A mass structure is a structure that is made by combining similar materials into a certain shape or design. Mountains are an example of a natural mass structure, and brick walls are an example of a man-made mass structure.
Mass structures are not necessarily solid, but can be layered and sometimes have a hollow area that helps the structure perform a specific function. The Great Pyramids of Egypt and a power dam are other examples of man-made mass structures. Another mass structure is a sandbag wall that people build to help prevent flooding. In order for that wall to perform as a mass structure, it must be heavy enough to stay in place but light enough so that it does not unevenly compact the ground below it. Additionally, the wall must be thick enough so that it can't be pushed out of place, and the wall must be firmly anchored to the ground.
Other types of structures include frame structures, shell structures and flexible structures. Some structures can be combined when building an object. Even though a power dam is a mass structure, it does incorporate a frame structure inside of the dam as a place to house the generators.