Design of Truss and Structure

RE: Design of Truss and Structure

Sye — Fri, 01 Sep 2017 15:34:01 GMT

The reason you are getting these zero stiffness warnings is that you have certain nodes where there are no stiffness associated with certain degrees of freedom. For example if you look at the node 10, all the members connecting to that node are trusses lying in the XY plane. So if there is a load in the Z direction at that node, there is no way for the structure to resist that load which is why you get the zero stiffness warning at that node for direction 3 ( which represents translation in Z ). In addition since all the members at that joint are trusses, there is no rotational stiffness about any of the global directions ( 4, 5 & 6 ).

In your model there are no abnormal displacements and the static check seems to be satisfied. So as far as I can see, the zero stiffness did not have an impact on the analysis.

You find more on zero stiffness at the following wiki

https://communities.bentley.com/products/ram-staad/w/structural_analysis_and_design__wiki/29020/zero-stiffness-warning

Instructions on uploading files in Bentley Communities forum is available at

https://communities.bentley.com/products/ram-staad/w/structural_analysis_and_design__wiki/28129/how-can-i-upload-files-in-the-bentley-communities-forum

RE: Design of Truss and Structure

Prad Adh — Fri, 01 Sep 2017 05:12:58 GMT

Dear Mr Sye,

I have some warning from STAAD as :

ZERO STIFFNESS IN DIRECTION 3 AT JOINT 10 EQN.NO. 51
LOADS APPLIED OR DISTRIBUTED HERE FROM ELEMENTS WILL BE IGNORED.
THIS MAY BE DUE TO ALL MEMBERS AT THIS JOINT BEING RELEASED OR
EFFECTIVELY RELEASED IN THIS DIRECTION.
**WARNING - THERE WERE MORE THAN 12 DOF WITH ZERO STIFFNESS.

Should I be worried about it? I applied TRUSS spec to only to intermediate truss element whereas not any spec to top and bottom chord as suggested by you.

Second, for the truss member, Does the member orientation ( beta angle ) affects much? I suppose truss has compression and tension only .. so I didn't care much about the rotation of each truss element ... will that be alright...?? My truss elements legs are parallel to global axis thus I applied BETA ANGLE, However, the orientation of unequal angle seems to be opposite.

I tried to upload my file here but doesn't know how.?

https://drive.google.com/open?id=0Bx8YDHIrp3cXMHBNYUw1Y0dsNzQ

RE: Design of Truss and Structure

Sye — Wed, 23 Aug 2017 07:44:56 GMT

Yes that should work.

RE: Design of Truss and Structure

Prad Adh — Wed, 23 Aug 2017 07:41:22 GMT

Thank you Mr. SYE.. Thats seems to be the solution. One more thing, Since my roof truss has attributary area little bit complex for hand calculation of wind load from roof covering to the member. I am thinking to make plate member with zero stiffness and apply the wind pressure to the plate member. Will this be the correct way of doing it ??? Once again thanks.

RE: Design of Truss and Structure

Sye — Wed, 23 Aug 2017 07:29:57 GMT

Since your truss support is 15 cm high, why don't you model these supports using short vertical members. These would connect your truss and the base frame.

RE: Design of Truss and Structure

Prad Adh — Wed, 23 Aug 2017 07:09:35 GMT

Dear Mr. SYE,

How can I deal with the connection between truss support and the base structure. My roof truss sits on the structure with 15cm gaps(because of truss support height). I am thinking to make the support point of truss and the node of base structure same. On doing so, the bottom member of truss and beam of the base frame becomes same and overlaps. How should I handle this situation ??

RE: Design of Truss and Structure

Jin Kazama — Sun, 13 Mar 2016 10:13:29 GMT

I got it! Thank you good sirs for helping me! Finally hahahaha

RE: Design of Truss and Structure

Rajat — Sun, 13 Mar 2016 10:05:16 GMT

Please go through the following post and modify your model accordingly

communities.bentley.com/.../21481.error-section-properties-not-entered-for-member

Thank you.

RE: Design of Truss and Structure

Jin Kazama — Sun, 13 Mar 2016 06:39:51 GMT

When I thought I was correct this error showed help please

communities.bentley.com/.../4645.Structure.std

RE: Design of Truss and Structure

Jin Kazama — Sun, 13 Mar 2016 04:08:37 GMT

Thank you again! I did what you said and these was the message I received

RE: Design of Truss and Structure

Sye — Sat, 12 Mar 2016 18:58:02 GMT

This error typically means that some of the members in your structure may not have a material assigned to it. Go to the menu option Select > By Missing Attributes > Missing Elasticity to see these members. Ensure that you assign the section property and material to all entities in your model. There is no need to recreate the file.

RE: Design of Truss and Structure

Jin Kazama — Sat, 12 Mar 2016 18:49:54 GMT

What can I do to correct this? I sometimes have this error in my analysis and the solution that I came up was to create a new file and do the process again which takes a lot of my time.

Thank you!

RE: Design of Truss and Structure

Jin Kazama — Sat, 12 Mar 2016 18:47:03 GMT

Thank you for answering and helping me!

RE: Design of Truss and Structure

Sye — Sat, 12 Mar 2016 18:35:46 GMT

There is no need to model the truss separately. You can model the truss on top of the supporting structure. The structure can be modeled as a space frame or a plane frame. Truss specification can be assigned to the braces or ties of the truss structure. Avoid assigning the truss specification to the top and bottom chords of the truss as these may actually take up bending. The loads applied on the trusses would automatically be transferred to the supporting structure. Depending on what forces (or moments ) are transferred by the truss to the supporting structure, you may need to specify appropriate member releases.