1 4 6 Design Illustration Concrete Bridge Design

PA Jackson, GM Walker page 1

DESIGN ILLUSTRATION – CONCRETE BRIDGE DESIGN Paul Jackson, Gifford, Southampton, UK Graeme Walker, Gifford, Southampton, UK

Abstract A worked example for an integral precast prestressed beam and in situ slab bridge designed to

the Eurocodes has been published as a design guide. The purpose of the guide is to help ease

the transition to designing to Eurocodes for UK bridge engineers, in conjunction with the

other guidance documents that have been produced. This paper introduces the design

example, discusses key issues it raised and makes comparisons with previous UK practice.

Introduction The Concrete Bridge Development Group decided that an advisory publication

[1] was needed

to help ease the transition to Eurocode design for UK bridge engineers. The guiding objective

of their publication was to help bridge engineers as much as possible, rather than to sell the

publication. Because of this, it was designed to complement, rather than compete with, other

publications. The only specifically concrete bridge publication known to be in preparation

was the Thomas Telford Guide[2]

. This is a clause by clause guide. It was felt the most

helpful complementary document would be a complete design example. It was further

decided that a precast prestressed beam and in situ slab structure would be best to illustrate as

many aspects of the code as possible. An integral bridge was used because current thinking

favoured such designs.

At the time work was started in 2004, EN 1992-2 itself was not fully finalised, its NA was in

very early phases and the decision to issue “PDs” had not been made. It was intended,

however, to publish the design example in time for implementation which it was still

anticipated would be in time for the intended two year “co-existence” period before

withdrawal date in 2010. In the event, implementation got delayed but some of the

documents were delayed further. In particular, despite delay in finalising the example, it

proved necessary to complete the design example with only a draft of PD 6694-1

Recommendations for the design of structures subject to traffic loading to BS EN 1997-

1:2004[3]

. This meant the coverage of specifically integral bridge issues could not be as

complete as originally hoped as it was this part of PD 6694-1 which was most subject to

change.

In this paper the design example will be described, some issues it raised discussed and

comparison will also be made with BS 5400 design. It is not possible to cover every stage of

the design process in this paper. Instead, it will concentrate on those areas which differ most

from previous practice or those where significant savings can potentially be made.

Scheme Design The scheme chosen for the design example was a two span integral bridge, with equal spans

each having a length of 20.0m. The bridge carries a 7.3m wide carriageway with a 2.0m wide

P A Jackson, G M Walker 2

footway on either side. The superstructure consists of eight standard precast, pretensioned

concrete Y beams with a 160 mm deep in-situ reinforced concrete deck slab cast on ribbed

permanent GRC formwork. There are in-situ diaphragms at the abutments and pier.

The superstructure is made integral with the substructure. The foundations for the bridge

consist of precast concrete piles with in-situ pile-caps. The pile-caps at the abutments are

integral with the end diaphragms, while the pier wall is rigidly fixed to both its pile cap and

the central diaphragm, avoiding the need for bearings altogether and simplifying the

construction.

The integral abutments are small and the piles relatively flexible in order to avoid excessive

reactions resulting from thermal expansion of the deck. However, there is still sufficient fill

behind the abutment diaphragms to resist longitudinal acceleration and braking forces.

Transverse Section

Elevation (Half in Section)

Figure 1. Details of integral bridge

Model and Analysis The global analysis of the deck was carried out using a grillage model with eight longitudinal

members at 1.5m centres representing the precast beams and associated sections of deck slab,

and transverse members at 1.85m centres. The restraint provided by the pier and abutments

were represented by rotational springs. The superstructure and sub-structure could have been

modelled together in a single 3D model, but the practicalities of the design process mean that

they may often be considered separately – in this case, the PD6694-1[3]

, which was

particularly relevant to the substructure design, was not available when the design of the

superstructure commenced.


The analysis also had to take into account the construction sequence and the resulting

distribution of load. As the bridge does not become continuous until the deck slab and

diaphragms are cast and the concrete set, all dead load for the main part of the bridge is

carried by the precast beams alone. Therefore, the load effects of the dead weight were

analysed using a simple line beam model, pinned at the abutments and pier, and the results

added to those of the grillage model. However, this approach was only considered for the

serviceability limit state (SLS) – at the ultimate limit state (ULS) the strain discontinuity

between the precast and in-situ concrete is not worth considering and all results are obtained

from the grillage model.

