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algorithmic modeling for Rhino

How to best define a mesh for the transmission of a wind load to facade nodes?

Hi again,

I am currently trying to apply a wind load to a portion of a buildings facade.

I have determined these nodes by applying the same methodology as the Karamba "Twisted Tower" tutorial.

If I have understood correctly, I now need to define a mesh comprising the node points on which the mesh load (of 'x' KN/m2, predetermined by Eurocode calculations) shall act. (Images 1 & 2)

I have tried several approaches without success, including:

- Patching a surface through the points in question (results in a (too) rough surface approximation which could then be meshed...) (Image 4)

- Generating a Delauney mesh from the points (resulting in a messy mesh, Image 3)

Would anyone with more experience happen to have any suggestions as to how to approach the definition of this mesh load?

Any tips are greatly appreciated!

Nathan

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Hi Nathan,

extract the surface where you want to apply the MeshLoad. In a second step you can use Grasshopper's built-in meshing components 'Mesh Surface' or 'Mesh Brep' or 'Mesh Breps' from Karamba3d.

Best,

Clemens

Hi Clemens, 

Thank you very much, I'll try what you've suggested.

Nathan

Hello again Clemens,

Sorry to bother you again; I just wanted to verify that I've implemented your tips correctly.

1) You said to use a mesh which is finer than the underlying structural geometry. Here I use one which has been refined with a 5 level subdivision...does it appear ok to you or would you recommend going even smaller? Green nodes are the ones loads act upon.

2) 'Local To Mesh' vs 'Global' vs 'Projected Global'

I am applying a positive wind pressure (0,729 kN/m2) in the direction of global positive y axis.

The façade mesh is 1505 m2, of which 1056 m2 runs parallel to the XZ plane.
The wind pressure is 0,729 kN/m2 acting in the positive y direction ---> expected resultant of +/- 1056 x 0,729 = 770 kN.

'Global' gives +1098 kN in positive y direction.

'Global Projected' gives a +770 kN resultant in the positive y direction.

'Local to mesh'...gives -770kN along x axis (parallel to the façade according to ModelView) & -215kN along y axis. This surprised me since 'Local to mesh' is indeed the option to chose according to the manual yet I can't see how a wind load perpendicular to the facade would result in the tower moving in a cross-wind direction. 'Global projected' appears to provide the most logical result.

In view of these results, do you think the mesh remains ill-defined, the tower's shape is the culprit...or my choice of coordinate system for the load? What would you recommend? 

Thanks again for your feedback, which is greatly appreciated.

Nathan

Dear Nathan,

the option 'Local to mesh' lets you create a surface load which can be e.g. always perpendicular to the mesh faces - like wind compression or suction on a facade.

In the above case there are two wall pieces perpendicular to the y axis which do not have the same area. Therefore a resultant windload in Y-direction results.

Best,

Clemens

Hi Clemens, thanks again, 

I feel silly for not understanding but I think the issue lies with a misunderstanding of how 'local to mesh' works. 

Karamba Manual :

" 'local to mesh': X-component of the force vector is at right angle to the mesh-face

(...)

This means a surface load with components only in X-direction acts like wind pressure. "

The largest part of the facade mesh is parallel to the XZ plane.

Wind is perpendicular to XZ plane. 

So if I plug a force vector of (0.729,0,0) into the input of the mesh load set to 'local to mesh', in this case the y component of the resultant must be predominant? (IF the resultant vector is indeed expressed in the world coordinate system.)

Yet the mesh load says I've applied a load in the z direction, and I am now getting a resultant with null x and y components and 1089 kN in the positive vertical (z) direction...

It appears that either something is not behaving as it should or I am severely misinterpreting the behaviour!

Apologies for taking up your time if this is basic/trivial for you!

I don't quite understand why, but results become much more intuitive when I 'flip' the mesh and input a force vector oriented along the +z axis:

Hello Nathan,

there is an error in the manual: the Z-component is perpendicular to the mesh in case of local mesh-loads.

Best,

Clemens

Ok, everything makes sense now!

Thank you Clemens, I really appreciate your feedback!

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