ADS 5 Bridges Report by rca-issuu

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CARBON COPIES

BRIDGES AND INFRASTRUCTURE

HOUSES TOWERS COMPLEX SHAPES MID-RISE AIRPORTS HOSPITALS/LABS BRIDGES

Andrew Reynolds Ching Yuet Ma Chloe Shang Daniah Basil Abdulazeez Al Mounajim Dario Biscaro Grant Donaldson Hayden Mills Janice Lo Lee Hei Yin Luca Luci Miles Elliott Mir Jetha Xinyi Shen Zhiting Jin Groupwork Royal College of Art

ACKNOWLEDGEMENTS

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CONTENTS 1.0

INTRODUCTION Purpose of Research

2.0

EXISTING CONDITION Introduction History Existing uses Photographs Existing Drawings Construction Carbon calculations Cost analysis Existing Condition Summary

3.0

CARBON COPY - TIMBER Introduction Cross Laminated Timber Form Development Proposed Drawings Construction Methodology Components Visualisation Carbon calculations Cost analysis Conclusion

4.0

CARBON COPY - STONE Introduction Tensile Stone Form Development Proposed Drawings Construction Methodology Components Visualisation Carbon calculations Cost analysis Conclusion

5.0

FINDINGS Carbon Cost Comparison

6.0

CATALOGUE OF MATERIAL COMPONENTS

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1 INTRODUCTION

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1.0 INTRODUCTION

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PURPOSE OF RESEARCH

M25 Overpass

BRIDGE TYPOLOGY

Bridges are a key element in transport infrastructure, providing more efficient access across various geographies. They are a simple, yet energy intensive structure, formed with safety and structural capacity as the key design drivers. This has led to a unanimous use of concrete and steel as the primary materials due to their strength and reliability. These are, however among the biggest contributors to carbon emissions, and within the current context of the climate crisis, we must look for alternative materials and construction methodologies which could reduce our environmental impact.

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This report analyses the existing conditions of bridges and infrastructure, focussing on the case study of London Bridge; a typical example of the construction materials and techniques used today within the typology.

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ABSTRACT

Our analysis focusses on the structural methodology and the environmental impact this causes through its use of high embodied carbon materials such as concrete and steel. Within the current context of the climate crisis, the construction industry must find alternative methods and materials which reduce their environmental impact. BRIDGES AND INFRASTRUCTURE

This report explores how two different materials could be used - timber and stone, as an alternative to concrete and steel to generate a ‘Carbon Copy’ of the existing form. We examine the difference in embodied carbon that these materials would offer, and the successes and difficulties that might arise from working with these elements, with the ultimate goal being an alternative solution to current construction methods which could transform the construction industry into a carbon sequestering industry.

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1 EXISTING CONDITION

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LONDON BRIDGE

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2.0 EXISTING CONDITION

PURPOSE OF CASE STUDY We are analysing a standard example of the bridge and infrastructure typology in order to provide a base for our alternative proposal. By looking at an existing example, we can propose a ‘Carbon Copy’ which provides an alternative solution to the brief.

BRIDGES AND INFRASTRUCTURE

WHY LONDON BRIDGE London Bridge represents the typology in both its construction being precast reinforced concrete and in its purpose as it provides both pedestrian and vehicular access across the Thames. LOCATION The bridge spans 269m, connecting the Northern and Southern banks of the Thames between the City of London and Southwark. DATE OF CONSTRUCTION The modern London Bridge was constructed between 1967 and 1972 and was opened to the public in 1973. It was designed by Architect Lord Holford and Engineers Mott, Hay and Anderson, with John Mowlem and Co. as contractor. CONSTRUCTION MATERIAL/METHODOLOGY

The bridge is a concrete and steel box girder construction which spans over the river using three arches with two concrete piers standing in the water. The largest arch spans 104m, giving a clearance of 8.9m above the maintained water level.

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HISTORY

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2.0 EXISTING CONDITION

ROMAN BRIDGES The first bridge was probably a Roman military pontoon type, giving easier access across the river to the Roman army. Around AD 50, the temporary bridge over the Thames was replaced by a permanent timber piled bridge, maintained and guarded by a small garrison.

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EARLY MEDIEVAL BRIDGES After the fall of the Roman Empire Londondinium was abandoned and the bridge destroyed. The river became a boundary between the kingdoms of Mercia and Wessex. The bridge was rebuilt around 990 and proceeded to be destroyed and rebuilt in timber multiple times until 1163 when the final timber bridge on the site was built. OLD LONDON BRIDGE King Henry II commissioned a new bridge to be built from stone which began construction in 1176. This was not completed until 1209The bridge was about 280m long, and had nineteen piers linked by nineteen arches and a wooden drawbridge. There were ‘starlings’ around the piers to protect them and the bridge, including the part occupied by houses, was from 6-7.5m wide. All the houses were shops, and the bridge was one of the City of London’s four or five main shopping streets. NEW LONDON BRIDGE

The New London Bridge was designed by John Rennie as part of a competition in 1799. Work began in 1824, with the old bridge still in use, and was demolished in 1831 when the new bridge was opened. Rennie’s bridge was 283m long and 15m wide and was constructed from Haytor granite. In 1896 the bridge was the busiest point in London, and one of its most congested; it was widened by 4m, using granite corbels. Subsequent surveys showed that the bridge was sinking an inch every eight years, and by 1924 it was decided that the bridge would have to be removed and replaced.

