When a petition was presented to Parliament in 1721 for a bridge to be built across the Thames at Westminster, the inhabitants of the City of London rose up as one, afraid that the effect would be to create a "Rival City" at Westminster. A new bridge would,

For centuries, London Bridge, its original wooden structure replaced by stone in 1209, had been the only Bridge across the Thames providing access to the capital, maintaining the City’s monopoly on trade.

For those wishing to reach the fashionable suburb, which was springing up at Westminster, centred on the law courts and royal palace, the lack of a more appropriately sited bridge was a great inconvenience.

In May 1736 an Act was passed authorising Westminster Bridge to be built from New Palace Yard. The Ninth Earl of Pembroke had the honour of laying the first stone in January 1739 and he laid what was intended to be the last stone in October 1746.

Sabotage attempts by watermen contributed to delays of the bridge. Accidents and damage to materials were part of an on-going campaign top hinder progress as much as possible, although the penalties if caught were severe:

The bridge was finally opened on 18 November 1750. It was greatly admired and was an inspiration to artists and poets. William Wordsworth was so struck by the view from the bridge that he wrote one of his best known sonnets, including the lines,

The sinking foundations brought the first bridge to the brink of disaster before it was even finished. However, constant attention and remedial work kept the bridge in service until a new bridge was built in 1862.

It was built to the design of Mr Thomas Page and at the time of its construction was considered to be "one of the handsomest structures that has ever crossed the waters of the Thames".

The bridge consists of 7 arched spans with an overall length of 250 metres, span lengths vary between 29m at the abutments to 37 m at the centre of the bridge. Each span is composed of 15 ribs consisting of a central wrought iron section connected to cast iron spandrel ribs at the piers.

The decking consists of buckle plates with concrete infill on top of timber and asphalt.

The bridge has an overall width of 26m.

One feature of the bridge was the incorporation of two 2m wide tramways.

The tramway tracks were removed in 1952 but the supporting beams and concrete infill still remain.

The stone piers are supported on timber piles with concrete infill within a cast iron cofferdam extending to 20ft below waterbed level.

A number of, now closed, flood arches are present behind the eastern abutment.

Westminster Bridge is a Grade I I * Listed Building.

The design load in the main arch ribs was 3 tons per square inch with wrought iron members being tested to more than 12 tons per square inch in compression. The average strength of cast iron in use in 1874 was 34 tons per square inch in compression and 6.8 tons per square inch in tension, and the recommended average ultimate strength of wrought iron was 21 tons per square inch in tension and 16 tons per square inch in compression.

The wrought iron buckle plates, which are a feature of the Westminster deck design, were apparently tested with a block of granite weighing 17 tons without any permanent deformation occurring.

Limited testing of a sample taken from the bridge in 1924 shows the cast iron to have an ultimate compressive strength of 40 tons per square inch and an ultimate tensile strength of 10.2 tons per square inch.

The bridge piers are made of large granite blocks placed on 14 inch diameter timber bearing piles made of elm. There are 145 such piles beneath each pier and the design load is 15 tons.

The original design assumes that the load on each span is carried independently by arching actions from the ribs. In addition, provision was made for thermal movements within the main longitudinal girders by incorporating vulcanised rubber between girder joints at the piers. The buckle plates are, however, continuous across the joint and no expansion joints are provided in the carriageway.

Movements of the girders is restrained at the abutments where the arch ribs bear directly onto the abutment wall.

A service bay carrying gas mains is provided, between ribs 7 and 8 of the deck. The enclosure is formed from wrought iron plates, riveted together and supported on crossbeams between the main girders.

Since 1914 there has been a 15 tons limit on the bridge. At peak period 3000 vehicles per hour cross the bridge in each direction, together with many hundreds of pedestrians crossing between Waterloo station and Westminster , Whitehall and Victoria Street.

River vessels have struck the bridge on many occasions during its life, and these have resulted in significant repairs to the structure.

A principal inspection was carried out by Rendel Palmer & Tritton, now called High-Point Rendel, on behalf of the City of Westminster in 1989, which revealed that the bridge was suffering from many durability problems mainly caused by water leakage through the deck. There was also a concern that the 15 tons weight limit was being exceeded and causing the buckle plates to be overstressed.

DTp Technical Memorandum BD 21/84, as amended, required that cast iron bridges be assessed by comparing resistance, based on permissible stresses, with the assessment load effect of BD 37/88. The load effects for cast iron being determined using f3 and the partial load factor f1 = 1.0, except in the case of the load effect of surfacing where f1 = 1.5. However, BD 21/84 requires wrought iron bridges to be assessed at the ultimate limit state by comparing resistance, based on yield strength m = 1.2 with the assess loads effects. The assessment loads effects for wrought iron being determined using f3 = 1.1 and partial load factor f1 > 1.0.

