Structural response of drystone Iron Age brochs

Abstract

Understanding the structural behaviour of the drystone Iron Age brochs – prehistoric circular towers in northern Scotland – is essential for their investigation and conservation, but their chaotic collapse patterns have been studied in a fragmentary manner, primarily as historic evidence by archaeologists. The response of brochs to structural action was simulated by building two scale models and testing them in settlement, a possible source of failure. With the key features carefully reproduced and overall identical dimensions, the effect of variation of basal style between ground-galleried and solid-based, the two main types, was examined. The tests indicated that solid-based brochs can withstand a horizontal displacement at the wall head of twice that of ground-galleried types. The discussion of these tests provides further insight into the effect of the form and features, such as restricted openings or the intramural void. The conoid drystone form showed substantial strength, as large settlement was required to cause the failure of a scale model, suggesting that structural actions alone cannot cause collapse.