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Artist’s impression of Roman London

The Frozen Thames (1677) by Abraham Hondius

New London Bridge in the late 19th century

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USE

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VEHICULAR

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The current London Bridge is a busy route for traffic, providing access for the A2 across the Thames. There are 2 traffic lanes and a bus lane on either side of the bridge, creating an overall road width of 9.75m on either side.

Aerial map of London Bridge Source: Bing Maps

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PEDESTRIAN Each side of the bridge has a wide pedestrian walkway - 6.8m on one side and 4.8 on the other, with granite clad steel handrails and a temporary steel barrier between road and pavement.

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Photograph of bridge carriages

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2.0 EXISTING CONDITION

PHOTOGRAPHS

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2.0 EXISTING CONDITION

CONSTRUCTION

HOW WAS IT BUILT? The bridge superstructure was designed as four pre-cast concrete box beams with steel reinforcement.

BRIDGES AND INFRASTRUCTURE

The existing London Bridge was used as the framework for the construction as well as a temporary gantry built across the river on column supports. The Existing bridge remained open to traffic, while they constructed a girder on either side, allowing traffic to then be rerouted to the new girders while the existing bridge was demolished and the final two central girders constructed. The traffic congestion and limited working space at the bridge meant that the pre-cast box parts were brought up the river by barge from the casting yard in Surrey Commercial Dock. Construction was carried out using the cantilever method, with segments being built outward from two piers, each segment tied to the previous one by high-strength steel tendons. In the centre the two cantilevers did not meet but stopped short, leaving a space into which the builders placed a concrete beam to complete the span.

1. Existing London Bridge traffic flow

2. Existing bridge used as framework for new girders on either side, maintaining traffic flow across the bridge. Temporary gantry erected to lift and move concrete box sections

3. Once completed, traffic flow redirected onto new girders. Existing bridge demolished and final two central girders constructed

4. Completed bridge comprised of 4 concrete box beam girders

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Images of the construction of the Modern London Bridge, taken from the film New London Bridge by British Pathe, 1969; accessible at {https://www.youtube.com/ watch?v=rtIlkcX9tjI}

The precast elements were transported to the site via boat from Surrey Quays

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The elements were craned into place by the temporary gantry

Steel framed elements craned into place

New precast concrete girders placed alongside the existing bridge

EXISTING DRAWINGS

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2.0 EXISTING CONDITION

BRIDGES AND INFRASTRUCTURE

STRUCTURAL SECTION The superstructure is made up of 4 pre-cast reinforced concrete box girders which span the river across 2 concrete piers. These hollow sections provide space for services and reduce the mass of concrete. On top of this structure sits a poured concrete slab and the road build up of aggregate, waterproofing and tarmac.

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2.0 EXISTING CONDITION

EXISTING DRAWINGS

ELEVATION

PLAN

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EMBODIED CARBON

This break down illustrates the different structural and non-structural elements that make up the existing bridge, and the embodied carbon of each element. The heavy use of reinforced concrete as the primary material has led to a high value of embodied carbon. This is due to the energy and carbon intensive processes of manufacturing cement and steel.

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2.0 EXISTING CONDITION

DETAIL OF SHORT SECTION THROUGH EXISTING

VOLUME M3

EMBODIED CARBON KGCO2E/M3

REINFORCED CONCRETE PIERS

2,424

1,619,232

REINFORCED CONCRETE DECK

8,628

5,763,504

REINFORCED CONCRETE PERPENDICULAR STRUCTURE

1167

779,556 TOTAL - 257,144

Granite Cladding

20,280

Stainless Steel Handrail

185,104

Steel Frame

0.25

5,785

Cement Blocks

142

45,975 TOTAL - 207,077

Steel Rail

185,104

Steel frame

0.25

5,785

Cement Base

16,188

ROAD SURFACE

TOTAL - 137,590

Tarmac Wearing Coat

732

129,543

Reclaimed Concrete Aggregate

732

7,218

Polyethylene Vapour Barrier

829

PAVEMENT SURFACE

TOTAL - 232,275

Sandstone tile

149

25,926

Concrete Pad

298

199,064

Concrete Curb

22.5

7,285

TOTAL

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PEDESTRIAN GUARDRAIL

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BALUSTRADE

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ELEMENT

8,996,378

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2.0 EXISTING CONDITION

COST ANALYSIS

This table illustrates the cost of each element of the existing bridge. The minimal use of materials has meant that the bridge is relatively cheap and is efficient in its use of materials.

