Notching joists.

Stick a brick under with some DPM on top. Oversite looks in good condition.

cdbe said:

Stick a brick under with some DPM on top. Oversite looks in good condition.

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One idea I had was to cut the base out of the brackets, fold up some channels in 2mm galvanised steel and weld them to the brackets. I'm a sheet metal worker so altering the brackets wouldn't be a problem. Cutting the base of the brackets out would be though. It will be very difficult to get in with a slitting disc on an angle grinder.
The oversite is mostly an old concrete path and the rest is concreted over. At the end where I've took the pictures the bottom of the joists are going to end up about 20mm off the concrete. The concrete slopes away so at the opposite end of the room there is a bigger gap under the joists. I think I'll notch the joists, screw them into the brackets and then pack up underneath the ends. It'll add extra support and is the easiest option.

Bouy said:

If you actually did the calc for a c16 47 x 54 joist it shows that it fails strength wise by 5% and deflection wise by 35%, shear wise its OK by 50%. Obviously the fact that its still 102mm deep in the majority of the joist suggests that strength and deflection would not be an issue. The only bug factor is that where the notch is all sorts of secondary stresses start to occur and this can lead to the propagation of splitting planes and ultimately to failure. Possibly by not notching at 90 degree but splaying off at 30 degrees (say) may help to prevent excessive secondary stresses occurring

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Does cutting a notch at an angle instead of 90 degrees really make it stronger? I've never heard of that before.

Gilesda said:

Does cutting a notch at an angle instead of 90 degrees really make it stronger? I've never heard of that before.

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Yes, an abrupt change of cross-section tends to cause increased localized stress at the internal corner. In the case of timber design, the Code of Practice recognises this by reducing the allowable shear stress at notched bearings.

(This is the reason the old Comet jet airliners of the 1950s failed catastrophically, because their windows were square. The 90° corners became highly-stressed during repeated pressurizations of the cabin, leading to cracking of the thin aluminium skin at the rivets and eventual rupture).

tony1851 said:

Yes, an abrupt change of cross-section tends to cause increased localized stress at the internal corner. In the case of timber design, the Code of Practice recognises this by reducing the allowable shear stress at notched bearings.

(This is the reason the old Comet jet airliners of the 1950s failed catastrophically, because their windows were square. The 90° corners became highly-stressed during repeated pressurizations of the cabin, leading to cracking of the thin aluminium skin at the rivets and eventual rupture).

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That's interesting, thanks.

Gilesda said:

I though about fixing new brackets alongside the originals set at the height I want. So the joists move along a few inches from where they are now. Fixing new brackets though I'm not sure how id do that given the originals were put in as the walls were built.. I suppose I'd have to cut mortar out of the joints and slide them in.

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Just get a face fix joist hangar and fix to the front of the brickwork at the right height with rawl bolts.

Gilesda said:

Does cutting a notch at an angle instead of 90 degrees really make it stronger? I've never heard of that before.

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If you really wanted to go to town, you could even use a 20-30mm holesaw to avoid internal corners completely, then cut the appropriate flat surfaces at the correct angles.