4A6 Dermot O’Dwyer
Pontypridd Arch Bridge
GROUP 20
Niamh Farrell
Eamonn Murphy
Anna O’Driscoll
James Reed
Introduction
Pontypridd is a town in Glamorgan, Wales. Pontypridd is located at the junction of the Rhondda
and Taff valleys where the River Rhondda flows into the Taff. The construction of the Old
Bridge started in the year 1746 by William Edwards. William Edwards was born in 1719 and
had little formal education. He taught himself masonry and was a keen learner, doing this
through self-study and practice. He gained a great talent for working with stone and this talent
soon gave him a good reputation in the area. Due to this, he was commissioned to design a
bridge spanning the River Taff and the maintenance of this structure was to last for a period of
no less than seven years. However, it took Edwards a total of four attempts before he was able to
construct a bridge to meet the requirements of the contract.
Basic Concepts of Arch Bridges
During early construction of bridges, the concept of arches was an important one. Early
construction technology was limited and the arch bridge was an efficient and simple way of
crossing rivers. The most abundant construction material during the time period of the
Pontypridd Bridge was masonry. Masonry has great compression strength but little tension
strength. This works well with arches because the forces involved are compression with little or
no tension forces. Figure 1 is a diagram of the typical parts of an arch bridge. The load of the
roadway is transferred down through the voussiors to the abutment in compression.
Figure 1 – Typical Parts of an Arch Bridge
Failure Modes of Single - Arch Bridges
The main failure mode for an arch bridge occurs when the horizontal load transferred to the
abutment (or haunches) is too great. This will cause either the haunches to fail in two manners:
To move closer to each other and cause the keystone (crown) to rise, or to move away from each
other and cause the crown to sink. The strength of the haunches can be increased by adding
more mass to the fill material, but this will make the voussoirs yield before the abutments start to
dislocate.
1st Attempt
DESIGN:
The bridge was to be a stone arch bridge and Edwards designed the initial bridge as a
conventional multi span, three or possibly four arch structure. Unfortunately, this bridge only
lasted for a period of 26 months.
FAILURE:
A severe storm rocked the town of Pontypridd which caused the River Taff to flood. Debris
from the flood built up around the supports at the base of the structure and finally the load
became too great and the bridge collapsed and was swept away in the floods. Weak supports
were Edwards’ first error; he was unsuccessful in predicting the fast flow and strength of the
river flowing beneath his bridge.
2nd Attempt
DESIGN:
Due to the failure of his first project, Edwards designed a more adequate single-span bridge
which would eliminate any chance of debris buildup at the structure’s supports.
FAILURE:
It is said that a contemporary of William Edwards, known as Thomas Morgan, commented on
this attempt saying that during construction, the timberwork in the centre of the span gave way,
resulting in collapse. The failure of construction methods could be attributed to another storm
that occurred during the construction or to the fact that the supporting frame was insufficient for
the load of the bridge. Records are unclear on the exact nature of this failure but the bridge did
collapse.
3rd Attempt
DESIGN:
Once again, undaunted by his previous mishaps, William Edwards designed a stone arch bridge
over the River Taff. Articles and reports from the time propose that in his third attempt,
Edwards succeeded in completing a single span arch bridge that lasted for an estimated 6 weeks
before collapse. This time, the bridge was of similar geometry to the present day bridge but with
slight differences.
FAILURE:
It is thought that the "weight of the bridge was either too great on the haunches, or too little on
the crown" according to the article, Theory of Arches and Pontypridd. However, as the bridge
collapsed shortly after the striking of the formwork it may be suggested that the stone beneath
the formwork was not yet stiff enough when the formwork was removed. Also to be noted during
this collapse is that it was not sudden and gave William Edwards time to observe the mechanics
of the failure.
4th Attempt
DESIGN:
Edwards had learned from his previous mistakes. He continued with the single arch concept and
implemented design features to resolve the problems of the previous failures. Two main features
were added to the bridge to lighten the bridge’s weight. The first was to use charcoal for the
back-fill in the bridge. Secondly Edwards created 6 holes in the spandrels of the arch: Two of
9ft diameter, two of 6ft diameter, and two of 4ft diameter. See Figure 2.
Figure 2 – William Edwards’ 4th and final Pontypridd Bridge
SUCCESS:
From Edwards’ past mistakes, and the slow collapse of the third attempt, he knew what had to be
accomplished to successfully construct a single arch bridge to span such a great length. With the
current technology and construction methods, Edwards had to devise a way of creating a lighter
bridge. To accomplish this, charcoal was used as the fill-material instead of the typical
substance. Under usual circumstances material removed while constructing the foundations of
the bridge was used as fill-material, but Edwards needed something lighter, thus the use of the
charcoal. Secondly the weight of the bridge was reduced significantly by creating the “holes” in
the spandrels of the bridge. Not only did these openings lessen the mass, they also allowed water
to pass through, reducing the hydraulic force created on the haunches and spandrels of the bridge
during a flood.
Conclusion:
William Edwards’ fourth and final bridge still stands today. Even though it took Edwards four
times to successfully build the bridge, the trial and errors he went through represents an
important engineering concept. Each time Edwards built a new bridge, he used lessons learned
from past mistakes. After the first failure, he realized that spanning this river would not be
possible with the traditional multi-span arch bridge. The river Taff had tendencies to flood and
flooding had the potential to destroy the foundations of a multi-span bridge. Thus Edwards
decided to design a single span bridge, so that even if the river flooded, the foundations would
not be in danger. The first attempt failed due to poor construction methods, and the second
failed due to Edwards’ underestimating the weight of the bridge. The fourth attempt used all
past lessons to create a single-span bridge that has stood the test of time and remains today.
Fortunately, Edwards’ mistakes were not harmful to any users of the bridge at the time.
Although the mistakes were costly, the failures provided useful information and insight about
arch bridges, not only to Edwards but to future bridge designers. At the time of construction,
building a single arch bridge across 140ft was unheard of. Edwards served as a pioneer for
stronger, longer, lighter arch bridges. Today, his bridge at Pontypridd is still considered an
engineering achievement.