#### Just Some Nerd

##### Structural

In my workplace here in Sydney, the overwhelming majority of our projects will be pad/strip/core footings embedded into rock. For the purposes of modelling structure behavior in finite element software (primarily ETABS, but others also) we need to model our larger footings with soil area springs for more correct behaviour of our structure.

The question I'd like to ask is what is the most appropriate method for calculating a footing spring when it comes to dense rock? Currently my workplace makes use of a calc sheet created for purpose at some point in the past by a 3rd party geotechnical engineer. The sheet was based upon some of Gazetas' equations for static stiffness (See tables 15.1, 15.2, etc. of the Foundation Engineering Handbook 1991 - Chapter 15). For our purposes these equations are very convenient as they only rely on knowing the Elastic Modulus & Poisson's ratio of the rock layer, and the dimensions of the footings we determine ourselves. While I don't have particular reason to doubt the engineer who provided this, neither I nor my company (we only deal with structure, not geotechnical matters) can be sure of the appropriateness of these equations with regards to a foundation embedded in rock, and much of the discussion in the chapter revolves around the dynamic behaviour of soils. Given our footings are almost always founded on stiff rock layers, are the static stiffness equations appropriate for use in our models? I am unconvinced concrete is going to be behaving like a suitably rigid foundation when sitting on rock with an elastic modulus in the 100s of MPa

A very basic rule of thumb alternative to our approach would be taking the allowable (service) bearing capacities for a rock layer + corresponding settlement provided by geotechnical reports and calculating an equivalent spring. Would this be more appropriate? Less accurate? Equally bad?

Would also welcome being pointed in the direction of alternative methods suited to foundations embedded in rock that are simple for us to calculate, given we never have much information beyond Elastic modulus, Poisson's ratio and allowable bearing capacity + the footing dimensions we determine ourselves