Improved appearance of bridge parapets

Highways Report HR5

Advice to designers on best practice forthe appearance of bridge parapets

IMPROVED APPEARANCE OF BRIDGE PARAPETS

Advice to designers on best practice for theappearance of bridge parapets

Contents

Page

1.0 Introduction 1

2.0 Parapet Analysis and Design 3

3.0 Construction and Appearance of Currently Accepted Parapets 5

4.0 Improving Standard Parapets 8

5.0 Best Practice in Parapet Design 13

6.0 Future Developments 17

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IMPROVED APPEARANCE OF BRIDGE PARAPETS

Advice to designers on best practice for theappearance of bridge parapets

1.0 Introduction

1.1 Preamble

The purpose of a bridge parapet is to containerrant vehicles or pedestrians using a bridge.To fulfil this function the parapet is situated atthe edge of the deck, a position where it has agreat impact on the appearance of the bridge.The Department of Transport’s Standard forThe Design of Highway Bridge Parapets(BD52/93) states in the design requirementsthat: “The aesthetic effects of the parapetconstruction including its details should beconsidered at the initial stage of the designof the structure.” This requirement isrecognised by those responsible for designingbridges but is often not reflected in finishedstructures.

Parapet forms have not evolved at the samepace as bridges. Standard designs are stillbased on styles that were developed during theexpansion of the motorway system forty yearsago. The development of new vehicle parapets,particularly in metal, is made difficult by theCodes of Practice that govern design. Theprescriptive rules which govern the placement,size and shape of the parapet elements limit thepossibilities open to the designer who is madeto work hard to create anything out of theordinary.

1. A standard parapet, familiar to most road users.

This document is intended to be used in twoways; to provide information on the types ofparapet available for use in particularsituations; to give examples of good parapetdesign and detailing and general advice on

improving parapet appearance. Some of thesuggestions contained within this report mayrequire the Codes of Practice to be interpretedin an unconventional manner. It is important tostress the role played by the TechnicalApproval Authority (TAA) for a bridge. Theirunderstanding and support is essential ifsignificant advances are to be made in theappearance of parapets.

1.2 Historical Influence

Parapets were an early enhancement of bridgestructures, introduced to give confidence to theuser or as a necessity on some suspendedstructures. The earliest vehicle parapets werepurely functional, relying on their mass toprevent anything leaving the bridge.

2. A Norwegian traditional parapet made up of boulders.

As bridge builders became more confidentparapets became more of a feature of thebridge often being adorned with decorationand artwork. This philosophy continued withthe development of new materials and types ofbridge. Of particular note was the developmentof iron bridges where parapets could be madelighter to demonstrate the relative strength ofthe new material and elaborate castings couldbe used to decorate the parapet.

The most commonly seen parapets in theUnited Kingdom today were developed as partof the motorway expansion in the 1950/60’s,

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and the associated increase in bridgepopulation. Designed to emphasise the speedand dynamism of the new highways whilstusing material efficiently, the familiar standardparapets are still widely used today.

1.3 Design Perspectives

The two perspectives of a bridge that aregenerally experienced are; on the bridge whentravelling over it; off the bridge when viewedremotely in elevation. In the former situationthe parapet will often be the only visuallydiscernible element of the bridge. In the lattersituation the parapet can have an enormousinfluence on the perception of the bridge andits proportions. The designer striving to createan attractive bridge must duly consider bothperspectives.

In designing for the view when crossing abridge the principal concern should be thebridge’s setting. In an urban setting where theview is of little interest, the bridge parapet canenhance and add interest to the roadsideenvironment with the clever use of colours,shapes and textures.

Alternatively if a bridge is set in anenvironment which the bridge user may wishto appreciate, the parapet is considered anobstruction. In this situation the designer facesthe challenge of making the parapetunobtrusive as well as aesthetically pleasing.

A bridge design will most often beconceptualised and evaluated with regard to itsside elevation. This view of the structure is

usually the most expressive and it is from thisviewpoint that the structural principle of thebridge is most obvious. When a designerstrives to make an elegant or striking bridge, itwill be this elevation that they are judging.

