Abstract
William Truesdell designed the Seventh Street Improvement Arches, which stand near Minnesota at St Paul. McArthur brothers, together with Michael O’Brien, were the construction contractors. After examination of several available design options, Truesdell settled for oblique method due to the nature of the intersection of the street at Duluth tracks and the St. Paul.
He used stones that were sourced from locally available quarries in the construction of the bridge. This paper reports on the various events that unfolded during the construction of Seventh Street Improvement Arches. It also discusses the methods, equipment, tools, and labor that were used in the construction process. Besides, it speculates on the ways that the construction would have been done if the bridge were to be made today.
Introduction
Seventh Street Improvement Arches refer to a slanted overpass that is situated near Minnesota in the United States of America. The double-arched bridge was built using the spiral method that is alternatively referred to as helicoidal viaduct-building methodology. Up to date, the Seventh Street Improvement Arches remain historically significant in civil engineering due to their intricate technical and architectural design that required much precision in terms of stonecutting.
During the time of designing and construction (1883-1884), the bridge stood as unique based on the few of such bridges that were in existence in the US. Today, the bridge constitutes the only known example of bi-arch brickwork helicoidal overpass in Minnesota. Hence, the artistic creativity of Engineer William Truesdell was necessary. McArthur brothers, together with Michael O’Brien, took up the challenge of the actual creation of the art piece of work (Minnesota’s Historic Bridges Para 3).
Their success in delivering the project derivable is evidenced by the up-to-date well-standing bridge at St Paul, Minnesota, although the overpass is not currently in use. This paper summarizes and discusses the 1883-1884 building process of the Seventh Street Improvement Arches. It also speculates and analyzes how the structure (whether a building, road, dam, canal, or bridge) would be made if it were built today.
Summary of Events
Seventh Street Improvement Arches as shown in fig. 1 below was among a number of constituents of a project that was proposed and scheduled for improving the ‘Seventh Street’ that was to connect various regions in 1883. Other tasks of the project included the construction of Iron Bridge that measured 300 ft in length at Northern Pacific Railway and the construction of masonry-arch for the septic tank at the Phalen Creek crossing. Seventh Street Improvement Arches comprised two main curves. The West curve has a span of 41ft. The East arch has a span of 30 ft. The arc barrel extends to 124ft.
At the dawn of 1883, the Minnesota administration announced that it had directed St. Paul to ensure the issuance of bonds for improving the ‘Seventh Street.’ The specific place of improvement of the avenue was where “it crossed the combined valley of Trout Brook and Phalen Creek that linked the downtown district with Dayton’s Bluff to the east” (Minnesota’s Historic Bridges Para 2).
At the time of construction of the Seventh Street Improvement Arches, civil engineering civilization drew most of its design configurations from French and Roman techniques of building arch-type overpasses.
Romans and French bridge designers did not have the skills for estimating the potency that was required for the arches. However, the knowledge they had inherited from Greek, especially in geometry, made it easy for early arch-type bridge builders to deploy extensions that took semicircular shapes.
The arches were mainly constructed through the deployment of stones that were shaped to form wedge segments that were alternatively called voussoirs. Although the construction was oblique, Truesdell’s design of the Seventh Street Improvement Arches reflects this technological civilization in the construction of arch-type bridges. The Seventh Street Improvement Arches follow the culture of construction of bridges using stones that characterize early ages of civil engineering works.
Building the Seventh Street Improvement Arches in 1883-1884
Among all the work elements that constitute the project, Seventh Street Improvement Arches presented the main challenges to civil engineers. In fact, the passage of the street at Duluth tracks and the St. Paul was at exactly 63 degrees (Minnesota’s Historic Bridges Par.4). In addition, it also needed to carry water lines together with drainage lines while at the same time corresponding to the profile of the hill at the ‘Seventh Street,’ which needed rebuilding, thus implying that a large amount of filling resources was required.
Exploration of different alternatives for building the bridge led to the ruling out of the ribbed-arches viaduct building approach. The method was inappropriate due to the large weight of the block that was to be supported. The second alternative was the twisted-arch approach that was developed by classical French civil engineers.
