# Correspondence on Stability, Strength and Thrustlines in Masonry Structures

*Mr. M. H. Rees (M)
Dr. Harvey’s paper is a model of clarity of exposition (refreshingly jargonless) and the economical setting-up of a method of structural analysis developed by him and his colleagues primarily for the very practical investigation of existing, often ancient, masonry arches and buttresses. The thrustline is the primary tool used, and its most revealing derivative, the thrustzone. The notion of packers placed between masonry blocks is introduced to simplify and clarify the analysis. Two further techniques are used to facilitate the method: a separate partial factor is postulated for material strength to take into account the reduction in stability arising from bending in the structure (the P-Ä effect for both elastic and creep displacement); and the equivalent rectangular pressure stress block under the packer in place of the more difficult-to-handle triangular block - this despite the otherwise working-load nature of the analysis. Some interesting effects are highlighted, such as that of scaling up a structure in equilibrium to produce one that is not (by deduction, the converse rule will also apply to render a scaled-down structure more stable).
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