The flexural stiffness of the concrete members used in the analysis was generally calculated

assuming uncracked cross-sections. However, BS EN 15050:2007[4]

, Annex D recommends

that cracked section properties be used for the reinforced concrete diaphragm over the pier for

the calculation of hogging moments, and this approach was used here.

Materials and Cover Class C50/60 concrete was used for the precast beams and C35/45 used for all in-situ

concrete, with the associated material properties taken from BS EN 1992-1-1:2004[5]

, 3.1.

The exposure classes are specified in Section 4 of BS EN 1992-1-1:2004 and BS EN 1992-

2:2005[6]

and in BS 8500-1:2006[7]

. The example bridge is assumed to be passing over a

carriageway, and so this is classified as XD3 (exposed to spray containing chlorides). The

bridge soffit is more than 5 metres above the carriageway and so according to the UK

National Annex to BS EN 1992-2[8]

, NA.4.2(106) does not have to be classified as XD3,

though it does not explicitly specify what it should be classified as. XD1 (exposed to airborne

chlorides) would appear most suitable and is confirmed by BS 8500-1: 2006. The top of the

deck is protected by waterproofing and so the UK National Annex to BS EN 1992-2,

NA.4.2(105) allows this to be classified as XC3.

The minimum cover for durability is a factor of exposure class and concrete class and type.

The UK National Annex to BS EN 1992-1-1[9]

specifies that minimum cover requirements

should be taken from BS 8500-1:2006, rather than Tables 4.3N, 4.4N and 4.5N of BS EN

1992-1-1:2004. To this minimum cover must be added an allowance for deviation. Values for

this were taken from the Highways Agency’s Interim Advice Note 95/07 Revised guidance

regarding the use of BS 8500(2006) for the design and construction of structures using

concrete[10]

, which is slightly more onerous than the minimum values given in the UK

National Annex to BS EN 1992-1-1:2004.

The proposed European standard for prestressing steel, EN 10138 Prestressing Steels, has yet

to receive a positive vote. While it is likely to be published in the future, in the meantime BS

5896:1980[11]

has been amended to cover 1860 grade 15.7mm strand.

Reinforcing steel is covered by BS EN 10080:2005[12]

and BS 4449:2005[13]

– it is the latter

that specifies the required properties for the standardised grades.


Actions Actions can be divided into three main types – permanent, variable and accidental actions.

Permanent actions include self weight, settlement and differential shrinkage. Material

densities for calculating self weight can be obtained from BS EN 1991-1-1:2002[14]

, Annex A.

Differential settlement was arbitrarily taken as 20mm for this example - A more rigorous

estimate should be calculated for a real structure. Generally, the effect of settlement need

only be taken into account at SLS, as there is sufficient ductility to justify ignoring it at ULS

(BS EN 1992-1-1:2004, 2.3.1.3(3)). Differential shrinkage, due to the in-situ deck being cast

after the precast beams and so shrinking more relative to them, causes tension within the deck

slab, compression within the beams and an overall sagging of the deck. As with settlement,

provided the bridge has adequate ductility, this effect can be neglected at the ultimate limit

state (BS EN 1992-1-1:2004, 2.3.2.2(2)). Shrinkage is discussed further later in this paper.

Variable actions comprise wind, thermal, construction and traffic loads. Wind loading on

heavy, short span bridges such as this is not a critical loadcase, and so was neglected in this

example.

BS EN 1991-1-5:2003[15]

gives two options subject to National Determination for differential

temperature in bridges but, with the options specified in the UK National Annex to BS EN

1991-1-5[16]

, it is very similar to BS 5400-2:2006[17]

, having largely been based on this code.

Traffic loads differ more compared to BS 5400-2:2006, but are generally simpler. Before any

traffic loads can be applied, the carriageway must be divided up into notional lanes, as

specified in BS EN 1991-2:2003[18]

, Table 4.1. For the 7.3m wide carriageway considered in

the example, two 3.0m wide notional lanes and a remaining area 1.3m wide are defined. As

with BS 5400-2:2006, the positions of these notional lanes do not have to correspond with the

lane markings on the bridge. Instead, the lanes and the remaining area are positioned to create

the most severe load effect for each element under consideration.

