The effect of precompression to masonry structures when subject to concentrated loads

Abstract

Unreinforced masonry (URM) has been the most predominant building type in the Maltese islands for a long period of time. The application of a concentrated load onto masonry walls is a very frequent case, and the ability to withstand a contact stress which is in excess of the wall’s uniaxial compressive strength has been known and studied for quite a long time. Existing knowledge, however, lacks when it comes to factor into the effect of precompression to masonry structures in relation to the strength enhancement phenomenon when subject to concentrated loads. An experimental programme was conducted to study this effect by representing masonry walls with scaled down homogenous concrete wall panels. This programme was split into 2 sets, implementing different loaded area ratios and design mixes. For each set, the precompressive stresses were varied in increments as a proportion of the previously found uniaxial compressive strength, and subsequently a central concentrated load was applied until failure. Results indicate that contrary to the original hypothesis, for the smaller precompressive stress ratios, the strength enhancement effect is even more amplified, which seemed to peak and decline back at higher ratios. No representative data was tested and collected for the higher precompressive stresses; however, one can hypothesise that the enhancement effect neutralises when the precompression approaches the uniaxial compressive bearing strength. Existing provisions given by masonry codes were also analysed and compared, highlighting possible future amendments to harmonise these codes by providing a holistic design equation including a modification factor to cater for the precompressive stresses.