Bren 1300025 h

Carbon calculator design toolfor bridges

&1 David A. Smith BEng, CEng, FICEStructures Service Director, Atkins, Epsom, UK

&2 Phillippa Spencer MSc, CEng, MICEBridge engineer, formerly Atkins, Epsom, UK

&3 Chris Dolling BSc, CEng, MICEManager, Technical Development, British Constructional SteelworkAssociation Ltd, Doncaster, UK

&4 Chris Hendy MA (Cantab), CEng, FICE, FREngHead of Bridge Design and Technology, Atkins, Epsom, UK

1 2 3 4

BCSA and Corus engaged Atkins to develop a simplified carbon calculator design tool, which could be used to

evaluate the carbon footprint of typical steelconcrete composite bridges. A key requirement was to allow rapid

evaluation of a range of preliminary design options based on minimal calculated quantities for each option, using

typical default assumptions. In addition, the tool needed to be flexible enough to allow the designer to enter more

detailed data to override default assumptions and quantities should they become known more accurately. This paper

describes the development of the design tool, assumptions used in setting the default parameters and commentary

on the sources of information used for the carbon data. An illustration of its use in comparing a typical steelconcrete

composite bridge with an equivalent concrete bridge is also provided. It is intended that the design tool will be used

by bridge designers to help quantify the embodied and operational carbon dioxide of steelconcrete composite

bridges at all stages in their life-cycle, facilitate reductions in the carbon dioxide footprint during construction and

maintenance by allowing alternatives to be rapidly assessed, and benchmark projects to facilitate continuous

improvement with respect to carbon dioxide.

1. Introduction and project background

With the introduction of climate change legislation and greenhouse

gas reduction targets, carbon dioxide emission reduction has

become a dominant aspect of sustainable design. To reduce car-

bon in designs engineers need to be able to measure it. While

quantifying embodied carbon in designs is possible through the

numerous carbon calculators that have been developed, such as the

UK Environment Agency calculator (EA, 2010), they do not lead

to a consistent assessment of carbon on their own. This is mostly

because of what they do not cover rather than what they do.

Within the authors own organisations it was noted that when

the same bridge was given to different engineers for determin-

ing its carbon footprint, vastly different answers were

returned. This was not because of incompetence or error but

simply because different engineers made different assumptions

about unknowns or, indeed, the boundaries defining the

carbon assessment itself. Partially as a result of this observa-

tion, Atkins was appointed by the British Constructional

Steelwork Association Ltd (BCSA) and Corus (now Tata

Steel), to develop a simplified carbon calculator design tool

that could be used to evaluate the carbon footprint of typical

steelconcrete composite bridges, based on the inventory of

carbon and energy database (Hammond and Jones, 2008) for

unit rates among other widely available carbon data values. A

key requirement for the design tool was to allow rapid

evaluation of a range of preliminary design options based on

minimal calculated quantities for each option. This was to be

achieved by including realistic default assumptions for aspects

of the project that may have very limited information available

at the design stage but that may have a significant impact on

overall carbon dioxide. In addition, the design tool needed to

be flexible enough to allow the designer to enter more detailed

data to override these default assumptions and quantities

should they become known more accurately.

This paper describes the development of the design tool, the

assumptions used in setting the default parameters and

Bridge Engineering

Carbon calculator design tool for bridgesSmith, Spencer, Dolling and Hendy

Proceedings of the Institution of Civil Engineers

http://dx.doi.org/10.1680/bren.13.00025

Paper 1300025

Received 16/10/2013 Accepted 06/06/2014

Keywords: bridges/design methods & aids/sustainability

ice | proceedings ICE Publishing: All rights reserved

1

commentary on the sources of information used for the carbon

data. An illustration of its use in comparing a typical steel

concrete composite bridge with an equivalent concrete bridge is

also provided.

