Throughout the world, reinforced concrete frame buildings with masonry infill walls house families, shelter school children, and provide offices for workers. These buildings are functional, durable, and economical. All too often, though, these buildings perform poorly in earthquakes. Some collapse and kill the people inside, and many are badly damaged, requiring demolition or expensive repairs. Sometimes, poor construction quality or a lack of engineering design is at fault. In many cases, though, the engineering design itself is to blame. Despite the stiffness and strength infill walls possess, building codes around the world lack guidance on modeling and designing infill walls as structural elements, and many engineers have been taught not to consider them as such. Engineers therefore often ignore infill walls during structural design or presume that they will have only beneficial effects. This simple yet fundamental oversight often dooms buildings to poor earthquake performance. For example, many multi-story reinforced concrete buildings with masonry infill walls collapsed at the ground level from the 1999 Chi-Chi, Taiwan earthquake. These buildings typically had commercial space or parking at the ground floor and infill walls in the stories above.