The parapets’ effect on the bridge elevationdepends very much on the type and scale ofthe bridge; on a large suspension bridge,always viewed from a distance, the parapetmay be barely discernible; on a smallfootbridge the parapet will contribute greatlyto the impression of the structure.

The criteria used to judge a bridge by remoteviewers and bridge users are therefore verydifferent and it is only by considering boththese viewpoints that the design ofaesthetically pleasing parapets can beachieved.

1.4 Design Presentation

If a bridge is to succeed visually the parapetmust be designed with the bridge, as opposedto being thought of and added later. Whilst thedifferent parts of the bridge, such as the deck,the pier and the abutments, may be inharmony, many bridges fail to succeed visuallydue to the lack of any clear relationshipbetween the structure and the parapet. Toencourage successful design it is important thatall drawings right from the concept stagereflect the scale and appearance of the parapet.The impact of the parapet on the bridgeproportions can readily be appreciated in thismanner.

3. The parapet of this bridge has been designed as its most striking feature

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2.0 Parapet Analysis and Design

2.1 Parapet Safety

The design criteria for parapets depend onwhich bridge users they are required tocontain. Pedestrian parapets are designed forthe static loads it is anticipated may act on thebarrier in a crowd situation. The design of astructure to withstand these loads is within thecapability of a competent engineer.

Highway parapets are designed for thecontainment of vehicles. This is achieved byabsorbing some of the impact energy anddeflecting the vehicle away from the edge.There are three levels of vehicle containmentprovided by highway parapets:

i) Low containment parapets are designed forurban environments where speeds arerestricted to less than 80km/hr.

ii) Normal containment parapets are

developed and tested to resist certainvehicles travelling at up to 113 km/hr.These are used on the motorway system.

iii) High and very high level containment

parapets are used in particularly sensitivelocations where any parapet failure wouldhave severe consequences.

With regard to the appearance of the parapetand the bridge overall, it is generally in thedesigner’s interest to use the lowestcontainment level acceptable. Gainingapproval for the use of a lower containmentlevel will usually require co-operation from theTAA with justification in the form of adetailed risk assessment.

During a collision a parapet may need towithstand extremely high peak loads. Thecollision normally results in permanentdeformation and damage to part of the parapetsystem. The dynamic nature of thevehicle/parapet response has historically beenvery difficult to accurately analyse. Theexception is very stiff concrete parapets, whichcan be considered to behave as rigid bodies.Conventional structural analysis is notconcerned with post failure behaviour, whichis often critical in parapet performance. Fullscale testing of parapet designs is thereforerequired to verify a particular design.

4. A parapet following impact testing.

2.2 Current Design Codes

At present there are three current design codesavailable for use in the design of parapets inthe UK, these are:

i) BD 52/93: The Design of Highway BridgeParapets.

ii) BS 6779: Highway Parapets for Bridgesand Other Structures.

iii) BS EN 1317: Road Restraint Systems.

2.3 Other forms of Vehicle Restraint

Not all vehicle restraint systems are classed asparapets, although the distinction maysometimes seem unclear to the layperson.

The difference between crash barriers andparapets is the extent of deformation theypermit; crash barriers deflect to a greaterextent. The wheel penetration may be up to2.0m on a crash barrier. If used on a bridge thedeck width may need to be increased to allowfor this.

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Other forms of restraint such as kerbs, crashcushions and bollards may be variouslyemployed. These elements will be required toconform to particular design codes and arebeyond the scope of this report. Their use inconjunction with parapets can be beneficial byreducing the containment requirement of theparapet, (eg; an anti-mount kerb may reduceimpact speed by around 15kph).

5. An anti mount kerb is used to protect an historicparapet

2.4 Road Safety Issues

Detailed information on road traffic accidentsis sparse. The Department of Transportpublishes default figures for accident rates,accident costs and accident casualties for usein road network analysis models. Thisinformation does not give an indication of thepart played by parapets in road safety.