It is intended that the design tool will be used by bridge

designers to help

& quantify the embodied and operational carbon dioxide ofsteelconcrete composite bridges at all stages in their

lifetime

& facilitate reductions in the carbon dioxide footprint duringconstruction and maintenance by allowing alternatives to be

rapidly assessed

& benchmark projects to facilitate continuous improvementwith respect to carbon dioxide.

2. Scope of carbon calculationsThe calculator covers all aspects of a highway bridge project,

including detailed consideration of

& site set up and shut down& materials used& construction activities& maintenance and inspection& demolition& highway traffic data& delay during construction& delay during maintenance.

The calculator covers rail bridges and any impact their con-

struction has on highway traffic, but the impact of any delay to

rail traffic itself is not covered.

During design, the bridge designer has little information on

many important aspects influencing carbon considerations,

such as

& site establishment how long will the project be on site, forexample, and what overheads are involved

& sourcing of materials where do the materials come from,for example

& construction equipment how will the bridge be built, forexample, and what do we use to build it

& traffic impacts during construction and maintenance willthe existing traffic be delayed, for example, and if it is

diverted where does it go

& maintenance and inspection how will the asset bemaintained during its life and what resources will we use

to maintain it.

The above unknowns may have far more impact on total carbon

dioxide than the embodied carbon dioxide in the materials

themselves and this is the reason why carbon predictions

can be so variable; different designers make different decisions,

inclusions and omissions.

The minimum required inputs to the calculator are kept

intentionally few in number and are initially limited to the few

drop-down options and numerical or text input cells illustrated

in Figure 1. This is achieved by providing over 50 default

assumptions within the remainder of the spreadsheet. How-

ever, when more detailed information becomes known, some

or all of these assumptions may be refined by the user to

improve accuracy.

3. Development and functionality of thedesign tool

3.1 Format and presentation

The design tool was designed such that it allows for a

comprehensive estimation of carbon dioxide with only minimal

high level information such as the type of bridge, dimensions,

concrete and steel quantities and the number of bearings. It

also mirrors the structure of a bridge project by having discrete

elements of the project life-cycle where information is inserted

for each project element design/construction, maintenance

and traffic delay. For each life-cycle phase this was further

divided into the key sections of the specific phase. For example,

the design/construction element was further split into founda-

tions, substructure and superstructure. This approach was

taken to allow users easily to adjust elements of a bridge

project within their control. Examples of this might include

proposals to change from driven piles to bored piles or from

steel parapets to concrete parapets. Does a ladder deck bridge

have a lower carbon footprint than a multigirder bridge deck,

or is a pair of column piers better than a leaf pier, for example?

The design tool is laid out using seven separate worksheets

& user guide& summary& user locked default assumptions& user values editable assumptions& traffic data& construction materials& information.

3.2 Basic initial input

To gain a rapid approximation of the carbon dioxide for a

bridge the user enters information into the simple front

summary sheet where preliminary material quantities are input

(see Figure 1). The number of cells where input is required is

limited to 16. Nine of these are global details of the bridge and

project and cover

Bridge Engineering Carbon calculator design toolfor bridgesSmith, Spencer, Dolling and Hendy

2

& bridge name& structure/project number& type of bridge (road (plus road classification), rail or

footbridge)

& type of obstacle crossed (plus classification if a road)& estimated construction duration& bridge length& bridge width.

The remaining seven required inputs relate to basic material

quantities from a preliminary design including reinforced

concrete, structural steel (painted and weathering) and the

number of bearings for each of the foundations, substructure

and superstructure bridge elements.

Having input this limited information, the design tool

automatically calculates the carbon footprint of the bridge

with a graphical breakdown showing the relative proportions

due to construction, maintenance and traffic delays. The con-

struction element is further subdivided to show the pro-

portions for the deck, substructures and foundations. Such a

presentation allows the bridge designer to see where the major

carbon dioxide burdens are, allowing the focus of design

development to be on the big issues in terms of reducing overall

emissions.

