As part of a 3-year longitudinal study, 136 sixth-grade students completed an engineering-based problem on earthquakes involving integrated STEM learning. Students employed engineering design processes and STEM disciplinary knowledge to plan, sketch, then construct a building designed to withstand earthquake damage, taking into account a number of constraints. On testing, students redesigned to build an improved structure. Using a framework of design processes, we report on the students' capabilities in planning, creating annotated sketches, and transforming these into 3D models. An understanding of core STEM concepts was apparent in their responses, including shape properties, stability, rigidity, balance, strength, and engineering techniques. Group problem solving involved moving iteratively between design phases, frequently revisiting problem scoping, being cognizant of the problem goal, boundaries and constraints, and appreciating design sketches as guiding constructions. Students were also observed to consider a number of problem components simultaneously, indicating their capabilities in handling the complexity of the task.