Although parapets are universally required onroad bridges, there is little published on howfrequently parapets are brought into use. Thiswould influence the risk assessments carriedout to decide the appropriate containmentlevel. Anecdotal evidence was sought for theAvonmouth Bridge on the M5 near Bristol (noactual records exist). This suggested anaverage of 2 parapet hits a year over the last 20years – only on one occasion was the parapetbreached. The road alignment is straight but,considering the traffic flow (over 100,000vehicles per day), speed of traffic and thelength of the bridge (over 1400m) this is a verylow accident rate.

The relationship between the view afforded todrivers using a bridge and road safety is thesubject of conjecture. Whilst a stunning viewfrom a bridge may distract a drivermomentarily from the road ahead, it may be

6. The Hume Bridge in Manchester has bollards assecondary protection

arguable that on long bridges or on scenicroads the mean traffic speed are reduced andhence accident rates are lower. Furtherresearch would be required to correlateaccident rates against parapet design.

One area where parapets may be able todemonstrably improve road safety is inproviding guidance to motorists. The use ofbright colours, reflective coatings or electriclighting could possibly assist motorists atcomplicated road interchanges and have apositive effect on accident rates.

Preventing glare from the headlamps ofoncoming vehicles may help to improve roadsafety at night. Panels or vanes fixed to the topof vehicle restraints, particularly in the centralreservations of dual carriageway roads arequite common. Obscuring barriers such asthese may also help to prevent ‘rubber-necking,’ which is a cause of some accidents.

7. Vanes protect the adjacent properties from glare.

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3.0 Construction and Appearance of Currently Accepted Parapets

3.1 Metal

The standard steel or aluminium parapetsdeveloped in the 1950’s are made up ofhorizontal rails spanning between verticalposts. An alternative, available for lowcontainment, is a parapet made up of panels ofvertical infill members at close centres. This isfrequently seen in urban environments.

If a metal parapet is specified, the choicebetween steel and aluminium will be governedby whole life costs of the parapet. The greatermaterial cost of aluminium is weighed againstits lower maintenance costs.

The rigorous testing procedure required fornormal and higher levels of containmentbefore a new all metal parapet can beintroduced has restricted the number of theseparapets which are available.

3.2 Concrete

The use of reinforced concrete for full heightparapets tends to be restricted to highcontainment. Its rigidity makes it ideal for thisapplication. There are some very goodexamples of its use in normal containmentsituations where it has given the concretebridge an homogenous finish, however weightoften precludes use on light bridges.

Concrete parapets are typically constructed aspre-cast units because of economic factors andthe higher quality finish available. It ispossible to introduce patterning on the outerface of the parapet.

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3.3 Combined Concrete/Steel

This form of construction gives the designerthe most freedom. A concrete plinth can becombined with metal posts and rails to form aparapet. There is no testing requirement of thesteel, which allows the designer scope todevelop a parapet of their own, so long asstrength stipulations are met. The plinth isrequired to be 400mm-800mm tall dependingon the code used and the containment levelrequired.

This form of parapet has not been widelyexploited in this country as yet, with theexception of some of the early motorwaybridges and some elevated urban motorways. Itis more common on the Continent where it hasbeen used on many landmark structures.

3.4 Masonry

The use of masonry as a parapet material has along history. In many environments, theappearance of brick or stone offers a morepublicly acceptable alternative to concretewhere a solid parapet is required. Recentdevelopments using masonry workingcompositely with a concrete core make itpossible to design low and mediumcontainment parapets in brick or stone.

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3.5 Timber

A relatively recent development in parapetdesign has been the introduction of a lowcontainment highway parapet of timberconstruction. Timber has a long history as abridge component, has environmental benefitsand is one of the few materials with anappearance that is almost universally favoured.Interestingly, the problem encountered withthis type of parapet is designing it to notdamage the bridge superstructure when failureoccurs. Timber parapets and crash barriers areused more extensively in the U.S.A., to verygood effect, where they are accepted by mostapproving authorities.

The use of timber for footbridge constructionis becoming more common as longer spansand more exciting profiles are possible withglued and laminated beams. Timber is afamiliar material that generally meets withpublic approval.