3.3 Default values and user-defined values

Behind this simple summary sheet is a huge amount of data

and considered assumptions. If the user wishes, they can delve

into all of the data and assumptions, amending them to suit

any better information that they may have, or testing the effect

of different design details. As a bridge design develops, the

amount of available project-specific information increases, so

Figure 1. Summary sheet showing minimal initial input

requirements


3

the carbon footprint calculation can be updated, becoming

more and more accurate with each iteration.

The user locked and user values worksheets expand each of

the project elements and provide significantly more detail on

each of the materials or plant used for each item that makes up

the bridge project. Both worksheets are identically laid out,

with the user-defined value functionality being the only

difference between them.

The user locked worksheet sets out default values that the

user cannot edit but can view to see what has been used for the

quantities, proportion of materials attributed to the bridge

element (e.g. the assumed split of concrete between piles and

pile caps), plant used and durations on site, transportation

distances, inspection regimes, maintenance intervals and the

associated traffic management arrangements during construc-

tion and maintenance of the structure. These default values are

underpinned by a series of assumptions discussed further in

Section 4.

The user values worksheet allows adjustment of some or all

the default values to refine the calculation as more information

becomes available. The type of plant, transportation method

and distances travelled can also be adjusted. The editable cells

are coloured green and blue to aid ease of finding and ad-

justing. An option to return all values to their default settings is

also included.

3.4 Construction materials carbon dioxide data

To support further the functionality and usefulness for users of

the tool, additional worksheets are provided that can override

the base carbon dioxide data assumed. This functionality was

included as it is believed that over time further research into

the carbon dioxide emissions will be carried out and the values

for each construction material will be refined. In addition, this

facility enables the user to cater for non-standard materials

such as bespoke concrete mix designs.

3.5 Traffic carbon dioxide data

Implementation of considerations of the impact of traffic delay

and diversion into the design tool is part of its core func-

tionality. This functionality also necessitated an additional

sheet on which the default parameters can be altered by the

user. The override sheet provides the user with options to

change the speed limit of the traffic if a contraflow is adopted

and to amend route diversion lengths if a full road closure is

required.

An option to return to the default settings is provided in this

worksheet.

4. Key assumptionsKey assumptions made in defining the default values in the

bridge carbon calculator design tool are discussed below.

4.1 Assumptions for materials

Over many years there has been a significant amount of

research and development undertaken in the area of quantify-

ing carbon dioxide emissions so the use of freely available

carbon dioxide data were integrated into the carbon calculator.

Carbon dioxide values were primarily sourced from the Bath

University inventory of carbon and energy (Hammond and

Jones, 2008) and the Department for Environment, Food and

Rural Affairs (Defra, 2009) for plant and vehicle emissions.

Carbon dioxide values for steelwork were provided by the

BCSA during several discussions as the commission pro-

gressed. Each of these sources has defined carbon boundaries

and it was vital to understand these boundaries so that the

carbon calculator consistently provided full life-cycle carbon

dioxide values for the life of a bridge.

The Bath University inventory of carbon and energy

(Hammond and Jones, 2008) has a default carbon boundary

of cradle to gate. This is a partial product life-cycle from

resource extraction (cradle) to the factory gate. This means

that transportation of the product from the manufacturers

factory to the site had to be added in separately to the carbon

calculator to provide a whole life value.

The Defra carbon dioxide data concentrate on carbon dioxide

emissions for engines, fossil fuels, electricity and emissions

resulting from different modes of transport and vehicles. From

these data carbon dioxide values per hour or per vehicle

kilometre were used to quantify the transportation of materials

and the duration and use of specific construction plant

throughout the construction and maintenance periods of the

bridge. Incorporation of these data values enabled a full

carbon dioxide life-cycle to be calculated, overcoming the

shortfalls of cradle to gate data values obtained separately.

