Value for acceleration of gravity used by Karamba

It appears that internally, Karamba uses a value of the acceleration of gravity of 10 m/s^2, rather than the standard 9.806 m/s^2. This appears to be true both for the Assemble Model component, when it computes the mass of the structure based on the weight of materials, and for the Natural Vibration component when it calculates frequencies of vibration based on weight of materials.

I tested this on a simple single-degree-of-freedom model, calculating the mass and vibration period directly, and comparing with Karamba results. The results match when the acceleration of gravity is set to 10 m/s^2. See attached.

Is this right? At about 1.9%, the resulting error is small, but not negligible. I've encountered some additional anomalies when imperial units are used, and will post those later.

Kirk Martini

dynamic-test-metric-02.gh

Replies to This Discussion

Permalink Reply by Karamba3D on September 21, 2015 at 1:17pm

Dear Kirk Martini,

you are right, for the conversion from weight to mass the acceleration of gravity is assumed to be 10 m/s^2. The advantage is that one can simply say that 100 kg correspond to 1 kN on earth. Given the uncertainties usually connected to load assumptions in structural engineering 1.9% is not much - and usually tends to be on the safe side. I think it's a good idea to include an option in the karamba.ini-file to make the value of g customizable.

A description of the anomalies you encountered with imperial units would be very helpful for removing possible bugs.

Best,

Clemens

Permalink Reply by Karamba3D on September 23, 2015 at 8:34am

Kirk,

thank you for your bug-report and the examples. I will fix that and give you notice when the new build is ready.

Best,

Clemens

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