Author Archives: Bill

Design of the Shoots Bridge Pylons

Foundations.

The main pier foundations for the cable stayed bridge were positioned on the English Stones, near a rocky outcrop known as Gruggy. They had to avoid nearby fault lines and keep clear of the deep and steep-sided channel that had been carved out over time and was now the main navigation channel, known as The Shoots, with its peak currents and turbulence.

Cable Stay fig 1 rescanned

Elevation of Shoots Bridge

The foundations had to withstand the dead and live loads from the bridge and be able to cope with possible ship impacts. They were carefully positioned following extensive investigations. A number of special navigation aids for the use of pilots on ships using the estuary were agreed with the Gloucester Harbour Trust, including a harbour radar system which replaced the previous unsightly proposals for ship protection islands in front of those piers that would be most at risk. These protection islands would have been capable of inflicting considerable damage to errant shipping.

Once again, caissons were deemed to be the most suitable choice for foundations in these conditions. They would be larger than those used for the viaduct, measuring 53 metres in length by 13 m wide and weighing 2000 tons each. They were designed to withstand impact from ships weighing 6500 t. The caissons would be heavy enough to prevent overturning and shear keys, in the form of 2 m dia. tubular steel piles cast into 2 m deep holes but allowing the upper parts of the piles to be encased in the concrete infill that was then poured into the bottom of the caissons, would resist any sliding which might otherwise result from ship impact.

The main piers of the bridge were designed to look similar to those of the viaduct. And although the caissons used in the main bridge foundations were larger than those for the viaduct, similar design criteria were applied. Back span piers were added to prevent the back spans from being lifted by loading on the central span. (see more information on Design of the Shoots Bridge Deck).

The Pylons.

The navigational channel in the vicinity of the crossing, known as The Shoots, is located in a deep but relatively narrow natural trench, little more than 300 m wide at river bed level. A symmetrical cable-stayed bridge, the centre-piece of the new crossing, would be constructed across this channel, with a main span of 456 m and a total length of 900 m. It is linked to the shore by major viaducts on either side, both 2 km in length, bringing the total length of crossing to 5 km. Following consultations with shipping interests, the clearance of the bridge was set at 37 m above mean high water level. This allowed for a notional rise in sea level (as a result of global warming) of 1 metre.

Cable Stay 2

Elevation of Pylon

Two twin-legged portal-frame pylons, rising 149 m above the river bed (101m above deck level) provide the main features of the bridge. Each pylon leg includes the upper anchorages for 60 cables, thirty of which radiate down on to the main span and 30 to the back span, making a total of 240 cables in all. The cables pass through openings in the concrete bridge deck, which is 34.6 m wide, but they are actually anchored on top of the deck steelwork.

The 456 m main span reflects the need to locate the pylons at a sufficient distance from the steep side slopes of the main channel to ensure the stability of the bridge foundations. Additional stiffness for each back-span is provided by two piers that are similar, in both appearance and spacing, to the piers supporting the viaduct.

The base of the pylon legs are 10.2 m wide, along the line of the bridge, reducing to 5.4 m at the top. Transversely, they are a standard 4.0 m wide. The legs of the pylons are constructed in reinforced concrete, using the “lift over lift” method, with the aid of self-climbing formwork. The legs are hollow, with lifts and steel stairs installed to provide access from the caissons to the deck level and to the top of the pylons. The importance of the pylon proportions to the appearance of the bridge was widely appreciated and was subject to much scrutiny from the architect, the engineers – and the Royal Fine Arts Commission.

Each pair of pylons is provided with two hollow, precast reinforced concrete cross beams for transverse stiffness. In order to reduce the amount of steel reinforcement required, these upper cross beams are post-tensioned. To cope with the tensile, splitting forces caused by the cables anchored on either side of the pylons, small precast beams have been fitted across the internal space between the front and rear faces of the pylons and post-tensioned. The tendons for this operation are contained within ducts and anchored outside the pylons. Hot wax is injected into the ducts to exclude air and water and so protect the tendons. The anchorages for these tendons are visible between the exit points of the cables.

There is a tendency, under certain loading conditions, for the weight of the deck to be lifted off the back span piers. This can produce significant bending moments in the pylons and, to cope with these moments, the pylon legs need to be post-tensioned vertically. 16 vertical tendons, each comprising 19 strands, are installed within HDPE (high density poly-ethylene) ducts in the front and rear walls of the hollow pylons, and tensioned. The tendons stretch from a point below the first cable anchorage, to the slab at the top of each pylon leg.

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Design of the Second Crossing Viaduct Deck.

The crossing includes a total of 4.2 km of viaduct in two sections, one on either side of the cable-stayed bridge, each comprising 20 standard spans of 98.12 m. Twin box girders were used in preference to a single, wider box, because the smaller, separate units would be easier to handle and would not need to be post-tensioned transversely. Also, there would be space between the twin box girders for an under-slung train to provide access for maintenance, along the whole length of the structure. Both box girders, in every span, contain 27 separate viaduct segments, each 3.5 m in length. In addition, a special segment of similar length was placed on every pier. Each of the twin box girder flanges would be 15.6 m wide and there would be a 2 m concrete stitch between them, bringing the total width of the viaduct deck up to 33.2 m.

The key to the load bearing strength of the viaduct spans is the use of the high tensile steel tendons that would be added during the actual construction of the viaduct deck. The tension would be introduced into the tendons and then locked in, using wedges or anchors in a process known as “post-tensioning” (because the stress is introduced after the individual deck segments have been constructed separately, and then positioned). Each new deck segment would be introduced and held in position while temporary tendons would be used to tension it sufficiently hard against its predecessor to ensure that it would remain secure when the lifting tackle was removed. Later, the temporary tendons would be removed and replaced by the permanent set.

Via Sup & Piers fig 1 copy

Close-up of tri-planar soffit to viaduct deck. Copyright; Neil Thomas of Photographic Engineering Services.

The soffit of the viaduct deck shown on the Illustrative Design was curved. However, the consortium rejected this idea in favour of the tri-planar form illustrated on the photograph. This would avoid having to use a large number of blocking devices around the upper surface of the lower flange to insure that the lower tendons followed the line of the curve fairly closely. With this change, the number of blockers or deflector diaphragms within each span was reduced to two, thus increasing the speed of production and reducing cost.

The Consortium had to comply with special government requirements relating to the use of tendons. When used for post-tensioning the precast concrete units together, tendons must be “external”, i.e. they must remain accessible and replaceable, even when enclosed within a hollow structure – and the free length of any such tendon must not exceed 40% of the span. SRC’s solution was to fit internal restraining devices, mounted on what could be described as a vertical diaphragm, at each of two locations within the span, to grip and hold all tendons passing those points and, in some cases, to act as a deflector to change the direction of the tendon.

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Internal vew of girder with pre-stressed tendons in place

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A scetch showing longitudinal layout of pre-stressed tendons.

