Having lived in Bath for almost three years and given that entry to the Roman Baths is free for Bath residents it is a wonder that I have not been to visit it sooner than Sunday just gone. I’ve been put off by all the tourists. This weekend we had a tourist amongst us in the form of Jess (Singapore) and so went along. It was so good I’ll be going again and again.

At the beginning I was randomly tuning into audio recordings (17 play, 11 play, 77 play, 84526 error, 10 play etc) such fun was the hand held device. Often I came across a rambling voice which I later realised was Bill Bryson’s. He has recently added numerous recordings to the audio tour.

A section of the barrel vault roof that covered the Great Bath is displayed by the Great Bath.

Billy boy is in awe of the weight of the roof, 75 play “Roof fragment”:

“This large piece of roof structure is worth a second look and given some thought too because I mean just imagine the weight that is in there and that this is just a very small part of one span across the roof. I mean look behind you at the space that had to be covered and to think of arches all the way along. Quite amazing the amount of weight that would be pressing down on the all sides. The genius of the Romans building these enclosures without interior support up the middle is really quite amazing. I was recently at the Pantheon in Rome which has the most fantastic ceiling because the interior is just this dome that seems to be impossible, just completely unsupported and a great thirty foot opening at the very apex of the stoned space. Quite amazing and to think something not dissimiliar is over this bath here is pretty amazing to think what it must have been like, er er, it was, it must have been quite an awesome sight. Particularly as you know remember that we were really on the very edge of the civilised world here. To an awful lot of people who saw this structure would never have seen anything of that grandeur before.”

On a scale of structural engineering feats the ‘weight of a barrel vaulted roof supporting itself’ ranks low. Really. For all practical purposes masonry can be assumed incompressible and infinitely strong (bricks at the bottom of a 1500m wall would be fine…) and so any limit state analysis on masonry is illogical when the masonry is in compression. To ensure the compression criteria is met the thrust must “flow through the material.” Therefore masonry design is all about geometry. For arches and vaults and buttresses etc design it thick enough to always contain the thrust line.

still working on this image…

Oh yeah baby I know my stuff! Go read The Stone Skeleton by Jacques Heyman. It’s about time I read more of his stuff…

Audio recording 44 mentions the hollow bricks used in the roof to reduce the weight and provide insulation.

The only benefit I can see in reducing the weight of the roof is reducing the required number of buttresses. Insulation is surely negated by the numerous and large unglazed (obviously) or unboarded windows.

In my opinion Billy boy going on about the weight of the roof is an example of public misconception of structural mechanics; the barrel vaulted roof is crude, structurally simple, ancient technology. Not that this misconception matters, unlike the misconception of engineers worth to society. No, it is not the weight of the roof that is impressive - you make the vaulting thick enough to contain the thrust line and it will stand up but what I am taken with is the level of thinking and detail put into it: hollow bricks!

As an aside it is funny how things are changed so the public can better relate with it, for example have you ever heard of the Millennium Tent?