The construction of Tower Bridge started in 1886 and was opened to public in 1894. The bridge is renowned for its two primary glaring towers constructed on the waterway docks. It has two more modest towers on the shore abutments through which the suspension chains are connected.
Key features:
- The span of Tower Bridge, including the length of abutments, is 1050 feet.
- Approach roads on the north and south side are 1260 feet and 780 feet, respectively.
- The width of the Tower Bridge between parapet walls is 65 feet.
- The gradients on the Tower Bridge are 1.67% and 2% on the north and south side, respectively.
The span between the two primary towers (also called as opening span) comprises two leaves. These leaves are pivoted on the piers so that they can rotate in vertical plane to allow the movement of vessels.
1. Main Towers of the Bridge
The four towers of the bridge were constructed using steel columns and the outer facing was covered with Portland stone and granite rock, whereas, the inner facing was made up of brick work. The shape of steel column towers located at each four corners is an octagon. The length of the steel columns is 120 feet and are spaced 60 feet apart transversely, and 33 feet apart longitudinally, to the line of the bridge. Steel columns were propped together by three sets of braces, which provide support to the outside walls.
Diagonal wind-supports were embedded between the landing portion of girders to resist a wind load of 56 Lbs exerted on the exposed faces of structure. Angle bars were used to construct the outer and inner side of the column at the corners of the octagon column. Also, T-stiffeners were provided at the center of each side plates.
Three side plates were provided between main landing, double between first and second landing, and single for last two landings. Horizontal diaphragm of 3 feet spacing was provided inside the column to increase the overall stiffness. These diaphragms have a circular hole which can be accessed from top to bottom. The 14 feet wide base of the column was connected to the gusset plate to distribute the weight of the column onto the granite bed of 16 ft2.
The granite bed is comprised of four stones with an area of each stone of 8 ft2. Further, the bed is resting upon Staffordshire blue brickwork. Layers of red lead and canvas were put between the base of the column and the granite bed to get a uniform bearing over the entire base surface.
The entire weight of the towers is supported by the columns. Also, the columns support the weight of high-level footways constructed on upper level of tower and some weight of the suspension-chains. If all of the weight acts together with the saturation capacity of the traffic, a total of 16 tons per square foot would be transferred to the granite bed and approximately 10 tons per square would act on the Staffordshire blue brickwork.
To accommodate contraction and expansion joint, the columns were wrapped with oiled canvas. It also helped to reduce the adhesion between concrete and steelwork.
2. Abutment Towers of the Bridge
Abutment towers were constructed in similar manner to the main tower, however on a smaller scale. The length and width of the columns of the abutment towers are 43 feet and 5 feet and it is also in octagonal shape. Corner-to-corner supports were provided between columns toward the suspension-chains direction in order to stiffen the arched ribs above the roadway.
Masonry and brickwork were provided all around the steel work and the walls of brickwork were carried above the steel columns to construct the living rooms for the authorities of the Tower Bridge. In case of the main towers, the suspension chains were fixed to the tie bars, whereas, in case of the abutment towers, the suspension chains were fixed with anchor bars by horizontal links of cleat angle, pins, and rockers at each end.
3. Suspension Chains and Rods
The suspension chains were built on the shore span side after the construction of the truss girder, to give adequate rigidity to shore span to withstand the unequal loading because of the substantial traffic of the Tower Bridge.
Each suspension chain comprises of two sections of unequal length. The longer length of the chain was fixed to the upper end of abutment tower with the tie bars, whereas, the shorter length was fixed to horizontal links on bottom end of the abutment tower.
The longer length of the chain subtends a chord of around 250 feet between the pins at each end and shorter length of the chain subtends a chord of around 120 feet. The longer section is 160 feet and the shorter section is 15 feet deep across the booms at the center. The vertical rods were provided at an interval of 18 feet between the booms to maintain the depth of the suspension chain.
A total of nine and five vertical rods were fixed to the longer and shorter sections, respectively. Then, diagonal braces were inserted between the vertical rods to resist either compression or tension.
The diagonal bracings were fixed to the booms on each side by interfacing plates. Further, the plates were riveted to webs by passing them through the slot developed on the flange plates. Also, the plates were joined to the rib-plates by angle bars. The plates connecting the bracings formed attachments for the upper ends of the rods with the bottom boom through the lower flange-plates.
Additionally, the plates protected the joints of booms in the web. The angle-bars and flange-plates were bent to give the required curvature to the booms, where the booms were straight.
4. Shore Spans
Each of the shore spans were constructed between adjoining pair with 15 primary transverse girders and nine longitudinal girders. The spacing between each shore spans is 20 feet. However, the spacing was restricted to 9 feet between the faces of the piers and the end girders. The shore spans are 64 feet in length and 4.5 feet deep at the center of the web-plates. Also, the shore spans were cambered 10 inches higher at web-plates locations to suit the curvature of the roadway.