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Another vew of pre-stressed tendons in place. Copyright, Neil Thomas of Photographic Engineering Services.

In total, 2,400 separate deck segments, each 3.5 m long, would be installed in the viaduct. They would be constructed on shore, in the form of a hollow reinforced concrete box open at both ends and weighing up to 200 t. In elevation, the viaduct segments are 7.0 m deep at the piers and 3.95 m deep through the central section of the span. See illustration. Care had to be taken to optimise this design because the tri-planar form is less pleasing to the eye than the curved option. The addition of a 2 m wide in-situ concrete stitch between the top flanges of the twin segments of the viaduct box girders, would enable the in-situ concrete deck slab to be laid right across the width of the deck in a single operation.

Via Sup & Piers fig 5 copy

Support beam for allowing transverse movement of launching gantry. Copyright; Neil Thomas of Photographic Engineering Services.

Access to the deck for maintenance is provided by a train that runs on rails under the central reservation, in the void between the two boxes. A number of conventional gantries with working platforms underneath the deck would also be provided and they would also allow for access up on to the top of the deck. These gantries would be suspended from both outer edges of the deck and they would be able to move horizontally along the deck.

Longitudinal movement joints are provided at the centre of every fifth span, i.e. 392.48 m apart, to restrict progressive collapse of the structure. This means that there are five separate, homogeneous sections of deck on each side of the cable-stayed bridge, each capable of small longitudinal movements, relative to its neighbours. The actual joints are inserted at mid-span to ease arrangements for maintenance. They comprise stainless steel/PTFE sliding surfaces which allow longitudinal deck movements, while restricting movements in other directions and, in doing so, they transmit vertical and horizontal shear forces from one side of the joint to the other. Similar joints are provided at the junctions between the viaduct and the main bridge, and at the two end abutments.

The viaduct deck is supported by elastomeric bearings (essentially, large thick rubber pads) located on each pier. These bearings transmit vertical and horizontal forces from the deck to the piers. Each section of deck therefore rests, or “floats”, on elastomeric bearings. No point of the deck is actually fixed and so each section can move a short distance, independently of its immediate neighbours. This enables temperature length changes, together with braking and seismic forces to be distributed over several piers.

Seismic activity provides the critical loading case for the bearings which are designed to restrict the pier’s horizontal acceleration. The bearings must be large; 1.3 m x 1.3 m and 500 mm deep, to cope with the seismic loading. Lateral deformations for the seismic design case are estimated at between 150 mm and 200 mm, while the maximum deformations for more normal loading conditions are expected to be in the region of 80 mm. A fail-safe shear key device is incorporated into the pier head to restrict movement in the case of a seismic event. Every bearing was shear-tested for normal in-service loads, and prototype bearings were subject to a full range of tests, including a shear load representing the full ‘design’ seismic load.

Every deck segment located over a pier must be vertically post-tensioned to that pier to provide the required stability against out-of-balance loadings that will occur during erection of the deck, especially during the construction of the balanced cantilevers, described later under “Construction of Viaduct Decks”.

The viaduct widens out by 13 m over the last 240 m at the Avon end, to accommodate the slip roads from the M49 junction. Here, the central part of the deck between the viaduct’s two concrete box girders, comprises an in-situ reinforced concrete slab supported on 2 m deep precast concrete beams which span between the inner webs of the box girders.

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Design of the Second Crossing Viaduct Piers.

Foundations

The viaduct required the building of more than 40 pier foundations. It was a major and spectacular operation that could be seen from both sides of the estuary. The ground conditions immediately beneath the piers varied from the exposed rocky out crops of the English Stones, in the east, to soft alluvial soils in the west.

Man of Const fig 4 copy

View of completed crossings with Welsh shore in background. Copyright; Neil Thomas of Photographic Engineering Services.

Another important and sometimes over-riding factor, was the strength of the tides for long periods, between high and low water. Caissons were ideal for the rocky conditions over the English Stones on the east side of the estuary, while the softer material near the Gwent shore, required piled or spread footings. SRC decided to use pre-cast caissons, hollow reinforced concrete shells weighing up to 2000 tons, for the majority of locations (33 out of 40), including all the more difficult locations.

The foundations are needed to deal with the dead weight of the viaduct itself, together with the live load of traffic, as well as a potential ship impact. The frictional shear resistance between the base of the caisson and the exposed bedrock, together with the shear strength in the bedrock beneath, would be crucial in resisting ship impact. This further reinforced the preference for large spread foundations, with high sliding resistance, especially in locations that might be at risk from ship impact.

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The view of crossing from Aust. Copyright; Neil Thomas of Photographic Engineering Services

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Vew of completed structure showing tri-planar soffit to deck. Copyright; Neil Thomas of Photographic Engineering Services

The forces on the caissons varied according to location and a decision was taken to develop three modules, differing in size and weight, to cope with these variations. The caissons were all cast in the Avon construction yard and transported to site on powered barges. They had to be buildable and robust in the extreme conditions that could occur on the Severn Estuary, with a tidal rise and fall of up to 14 metres and current velocities up to 5m a second.

Special attention was paid to the two piers, located either side of, and adjacent to, the Severn Railway Tunnel near the Avon Bank. The tunnel is 140 years old and it carries the main line from London to South Wales, so any risk of damage to the structure would be unacceptable. In order to protect the tunnel, an exclusion zone, centred on the tunnel and 30 m wide, was established to prevent additional loading or vibration that might damage the tunnel.

The problem is illustrated on the drawrings below. The tender drawing is on the left, with the centreline of the viaduct passing horizontally across the centres of the two caissons. SRC’s modification is shown on the right.

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Arrangement shown in the tender drawings

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SRC’s modified arrangement

SRC’s first step was to amend the illustrative design provided with the Invitation to Tender, by standardising all viaduct spans at 98m.  Then, a special pile layout was designed for the two piers on either side of the tunnel, with a platform above the piles in the form of a modified caisson. As shown on the diagram, this allowed the edge of the caisson to be cantilevered out above the safety area designated by British Rail around the tunnel, without imposing any additional pressure or stress on the tunnel, either from the piling or the caisson. After lengthy negotiations that concluded with agreement to install a comprehensive system of instrumentation in the tunnel to continually monitor for possible movements, SRC were allowed to proceed with their proposals.

Des Con Found fig 5 copy

Elevation showing the effect of SRC’s modification

An interesting fact emerged from this story. When the monitoring instruments, mentioned above, were installed in the tunnel and the whole installation was switched on for testing prior to the commencement of work, it became clear that the shape of the tunnel is not constant. Its internal height varies by an amount that can approach a centimetre, according to the state of the tide. Measured from the base of the tunnel, the soffit (top) moves up and down with every tide. Presumably, it has been doing this, twice a day for nearly 150 years! The tidal range that occurs at this very point, up to 14m, is the second highest in the world.

