Double Door Lintel on Load Bearing Wall Advice

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Interesting conclusion, if I had a lintel that was taking a load y then the deflection would be 10mm( say) if I halved the load the deflection would be 5mm. I then take the load away and place a similar lintel underneath the original one and reapply the load y. The top lintel deflects as does the bottom one in equal measure. When the top lintel has deflected 5mm it will be resisting a load of y/2, similarly the bottom one will be resisting a load of y/2 hence the total load resisted will be y. Taking this through to a deflection of 10mm then each lintel will be resisting a force of y giving a total resisting force of 2y. The lintel weights are ignored for this simplification. To me it is the same as put a scaffolding plank on top of another which makes things less bouncy which reflects the decrease in loading per board. Obviously a single lintel of twice the depth increases the available resisting force more than 2 laid together. Thoughts?
All things being equal, strength & deflection have a cubic relationship to depth. So a 100x140 lintel is ~8 times stronger/stiffer than a single 100x65mm lintel. Whereas doubling them up (which is pretty much the same size and cost as the 100x140) is only twice as strong/stiff.

So I was being pessimistic saying it'd only be load distribution, but you are relying on the mortar bed giving a good load transfer between them. If the load was too much for a single 65mm, I'm not aware of any manufacturer who'd instruct you to stack up as opposed to just using a larger lintel.
 
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Gosh thank you all - I will have a think and maybe phone around for some quotes for a spec. What I'll do is report back which may be helpful to other readers :)
 
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Sometimes Woody its best to follow your mothers stock answer to inquisitive questions " because thats why" than try to understand the intricacies of what is being asked
What?

But sitting in front of the TV did not give me square eyes, and I did not catch a cold by not wearing my big coat when I went outside.
 
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All things being equal, strength & deflection have a cubic relationship to depth. So a 100x140 lintel is ~8 times stronger/stiffer than a single 100x65mm lintel. Whereas doubling them up (which is pretty much the same size and cost as the 100x140) is only twice as strong/stiff.

So I was being pessimistic saying it'd only be load distribution, but you are relying on the mortar bed giving a good load transfer between them. If the load was too much for a single 65mm, I'm not aware of any manufacturer who'd instruct you to stack up as opposed to just using a larger lintel.
I think it's quite simple.

If a single lintel is not quite good enough, or borderline, use two.

Again, this is a simple domestic loading situation.
 
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The advantage of course in using two smaller section lintels is you don't need 3 weetabix to position...
 
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All things being equal, strength & deflection have a cubic relationship to depth. So a 100x140 lintel is ~8 times stronger/stiffer than a single 100x65mm lintel. Whereas doubling them up (which is pretty much the same size and cost as the 100x140) is only twice as strong/stiff.

So I was being pessimistic saying it'd only be load distribution, but you are relying on the mortar bed giving a good load transfer between them. If the load was too much for a single 65mm, I'm not aware of any manufacturer who'd instruct you to stack up as opposed to just using a larger lintel.
Totally agree but it doesn't matter if there is a mortar bed or not, the load will still be transferred by numerous contact points between the upper and lower lintel.
 

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