Three traffic load models were applied to the bridge – Load Model 1 (LM1), which is the

equivalent to HA loading in BS 5400-2:2006, LM 2, a single axle load, and LM 3, which

represents abnormal vehicles.

LM1 comprises a UDL and a double axle loading, referred to as a “tandem system” or “TS”

in each lane, and a UDL in the remaining area. Thanks to the nationally determined

adjustment factors given in the UK National Annex to BS EN 1991-2[19]

, this UDL takes a

constant value of 5.5kN/m2 across all lanes and the remaining area, irrespective of the number

of lanes or the loaded length. This greatly simplifies analysis.

Load Model 2 (LM2) is a single axle load, 33% heavier than one of the axles of the LM1

tandem system. However, LM2 is not combined with other traffic models so is generally only

critical for local effects – for example, when considering transverse flexure or punching shear

in the deck slab of this design example.

Load Model 3 (LM3) represents Abnormal Vehicles. A series of load models are defined in

the UK National Annex to BS EN 1991-2:2003 for the design of UK road bridges. These will


be familiar to those who have used BD 86/07[20]

. There are three SV80, SV100 and SV196

representing STGO vehicles with maximum gross weights of 80t, 100t and 196t gross train

weight respectively, the last being a 150t vehicle with a separate tractor. There is also a

further series of models representing heavier Special Order Vehicles which are equivalent to

the old “AIL” vehicles whish were not covered by BS 5400. It should be noted that dynamic

amplification factors must be applied to the axle loads of LM3 – these are already

incorporated into the other load models. The UK National Annex states that the choice of

load model should be determined for each individual project. The choice of SV196 for this

design example, which is assumed to be carrying a trunk road, is in line with the best current

guidance, the Highway Agency’s Interim Advice Note 123/10 Use of Eurocodes for the

design of highway structures[21]

.

There is a fourth load model, LM4, which represents crowd loading. By inspection, this load

model, which comprises a UDL of 5kN/m2 across the entire deck, was not critical for this

bridge and so was not considered. It appears in the UK it will rarely be critical. It is required

in BS EN 1991-2:2003 because with the “recommended values” the LM1 UDL is less severe

than LM4 except in Lane 1.

Creep and Shrinkage The analysis of creep and shrinkage is little changed from BS 5400-4:1990

[22] but the

derivation of the free strains, particularly for shrinkage, is. BS 5400-4:1990 contained simple

figures in the main text for estimating prestress losses and a more detailed appendix which

often gave very different results, particularly for timescale. The approach in the BS 5400-

4:1990 appendix was based on the 1978 edition of the CEB/FIP model code but that in EN

1992 is based on the 1990 edition[23]

. It splits shrinkage into “drying” and “autogenous”

components. This gives more realistic results as the two components have very different

sensitivities to the various variables considered. The newer approach also uses formulae

rather than diagrams which is more convenient when using spreadsheets.

SLS Design The serviceability limit state (SLS) governs for most prestressed structures. Three sets of

checks can be required – decompression, crack widths and stress limits. Reinforced concrete

elements only require crack widths and stress limits to be checked. However, SLS

considerations will rarely govern for reinforced concrete, particularly as the crack width

criteria are less onerous than in BS 5400-4:1990.

In a departure from previous practice, SLS checks even in prestressed concrete, are carried

out using cracked section analysis whenever the tensile flexural stress exceeds the effective

tensile strength, fct,eff. However, this will normally only arise when either there is no chloride

exposure so decompression does not have to be checked or when sections are being checked

which do not have tendons close to the tension face. Sections which are checked for

decompression and have tendons close to the tension face, such as here, will usually continue

to be analysed using an uncracked section. Where cracked sections are considered, it makes

the calculations more involved but this is generally not a problem as the cracked section

analysis can be done by computer. However, it does mean that the prestress can no longer be

calculated directly from a set of equations: as with crack width checks of RC, it is necessary

to assume a design and check it


Decompression For XD (chloride) exposure, the decompression limit is checked under the frequent load

combination (i.e. with a reduced “frequent” value of Load Model 1, and no Load Model 3

(abnormal vehicles), as specified in BS EN 1991-2, Table 4.4b). All concrete within a

distance from the tendons equal to the minimum cover distance required for durability

(cmin,dur) must remain in compression (UK National annex to BS EN 1992-2:2005, NA.2.2).