British Rail’s acceptance of the modifications to the these pier foundations, proposed by SRC and described above, opened the way for the viaduct span over the Railway Tunnel to be reduced to 98m, which SRC then chose to adopt as the standard to be used for the whole viaduct.  The agreement reached on this particular item, in turn,  opened the way for SCR to gain acceptance for a further modification that was very important to them.  The terms of both the Concession Agreement, and the formal contract that followed , required SRC to obtain approval from the Royal Fine Arts Commission (RFAC) for all modifications they intended to make to the Illustrative Design for the River Crossing.  Most significantly, SRC wished to replace the curved soffit of all the viaduct spans by a tri-planar shape which would be much more efficient from a structural point of view, less expensive and quicker to build.  Internal structural maintenance, especially the replacement of internal pre-stressed cables would also be safer and easier to manage.

It became clear from the start of discussions on this subject that the RFAC had strong reservations to both tri-planar soffits and to the use of a longer viaduct span over the Railway Tunnel.  It was only when SRC managed to find a practical solution, acceptable to British Railways, for reducing the length of the span over the Railway, that their attitude began to soften.  Officials in the Government Agent’s office gained the impression that the use of tri-planar soffits for the viaduct spans would not have been acceptable to the Royal Fine Arts Commission, without prior agreement to the use of a standard span throughout the whole length of viaduct. However, having gained permission to proceed with their proposal for revising the pier foundations adjacent to the railway tunnel, SRTC were able to opt for a standard viaduct span throughout and this led to permission from the RFAC to build the viaduct units with tri-planar soffits.

The Piers.

Des Con Found fig 3 copy

Elevation of a pier over its cassion

The illustrative design for the viaduct piers prepared by the government’s consultants, was based on the use of in-situ concrete piers, each standing on a caisson. However, in order to maximise the amount of work done off-site, the consortium elected to use hollow precast concrete units for the 37 piers that were not accessible from shore-based plant. The revised piers would vary in height from 7m to 40m, with a footprint of 6m x 3.5 m and with walls 500 mm thick. When completed, they would need to be post-tensioned, vertically, to the caissons below. Each pier would stand on a 1 m deep starter unit, cast on top of the caisson while, at the top of the pier, a standard 5.5 m high pier head unit would support the plinth that would carry the viaduct bearings. The balance would be made up of standard 6.5 m units and a unit of variable height. The maximum weight for any precast unit was set at 180 t to suit the lifting plant.

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Another vew of the completed structure looking west. Copyright; Neil Thomas of Photographic Engineering Services.

Before leaving “Design of Viaduct Piers”, it is worth noting that adjacent piers are linked together to enable a number of them to work together in the event of ship impact. The next section of more detailed information, on the design of the viaduct deck (go to botton of current section and click on link to return to Main Text. Then click on the next available link in that text, “For more on Design of the Second Crossing Viaduct Deck”). This second tier element describes how the deck has been divided into eight separate sections, each four spans in length on either side of Shoots Bridge.  Each four span section has been prestressed longitudinally to form a homogeneous beam. The sections are separated from their immediate neighbours by longitudinal movement joints located mid- way between two piers.  These joints will reduce the effect of an impact on any one pier because it would be resisted by several piers acting in unison.

SRC decided to strengthen the piers by inserting pre-stress tendons into the side walls of the pier units, where they would be most effective in resisting lateral forces. The difficulty of anchoring the tendons below the pier, while ensuring that they would still be available for stressing and eventual replacement, was overcome by setting a U-shaped steel duct into the concrete at the top of the caissons. More information about this feature is given under “Building the Viaduct Piers”.

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Concession and Concessionaires’ Strategy.

The concession for the design, construction, financing and operation of the Second Crossing was awarded to the consortium, Severn River Crossing, plc (SRC) at the end of 1989 and the Agreement was signed in October 1990. Construction work could not start until the necessary legal powers had been obtained from Parliament. This was a complex and lengthy process requiring a major input from SRC.

The Severn Bridges Bill was put before Parliament in the 1990/91 session and details were made public. Consultations took place, especially with those directly affected. The Bill was subject to two readings in the House of Commons and a Select Committee was deputed to consider the various petitions made against it. A number of concessions and amendments were introduced and the Bill received Royal Assent on 13 February 1992. The four year construction period and the much longer concession period both started on 26th April 1992.

The consortium, Severn River Crossing (SRC), had been established by two leading European contracting companies, Laing from Britain and GTM from France. They relied on SEEE, the highly respected design arm of GTM, for the pre-tender designs.  Halcrow, engineering consultants with long experience of British design and checking procedures, then joined the consortium to work with SEEE. All calculations for the final designs were checked independently by other approved consultants, in accordance with normal British practice.

The terms of the concession made SRC responsible for both construction, and the day-to-day supervision of construction. The two functions had to be separated within SRC to ensure that the interface, involving compliance with technical specifications and conditions of contract, was properly managed. Consultants Maunsell were appointed as Government Agent to oversee this interface and to ensure that SRC complied with the terms of the concession agreement which included a general technical specification and a number of special requirements for this particular project.

Heads of Agreement had been signed in April 1990, prior to the completion of negotiations over the concession agreement. This enabled SRC to commit significant resources to their preliminary investigations, in the knowledge that all essential expenditure would have been reimbursed by the government in the event that the Government failed to procure the necessary legal powers.  It provided SRC with two years to refine their designs and complete their preparations for construction before work started.  They made good use of that time, scouring the globe to source the plant and equipment they would need, especially marine plant.  They were also able to order, and have delivered, a bespoke gantry for transporting the viaduct deck units over the last leg of their journey from the casting yard to their position in the deck.  It was a luxury not afforded to many contractors and was undoubtedly a factor in the high quality of the work throughout and the quality of the finished product.

SRC’s Preparations and Strategic Approach.

While awaiting completion of the parliamentary process, SRC established a number of key principles that would be applied throughout the job. They included:-:-
1. Maximum use would be made of pre-casting and prefabricating on dry land, to minimise work done off-shore.
2. Where possible, prefabricated units would be standardised.
3. Where possible, the use of in-situ concrete would be avoided.
4. High strength friction grip bolts would replace site welding where possible.
5. Maximum size and weight limits would be established for all items that would be lifted on site or transported by river barge, to reflect the capacities of the lifting equipment and the ships acquired.

Details of the Illustrative Design
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Elevation of Shoots Bridge

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Elevation of Pylon

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Cross Section of the Deck

The Government’s tender documents included a detailed “illustrative design” for the whole structure that would provide the river crossing. It had been prepared by the government’s consultants, to assist tenderers in their initial assimilation of the problems to be faced. Use of this design was advisory, not mandatory, its sole purpose being to give tenderers a helpful start in working up their own proposals. In the event, all the tenderers found it helpful and made considerable use of it, while introducing different amendments of their own.  Some of the changes introduced by the consortium were revealed in their submitted tender, others were made after the tender had been accepted, so the job, as built, was not necesarily identical to the tender drawings.  Details of the illustrative design provided by the government are displayed below.

