Use STAAD to model the support. You can model the support only if you know the exact members that will be used to build it and the geometry of the supporting frame ( goal post, T-post, cantilever etc. whatever it is ...). Then apply a unit force ( 1kg/1N/100 kg whatever you are comfortable with)at the point where you want the stiffness to be calculated i.e. the point where the pipe will be supported on the member. Run the model and check the displacements at the point where you applied the force. The stiffness at that point will be
= Unit force applied / deflection.
This is the simplest method.
John
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John Wright