Support Types in Truss Model - Engissol

[AdamJSR]

Just wondering what support types you guys use when modeling roof trusses? I've been using fixed/roller when solving by hand, but is there a more realistic, or accepted method when using software to model?

Thanks so much for the help.

 

[XR250]

I generally use fixed-roller, however, it is not entirely accurate.
The horizontal resistance for a roof truss typically comes from the roof diaphragm (under wind or seismic loading, that is).
Hard to model that easily.

 

[johnbridge231]

The support conditions should conform with the actual ones (construction). There is not any general receipt that can cover all cases together. Experience, good knowledge of the software you qre using and adequate supervison during the construction are the keys.

Regards

Analysis and Design of arbitrary cross sections
Reinforcement design to all major codes
Moment Curvature analysis

http://www.engissol.com/cross-section-analysis-design.html

 

[XR250]

Johnbridge231;

The majority of common 2x trusses are typically modeled with fixed-roller. Other than special circumstances, such as collector trusses, when would you feel that you would not model it that way?

 

[BAretired]

Fixed-roller? Don't you mean hinged-roller?

BA

 

[johnbridge231]

ExcelEngineering,

As I said, experience, software knowledge and supervision.
If you think you lack one of those, I can kindly suggest you where to start from.
We are here to help.


Analysis and Design of arbitrary cross sections
Reinforcement design to all major codes
Moment Curvature analysis

http://www.engissol.com/cross-section-analysis-design.html

 

[XR250]

@BA - Yup, of course I meant pinned-roller. Fixed would be tough to accomplish easily :>
@Johnbridge231 - you did not answer my question

 

[johnbridge231]

Excel engineering, do you uave sufficient engineering knowledge regarding trusses?!

Analysis and Design of arbitrary cross sections
Reinforcement design to all major codes
Moment Curvature analysis

http://www.engissol.com/cross-section-analysis-design.html

 

[XR250]

Probably more so than you - from a practical standpoint.
You still did not answer my question.

 

[johnbridge231]

omg. It's getting funny. You want us to solve your problems? Do you have an idea of boundary conditions? Do you know what the term diaphragm means? Do you know what the term flexibility is? Do you know what stiffness is? Do you know what springs are? Things are not so straight forward as you may think. There are many parameters that make each case unique. Therefore we need to adapt ourselves to the conditions of each case and not rely on improper guidelines, eg apply standard restraints without knowing how they are implemented. We are engineers (I hope that for you too) and must have sufficient knowledge and experience.

As for your question, in order to account for thermal loads and their effect on the truss members, a fixed roller as you suggest is wrong.
Sorry to say that but you wanted to know.

Happy reading and many many regards.

Analysis and Design of arbitrary cross sections
Reinforcement design to all major codes
Moment Curvature analysis

http://www.engissol.com/cross-section-analysis-design.html

 

[XR250]

You are correct, this is VERY funny!

 

[BAretired]

Getting back to the original question, it is acceptable to model the truss with vertical reactions only at the ends and a horizontal reaction only at mid-span. That is often more realistic with respect to horizontal deflections, but member forces should be the same as the pin/roller model.

Design of the roof diaphragm is a separate issue.

BA

 

[johnbridge231]

AdamJSR, to sum up, the software you are referring to, covers fully every type of truss efficiently. As for the restraints,
the user can easily assign any type of them at any joint of the truss.

PS: This post is very educational, as it seems that terms like "restraint", "stiffness", "thermal loads" etc. are explained in deep.

 

[charliealphabravo]

I would say that more than 99% of single span trusses are designed pin-roller. This especially true in wood truss and light gauge truss design even tho the roller support is rarely constructed as a true roller. The roller behavior more often comes from lateral deflection of the top of the support walls (wood frame, masonry). As BAr noted this can be modeled by using rollers at both ends and a vertical slider at the middle. Even this is only an approximation since the walls provide partial restraint.

In my experience the goal in design is not to simulate the field conditions as much as it is to find a conservative simplified model.