day to day stuff but watch out when you go for the longer spans as the deflections could cause other issues with say bifold doors below or sensitive renders. Always check to see if the lintel manufacture shows their loads as the total load or a UDL load (load per metre run of lintel)

 

In the round I often use non composite lintels as the extra cost outweighs the on site risks of bad workmanship, folk making late changes (an enthusiastic heating Enginner coring / slapping holes in the theoretical arch etc) to the design or propping an faffing about.

 

 

[Iceverge]

Ok, learning here. 

 

Correct my understanding if I'm wrong. 

 

Both composite lintels and non composite lintels operate effectively the same way. They enclose some steel to do the "tension" bit. They have some concrete above to do the "compression bit"

 

However a composite lintel gains lots of strength by having some masonry built above, all bonded together, acting monolithically to  do more "compression" increasing the load that can be carried? 

Edited February 23 by Iceverge

[bontwoody]

Thanks everyone for the contributions, Ill pop down to the builders yard now and see if they have non composite lintels in stock. Good to understand the theory behind the choices. 🙂

[bontwoody]

Tried two builders merchants now, neither has a clue what Im going on about. Makes you wonder how many lintels are put in using the wrong calculation? 🤔

[kandgmitchell]

Builder's merchants are there to sell you stuff, particularly the stuff they stock! Ask them whose concrete lintels they stock then refer to the manufacturers advice, decide what you need and then just tell them which one you want.

[Canski]

4 hours ago, bontwoody said:

Tried two builders merchants now, neither has a clue what Im going on about. Makes you wonder how many lintels are put in using the wrong calculation? 🤔

What are you asking for ? I have no clue either.  I’d just put a 145 x 100 mm prestressed lintel in. If you want to over egg it go for 2.1 m long instead of the min 1.8. Is the blockwork going to be rendered or timber clad ? I’ve had many a discussion with the NHBC about the need for cavity trays and weep vents with walls that are faced with waterproof render especially when using a PC lintel to the outer skin. If you put a tray in then your weep holes end up 150 mm above the top of your opening. Not pretty things these weep vents. NHBC are 50/50 on the need for cavity trays in this situation. 

Edited February 24 by Canski

[bontwoody]

15 hours ago, kandgmitchell said:

Builder's merchants are there to sell you stuff, particularly the stuff they stock! Ask them whose concrete lintels they stock then refer to the manufacturers advice, decide what you need and then just tell them which one you want.

I asked if they had span tables (which they didnt) but didnt think to ask whose lintels they were . Thanks

[bontwoody]

13 hours ago, Canski said:

What are you asking for ? I have no clue either.  I’d just put a 145 x 100 mm prestressed lintel in. If you want to over egg it go for 2.1 m long instead of the min 1.8. Is the blockwork going to be rendered or timber clad ? I’ve had many a discussion with the NHBC about the need for cavity trays and weep vents with walls that are faced with waterproof render especially when using a PC lintel to the outer skin. If you put a tray in then your weep holes end up 150 mm above the top of your opening. Not pretty things these weep vents. NHBC are 50/50 on the need for cavity trays in this situation. 

Its going to be rendered, thanks for the information. 🙂