AMOUNT

COST

REINFORCED CONCRETE PIERS

2,424m3

£4,040,000

REINFORCED CONCRETE DECK

8,628m3

£5,752,000

REINFORCED CONCRETE PERPENDICULAR STRUCTURE

1,167m3

£778,000 TOTAL - £ 2,202,138

Granite Cladding

81m3

£2,025,000

Stainless Steel Handrail

63,200 kg

£158,000

Stainless Steel Frame

1,975 kg

£4,938

Concrete Blocks 40MPa

142 m3

£14,200 TOTAL - £ 167,938

Steel Rail

63,200 kg

£158,000

Steel frame

1,975 kg

£4,938

Cement Base

50 m3

£5,000

ROAD SURFACE

TOTAL - £ 552,627

Tarmac Wearing Coat

4,855.87 m2

£291,352

Reclaimed Concrete Aggregate

1,369,000 kg

£18,481

Bitumen Elastomer

4,855.87 m2

£242,794

PAVEMENT SURFACE

TOTAL - £76,750

Sandstone tile

149m3

£44,700

Concrete Pad

298m3

£29,800

Concrete Curb

22.5m3

£2,250

TOTAL

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PEDESTRIAN GUARDRAIL

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BALUSTRADE

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ELEMENT

£ 13,569,453

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2.0 EXISTING CONDITION

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CONCLUSIONS

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EXISTING CONDITION SUMMARY

London Bridge provides an essential route across the Thames for vehicles and pedestrians alike. It is a simple, minimal design which has very little visual impact. It’s construction is elegant and austere in the amount of material used, but the reinforced concrete and steel materials have a significant environmental impact. SET OUT OF BRIEF

TIMBER AND STONE

BRIDGES AND INFRASTRUCTURE

In order to create a ‘Carbon Copy’ of the bridge, various factors must remain constant. The span of the bridge and its position must remain, along with the placement of the piers in order not to impact river traffic. The air draft underneath the bridge must also be kept the same for this reason. The bridge width and the number of carriages and pavement area must remain the same in order to allow the same amount of traffic to pass over the bridge. The road level must be retained and the visual impact of the bridge’s height should be kept as low as possible. The same safety measures should also remain in place; guard rails at the edges of the bridge and between the pedestrian and vehicle zones. Due to a lack of accessible information of the bridge’s foundations, this study will address the parts of the bridge above the mean low water line.

The proposed materials are timber and stone; two naturally sourced materials which have a low embodied carbon value thanks to their natural structural qualities and lack of need for additional processing, as well as their carbon sequestering characteristics. Each material option will seek to use as little steel and concrete as possible, creating a bridge which mimics the existing, maintaining the factors set out in the above brief, but using materials which have a significantly lower environmental impact.

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CARBON COPIES

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PROPOSALS

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CARBON COPIES

Following the analysis of the existing structure and its environmental and financial costs, this report begins to look at alternative structural scenarios which make use of low embodied carbon materials in order to propose a ‘Carbon Copy’ which improves the structure’s environmental and financial credentials. The two solutions explored are:

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Carbon Copy A: Timber Carbon Copy B: Stone

CARBON COPY A: TIMBER

126%

41%

IN EMBODIED CARBON

IN PROJECT COST

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83%

IN EMBODIED CARBON

IN PROJECT COST

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77%

CARBON COPY B: STONE

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CARBON COPY: TIMBER

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TIMBER

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3.0 CARBON COPY - TIMBER

INTRODUCTION Timber has long been a common material used in bridge construction, from the first ever trees which fell across rivers and ravines, to the great Roman pontoon bridges, timber has provided a readily available solution to spanning across inaccessible terrain.

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Bridge innovation has led to the development of complex forms and shapes which utilise the natural properties of the material to carry load over longer distances. GRUBENMANN BROTHERS The Grubenmann brothers were two Swiss carpenters who operated in the second half of the eighteenth century, creating several examples of timber bridges which spanned significant lengths. The bridge at Schaffhausen over the Rhine, in 1755, comprised of two spans of 52m and 59m that met at an angle in midstream, resting on a central stone pier. This was achieved through using the principles of the arch and the truss in timber. BEAM TRUSS It was in the first half of the nineteenth century that the beam-like truss as now known really emerged, particularly in North America under the stimulus of an almost insatiable demand first for road and then for railway bridges. The drawing below shows a few of the most significant from among the very large number of designs proposed and constructed up to 1848. The first three were intended for construction wholly or primarily in timber; the others marked the transition to iron.

- Developments in Structural Form by Rowland Mainstone

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Side elevations (top and bottom) of the timber framing of the two spans of the Grubenmann brothers’ covered bridge at Schaffhausen, together with typical cross sections and a plan of the lateral bracing at floor level of the shorter span, from J. Rondelet, Traite theorique et pratique de I’art de bdtir, vol.4, part 1, Paris, 1814.

Bridge trusses of the first half of the nineteenth century: (a) Burr, 1804; (b) Town, 1820; (c) Howe, 1841; (d) Pratt, 1844; (e) Whipple, 1846; (f) Warren, 1848. Single lines represent the ties in (c) to (e).

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3.0 CARBON COPY - TIMBER

CROSS LAMINATED TIMBER Cross Laminated Timber is manufactured by gluing multiple layers of timber together perpendicular to each other to create a thicker and stronger compound beam.