Key for the BSCA in the development of the carbon calculator

was the use of accurate carbon dioxide figures for steel. The

figures promoted by the BCSA (Anon, 2010) are used in the

bridges carbon calculator and are based on the system

expansion method used by the World Steel Association. The

system expansion method is the most comprehensive assess-

ment method currently available and is the preferred approach

of the BS EN ISO 14040 (BSI, 2006) series of environmental

standards. The method credits the manufacturing processes for

co-products that save energy and emissions, such as process

gases being used to generate electricity. Credit is also given for

the net carbon dioxide that is saved when a product is reused or

recycled. In particular, the method credits the manufacturing

process for the creation of by-products, such as ground


4

granulated blast-furnace slag (GGBS), which can be used as

cement replacement.

The system expansion method is used in the design tool by

default. However, an option is given to use the 50 : 50

approach, which is an average of the system expansion

method and the recycled content method from the Bath

database. For concrete, the use of 100% ordinary Portland

cement (OPC) is taken as the default to avoid double-counting

the benefit of cement replacements, which are already taken

into account in the system expansion method. A warning is

provided to users that reducing the OPC content by includ-

ing GGBS, for example, or other cement replacements will

introduce double-counting unless the steel value is also adjusted

accordingly.

Default assumptions are also made for the transportation and

delivery of construction materials. For example, 200 km is

taken as the default distance for transporting structural steel,

bearings and parapets, and 50 km is taken as the default for

concrete, formwork and surfacing.

There is currently very limited information available for most

proprietary bridge products such as bearings, parapets and joints.

Hence carbon dioxide estimates for these are based on constituent

materials together with simple bridge dimensions such as the

bridge width for joints, bridge length for parapets and deck area

for bearings. Other materials used to construct the bridge, such as

formwork, also need not be explicitly calculated, as default

quantities are generated based on approximate relationships

between the volume of concrete and volume of formwork.

Greater accuracy is seldom warranted because they typically

represent only a small proportion of the total carbon footprint.

4.2 Assumptions for construction plant and traffic

delays

Default assumptions are made for the plant used to construct

the bridge based on the input estimated duration on site. Any

traffic management requirements, commensurate with the type

of road being crossed, have a choice of two defaults

& full closure with night-time working and a diversion for thetraffic (the default diversion length is a function of the road

type selected).

& contraflow incorporating slowing speeds down by 10 mile/hfor a specified distance to account for the lengths of traffic

management required.

The impact of the traffic disruption in terms of carbon

footprint is calculated within the design tool using national

traffic statistics (Defra, 2009) for different types of vehicles and

traffic speeds. Traffic diversions can have a significant impact

on increasing carbon dioxide, which highlights the need to

design for bridge erection and construction with the minimum

of delay to traffic.

4.3 Assumptions for maintenance and inspection

The following default assumptions are made for maintenance.

& There will be four maintenance closures (with defaultduration of five nights) over the lifetime of the bridge with

replacement of bearings every 30 years.

& All major maintenance is undertaken during these closures.& No strengthening to the deck is required for bearing

replacement.

& All ancillary items are replaced like for like.& Concrete repairs are minimal.& One full replacement of the parapets is undertaken in the

bridges life together with one panel replacement each

year to account for accidental impacts during the lifetime of

the bridge.

& All painted steelwork is repainted at each maintenanceclosure.

While the assumption of being able to conduct all major

maintenance conveniently at the same time is optimistic, the

amount of maintenance to be performed is arguably con-

servative to offset some of this.

The following default assumptions are made for inspection.

& Inspections are based on current UK requirements specifiedin the UK design manual for roads and bridges.

& General inspections each of 1 d duration are undertakenevery 2 years.

& Principal inspections each of 2 d duration are undertakenevery 6 years.

It is also assumed that no remote monitoring of the structure is

necessary or provided.