With limited experienced stonecutting labor supply, the precision required by this method meant that labor costs could be exorbitant (Minnesota’s Historic Bridges Par.4). The method also involved making many different types of shapes that fitted in the arch configurations. To produce different shapes, a large amount of work was required in the making of patterns. This necessity amplified the costs of construction beyond the projected scope.
Truesdell considered Peter Nicholson’s helicoidal approach the third alternative. Although the approach was satisfactory, it involved rigorous and hefty mathematical computations. However, Truesdell had studied mathematics. Hence, he believed he could successfully complete the challenging task. Initially, he encountered several challenges in the computation of the required curvature side view.
However, amid the challenges, upon successful computation of the curvature side view, all voussoirs were possible to generate from one pattern with the exception of ring stones. Indeed, all stones were of the same breadth and form (DuPaul 31).
This suggested that although there was limited availability of skilled stonecutting labor for a new project that had never been done before, successful training on the process of cutting one stone would ensure precision in terms of cutting all other required rocks to build the arch side view. However, Truesdell noted that the main challenge was insisting on the stonecutters the importance of doing their work more carefully than when cutting stones for other applications (Minnesota’s Historic Bridges Par. 4).
Voussoirs stones were cut such that the curved surfaces made a pattern of matching spiral courses. The generation of spirals was done through straight lines that met with the arch’s axes. The spirals formed continuous right angles with the axes. They also “moved uniformly along that axes while at the same time revolving uniformly around it” (Minnesota’s Historic Bridges par.6).
The precision that was required in cutting the stones implied that the stonecutters needed to work with greater precision, which they were not accustomed to when cutting stones for other civil works. However, the builders of the bridge had a highly skilled supervisor who was capable of supervising and teaching the stonecutters the necessary procedures and checks to ensure compliance with precision requirements (DuPaul 36).
Thus, one of the major constraints of building the Seventh Street Improvement Arches was the availability of labor that required minimal training. Labor costs were higher due to the time taken in learning compared to when skilled labor was available immediately when a certain task that formed part of the entire project required implementation. Such costs were uncontrollable since alternative design options that called for high precision level in stonecutting could not apply to the Seventh Street Improvement Arches. The construction culture in the period of building the arches was also dominated by brickwork structures with evident sparing use of cast steel bridges.
Several materials were required in the assembly of the Seventh Street Improvement Arches. DuPaul informs, “The abutments, piers, and wing walls were built with a variety of gray limestone locally quarried in St. Paul, while the voussoirs, ring stones, coping and spandrel walls were built with a buff-colored limestone quarried in Kasota, Minnesota” (53). The actual bridge construction was initiated in September 1883. McArthur brothers, together with Michael O’Brien, engaged in the construction process.
Michael O’Brien did the digging, abutment, and the setting of the bridge groundwork. McArthur brothers did the entire construction of the bridge. The shaping of the stones was done manually at the quarries. Hence, no special equipment was deployed in shaping all the Voussoirs stones.
Indeed, masons who were hired by contractors, also did the blue-collar bridge building. It was ready for handling traffic by 18 December 1884. As depicted in fig. 2 below, the Seventh Street Improvement Arches were an outstanding piece of civil engineering work since the bridge remains intact even today, although it is no longer in use. Its inner surfaces have not even developed cracks.
Building the Seventh Street Improvement Arches Bridge Today
People’s artistic creativity develops as time progresses. This claim is perhaps true in the case of bridge construction. During the most primitive times, people deployed fallen tree and fallen rocks when they needed to cross rivers. Suspended ropes also formed an important alternative for crossing rivers and valleys. The structure of modern bridges borrows from these primitive types of bridges. Overpasses can take the makeup of arches, beams, or suspensions.
While these structures remain the main alternatives that were utilized by civil engineers in the construction of bridges for road and railway crossings, linking buildings, and/or carrying trains and road traffic over water bodies, materials and methodologies of construction have changed. Equipment that is used in the construction has also changed so that if the Seventh Street Improvement Arches were built today, the bridge would probably involve heavy use of the equipment and different materials rather than stones.