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CLT provides a new structural solution for larger spans and heavier loads. Although it is yet to be tested at the scale of London Bridge, some examples such as the Mistissini Bridge in Quebec and the Duchesnay Creek bridge in Ontario have used CLT as the superstructure, spanning distances of up to 43m.

Mistissini Bridge

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Duchesnay Creek Bridge, Ontario uses timber arches bolted in between the beams to achieve its span.

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3.0 CARBON COPY - TIMBER

With help from the structural engineer, the cross sectional area needed for a timber bridge spanning the same distances and holding the same load was established as being 30m x 1.5m of solid timber. This area could then be divided into beams capable of performing the same function of the existing bridge in timber. In order to create a form which closely resembles the existing London Bridge in timber, shallow arches and extremely deep CLT beams would need to be used.

Existing cross section: Reinforced concrete hollow structural girders

17m2

Cross sectional area needed for equivalent structural support in timber

45m2

Cross sectional area divided into CLT beams to achieve bridge span

45m2

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Timber does not perform well in an arched form and for the spans needed, 3.5m deep beams would have to be manufactured. This combination of inefficient form and amount of material means that this option would not be viable and an alternative form must be found which responds better to the properties of the material. The most effective way of spanning these distances in timber would be to employ a truss beam. This, however, would have a large visual impact and would not be an exact carbon copy. In response to the feedback from the Structural Engineer, a better structural solution to form these spans in timber would be to use steel pylons and cables which could be set to the same height as the existing street lamps in order to minimise any visual impact.

Chart Datum

61720m

8773m 15973m

Mean High Water Springs

103730m

93520m

Existing elevation

258980m

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existing street lamp level

Timber arch form

existing street lamp level Fink truss timber suspension bridge

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TIMBER PROPOSAL

The proposed form employs a Fink Truss, locating steel pylons at the joins between each CLT beam and connecting these to each other using steel cables. This allows the beams to be supported at the key junctions where they join and reduces the need for the larger CLT beams previously explored, instead using smaller beams as a secondary structure spanning between the large beams supported by the cables in tension.

BRIDGES AND INFRASTRUCTURE

This has allowed the constraints set out in the brief to be adhered to, maintaining the span, height, air draft and width of the bridge. This allows the existing use of the bridge to be maintained, supporting 3 traffic lanes and a pedestrian walkway either side, and allowing river traffic to pass beneath.

62m

ELEVATION

103.7m

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93.8m

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3.0 CARBON COPY - TIMBER

62m

PLAN 10

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93.8m 03.7m

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3.0 CARBON COPY - TIMBER

1. Limestone piers

2. CLT Beams

3. Steel fixings

4. Steel pylons

5. Steel cables

6. CLT secondary structure

7. Timber railings

8. Road and pavement build-up

SUMMARY OF COMPONENTS

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AXONOMETRIC SECTION - ABOVE

AXONOMETRIC SECTION - BELOW

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3.0 CARBON COPY - TIMBER

CONSTRUCTION METHODOLOGY The superstructure is made up of 3 large CLT beams formed out of a series of smaller beams bolted together. These are made up of central thicker beams of 3.5m depth and smaller beams of 2m depth either side. The larger central beams are joined through a steel shoe which wraps around where the beams are joined at the piers and pylon junctions, and the tensile steel cables are connected to these steel joints. The steel pylons also sit on top of the steel shoe and are welded in. Cables are connected at the top and bottom of the 11m verticals. The secondary structure is formed of 500mm deep CLT beams which run perpendicular to the large beams and are supported on top of the smaller beam sections. These are also supported underneath using diagonal timber struts.

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The road build up above these is made up of timber panels, limestone aggregate and sandstone paving.

11000mm

3720mm

3000mm

4000mm

3000mm

mean high water

7170mm

mean low water

SHORT SECTION

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800mm

1 1. Steel cables connect to top of pylon 2. Steel pylon 3. Cable connection at base of pylon 4. Steel shoe 5. Road build-up 6. CLT beams primary structure 7. CLT secondary structure 8. Timber hand rail 9. Limestone pier

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1000mm

3 4

11000mm

300mm 500mm 3500mm

2000mm

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1000mm 1000mm 1000mm

5410mm

4665mm

SHORT SECTION 51

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3.0 CARBON COPY - TIMBER

1. Steel pylon 2. Steel cables 3. Steel shoe 4. CLT primary structure 5. CLT secondary structure 6. Road build-up 7. Limestone pier 8. Steel plate and bolts

300mm

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2 1

900mm 500mm 2000mm

250mm

8 5000mm

2000mm

7 8600mm

2950mm

3950mm

SECTION THROUGH PIER 52

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AXONOMETRIC DETAIL THROUGH PIER JUNCTION

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3.0 CARBON COPY - TIMBER

DETAIL A

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DETAIL B

5 3

1. Steel pylon 2. Steel cables 3. Steel shoe 4. CLT primary structure 5. CLT secondary structure 6. Steel plate and bolts

AXONOMETRIC DETAIL THROUGH PYLON BEAM JUNCTION

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DETAIL A At the top of each pylon, a welded steel plate provides a connection point for the cables.

