Moment Releases On horizontal beams 1

[Leftwow]

Good Morning All,

I have always been told to release my horizontal simple shear connection members with in the My and MZ for Staadpro. However a clients staad just has the Mz released.
Number one is this correct?
Number two, where can I find documentation regarding this?

 

[Shu Jiang PEng SE]

Releasing the beam end moments is used to simulate the simple shear connections. Mx, My and Mz refers to the beam local coordinate system. Mx refers the rotation along the member axis, My, Mz refer to the beam strong and weak axial bending. Which moments to be released is depends on the real connection geometry.

 

[Leftwow]

I get the concept I'm just wondering if there is documentation referencing this to prove it

 

[TehMightyEngineer]

Not sure if I understand your question Leftwow. The moment release dialog is discussed in the user manual and help file in Staad. If you're asking how they're justifying not releasing the simple shear connection then we'd need to know what their connection geometry is. Some connections could transmit limited weak-axis moments so it's not entirely unreasonable. As for documentation I've seen some stuff on it but, again, it really depends on what the connection is.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

http://www.americanconcrete.com

 

[Leftwow]

My boss asked me to check another engineers work, he only released the Mz component on his shear connections. There is also no horizontal bracing at a large platform 112' in the air. I think that the shear connections should be released in My and Mz, and then the correct instabilities will occur on the top, which will require horizontal bracing. However, I was wondering if there was a standard in place that said to actually release the moment in the axis y and z on my beam.

 

[TehMightyEngineer]

However, I was wondering if there was a standard in place that said to actually release the moment in the axis y and z on my beam.

The short answer to your questions is "no".

If it's a typical shear connection then I'd release both the my and mz. As for bracing; did they properly set the unbraced length to the length of beam in Staad? Is there actually any weak-axis moment on the beam? Are there any items that could potentially be bracing the compression flange of the beam (such as metal decking)? Some pictures or details showing these beams will greatly help us tailor our responses to your question.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

http://www.americanconcrete.com

 

[BAretired]

What is the standard convention for naming axes in Staadpro? I assume X is the positive direction of the beam, Y is at right angles to the web and Z is the vertical axis. Is that correct? The axes are oriented such that rotating X into Y is positive Z, Y into Z is positive X and Z into X is positive Y, using the right hand rule, correct?

BA

 

[RWW0002]

I doubt you are going to find technical information on what is the "correct" way to model simple-span beams. For a simple shear connection the members should be modeled to (ideally) only transfer vertical shear. However, in order to eliminate irrelevant errors and "zero stiffness" warnings, modelers will sometimes choose to "fix" the weak axis and torsional moments at some, or all beam ends in the model. Although this method can be risky from an error-catching standpoint, so long as the members are not loaded in such a way to induce torsional or weak axis moment at the member ends, the model may in fact be fine.

The easiest way to check whether the model is handling the loads in the assumed manner is to print member end forces (or turn on the member force diagrams in the GUI) and see if any members are picking up (or delivering) unintended load to other members. If they are, then this is your justification to the other engineer (ex. Beam XXX looks like it is currently transferring significant torsion to column XX, did you intend to release My for that member or are you planning on somehow transferring that force through the connection..) That should go over a little better than a blanket statement on mow member ends are supposed to be modeled.

At the end of the day it does not matter so much just how it is modeled so long as there are simple clear load paths and connections to resist the applied forces, and the model matches the detailing in the drawings.

Regarding the "instabilities" do a search of Bentley's forums and help files. In many cases this is actually a member that is technically able to rotate about some axis but may not be loaded in a way that induces the rotation (i.e. a simple span beam loaded in the vertical direction with Mx released on both ends will give a instability warning due to the fact that it is modeled like it is free to twist - While this is technically correct, the minimal torsional resistance from a standard shear connection is adequate to maintain stability so long as there are no loads that induce torsion.)

 

[TehMightyEngineer]

BA: Staad has it so that for a typical beam Mx = torsion, My = weak-axis bending, Mz = strong-axis bending.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

http://www.americanconcrete.com

 

[BAretired]

Thanks, TME; in that case, it would seem to me that My and Mz should both be released. Otherwise, the beam would be deemed to be fixed against weak axis bending which would affect the spacing of lateral bracing, producing nonconservative results.

BA

 

[Leftwow]

RWW0002,

The instabilities and zero stiffness warnings are there because you either aren't planning your beams out properly or because you require bracing.

 

[RWW0002]

Leftwow
Thank you for your comments. I understand the purpose and meaning of the zero stiffness warnings and potential instabilities. But this does not always mean that bracing is needed or that the beams are not "planned out" properly. You may want to do some additional reading on the topic.

These warnings are generally resolvable (with some thought) and may point to potential problems in a model, but they do not necessarily mean that there is a problem. See the following FAQ for additional information

http://communities.bentley.com/prod....staad-pro-instability-and-zero-stiffness-faq

The point of my original post was simply that the designer must understand the load paths through the structure and ensure that the model is behaving as expected and detail the joints accordingly. While paying attention to potential instabilities and warnings is part of this process, it it not the whole game...

 

[SethGuthrie]

Keeping the MY fixed can provide also provide diaphragm stability when a diaphragm or master/slave is not used (i.e prevents racking).

 

[Leftwow]

I am confused how one can say "oh, I am getting these strange warnings let me unrelease these beams and see what happens". By no means am I the master, but I never get those warnings anymore.