Boundary Conditions of welded HSS Steel truss

[VTAdam]

I am analyzing an already designed Steel truss. I am not 100% sure what the boundary conditions should be in Risa3D. The HSS sections are welded with a 6mm (0.25 in) fillet weld. The truss as shown is in the XY plane. What do you suggest the boundary conditions of the nodes should be? By default, all the boundary conditions are set free. This causes some instabilities in the model. The options for the boundary conditions are: fixed, reaction, spring, free..etc. Thanks in advance

 

[mike20793]

Are you referring to the HSS to HSS connection or the support boundary conditions? I assume you're referring to the supports. What's supporting the truss on each end? Do you know what the connection to the support consists of, ie bearing plates, etc.? I'm guessing it is probably going to a pinned connection each end based on the what I think is going on there, but we really need some more information to know for sure.

 

[CBEngi]

Since the connection is fully welded with a HSS web and HSS Chord (T/B), I would consider this fully fixed.

Edit:
Hollow Structural Sections, Connections and Trusses is a good reference for this.

Also you should consider joint eccentricity, as this could increase the moment in both the chord and web members.

 

[mike20793]

I agree if the OP is referring to HSS to HSS connections. The way he asked his question leads me to think that he is referring to the supports since member to member connections are defaulted as fixed in RISA and he said they are defaulted to free.

 

[VTAdam]

Thank you so much for the reply. I was referring to the connection between the chords and the web members. I am not asking about the supports. Since you brought it up, the connection to the support is designed using steel plates. The columns are I shaped members with a steel plate on each end. The truss has a steel plate on each end as well. The two plates are bolted together. I used two pins for safety even though they are closer to a fixed connection.

The way I think the connections should be assumed is: (Assuming a two dimensional model of the truss in the XY plane)

X Translation - Free
Y Translation - Free
Z Translation - Fixed

X Rotation - Fixed
Y Rotation - Free
Z Rotation - Free

 

[KootK]

For the joints that are not at the support locations:

X Translation - Free
Y Translation - Free
Z Translation - Fixed

X Rotation - Fixed
Y Rotation - Free Fixed
Z Rotation - Free

For one support location (pin):

X Translation - Fixed
Y Translation - Fixed
Z Translation - Fixed

X Rotation - Fixed
Y Rotation - Free Fixed
Z Rotation - Free

For other support location (roller):

X Translation - Free
Y Translation - Fixed
Z Translation - Fixed

X Rotation - Fixed
Y Rotation - Free Fixed
Z Rotation - Free

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.