Oak beam calculation to support existing joists - help


Postby ianross » Mon Dec 29, 2008 6:25 pm

Any help would be appreciated.

Property dates from 1850s and as such the joists in the old part of house do not confirm to current regs - not a problem as they are sturdy enough.

Problem has come as have just built new extension and cut door on first floor into new extension. To esnure the floor height upstairs is te same as existing house and that the ceiling height downstairs is not too low (remember this is an old cottage with low ceilings !) I have used C24 graded joists.

The joists span a distance of 3800mm and centres are 400mm apart. The joists measure 45mm wide by 120mm deep . The building Inspector is refusing to pass the beams as states they are not suitable for the span and I should have installed deaper beams (That would have meant that joist would have been below height of windows and meant the rear door wouldnt open!!)

An option that I have would be to install one, thick green oak beam running at right angles to the existing beams. I would like to cut this beam into the existing joists albeit not fully only by 2 inch or so, so that I can then plasterboard between joists and keep the cottage look as per rest of house. The ideal centre for the beam would be 1950mm from one edge of the 3800 span ie,just off centre. The beam would have to span a distance of 3200mm to run at right angles to existing joists and be fixed into wall at each end, one wall is stone, the other concrete block.

Building Inspector states I should take into account the fire superficial timber when calculating the size of the beam ??

Is there a simple table that anyone can direct me too or assist with calculations required.

thanks in advance. Ian
ianross
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Postby stoneyboy » Tue Dec 30, 2008 9:41 am

ianross,
The BI is correct, the maximum span for the joists you describe is about 2.6m - + or - depending on the loading.
You will need a structural engineer with the appropriate computer software to do the calcs for you, even then I am not sure strength details are avilable for grren oak. How about using an rsj and encasing this so it looks like an old oak beam?
end
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Postby ianross » Tue Dec 30, 2008 11:05 am

stoneyboy - thanks for the reply. is it easier to find calculations for rsj or will it still be a cheque to the structural engineer.
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Postby stoneyboy » Wed Dec 31, 2008 4:56 pm

ianross,
You should be able to get hold of load performance of universal beams (rsjs) from Corus (now TATA). Whether your building inspector will accept these I do not know - you will have to calculate the udl + safety margin, which with the span should enable you to select the correct beam size.
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