Errors about Geometric Transformation?

ingLuca · Post by **ingLuca** » Tue Apr 03, 2007 5:24 am

I'm a new user, and having some trouble about geometric transformation I looked among posts and found a discussion of two years ago about this matter, and some problems with user manual figures. There was the correction or not? Because I can't understand well....

Looking in on-line manual you can read that "local x is defined from node i and node j, and that vecxz is in the plane of local x and local z, and local y is the cross product x^vecxz. "

Now, from the figures with element 1 and 2 it seems to be right what is written; from the first figure of the paragraph, the one with only the axes in the space, it seems right the contrary: local y is vecxz^x.

Having tried to load element I think that the right figure is the first of the paragraph, not the ones with the elements showed in 3D. I'm right?

ps: I don't know if you could use another convention, I'm from Italy, and for me the cross product is such that Z=X^Y (where I use ^ as cross product symbol).
Even if you use a different convention the two figures mentioned above seems differents to me, then there must be some problem.

Thanks

silvia · Post by **silvia** » Tue Apr 03, 2007 8:20 am

i'll look it over.
here in the us we follow the right-hand rule, too. x^y=z, z^x=y

silvia · Post by **silvia** » Tue Apr 03, 2007 10:31 am

okey, check it out now.
all i did in the first figure of the sections, rotate them so the y points up, but that's just to be consistent with what i have in the examples manual.

then the next figure is okey.
in the figure after that i rotated the vectors parallel to the vecxz
in the figure with the equations i, therefore, changed the sign of the vectors.

funny 'cause it's an easy concept, but hard to put down on paper.

ingLuca · Post by **ingLuca** » Wed Apr 04, 2007 6:24 am

you're right, it's a troubling matter to explain.

Now the figure seems ok, y=vecxz^x

You don't see any problem until you load the beam, but when with gravity load the beam moves up, you find there's something wrong

Thanks, you all of the them are very kind.

silvia · Post by **silvia** » Wed Apr 04, 2007 6:36 am

most of the time it's not a problem because people don't check!

ingLuca · Post by **ingLuca** » Mon Mar 31, 2008 3:14 am

I'm working on a small "Quick reference" on OpenSEES for my staff, and focusing on the Transformation problem I came back to read this topic. I had a flash: if vecxz is the vector that make

y=vecxz^x

why don't say that for Geometric Transformation we have to pass to the program the local z vector? Isn't the same thing? I'm sure it would be simplier for users to see the problem this way, without thinking about cross product.

Thanks for the support

Luca

silvia · Post by **silvia** » Mon Mar 31, 2008 7:43 am

typically, yes, you just give a direction parallel to the z axis of your element (remember, my vector starts at the global-axis origin, the local z axis doesn't).
What is in opensees is a bit more general, since any vector on that plane crossed with the x axis gives you the z axis. -- sometimes it is not convenient to give the local z axis.
I have high expectations for OpenSees users, YES I do!!!

silvia · Post by **silvia** » Mon Mar 31, 2008 7:44 am

p.s. do you use OpenSees within your company? I'm curious about the use of OpenSees outside of academics.

ingLuca · Post by **ingLuca** » Mon Mar 31, 2008 10:03 am

I understand what you say, local-z is not the only vector wich correctly defines the x-z plane. I was just searching the simpliest way to write what I have to explain.

I'm sorry for your curiosity, but the "staff" I was speaking of is not a "company", but a few researchers using OpenSEES to investigate seismic response of plan-asymmetric structures, and trying to develop a graphical interface to create the model. My world is university too!!!

Buon lavoro, e ancora grazie!

Luca

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