Plate Element Stress - Mx/My vs Global Moment

[stallion0002]

I'm new to using STAAD for plate element design. I am trying to design a mat foundation using plate elements. In post-processing, there are Mx and My moments as well as 'Global Moment' as options for the stress types.

The Mx and My can be used to determine the appropriate rebar needed to resist the moments in thier respective directions for the plate element. But where does the gloabl moment come into play? At first I thought that it was a resultant moment, but the Mxy is the resultant moment leading to torsion in the plate element. Any clarification would be greatly appreciated.

Don't know if it will help, but I am attaching screen clips for Mx, My, and Global Moment post-processing stresses.

Mx Local Moment

http://files.engineering.com/getfil...e3-8653-f0628fa06339&file=Mx_Local_Moment.bmp

My Local Moment

http://files.engineering.com/getfil...e5-893d-2fc11e5fc7a1&file=My_Local_Moment.bmp

Global Moment

http://files.engineering.com/getfil...40f2-b435-977e78be08a5&file=Global_Moment.bmp

Thanks

 

[rook1]

Hi!

Global moment should have the same answer with the local Mx and My. I think global moment is convenient when the plates are divided triangularly.

 

[JoshPlumSE]

Actually, global moments are not quite the same as the Mx and My. This is because of the effect of the Mxy moment as you have mentioned.

Refer to the Wood-Armer method of combining plate moments together to get design moments. That will give true design moments in the Mx and My local directions.

See this thread for more detail:

http://www.eng-tips.com/viewthread.cfm?qid=24256

Josh

PS Why am I (a RISA guy) answering questions on STAAD's forum?

 

[stallion0002]

Thanks for the information on the Wood-Armer method. That should help out a lot.

As for you question Josh, from a STAAD guy to a RISA guy, maybe this is the first step to program compatibility. LOL.

Thanks again.

 

[accountclosed]

I know this is a little after the fact, but better late than never...

When refering to Mx and My for plate moments, be sure to keep the nomenclature straight. Mx does not mean moment about the plate local x axis. It actually means moment on the X face, where the x face is the edge that the local x axis "pokes out of". It turns out that Mx is actually the moment ABOUT the plate local y axis, which can be a little confusing. But it may help to think of it this way...Mx is the moment you want to use to design the rebar that runs parallel to the plate local x axis.

 

[SteelPE]

STAAD

So do you mean to say that using the Wood-Armer method as JoshPlum referenced is incorrect?

 

[accountclosed]

Wood-Armer is a design method.

Mx, My, Mxy, and Global Moments are analysis results.

Let's say we have a mesh of quad plates that is oriented as follows:

plate local x axes point in the positive global X direction

plate local y axes point in the negative global Z direction

plate local z axes point in the positive global Y direction

Then Mx is the moment on the x-face of the plate. This is moment ABOUT the plate local y axis, which is the same as describing moment ABOUT the global Z axis for the orientation described above. Now a moment ABOUT the global Z axis produces tensile and compressive stresses in the global X direction. So if you were to review the global moments for the same plate, look in the columns that relate to the Global X stress direction, use the column that is coordinated with our plate "top" orientation of Y (+ve), you will read the exact same moment as was reported for Mx for the same plate.

Hope that helps.