Estándares de Codificación

Transport for London

In 2008 Transport for London (TfL) Major Projects commissioned Atkins to produce a structures options report to present the preferred structural form for a new River Roding Crossing – Figure 4 shows the location.

The River Roding structure was required to accommodate buses, cyclists, and pedestrians. The structural options developed had to address several key issues as identifi ed by TfL. These were:

Clearance and alignment

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constraints Health and Safety

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Aesthetics

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Buildability

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Cost (including whole life cost)

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The issues highlighted had a clear synergy with the attributes of the bridges sustainability index and therefore, an assessment of the proposed options for sustainability to demonstrate the preferred solution was undertaken using this tool.

River Roding Crossing design options

Five options were proposed with the fi nal recommendation of a twin tied arch bridge being adopted. Two of the proposed options, and the way they were assessed using the sustainability index, are discussed below: a full width half through girder

deck and a twin tied arch bridge as shown in Figures 5 and 6 respectively.

The specifi c aspects discussed are the initial cost, whole life cost, aesthetics, consumption of natural resources and carbon footprint. All other sustainability attributes assessed showed little variation between the two proposed options or were not a key requirement of the client and therefore are not discussed further.

Initial cost and whole life cost For any structure, the main choice of structural form will have a fundamental effect on the initial and whole life cost. Architectural features with unusual materials may increase the cost of the structure, but may be justifi ed by the visual enhancements they bring. Similarly, some additional initial cost may make the structure more maintainable, thus reducing its whole life cost. The index determines a sustainability rating for both initial cost and whole life cost per square metre of deck area. The sustainability index output for the two options shows that the tied arch structure not only has a considerably higher initial cost than the through girder option, but also that this cost is very high compared to the benchmark from all bridges because the coordinate is right on the boundary of the diagram. This is due to the tied arch having more complex fabrication and erection requirements compared to through girder bridge construction or, indeed, typical plate girder solutions.

Quantifi cation of sustainability principles in bridge projects 80

Figure 3 - Example Sustainability Index drawing

Figure 4 - Site location River Roding crossing

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As shown in Figures 5 and 6, the tied arch had the better sustainability rating for aesthetics. The openness of the tied arch was preferred visually to the deep solid elevation of the through girder option. The latter also signifi cantly reduces the bridge users’ visual experience when crossing the bridge compared to the views obtained from the arch bridge.

Consumption of resources and carbon footprint

The index attribute for consumption of natural resources quantifi es the tonnage of concrete and steel components of a structure per square metre of deck and benchmarks this against typical existing steel, concrete and steel-concrete composite structures. The through girder option actually contained the greater amount of materials, albeit Both options have bearings and

expansion joints, requiring regular maintenance. The assessed whole life cost for the through girder option, however, was slightly higher than that for the tied arch due to maintenance requirements of the connections between the main longitudinal beams and the cross beams. These would be more diffi cult to inspect and maintain, due to confi ned access and lack of space between the parapets and the main beams, in comparison to the relatively open structural form of the tied arch. In addition, this connection zone would be particularly vulnerable to corrosion due to de-icing salts during its lifetime which increased the assessed potential maintenance costs of this option. Aspects of the tied arch design option, such as arch ribs and hangers, were, however, still considered to require signifi cant

an adverse impact on the ease of modifi cation and demolition, which is specifi cally refl ected in the “Ease of modifi cation, demolition”.

Aesthetics

Aesthetics was a key aspect in infl uencing the form of structure for this new regeneration project.

Structural choices were reviewed for suitability within the proposed developments, using appropriate materials for its urban location. The bridge needs to fi t in with the river front redevelopment and should be appealing to those viewing it.

