SIMPLE LATERAL MECHANISM ANALYSIS (SLaMA) FOR THE SEISMIC ASSESSMENT OF UNREINFORCED MASONRY STRUCTURES

Of the broad range of displacement-based assessment methods available to structural engineers, one that has seen recent uptake on a design code level is the Simple Lateral Mechanism Analysis (SLaMA). In particular, the 2017 New Zealand Technical Guidelines for (seismic) Engineering Assessments and the Dutch practice note NPR9998:2018 both enable the use of SLaMA for the seismic assessment of unreinforced masonry structures. This paper investigates the merits and drawbacks of SLaMA as a seismic assessment method for unreinforced masonry structures based on practical experience in New Zealand and the Netherlands. In general, the SLaMA method focuses on calculating the force and displacement capacities of individual structural elements and then identifying a global failure mechanism. The global displacement capacity of the structural system is then evaluated against the displacement demand on an equivalent single-degree-of-freedom system. This approach can easily be facilitated by hand or spreadsheet-based calculation and alleviates the need to develop a numerical model, which can often be complex for masonry structures. Consequently, SLaMA is generally found to be a cost-effective assessment methodology. This is well suited to masonry structures, which, due to their typically historic nature and limitations of the construction material, are often low-rise structures with relatively lower asset values. Following an overview of the SLaMA methodology, its specific application to masonry structures is discussed and a number of key considerations identified. It is then shown that there are a number of merits and drawbacks in utilizing SLaMA for the assessment of masonry structures. The paper also provides an example application of SLaMA to a typical Dutch unreinforced masonry terraced house. This example helps to elucidate the peculiarities of masonry structures and some of the technical challenges the engineer might face. Furthermore, the example demonstrates that SLaMA is very efficient when applied to relatively simple masonry buildings and when a reasonable set of starting assumptions are used.