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Part elevation of the viaduct – over two km long on both sides of cable-stayed bridge

SRC’s designs were similar to that of the illustrative design and attention will be drawn to major departures. One important change was to adopt 98m as a standard for all viaduct spans, rather than using two modules, as suggested on the illustrative design. This reduced the numbers of caissons and piers, and the number of deck gantry movements. It also standardised, still further, the manufacture of the deck units and it enabled the contractor to complete the whole viaduct without having to undertake the modifications to the gantry that would have been necessary, if certain spans had been longer than others. The change, which was welcomed by the Royal Fine Arts Commission, was made feasible through the introduction of an innovative redesign for the two viaduct foundations immediately adjacent to the line of the Severn Railway Tunnel. That, and other less dramatic changes to the illustrative design, will be explained in the following sections.

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Init stud fig 6 copy2

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Subsequent Problems in main cables of Severn Bridge

Inspections of the main cables on the Forth Bridge, followed by inspections on the Severn and the Humber, were undertaken in accordance with guidelines developed by the American authorities. This process involved removing the external wire wrapping between cable clamp positions and driving wedges into the body of the cable to enable inspection of as many wires as practically possible in eight radial positions around the circumference

The first inspection of the Severn cables took place in the summer of 2006, some 40 years after opening and approximately one-third through the original 120 year design life of the bridge. Eight sections between cable clamps were opened up. A number of broken wires were discovered and the condition of the remainder were carefully logged on a scale of 1 to 4 (1 being ‘very little corrosion’ and 4 being ‘widespread corrosion with rust over most of the surface of the wires’). Locations for inspection had been chosen at high, medium and low levels of both the main span and the side spans. It was no surprise that the sections at the lowest level were the worse for wear, because any moisture entering the cable would tend to run down and collect at the bottom.

A statistical analysis of the potential loss of strength was undertaken. This indicated a significant reduction in the factor of safety against failure compared to the original design intent. The only part of the loading that was controllable to any extent was the weight imposed by the traffic, so a regime of traffic monitoring and control was imposed, having particular regard to heavy goods vehicles. The main control measure was to prohibit HGVs from using Lanes 2 of the carriageway; this prohibition is still in force. The monitoring by Weigh-in-Motion (WiM) sensors, assisted by video camera monitoring of flows, is used to confirm that the traffic volumes and mixes remain within the assessment design loadings.

Most of the broken wires that had been found were repaired by cutting out those corroded lengths and inserting new wire that was re-tensioned by the use of specially developed swaged connections, incorporating turn-buckles. The opened sections were re-wrapped and re-painted. In addition a system of acoustic monitoring was installed to record on-going wire breaks. Measures were also put in hand to completely seal the cables in a membrane and, in time, to dry and de-humidify the interior of the cables by forcing dry air through them.

By 2011, when a second intrusive inspection was carried out, it was apparent that the measures imposed were taking effect. The humidity in the voids between individual wires was below 40% relative humidity, at which no corrosion of steel takes place, and the rate of wire breaks had been reduced to a very low level.

The monitoring and control of traffic and the de-humidification and acoustic monitoring is on-going and these measures are likely to be needed for the remaining life of the bridge. At the time of writing, the situation appears to have stabilised and, although the condition of the cables is considered unlikely to deteriorate significantly further, another examination is scheduled for 2016. Should this inspection confirm that a stable state has been reached there is scope for increasing the time between periodic inspections, inspecting only the most vulnerable (lower) panels, and allowing the relative humidity of the injected air to rise somewhat thereby saving some cost of the energy required to dry and warm the air.

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Modification and strengthening of Severn Bridge

A. Tower Legs

The towers supporting the main cables were originally constructed as hollow steel boxes, cantilevered from the foundation. They carry both the vertical load from the weight of the structure and the bending moments that arise from different distributions of live loading. As a single heavy vehicle moves on to the bridge, the main cable will sag a small fraction towards the nearer tower (like a garment on the clothes-line), accompanied by similar reactions from all the other vehicles on the bridge at that time. The top of the tower will be pulled backwards and forwards, depending upon the aggregate of all the individual reactions, and it must be capable of resisting the impact of the worst case scenario, including any co-existing transverse wind loading.

Single plates of mild steel had been used in the construction of the tower walls, each being stiffened, off-site, by the addition of vertical stiffeners on what would become the interior face of the tower wall.

828-C-021 Severn tower strengthening

Column and guide inside tower leg.

 

To relieve the box walls of some of their vertical stress, four tubular steel columns were installed in each tower leg and then jacked up, simultaneously. These columns are about 120 metres high and 400 mm diameter with a wall thickness of 30 mm. Each column has 19 sections, which were threaded, one by one, through a small access door at walkway level.

828-C-022 Severn tower strengthening

Inserting column section into tower at deck level.

The door could not be enlarged because of the high stresses in the tower walls. A grillage of steel beams and columns at the inside at the top of the tower legs transfers the jacking load from the columns to the underside of the main cable as it passes over the tower.

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Top of columns under grillage, inside cable saddle.

 

The columns were jacked upwards from the bottom with a total force of 2000 tonnes per tower leg. Special sliding bearings locate the columns at nineteen positions within the tower to prevent buckling. The jacking lifted the bottoms of the columns by 100mm. The columns shortened in compression by about 75mm and the saddles that carry the main cables over the tops of the towers were raised by some 25mm. The Severn Bridge is an inch taller than it was before the strengthening!

B. Middle Portal Connections
The connections between the horizontal portal beams and the vertical tower legs were strengthened, both internally and externally to cope with the transverse wind loading.

C. Tower Saddle
The main cables pass over the top of each tower leg in a saddle. During construction, the individual strands of the main cable were laid in this saddle as the spinning continued. As the main cables carry the whole weight of the bridge deck and the traffic, the stresses in the saddle are very considerable, especially the splitting effect that tends to separate the two sides. The saddles were upgraded with C-clamps – similar in principle to a G-clamp used in carpentry – to help resist the splitting force.

New access platforms were also installed at the tops of the towers to assist with inspection and maintenance.

D&E. Cables
New hand-strand ropes and posts were provided above the main cables. As well as providing access, these support new maintenance cradles for inspecting and painting the main cable. The main cable itself did not need strengthening but the bolts in the cable clamps, at the tops of the hangers, were renewed and re-tensioned.

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Deck/hanger configuration after strengthening.

