connection of timber beams - shear force? 6

[greznik91]

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?

 

[hokie66]

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

 

[greznik91]

@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)?

 

[KootK]

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.

 

[mike20793]

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.

 

[kipfoot]

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.

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:

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.

 

[mike20793]

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

 

[KootK]

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.

 

[Teguci]

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