Wood building truss forces

[AuEng99]

I have developed a model utilizing the recommendations from the previous post. They include using a quad mesh to get the plate load close to one foot increments, designating the truss nodes as reactions, and showing the plates as fixed.

Some of the attached design (flat roof.pdf) may not make sense material wise. For example, I have a concrete flat roof on top of a truss, but this was the best way that I could use the program to produce a result that I could compare with my hand calculations.

A 20 psf load is intended for application to the concrete top and then transfer down into the truss.

I have also attached a drawing of the member force hand calculations. It is shown as (Truss.jpg).

The problem is that the RISA results are not matching up with my hand calculations. Can someone help me find out what is wrong with my model?

 

[AuEng99]

Sorry, this is the attached Flat roof model that I referred to in the previous post (Flat roof Wood barn.pdf).

 

[JoshPlumSE]

AuEng -

Your model demonstrates that you're having some confusion understanding the difference between end releases and boundary conditions.

1) Boundary Conditions (at least for this model) should only be defined at the base of your columns. these define how those joints are connected to the foundation. You have accidentally defined "Fixed" boundary conditions along the length of your truss chord.

2) End Releases are member properties that dictate how members are connected to adjacent members. Like a BenPin end release on the end of a beam says that that beam can transfer shear and axial force into the column it connects to. But, it cannot transfer bending moment.

In your case, I believe the interior members of the truss should have "fixed" end releases (not boundary conditions) and only the ends of the truss should have BenPins defined. That is assuming that the chord is a continuous member without any splices along its length. If it does have splices then you may want to introduce a BenPin at the splice location as well.