Drilling an oversized joist

[JohnW2]

A number of people here seem to be assuming that a joist larger than the minimum can have holes drilled or notches cut in it and only their incursion into the minimum "zone" taken to be of structural significance. ... I am not a structural engineer, and I suspect neither are they, but I very much doubt that they are right.

I'm certainly no structural engineer, but ...

...consider a joist which is of just the minimum permitted dimensions for the application/span, with a hole of permitted size drilled at its vertical mid-point (at a point in a span where drilling is permitted). This is compliant. Now imagine 'perfectly bonding' a substantial additional piece of timber to the top of the joist. The drilled hole is now considerably below the midpoint of the joist (hence maybe not compliant) - but is it conceivable that addition of that extra timber has actually made the joist structurally weaker? That appears to have been the basis of the OP's point - and my non-expert intuition finds it hard not to sympathsise (unless/until I am otherwise educated!).

Kind Regards, John

 

[bernardgreen]

but is it conceivable that addition of that extra timber has actually made the joist structurally weaker?

Possibly it would. there have been instances where adding bracing to structures has moved stress from nodes designed for stress to nodes not designed for stress ( or a different mode of stress ) and those nodes have failed.

 

[DetlefSchmitz]

I am working on the basis of instructions I received from a building control officer. I needed to top notch near the wall for underfloor heating and was told to over-specify the height of the joists by the amount I would need to remove.

 

[chapeau]

but is it conceivable that addition of that extra timber has actually made the joist structurally weaker?

No

Possibly it would. there have been instances where adding bracing to structures has moved stress from nodes designed for stress to nodes not designed for stress ( or a different mode of stress ) and those nodes have failed.

We are talking about an already overengineed simple joist.

Just notch the thing and move onto the next job.

 

[Scoby_Beasley]

The MIDDLE of the joist is ""redundant.""

When the joist is trying to bend under the weight it is supporting the top is being compressed ( along its length ) and the bottom is being stretched.

^^^What he said^^^

Plumbers seem to make their own rules up

Be fair - it's a lot harder to thread copper pipe through holes in joists than it is cable.

Copper pipe?? They seem to manage to wedge my cables in my holes with their plastic pipework OK!!

 

[JohnW2]

Possibly it would. there have been instances where adding bracing to structures has moved stress from nodes designed for stress to nodes not designed for stress ( or a different mode of stress ) and those nodes have failed.

I can understand that, but we are talking about the situation of a single simple joist, which has merely been 'strengthened' by the addition of material. I find it hard to believe that can have a detrimental effect.

Kind Regards, John

 

[ban-all-sheds]

The drilled hole is now considerably below the midpoint of the joist (hence maybe not compliant)

And therefore the new joist is going to perform just like a joist which has been drilled in the wrong place.

The stresses on it will be in different places. The added material will not be stress free.

but is it conceivable that addition of that extra timber has actually made the joist structurally weaker?

It is to me, because I am sensible enough to recognise that the extent of my ignorance in this field is unquantifiable.

That appears to have been the basis of the OP's point - and my non-expert intuition finds it hard not to sympathsise (unless/until I am otherwise educated!).

It's because I am not an expert, and because the consequences of being wrong could be catastrophic, that I will not use intuition as a substitute for expertise.

chapeau is not an expert either.

But it's Adrian's choice - if he wants to use a stranger's non-expert intuition as a substitute for proper advice, that's up to him - they are his timbers and it will be his house which suffers structural damage and he who will face enforcement orders from his local Building Control if chapeau is wrong.

 

[JohnW2]

The drilled hole is now considerably below the midpoint of the joist (hence maybe not compliant)

And therefore the new joist is going to perform just like a joist which has been drilled in the wrong place. The stresses on it will be in different places. The added material will not be stress free.

I have no argument with what you say, but I still find it very difficult to understand how (in the simple situation we're discussing) the effect could be structurally detrimental.

The corollary would be even more counter-intuitive. If one had a joist 4" deeper than it needed to be, with a hole drilled in it 2" below the mid-point, can you believe that could be structurally unsound, but that it would become structurally satisfactory if one chopped the top 4" off the joist.

I obviously fully accept that neither of us are experts in this field, but that doesn't stop us trying to apply common sense to it. However, as you say, the OP must (and I'm sure does) understand that none of the people who have commented here are relevant experts, and make his decisions accordingly.

...I suggested to the OP early on in the discussion that the only 'safe' approach (in terms of regulations and inspectors) is to follow the 'word' of the regs, which is the only thing which some inspectors will see, or be capable of understanding.

Kind Regards, John

 

[securespark]

But it's Adrian's choice - if he wants to use a stranger's non-expert intuition as a substitute for proper advice, that's up to him - they are his timbers and it will be his house which suffers structural damage and he who will face enforcement orders from his local Building Control if chapeau is wrong.

That's unfair Ban. He actually said earlier:

I think i will speak to the bco next he comes round, and see what hes got to say on the matter.

So, he's not going to take the advice of the great unwashed.

 

[big-all]

the way i look at it if your joist is over sized rather than assume the full over size is yours to cut and play with aim for 50% as being yours to allow for the fact a hole drilled central in the correct place would affect the strength so belt and braces
this will give you 30mm plus the 15 permitted so 45

 

[ban-all-sheds]

That's unfair Ban. He actually said earlier:

I think i will speak to the bco next he comes round, and see what hes got to say on the matter.

Sorry - forgot that.

 

[stillp]

Easy enough to check. Take a few pieces of timber of different sizes, with and without notches, place each one across a couple of bricks, and stand on it. You'll find that a 3" joist notched to 2" is much weaker than a 2" joist, because all the bending stress is concentrated in the area under the notch.

 

[JohnW2]

Easy enough to check. Take a few pieces of timber of different sizes, with and without notches, place each one across a couple of bricks, and stand on it. You'll find that a 3" joist notched to 2" is much weaker than a 2" joist, because all the bending stress is concentrated in the area under the notch.

Interesting! For a start, I was talking and thinking about ('central') holes (per OP), not notches, but I guess there is a similar argument, given that the bending stress will be zero round about the centre of the joist.

On quick reading, what you say sounds very reasonable - but I have to say that I still have a bit of an intuition problem when I start thinking! If one starts with a 2" joist and stands on it, it does 'feel wrong' that attaching ('perfectly') extra timber to some, but not quite all (i.e. the notch), of the top of the joist would cause it to bend more.

If there were a point load near to the notch, what you suggest would make more sense to me. However, if the load is evenly distributed across the span (which I think is the usual assumption in structural calculations), I'm far less convinced. If, for example, one notches 2% of the span, I would have though that 98% of the load would simply be transmitted through the 'extra wood' to the 'smaller' (but adequate) joist below.

It sounds as if some experiments (and reading) is in order!

Kind Regards, John

 

[stillp]

If there were a point load near to the notch, what you suggest would make more sense to me. However, if the load is evenly distributed across the span (which I think is the usual assumption in structural calculations), I'm far less convinced. If, for example, one notches 2% of the span, I would have though that 98% of the load would simply be transmitted through the 'extra wood' to the 'smaller' (but adequate) joist below.

The distribution of the load matters less in the case of the notched joist, since the additional material will concentrate the bending stress into the notched section.