I am tasked with designing a mat foundation for a lime silo. Since this isn't a building, what code/standard do I need to use to calculate the lateral seismic forces in order to design the mat foundation.
What country are you in? The Australian standard AS 3774 Loads on bulk solids containers provides guidance on seismic loads for silos. My understanding is that in the United States, the requirements for non-building structures of ASCE 7-02 apply to silos. The ASCE's Guidelines for Seismic Evaluation and Design of Petrochemical Facilities may provide some guidance.