Experimental assessment of the dynamic behavior of timber trusses: a not so straightforward task

Several FE models of timber trusses can be found in the scientific literature aiming to predict their static and dynamic behaviors. On the contrary, surveys aimed at experimentally assessing those behaviors and tuning the models are far rarer. We present the operational modal analysis of 18 XVII century timber trusses, acknowledged among the longest in Europe, from the San Pietro Cathedral in Bologna (Italy). We show that the experimental assessment of their natural modes is not straightforward due to the forcing vibrations induced by directly linked (the cathedral walls) and indirectly linked (the vault, the belltower, the ground) elements. We show how to separate the different contributions and discuss the extent to which the tie beams’ dynamic properties can be used to assess their tension level.