3.6 Glass

The range of laminated and toughened glassnow available is increasing the possibilities forthe use this material in pedestrian parapets.Commonly used for enclosed footbridges suchas link bridges between buildings, it presentsan aesthetically pleasing form of screenespecially where full protection of this kind isdesired.

Issues to be addressed when specifying glassinclude maintenance (scratched and dirty glassis far more noticeable than scratched and dirtysteel) and the prevention of glare in highwayapplications. Glass technology is advancing atsuch a rate that these issues are nowmanageable.

3.7 Composite Materials

At present the use of composite materials forparapets has been restricted to footbridges,although there are advances being made insome European countries which extend thepossibilities. This family of materials hasgreat potential for future use in parapets.

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4.0 Improving Standard Parapets

4.1 Provision of Secondary Safety Barriers

In producing an out-of-the-ordinary schemethe designer will need to go beyond “off theshelf” parapets. The greatest freedom ofexpression is possible in pedestrian parapetsfor which destructive testing is not required togain approval. However, in order to have thisoption on a highway bridge, it is necessary tobuild a traffic containment barrier in additionto the parapet. This solution is also widelyemployed to protect existing parapets whichare considered to be under strength. The crashbarrier then provides vehicle restraint and theparapet contains pedestrians only.

19. Crash barrier protects parapet and cable stays

Before adopting a crash barrier/parapetcombination it is necessary to consider theperception of the pedestrian using the bridge.The presence of a continuous crash barrierdeters pedestrians from crossing the road, thismay be desirable in many urban designsituations where efforts are made to control“jay walking”, but may also intimidatepedestrians. The length of the bridge as well asits location will have a bearing on whether thepedestrian feels trapped in an alleyway. Thisproblem can be overcome by the imaginativedesign of the crash barrier, (eg; a concretebarrier can be disguised as a bench or a planterto reduce its visual impact). It is also possibleto use a discontinuous crash barrier or bollardsto alleviate the feeling of entrapment. Thesolution may require the assistance of theTAA.

The design of the bridge can also benefit fromthe introduction of a secondary barrier sinceload on the footpath will be reduced. Vehiclesare prevented from mounting the footway. Theonly loading to be considered in this area ispedestrian live load. The less onerous loadingon the footpath results in a lighter structure atthe edge of the bridge, although extra widthmay be required to accommodate theadditional barrier.

20. Crash barrier does not trap pedestrians

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4.2 Detailing of the parapet 4.2.2 Bridge/Parapet Interaction

Within the prescriptions of the Codes ofPractice it is possible to improve the visualeffectiveness of parapets without altering theirstructural performance. These improvementsmust be justifiable in terms of cost. As oneimportant aspect of appearance is the longevityand durability of the finishes, the ease and costof maintainability becomes paramount. Manyexisting parapets have deteriorated visuallybecause their long term appearance was notadequately considered at the design stage.

4.2.1 Proportions

Good bridge design will ensure that anyparticular element or relationship betweenelements does not spoil the aesthetics of thebridge as a whole. It is essential that the sizeand scale of the parapet construction isappropriate to the bridge. Rules of thumb exist

21. Edge beam reduces the impact of the parapet

for successful bridge design in terms of correctproportioning of the bridge elements such asthe piers, deck edge and cantilevers. Theseprinciples need to be extended to the choice ofparapet design particularly with regard to thenumber and spacing of posts, the height ofplinths and the slenderness of rails. Nonstructural additions such as dummy rails orrelief patterns can be considered to give therequired effect of scale to the parapet/bridgecombination. The use of shading and edgebeams can also be effective in altering theapparent size and proportion of parapets to suitthose of the bridge.

On bridges viewed from the side, the bridgestructure and the parapet can sometimes appearto be very distinct which can spoil theaesthetics of the bridge. This can be overcomeby applying cladding panels to the edge of thebridge structure that mask the parapet. Thissolution removes the delineation betweenbridge and parapet and therefore gives thebridge an overall streamlined, unclutteredappearance. Perforations in the portion of thepanels extending above the bridge deckprovide transparency and afford some views tothose using the bridge.

