Does a cantilever exist in shell type structure?

Dear Karamba Team,

I am using Karamba to do a hanging model and took the reference from your website:

http://www.karamba3d.com/large-deformation-of-a-triangular-grid/

Based on my understanding, the shell structure is mostly compression. However in the example on the web(pls see the attached pic with red dot mark), the function 'LARGE DEFORMATION' caused shape transformation to form which whin the 4 support zone, a compression shell, but as long as the contilever, what is the main type of force? or, if let`s assume the cantilever part is possed by compression, where is the third support to pass the load?

I feel confusing because if the hanging model is to find out the perfect shell form, a cantilever which in that example, for me, is more like possed by tensile force, which seems like bit of against the concept of compression. So, is it really rational and logic to use 'LARGE DEFORMATION' for form finding when dealing with a structure not only consist of compression shell, but cantilevers?

Thanks for in advance for any explaination and insights!

All the best,

Lei

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Replies to This Discussion

Permalink Reply by Lei Yang on September 29, 2016 at 2:24am

Dear Karamba Team,

In you Karamba Manual introduction book, says:'The more and the smaller the steps the better the approximation of geometric non-linearity.' So for the input of function 'Large Deformation', Inc; MaxDisp; how does it work refering to the quoted above? does the decrease value of Inc means smaller?

Permalink Reply by Karamba3D on September 30, 2016 at 1:14pm

Dear Lei,

increasing the number of increments (i.e. increasing 'Inc') makes the load steps smaller and gives more accurate results. However in case of form finding for shells excessive accuracy of the displacements is generally not necessary.

Best,

Clemens

Permalink Reply by Karamba3D on September 30, 2016 at 1:10pm

Dear Lei,

shells are structures which can carry external forces mainly by in-plane normal forces. Moments play only a minor role and that is why shells are so effective when it comes to bridging large spans.

Arches can also be shaped in such a way that they carry external loads by normal forces only. However for arches this can be achieved for one predefined set of loads only. Otherwise they perform hardly better than straight beams. In case of a stone arch the dead weight is normally far larger than the live load. Therefore they are optimized for carrying their own weight.

Shells however can be visualized as structures where an infinite number of arches passes through each internal point. This is why shells perform well irrespective of the distribution of external loads. The only condition is that they are double curved. Heinz Isler for example was inspired by the shape of his pillow.

The weak points of shells are their edges especially their free edges: there shells behave locally like arches and bending moments occur. Bending moments are also hard to avoid at the supports: there the resulting support forces would have to be tangent to the shells middle surface in order to avoid bending. Shells with positive Gaussian curvature perform better at the supports than those with negative Gaussian curvature where the bending moments reach farther into the interior of the shell.

To sum it up: avoid free edges as much as possible (or increase the stiffness there by adding an edge beam) and make the shell double curved - then you should get a good load bearing behavior.

Best,

Clemens

Permalink Reply by Lei Yang on October 10, 2016 at 12:46am

Thanks so much Clemens,

that helps a lot!

Best,

Lei

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