F. Hangers
All 376 hangers were replaced, one by one, at night while traffic was restricted to one lane in each direction. The new hangers are slightly thicker that the originals and the design of the end sockets is improved to reduce the risks of kinking and corrosion

The original hangers were terminated in metal sockets filled with white metal. However, there was no hole in the socket for drainage which led to some speculative concern about the condition of some of the wires making up the hanger. In addition, just before the Royal Opening a “man with a file” was sent around to tidy up the white metal protruding from the tops of the sockets. Unfortunately he also broke through the anti-corrosion coating in several places and a number of the outside peripheral wires failed. This posed the question about the condition of the inner wires. Ultimately all the hangers were replaced, although mainly for reasons concerned with the bridge loading.

G, J & K. Steel Box Deck
The top of the steel box girder deck, over which the traffic drives, is 12mm plate stiffened by steel troughs welded to the underside. Repeated loads from the wheels of heavy goods vehicles had tended to cause local weld damage and it was decided to grind out and replace the welds immediately under the lane wheel tracks. The original fillet welds were replaced with a partial penetration weld requiring three passes. Changing just this one weld detail required 48 miles of welding (3 passes/weld, 2 welds/wheel track, 2 wheel tracks/carriageway, 2 carriageways, length of crossing 2 miles).

New under-deck travelling maintenance gantries were provided for inspection and painting. Also, new manholes were installed in the deck boxes to assist access for the strengthening work and for future inspection and maintenance. When the bridge was originally built, it was intended that the box girder deck would be sealed and kept dry with silica gel. For this reason the original access manholes were very small. Hence it is rumoured that a one-time Chief Highway Engineer of generous proportions was unable to visit the inside of the bridge deck.
Prior to strengthening, there was only a thin layer of asphalt surfacing on the steel deck units. A thicker layer was desirable but had to be rejected because of the effect of the extra weight on the cables.

H. Deck Bearings
Lateral bearings resist sideways forces. Rocker bearings resist vertical forces at the towers to keep the deck boxes at the correct level.

It was discovered at an early stage that the two rocker bearings at each of the towers had been designed to carry half the maximum reaction from a fully loaded deck. However, when the deck was eccentrically loaded, one of the rockers went into tension and the other carried a greater load than it had been designed for. All were replaced.
New buffers were provided to transfer longitudinal force if deck movement becomes excessive.

I. New Air and Water Mains
To help with future maintenance, a compressed air main and a water main were installed over the full length of the crossing. The water main was to be used to fight any substantial fire on the bridge, although subsequently it has been decommissioned.

Other Work on Severn Bridge
Movements of the deck due to thermal expansion and loading are catered for by “Demag” expansion joints in the carriageway at each tower. Increases in the required longitudinal expansion and contraction temperature allowances had to be catered for. Also it had not been recognised that the joint would need to cope with extreme racking movements due to the side sway of the main span from increased high wind design speeds. For these reasons, the joints had to be modified significantly.

It was also discovered that the large chambers in the concrete anchorages, where the individual strands of the main cable were splayed out and anchored separately, had a damp atmosphere and that the wires were beginning to show early signs of corrosion. The decision was that the chambers had to be de-humidified.

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The Steel Box Girder Problem

The background.
The failure of four steel box girder bridges during construction caused quite a furore. The use of such girders in bridge construction had been a fairly recent phenomenon, partly because the mathematical analysis was quite daunting before the advent of electronic computers. Their popularity came at a time when the post-war surge in bridge building was getting underway. They required less steel for the construction of a bridge deck than the truss or the plate girder that had been the popular choice previously.

A cantilever built using box girders poses problems for the designer. The stresses will be distributed between the various constituent plates of the boxes depending upon the accuracy of the individual elements and the precision of the fixings involved. Distortions of an individual plate could cause havoc especially during the construction of a cantilever and they are difficult to spot and to eliminate. Try to imagine how a box girder would crumple if tested to destruction by the continual adding of new units, i.e. more weight, to the unsupported end of the cantilever.

The government’s response.
In 1970, the government responded to the box girder crisis by setting up an advisory committee under the chairmanship of mathematician, Dr. (later Sir) Alec Merrison, then Vice Chancellor of Bristol University, to investigate the circumstances of the previous failures and to make recommendations for the safe re-construction of the Milford Haven bridge and the safe completion of the Avonmouth Bridge (under construction at the time). The committee also advised changes to the design rules and on the procedures to be adopted for future applications. As part of this process, a major programme of research was initiated to study the behaviour of the various components of a box girder and to consider how the mathematical analyses, then in circulation, might be improved.

The committee delivered its final report in 1974 after the results of the research studies became available. It concluded that the primary causes for two out of the four failures were inadequate organisational procedures and poor communications. It provided an improved set of rules for the design of box girders, incorporating the findings of the latest research. It also stressed the need for the introduction of an independent check on methods of erection and on the design of temporary works, together with the need for clearer distinctions between the respective responsibilities of the Engineer and the Contractor,

It is true to say that only one of the four box girder failures was strictly the result of lack of technical knowledge of steel box girder behaviour, although that was not apparent when the failures originally occurred. In three cases, the primary cause of failure was human error during construction and can arguably be attributed to the rapid growth of the design and construction programme out-stripping the supply of suitably qualified and experienced engineers to adequately supervise these works.

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Highway Bridge Loadings

Introduction.

At the time of the design of the first Severn Crossing, the relevant live loading for such a bridge would be based on the assumption that the proportion of heavy vehicles in the traffic flow would be about 15%. This figure was based on observations undertaken in the late 1950s. It was also assumed that the heaviest loading would occur when traffic became halted on the bridge, forming a stationary queue with minimal distances between adjacent vehicles. In such circumstances, the impact on the bridge from heavy vehicles would be softened by the presence of lighter vehicles interspersed between them.sbtraffic2

 

By the early 1980s, traffic counts were showing a much higher percentage of HGVs on the majority of the country’s strategic road network. It was then averaging between 30% and 33%. This was due to the increased value that operators were then attributing to two of the advantages that the motorways and improved trunk roads provided. The first was the actual savings in journey times. The second, possibly more welcome, was the increased reliability of those journey times, which is so important when operators are trying to work to tight schedules.

The widespread availability of computers by the 1970s led to the development of mathematical traffic models which provided data to inform decisions on many problems in the landuse/transportation field. Typically, these models would consist of a representation of the road network covering the area under investigation, a breakdown of that area into a number of discreet zones, a data base containing information on the traffic flows from each zone to every other zone (compiled from roadside interviews) and various tools for assigning traffic from the data base to the road network, etc. These models could be validated by comparing their forecasts for “the present day” against an independent set of data.sbtraffic

A model of the type described above was used to forecast the worst case loading on the Severn Bridge, prior to the strengthening work. An assessment was required of the aggregate weight of a stationary queue of traffic on the bridge, with minimal distances between adjacent vehicles. The traffic model provided forecasts of the proportions of the various types of vehicle that would use the bridge during a particular period. A separate application then selected vehicles from this mix, randomly, to generate many different queues, each conforming to the vehicle type proportions and spacing requirements, and then it calculated the load represented by each queue in terms of kN per lane meter. The figure taken forward was the load exceeded by 5% of the queues. For the Severn Bridge in the mid 1980s, this figure proved to be 9 kN per lane metre, which happened to be twice that for which the bridge had originally been designed.

