What can I use these joists for?

[ProDave]

I got these from my neighbour, who in turn got them from an office unit they dismantled.  They were spanning the ceiling in that.

 

10" by 3" timber joists 6 metres long.  Quite old, but were used indoors and dry (until removed)

 

Each has a scarf (I think that is the name) joint at about mid point.

 

No markings found yet so say any grading.

 

7

 

I would like to use them for my car port that I would like to have a 6 metre span.  That's why these 6 metre joists caught my attention.

 

The car port will be 6 metres square, roofed in box profile steel sheet. I want to keep the span open, the side joining the garage can have as much support from the garage as it needs.  the opposite side can have multiple support posts as can the back.

 

Initial thinking is to use these 4 big joists to span side to side, which would put them at 1.5 metre centres, and then span front to back with something smaller.

 

Thoughts please?  Particularly about those joints, would I be wise bridging those with something like a steel plate bolted on?

[Mr Punter]

I think they may be finger jointed.  You see it on window frames etc.

[saveasteading]

My thoughts. These precision scarf joints are amazing. But I don't know that they are ever used structurally.

I've smashed some up and they failed elsewhere than the joints,  but that's not proof of being strong as joists.

 

If there is no need for proof, and it isn't  an inhabited space, then I agree to add some extra splicing. Steel plates will look deliberate but will cost about £30 each. Nail plates will do but may look crummy. Plywood will do too, but should be longer.

Lots of nails.

[SteamyTea]

Finger joints, which those are, are used structurally.

They won't fail in normal usage.

[ProDave]

I worry about their integrity when wet.  As soon as I can after this current wet spell, I want to empty the storage under the static caravan (mostly ladders) and try and get these under there in dry storage until I use them.  Then in use, I will treat them with something and they should be mostly dry under the box profile roofing.

[SteamyTea]

43 minutes ago, ProDave said:

worry about their integrity when wet

Finger joints are usually bonded with an adhesive.

The 'fingers' are used to massively increase the contact surface area.

The angle allows for self locking of the joint.

[TonyT]

Leave to dry and sit them on breeze block at either end to check the defelction

[joe90]

I think that’s a finger joint not a scarf joint, not sure of its integrity structurally, really depends on the glue used I guess. The SE in me says put it on blocks either end and jump up and down on it in the middle 🤣

[markc]

The joint will be stronger than the timber, look at any trussed rafter, glulam beam or other structural timber and you will see the joints. In the mill knots are cut out leaving timber lengths from 500mm (ish) that are then jointed and run together to make a continuous piece that is then cut to the pack lengths for shipping

[SteamyTea]

 

[FuerteStu]

25 minutes ago, SteamyTea said:

 

There's something so satisfying about seeing natural materials being manipulated to perfection by machines.. 

 

It's like that scene in the matrix where neo suddenly realises were all just batteries. 

[saveasteading]

I'm interested, having never seen this advertised other than as furniture. 

Youd think it would be commonplace to reduce waste.

Can't see anything definitive yet.

This one thing contra.

Finger-jointed lumber and timber can be fabricated to have 75% of the strength of clear wood

[SteamyTea]

7 hours ago, saveasteading said:

Finger-jointed lumber and timber can be fabricated to have 75% of the strength of clear wood

As SEs over engineer for safety, it is probably well within tolerances.

 

Below is a scarf joint.  It works on the same principle of increasing the contact surface area as finger joints.

Edited October 8, 2023 by SteamyTea

[saveasteading]

2 hours ago, SteamyTea said:

Below is a scarf joint.

Positioned over a prop, to avoid a joint in mid span. Interesting cracks.

 

2 hours ago, SteamyTea said:

As SEs over engineer for safety

Engineers design with safety factors. That isn't over-engineering.

 

But my original point stands. For use in an ancillary building, these jointed beams are OK, but splicing wouldn't be a bad idea. If the 75% is anything like accurate, it is easy enough to consider them  spanning 20% shorter than for the equivalent natural C16.

[SteamyTea]

38 minutes ago, saveasteading said:

Engineers design with safety factors. That isn't over-engineering.

Yes.

But there are different safety factors for different industries, and then longevity has to be taken into account.

I don't think that finger joints are a problem in structural elements, as long as they are known about.

I worked some someone that made an automated jointing machine for Barratt Homes, back in 1987, just before the timber frame scandal hit, that we are still paying the price for.

 

More Scarfs, they can be engineered quite cleverly.

 

 

 

 

Edited October 8, 2023 by SteamyTea

[ProDave]

This is a non habitable structure and not going to be subject to inspection.

 

So reading the above, the joint can be assumed to be no more than 75% of the native timber strength.  In due course (i.e. not straight away) I will start sketching some possible joist plans, and we will see if anyone here is able to calculate how close I am to the maximum strength of the joists.

 

Thinking about reinforcing the joint, a full steel plate would be inconvenient as I will probably be attaching other joists to the side of these.  But those other joists would be smaller.  I have some lengths of thick 2" wide  steel straps.  Working on the basis that the top of the joint will be in compression and the bottom will be in tension, it would seem logical that I bolt a 2" steel strap across both sides of the joist at the bottom through bolted with stud and nuts.