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These are bolted together through the plate and allow for any movement.

DETAIL B At the base of the pylon, it is welded to the steel shoe which connects two primary CLT beams. Here, there is another plate which provides the connection for the steel cables at low level in the same way as the at the top of the pylons.

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3.0 CARBON COPY - TIMBER

VISUALISATION: VIEW FROM SOUTH BANK

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The analysis of embodied carbon of the proposed timber bridge has shown a 126% reduction from the existing bridge. The majority of this has come from the use of timber as the primary material. Because it sequesters carbon throughout its lifetime, timber can dramatically offset the carbon that is produced in the production of steel and stone, yielding a negative carbon value.

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EMBODIED CARBON

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ELEMENT

VOLUME M3

EMBODIED CARBON KGCO2E/M3

EMBODIED CARBON DIFFERENCE

LIMESTONE PIERS

3,413

645,057

-974,175

SUSTAINABLY SOURCED CLT SUPERSTRUCTURE

7,285

-6,129,059

-12,672,119

TIMBER BALUSTRADE

134

-137,122

-394,266

TIMBER PEDESTRIAN GUARDRAIL

-207,077

STEEL FIXINGS

1,097,341

STEEL CABLES

405,932

STEEL PYLONS

129

2,177,799

-345,503

-483,093

ROAD SURFACE Sandstone slabs

525

91,350

Quarried Sandstone aggregate

525

4,843

Timber load protection layer

525

-441,696

PAVEMENT SURFACE

-126,356

Sandstone tile

192

33,408

Quarried Sandstone aggregate

192

1771

Timber load protection layer

192

-161,535

TOTAL

-2,411,911

-358,631

-11,408,289

126% reduction in Embodied Carbon

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COST

AMOUNT

COST £

COST DIFFERENCE

LIMESTONE PIERS

3,413m3

£1,023,900

-3,016,100

SUSTAINABLY SOURCED CLT SUPERSTRUCTURE

48,566m2

£12,141,500

+5,611,500

TIMBER BALUSTRADE

986.1m2

£98,610

-2,103,528

STEEL FIXINGS

513.5t

£1,283,750

+1,283,750

STEEL CABLES

189.6t

£474,000

+474,000

STEEL PYLONS

1,019.1t

£2,547,750

+2,547,750

£1,190,000

+637,373

ROAD SURFACE Sandstone slabs

525m3

£157,500

Quarried Sandstone aggregate

525m3

£157,500

Timber load protection layer

3,500m2

£875,000

PAVEMENT SURFACE

£435,200

Sandstone tile

192m3

£57,600

Quarried Sandstone aggregate

192m3

£57,600

Timber load protection layer

1,280m2

£320,000

TOTAL

£19,194,710

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ELEMENT

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The cost of the proposed timber bridge is 41% higher than the existing concrete bridge. The analysis below shows that this cost predominantly comes from the CLT beams which form the superstructure of the bridge. The added steel also accounts for much of the added cost. The alternative stone piers and timber balustrade, however, provide a much cheaper alternative to the existing elements.

+358,450

+£5,625,257 41% increase in cost

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3.0 CARBON COPY - TIMBER

CONCLUSION

This exploration has shown how timber could be used as the primary material in bridge design, providing a solution to large spans for heavy loads which is carbon sequestering.

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However, due to the loads, spans and weathering factors, the amount of timber that would need to be used would not make the option realistically viable. Although the bridge satisfies the brief in terms of use and dimensions, it does not have the same visual simplicity and elegance of the existing London Bridge. The pylons and cables do create a visibility barrier, which could pose a problem for the visitors to the city, obstructing views of the surrounding buildings and bridges. This is not to say, however, that timber should not be used in bridge design. The large 103m span of London Bridge is what has led to the amount of timber needed, and with a smaller span or lighter load, timber could provide a solution, fulfilling all areas of the brief. This has been shown in the precedents of Duchesnay Creek and Mistissini where the shorter span has been achieved without the use of pylons. WHAT IF A more efficient timber bridge form would employ a truss form, creating a beam which could span longer distances without the use of pylons and cables. The problem with creating a timber truss bridge in this context is the visual impact that it would have. For the sake of creating a ‘Carbon Copy’, this option has not been explored further.

This is an example of efficiency of form in relation to particular materials having the greatest influence on the design of the bridge. In future bridge design, a CLT truss could provide the required span and hold the required load.

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4 CARBON COPY - STONE

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STONE

THE SHALLOW STONE ARCH

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The Romans are most commonly attributed to be the greatest masonry bridge builders of antiquity. They built passages and aqueducts still standing today. These immensely robust structures would only get more robust as they carried heavier loads, thanks to their great understanding of forces and the art of cutting stone. Semicircular arches would transfer their loads on their typically thick piers. “The legacy of the Roman bridge builders is encompassed within the six span bridge over the river Tagus at Alcantara in Spain, which contains individual spans of up to 30m.” Roman masonry bridge building was extremely expensive due to the shear mass and amount of material necessary. While roman bridges became an icon representing masonry bridge construction, a whole new revolution of masonry bridge building was happening in China. “the Chinese were also developing bridges with new geometries (segmental, elliptical, parabolic, etc.) and the Zhaozhou Bridge, built from 595 to 605 AD during the Sui Dynasty in China, is historically significant as it is the world’s oldest open-spandrel, stone, segmental arch bridge. This bridge demonstrates an advanced level of understanding of the forces exerted on the arch, piers and abutments, which was not demonstrated in Europe until the Renaissance period.” Segmental bridge building has had a more visible impact on how we build bridges today in both reducing the amount of material and keeping the walkway relatively low in respect to the span more commonly having to go up by 1 meter in height for every 8 meters in span.

