The construction of Tower Bridge, known for its iconic twin towers, began in 1886 and was completed in 1894, opening to the public. The bridge spans a total length of 1050 feet, including the abutments that connect to the shore. The approach roads on the north and south sides of the bridge are 1260 feet and 780 feet long, respectively. The width of the bridge, measured between the parapet walls, is 65 feet. The gradients of the bridge are 1.67% on the north side and 2% on the south side. Tower Bridge also features two smaller towers on the shore abutments, from which the suspension chains are connected.
London’s famous Tower Bridge
The primary towers, also known as the opening span, are connected by a span that consists of two leaves. These leaves are hinged on the piers, allowing them to rotate in a vertical plane. This unique design enables the movement of vessels by opening and closing the leaves to create a passageway.
1. Main Towers of the Bridge
The construction of the four towers of the bridge involved the use of steel columns covered with Portland stone and granite rock on the outer facing, and brickwork on the inner facing. The steel columns were shaped like octagons and had a length of 120 feet. They were spaced 60 feet apart transversely and 33 feet apart longitudinally to the line of the bridge. The steel columns were propped together using three sets of braces to provide support to the outside walls.
Diagonal wind-supports were embedded between the landing portions of the girders to resist a wind load of 56 lbs on the exposed faces of the structure. Angle bars and T-stiffeners were used in the construction of the outer and inner sides of the column at the corners of the octagon column. Three side plates were provided between the main landing, double between the first and second landing, and single for the last two landings.
To increase overall stiffness, horizontal diaphragms with a spacing of 3 feet were provided inside the column. These diaphragms had circular holes that could 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 16 ft2 granite bed.
North and South towers of the Tower Bridge
The granite bed consists of four stones, each with an area of 8 ft2. It is supported by Staffordshire blue brickwork. To ensure a uniform bearing over the entire base surface, layers of red lead and canvas were placed between the base of the column and the granite bed.
The columns bear the entire weight of the towers, including the high-level footways constructed on the upper level of the tower and some weight of the suspension chains. When combined with the saturation capacity of the traffic, a total weight of 16 tons per square foot is transferred to the granite bed, while approximately 10 tons per square foot acts on the Staffordshire blue brickwork.
To accommodate contraction and expansion joints, the columns were wrapped with oiled canvas. This serves the dual purpose of reducing adhesion between the concrete and steelwork.
2. Abutment Towers of the Bridge
The abutment towers of the Tower Bridge were constructed in a similar manner to the main tower, but on a smaller scale. The columns of the abutment towers were octagonal in shape, with a length and width of 43 feet and 5 feet, respectively. In order to stiffen the arched ribs above the roadway, corner-to-corner supports were provided between columns in the direction of the suspension chains.
To provide additional support, masonry and brickwork were added around the steel work, and the walls of brickwork were extended above the steel columns to create living quarters for the Tower Bridge authorities. Unlike the main towers, where the suspension chains were fixed to the tie bars, the suspension chains in the abutment towers were fixed with anchor bars through horizontal links of cleat angle, pins, and rockers at each end.
3. Suspension Chains and Rods
The suspension chains for the Tower Bridge were constructed on the shore span side after the truss girder was built, in order to provide sufficient rigidity to withstand the unequal loading caused by the heavy traffic. Each suspension chain consisted of two sections of different lengths. The longer length of the chain was attached to the upper end of the abutment tower with tie bars, while the shorter length was fixed to horizontal links on the bottom end of the abutment tower.
The longer section of the chain spanned a chord of approximately 250 feet between the pins at each end, while the shorter section spanned a chord of around 120 feet. The longer section was 160 feet deep across the booms at the center, while the shorter section was 15 feet deep. Vertical rods were placed at intervals of 18 feet between the booms to maintain the depth of the suspension chain, with a total of nine rods fixed to the longer section and five rods fixed to the shorter section. Diagonal braces were then inserted between the vertical rods to provide resistance against compression or tension.
Suspension chains and rods of the Tower Bridge
The diagonal bracings were securely attached to the booms on each side using interfacing plates. These plates were riveted to the webs by passing them through slots in the flange plates. Angle bars were used to join the plates to the rib-plates. These plates also served as attachments for the upper ends of the rods, which were connected to the bottom boom through lower flange-plates.
Moreover, in addition to their structural role, the plates also provided protection for the joints of the booms in the web. To ensure the correct curvature of the booms, the angle-bars and flange-plates were bent as needed. This was done to give the booms the required curvature, even though they were originally straight.
4. Shore Spans
The construction of each shore span involved the use of 15 primary transverse girders and nine longitudinal girders, spanning between adjoining pairs. The spacing between each shore span was set at 20 feet, but it was limited to 9 feet between the faces of the piers and the end girders. The shore spans themselves were 64 feet in length and had a depth of 4.5 feet at the center of the web-plates. Additionally, the shore spans were cambered, with a rise of 10 inches at the web-plate locations, to accommodate the curvature of the roadway.
In order to increase the thickness of the lower flange, plates were added to the center of the girders, which deviates from the usual practice of adding plates to the undersides of the girders. Similarly, the transverse web girder’s thickness was increased by adding plates in the same manner as the suspension rods, which were connected to the girder using link-plates.
Cross-section of opening span and Surrey shore span
The longitudinal girders were utilized to add stiffness to the flanges of the bridge, and they were spaced at intervals of 8 feet. Furthermore, intermediate stiffeners were incorporated between the longitudinal girders on all sides of the bridge, except for the outer faces. This was because the outer faces were already reinforced by angle-bars at the bottom and flange-plates at the top, making additional intermediate stiffeners unnecessary.