5. Typical steelconcrete composite bridgeexample and comparison

To test the bridge carbon calculator design tool, a comparative

assessment of two bridge designs was carried out between a

steelconcrete composite bridge and a pre-tensioned precast

concrete beam bridge equivalent. This assessment was based on

preliminary design quantities for a typical three-span over-

bridge with open side spans carrying a single carriageway. Both

bridges were designed to the Eurocodes. Details of the two

bridges are discussed below, with commentary on the results

and conclusions arising.

5.1 Common bridge details

The following details were assumed for both bridge options

considered


5

& road overbridge (urban A road classification)& crossing an existing road (urban A road classification)& construction duration of 24 weeks& bridge length of 84 m& bridge width of 17?5 m.

5.2 Steelconcrete composite bridge example

The example steelconcrete composite option considered was of

ladder deck construction with substructures and foundations

representative of current UK best economic practice. This

included adopting single columns under each main girder and

bankseat abutments. Figures 2, 3 and 4 show the general

arrangement of the steelconcrete composite ladder deck bridge.

The steelconcrete composite ladder deck bridge was designed

using the BCSA/Tata Steel preliminary steel composite bridge

design charts (BCSA, 2010; Mitchell et al., 2011), which gave

preliminary but optimised material quantities directly.

5.3 Pre-tensioned precast concrete beam bridge

In order to obtain a like-for-like comparison with the steel

concrete composite design, a pre-tensioned precast concrete

beam bridge of similar dimensions was also considered. Stru-

ctural general arrangements are shown in Figures 58. The

particular form of the pre-tensioned precast concrete beam

bridge chosen represented a bridge recently designed by Atkins

to BS 5400 (BSI, 1990) for another scheme. It was not expected

that the concrete outline or reinforcement quantities would

change significantly when reworked to the Eurocodes.

5.4 Preliminary design quantities and carboncalculations

Table 1 summarises the key preliminary design quantities for

each bridge option considered.

The results obtained from the bridge carbon calculator design

tool are shown in Figures 9 and 10 and are discussed in the

following sections.

5.5 Carbon dioxide emissions for design and

construction material quantities

The preliminary bridge details and the materials quantities

listed in Table 1 were inserted into the summary sheet of the

bridge carbon calculator for each bridge option. The default

values were then overridden with user-defined values to adjust

the details of the reinforcement content percentages and the

number and type of bearings used. These adjustments were

Steelconcrete composite ladder deck solution

B

B

End span

0 1 1 1 1 1 1 11 1 00* 0* 0 0* 0* 0* 0* 0*0 01

24 m

End span

24 m

Internal span

Elevation

*Spread footing asrequired in brief??

36 m

A

A

Figure 2. Steelconcrete composite ladder deck example general

arrangement: elevation

500 5002000

1500 1500

1600 1600

2000

Section AA (NTS)With intermediate

cross-girder

Section BB (NTS)With main

cross-girder

17 464

12 464

14 264


arrangement: sections


6

made in the user values worksheet. The total carbon dioxide

emissions (for material quantities only) for both bridges are

shown in Table 2, illustrating both the default and user-defined

value results for both bridge options.

The comparative assessment for material quantities only, using

the default assumptions and values, suggested that the steel

concrete composite bridge had a 13% lower carbon footprint

than the equivalent precast pre-tensioned concrete bridge.

