| Name | Second Severn Crossing, England |
|---|---|
| Who | |
| Owner | The UK Severn Bridges Act of 1992 enabled the concession for both Severn Bridges which is now managed by Severn Crossing plc. |
| Design | Halcrow SEEE Joint Venture (JV) |
| Contractor | Laing, GTM-Entrepose |
| Where | |
| Latitude | N 51 34' 11" |
| Longitude | W 2 41' 00" |
| Why | Takes M4 over the River Severn from England to Wales |
| What | Read more..... |
| How to read the bridge | Read more..... |
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| What | |
|---|---|
| Overall type | Hanging - Cable -stayed Bridge |
| Width | 34.6 m. |
| Length | Total length including approach spans - 5.128 km. |
| Spans | Main span 456 m. with 3 anchor spans either side main span all 98.118 m. making total of 1044.7 m. 24 approach spans on Welsh side and 25 on the English side. |
| Height of tower | 149 m from river bed level, 101 m. from deck level. |
| Materials | Towers: Prestressed concrete portals Deck: steel plate girders with composite reinforced concrete slab Cables: steel |
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| How to read the bridge | Read more about the book metaphor... |
|---|---|
| Chapter 1 | Suspension system |
| Paragraphs | Towers/Pylons: H frame portals with two cross beams. Supporting cables: Each group of cables either side of the pylons. |
| Sentences | Tower/Pylons: Hollow legs with lifts and steel ladders.
Legs are a constant 4 m. wide viewed from the side but taper from 10.2 m. to 5.4 m. longitudinally.
Upper regions are post-tensioned vertically.
Cross beams are precast hollow reinforced concrete box girders. Cables: Cables are 19 to 75 strands depending on location. |
| Words | Towers/Pylons: Cables anchored to pylons with precast anchorage tie beams to resist cable splitting forces.
The cables are inclined longitudinally and laterally so accurate positioning needed - the prestressing bars have
external anchorages which are protected by a wax medium. Cables spaced at ~7.3 m.
Cables: Parallel strands made up of 7 galvanised wires. Tapered wedge anchorages in wax-filled HDPE (High-Density PolyEthylene) ducts |
| Letters | Steel: Iron, carbon with other additives such as chromium etc Concrete: Cement, sand, aggregate and water Reinforced concrete: Concrete is strong in compression but weak in tension where it has to be reinforced with steel bars |
| Chapter 2 | Bridge deck |
| Paragraphs |
Main span deck wider than approach spans to allow for anchorages.
Deck girder is two steel plate girders (2.15 m. deep) which act compositely with RC slab 200 to 470 mm. thick. |
| Sentences | Transverse truss girders at 3.6 m. centres - but solid girders at at back span piers and connectiosn with approach spans.. |
| Words | Individual steel plates for girders. Trusses elements were rolled T and I beams 25 mm. diameter shear studs, high strength friction grip bolts (HSFG) were used to avoid site welding. |
| Letters | Steel: Iron, carbon with other additives such as chromium etc Concrete: Cement, sand, aggregate and water Reinforced concrete: Concrete is strong in compression but weak in tension where it has to be reinforced with steel bars |
| Chapter 3 | Foundations |
| Paragraphs | There is bedrock at the surface of much of the site. A rail tunnel is nearby and crosses the line of the bridge.
The foundations have to cope with possible ship impact on the bridge. The tidal range is large with high flow velocities.
Much of the estuary bed is exposed at low tide. Caissons: Spread foundations preferred for many of the piers using precast concrete caissons with high sliding resistance. Piles: Used over part of the site near the tunnel and areas where alluvial ddeposits overlay the rock Bearings: Hydraulic shock transmission units were located at each pylon allow restrained longitudinal movement of the bridge due to slow temperature or creep changes as well as instantaneous braking loads or seismic loads. Elastomeric springs keep the deck central between the pylons and transfer lateral wind loads. Vertical forces are taken by sliding pot bearings |
| Sentences |
Caissons: Precast concrete shell with open bottom taken out to position, placed by supporting on inflated grout filled bags
to get even ditribution of load, then quickly filled with mass concrete 'plug'. Piles: 2 m. diameter bored cast in situ concrete piles. |
| Words |
Caissons: The caisson was pumped dry and reinforced concrete based cast followed by topping piece and pier. Piles: An earth bund used to cretae a coffer dam for drilling some of the piles - others were constructed in the precast shell of a caisson |
| Letters | Steel: Iron, carbon with other additives such as chromium etc Concrete: Cement, sand, aggregate and water Reinforced concrete: Concrete is strong in compression but weak in tension where it has to be reinforced with steel bars |
| Grammar | Technically the bridge is a way of taking forces from up in the air down to the ground.
So imagine the flow of those forces through the structure.
Think of a truck standing on the brdge and how its weight is transmitted through the bridge to the ground. The bridge must be able to function with any one cable removed on full loading and any two removed with dead load and 10% of live loading. Secondary cables or 'aiguilles' of two 7 wire strands link the main cables in plane to reduce cable oscillations. Read more about hanging bridges..... |
| Erection | The work in the estury was reduced to a minimum. Reinforcement cages, cross beams, deck units were all assembled on shore. The pylons were built using self climbing formwork. The deck units were taken out at low tide and lifted and bolted or 'stitched' into place. The deck was built by the balanced cantilever method. The units were carefully matched with their 'neighbours' on shore before being erected. |
| References | Mizon D H, Smith N, Yeoward A J, "Second Severn Crossing - Cable stayed Bridge",
Proc Instn Civ Engs Civil Engng, Second Severn Crossing, 1997, 49-63 Kitchener J N, Ellison S J, "Second Severn crossing - design and construction of the foundations", Proc Instn Civ Engs Civil Engng, Second Severn Crossing, 1997, 22-34 |
| Links | Read more at the Severn River Crossing plc web site. |
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