The weights of the actual vehicles crossing the Severn Bridge were also measured by placing a weighbridge in the carriageway. This showed that the flows were not entirely random. Many operators of heavy lorries favoured an early morning start, when there would be relatively few cars and other light vehicles on the road to dilute concentrations of lorries. Unless that kind of situation were managed, it could result in queues being formed on the bridge with more than 30% of heavy vehicles, generating a 5-percentile loading of up to 12 kN per lane metre. Fortunately, the timely arrival of the Second Severn Crossing, in the mid-1990s, took much of the sting out of such speculations.

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Developing the cable stayed design for Wye Bridge

Modern cable-stayed bridges were only starting to be considered in the UK at the time that the first Severn road crossing was being built. And it was not until later, when electronic computers became widely available for engineering calculations, that the load bearing capacity of the more complicated cable stayed bridges could be analysed with any real confidence.  In the earlier circumstances, engineers were reluctant to break new ground. In fact, 25 years later, when construction of the Second Crossing got underway, no cable stayed bridge in the world had been completed with a longer span than the one that was then under construction on the Severn. However, before the Second Severn Crossing was completed, the French had taken a great leap forward with the opening of the Pont de Normandie (main span 856 m). And since then, with developments in design and stronger materials, further great strides have been made with main spans now 1,000m and longer. Having said all that, the Wye Bridge is a simple, but very important, early example of a cable stayed bridge.

The deck units chosen for the viaduct enabled the contractor, Cleveland Bridge of Darlington to complete each full span of 64 m using a simple cantilever method of construction.

WyeBoxErection

Wye Bridge typical box erection

Calculations confirmed that the same method could be used to erect the side spans of the bridge. However, some additional strengthening of the deck would be required if the same cross section of deck unit were to be used for the main span of 235 m. The obvious way to proceed with this span would be to work from both sides, building two half span cantilevers that would meet each other in the middle. Some method would then be needed to lift both the leading edges to the correct height (even when stiffened, both sections of unsupported deck would have drooped, to a significant extent, under their own weights).

WyeCantErection

Wye Bridge cantilever erection

The addition of two simple elements of “cable-staying” would kill both birds with one stone, stiffening the whole of the bridge deck sufficiently, and providing means of lifting the leading edges of the cantilevers when required.  It was achieved through the addition of two towers and two long lengths of high tensile steel cable. This well targeted response provided a very efficient solution, enabling the Wye Bridge to be completed successfully and in accordance with the loading criteria of that period.

It is interesting to note that when the nearby M4 Avonmouth Bridge came to be strengthened in the 1990s (for the same reasons as the Severn and Wye Bridges), the method chosen to upgrade its load carrying capacity was, in principle, exactly the same as that used to build the Wye Bridge.  However, in that case, the original box girders were deeper than over the Wye and so it was possible to achieve the additional strengthening using high tensile steel cables that were post-tensioned and fixed entirely within the original box girder.

The cable stays on the Wye Bridge do not carry the whole weight of the bridge deck and the traffic, but they effectively take part of the load by adding a new set of supports. The tension in each cable has an upward component of force, which gives support to the bridge deck, and a component of compressive horizontal force, that is absorbed by the bridge deck.

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The Decision to use Inclined Hangers

 

Choice of Deck Cross Section.

The previous item of ‘more detailed information’, entitled “Background on choice of design for First Road Crossing”, describes the circumstances which led to the catastrophic collapse of the Tacoma Narrows Bridge in the USA in 1940. Initial work on the Severn Bridge started shortly after that and there can be no doubt that its designers were well aware of the potential consequences of neglecting the lessons learned.

In the late 1950s, using an anemometer mounted on a mast 110 feet (33 m) high (the height of the proposed Severn Bridge deck) and an array of instruments mounted on the then existing Severn Railway Bridge, some 3 miles (5 km) upstream, a spectrum of data on local wind speed, direction and inclination, was established over a width of about 300 feet (90 m). This information would be put to good use when the 1/100-scale model options for the Severn Bridge deck were tested in a wind tunnel at Bedford. At that stage, it was assumed that the deck design would follow American practice with a stiffening girder, about 33 feet (10 m) deep and of open-lattice truss construction, as had been proposed in the 1930s.

However, in the late 1950s, the Government stipulated that the new Forth Road Bridge would take priority over the Severn. The Forth Bridge would, not surprisingly, be designed on the same principles as those being developed for the Severn Bridge, although the main span across the Forth would be slightly longer, at 3300 feet (1008 m). Mott Hay and Anderson were appointed in association with Freeman Fox and Partners to design the Forth crossing using a conventional design for the stiffening girder, consisting of a horizontal concrete slab, above a latticed steel-work construction (a truss) that supports it.

There was a new development on the Forth Bridge in that the road deck was made of stiffened steel panels resting on cross girders at the top chords of the trusses. These panels were light in themselves and the overall dead weight was also kept to a minimum by specifying a thin layer (c 1 ½ inch or 40 mm) of mastic asphalt as the running surface. Nevertheless, the total dead weight of the Forth deck was some 18,000 tons which required the towers, main cables and anchorages to be designed to cope with this load plus the weight of the traffic on the deck. Construction on the Forth proceeded about two years ahead of the Severn.

This delay allowed different deck cross-section to be considered for the Severn Bridge, 4 of which are illustrated below.

At an early stage in the design process for the deck, it was necessary to test the 1/100 scale models of the various options being considered for this crucial item, using a wind tunnel to check the strength of the deck, when exposed to the strongest side winds that might occur.  It was also necessary to ensure that the deck would not start oscillating during a less powerful but steadier wind flow. However, very early in the first part of this procedure, the model fixings in the wind tunnel failed and the model was completely destroyed. During the delay that followed, the Consultants, Freeman Fox and Partners, were moved by inspiration to develop an unprecedented design for the deck section, quite different from anything that had been tried before.  It was based on the technology developed to design aircraft wings.

A problem.  The box girder would clearly have no difficulty in coping with anticipated lateral wind forces but during the later stages of the tests, a slight movement had been seen from the model while it was being subjected to a less powerful but steady wind. This led to concerns, and then to doubts about the box girder’s ability to resist oscillations. The presence of this small amount of movement was worrying, given the absence of any obvious source of damping.  The wind-flow over the welded box girder was so smooth and undisturbed that there was very little chance that the deck cross-section itself would be able to provide a worth-while contribution to any damping.