500 1600 1600

5800 4000

Foundation arrangement

1200

1500 Pier1500 Pier

1

1

Section 111600 1600 500

500 5001500 1500


arrangement: piers

* Bridge square in plan

B

B Five beams Nine beams Five beams

A

End span

24.22 m

Internal span

36.079 m

End span

24.22 m

Figure 5. Pre-tensioned precast concrete beam bridge example

general arrangement: elevation


7

The key items that differ between the two bridges are the

bearings and the reinforcement percentages. Within the design

tool the default assumption for the reinforcement percentage in

reinforced concrete is 2?5%. For the concrete bridge option the

reinforcement percentage value used for the total concrete

volume of the deck and the beams combined was 3?5%. This

takes into account an average between a higher density (5% in

the deck) and lower density (1?9% in the beams) as noted in

Table 1, because the reinforced concrete volumes for deck and

beams are not separate entries in the tool. The equivalent value

of reinforcement for the concrete beam includes links and an

allowance for prestressing strand (using an effective increased

Summary of quantities

Steelconcrete composite bridge Pre-tensioned precast concrete beam bridge

Quantities Reinforcement content: % Quantities Reinforcement content: %

Foundations: m3

Pile cap 261 1 319 1

Precast piles 301 1 315 1

Total 562 634

Substructure: m3

Abutments 212 2 212 2

Piers (incl. diaphragms) 49 2 250 2

Wingwalls 22 2 22 2

Total 283 483

Superstructure

Articulation: number of

bearings

Mechanical 8 0

Elastomeric 0 10

Total 8 10

Structural steel: t

Painted steel 222 0

Weathering steel 0 0

Total 222 0

Reinforced concrete: m3

Deck 346 5 346 5

Beams 0 0 362 1?9a

Total 346 708

aEquivalent value of reinforcement includes links and allowance for prestressing strand (using increased weight of strand toaccount for increased carbon dioxide unit rate of steel coil)

Table 1. Quantities from preliminary design

Carbon dioxide emissions for steelconcrete

composite bridge: tCO2

Carbon dioxide emissions for pre-tensioned precast

concrete beam bridge: tCO2

Default values User-defined values Default values User-defined values

Foundation 313?6 234?6 352?4 263?2

Substructure 221?8 208?6 400?9 378?4

Superstructure 673?6 753?5 636?6 633?5

Total 1209?0 1196?7 1389?9 1275?1

Table 2. Total tonnes of carbon dioxide emissions for design and

construction (materials only)


8

weight of strand to account for the increased carbon dioxide unit

rate of steel coil). Adjusting the user-defined values for rein-

forcement content and bearing type (elastomeric for the concrete

option and mechanical for the steel, as noted in Tables 1 and 2)

further refines the carbon footprint for both designs.

When the default values were altered to more accurately

known values, the totals showed that the carbon dioxide

difference between the two bridges reduced from 13% to

around 6%. There was a negligible difference between the

default and user-defined values for the steelconcrete compo-

site bridge; differences for the precast concrete bridge arose

mainly from adjusting the default reinforcement quantities

where they are less appropriate for precast beams.

It should also be noted that mechanical bearings (used for the

steelconcrete composite bridge) have higher embodied carbon

dioxide than elastomeric bearings (used in the pre-tensioned

concrete bridge). In this particular example, the elastomeric

bearings had only 37% the emissions of the mechanical bearing

even though a greater number had been assumed to be required

(ten compared to eight).

5.6 Carbon dioxide emissions for inspection and

maintenance

The total carbon dioxide emissions for the inspection and

maintenance phases of each option are shown in Table 3, again

illustrating both the default and user-defined value results for

both bridge options.

For inspections, both bridges have the same total carbon

dioxide emissions due to the general requirements used for the

assumptions, as set out in BD 63/07 (HA, 2007).

For the maintenance of each bridge option, considering just the

default assumptions and values, the total carbon dioxide

Carbon dioxide emissions for steel concrete

composite bridge: tCO2

Carbon dioxide emissions for pre-tensioned precast

concrete beam bridge: tCO2

Default values User-defined values Default values User-defined values

Inspection 17?4 17?4 17?4 17?4

Maintenance 288?7 268?5 351?3 144?6

Total 306?1 285?9 368?7 162?0

Table 3. Total tonnes of carbon dioxide emissions for inspection

and maintenance

500 2000

Footway

Precastpre-tensionedU-beamusing existingbeamdimensions

Detail A

1600 1783 1783 1783 1783 1783

Section AA (NTS)

