Methods, systems, and apparatus, including medium-encoded computer program products, for computer aided design of physical structures using generative design processes. A method includes obtaining one or more load cases and one or more design criteria for a modeled object, the one or more design criteria comprising a build material strength model indicating strength relationships between thickness of an object feature and build angle for that object feature resulting from additive manufacturing; iteratively modifying a three dimensional shape of the modeled object in accordance with the one or more design criteria and the one or more load cases, including applying the strength relationships between the thickness of the object feature and the build angle for that object feature on a per-element basis during numerical simulation of the modeled object; and providing the three dimensional shape of the modeled object for use in manufacturing a physical structure.