G&M wrote:

Also if you have more than three courses of brick above a 80cm
opening all loads above and outside the triangle are taken by the
side walls so the only loading on the lintel is a few bricks.

I think that's crap of IMM proportions from yourself, actually. I've
got in front of me the BRE's 'Good Building Guide' no. 10 (dated
November '91 - so probably not the latest version). This gives
simplified methods for estimating loadings where temporary openings are
to be made.

The BRE publication defines two triangles, both with a base 10% wider
than the clear span of the opening - 880 mm in this case. The load
triangle has base angles of 45 deg. and the weight of the masonry plus
any point loads supported within this triangle are considered to be
supported 100% by the lintel. The height of this triangle is clearly
equal to half its base, so extends up to 440 mm above the opening -
that's nearly 6 courses of brick, not 3.

Then there is the interaction triangle, which has 60 deg. base angles
and the apex at half the base times sqrt(3) - 762 mm. 50% of any loads
placed within the interaction zone, being the area that's inside the
interaction triangle but not the load triangle, are considered to be
supported by the lintel.

Thus only loads higher than 10-and-a-bit courses above the lintel can be
safely ignored without resorting to more precise calculation methods.
That's not quite what you said above.

I have been presuming you were a real live BCO but if that is the case I
would have expected you to have a much better understanding of loading
calculations.

Hmmm...

--
Andy