th

Pontypridd is a town in Wales located 12 miles North West of Cardiff at the confluence of the Rivers Taff and Rhondda. As one would expect due to the town’s location at a river crossing there have been numerous attempts over the years to bridge the river. This report will examine the different bridges used over the years with particular emphasis on the third arch bridge which collapsed and then the final bridge built which stands to this very day.
The present arch bridge is a Georgian pedestrian bridge which spans 43 metres across the
River Taff. It was the world’s largest single-span arch bridge for 40 years. The bridge itself is a segmented arch bridge with a chord length of 140 feet and a height of 35 feet.
The bridge is part of an almost perfect circular arc with a radius of 89 feet.
The first bridge in the town was built by William Edwards in 1746 and was a three arch structure. This was washed away by debris during a flood after only 26 months. The second attempt once more built by Edwards had no central pillars midstream and failed as the bridge collapsed as the structure used to support the bridge during the construction phase was not able to take the loads imposed upon it. The third and final bridge to collapse was an arch bridge however this particular bridge collapsed as the central section was unable to support the large pressure imposed upon it by the massive side haunches.
In other words the pressure from the abutments was too great for the relatively light crown of the bridge to counteract and as a result the crown popped out and hence collapse occurred.

By observing the mode of the final collapse Edwards was able to design his final bridge to be able to withstand all the loads imposed upon it and thus see through its design life.
Edwards understood that the previous bridge collapsed due to excessive pressure on the crown and realised that if he could reduce the weight of the haunches he would in turn reduce the pressure on the central crown thus meaning that it was more likely for the bridge to remain standing. In order to reduce the weight Edwards constructed 3 large cylindrical tunnels through the spandrels. These tunnels were 9 feet, 6 feet and 3 feet in diameter respectfully getting smaller as you moved towards the centre of the arch.
Edwards also took the step of using charcoal as a lightweight infill material in the spandrels to further help with weight distribution and ensuring that collapse did not occur for a fourth time.
As a result of these steps taken by Edwards his fourth bridge has stood the test of time and remains standing to this day and is a testament to his ingenuity and perseverance.
Due to the steepness of the arch bridge it was difficult for horse drawn wagons to use the bridge and they often needed to use a chain in order to prevent the load picking up momentum as it rushed down the bridge. As a result of this and due to increased traffic use the bridge was supplemented by a second crossing. The road surface of the original arch bridge has since been repaired with steps added to now be used by pedestrians only.
There are numerous lessons to be learned by engineers by looking at the three bridge collapses in Pontypridd but specifically at the third and final arch bridge collapse. For

example by looking at the collapse of the first bridge it is apparent that a bridge or structure must be designed to be able to take any loading that may reasonably be expected to be imposed upon it. In this case the bridge should be designed to take the loading due to flooding as it crosses a river which could conceivably flood over the lfe of the bridge. Upon examining the second collapse it is clear that the engineer must not attempt to be too daring in his design as the more daring a design becomes the greater the chance of failure. This phenomenon is still seen to this day whereby structures get more and more daring until a catastrophic failure occurs and then for a period following that all structures are very conservative to ensure a similar failure does not occur. In addition this failure highlighted the importance of ensuring that temporary works are designed correctly to be able to support all the loads imposed during construction. The engineer must also learn from the manner in which a failure occurs and ensure that the error is never repeated, for example Edwards realised that the third bridge collapsed due to the excessive pressure from the abutments and was able to rectify this in his fourth design by inserting the six cylindrical tunnels and using charcoal to improve the weight distribution.
In addition it is obvious that small details can make a large difference. In this case by constructing 6 tunnels which to the lay person may seem decorative Edwards was able to ensure the bridge didn’t collapse. Finally this example shows that perseverance does pay off as Edwards finally managed to settle on a design that worked on his fourth attempt which is still standing to this day. The design of the current arch bridge also illustrates the importance of designing a structure that is functional for everyday use. The bridge as designed it is much too steep and posed numerous difficulties for horse drawn wagons trying to negotiate it. Modern engineers should be mindful of this and ensure that the

structures they design and build are functional and can be used by everyone that needs to use them (Universal Design).
1.
text, [WWW Document], Pontypridd, available from http://webapps.rhonddacynon-taf.gov.uk/libraries/heritagetrail/taff/pontypridd/Pontypridd.htm
, assessed on 25 th
October 2005
2.
text, [WWW Document], The Architecture of Wales - Architectural Drawings in
The National Library of Wales, available from http://www.llgc.org.uk/ardd/pensaeri/arch008.htm
, assessed on 25 th
October 2005
3.
text, [WWW Document], The Engineering Timelines Map of The British Isles, available from http://www.engineeringtimelines.com/scripts/engineeringItem.asp?id=284 , assessed on 25 th
October 2005