5. Opening Span
The opening span of the bridge is composed of four main girders. Each girder extends 100 feet beyond the pier face and 60 feet towards the roadway. The spacing between each girder is 15 feet. Transverse girders were utilized to connect the main girders, with bracing at a spacing of 12 feet. The overall structure of the opening span resembles an open-formed lattice.
The transverse girders were fitted between the longitudinal girders, with small intermediate transverse girders between them. This method of construction created partitioned spaces in the bridge floor, each with an area of approximately 4 feet by 3 feet. The floor of the bridge is covered by a 6-inch thick buckled floor-plate.
To provide safety, parapets were constructed on the outer edge of the longitudinal girders. A width of 49 feet was maintained between the parapets through bracketing out from the external primary supports. The parapets consist of short columns spaced at 6 feet, with light cast-iron panels between them.
6. Anchor-Ties and Girders
The anchor ties were connected to the upper end of the suspension chains to resist the stresses generated by the chains. This connection was made by linking their upper end with ties across the abutments and attaching their lower end to the concrete blocks of the foundations of the approach viaducts. The anchor ties were constructed as box-girders between the abutments and the road-level to ensure they were stiff enough to support themselves over the 80 feet span between those points.
Underneath each tie at the roadway level, a rocker support was placed, and other supports were already provided beneath it. The ties beneath the roadway level were made up of flat plates bolted together and encased in the thickness of the approach side walls.
Below the ground level where the approaches were constructed, ties branched out on each side of the main ties, allowing for three points of connection to the anchor-girders for each tie. These lower portions of the ties consisted of a double set of plates braced together and riveted to the double webs of the anchor-girders.
Anchor ties supporting the abutment towers
The Surrey side of the ties undergoes more stress compared to the Middlesex side due to the action of the stiffening girders. The maximum tension calculated for the Surrey side ties is 1,900 tons, while for the Middlesex side ties, it is 1,200 tons. The anchor-girders on the Surrey side are 50 feet long, 4 feet deep, and 5 feet wide across the upper flanges, whereas the Middlesex girders are 38 feet by 3 feet by 5 feet in dimensions.
The upper flanges of the anchor-girders are responsible for transmitting the stress from the ties onto the embedded concrete. To allow for inspection and painting, all parts of the girders, except for the bearing surfaces of the upper flanges, are made accessible by surrounding them with cast-iron subways that are bolted to the upper flanges. The cast-iron segments are positioned around the ties, and access to them is provided either from the upper ends of the ties or through manholes cut in the top flange-plates of the anchor-girders from the girder-subway. The subways are designed to function as both inlets and outlets, ensuring proper air circulation and ventilation.
7. Foundation Details
The Tower Bridge is comprised of three spans: a middle span that is 200 feet long, and two spans on either side of the middle span that are 270 feet long. To accommodate the passage of vessels, a bascule bridge system was used for the construction of the middle span, which includes two movable platforms projecting 100 feet from the pier face.
The total weight of the main tower, roadway, and movable platforms with projections was considerable for a short span bridge. The weight distribution on each foundation of the main tower piers was determined to be 4 tons per foot, which necessitated a wide isolated footing with a width of 100 feet.
Caisson foundation used in the Tower Bridge construction
The main tower piers required a caisson foundation due to the massive weight they had to support. The reliable nature of the London-clay, which had good compressive strength, limited the width of the caisson foundation, providing sufficient bearing strength. The caisson foundation had a diameter of 90 feet and a length of 2000 feet, and it had to pass through the London-clay layer.
Instead of sinking one large caisson that extended across the entire pier, the geotechnical designers opted to construct four rows of smaller caissons. These smaller caissons were square in shape with dimensions of 28 feet each. Additionally, a triangular-shaped caisson was constructed at the end of each row on either side.
The smaller caissons were spaced at a distance of 3 feet from each other, which was the minimum spacing required to allow workmen to effectively work in between the caissons. This arrangement allowed for efficient construction and ensured that the caissons were placed at appropriate intervals while passing through the London-clay layer, which had good compressive strength.
FAQs
What is the type of foundation used in the Tower Bridge construction?
The piers of the Tower Bridge were constructed using a caisson foundation.
When did the construction of the Tower Bridge begin?
I’m sorry, but there seems to be an error in the date you provided. If you meant to say that the construction of the Tower Bridge started on June 10th, 1892, then here is a rewritten version of the given context:
The construction of the Tower Bridge, an iconic symbol of London, began on June 10th, 1892. The bridge was designed by Sir Horace Jones and Sir John Wolfe Barry, and it took eight years to complete. The bridge was officially opened on June 30th, 1894, by the Prince and Princess of Wales.
The Tower Bridge is a suspension bridge that crosses the River Thames and connects the boroughs of Tower Hamlets and Southwark. It is known for its distinctive Victorian Gothic style and the two towers that support the bridge. The bridge is 800 feet long and has a roadway that is 140 feet above the river at high tide.
Today, the Tower Bridge is one of the most recognizable landmarks in London and is visited by millions of tourists every year. It is also an important part of London’s transportation infrastructure, with thousands of vehicles and pedestrians crossing it every day. The bridge has undergone several renovations and upgrades over the years, but it remains an enduring symbol of London’s history and engineering prowess.
Who was the design engineer of the Tower Bridge?
The engineer responsible for designing Tower Bridge was Horace Jones.
What was the cost of construction of the Tower Bridge?
The Tower Bridge, a prominent landmark in London, was built with a significant financial investment of £1,184,000. This amount was spent during the construction period, which took place in the late 19th century. When adjusted for inflation, this cost would amount to an equivalent of £137 million in 2020, highlighting the considerable expense incurred for the construction of this iconic bridge.