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?

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?
RE: connection of timber beams - shear force?
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?
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?
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?
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?
Dik
RE: connection of timber beams - shear force?
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?
RE: connection of timber beams - shear force?
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?
Gotcha. Thanks for the clarification.
RE: connection of timber beams - shear force?
RE: connection of timber beams - shear force?
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?
Exactly the point which molibden made, and I agree.
RE: connection of timber beams - shear force?
RE: connection of timber beams - shear force?
There’s nothing wrong with a little ADD(ition) here and there. Just don’t be subtractin from our effort here.
RE: connection of timber beams - shear force?
RE: connection of timber beams - shear force?
RE: connection of timber beams - shear force?
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?
RE: connection of timber beams - shear force?
RE: connection of timber beams - shear force?
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?
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?
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?
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:
RE: connection of timber beams - shear force?
RE: connection of timber beams - shear force?
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?