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.