Three lessons from the Mapungubwe shells

Abstract

Shell structures show several unique structural attributes, which promote their use when material and economic efficiency is required. The principal advantage of shells is that forces are largely transferred through in-plane action (i.e. axial forces), with only limited bending and shear forces. This has endorsed the use of unreinforced shells which can been designed through numerous methodologies - i.e. thrust line analysis (O'Dwyer 1999), shell theory (Farshad 1977), finite element analysis (Ramm & Mehlhorn 1991) and numerous physical modelling techniques - to ensure that only compressive stresses exist in the structure. Of specific discussion in this paper are the shells of the Mapungubwe Interpretation Centre, which are built exclusively from unreinforced earth tiles. Because the shells are unreinforced and earth tiles are susceptible to tension cracking, the shape of the structure and the construction practice are of paramount importance. Unfortunately, the Mapungubwe shells have cracked extensively, most notably at the openings. The causes of this cracking have been postulated in order to propose principles of good practice in designing and constructing unreinforced masonry shells. These guidelines were identified as follows: lips and eyebrows at shell openings should be avoided, shells should be unrestrained against expansion and contraction, and openings at the base should be of a catenary shape to match the flow path of compressive stress.