Pontypridd Arch Bridge
Disasters
Group 20
Introduction
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Pontypridd is situated on the confluence of the rivers Taff and Rhondda
approximately 12 miles north west of Cardiff, Wales.
The bridge was built in a rural area and it is thought that this was due to a
previous religious crossing located there.
William Edwards was commissioned to build a bridge in Pontypridd for £500
provided the structure stood for a minimum of 7 years..
Design
• The bridge that stands today is a single span stone arch
bridge spanning 43 metres across the river Taff.
• For 40 years this bridge remained the world’s largest
single span arch bridge.
• The bridge has a height of approximately 10.7 metres
and a chord length of 42.7 metres.
• 4 circular tunnels of diameters 9, 6 and 3 feet were
constructed in the abutments to decrease the pressure
on the midsection. The fourth is walled in and cannot be
seen.
First Attempt
• The first bridge constructed of wood, with
three middle piers was washed away in a
flood after two years.
• The are no records on this bridge that we
could find. It is most probable that the
bridge was just too lightly constructed for
the one hundred and forty foot span.
Second Attempt
• After the first bridge being washed away
Edwards was prompted to design a single span
bridge.
• The bridge was nearly completed when the
centring dislodged, allowing the bridge to
collapse.
• Here we see the first of Edwards problems with
vertical load distribution.
• Remember at this time there were few
mathematical models and no real design
standards.
Third Attempt
• . On this bridge he builds an abnormally thin
arch for that time, to save materials, cost and
time.
• He would have got away this time with the thin
arch had the spandrel weight be reduced
accordingly. To spread the vertical loads evenly.
• Unfortunately for Edwards this model only lasted
six weeks before the crown was forced out by
the weigh of the spandrels.
• The parapets had not been put on before it
collapsed. Had they been constructed the bridge
lifespan, while not healthy would have been
stronger from the strap effect of the parapets
Final Attempt
• There are no records of Edward’s
correspondence and relationship with his
financial backers. It most have been very
good or they forced him to build a forth.
• Edwards consulted John Seaton another
up and coming engineer of the time.
• He comes up wth a imprecise design for
the spanrels, by putting four circular voids
through them to reduce weight.
Design
• 4 circular tunnels of diameters 9, 6 and 3 feet were
constructed in the abutments to decrease the pressure
on the midsection. The fourth is walled in and cannot be
seen.
• charcoal was used as a lighter filling option than gravel.
There is no written record of this and it has not been
examined in case of irreversible consequences
• The light spandrels know match the thin arch structure,
to produce a very fine and elegant bridge with equally
balanced loads. Previously the load had been too strong
on the hunches or to light on the on the crown.
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Its slim build is quit amazing given it is built from
Rubble masonry and small stones. However
they are slaty stones.
• Therefore very hard when perpendicular to the
cleavage plane, so when aligned correctly like
this case, these stones are very strong. They are
bonded with Aberthour lime mortor.
• From a practical point of view the bridge was not
brilliant. It was only eleven foot wide between
the parapets, secondly it was to steep for a
wagon or couch to descend. .
• A chain and drag system had to be used. When
one wagon got to the top a chain was attached
to the back of the wagon and the other end
attached to one going up.
• In later bridges Edward’s raised the approaches
to reduce this problem.
• However as a visual aspect it looks fantastic and
very elegantly designed.
• Edwards has to be commended for his
perseverance, he also revolutionised the
traditional style of Welsh bridge and made it
span one hundred and forty feet.
• He set up a trend in bridge design with the us of
spandrel voids. Which was used through the
twentieth century.
•The bridge was final completed in 1756 that stands today
is a single span stone arch bridge spanning 43 metres
across the river Taff.
•For 40 years this bridge remained the world’s largest
single span arch bridge.
•The bridge has a height of approximately 10.7 metres and
a chord length of 42.7 metres.
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 must remember that when we undertake
complicated and daring designs we must first
consider all the factors of loading that the structure
must withstand.
• It is relatively simple to sit in an office and design
beautiful and daring structures, however these
designs must then be practically built.
Pontypridd Arch Bridge
References
• Arch Bridgs and Their Builders 1735-1835 by
Ruddock
• http://webapps.rhondda-cynontaf.gov.uk/heritagetrail/taff/pontypridd/Pontypridd
.htm) Accessed on: 21st of October 2006; 15:45.
• (http://www.llgc.org.uk/ardd/pensaeri/arch008.ht
m)
Accessed on: 21st of October 2006; 16;22.