500

500

250

Parapet

Edge beam

Detail A (NTS)

1783 1783 1783 1600

Footway

2000 500

17 464

12 464

Carriageway


general arrangement: main span section


9

burden is greater (by 20%) in the concrete design in com-

parison to the steel. This is mainly due to the input of extra

bearing numbers being specified (ten compared to eight) and

thus being assumed to require replacement. By default, the

bearings are assumed to be mechanical types for both options

and there is an assumed allowance for concrete removal and

repair associated with each bearing replacement. In addition,

due to the higher total concrete quantities required for the

concrete bridge option, there is a higher carbon dioxide burden

for concrete repairs over the design life of the structure. When

using the user-defined values to specify elastomeric bearings,

however, for the concrete bridge option, the carbon dioxide

burden of maintaining the bridge is reduced significantly below

that for the steel option (46% lower).

5.7 Carbon dioxide emissions for site set up

For both bridge options the site set-up assumptions are the

same and have a minimal effect on the total carbon footprint

for a project. At the preliminary design stage, very little, if any,

information would be known for making major adjustments to

the default assumptions built into the design tool, but the

influence is not large enough to warrant further effort until

much later in the project.

5.8 Carbon dioxide emissions for traffic delay

For both bridge options the traffic delay is similar due to the

classification of roads being crossed and a new road being built

on the bridge. This aspect of the total carbon footprint is the

most significant, accounting for around 75% of the total

carbon dioxide emissions for these particular examples. It

should be noted that the road classification adopted for this

comparison study is urban A roads only. Altering these road

1600

Precastpre-tensioned

U-beam 1600

500

3566

Section BB (NTS)

Carriageway20002000500

Footway Footway

17 464

12 464

3566 3566 3566


general arrangement: back span section

Use diaphragmand beam depthfrom existingdesign

1200 mmdiameterpier

Pier

5955

732

1600Typical Typical

1600

1200

500 1500 1500

4000

12.0 mto foundinglevel

500

18.468 m

732

5955

17 464

Pier Deck

Pre-tensioned beamDeck/pier

Pierpilecap

Diaphragm


general arrangement: piers


10

classifications to higher trafficked roads will have an even more

significant effect on a total project and in some cases may

render the total carbon dioxide embodied in the construction

materials insignificant by comparison, particularly for motor-

way construction. This highlights that choosing erection and

construction methods to minimise traffic disruption can be key

to minimising the burden of carbon dioxide emissions on

bridge projects. The same logic and conclusions apply to

eliminating the need for significant traffic management during

operation and maintenance.

5.9 Conclusions from the comparative assessment

The embodied carbon dioxide in the steelconcrete composite

bridge was shown to be marginally lower than that in the

similar pre-tensioned precast concrete beam bridge. The

difference was, however, very small and cannot necessarily be

regarded as a general conclusion for all such bridges for a

number of reasons, including the following.

& The preliminary steelconcrete composite bridge design chartsoptimise the design of the steel sections for the steelconcrete

composite bridge option, but the same level of optimisation

could not be achieved for the pre-tensioned precast concrete

beam design in the limited time available for this study.

& An independent design team undertook the concrete bridgedesign, which may have led to minor variations in design

assumptions.

& Good design and detailing to provide an efficient use ofmaterials and ease of construction and maintainablity is key

to minimising carbon emissions and is much more

important than the actual choice of materials.

Notwithstanding the material quantities, the comparative

assessment illustrated that traffic delay was the main con-

tributor to carbon dioxide emissions for the construction and

maintenance of the bridge, accounting for around 75% of the

total emissions. Investigating options for managing the traffic

and keeping it free flowing is therefore fundamental to

reducing the overall emission burden. Such options might

include night working, alternative diversion route planning

(both of which are provided as options to change to from

the default assumptions in the design tool), or looking at

Figure 9. Results for steelconcrete composite bridge example


11

high-performance materials and low-maintenance designs to

reduce the maintenance intervention requirements and sub-

sequent impact on traffic.