The New Stone Age Exibition at the Building Centre

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La Balme Bridge Source: https:// structurae.net/en/ structures/la-balmebridge

NEED FOR IT Highways agency BD91/04: ‘Experience has shown that arch bridges are very durable structures requiring little maintenance in comparison to other bridge forms. BD 57 (DMRB 1.3.7) says their use should be considered. However, there has not previously been a standard for the design of new unreinforced arch bridges. The objective of this Standard is to encourage a renaissance in arch building using unreinforced masonry materials.’ https://www.thenbs.com/PublicationIndex/documents/ details?Pub=HA&DocID=329043

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TENSIONED STONE

HOW DOES IT WORK The concept of tensioned stone capitalizes on the stone’s unparalleled ability to perform in compression. A bar is threaded through a series of blocks, stretched and secured at the ends by a metal plate. As the bar attempts to return to its former length, the stone will be compressed by the metal ends that are trying to come closer to each other.

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This allows us to build elements like beams out of stone, that unlike a column are affected by tension as well as compression. Wile this technique is not new in itself and is widely used to stiffen spanning concrete elements, it’s application in combination with cut stone is uncommon with only a few instances in construction where it has been used structurally. A reevaluation of this technique could allow us to treat stone with the same freedom as we do with concrete.

The New Stone Age Exibition at the Building Centre

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PERFORMANCE Coupled with the form of the arch tensioning stone is then able to achieve much wider spans while also performing a structural role. The tensioning increases the failure load of the stone in bending, while the form of the arch minimizes the bending stress by transferring it onto the piers. Tensioning stone just as we pre/post tension concrete can amplify and empower the architectural language of the material just as reinforcing concrete with rebar did for Modernism. Photograph of assembly of a masonry arch. An arch a framework and its drilled masonry elements in the foreground.

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Photograph of a structural threaded masonry arch inside Padre Pio Pilgrimage Church

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ASSEMBLY

TRANSPORTATION AND PHASING In the construction of the six central towers of the Sagrada Famìlia, tensioned stone panels played a central role in making the building process quicker and safer, than laying each stone individually. It made it possible to safely assemble the sections at ground level and review the work before lifting it in place. It minimizes the work done on site, making it easier to schedule tasks, as the finished panels can be stored before using. This allowed the type of work to be standardised both on site and in the manufacturing plant.

AFTERLIFE Tensioning stone makes it possible to have dry joinery between elements making it easier to re purpose or replace individual blocks if necessary. Its lighter structure lends itself to being easily demountable. Just as the previous stone Bridge had a second life in Arizona, the proposed tensioned stone London Bridge could be dismantled and reassembled with much more easr somewhere else if necessary.

A postcard of the Old London Bridge reassembled in Lake Havasu City, Arizona , U.S.A.

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Photograph of the stone panels being stored off-site in preparation of assembly.

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Photograph of a worker tensioning the stone elements of a structural panel for the construction of one of the central towers of the Sagrada Famìlia.

Photograph of the tensioned stone panels being guided into place by two workers on site.

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PROPOSAL

The proposal for a light weight stone bridge that spans a total of 150m with only two piers has to be achieved through the strategic use of parabolic geometry and the tensioning of the primary structure. In the proposal the arch form a parabolic shape to account for the uneven loading of the road above. From a visual perspective the proposal assumes an even slimmer profile than the existing pre cast bridge while still having the same pier position. The arches pierce through the deck and making the bridge even slimmer and the open spandrels create unprecedented views on the Themes.

LONG ELEVATION LOOKING EAST

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SKETCH OF STRUCTURE

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PROPOSAL

PLAN OF BRIDGE

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ELEVATION USE OF BRIDGE The bridge retains the same profile of the existing bridge providing an additional 3m height throughout most of the central span. While the bridge does provide this additional height it does so without compromising on the camber of the road surface above, the road surface maintains the same pitch as the existing bridge, so that not only vehicular but also pedestrian access is effortless even for those with impaired mobility. The structural arches pierce the road surface making the profile exceptionally thin. And the loading of the arch is made uniform through the parabolic shape of the arch as well as secondary parabolic arches to distribute the weight that would otherwise go directly on the piers.

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This allows a light profile design that works with open spandrels.

DETAILED ELEVATION WITH TRANSIT

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SECTION

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SHORT SECTION THROUGH TOP OF MAIN ARCH

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RELEVANCE The menu of components represents an order of structure going from the interface with the water to the touch of the hand. 1. The Pier to Arch component presents the tensioning strategy for the primary structure and a pier design which is friendlier to the water life of the Themes. 2. The Arch to Beam component is representative of how a post tensioned system can work at different scales and the way these come together. How secondary beams can be tied to the primary structure through he use of the tensioning element on itself.