The sustainability index uses a combination of a quantitative and qualitative assessment to determine the aesthetic merits of the design. The considerations comprising the overall rating include whether aesthetic considerations are relevant at all, the

Quantifi cation of sustainability principles in bridge projects 80

Figure 6 - Twin tied arch deck and Sustainability Index Output Figure 5 - Full width through girder deck and Sustainability Index Output

SUS TAINABILIT Y

The increased quantity of construction materials used in the through girder option also has a direct associated impact on the carbon footprint of the structure. However, as well as the materials themselves, the method of construction and plant and labour requirements are also included in the carbon calculation. The sustainability index attribute is then based on the tonnes of CO₂ embodied in the design and construction per square metre of deck area. The carbon footprint was found to be higher for the through girder than for the tie arch.

As noted above, the through girder has an increased whole life cost in comparison to the tied arch which, again, has a direct impact on the carbon footprint for the maintenance of the structure. The through girder has a greater adverse effect in comparison to the tied arch option.

It is worth noting here that the calculation of carbon footprint is open to much interpretation.

The Environment Agency’s carbon calculator was used for this project but different engineers can make different assumptions in its use, such as those relating to the transport distances of materials or the traffi c disruption resulting from the bridge construction. It is therefore vital that a consistent set of assumptions is used when comparing options until such time as a more bridge-specifi c industry standard carbon calculator is produced.

Summary

The sustainability index calculated an overall sustainability rating for the two River Roding Crossing options. The ratings are 0.507 and 0.460 for the through girder and tied arch options respectively. The sustainability index rating shows that the tied arch option is more sustainable in comparison to the through girder, given the evaluation criteria incorporated in the tool.

The sustainability index verifi ed the extent to which each of the proposed options impacted on society, the environment and the economy and allowed the client to make a more informed choice on preferred option taking all of these criteria into account. The output demonstrated that initial cost, whole life cost, aesthetics, consumption of natural resources and carbon footprint were the key aspects driving the fi nal choice of structural form and it also identifi ed the areas which could be refi ned further to improve the overall impact on sustainability for the fi nal design.

Figure 7 shows a virtual reality model of the fi nal option selected for the River Roding Crossing.

Conclusions

Quantifying and assessing civil engineering projects in terms of sustainability and meeting carbon reduction targets is a new challenge for engineers and the civil engineering industry. Whilst the development of carbon accounting tools identifi es areas of bridge design and construction that have the greatest contribution to carbon emissions, quantifying sustainability overall has been less well studied.

The sustainability index for bridges is a signifi cant step towards facilitating the systematic quantifi cation of the sustainability of schemes through a simple and graphical tool.

The output facilitates identifying where improvements can be made on a particular design and it allows designers and clients to objectively compare alternative design solutions.

Further, it can be used throughout the design process to monitor the strength of decisions and the success of changes to the design and to inform decisions on future projects with a view to improving the sustainability of designs.

The overall sustainability index rating provides a useful means of benchmarking designs. Targets can be set for the desired performance for a particular group of bridges on one project or for all the bridges produced by an organisation. The format of the tool is fl exible enough to allow different projects to weight attributes differently; in such cases, the benchmarking can obviously only take place within that pool of structures for which the weighting has been altered. The methodology lends itself to adoption by Clients so that, once the key attributes are set, designs can be benchmarked across their whole asset pool of bridges.

Acknowledgements

The authors would like to thank the bridge engineers throughout Atkins who contributed to the development of the Sustainability Index for Bridges. The tool was developed collectively by the Atkins Bridge Engineering Working Group, which links all 41 Atkins offi ces engaged in bridge design and engineering.

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Figure 7 - River Roding Crossing Tied Twin Arch

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References

Report of the World Commission on Environment and Development, General 1.

Assembly Resolution 42/187, United Nations, 11 December 1987.

Environment Agency Carbon Calculator for Construction Activities Available at 2.

http://www.environment-agency.gov.uk/static/documents/Business/Carbon_calculator_v3_1_1.xls New Model Code 2010, Bulletin 55, International Federation for Structural

3.

Concrete (fi b), March 2010, ISBN 978-2-88394-095-6

New Model Code 2010, Bulletin 56, International Federation for Structural 4.

Concrete (fi b), April 2010, ISBN 978-2-88394-096-3

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