Pontypridd Arch Bridge
Disaster
Group 20
Patrick Morrissey
Dermott MacTernan
Craig Goodbody
Patrick Temple
Background
Pontypridd is situated on the confluence of the rivers Taff and Rhondda
approximately 12 miles north west of Cardiff, Wales. In the year 1746 William
Edwards was commissioned to build a bridge in Pontypridd for £500 provided the
structure stood for a minimum of 7 years.
William Edwards was born at Ty Canol, Groeswen. When William was 7 years old
his father was drowned fording the river Taff while returning from Llantrisant Fair on
horseback. Was this the catalyst for his later obsession with the bridge at Pontypridd?
Edwards began building at an early age and one of his many projects was a mill at
Craig y Fedw just east of Abertridwr. During the period when he lived in Cardiff he
also
built
a
furnace
at
Cardiff
Castle.
It took 3 failures while constructing the bridge until on the 4th attempt Edwards
learned from his mistakes and completed the bridge in 1756. The bridge still stands to
this day as a monument to William Edwards’ efforts and for 40 years remained the
world’s largest single span arch bridge.
Figure 1
Design
The most common cause of arch bridge collapse at this time was the collapse of the
piers or the dislodgment of the crown. This was due to imbalanced design of
equilateral and vertical loading. The first attempt was a three arch structure, which
stood for 26 months before being washed away by debris carried by the river Taff in
flood. The second attempt was to span the Taff without any supporting pillars
midstream. Staging collapse caused the failure of this attempt when the structure that
was used to support the heavy bridge during the building phase gave way. The next
attempt was also to end in failure when the centre section collapsed because it could
not support the weight of the large side haunches. The midsection under the massive
forces pushing in from the haunches was pushed up and the bridge collapsed as a
result. Edwards witnessed the collapse and realised the problem with his design. He
modified the design and created large tunnels through the haunches on each side to
reduce the weight transfer to the centre section. These tunnels measuring 9 feet for the
bottom, 6 feet for the middle and 3 feet for the top were to prove successful and the
bridge was finally completed in 1756. Charcoal was also used as a lightweight infill
further reducing the forces. As a result of Edwards’ persistence and resourcefulness
the bridge still stands after 250 years.
Figure 2
The bridge that stands today is a single span stone arch bridge spanning 43 metres
across the river Taff. The bridge has a height of approximately 10.7 metres and a
chord length of 42.7 metres. 3 circular tunnels of diameters 9, 6 and 3 feet were
constructed in the abutments to decrease the pressure on the midsection.
The steep nature of the bridge made it acceptable for livestock but difficult for heavily
loaded horse drawn vehicles. Heavy chains were dragged behind to slow the wagons
down on descent from the bridge. This caused continual wear on the bridge surfacing.
In 1857 another bridge was constructed alongside Edwards’ bridge to accommodate
for increased traffic. Because of the increase in traffic over the river Taff a second
bridge was constructed alongside in 1857. The structure still stands to this day as a
monument to William Edward's ingenuity and perseverance around 1900 the arch
bridge was resurfaced with steps and restricted to pedestrian use.
Conclusion
As professional engineers we must strive to be aware of every possible failure mode
of any structure that we design. We must also be willing to learn from our mistakes as
well as the mistakes of others.
We can learn from this triumph over disaster. We must remember that when we
undertake complicated and daring designs we must first consider all the factors of
loading that the structure must withstand. We can see from the example of the
Pontypridd bridge that this approach was not adopted as the first attempt was washed
away by river debris. If time is taken to consider all of the possible modes of failure
then measures can be taken to accommodate them into the design and so prevent an
unexpected and sudden collapse. This can not only create a more structurally sound
and robust structure but it can also prevent disasters such as this one.
We must also consider in our design considerations the buildability of the structure. It
is relatively simple to sit in an office and design beautiful and daring structures,
however these designs must then be practically built. This is another lesson that we
can learn from Edwards’ mistakes. The second collapse was due to the failure of the
staging that was supporting the building works. If we do not consider how the
structure will be built in our design it will cause huge problems in the construction
phase. The structure will become more expensive and it may lead to accidents and
disasters such as the one which Edwards was involved in.
Edwards’ bridge is no longer in use today as it was too steep to accommodate
increased traffic. We as professional engineers must be farsighted and considerate in
our designs. Edwards had become so determined on succeeding in his quest that he
ignored the fact that the bridge was far too steep even for the horse drawn traffic of
the time. This is an example of a bad and inappropriate design. We must look at a
structure and design it to facilitate all of the possible uses that it may be required for
and we must also look ahead and think of what it may be used for in the future.
However, though the design of the bridge was not functional and it took several
attempts to complete, the story behind it is an example of perseverance and
determination in the engineering world. William Edwards’ innovation with the use of
tunnels and charcoal also shows his ingenuity and resourcefulness in a time before
computers and design programmes. We must strive to become this resolute in every
project that we undertake.
References
(http://webapps.rhondda-cynon-taf.gov.uk/heritagetrail/taff/pontypridd/Pontypridd.htm)
Accessed on 21st of October 2006; 15:45.
(http://www.llgc.org.uk/ardd/pensaeri/arch008.htm)
Accessed on 21st of October 2006; 16;22.
(www.wikipedia.com)
Accessed on 18th of October 2006; 16;22