3. The Beam to Balustrade component shows an elegant way to anchor the tensioning system in such a way to hold the balustrade in place. This simplifies the detailing and reduces the amount of joinery required.

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PIER TO ARCH ARCH TO BEAM BEAM TO BALUSTRADE

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PIER TO ARCH

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2. 3. 4.

10.

The interface to the pier is the most vital part of the tensioned arch as it has to withstand forces coming from different directions and contain the space for the tensioning component at the base of the arch as well.

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The stone piers sit within the same footprint as the previous concrete one, with ashlar construction to support the arches and rough stone crib construction as infill. The additional ashlar on the ends holds the cross bracing in place and prevents possible damage from impact with boats. The stone crib construction decreases the amount of material used while providing shelter for the resilient water life of the Themes.

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HOW DOES IT WORK

The stone arches taper at the base to redistribute the forces onto the piers more evenly. The base of the arches contains the elements of a Fressinet inspired tensioning system. This comprises of a steel anchor, a bearing plate and cable grips to keep the tendons in place. BRIDGES AND INFRASTRUCTURE

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1. Ashlar Arch 2. Limestone Arch Base 3. Cable Guide 7.

4. Bearing Plate and Cable Grips 5. Access for Tensioning 6. Steel Anchoring Plate 7. 19 x 12,7mm Steel Tendons 8. Cross Bracing Steel Tendon 9. Stone Pier 10. Connection to Pier 11. Stone Crib Infill

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ARCH TO BEAM

PRESSED METAL PLATE CONNECTION The connection between the secondary beam and the arch primary structure relies on the compression of the tendons running through the beams. A metal plate is pressed between the beam and the arch to compensate for small variances in beam length. The metal plate moulds to the texture of the arch, in turn creating a firm grip.

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KEYSTONE BEAMS While an arch typology works entirely in compression, beams need to withstand bending. While this could be achieved with multiple tendons in the stone, to decrease the amount of steel necessary the stones are cut in rhombus and trapezoidal forms to keystone in each other. This redistributes the live loads in the beam.

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1. Limestone Ashlar Primary Arch Structure 1600mm x 1250mm 2. Limestone Ashlar Beam 600mm x 400mm 3. Limestone Ashlar Cantilever Beam 300-600mm x 400mm 4. 12,7mm Diameter Steel Tendon 5. 19 x 12,7mm Diameter Steel Tendon 6. Hot Rolled Metal Plate 7. Hot Rolled Metal Cable Sheath

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BEAM TO BALUSTRADE

FIXINGS The design of the balustrade integrates the anchorage of the tendons and the fixing of the handrail/safety net into one detail.

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Metal plates press on the vertical stone element which supports the handrail and hold the bottom fixing for the safety net. The cable safety net is kept tort by the handrail cable and the bottom fixing attached to the metal plates. The metal plate acts as the anchor for the tensioning system, on which washers and nuts are threaded. The tensioning system employed for the secondary beams which support the carriage way is a Lee McAll Tensioning System. This system employs the use of a threaded nut to tension the tendon, and is much simpler than other analogous tensioning systems.

POST TENSIONING CABLE Sandstone planks act as the tertiary structure for both the walkway and the vehicular lanes. The choice of sandstone over other types of stone is justified by the coarse texture of sandstone that does not wear with use. It is important for sandstone to be scanned for fissures and cracks. This is especially important due to the change in temperatures and humidity in the London climate.

Stone is proportionally a lot less carbon intensive than steel, this proposal tried to minimise the amount of steel necessary. This was achieved by using steel only when used in tension in the form of steel tendons. The use of galvanized steel has also been reduced in the joinery as the secondary structure is threaded to the primary through the same tendons that keep the structure in compression.

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2. 4. 3.

7. 8.

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1. Handrail Cable 2. Cable Mesh 3. Stone Balustrade 4. Rough Sandstone Plank 1500mm x 150mm 5. Hot Rolled Metal Sheath 6. Washer and Nut 7. 12,7 mm Steel Tendon 8. Limestone Cantilever beam 600-300mm x 400mm

