How to calculate which lintel to use

[lindsey hodges]

We got build regs approval to put 2 small windows (630mm w x 1200mm h) in the gable end (ground floor) wall to let some light in. It is a timber frame house with cavity wall and brick faced. We are doing it ourselves as we're good at DIY but council are saying they want the calculations for the lintel we will be using. They haven't even mentioned the timber frame which I find odd as that's the main load bearing wall. I have weighed bricks they weigh 2141g each there are 80 bricks above opening. Bricks measure 8cm x 21cm. I was going to put in a lintel at 1050mm wide load max of it is 12kn. Can anyone show me how to work it out ? As structural engineer just quoted me £950 to do the calculation!!! Many thanks for any help you can enlighten me with! Lindsey

 

[Alastairreid]

When they ask for calculations it would generally be from the lintel manufacturer or a structural engineer...try catnic.

 

[jeds]

80 bricks = 1.25m². 1 square metre of brick (laid) = 130kg = 1.3kN/m² - 1.25 x 1.3 = 1.63kN. Multiply by 630mm = 1kN load on beam.

This is the why calculating loads on domestic openings up to, say, 3m is unnecessary. (because lintels are way, way, way over what is needed.)

You'd think building control would know that wouldn't you?

 

[mrrusty]

Surely if its only 630 wide, the only brick load on the lintel is the triangle above the opening but I'm no structural expert. Timber frame surely is going to depend on where the structural uprights are? or is it SIP?

 

[lindsey hodges]

Thanks guys, the catnic timber frame lintel is 5kn whereas the cavity wall 1 from buildbase I was considering is 12kn. Jeds I didn't communicate my 80 bricks very well I meant 80 bricks high above window to under roof so that would be about 6.4m high by 1m width ? or do I only calculate the open space window width of 63cm ? Not a metre which would be total lintel width ? Either way that should still be under the 12kn lintel max I think...

 

[lindsey hodges]

I also wondered if its just a triangular load above ? Then my 15 year old confused my by pointing out if I turned the triangle upside down all load would be on 1 brick ???

 

[^woody^]

You just specify a suitable standard lintel for the opening - IG L7/50 x900mm

Give that to the inspector. It can't be refused, no calculation nonsense.

For the inside with the addition of a new header plate into the frame that should be acceptable as per the LABC standard detail. Or either double 50x150 timbers bolted together as a beam, or a similar size box beam of 50 x75mm timber and 11mm ply sheathing. insulate the void.

 

[lindsey hodges]

Thanks woody I will print your response out and give it to them ! Much appreciate everyone's advice thankyou

 

[tony1851]

Thanks guys, the catnic timber frame lintel is 5kn whereas the cavity wall 1 from buildbase I was considering is 12kn. Jeds I didn't communicate my 80 bricks very well I meant 80 bricks high above window to under roof so that would be about 6.4m high by 1m width ? or do I only calculate the open space window width of 63cm ? Not a metre which would be total lintel width ? Either way that should still be under the 12kn lintel max I think...

If it was 800, or even 8,000 rather than 80 bricks to the top of the wall it would make not one jot of difference to the load on the beam, due to the 'arching' effect of brickwork.
The load on the lintel will be triangular; if it's 630 mm wide, the lintel will support 3 bricks on the first course, 2 on the second course and 1 on the third and final course, that's 6 bricks-weight - say 7or 8 bricks weight tops to be on the safe side; a small off the peg catnic or IG lintel, with 100 bearing each side would do.
That'll be £850, please (I've given you a £100 discount).

PS the SE who quoted you £950 is just having a laugh.

 

[lindsey hodges]

Lol thanks Tony, great explanation for a diy person like me, yeah I did chuckle slightly whilst telling the SE it was a little more than I expected...

 

[mrrusty]

Is the BC concern more about cutting new openings in the structural timber frame than the non-structural brick skin even though they haven't mentioned it? Or perhaps they haven't realised it's timber frame. A quick look around shows little information is available on this subject and how to carry it out in different types of frame construction.

 

[lindsey hodges]

I told them it was timber frame therefor the brick was not load bearing, they just said to replace 1 upright wood with 1 either side and a double piece above. But that for brick work I would need structural engineer calculations to prove lintel was adequate as the inspection guys do not know whether its correct and it has to have written calcs so it can be passed to SE for approval...

 

[^woody^]

I told them it was timber frame therefor the brick was not load bearing, they just said to replace 1 upright wood with 1 either side and a double piece above. But that for brick work I would need structural engineer calculations to prove lintel was adequate as the inspection guys do not know whether its correct and it has to have written calcs so it can be passed to SE for approval...

Did you tell them the name of the actual lintel you were going to use?

 

[lindsey hodges]

No I didn't get the chance, the last planning inspector said he would come out before I started to let me know requirements, he has left and the new one is not helpful at all

 

[^woody^]

Well you need to, that's how it's done. You specify a standard lintel, they approve it.