aircrete, lintels and joist hangers

Standard single storey extension, brick outer, aircrete inner, walls holding up 1 tonne shallow pitch tiled roof. In a standard build like this, is there any requirement to increase the catnic style lintel bearings from the minimum 150mm ? Since it costs not a lot more, I thought I'd increse bearings to, say, 200mm just to be sure. Any point in doing this ?

Also, any problem with the 75mm wide contact area for joist hangers or purlins built into the aircrete wall ?

Also, sometimes engineering bricks are recommended as a spreader, but this will be reducing the insulation of the wall. In these types of conflict in regs, what is the usual BCO view ? (Bit like the conflict between insulation and ventilation at the moment) Cheers, Simon.

Standard single storey extension, brick outer, aircrete inner, walls holding up 1 tonne shallow pitch tiled roof. In a standard build like this, is there any requirement to increase the catnic style lintel bearings from the minimum 150mm ? Since it costs not a lot more, I thought I'd increse bearings to, say, 200mm just to be sure. Any point in doing this ?

Also, any problem with the 75mm wide contact area for joist hangers or purlins built into the aircrete wall ?

Also, sometimes engineering bricks are recommended as a spreader, but this will be reducing the insulation of the wall. In these types of conflict in regs, what is the usual BCO view ? (Bit like the conflict between insulation and ventilation at the moment) Cheers, Simon.

Probably not, it sounds like the loading on them is going to be minimal.

What strength are the blocks rated at?

I would expect that staying up, overrides staying warm! ;-)

Blocks are Thermalite Shield, 4N/mm^2. Thanks, Simon.

So with a contact area of 75x150 and x4 you need 45kN to crush the blocks. That sounds like it should only be a real problem for loads exceeding 9 tonne evenly distributed on the lintel.

On Wed, 29 Apr 2009 00:45:18 -0700 (PDT), a certain chimpanzee, sm_jamieson randomly hit a keyboard and produced:

The 150mm end bearing for pressed steel lintels is a rule of thumb to avoid over stressing the end of the lintel. For a lightly-loaded lintel (just holding up a short lean-to roof), it could be reduced to

Timber would probably crush, or more likely start bowing in the middle, before it had an effect on aircrete.

Spreaders are really only needed for steel beams (RSJs) or very heavily loaded pressed steel lintels on aircrete. A smaller area of a higher strength blockwork would be permissible if required, so long as the wall's U-value didn't exceed 0.7W/m^2K at that point. If you really wanted, you could work out the average U-value of the whole wall.

Haven't you learned by now every BCO has their own version of 'The Regs'?