Splits in loft timber joist

[dave]

Here;s a pic of the splits in one of the loft joist in our house.

such splits normal?

Those splits has been there for several years. Thing is, we have recently heard an occasional "timber creak thud" and the only problem thing visible up there is as in the pic.

Are there steel U piece available for this kind of thing so I can bolt two U'stogether at intervals along the joist? If so is a builder merchant the place to go for them? They'd need to be pretty beefy to do any good. Maybe (guess) 10mm steel so they can take some of the load.

I thought flat plates would be easier to fit (bolt them through an to the joist), but then I have to drill the joist - which given the condition it is in - I'd rather not.

I could have some made up U brackets made up (expensive but cheaper than a new roof!)

[Cash]

Yes!

Timber is a natural product and will move with variations of temperature - often making some rather scary sounds in the process. If there are no serious cracks appearing in the walls, rot or beetle infestation - then don't worry about the noises

From your picture - it's not worth the cost or time to do anything at all and in fact, there is no need to do anything at all!

Again, from your picture, you are worrying for nothing - that's natural movement.

Then to fit the U brackets you would have to start opening the roof up - with the likelyhood of causing far more serious and expensive damage to the sarking felt, battens and tiles (or slates).

If there are no worse splits in the rafters that you have not showed us, then there really is nothing to worry about - other than paranoia that the roof is about to fall in.

Cash

[meow2222]

Even if you sawed right through that joist, nothing would happen. Chill.

NT

[Matty F]

house.http://tinyurl.com/ryr4ol>> Are such splits normal?

Nothing to worry about. I'm wondering how the timber is attached. There appear to be two nail heads at the top left. It looks like the timber is a tie to keep the roof from spreading apart, so the timber is in tension, and more than adequate for the job. Except that I'd use more nails or preferably a bolt to attach it.

[The Natural Philosopher]

Yes, and allowed for in structural calculations.

[TheScullster]

"The Natural Philosopher" wrote

Can you elaborate on that a bit please? I did HNC structures and don't remember that bit - to be fair the timber bit passed me by to some extent (more interested in steel at the time). I do remember stresses across the grain/with the grain and moisture content levels playing a part, but that's about it :(.

Phil

[JTM]

[snip] I wish mine looked so good, but as they've been attacked by something called 'capricorn' (long horn beetle) they are a bit chewed in places

John

[The Natural Philosopher]

ER..you misunderstood me.

What I meant was that structural timber of a certain grade is expected to have a certain amount of checks and knots, and therefore the tables that architects and builders use, do not refer to 'a good piece of wood in a laboratory' when it comes to the wood strength properties but 'an average piece of builders merchant shit that's been in the rain too long'.

Funnily enough, longitudinal splits don't affect the load bearing capacity, but they do affect the stiffness. Think of leaf springs versus solid beams..:-) However the splits need to extend a long way across the total length before this makes any real difference.

Most timber structures are way too strong, they are more built to a stiffness criterion. I.e bouncy floors are out, even if they can support a grand piano.

So, basically when building, it being the case that by and large unless you are prefabbing trusses, labour cost is way over material costs, there is little incentive to underspecify timber to the point where its 'on the limit'.

Or select 'top grade timber' - which costs more anyway because it's hand selected.

Most builders will cut off dodgy ends and use them for noggins or firewood anyway!

[Fredxx]

Errrr - are you sure? Stress was an inverse cube function of depth. Halve the depth and you have 2 discrete pieces of timber with 1/4 strength of the original piece. Where am I going wrong?

I do agree that generally, as long as the cracks don't join, the OP has nothing to worry about.

[RubberBiker]

Lots of new structural timber has that level of splitting in it.

Structural timber is automatically tested/graded/marked by the supplier, i.e. each beam is rolled through a machine that measures its deflection under load.

See very lovely little animation of it happening:

the past, timber was visually graded (well, it still is for things like green oak timber frames, or using timber in the round) - but nowadays each piece has been tested (though it's still smart to select your best timbers for the most demanding locations, and juggle slicing up your stock to eliminate any uncertain-looking areas).

[The Natural Philosopher]

1/4 the stiffness. Not strength.

Two beams one on top of each other at same load:

Each beam takes 1/2 the bending, over 1/2 the depth. Same tension /compression load in as original.

So 4 times the deflection, but same ultimate breaking load.

[Rob G]

So the possible answer in the end depends on whether this is a rafter or a joist, as the former will have a deflection load while the latter, depending on the structure, is essentially in tension.

Rob

[Clot]

Agreed. Don't worry, Captain Mainwaring!

[Fredxx]

Same shear stress I will agree with you, but not the same tensile stress, assuming that the 2 beams are allowed to perfectly slide on each other.

[The Natural Philosopher]

No, the answer is that strength is a particular quality in engineering which is not the same as elasticity.

Strength is where it breaks. Elasticity is how far it moves.

[The Natural Philosopher]

Eh? of course it is. Half the load over half the depth gives the same bending moment, so the tensile and compressive stresses at the top and bottom are the same.

[Fredxx]

No. Look up second moment of inertia and work out how to calculate it.

Anyone who's ever done half a course on stress analysis would know.

[The Natural Philosopher]

Second moment of inertia is not about breaking strain, its about elasticity.

Read it up yourself.

[The Natural Philosopher]

Or to put it in terms even an idiot can understand, you are telling me that if a given beam takes a given weight before breaking, two beams together, one above the other or side by side, can take more than twice the weight before breaking? Glueing them together makes theme stiffer, but not stronger.

[Fredxx]

Your suggestion that half the depth of a beam gives 1/2 the deflection is so far off the mark. Half the width and I'd agree with you.

This is a sag calculator, mentioned in this groupe before, where you can try halving the thickness of a shelf to check that deflection is x 8.

I can't be arsed to find bending stress calculators or tables for beams, perhaps you can can find one yourself?

Do you actually know the formulae for calculating bending stress in a beam?