22. Interface between bridge and parapet is blurred

By reflecting the pitch of the parapet postswith patterning on the edge of the bridge atsimilar centres, the bridge and parapet areunited. There are examples where this hasbeen effectively achieved by creating a facetedsurface on the side of the deck. The facetedsurface creates shadowing effects lendingdepth to the patterned elements.

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23. Edge detail complements parapet posts

4.2.3 Mesh/Sheeting

Given the widespread use of mesh panellingon existing parapets, there is a need to developand introduce more attractive and easilymaintainable panel types. The unattractivesight of rusting panels can be avoidedcompletely if aluminium or stainless steel isused. Sheets of these materials punched orsheared to the solidity desired by the designerare a viable alternative to wire mesh if wholelife costs are considered.

24. Plastic coated mesh after the coating is breached

25. Plastic panels provide an alternative screen

Vertical vanes oriented perpendicularly to theroadway can be effective in enhancing theouter face of the parapet whilst retainingvisibility for bridge users. Since views aremost often glimpsed by motorists to their side,the orientation of these louvres does notunduly obscure these sideways views,particularly from a vehicle travelling at speed.The louvres are often made from compositematerial such as G.R.P. that can be brightlycoloured and has very low maintenancerequirements. The open louvre arrangementdoes not attract unnecessary wind load to theparapet and also reduces some of the noise andlight pollution emanating from the roadway.

4.2.4 Texturing of Concrete Parapets

Unless carefully detailed and well finished,concrete can be bland and uninteresting.Decoration of the outside face of the parapetand indeed the bridge structure in the form ofrelief patterns can give interesting effects. Thetraffic faces of parapets are not usuallyprovided with relief decoration for safetyreasons. The design codes dictate that thetraffic face of the parapet must be free ofprojections above 20mm.

A further benefit to a textured concrete finishis in maintenance, a relief pattern is effectivein reducing the incidence of graffiti. Texturedconcrete also tends to weather moreattractively, staining actually exaggerates thesurface patterns, rather than spoiling a smoothsurface of concrete.

A design method that may allow relief patternsto be detailed on the traffic face of concreteparapets is the addition of a metal mainlongitudinal member to the parapet at theheight that collision occurs. This metalmember will prevent any snagging andtherefore allows the traffic face to be textured.

26. Textured internal face of concrete parapet

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4.2.5 Colouring of Parapets

The use of colour on bridges can help thedesigner achieve a range of desirable effects.Some colours can be used to lend individualityto bridges or families of bridges, others tend tocamouflage and understate bridge elements.The particular desired effect can be achievedthrough careful consideration and consultation.The use of bold colours also helps to orientatemotorists and can make bridges andinterchanges into landmarks.

27. Its colour makes this bridge stand out

Coloured concrete may be appropriate anddesirable in many circumstances. Colour canbe introduced by careful sourcing of colouredaggregates or by the addition of dyeadmixtures. Concrete is prone to staining as itages which can have a very unattractiveappearance. Careful detailing and specificationof surface finish and concrete mix constituentscan reduce the severity of staining.

Painting has been the traditional method ofsurface decoration, a wide range of colours andfinishes are available. Both aluminium andsteel can be satisfactorily coated, cost savingsthrough factory application are possiblealthough handling of painted components thenbecomes more important. Modern paints offergood adhesion and resistance to weatheringand abrasion. If the component is notgalvanised steel or aluminium, then corrosionwill occur once the paint film is fullybreached. Paint cannot be recommended forcomponents such as mesh, which cannot bepainted quickly or effectively. The possibilityof applying paint completely on site lends it torejuvenating existing parapets of all materials.

Vitreous Enamel coating of mild steel andaluminium parapet components can be used togive a pleasing appearance. The porcelaincoating is factory applied and results in asurface that is resistant to weather, abrasionand light. The coating provides corrosion

protection until it is breached, most commonlyby heavy impacts. The treatment is not suitablefor components with sharp edges but rails andposts could be suitably designed for ceramiccoating. The coating can achieve a life of up tofifty years, however repairs to the coating maynot achieve the durability or colour match ofthe original.