So the search was on for a fresh energy-absorbing element that could be relied upon to eliminate any embryonic oscillation that might occur. The Consultants were seeking an independent source of energy which, when transferred to the deck, would be capable of denying embryonic oscillations the opportunity of becoming uncontrolled and divergent.

The introduction of Inclined Hangers

The choice of the welded box girder for the bridge deck, was widely welcomed, although it would not have been accepted, if there had been any real doubt about finding a reliable source of energy which could be transferred into the deck to ensure that there would be sufficient dampening of all the oscillations that might seek to establish a foothold.  In a second major innovative move, the Consultants decided that all the hangers used to suspend the deck from the main cables, would be hung in an inclined pattern, as shown on the following diagram. The purpose of this move was to increase the structure’s ability to prevent the deck from oscillating.

The pattern for inclined hangers.

The introduction of inclined hangers gave the Consultants a game-changing opportunity to prevent any embryonic oscillation that might form in the deck from developing into an uncontrollable and divergent oscillation. Engineers have long known that certain elements of a suspension bridge act in a very similar manner to the equivalent items on a washing line. They have been aware that, if an additional garment is hung on a washing line, away from the centre, the point on which it hangs, will sag and, more importantly, it will move a short distance towards the nearest point on the washing line that is held fast by a fixed support.

Similarly, when a vehicle travels over a suspension bridge, the deck will not only dip down a little at the moving point of the load, the whole deck will move a short distance longitudinally towards the nearest tower, whether the vehicle is travelling towards, or away from, that tower. The bridge deck is often called the stiffening girder because it spreads out the tensions among the hangers, in order to avoid excessive local sag under an exceptional load.

The purpose of employing inclined hangers is to take advantage of the opportunity to generate more frequent switching of the tensions between each pair of hangers that is connected directly to any of the cable clamps. As a result, the tensions in each of the hangers, and the bridge deck, will be in an almost-continual state of change. These changes will be related to the lengths of the hangers involved, and the angle between each pair, in each case.  This means that the longer hangers on each side of the towers are effectively only subject to a change in vertical load from passing vehicles. Those near the centre of the main span and those towards the ends of the side spans suffer, not only the changing vehicle loads, but the much more significant reversals in tension due to the relative longitudinal movement of the deck in relation to the cable clamps to which they are attached.

Preparations. The deck of the bridge is freely suspended from the two catenary cables, using 172 hangers on each cable. And because the cables hang in catenary in the vertical plane, the two lines of lower anchorages, one on each side of the deck, that are attached to the surface of the deck to receive the hangers, are also in the same vertical plane. The next step was to wrap clamps around the cables at locations set at positions along the cables equivalent to 60ft apart horizontally.  Each clamp was provided with a double eye to secure the top ends of the two hangers in each pair together, before they are allowed to diverge from each other as they go down to meet the deck. The lower ends of the hangers have to be moved 30 ft longitudinally, but in opposite directions, in order to be fastened into the two double deck anchorage eyes that had been fixed to the surface of the deck to receive them, and to provide the hangers with their relevant inclinations.  The other halves of both these deck anchorages will be occupied by hangers from the cable clamps that are their nearest neighbours on either side

Both sides of the deck were treated as described in the above paragraph and, as a result, there is a single line of double sockets at 60ft centres on each side of the bridge deck. These lines are parallel to each other and identical in other respects. In every case, the cable clamp is located, horizontally, at the mid-point between each pair of sockets from one of the lines on the bridge deck.

Lateral restraints and vertical rockers have been provided at each tower and anchorage, to restrain the horizontal and vertical movement of the ends of the deck but permit longitudinal movements of the deck, unhindered. So the deck is restrained laterally and vertically, at both sides of the towers, but permitted the move with traffic loading and thermal expansion. The ends of the side spans are restrained vertically, laterally and longitudinally at the anchorages.

What actually happens?  To cover the various activities that ensue, it is worth starting from a period of shutdown, when the deck will be lying in its position of equilibrium and before the traffic arrives, any two hangers involved will each carry half of the total tension that goes up through the cable clamp to the main catenary cable above. The total tensions carried by both of the cable clamps will remain similar at all times because the dead loads they carry are constant and virtually identical.

The process of additional damping will start with the build-up of vehicles running across the bridge in each direction. Each time that happens, the bridge deck will dip slightly and make a small longitudinal movement towards the nearest tower, similar to the garment on the washing line. The displacement will be proportional to the weight of the vehicle. The first tranche of vehicles will add to the tensions in the hangers, and provide many small longitudinal movements of the deck, which will soon coalesce into a distinct movement in one direction. The actual distances moved by the deck would be more correctly described as being the algebraic sum of the movements in a particular direction. Quite quickly, the deck will have moved a significant amount longitudinally, one way or the other, and this will enable the system to click, autonomously, into a higher gear. (Remember that the direction of these additional movements is governed by its location in relation to the positions of the towers, not by the direction in which the vehicle was travelling).

Now consider what will happen when the deck has made a fairly short movement in a particular direction. The system will react to a longitudinal movement of the deck, destroying the parity between the tensions in the two adjacent hangers. The system will move from point A to point B, in the time that the deck took to move to its new location, just like when an extra garment is added to a washing line, the difference between them may not appear to be very significant but it must be remembered that the crucial triangle at the centre of the issue is wholly comprised of steel which can be very demanding in such circumstances, a fact that is borne out by the contents of the next paragraph.  And it must be remembered that the effect of the movement will probably impinge upon every pair of hangers that serves the deck – the whole caboodle. Think about it!

There has however been an important down-side to the use of inclined hangers.  Depending upon their length and position on the deck, some of the shorter hangers are subject to increased fatigue damage, due to the frequency of the switching of the tensile stresses within each pair of hangers that shares a common cable clamp on one of the main cables.   This is a sure sign that there has been ample switching of tension within the pairs involved. The shorter hangers might need to be replaced after a fairly short time in service. No traffic restrictions are needed, while the hangers are being replaced but the additional maintenance charges are significant.

The previous paragraphs illustrate how the idea of suspending the deck of the Severn Bridge using hangers erected in an inclined pattern, had made it possible for large tensile stresses to be redistributed within each pair of hangers that share an upper cable clamp.  It was exactly the kind of new energy source that the consultants had been  seeking to combat the possibility of an oscillation developing within the completed structure, similar to the one that destroyed the Tacoma Narrows bridge in 1940.  It is particularly intriguing that the actual source of that additional energy should be the ‘live load’ that crosses the bridge.  However, the consultants concluded that the most sensible approach would be to avoid situations that could lead to a hanger becoming slack, or otherwise overstretched under extreme live loading conditions, and they were supported in that decision by the government Department for Transport. 

