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connection of timber beams - shear force?
6

connection of timber beams - shear force?

connection of timber beams - shear force?

(OP)
Hi, I have a question about connecting timber roof beams.

Since its pretty common, that roof beam consists from more than 1 element (span of several meters) we have to connect its elements.
So if we have 3 elements we can connect them at/above supports (walls/columns)- MODEL 3 in picture. Other option is to make a connection at a point where bending moment diagram is zero - MODEL 2 in picture.

I found out that in MODEL 3 there are always bigger deflections which often causes problems and demands bigger beams. MODEL 2 has much lesser deflections at same load and geometry...

But what Im wondering about is what do we do about shear forces that acts AT connection (node) in MODEL 2?
How do we control it on shear force? in picture we have a shear force of 25 kN there...Element is 20/24 cm, timber C24.

I have never seen any project where engineer made a control of connection like this for a shear force. Why is that?



RE: connection of timber beams - shear force?

Unless you can get one piece long enough for continuous spans, we typically design wood beams as simply supported. So your model 3, but not normally connected that way.

RE: connection of timber beams - shear force?

Greznik91:
Take a look at a few good Wood/Timber Design textbooks or other reference materials for wood construction. What details and arrangements do you see being used out in the real world, have you ever looked, studied, thought about what you see? Which model gives you the smallest beam sizes, the lowest stresses and deflections for a given loading and span arrangement? Model 2 is a fairly common framing model because is gives shorter beam lengths, smaller beam sizes, fairly clean connection details and clean construction/erection. I’ve seen the connection detail you show, but I would not normally use it either.

RE: connection of timber beams - shear force?

This is typical connection in my area. You turn your connection in a way that the bolt is under tension. Turn it around and you can get splitting in both timber members. So basically you need to design the bolt and the washer so it is big enough based on compression resistance perpendicular to grain. Most of the time there are two bolts.

Edit: I'm describing model 2. I design such connections. Maybe because I am more of a wood guy. Other engineers that mostly do concrete probably don't go in so much trouble and let the carpenter do his thing.

RE: connection of timber beams - shear force?

Are these connections intended to be link - span connections?

I have seen them in older structures, but metal connectors are cheaper and faster.

Mike McCann, PE, SE (WA)


RE: connection of timber beams - shear force?

molibden,

With the bolts in tension due to gravity loading, the drop in span has to be lifted into place, rather than "dropping in". If I designed it and detailed it that way, I'm sure the builder would get it wrong. I understand why the wood would split, but consider this detail archaic.

RE: connection of timber beams - shear force?

Hokie... I did a report on a timberframed barn once and the floor timbers were 14"dp x 16"wide x 75' long... each 'stick' was at 12' o/c with floor purling between... no spices. Barn was constructed about 1850.

Dik

RE: connection of timber beams - shear force?

You might try "model 4" which would be similar to Model 2. To state the obvious, model two is a double cantilever with a simply supported beam on each end. You state that you've put the location of the connection "a point where bending moment diagram is zero," but in reality any location you choose for this connection will be a point of zero moment. So, you can tweak the length of the cantilevers so that the members will have manageable lengths.

I agree with msquared that a steel hanger would probably be less expensive, but if you choose to use a scarf joint, then I'd agree with hokie66 that the cantilevered end should be the lower piece. Then the reaction of the simply supported beam is transferred through bearing. Search images "timber scarf joint design" and you'll see some configurations.

RE: connection of timber beams - shear force?

To clarify, I did not say the cantilevered beam should be the lower piece. I was agreeing with molibden that splitting can occur with that arrangement, but the contractor wouldn't like his way.

RE: connection of timber beams - shear force?

hokie66, I did a lot of roofs with this detail. Some contractors just ignore it, but most of them want to do it right when I clarify the theory behind it. Erection is not so different because all of them use cranes, they crane the longer piece from side under the cantilever part and bolt it right away with crane holding it in position. Beams are light and there is no problem whatsoever.

In my area carpenters love these kind of details. They all use CNC machines to cut wood so basically it is cheaper and easier for them comparing to metal parts. Also it is nice to look at wood. I always try and conceal metal as much as possible.

RE: connection of timber beams - shear force?