Assuming the intersection of the street at Duluth tracks and the St. Paul at a right angle, if the Seventh Street Improvement Arches were built today, beam bridge from pre-stressed concrete would be the most appropriate to minimize the time required to complete the construction from about one and half years to a few months. Pre-stressing is one of the methodologies that are deployed by modern civil engineers to mitigate natural drawbacks of steel concrete structures.
The methodology is employed to produce various structures for commercial utilization such as floors, bridges, and beams. Concrete withstands more loads while subjected to compressive loads than while exposed to tensile loads. This behavior implies that, when used to produce columns that are to be subjected to compressive loads, concrete can withstand more loads in relation to when it supports loads that subject it to tension.
A similar scenario is experienced when concrete is deployed to produce a beam. For instance, when a beam is simply supported and loaded, the dead load (load due to the heaviness of the beam) and the applied load subject the upper portion of the beam to compressive deformation. The lower side is subjected to tensile strain, which induces tensile stress. Since non-reinforced concrete is stronger in compression than in tension, the beam can only support a limited amount of load in tension.
When the span of the beam is increased, the load that can be supported reduces because longer spans buckle more than shorter ones. One way of dealing with this challenge is by providing more support to the beam. However, this strategy is inconvenient, especially when a beam spans over water bodies or valleys. The amount of concrete that is used to make a beam support a given amount of load is also higher than in the case of a reinforced beam.
Hence, the cost of constructing bridges using plain concrete becomes prohibitive. Consequently, reinforcing becomes necessary. Traditionally, reinforcing was done using steel bars, which provided the required strength in tension.
With a reinforced beam, the span that can support an equivalent load with a non-reinforced concrete beam with equal cross-sectional dimensions is higher. The need to increase such a span even higher gives rise to the need of utilizing pre-stressed concrete, which will be most applicable in the constructions of the Seventh Street Improvement Arches if the bridge were to be made today.
Constructing the arches for the Seventh Street Improvement Arches would not make use of stones as one of the material selection alternatives. Concrete is the prime material that is deployed in the construction of bridges.
One of the advantages of using it in building the Seventh Street Improvement Arches is that it can transfer loads effectively to the abutments compared to stones in situations where supporting materials have adequate strength to hold horizontal loads. Pre-stressed concrete and steel would permit the construction of the Seventh Street Improvement Arches in a more elegant way, even if the spans were increased from 124ft to 800 ft.
While erecting the arches, the approach that was used in the Seventh Street Improvement Arches would still be important since the bridge arches would require construction using precast concrete. Voussoirs that take a wedge shape that is built using temporary supports would be necessary. Provision of the temporary supports from below the bridge and the tower constitutes the important alternative for providing the necessary temporary support while building the Seventh Street Improvement Arches today using precast concrete.
Building it today will require the reflection of cost, strength requirements, and material availability. This situation will create the necessity for the deployment of pre-stressed concrete instead of precast concrete. Truesdell would definitely consider the knowledge and achievement of Freyssinet in terms of improving bridge-building technology. From 1928 to 1933, Freyssinet made the most significant achievements in the development of pre-stressed concrete.
These achievements were due to “the development of vibration techniques for the production of high-strength concrete and the invention of the double-acting jack for stressing high-tensile steel wires” (Raju 2). These discoveries marked the beginning of the intensive spreading of the practical applicability of the pre-stressed steel as from 1935.
Using the Freyssinet methodology for pre-stressing concrete, civil engineers in the US and Europe began to construct long-span bridges from 1945 to 1950. A good example of such a bridge is shown in fig. Three below. Christian Menn designed and fabricated it in 1962. The bridge is found in Tamins-Reichenau in Switzerland.
Instead of using stones, the Seventh Street Improvement Arches would probably use pre-stressed concrete while taking shape and configurations of Christian Menn’s bridge that is shown above. Building the Seventh Street Improvement Arches using the pre-stressed concrete today requires Truesdell first to manufacture prestressed beams just as he sourced stones for the quarry and shaped them accordingly. Manufacturing of pre-stressed concrete is done at a pre-stressing concrete plant.
