is the joist weakened?

I

imamartian

if i have a 100mm x 47mm joist, is there a calculation to work out it's strength? or what weight it can support?

And what if i drill a 10mm hole in the middle of that joist, do i compromise it's strength?

and if i drill two 12.5mm holes above each other, have i reduced the strength of the joist to that of a 50mm joist?

and finally, if i drill one 10mm hole and put a 10mm bolt through, can i expect the strength of the joist to be unaffected?
 
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what is it you are trying to achieve it may be easier for an answer.
 
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i thnik your question is well above that of DIY level, thats more something that a structural engineer may know about.
 
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imamartian

ok, to bring it back to a DIY question.... what's the biggest hole i should drill in a 4" joist to run a cable through?

My original question was aimed at the purist.... :rolleyes: lol sorry
 
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i wouldn't personaly drill any hole in a 4" joist? where is this joist located? whats the span? if its only one cable then drill the smallest possible without it being too tight.
 
H

hotrod

what's the biggest hole i should drill in a 4" joist to run a cable through?

25mm hole drilled at the joist centreline and located between 0.25 and 0.4 times the span from the support. Multiple holes should not be less than 3 diameters (centre to centre) apart.
 
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if i have a 100mm x 47mm joist, is there a calculation to work out it's strength? or what weight it can support?
Yes, it's a straightforward number-crunch

And what if i drill a 10mm hole in the middle of that joist, do i compromise it's strength?
No

and if i drill two 12.5mm holes above each other, have i reduced the strength of the joist to that of a 50mm joist?
No, but there are rules for positions and spacing of holes and slots, generally totally ignored by sparkies and plumbers.

and finally, if i drill one 10mm hole and put a 10mm bolt through, can i expect the strength of the joist to be unaffected?
Yes
 

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