Manufacture of the hangers.  The above demonstration of the ability of the inclined hangers to provide a new source of energy that must be generated from the live load of traffic, raises questions about the construction of individual  hangers.  The dead load of the bridge deck is extremely uniform throughout its length and so all cable clamps (at 60 ft centres) would carry basically the same share of that weight.  At the same time, the average daily live load borne by each cable clamp  would be virtually identical.   It was also clear, at that stage, that more than 50% of all the hangers would be long enough to be treated in exactly the same manner as if they had been hung vertically.  This all suggested that the current practice of the day, based on a multi-strand configuration, would probably be adequate.  At the same time, there was strong support for developing a strand that would further strengthen the structure against the possibility of oscillations by maximising the amount of hysteresis in the hangers (this work was undertaken but very little has been said about it since).  These deliberations resulted in a strand that contained a total of 178 separate wires, made up from the following three sizes, 0.118 ins, 0.133 ins and 0.339 ins with a lay of 7.5 d.  The resulting strand would provide a maximum working load of 100 tons and a breaking load of 225 tons.  At this stage the way forward was clear for the suspending of all the longer hangers from cable clamps and making the necessary connections to the sockets that would be fixed to the deck to receive them.  But before that work started the possible need for a modification to the system outlined above, in order to cope properly with all the shorter hangers, was undertaken. 

The initial system for supporting the bridge deck that was used to suspend all the longer hangers is described in the above with a section of main cable on one side of the deck, with the two hangers in one pair, coming together directly underneath the main cable to which they are both fixed using a special connection attached to the base of the cable clamp.  Lower down, on the top of the deck, a line of sockets had been fixed to the deck  at 60 ft centres, to secure the lower ends of the hangers.  Each cable clamp was positioned directly over the mid-point between two sockets.  An initial examination confirmed that that this arrangement, in the form described above, would not be able to cope with the shortest hangers.  This meant that a modification was required together with a decision concerning the point at which the modification would need to be introduced.  

The Included angle.  It soon became clear that the critical item when dealing with shorter hangers is the angleincluded’ between the two hangers at the point where they come together under the cable clamp to which they are both fixed..  This angle will be quite small In the case of the longer hangers but consider what would happen if it reached the 60 degrees shown on the diagram above (note, an angle of 60 degrees has been chosen just to simplify the mathematics).  The triangle that includes both of the hangers and the line of the deck beneath them, approximates to an isosceles triangle.  This means that these hangers are inclined at an angle of 30 degrees to the vertical.  Because of that, only 86% of the tensions in the hangers (i.e., the sum of the vertical components of these tensions) would be used to support the weight of the bridge deck, together with the live load of traffic on it.  The other 14%, the horizontal components of the tensions in the two hangers, would cancel each other out, being dissipated into the bridge deck.  And at the mid point of the span where the main cable drops lower, approaching the centre of the bridge, the included angle between the two hangers would need to be larger still in order to conform with the model.  

The chosen modification involved a simple change to the rules that are implied in the model that is described above.  Having reached the point chosen to make the change, the consultants extracted the included angle at that point and decreed that it should apply to all subsequent operations that would otherwise have adopted a wider angle between hangers.

Changes in hanger tensions due to longitudinal movements of the deck

This diagram shows an elevation of a short length of the support mechanism developed for connecting the bridge deck to the main catenary cable when viewed from one side.  The cable clamps, that transfer the loads from the hangers to the main cable, are positioned along the cable at 60 ft intervals, horizontally, and they each have to secure two hangers, one from either side.   The eyes that are used to fix the hangers to the deck are located mid-way, horizontally, between the cable clamps to either side.  They are fitted along a line just off the flat area on top of the deck, where an additional section of box girder has been welded on to the main box girder, to provide the short cantilever that  carries the footpath and cycle track at the side of the deck.  (see Deck Section Diagram in Main Elements).

  Once decisions were taken on the dimensions of the spans and the towers, together with the shape of the main cable and the horizontal space between the cable clamps, there will be no scope for adjusting hanger lengths.  They will simply be a matter for calculation   

The purpose behind the introduction of inclined hangers is to produce a situation in which the energy stored in the hangers will increase and decrease much more rapidly and by larger amounts than would be the case with vertical hangers. This should provide the structure with much greater potential for dampening embryonic oscillations of the type that destroyed the bridge at Tacoma Narrows. The Severn Bridge has already given good service for more than 50 years and, with adequate maintenance, should continue to do so for the full 120 years of its design life, and beyond.

Unfortunately, there is a downside to the introduction of inclined hangers. It is extremely important that components of the structure that are active in suspending the deck, should remain in service for long periods. And when necessary, they must be capable of being removed and replaced, without requiring traffic restrictions. The rapid fluctuations in traffic loading associated with inclined hangers, and the additional loads that are imposed, make these hangers especially vulnerable to fatigue failure. This is particularly true of the shorter hangers located in the central area of the bridge. The situation is manageable but it requires significantly more maintenance than was originally planned.

It has been shown that the level of stress for hangers in an inclined pattern could rise to at least twice the maximum level experienced in a pattern of vertical hangers, especially where the hangers are short and near the centre of the main span. And it has been established that the relative fatigue life of an individual hanger on that bridge can be expressed, mathematically, as being inversely proportional to the fourth power of the range of stress involved. When this formula is used to explore the effect of doubling the range of stress in a hanger, as in the present circumstances on Severn Bridge, it confirms that we should expect the fatigue life of some of these hangers to be reduced to one sixteenth of the life that would apply if the hangers had been in a vertical pattern (the fourth power of 2 being 2 x 2 x 2 x 2, which equals 16).

It should have been no surprise, therefore, to find fatigue failures of wires within hangers on the Severn Bridge after only six to ten years service. After a period during which individual hangers that had failed, were replaced ad hoc, a wholesale strengthening programme for the bridge put in hand. All hangers were replaced using stronger and less fatigue-prone strand construction, and new designs of socket were installed.

Mott Hay and Anderson continued in their role to design and supervise the construction of the crossing, but now jointly with Freeman Fox and Partners. Motts led overall and designed all the foundations. Freeman Fox were responsible for the steel superstructures. The superstructure of the main suspension bridge was a major technical advance on all previous long-span road bridge designs.

A total of three suspension bridges were built with inclined hangers, between 1965 and 1980, all with Freeman Fox and Partners as main consultants. It is interesting to note that the inclined hangers on the Humber Bridge have proved less prone to such problems, possible because of a less severe traffic loading pattern. On the other hand, those of the first Bosphorus Bridge have been so fatigue prone, probably because of a traffic loading pattern even more extreme than on Severn, that they were all recently replaced by new vertical ones. Suffice to say that, to date, no other major suspension bridge has been fitted with a system of inclined hangers.

At the time, the Severn Bridge was hailed not only for its majestic appearance but also for its technical excellence. The all-up weight of steel in the Severn Bridge, 3,240 feet (988 m) span, was 19,000 tons for the deck, cable suspension system and towers compared with 39,000 tons for the earlier Forth Bridge which had a main span only 60 feet (20 m) longer.

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