Nothching joist to reduce roof height

[tony1851]

That would be the easier way, but the builiding inspector may raise an eybrow or two - Is it documented in building regs?

No. The Building Regs for the structural use of timber just points us to Guidance, eg the TRADA tables, which are often overly-conservative.

But is also acceptable to work to the standards in the traditional timber design code, BS 5268, or the Eurocode, EC5.

Among other things, these allow notching at the bearings, giving simple equations to follow; in some cases, it is possible to notch up to half the depth of a beam.
On a 3m roof spanned with 150 x 50 joists, a 25-30mm notch on the underside would be no problem.

 

[Dereekoo]

As Tony has stated no problem with notching and to alleviate any undue stress concentrations at the corner of the notch
it is advised to cut at a 1 in 10 angle,

 

[^woody^]

All well and good but the point is, why notch, why reduce any performance at all when there is no need because other options are available?

That is not good design

 

[cdbe]

Looks impressive. But why would you bother doing all that (drilling steel, bolts, washers, hangers, nails, timber) when you could just do this:

I did read (so correct me if I'm wrong) that a row of noggins should be inserted to restrain the joists, I used the offcuts.

 

[tony1851]

All well and good but the point is, why notch, why reduce any performance at all when there is no need because other options are available?

That is not good design

It doesn't reduce performance in terms of bending stress or deflection, neither of which are of any consequence at the bearing.
It does reduce the allowable shear stress at the bearing, but this is rarely an issue in domestic joists, which are not heavily-loaded.

 

[^woody^]

It doesn't reduce performance in terms of bending stress or deflection, neither of which are of any consequence at the bearing.
It does reduce the allowable shear stress at the bearing, but this is rarely an issue in domestic joists, which are not heavily-loaded.

But it does reduce performance, irrespective of your personal opinion of whether it matters or not (an without you knowing if it will matter or not at some future date), so the question remains "Why would you want to purposely do that when there is an option to construct the roof with maximum design performance?"

So as I said its just plain poor design

 

[OrangeJuice09]

Among other things, these allow notching at the bearings, giving simple equations to follow; in some cases, it is possible to notch up to half the depth of a beam.
On a 3m roof spanned with 150 x 50 joists, a 25-30mm notch on the underside would be no problem.

I looked up EC5 document, the formula looks complicated - I don't have a structural engineer, is there a simple way to work this out? (perhaps an online tool)?

Alternatively, I can oversize the beam and then notch.

Thanks

 

[^woody^]

Looks impressive. But why would you bother doing all that (drilling steel, bolts, washers, hangers, nails, timber) when you could just do this:

Because webs should carry the load, not flanges. Like something lese in this thread, bad practice.

 

[tony1851]

Because webs should carry the load, not flanges. Like something lese in this thread, bad practice.

Yes, because putting load on the flanges induces torsion in the beam, which adds to the bending stress. So doing it by the book,you bolt/weld/screw to the web, to keep the load central.

However, for the small loads encountered in domestic settings, it's perfectly feasible to apply a load to the bottom flange without ill effect.
As an example, in domestic work when doing a 'T'-beam arrangement, I often set an incoming 152 deep beam directly on the bottom flange of a 203 deep beam with no fixings; never had any issues at all. Sometimes, we need to think outside the box a little.

 

[tony1851]

I looked up EC5 document, the formula looks complicated - I don't have a structural engineer, is there a simple way to work this out? (perhaps an online tool)?

Alternatively, I can oversize the beam and then notch.

Thanks

The eurocodes are complex, the BS code less so. What are your roof build-up, joist spacings and timber grades?

 

[OrangeJuice09]

The original passed off drawings had a 'lean to' roof with a vaulted ceiling. Because it seems too high (I have neighbour and the extension is built close to the boundary) I'm looking at a flat roof. The inspector seems ok with a flat roof (in principle).

Hence, I'm working to the same joist size 6 x 2 but rather than C16 I'm putting in C24. The span is 3m to the outer wall of the extension, though I may overhang by 300mm. The distance between the joist is 450mm. Planning on a cold roof system, to maintain minimum height.

 

[Dereekoo]

I looked up EC5 document, the formula looks complicated - I don't have a structural engineer, is there a simple way to work this out? (perhaps an online tool)?

Alternatively, I can oversize the beam and then notch.

Thanks

Out of interest I have put that formula into an XL spreadsheet but must admit that I am not 100% convinced on the output as a 90 degree notch give a very low Kv value and blows allowable stresses out of the water, in your case 10% of those allowed in a full depth beam (maybe my understanding of the formula is not up to scratch?). Unfortunately I cannot drop the spreadsheet onto the forum but can drop a photo of the formula in each cell which you could copy onto your own spreadsheet if you are OK with spreadsheet inputs and the like, many people aren't.

 

[OrangeJuice09]

Perhaps send a screenshot of a worked formula and I should be able to work it out. If you could annotate, that would be helpful.

 

[tony1851]

Out of interest I have put that formula into an XL spreadsheet but must admit that I am not 100% convinced on the output as a 90 degree notch give a very low Kv value and blows allowable stresses out of the water, in your case 10% of those allowed in a full depth beam (maybe my understanding of the formula is not up to scratch?). Unfortunately I cannot drop the spreadsheet onto the forum but can drop a photo of the formula in each cell which you could copy onto your own spreadsheet if you are OK with spreadsheet inputs and the like, many people aren't.

Eurocodes I think are designed for spreadsheets rather than hand calcs. Though they can give slightly lighter sections than the BS codes if you are into pain and only use a Casio.
The BS timber code is conservative and easier for oldtimers like me to use.
OP could easily do a 35mm notch in a 147 deep joist:

 

[Dereekoo]

Perhaps send a screenshot of a worked formula and I should be able to work it out. If you could annotate, that would be helpful.

Herewith spreadsheets showing figures and formula behind the figures. Red cell figures are typed in to suit, all other cells not touched. Formula 6.62 determination shown in blue cells. Ignore orange cells. Hope it all makes sense.