For the strand pattern shown in Figure 2, the concrete located cmin,dur (30mm) below the lowest

tendons remained in compression at midspan. If prestressing tendons had been included near

the top of the beam, then decompression of the concrete around these tendons due to hogging

over the pier would have had to have been checked as well.

In parallel with the development of the design example, the same bridge was designed to BS

5400 for comparison. It was found that in terms of prestressing requirements, the two designs

came out almost identical. However, if the bridge had been passing over a train track or fresh

water river, the soffit of the beams would not be exposed to chlorides. Under these

circumstances the UK National annex to BS EN 1992-2:2005, NA.2.2 does not require

decompression to be checked under the frequent load combination. Instead, only crack widths

need be checked under this load combination, with decompression being checked under the

less onerous quasi-permanent load combination (i.e. no traffic load since 2 = 0 for traffic

actions). It was found that in this case, the level of prestress could be reduced by

approximately 25%, or the beam section size reduced, either of which would result in

significant savings.

Transfer and tendon arrangement Along with decompression in service, transfer will often govern the design. BS 5400 had an

explicit requirement for concrete that goes into tension at transfer, namely that the tension

should not exceed 1N/mm2. This normally required strand in the top of pretensioned beams.

EN 1992 has no equivalent and so the normal rules apply. Without the 1N/mm2 tension

allowance, this can be marginally more restrictive. However, there is an irony that the top

strand is there to comply with decompression criterion which only has to be complied with

because the top strand is there. If the strand is not provided, it would only be necessary to

check crack width at the top of the beam. This would enable you to dispense with the top

strand completely, but you would have to provide reinforcement in its place to control crack

widths. In purely material terms, this would probably be more economic, if only because the

reinforcement could easily be curtailed where it was not needed. However, in the practical

situation of precasting, it is not clear this would really represent a saving. For this particular

case, it was found that the use of reinforcement in the top of the beam made it possible to

eliminate debonding of the lower tendons, which would result in further cost savings. An

alternative approach would be to limit the top stress to fct,eff using strands nearer the top than

in Figure 2 but still located low enough down so that the decompression requirement is

complied with at transfer.


Figure 2. Beam section showing strand positions

BS EN 1992-1-1:2004, 5.10.2.2 places a limit on the maximum concrete stress at transfer of

60% of the characteristic concrete strength at the time of transfer. This may be increased to

70% if “it can be justified by tests or experience that longitudinal cracking is prevented.” It is

expected that precast concrete manufacturers will use this increase and it was used in this

example

BS EN 1992-1-1:2004 explicitly states that the maximum transfer stress is derived from a

characteristic value of concrete strength – this is a departure from previous practice, where

BS 5400 was less specific and a value derived from a small number of cubes (often the lowest

of three) was commonly used. It is expected that in practice, rather than casting enough cubes

to derive an accurate characteristic strength each time, precasters will use a small number of

cubes in conjunction with historic records of the concrete variability that they achieve.

Stress limits Because the same limit to compressive stress is used in prestressed and RC, the compression

limit in prestressed is higher than before. However, this will rarely give any advantage in this

type of structure. Cracked section properties will have to be used for prestressed sections

where the effective tensile stress is exceeded, but in practice this will cause little extra work

for most engineers who routinely use computer programs for such calculations anyway.

Crack widths For the bridge deck in this example, crack widths only had to be checked in the hogging

region of the deck over the central pier (the hogging moment at the abutments being lower)

and in the top of the beam at transfer. In principle, crack width in the deck slab in the

transverse direction would also have to be considered but, because of the quasi permanent

combination considered, this would clearly not be critical. Two methods are presented for

checking crack widths. The method involving direct calculation looks somewhat complicated

due to the large number of parameters and so this method was used in the design example to

demonstrate and, hopefully, clarify the procedure.


There is also a simplified method that avoids direct calculation and it is believed that this

method will be used by engineers in most instances. This method tends to be less

conservative, apparently because it is based on a lower cover value than is usual on bridges,

but for this particular example would have given the same bar spacing as the calculation

method. However, ULS and fatigue considerations governed rather than crack widths, as

might be expected for the reinforced concrete deck and diaphragms.

ULS Design

Flexure and shear EN 1992 differs little from previous practice with regards to flexure – unsurprising given that

flexure is the best understood aspect of concrete structural design.

