Pontypridd Arch Bridge
Collapse
Introduction:
The Welsh town of Pontypridd is located on the confluence of the rivers Taff and Rhondda,
approximately 12 miles north west of Cardiff. The name may come from a contraction of Pont-y-typridd, the Welsh for "bridge of earth", since in earlier centuries, people took advantage of the
shallowness of the River Taff here to cross it. In 1746 William Edwards was commissioned to build a
bridge in Pontypridd (crossing the River Taff) for £500. One of the conditions included in his
commission was that he also had to maintain it for a minimum of 7 years.
William Edwards (February 1719 – 7 August 1789), was born at Ty Canol, Groeswen. Edwards began
building at an early age and built bridges in Aberafan, Betws and Usk. Edwards' most famous
creation was the Old Bridge at Pontyridd. The contract included a "guarantee" clause, and as a result
Edwards actually constructed three successive bridges at the same site, with only the last surviving
the torrential waters of the River Taff. The last bridge was the largest single span in the world when
it was constructed measuring 140ft. Edwards' master-stroke was to lighten the weight of the stone
by leaving voids in the lower part of the structure. The bridge opened in 1755 and celebrated its
250th anniversary in 2005. The resulting construction still stands today, although another bridge
next
to
it
carries
modern
traffic.
By the time the fourth and final bridge was constructed Edwards had learned from his mistakes and
the project was completed in 1756. The bridge still stands to this day as a monument to William
Edwards’ efforts and remained the world’s largest single span arch bridge for 40 years.
Discussion:
1st Attempt
William edwards first attempt at building the bridge to cross the Taff river was to contruct a stone
arch bridge. Edwards designed this first bridge as a three arch structure. However this bridge only
stood for 26 months.
Design failure
26 months after Edwards first bridge design was completed a severe strom hit the town of
Pontyprid, this heavy storm caused the Taff river to flood. This was to be the downfall of William
Edwards first attempt at forging Taff river. The flood caused debris to be washed downstream and
this began to build up against the supports at the base of the structure. Edward had failed to
correctly predict the strength of the flash flood and the loading this would cause on his bridge. The
loading was too great on the supports of the bridge which collapsed and was swept away in the
floods.
2nd Attempt
After the failure of his first attempt William Edwards examined the mode of failure and resolved to
design a single span bridge that would eliminate the possible failure loading on the supports of the
structure.
Design Failure
There is disagreement in records of how exactly this attempt ended in failure. Some records say that
there was a flood during the construction in which the bridge was again swept away. Other records
say that there was a staging failure, that the formwork collapsed during construction. However
records agree that failure of this second attempt occured during construction.
3rd Attempt
Once again William Edwards attempted to bridge the river Taff, on his third attempt he continued
with his design of a single arch stone bridge. This bridge also collapsed over a period of six months
from when the formwork was removed.
Design Failure
It was suggested that this bridge failed because the centre second collapsed. This could have accord
either because the weight of the side haunches was too great causing the crown to rise, or the
weight of the haunches was not great enough causing the crown to sink. The result was that
Edwards third attempt collapsed, this failure however was a slow process so William Edwards was
able to observe how the bridge collapsed and learn from it.
4th Attempt
For this attempt Edward had learnt from his past mistakes and was able to use that to his benefit.
Again he designed a single span bridge to cross the Taff river, with some added features. These
features were added to decrease the weight of the bridge. The first of these features and most
obvious one, is that William Edwards designed three visible cylindrical voids on both sides of the
bridge. These cylindrical voids were 9ft, 6ft, and 3ft in diameter and succeed in lighten the load of
the bridge. In an effort to further reduce the weight of the bridge it is said that William Edwards
used charcoal for back-fill in the bridge.
Design Success
After learning from his initial failures William Edwards succeed in building a bridge that not only
accomplised the required contract lifetime of seven years but is still standing and in use today.
Against construction methods of the time William Edwards had to devise ways to reduce the mass of
the bridge, such as not using the material removed during construction for in-fill but using less dense
charcoal. The Cylindrical voids where inovations for lightning the load of the bridge, not only did
they lessen the overall weight of the bridge, but also allowed water to pass through them at high
flood levels decreasing the hydraulic loading on the bridge.
Conclusion:
We found the collapse of the Pontypridd bridge collapse to be quite interesting. As Edwards didn’t
have design codes available to him at the time such as what is available in the present day, he found
it difficult to successfully design and a build the bridge on his first three attempts as the location of
the bridge was situated in a challenging environment. It was the lack of theses design codes which
ultimately led to the failure of the bridge, particularly on the third occasion where the weight of the
bridge was too great for its span.
Even though it took Edwards Four times to successfully complete the project, he showed great
Engineering skills and didnt let the original failures get to him. Instead he assessed his previous
design/ designs to come up with new methods to make the bridge work effectively. He done this
through a trial and error basis and although his mistakes and failures were costly to the local
community, his final solution provided useful information and insight about arch bridges for future
bridge engineering generations.
Although many regard the final Pontypridd bridge an Engineering failure, (due to its narrow lanes
and steep gradients), it was still a great Engineering feat to be able to produce such a long spannnig
bridge at that time which wouldn’t be affected by rising flood flows.
William Edwards served as a pioneer for stronger, longer and lighter arch bridges for his future
colleagues. His fourth bridge still remains today however it has been pedestrianised due to it being a
listed structure.