28. Strong colours extend to the mesh.

Anodising of aluminium components is a verysuitable and appropriate method of improvingtheir appearance. Although the appearance ofuntreated aluminium is often acceptable,anodising can impart a range of colourspermanently. The electrolysis process forms anartificially thick coat of aluminium oxide thatcan be given lasting colour through the use ofinorganic or metallic pigments. The surface isattractive, light resistant, very hard anddurable. The range of colours is large, butdepends upon the particular alloy in use.

29. Coloured parapet set back behind crash barrier.

4.2.6 Decoration

Adorning bridge structures with artwork bothadds interest to the motorway environment andgives bridge structures identity. This can beparticularly well exploited in a regional senseby using artwork to make statements tomotorists about the district through which theyare travelling. Regional Bodies, other than theowners of the structure may be willing to fundsuch improvements. Possibly the mostappropriate medium for decoration is ceramictiles which benefit from vivid colours and lowmaintenance whilst also deterring graffiti.

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30. Tiling conveys regional identity

Tiling is most often applied to other elementsof the bridge structure such as piers andabutments but there is no reason that it couldnot be used to decorate both faces of concreteparapets provided secure attachment of thetiles is ensured. The possibility of loose orshattered tiles falling onto passing trafficfollowing a collision must be reduced to thesatisfaction of the TAA.

4.2.7 Vegetation

31. The Green Bridge in London

There has been a move in recent years topromote green bridges (that is bridges onwhich plants are grown), particularly in urbanenvironments. It has been widely recognisedthat careful planting can be used to improvethe appearance of bridge abutments. A naturalprogression from this is to cultivate plantsaround the parapet using the parapet as aframework or trellis. Careful selection ofplants could lead to different effects rangingfrom well maintained and manicured to wildand unkempt (in appearance – not practice).This would undoubtedly make bridges moreinteresting.

32. The feeling is not of being on a bridge.

4.2.8 Lighting Columns

The interaction of lighting columns andparapets can be difficult. If the lighting columnis positioned outside the line of the parapet itcan break up the lines of the structure, insidethe parapet it is an obstruction and a litter trap.The best solution is therefore to incorporatelighting columns into the line of the parapet,either built off the top of the parapet orincorporated into the structural lines of it.

33. A feature made of the lighting columns

34. Light standards built into this parapet.

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5.0 Best Practice in Parapet Design

5.1 Suspension Bridges

The sheer magnitude of these structures oftencauses the parapet to pass unnoticed. Thisshould not diminish the designersresponsibility, these grand structures deservecarefully considered parapets. If the walkwayis positioned outside the cable line a secondarybarrier will normally have been used. Thedesigner therefore has the freedom to designthe outer pedestrian parapet in a formappropriate to the bridge, maximising theviews from the bridge.

35. The crash barrier and parapet on Humber Bridge

5.2 Cable-Stayed Bridges

These landmark structures often require theuse of a crash barrier to protect the cables fromvehicle strikes. The outer parapet may then bedesigned for pedestrian restraint only, enablingit to be detailed in sympathy with the locationand architecture of the bridge. An area whichrequires particular consideration on cablestayed bridges is the interaction of the slopingcables with the lines of the parapet members.Orienting the vertical members to reflect theangle of the stays is an effective way ofreducing the visual clutter.

36. Parapet stanchions angled to suit cables

5.3 Motorway Over Bridges

The standard metal traffic parapets weredeveloped as part of the massive expansion ofthe motorway system in the 1960’s. Thearrangement of horizontal members wasdesigned to emphasise the speed anddynamism of the then new highways. The useof these parapets is still valid on motorwaybridges, in the context for which they weredesigned. Some improvements can be made tothe finishing of the parapets in order toimprove their appearance.

37. A brightly coloured overbridge

When a bridge parapet is painted, or colouredby anodising or other techniques mentionedearlier, it can stand out dramatically. Colourcan be used to give a regional identity orcoordinate with other street furniture. This hasthe benefit of giving the driver a sense oflocation in what may otherwise be a blandcorridor.