The plates were added on the center of the girder to increase the thickness of the lower flange. However, the usual practice is to add the plates on the undersides of the girders. The thickness of the transverse web girder was increased in the same way as of the suspension rods with which the girder was connected by link-plates.
The longitudinal girders were used for stiffening the flanges and were provided at a spacing of 8 feet. Additionally, intermediate stiffeners were provided between longitudinal girders on all of the faces expect on the outside faces of the bridge because the outer faces were already stiffened by the angle-bars at the bottom and by flange-plate at the top.
5. Opening Span
The opening span consists of the four main girders. Every girder is projecting 100 feet beyond the pier face and is extending backwards around 60 feet towards the roadway. The spacing between each girder is 15 feet.
Transverse girders were provided between the main girder and bracing were used at a spacing of 12 feet to connect them. Overall, the opening span looked like an open formed lattice.
The transverse girders were fit between longitudinal girders and small intermediate transverse girders between the longitudinal. In this way, the floor of the bridge was partitioned into spaces of area about 4 feet by 3 feet, which were covered by the 6-inch thick buckled floor-plate.
Parapets were constructed on the outer edge of the longitudinal girder. A width of 49 feet was maintained between parapets by bracketing out from the external primary supports. The parapets comprise of short columns spaced at 6 feet with light cast-iron panels between them.
6. Anchor-Ties and Girders
The anchor ties were connected at the upper end of the suspension chains to resist the stresses generated by the chains. The connection of anchor ties with the suspension chains was provided by connecting their upper end with ties across the abutments and by connecting their lower end to the concrete blocks of the foundations of the approach viaducts. The anchor-ties were constructed between the abutments and the road-level, as box-girders, in order to render them sufficiently stiff to support themselves throughout the 80 feet intervening between those points.
A rocker support was put underneath each tie at the roadway level, beneath which different supports were already provided. The ties underneath the roadway level comprise of flat plates bolted together and encased in the thickness of the approach side walls.
Beneath the ground level on which the approaches were constructed, ties branch out on each side of the focal ties, with the goal that three points of connection to the anchor-girders can be obtained for each tie. These lower portions of the ties consist each of a double set of plates braced together and riveted to the double webs of the anchor-girders.
The ties on the Surrey side are subjected to greater stress than those on the Middlesex side by the action of the stiffening girders. The calculated maximum tension upon each of the former was 1,900 tons, and upon each of the latter 1,200 tons. The Surrey anchor-girders are each 50 feet long, 4 feet deep, and 5 feet wide across the upper flanges. The dimensions of the Middlesex girders are 38 feet by 3 feet by 5 feet.
The upper flanges of the anchor-girders transmit the stress from the ties on to the concrete in which they were embedded. With the exception of the bearing surfaces of the upper flanges, all parts of the girders were rendered accessible for inspection and painting, by surrounding them with cast-iron subways bolted to the upper flanges.
The cast-iron segments were provided around the ties and access to them was provided either from their upper ends, or from the girder-subway through manholes cut in the top flange-plates of the anchor-girders. The subways act somewhere as inlet and somewhere as outlet so that the proper air circulation can be obtained and good ventilation can be obtained.
7. Foundation Details
The tower bridge consists of three spans. The length of the middle span is 200 feet and the length of other two spans, on either side of middle span, is 270 feet. Bascule bridge system was adopted for the construction of middle span with two movable platform so that the vessels can pass the bridge easily. Both movable platforms are projecting 100 feet from the pier face.
The total weight of the main tower, roadway way, and movable platforms with projections was very huge for a short span bridge. Proportion of the weight on each foundation of the main tower piers was 4 tons per foot. This much of load led to the 100 feet width of the isolated footing.
To encounter such a huge weight, caisson foundation was adopted for the main tower piers. The caisson foundation had to be passed through London-clay, which had good compressive strength. Thus, the reliable nature of London-clay limited the width of caisson foundation by providing sufficient bearing strength. The diameter and the length of caisson foundation was 90 feet and 2000 feet, respectively.
The geotechnical designers constructed four rows of smaller caissons instead of sinking one large caisson extending right across the pier. Smaller caissons were provided in square shape with dimension of 28 feet. Also, at the end of four rows on either side, a triangular shaped caisson was constructed. Each caisson was constructed at a spacing of 3 feet to the nearest caisson. This was the minimum spacing provided so that workmen could be employed effectively in between the caissons.
FAQs
Caisson foundation was adopted for the piers of the Tower Bridge.
The construction of the Tower Bridge started on 10th June 1986.
Horace Jones was the design engineer of the Tower Bridge.
The Tower Bridge was constructed at a cost of £1,184,000 (equivalent to £137 million in 2020).