There is a significant difference between EN 1992 and BS 5400-4:1990 with regards the

calculation of shear resistance. As will have been discussed elsewhere in this conference, the

EN 1992 approach will generally result in prestressed members having more shear links than

they would have required if designed to BS 5400-4:1990. However, for the design example,

this difference was not significant, as it was the requirement for interface shear reinforcement

between the precast beams and deck that governed the spacing of the shear links, with B12

bars at 75mm centres required close to the supports, compared to 250mm centres for flexural

shear resistance

Fatigue EN 1992 provides three methods for verifying the fatigue resistance of the reinforcement.

The first is to calculate the cumulative fatigue damage due to the predicted cyclic load history

of the structure. This is unlikely to be practical for most structures.

BS EN 1992-1-1, 6.8.6(2) allows a simplified method (which cannot be used for the tendons)

whereby provided the stress range in the bars due to the cyclic (live load) portion of the

frequent load combination is less than a certain value, k1, the fatigue resistance can be

considered adequate. The recommended value of k1 = 70 MPa is retained by the UK National

Annex, but is increased (albeit in this particular case only to 85MPa) in PD 6687-2:2008[24]

,

Table 2A as discussed in the earlier paper on that document. For the design example, the

stress in the deck reinforcement over the pier was 128MPa, indicating that the area of steel

required for flexural resistance needs to be increased by 50% to provide adequate fatigue

resistance.

However, the alternative “damage equivalent stress range” method is significantly less

conservative, if more time consuming to carry out. The stress range in the bars or tendons due

to a special load model (Fatigue Load Model 3) is calculated. This stress range is then

multiplied by a series of factors that allow for site specific factors such as traffic volume or

design life. The fatigue resistance is deemed adequate if this modified stress range is less than

the permissible stress range for the bar or tendon at N* cycles. For the reinforcement over the

pier the permissible stress range is 141 MPa and the calculated damage equivalent stress

range is just lower than this, at 140 MPa. It can therefore be seen that significant reductions in

reinforcement may be possible through the use of the damage equivalent stress range method

compared to the simplified method.


Pier Design When considering the pier, the forces due to the loading on the bridge deck can be obtained

from the support reactions on the grillage model of the deck. In addition, the effects of

accidental vehicle impact on the pier must be considered. The loading used is similar in

principle to BD 60/04[25]

and comprises two components which must be considered together,

a main and residual force. Two directions need to be considered – forces parallel to and

perpendicular to the carriageway. However, only one direction should be considered at a

time. When considering the accidental loads, it must be remembered that action factors are

not used and different material partial factors are specified for accidental loads from those

used for ULS cases.

Imperfection in the construction of the pier means that axial loads can produce additional

moments. BS EN 1992-2:2005 requires that this is taken into account in the analysis in two

ways. Firstly, it is assumed that any axial load is applied with an eccentricity of 1/30th

of the

depth of the section (but not less than 20mm). Secondly, the column is assumed to deviate

slightly from the vertical, while the axial force remains vertical. These effects produce

additional moments that are additive to those obtained from the grillage model of the deck.

Tall, thin piers may buckle under axial loading. The likelihood of this is related to the

slenderness of the member. Provided the slenderness of an axially loaded member is below a

limiting value then buckling does not need to be considered – the pier considered in the

design example was relatively stocky, with a length to depth ratio of 10.2 and so buckling

could be neglected.

Conclusion and Comments This paper has considered the design of a prestressed beam and slab concrete integral bridge

designed to the Eurocodes. This design has been published as a worked example and

accompanying commentary to assist UK bridge engineers in the transition to Eurocodes.

Some of the key conclusions that can be drawn from the design example are:

i) For structures subject to de-icing salt or marine environments, the design of

prestressed sections of highway bridges to BS EN 1992-2:2005 comes out very similar

to BS5400. It appears from other calibration work that this will always be the case for

“normal” structures where the tendons are reasonably close to the tension face at the

critical sections.

ii) Where not exposed to chlorides, significant savings can be made in the amount of

prestress required when compared to BS 5400.

iii) The simplified method for carrying out fatigue checks appears to be very conservative,

and significant savings in reinforcement quantities will be possible in many situations

if the “damage equivalent stress range” method is used instead.

However, the most important conclusion is that the design of concrete bridges to the

Eurocodes is no harder than it was to BS 5400, it’s just different.