Very careful consideration should be givenwhen infill is required on the traffic face. Theuse of stainless steel or aluminium perforatedpanels provides a better more enduringappearance than mesh. The maintenance ofmesh has been a problem in the past withregular replacement the most viable solution.

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As an alternative to the standard parapets theuse of combined metal and concrete parapets isrecommended, they can give reasonablevisibility for bridge users and lend uncluttered,clean lines to the elevation. These parapetsalso give the designer greater freedom ofexpression.

5.4 Motorway Underbridges

In general motorway environments are bothbland and sterile, those using the roads aredetached from the landscape that they aretravelling through. Bridges offer views thatbreak the monotony of embankments androadside furniture. Designers should attempt toenhance the motorist’s journey by maximisingthe sights from the road. The parapet can beused to emphasis the presence of the bridge bydrawing one’s attention to it. Colour can beused to distinguish the parapet from thestandard grey colour of a crash barrier.

Often the only indication that a river is beingcrossed is a small signpost at the start of thebridge and these are easily missed. Anapproach that can be borrowed from historicalbridge builders is to emphasise the ends of thebridge and the start of the parapet with afeature or standard. This can be achievedwithout contravening the parapet designregulations.

38. Ends of the bridge are emphasised

5.5 Major Highway Footbridges

Footbridges over large roads and motorwayscan be generally divided into two sections.Firstly, typical beam bridges, where theparapet sits on top of the superstructure and isall the persons crossing the bridge willexperience. The second type of footbridgeoften seen in this situation is the through truss.

Both of these bridge types are basic structuresthat have clear structural expression if donewell.

39. Beam bridge with simple parapet.

The beam bridge is a very simple structurewhich benefits from a simple, plain parapet. Aparapet made up of small members such as avertical railing is more in context than ahorizontal rail parapet whose members seemdisproportionately large. A fine verticalrailing usually avoids the need for mesh, whichis a further advantage.

40. Mesh enhances shape of vierendeel truss

Through truss footbridges are striking becauseof their magnitude. It is not possible tounderstate this structure and so it may as wellbe emphasised. The parapet top will often bearound the mid-height of the truss, a positionwhere it will confuse the viewer from the cleanlines of the truss. A truss that has full heightmesh or railings without a discernible handraillooks far more attractive than one that iscomplicated by a standard parapet within thetruss structure. Solid panel infill to the trussopenings can work well for bridges overrailway lines.

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5.6 Link Bridges

Another common form of footbridge more inevidence in recent years is the link bridgejoining buildings. These are normally fullyenclosed to act as a continuation of thebuildings they link. The use of glass parapetsin this situation is appropriate and attractive.Glass protects the bridge user from theelements and its transparency alleviatesfeelings of claustrophobia. Where a link bridgeis clad with glass or panels, the joints mustcoincide with any visual elements of thestructure to avoid a disjointed appearance.

41. Framing is minimised

5.7 Bridges in Urban Environments

Generally an urban bridge will have a speedlimit below 80km/h and hence require only alow containment parapet. This has the effect ofminimising the structural weight of theparapet. The use of a combined metal andconcrete parapet allows the designer tointroduce elements of individuality into theparapet, possibly to reflect the environment ofthe bridge.

The use of secondary protection to reduceparapet requirements should be investigatedwith the TAA. A pedestrian standardrequirement gives the designer absolutefreedom to develop the parapet.

Urban bridge parapets must be detailed toreduce the risks of vandalism. Enhancingbridges through the use of colours or ceramicartwork should be considered.

5.8 Bridges in Rural Environments

An appropriate material for a rural bridge istimber. An earlier section described how lowcontainment bridge parapets have beendeveloped in hardwood. In America the use oftimber for parapets and crash barriers iswidespread. The TAAs in the UnitedKingdom are gradually accepting it as amaterial in highway bridge construction.

42. Simple timber parapet

Although unlikely to be financiallycompetitive, locally won materials such asstone or rock can be attractive. There existfine examples of modern road bridges clad instone.