6. ConclusionThe development of a bridge-specific carbon calculator design

tool has enabled a consistent approach to be taken in assessing

the carbon footprint of various bridge options at the preliminary

design stage of a project. The tool has been written to enable

rapid calculation of the carbon dioxide burden for bridge

schemes with minimal, basic input required, yet also be flexible

enough to incorporate new data and adjust the default as-

sumptions as the project develops and more detailed informa-

tion becomes available. It is intended that the design tool will be

used by bridge designers to help

& quantify the embodied and operational carbon dioxide ofsteelconcrete composite bridges at all stages in their life-cycle

& facilitate reductions in the carbon dioxide footprint duringconstruction and maintenance by allowing alternatives to be

rapidly assessed

& benchmark projects to facilitate continuous improvementwith respect to carbon dioxide.

While the design tool was initially developed for typical

highway bridges in the UK, it was written to be flexible enough

for use with rail bridges, footbridges and smaller highway

bridges. The flexibility of the tool also allows international use,

with careful adjustment and consideration of some of the

default assumptions, including default distances for travel. The

design tool is principally intended for concept and preliminary

design to assist with evaluating the benefits and disbenefits of a

range of possible bridge options.

Some of the key findings from assessing the carbon foot-

prints of a number of bridges with the design tool include the

following.

& Carbon dioxide per square metre of deck area forconstruction of a typical bridge is somewhere between 1 and

3 t if traffic impacts are ignored; typically 2 t/m2 for routine

construction.

Figure 10. Results for pre-tensioned precast concrete beam bridge

example


12

& Typically one-third to half the carbon dioxide emissionsfor a bridge is produced during maintenance, so

consideration of good design for maintenance is

important.

& Minimising traffic delay is key for the construction phaseand maintenance in terms of limiting the effects of traffic

delay on carbon dioxide. This is usually much more

significant in terms of carbon dioxide than the embodied

carbon in the materials themselves, so good design with

respect to ease of construction and maintenance is therefore

important.

& Comparisons between steel, concrete and steelconcretecomposite indicate that their embodied carbon contents are

similar and that the benefits of an efficient design are much

more significant than the choice of material.

& Embodied carbon dioxide in the design itself is roughlyproportional to total bridge cost, so advances in analysis

techniques and codes lead to more efficient designs in terms

of both carbon and cost.

A freely downloadable version of the design tool is available

from http://www.steelconstruction.org/resources/sustainability/

bridges-carbon-calculator.html.

AcknowledgementsThis paper is published with the permission of the BCSA and

Tata Steel. The authors acknowledge John Dowling (BCSA)

for his valuable input during the development phase and in

checking and testing, and Louisa Man (Atkins) for providing

subsequent thorough reviews of the completed tool.

REFERENCES

Anon (2010) The Carbon footprint of steel. New Steel

Construction Magazine, January: 3233.

BCSA (British Constructional Steel Association) (2010)

Preliminary Steel Composite Bridge Design Charts. see

http://www.steelconstruction.org/resources/technical/

bridges-preliminary-design.html (accessed 13/01/2013).

BSI (1990) BS 5400-4:1990: Steel, concrete and composite

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SearchResults.aspx?name5GEHO0712BWTW-E-X

(accessed 12/02/2013).

HA (Highways Agency) (2007) DMRB (Design Manual for

Roads and Bridges) BD 63/07 Volume, Section 2.

Highway Structures: Inspection and Maintenance

Inspection of Highway Structures. Highways Agency,

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Hammond G and Jones C (2008) Inventory of Carbon & Energy

(ICE). University of Bath, UK, see www.bath.ac.uk/mech-

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Mitchell RP, Smith DA and Dolling C (2011) Updating preliminary

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Proceedings of the ICE Bridge Engineering 164(4):

185194.

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