Fiber Section

[Mohammadf]

Hi Dear all
when we are defining a section by fiber section command, we have to use local coordinates. but i have a question. is it important to locate points through center of area or mass of the section or it can be define from center of mass of the section? i mean that is it possible to define corner points of the section from one of corners or we have to define corner nodes from center of section? does opensees calculate center of section automatically?

Regards

[vesna]

OpenSees does not calculate anything automatically. Every calculation at the sectional level is tied to the origin of the local coordinate system.

[linguan118]

Opensees calculates the centroid of the section which is independent of what local coordinate system you use. I think it is fine to use any local coordinate system.
Not sure although! Need to check the source code.

[Mohammadf]

thanks dear friends
is it important to use centroid of the section as the source of coordinate in patch command or not? is it possible to use one of corners of the section as the center of coordinate system? if we use another point for center of coordinate what happens to the longitudinal line of the element?

best

[linguan118]

If it is fiber section (2d or 3d), I think we can use any point as the center of coordinate system. Actually, there is no longitudinal line, it is fictitious.
Am I right, Vesna?

[Mohammadf]

Dear friend
I think that when we select a point as center of coordinate for a fiber section, longitudinal line of the element passing this point and because of that we have to chose centroid of the section as the center of coordinate system. dear vesna please solve this problem.

Regards

[vesna]

You can set the origin of the local coordinate system wherever you want but need to be aware that all calculations will be relative to that origin.

In modeling we tend to simplify things by modeling frame elements with line elements going through the element's centerline. Thus, to represent the reality closely we set the origin of the local coordinate system at the centroid of the section.

[aodonne1]

Vesna,

I am creating an asymmetrical cross section in 2D space. I am connecting a Nonlinear Beam Column Element with an asymmetrical cross section to the bottom edge of a rectangular panel frame. For simplicity of the model I would like to connect the NL BCE to the centerline of the frame. However, given the asymmetry of the cross section this does not coincide with the centroid of the BCE section. If I specify the fiber cross-section geometry with respect to the centerline of the frame panel (i.e. y = 0 z = 0 is at the centerline of the frame panel and not the controid of the section) OpenSEES will automatically calculate the location of the centroid of the NL BCE section and locate it away from the local x-axis of the created fiber section Beam Column Element. Correct?

Thanks,
Andrew