Concrete Block Strength

6 Aug 2019
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United Kingdom
Hi All...

Possibly a stupid question, but here goes - how much weight can a concrete block (7.3N) and a Thermalite or Fibotherm (3.6N) actually support before it crushes?

Reason for asking is that I have 4m lintel (Catnic extra heavy duty) going in above some bi-fold doors - its a single storey extension, so the lean to roof will be sitting on the lintel. Wall is concrete blocks on outside, fibotherm on inside and the SE has just said to plonk the lintel straight on the blocks with a 150mm bearing with no padstone or anything... does that sound right?

Out of interest, how much weight could a 150mm bearing on fibotherm take? Is the calculation as follows:

150mm x 100mm = 15,000mm2

15,000mm2 x 3.6N = 54,000N

54,000N / 9.8 = 5,510kg

So you could sit 5.5 Tonnes on that little bit of block could you? Seems like a hell of a lot, but great if so...!

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The SE is correct, just rest straight on the blockwork.

The figure of 7.3N is known as the characteristic compressive strength of the block but in practice that figure is reduced by a 'safety factor.' In the case of new blockwork, the safety factor is 3.1. The safety factor is required to allow for faults in the block, and faulty workmanship, eg rubbish mortar or setting the block slightly out of vertical etc. When working out the permissible load per square mm, you can also increase the compressive strength by a further factor - which is 1.25 for most types of bearing.

So the maximum allowable stress on your 7.3N block will be (1.25 x 7.3)/3.1 = 2.9 N/mm2, which is a lot less. For the Thermalite, the maximum permissible stress will be (1.25 x 3.7)/3.1 = 1.49 N/mm2.
Interesting, many thanks.

So taking the Fibotherm then and using my calc above - 1.49N/mm2 would equate to 2,280kg safe load on a 150mm end bearing.. is that right? Still a decent amount of weight....
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Your figure of 2,280Kg is correct, but that will be a 'factored' load.
When the SE calculates the loads on the beam, he has to increase them by certain factors (usually 1.6 for live loads and 1.4 for dead loads - a little less if using Eurocodes). We dont know the actual loads, but as a rough guide, divide your 2280 by approximately 1.5, to give you the actual maximum (unfactored) load allowed.

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