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advice required On beam strength loss

Discuss advice required On beam strength loss in the Electrical Forum area at ElectriciansForums.net

Bernard1991

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Hey guys I’m currently re wiring a dormer bungalow and decided to check the bathroom fitters electrical work

To my horror the dope decided to drill out notches in the beams instead of moving these four lights to a clear location

Can anyone help on how much strength has been lost due to this?

Also these lights don’t look fire rated to me as they look to be all plastic with a metal effect plastic front/bezel and advice to this?

Thanks
Bernie

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diditrain

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Arms
Some numpty has seriously got their measurements wrong, I would also say, that is a fire hazard waiting to happen. What type of lamps are in the fitting and have you checked to see how much heat they are generating, at the point where they are cut into the joists?

Charlie_

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Arms
That is dog rough..
If that is the loft of the dormer then the fittings don’t need to be fire rated..
Manufacturers state a minimum distance for fittings from timbers, usually >50mm

ELECNEWT

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The strength of a rectangular beam loaded in the vertical direction depends on the second moment of inertia I.
Where I= B*D(cubed)/12 . Where B is the width of beam and D is the depth.
You need to know the exact dimensions but as an example: for a six-inch-deep beam with a one-inch- deep notch, the beam becomes effectively five inches deep.
So, the original D(cubed) was 216 and the modified D(cubed) becomes 125. The strength is now 125/216 or 58% of the original.
For changes in stress levels, other factors such as distance from supports, are relevant.

For interest:
If you halve the width of a beam you reduce its strength by a factor of two.
If you halve the depth of a beam you reduce its strength by a factor of eight.
If you halve both width and depth you reduce its strength by a factor of sixteen.

DPG

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Arms
Esteemed
elecnewt - I bet you've been waiting for someone to ask that question eh!

Good post!

James

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Mentor
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That bathroom fitter is as good at electrics as I am at plumbing and plastering. Why do I think there is no cert. for the electrical work?
Tell the customer it’s a concern and your not willing to connect to new DB until it has been put right.

Wilko

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Arms
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Hi - Unfortunately we see this all too often. A bit of thought and the beams can be avoided. I don’t think the ceiling will collapse but it’s clearly weaker now and may flex if you crawl about on it, for example. If that’s a worry another joist can be fish plated to it perhaps. The possible increased fire risk due to heat build up is another issue to be resolved. Do you know the make / model of the lights fitted?

Zerax

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The dimensions of modern build ceiling joists always look scarily small to me... and the spacing is too great too !

ELECNEWT

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elecnewt - I bet you've been waiting for someone to ask that question eh!

Good post!
Yes. Now I can move on the IET forum and lurk there.

Bernard1991

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I don’t have the specifics to hand at the moment I.e lamp makes and beam dimensions

It is right behind the dry line wall of the landing at the top of the stairs in the crawl space, the down lights are some cheap nasty looking leds fully made of plastic so any fire in the bathroom they would just fly up into the loft space in no time at all (well if they weren’t blocked by beams that is) don’t even know if there ip rated, granted there likely outside of zones with the Celing height but there’s a shower and no fan so wouldn’t shock me if steam gets in a wrecks them

ELECNEWT

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I don’t have the specifics to hand at the moment I.e lamp makes and beam dimensions
Just to clarify, the formula I referred to in my post was purely to highlight the effect of reducing the depth of a beam as opposed to the width.
To carry out any valid assessment of the effect of a notch or notches would require lots of data, advanced techniques and relevant expertise.

Meanwhile the Building Regulations lay down the permissible limits for notches in beams.

telectrix

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looking at the size of those joists, I can't see any appreciable loss of strength in them. however, the lights themselves and the possibility of heat damage is another matter.

ELECNEWT

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I don’t have the specifics to hand at the moment I.e lamp makes and beam dimensionthat is)
So what was the outcome ?

johnduffell

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The strength is unlikely to be an issue for joists, the limiting factor is usually the deflection. As newts excellent post the beam will be deflecting about twice as much.
If you notice how thin tree branches are and they bend all over without breaking, but you couldn't plasterboard a tree without the joints cracking. So the issue forr the ceiling will be cracking. If that space is just a void then it might be ok, but if there are people and furniture using those joists there could be cracking especially if the notches are closer to mid span where the bending moment is greatest.

telectrix

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so this is a no-no:

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