VISUALISATION: VIEW FROM SOUTH BANK

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CARBON CALCULATION Element

Volume m3

Primary Arches

Total - 457,639

Lime Stone

1522.00

287,658

Galvanized Steel Plates

7.20

121,552

Cables

2.86

48,429

Spandrels

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Embodied Carbon kgCO2e/m3

Total - 177,127

Lime stone

774.00

146,286

Cables

1.82

30,841

Secondary Beams

Total - 328,508

Lime Stone

1090.00

206,010

Galvanized Steel Plates

6.70

113,111

Cables

0.55

9,387

Path Slabs Sand Stone

Total - 202,884 1166.00

Balustrade

202,884 Total - 28,693

Stone

28.00

5,292

Cable

0.05

846

Netting

0.03

567

Galvanizes Steel Plates

1.30

21,988

Total - 1,198,227

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Element

Amount

Primary Arches

Cost Total - £655,285

1,522.00 m3

£456,600

Metal Plates

56,880 kg

£142,200

Cables

22,594 kg

£56,485

Spandrels

Total - £268,145 774.00 m3

£232,200

Cables

14,378 kg

£35,945

Secondary Beams

Total - £470,188

Lime Stone

1,090.00 m3

£327,000

Metal Plates

52,930 kg

£132,325

Cables

4,345 kg

£10,863

Path Slabs

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Lime stone

Sand Stone

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Lime stone

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COST CALCULATION

Total - £349,800 1,166.00 m3

Balustrade

£349,800 Total - £35,656

Stone

28.00 m3

£8,400

Cable

395 kg

£988

Netting

237 kg

£593

Metal Plates

10,270 kg

£25,675 Total - £ 2,305,874

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EMBODIED CARBON

It is clear that both options dramatically reduce the embodied carbon of the bridge through the use of alternative materials. The natural carbon sequestering values of timber and stone allow for reductions of 126% and 77% respectively.

10,000,000

8,000,000

EMBODIED CARBON KGCO2E/M3

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6,000,000

4,000,000

2,000,000

-2,000,000

-4,000,000

EXISTING

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COST

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The material costs of each option provide another view of each proposal. The inadequacy of timber for large spans and heavy loads is emphasized in the cost projected, highlighting the need for a much larger amount of material with a 41% cost increase. The stone, however, illustrates the strength of the material through the reduction of material and cost, providing a solution which is 83% cheaper. This figure, however appears to be far too low which could be due to an inaccurate cost value of Limestone, or an error in calculations. Illustrated below is also a stone value of £1500/ m3 which seems to provide a more realistic value.

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25,000,000

20,000,000

COST £

15,000,000

10,000,000 If the cost of Limestone was assumed to be £1500/ m3, this would seem to be a more reliable figure

5,000,000

Cost of Limestone assumed as £300/m3. This figure seems too low

EXISTING

TIMBER

STONE

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5.0 FINDINGS

COMPARISON TIMBER

126%

41%

IN EMBODIED CARBON

IN PROJECT COST

PROS Dramatic reduction in embodied carbon CONS

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Visual impact increased due to steel pylons Material qualities not suited for the existing form and span An excessive amount of material is needed, leading to an increase in project cost

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77%

83%

IN EMBODIED CARBON

IN PROJECT COST

PROS Large reduction in embodied carbon Large reduction in project cost Material suited to shallow arch form and span, providing a more efficient alternative to concrete CONS

More skilled labour and construction techniques required

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Existing photograph

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Proposed timber bridge

Proposed stone bridge

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CONCLUSION

In conclusion, these two options have illustrated the possibilities of using alternative, low carbon materials to create bridges with large spans and heavy loads. This is an important intervention into the bridge typology which is dominated by reinforced concrete and steel construction.

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For the constraints given by London Bridge, the stone bridge has provided a more viable option in terms of material use, cost, and fitness for purpose in its material choice. This is due to the natural compressive strength of stone which is what is required to carry the heavy traffic load. Although the timber bridge provided a possible solution, the sheer amount of material and the weathering and structural properties of timber would mean that it is less applicable. However, if the spans were to be smaller and the load lighter, timber could provide a better alternative to stone, as is the case with many footbridges and short crossings around the world. The stone option provides a possible future to masonry arch building which integrates tendons to achieve shallow arches that work entirely in compression just like a full arch. It is very difficult to predict the amount of tendons necessary for the arches to work and they would need to be protected from weathering and rusting. Stone becomes a viable option although it would require a new set of engineering knowledge for the tensioning of the arches.

Overall this exercise has been extremely useful in analysing the common construction techniques which are taken for granted and exploring how alternative materials and techniques could provide better solutions.

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1. Steel pylon 2. Steel cables 3. Steel shoe 4. CLT primary structure 5. CLT secondary structure 6. Road build-up 7. Limestone pier 8. Steel plate and bolts

300mm

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900mm 500mm 2000mm

250mm

8 5000mm

2000mm

7 8600mm

2950mm

3950mm

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DETAIL A

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DETAIL B

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1. Steel pylon 2. Steel cables 3. Steel shoe 4. CLT primary structure 5. CLT secondary structure 6. Steel plate and bolts

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AXONOMETRIC DETAIL THROUGH PYLON BEAM JUNCTION

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DETAIL A At the top of each pylon, a welded steel plate provides a connection point for the cables.

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These are bolted together through the plate and allow for any movement.

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AXONOMETRIC DETAIL - PIER TO ARCH

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1. Ashlar Arch 2. Limestone Arch Base 3. Cable Guide 7.

4. Bearing Plate and Cable Grips

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5. Access for Tensioning 6. Steel Anchoring Plate 7. 19 x 12,7mm Steel Tendons 8. Cross Bracing Steel Tendon 9. Stone Pier 10. Connection to Pier 11. Stone Crib Infill

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AXONOMETRIC DETAIL - ARCH TO BEAM

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