A couple of days ago, Nat and Stuart Witt dropped by for a visit, over from BC in Canada. We had a nice day together with loads of coffee and lots of chatting. One of the things we did was that we traipsed up the tower of the Münster, the cathedral in Basel. Considering it was built in the twelfth century in a earthquake zone, it has weathered the passage of time pretty well. OK, granted, during the great earthquake of 1356 the third tower fell in the river Rhine – but apart from that, I mean…
We were up the tower at noon when the bells start ringing. Stuart pointed out that the whole tower was moving, which I rubbished. To then realise… that the tower was moving! Quite noticeably so in fact. The movement of the bells cause the whole stone structure to move from left to right. For this reason, amongst others, the building was constructed with grouting which was softer than the red sandstone used for the construction of the cathedral – in the wind the towers sway up to a meter from side to side. Quite impressive considering they are 65m high, that is some oscillation!
I cannot even imagine how a medieval engineer or architect would have calculated all this, but it would seem they got their maths right.
Similar to aircraft wings or the towers described above, the movement of trees in the wind serves to dissipate energy. Rather than the movement being being unnerving, on the contrary, were the structure to be immobile and rigid there would be cause for concern. All the examples above are strategies in resilience, allowing the structure to deform under load and thereby withstand it, to thereafter return to a state of relaxation, the original form.
Like the aircraft engineer or the medieval architect, when the arborist considers an anchor point for rigging or climbing, for instance, he or she will be estimating the range within which the structure will be able to withstand load before irreversible or catastrophic change occurs.
After all, we want those towers to sway nicely, not fall in the river, metaphorically speaking! 😉