Wednesday, December 21, 2011

Corbelling Arches and Bridges in Irelend



In response to a comment John Scott made on a post I wrote last week concerning Arch Chronology, Patrick McAfee went on to add some other very interesting comments.


"I'd like to mention that we love to corbel here in Ireland and had done so for about 6,000 years when eventually we began to shape the underside of lintels to look like arches. We were happy enough with this until one day quite unexpectedly we were struck by the arrival of the real arch. Although this magically worked only in compression it was a bit too far for us because we were hung up on creating tension. It took many years for us to succumb to the show-off easy ways of compression. But every so often, quite unexpectedly, there is an outbreak of corbelling, I've done it myself, even on your continent. I cant help it, it's a neolithic gene thing.








.... I'd like to add a lesser known fact (probably only of interest to myself) to do with corbelling. It is to be found on some medieval (say 15th century) stone bridges here. They are arched, (pointed or slightly pointed is common) but from the springing stone up say a quarter to one third the rise of the arch the voussoirs are corbelled and not radiated towards centre points. Above that they are are radiated. I have not come across a good reason for it yet but it's common enough and must have had a purpose. "


Does anyone have any thoughts about this?

4 comments:

  1. These Irish bridges seem to be over fairly narrow streams but the arch is fairly high over the water.

    Might it be that the high arch is necessary because from time to time the river floods and the best way of getting a high arch over a narrow stream is to build up with corbelling before putting in the radiated voissures? Pat admits they were not comfortable with arches, perhaps the builders would leave the construction of the radiated voissures until corbelling in the bridge was getting 'risky'.

    When you think about it, it is not really that much different from a viaduct, the arch does not actually 'happen' until it is necessary.

    I'd love to hear the various solutions to the query.

    Best wishes,

    Nick

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  2. I suppose the lower corbels reduce the span and thus require less arching. This also reduces the need for a meaty spandrel, plus the spandrel load can double as counterweight for the corbels(?) Of course these bridges have a lot of spandrel mass so that blows that theory. js

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  3. another thought... maybe they were originally corbelled arches that were rebuilt? Probably by Pat. js

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  4. The bridge in the bottom photo is Garfinny bridge, Dingle, County Kerry, the only bridge (of 250,000) in Ireland declared a national monument and reputed to be the oldest bridge in the country.
    It is said to have carried 800 troops under Lord Grey in 1580 to massacre 600 civilians who had already surrendered at Smerwick Harbour in County Kerry.
    Although bridges are usually seen as benign and serving a functional and aesthetic purpose, historically many of them here are associated with rather unpleasant occurrences.
    Garfinny would be much older than 1580 to qualify as the oldest bridge in the country but I can only guess that it is difficult to date because it is dry stone. Lime mortar can generally be carbon dated.
    The bridge is narrow (common in medieval times. Being narrow it made it vulnerable to being turned over in a high flood. To counteract this the bridge has little or no parapets.
    You can see some corbelling of the voussoirs near the springing points of the arch barrel. There are better examples of this on some other medieval bridges. Dry stone bridges are rare in Ireland.
    I believe there are quite a few on the east coast of the US. I'm not sure about Canada.

    Nick, I'd say yes the river floods and that is why the bridge is so high. The pointed arch too allows a large volume of water to pass through for a relatively small span.

    I don't recognize the photo at the top of the page.
    Pat

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