The relevance of frictional resistances in out-of-plane mechanisms of block masonry structures

In the last decades increasing attention has been devoted to the problems of structural strengthening and repair for those historical buildings under threat from several risks, such as general decay, deterioration, earthquakes and other natural hazards. In particular, the lack of out-of-plane strength of masonry structures has been identified as the weak link in masonry seismic capacity, being a primary cause of failure in different forms. Within this framework the more recent studies in block masonry structures either with dry contacts or with connecting poor mortar have pointed out the great importance of the stabilising role of friction between blocks interlocked. This topic is examined in this paper with reference to the out-of-plane behaviour of a masonry wall, weakly connected to sidewalls, all arranged in a stretcher bond pattern and whose ability to carry load is dominated by friction and self weight. The frictional resistance due to the interlocking between orthogonal walls is compared to other extrinsic or intrinsic structural capacities, e.g. the effect of tie-rods and the frictional resistance due to the presence of a simply supported horizontal diaphragm. The sensitivity of the load multiplier to these strength parameters is investigated at the onset of the rocking mechanism. The results of the parametric analysis are obtained with reference to different combinations of loading conditions, including the detrimental effect of the static thrust of masonry vaults.