Westminster Bridge comprises both wrought and cast iron components and , in the case of the main arch ribs, use both materials in the same component. Notwithstanding this, the cast iron and wrought iron sections have been assessed in accordance with BD 21/84, the load distribution analysis being undertaken using nominal loads with their effects being enhanced by f1 factors at wrought irons sections. The permissible stresses in cast iron are taken directly from BD21/84 (section 6.2.5) and the yield strength of wrought iron is taken to be 220N/mm².

For the computer analysis of statically indeterminate parts of the bridge the Young’s Modulus of cast iron has been taken as 90,000 N/mm². This is at the bottom of the range permitted in BD 21/84 and reflects the results of 3 test on samples taken from the bridge in 1924, which indicates a Young’s Modulus of 73,000 N/mm².

Young’s Modulus of wrought iron component has been taken as 200,000 N/mm² as given in BD 21/84.

A load test undertaken to confirm, or otherwise, the structural action of the arched ribs of the bridge assumed in the assessment. Water filled army pillow tanks were selected as they permit a uniformly distributed load to be applied in a controlled fashion. The uniformity of the pillow tank loading prevented the risk of overstressing local deck members and simplified the post load test analysis. Each 13.5m long, 8m wide tank was capable of applying a load of 130 tons, when completely filled with water.

For the load test two loads were incrementally applied. Initially, a single tank was filled to a depth of 1.0m emptied and then two adjacent tanks were filled to a depth of 0.65m. Both the test loads were approximately equivalent to 17.0 tons assessment loading applied to the area of the bridge covered by the tanks.

The load test and the subsequent monitoring gave confidence that the 7.5 tons limit imposed on the bridge prior to the load test was safe, as they demonstrated that high strains were not developed at the critical locations of the arched ribs under either the test load or actual traffic.

The objectives of the design were to :

• Raise the strength of all bridge components found to be sub-standard by the assessment so that they could support 40 tons vehicles

• Address the cause of damage to the bridge, typically corrosion of metal work

• Enable the strengthening works to be undertaken with the minimum disruption to vehicular, pedestrian and river traffic

• Be sensitive to the heritage value of the Grade I I * listed structure.

It was decided to replace the existing timber and concrete decking with a structural reinforced deck. A 40N/mm² concrete utilising a sintered pulverised fuel ash (Lytag) lightweight coarse aggregate with a target insitu density of 1950kg/m³, was selected to provide the required strength with a negligible change in dead loading.

It was decided to replace the spandrel end girders with steel fabrications and to pre-stress them down to the underlying pier castings to control the level of tensile stress arising in the castings.

The ornate bridge parapet was of brittle cast iron construction, weakly connected to the underlying slender fascia girder and was showing signs of distress. It was decided to replace the parapets with visually identical parapets, but in a ductile material, designed to withstand vehicle impact loading appropriate to the class of road carried by the bridge. The material chosen was a spheroidal graphite cast iron grade 400/18L20.

To remove the dependency of the parapet for support on the underlying fascia girder, the replacement parapet was to be attached to the new concrete deck slab.

Note 1: Some buckle plates, particularly those at the kerb lines over the piers, have corroded and have negligible residual strength.

Note 2: The analysis has shown that localised areas of the cast iron spandrel ribs will be overstressed under the 7.5 tons Assessment Live Load acting in combination with temperature effects.

Note 3: The critical cast iron spandrel ribs and wrought iron ribs are those subjected to maximum dead load i.e. ribs 10,11,12 and 13, which carry the tramway concrete reinforcement.

Note 4: The capacity of type 1 wrought iron cross girders are limited by the strength of their riveted end connections. The girders themselves have full live load capacity.

Note 5: The piers and abutments appear to be in generally sound condition and can therefore be considered adequate. Repair of minor cracking and scour damage will be required.

The refurbishment included installation of proprietary expansion joints at each pier and abutment, a new deck waterproofing system, and a complete rationalisation of services, so they were either accommodated in the bridge footpath above the new waterproofing membrane or completely beneath the deck slab.

The areas of unrepaired boat impact damage to the bridge were repaired using the Granges Metalock stitch repair system.

Refurbishment included painting repaired areas of the bridge and new steel components using a six-coat system, returning the bridge to its original paint colours.