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Wood Truss Design

Wood Truss Design

(OP)
I am currently working on a design for a wood king post truss. I have never designed a wood truss before. I know how to check the members for the allowable tension and compression forces in the wood members but how do you check the joints. What reference should I use to check the joints? The client wants two option choices. One with steel plates and bolts at the joints. The other with wood dowels. These are heavy timber trusses and seems to me that the joint would control. Thanks

RE: Wood Truss Design

If this is the configuration I think it is, then the bottom chord acts as a single beam to support the center post that supports the rafter structure above. So designing the bottom chord to take the load from the king post is the meat of the problem. There should be no splices in the bottom chord, only the connection of the king post to the bottom chord and the roof structure to the post.

Mike McCann
MMC Engineering

RE: Wood Truss Design

Lake:

Refer to the web site at the link:

Link

They have info on engineering heavy trusses.

Regards

DB

RE: Wood Truss Design

You can use that software to get the forces of the truss elements and this one to design/check the adequacy of its cross sections.

RE: Wood Truss Design

(OP)
Johnbridge231,

Thanks. We have Stadd here in our office. I may have to look into that program if this does not work out. Stadd has the following options to model and they are;

SPACE
PLANE
FLOOR
TRUSS

I first tried utilizing the option for TRUSS, however I did not get any forces in the lower chord. I think this is due to the loads not being at the panel points like a true truss. So I believe this should be modeled as a ridge frame and I tried modeling it with the Plane option. Have you used the program Stadd before?

RE: Wood Truss Design

(OP)
It appears from my model that the lower cord has virtually no axial forces for the balanced snow load and dead load condition which I believe confirms what msquared48 stated about the lower cord being a beam to support the king post. Just to clarify there are two purlins to the left of the centerline of truss and two purlins to the right of the centerline of truss, which are framed into the top cord. One on each side of the strut for both struts.

RE: Wood Truss Design

Lake06:
Why don’t you let the people who are going to be building those timber frame trusses design them for you, before you hurt someone. There are some little tricks to that trade which different framers will handle slightly differently, partly because of their experience and partly because of their shop equipment. But, they should all get about the same member sizes, forces and stresses prior to any changes for joinery methods. I would show the client what the two different joinery schemes look like with some simple sketches or photos, and tell them to pick one, unless they will pay you to do both. Bolts and wooden pegs are not generally interchangeable, except one is without the side plates.

You said you knew how to size the members and find the forces and stresses. Why didn’t you show member sizes, forces, stresses, purlins, loadings, etc. on your sketch? That way the sketch would be much more meaningful to experienced engineers for a critique. If you are not finding any tension in your bottom chord, you should probably throw that one away, and get another one which has considerable tension in it. Your truss is nothing more than two rafters, which cause considerable thrust at their bearing points; and need a bottom chord or ceiling jsts. to carry/react that thrust. Draw the free body diagrams for this simple roof and put reactions, forces, etc. on these diags. Then, you add a decorative king post which really can’t/doesn’t take much load, because the bottom chord can’t resist that point loading on the 28.68' span. Then, you add two diags. which are intended to reduce the bending length (or size) of the rafters. They do finally load the king post, in tension, and it must hang off the joint at the ridge. The king post also has the affect of preventing the bottom chord from sagging under any vert. loads. That’s a slight simplification, but not by much. And, you should think through the problem that way, and size things longhand. Then some software will refine things a bit, taking into account relative member stiffnesses and top chord bending, etc. And, you can look at unbalanced loadings too. How did you arrive at your 5.28' and .75' vert. dimensions?

What is your engineering background, did you study truss/frame analysis, you seem pretty lost on this? You should really get some help from your boss or a senior engineer in (or out of) your office, who might also become a mentor for you. Your boss should know what you do and don’t know, so he can help keep you and the company out of trouble. And, when you are starting out you should not feel embarrassed about not knowing some of this stuff, unless you oversold yourself in the first place. And, we won’t help you if the latter is the case.

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