Quote (hokie66)

To clarify, I did not say the cantilevered beam should be the lower piece.

Gotcha. Thanks for the clarification.

RE: connection of timber beams - shear force?

Quote (kipfoot)

but if you choose to use a scarf joint, then I'd agree with hokie66 that the cantilevered end should be the lower piece. Then the reaction of the simply supported beam is transferred through bearing. Search images "timber scarf joint design" and you'll see some configurations.
Seems if the cant. end was the upper piece, it would be less likely to split as the notches would be in compression.

RE: connection of timber beams - shear force?

Quote (XR250)

Seems if the cant. end was the upper piece, it would be less likely to split...

I see your point. I'll also add that some folks have done testing to try to determine the bending capacity of a scarf joint, so not everyone is assuming it's a hinge as I suggested.

RE: connection of timber beams - shear force?

XR250,

Exactly the point which molibden made, and I agree.

RE: connection of timber beams - shear force?

Quote (hokie66)

Exactly the point which molibden made, and I agree.
I guess my ADD is really shining thru :>

RE: connection of timber beams - shear force?

XR250:
There’s nothing wrong with a little ADD(ition) here and there. Just don’t be subtractin from our effort here. smile

RE: connection of timber beams - shear force?

(OP)
tnx for answers, but Im still confused - is a cantilevered beam lower piece or not?

RE: connection of timber beams - shear force?

Read again. It's all there, but you have to read to the end. If you just want to know the answer to your puzzle, you have come to the wrong place.

RE: connection of timber beams - shear force?

(OP)
i did read it again. sorry...
Cantilever beam is upper piece.



However im posting few more pictures. Some are done that way, others are not...



RE: connection of timber beams - shear force?

Read some Timber engineering books, some German ones have exactly this connection as an example. You can see the splitting of timber in your second picture under zero load. You can imagine what follows under extreme load. It works either way but do you want to have a weak connection in your structure? If the loads are low it doesn't really matter how you connect it...

RE: connection of timber beams - shear force?

Your second picture illustrates a point I made earlier. Using this detail, sooner of later a builder will get it reversed, unless properly supervised.

RE: connection of timber beams - shear force?

(OP)
@molibden - you said you designed many joints like this.

My question: is a tensile force (shear force from diagram)in bolts the only force you consider when designing a joint? Any other controlls you make beside contact pressures perpendicular to the grain flow (washers)?



RE: connection of timber beams - shear force?

Fascinating thread. It seems to me that, installed snug, these connections would transfer quite a bit of unintended moment, resulting in the tendency for splitting in one or the other of the members. Is any consideration given to that in the design of these joints? Or does wood shrinkage introduce a bit of slack? Things always look pretty tight in the photos.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: connection of timber beams - shear force?

Is the second picture actually splits? It seems unlikely that under just self weight that it would split to that extent. They kind of look like surface shakes/checks and may just be a poorly graded piece of lumber.

This may be a dumb question because I think I know the answer, but why not use a specialty hinge connector and be done with it? It would save on the field work and they are much easier and more reliable to install.

RE: connection of timber beams - shear force?

3

Quote (mike20793)

Is the second picture actually splits?
In my opinion, no. I think that what the second photo shows is checks in the timber. This shouldn't be confused with horizontal shear failure that you can get in a loaded member if you don't account for notches in wood.

Quote (KootK)

these connections would transfer quite a bit of unintended moment...

My initial thought was that this should be treated as a hinge, but sometimes the moment capacity is intended.
Testing Scarf Joints in Bending

from the conclusion:

Quote:

For the best designs, the theoretical maximum limit for moment
capacity of a simple face-halved scarf joint is 50 percent of a like-
sized, solid sawn timber. For a simple edge-halved scarf joint, the
theoretical maximum is one-quarter. The rule of thumb that a well-
designed and well-crafted scarf joint’s moment carrying capacity is
one-third of a solid-sawn timber’s is consistent with our results,
assuming the joint orientation is designed for the load orientation.

RE: connection of timber beams - shear force?

Thanks for sharing the reference. I thought they also looked like surface checks.

RE: connection of timber beams - shear force?

Thanks for the reference indeed kipfoot!

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: connection of timber beams - shear force?

Well if they are going to CNC the connection...

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