Acknowledgements The authors would like to acknowledge the support of the Concrete Bridge Development

Group, and Alan Tovey in particular, in the development of this design example. Thanks also

go to the CBDG Technical Committee and to Barry Skinner of Bestech and Owen Brooker

and Charles Goodchild of TCC and others for their comments and input into the design

example as well as to Peter Takács (now of Grontmij) for early work on the design and to

Stephen West for work on the geotechnical aspects.

References

[1] Jackson, P.A, Takács, P.F., Walker, G.M. and West S. (2010). Integral Concrete

Bridges to Eurocode 2. Concrete Bridge Development Group and the Concrete

Society, UK.

[2] Hendy, C.R. and Smith, D.A. (2007). Designers’ Guide to EN 1992. Eurocode 2:

Design of concrete structures. Part 2: concrete bridges. Thomas Telford, UK.

[3] PD 6694-1 (2008 – Draft for public comment). Recommendations for the design of

structures subject to traffic loading to BS EN 1997-1:2004. BSi, London, UK.

[4] BS EN 15050:2007. Precast concrete bridge elements. BSi, London, UK.

[5] BS EN 1992-1-1:2004 incorporating Corrigendum January 2008. Eurocode 2: Design

of concrete structures – Part 1-1: General rules and rules for buildings. BSi, London,

UK.

[6] BS EN 1992-2:2005 incorporating Corrigendum July 2008. Eurocode 2: Design of

concrete structures – Part 2: Concrete bridges – Design and detailing rules. BSi,

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[7] BS 8500-1:2006. Concrete – Complementary British Standard to BS EN 206-1 – Part

1: Method of specifying and guidance for the specifier. BSi, London, UK.

[8] UK National Annex to BS EN 1992-2 (2007). UK National Annex to Eurocode 2:

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BSi, London, UK.

[9] UK National Annex to BS EN 1992-1-1:2004 incorporating National Amendment

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[10] Interim Advice Note 95/07 (2007). Revised guidance regarding the use of BS

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[11] BS 5896:1980 incorporating Amendment No. 1 (2007). Specification for high tensile

steel wire and strand for the prestressing of concrete. BSi, London, UK.

[12] BS EN 10080:2005. Steel for the reinforcement of concrete. BSi, London, UK.

[13] BS 4449:2005. Steel for the reinforcement of concrete – Weldable reinforcing steel –

Bar, coil and decoiled product – Specification. BSi, London, UK.

[14] BS EN 1991-1-1:2002 incorporating Corrigendum No. 1 (2004). Eurocode 1: Actions

on structures – Part 1-1: General actions – Densities, self-weight, imposed loads for

buildings. BSi, London, UK.

[15] BS EN 1991-1-5:2003 incorporating Corrigendum No. 1 (2004). Eurocode 1: Actions

on structures – Part 1-5: General actions – Thermal actions. BSi, London, UK.


[16] UK National Annex to BS EN 1991-1-5 (2007). UK National Annex to Eurocode 1:

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[17] BS 5400-2:2006. Steel, concrete and composite bridges – Part 2: Specification for

loads. BSi, London, UK.

[18] BS EN 1991-2:2003 incorporating Corrigendum No. 1 (2004). Eurocode 1: Actions on

structures – Part 2: Traffic loads on bridges. BSi, London, UK.

[19] UK National Annex to BS EN 1991-2:2003 incorporating Corrigendum No. 1 (2008).

UK National Annex to Eurocode 1: Actions on structures – Part 2: Traffic loads on

bridges. BSi, London, UK.

[20] BD 86/07 (2007). The assessment of highway bridges and structures for the effects of

special types general order (STGO) and special order (SO) vehicles. The Highways

Agency, UK.

[21] Interim Advice Note 123/10 (2010). Use of Eurocodes for the design of highway

structures. The Highway Agency, UK.

[22] BS 5400-4:1990. Steel, concrete and composite bridges – Part 4: Code of practice for

design of concrete bridges. BSi, London, UK.

[23] CEB/FIP model code 1990 (1993). Design Code. Thomas Telford, UK.

[24] PD 6687-2:2008. Recommendations for the design of structures to BS EN 1992-

2:2005. BSi, London, UK.

[25] BD 60/04 (2004). The design of highway bridges for vehicle collision loads. The

Highways Agency, UK,

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