Since the arches and beams would be required only in constructing the bridge, setting such a plant would make the costs of constructing the bridge prohibitive. The best alternative entails contracting the manufacture of the beams and arches.
During the manufacture of the pre-stressed concrete, the contracted civil engineers need to apply two ways for inducing compressive stresses in pre-stressed concrete. The first approach encompasses pre-tensioning, while the second approach involves post-tensioning. In the pre-tensioning process, the concrete has to be positioned after stretching of the tendons.
The force used to pre-stress concrete must be relocated to the concrete via a bond. In the pre-tensioning method, concrete is placed on stretching steel. Mutsuyoshi and Hai confirm, “To strengthen the beam, steel tendons with high strength are put in between two abutments to be tensioned to around 70 to 80 percent of their overall strength” (167). Tendons are held in their respective positions by means of a tensioning force before concrete is introduced into a mold.
Time will then be provided for the concrete to cure to gain the necessary strength. Tensioning forces will then be on the loose. Steel produces a reaction after attaining the required strength from the concrete.
This observation makes it gain the length that it had before. Consequently, tensile stresses are transformed into compressive stresses. Upon their complete curing, the concrete becomes very firm. Fig.4 below shows an example of a beam that can be deployed for constructing the Seventh Street Improvement Arches today. It is manufactured through the pre-tensioning methodology.
Instead of doing the pre-tensioning process, manufacturers of the beams and arches that were used to build the Seventh Street Improvement Arches today would consider using the post-tensioning methodology to induce the necessary strength in the concrete. In this approach, the concrete would be put after the tensioning and hardening of the tendons before the steel is stretched.
The force causing the pre-stressing would then be passed via the terminal ports to the concrete (Mutsuyoshi and Hai 171). Concrete would then be cast around.
Civil engineers would make ducts in the concrete body as the process progresses. This goal would be accomplished with the aid of steel rods, which are then removed later. In the next step, with regard to Raju, “after the concrete is hardened until it gains the required strength, engineers would place and stretch the steel tendons towards the end of the unit by externally anchoring them off to put the concrete into a state of compression” (57).
Both pre-tensioning and post-tensioning produce good pre-stressed concrete. However, they are different. One of the striking differences is that post-tensioning is only possible to do in a manufacturing plant.
Post-tensioning is done on the job site using applications, which are cast in place. Hence, once presented with post-tensioning and pre-tensioning alternatives, Truesdell and other engineers would select the post-tensioning method if they were building the Seventh Street Improvement Arches today. In today’s civil construction engineering works, lifting of the post-tensioned beams and arches would require lifting machinery.
Conclusion
William Truesdell designed the Seventh Street Improvement Arches while McArthur brothers and Michael O’Brien took its construction challenge. The arches were made from stones, which required high precision in terms of shaping them. A major challenge was experienced such no other bridge that required such stone-shaping strategies had been done before within the Minnesota locality. This claim implies that no one had seen such a bridge before.
Thus, the stonecutters needed training and supervision to ensure precision in their work. Consequently, the construction process was mainly in labor-intensive. Constructing the Seventh Street Improvement Arches today would probably require the use of concrete either in precast or pre-stressed state with significant use of lifting machinery.
The paper held that if Truesdell and other engineers were to construct the Seventh Street Improvement Arches today, they would possibly opt for post-tensioned pre-stressed concrete instead of stones. The design could also change significantly.
Works Cited
Billington, Duncan. “Historical Perspective on Pre-stressed Concrete.” PCI Journal 1.3(2004): 14-30. Print.
DuPaul, Angela. An Engineering Marvel Beneath Your Feet. London: Thomas Hurst, 2001. Print.
Minnesota’s Historic Bridges. Seventh Street Improvement Arches, 2012. Web.
Mutsuyoshi, Haiyu, and Nier Hai. Recent Technology of Pre-Stressed Concrete Bridges in Japan. Tokyo: Saitama University, 2010. Print.
Raju, Kingston. Pre-Stressed Concrete. New York, NY: McGraw Hill, 2009. Print.