43. Modern stone bridge

Footbridges in a rural environment shouldsimilarly not look out of place. There are anumber of timber kit bridges available forshort and medium spans that provide a verynatural solution with parapets in the samematerial as the structure.

44. Timber kit footbridge

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5.9 Railway Overbridges 5.10 Canal Bridges

Railway parapets are the subject of the moststrict stipulations in the codes of practice. Atpresent the design codes require these parapetsto be solid. This requirement is for twopurposes, to prevent vandals from aimingmissiles at a train easily, and to prevent thetrain driver from confusing car brake lightswith red signal lights. The restrictions are moststringent for high speed lines with overheadelectrification equipment.

45. Well managed transition between highway and rail parapet

In Continental Europe, the railway andassociated electric equipment are sometimesprotected by means of a shelf cantileveringoutwards from the bridge above the sensitiveareas. If the parapet has the requiredcontainment level, this much less imposingsolution may be feasible in some situations.

Some recent metal railway parapets havegained approval for the use of perforatedsheeting, this may represent a rationalisation ofparapet design by the approving authorities.

48. Attractive panelling over railway

Many regions are now realising the potentialof the canal system of the country which haslain redundant for many years. Use of thewaterways, primarily for recreation, is on theincrease and cities and towns are seeing thebanks of canals redeveloped as desirable areasto live.

47. Modern canal bridge

Canals are generally considered peaceful,tranquil places removed from the bustle ofeveryday life. The design of the parapet shouldreflect this, emphasising vertical members asopposed to the horizontal lines on motorwayoverbridges. The short span and steep verticalcurve of these structures lends itself to thistype of parapet.

5.11 Equestrian & Cycle Bridges

The vertical height required for these parapetsis greater than that for an equivalent pedestrianbridge. Unfortunately this is often achieved byadding a disparate structure on top of astandard parapet design. The height of theseparapets can be emphasised using verticalmembers to create a more uniform finish.

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6.0 Future Developments

The standard parapets which have beenavailable since the 1950’s are still widely usedon new bridges today. In the right context,such as motorway overbridges, these designsare still valid and do not appear incongruous.The harmonisation of European Standardsshould have the effect of widening the marketand increasing the choice available to thespecifier.

What this report has aimed to emphasise arethe alternatives which are available within thecurrent codes of practice, which may not beimmediately obvious. The largest area offreedom to design is with parapets ofcombined concrete and steel, a parapet typewhich has not been fully exploited in recentyears.

An area where additional investigation wouldbe of benefit is the development of parapetsfrom currently non-standard materials such astimber and composite materials. The use oftimber, as both a popular material aestheticallyas well as environmentally sustainable is onewhich should be more widely practised. Thedevelopment of standard parapets wouldundoubtedly increase the use of these materialsand should not be too onerous following workdone in a number of other countries. The newarrangement of structural design Eurocodesshould simplify this process.

50. Timber crash barriers in USA

Composite materials offer a colourful, lowmaintenance future for parapets. Previous usesof these materials has tended to mimic whathas been previously done in steel, there is greatpotential for using this easily formed materialto create exciting new forms of parapet.

51. GRP handrails

Photo credits

All Hyder Consulting Limited except:

1. Varley & Gulliver Limited2. Jan A. Martinsen3. Jean Muller International4. TRL Limited5. Lafarge Concrete Products6. Arup9. Varley & Gulliver Limited10. Tarmac Precast Concrete11. Scott Wilson Kirkpatrick14. Prof. Brian Hobbs, University of Teeside15. Eskdale Stone Limited16. CTS Limited17. Sarum Hardwood Structures Limited18. Dane Group20. Jon Wallsgrove, HA25. Marc Mimram28. Markus Kaiser29. Markus Kaiser33. Parsons Brinckerhoff34. Markus Kaiser36. Flint & Neill Partnership38. British Waterways41. Jon Wallsgrove, HA42. Sarum Hardwood Structures Limited43. Eskdale Stone Limited44. Technical Timber Services Limited45. Arup46. John Evans49. Varley & Gulliver Limited50. Jon Wallsgrove, HA