Linear static analysis is the easiest method for the seismic design of unreinforced masonry buildings. What makes this method particularly convenient to apply is that the fundamental vibration period of the structure may be obtained conventionally from a formula given by the codes of practice, rather than being calculated rigorously. Usually, the code-based formula overestimates – sometimes significantly – the rigorous value of the fundamental period of vibration of masonry structures. By referring to a series of unreinforced masonry buildings, the paper shows that the conventional period may be even 4-9 times greater than the rigorous one. However, overestimating the fundamental period may lead to a reduction of the design loads to be applied in the static method. This occurs, in particular, when the design response spectrum decreases in the short-period range, where natural periods of masonry buildings typically fall. Some codes do not allow design spectra to be negatively sloped in the short-period range. While others, such as the Italian code of practice, do allow it, and in so doing, they make the linear static method non-conservative (in the given examples the shear at the base of masonry buildings may be underestimated by even as much as 35% when compared to the values obtained from the more precise linear dynamic method). The paper gives some hints towards making the code-based static method for seismic design of masonry buildings safe.
