Bridge Design Selection1 INTRODUCTIONEver since the earliest bridges, whether it was the first simple stone arch or a modern gravity defying

marvel, all types have be based on making certain that the fundamental forces that affect bridge design

are overcome and in some cases exploited. Dead loads, live loads, wind, thermal, length, height, shape,

etc. have effected every bridge ever designed, these loads have varying actions such as tension and

compression, shear and torsion. Consequently material strength has played a major part in bridge

design. Some materials are more resilient to forces acting tensional or in compression. For example

stone is great for compression but suffers in tension, because of this the arch design was prominent with

the use of stone as the shape keeps the loading in compression. Flexible vines where widespread across

the southern continent resulting in the design of bridges that exploited the tensile force due to the fact

this type of material was better than stone in tension. Material strength therefore is a paramount factor

in bridge design. Today this fact still reigns, however the foremost popular kinds of material used remain

steel and concrete. These resources allow the designer to create a variety of bridges which structurally

can either be solely steel, or concrete, or a mixture of the two. Because of this, a variety of bridge styles

and there materials will be analyzed in this report. Where each bridge type shall be equated with a

range of factors that will help decide which design is best.

2 EVALUATING BRIDGE DECK TYPES ON MATERIAL CHARACTERISTICS

All structures are susceptible to environmental attack, however bridge structures are in some cases

more vulnerable to weathering than other types of infrastructure. The main suspects to attack are salts

from grit spreaders or the sea, oxygen, water/moisture, sulphur and carbon dioxide. These penetrate

the material and overtime causes corrosion, cracking, rust, and spalling. Different types of bridges offer

more protection than others due to their shape and design this will be discussed later.

2.1 CONCRETE

Concretes properties are very good in compression but weak in tension, which why members are

reinforced with steel or use of tendons for prestressed units. Concrete is a material which is formed

during the hydration process caused by cement and mainly water. It forms a gel or paste which holds

together a range of aggregate sizes. A bridge deck has little cover from the elements therefore the

concrete needs to have a low water/cement ratio to increase overall strength and decrease

permeability. This needs to be balanced with the workability aspect of concrete. For the reason that the

bridge has little cover from the elements it is required to be strong, resistant to penetration of liquids

(especially saltwater) chemical attack and possess a low thermal conductivity.1. Because concrete

structures mainly act as a composite material with steel the effectiveness depends upon the bonding of

the steel and concrete and the concrete acting as a shield for the steel.

2.1.1 Carbonation

Concrete contains pores. These are caused by either poor production for failure to compact or capillary

and gel pores created during the hydration process. These pores allow carbon dioxide to dissolve into

the pores with the aid of water to create an acidic solution. This process reduces the overall alkalinity, if

this occurs it provides more access to the steel where corrosion can follow.

Air entraining concrete has been developed which creates small air bubbles in the concrete that allow

substances to enter without damaging the concrete, it also allows enough room for freezing to occur

without expanding the concrete.

2.1.2 Chloride salts

Salt can enter the concrete in a variety of ways. During the construction process some aggregates may

have remnants of salt, if they are sourced near the sea. Or in a more common case the use of de-icing

salts spread along the bridge deck. Cracks and spalling may occur causing structural damage to the

concrete. Also once chloride salts reach the reinforcement corrosion occurs within the steel.

The best method to prevent salts from attacking concrete is to use waterproof sheeting and an asphalt

layer on top of the slab to create a barrier between the salt and the structural concrete. Splash back

from bridges also give the opportunity for salt to attack the sides and lower parts of the bridges. The

bridge design should have some form of cantilever so water drips off the deck without causing surface

runoff. The best bridge shape for such a prevention would be a trapezoidal box girder.

2.1.3 Freeze thaw

Unless the pores are large to start with freeze thaw is an unlikely occurrence for concrete. However

actions by other corrosive cohorts create larger gaps within the concrete, for which water can enter,

freeze and expand. If this cyclic action continues overtime the gap will continue to grow.

Air entraining agents as discussed in 2.1.1 offer a similar solution to freeze thaw.

2.1.4 Cracks caused by loading

Structures are subject to changing loads throughout the day; this kind of cyclic loading causes the

concrete over time to crack and eventually failure due to fatigue. If the member is prestressed or

reinforced the steel itself carries a large portion of the tensile forces, however large cracks still form in

normal reinforced members, which on their own won’t cause failure but will provide an opportunity for

harmful substances to enter and attack the steel. Due to sustained loads concrete is more susceptible to

creep. Creep can offer both an advantage and a disadvantage to a structure. In the case of a prestressed

structure creep can be detrimental however in other creep can provide a relief of stress concentrations

which is seen as beneficial.2. To diminish this kind of cracking (virtually crack free) the technique of

prestressing the concrete by using tendons is a well-respected method.3. Clearly this will have

imperative implications for durability.

2.2 STEEL

Steel is greater in all aspects of strength when compared to concrete. However in some cases the use of

a composite reinforced concrete beam can meet the required load specifications at a cheaper cost.

When using primarily steel one of the most important considerations a designer should anticipate is

corrosion. The design needs to consider steels protection from corrosion, and its accessibility for future

maintenance and inspection to help prevent corrosion. Steel is made from a combination of metals, the

process requires large amounts of energy and heat to combine these substances together. This means

the steel is thermodynamically unstable, overtime it will reverse back to its lowest energy state, and this

process can be decelerated by using physical or chemical means. The process is more commonly known

as corrosion.4. Steels which are of higher quality and degrade slower than cheaper options have their

composition altered by using less reactive metals, because they are low in reactivity corrosion is

reduced. Below shows how corrosion occurs and how it can be prevented:

2.2.1 Atmospheric corrosion

Steel stored inside a dry building will not rust as much as steel living in an external environment. Even

though oxidation requires oxygen to occur this is not the most significant driving force for corrosion.

Corrosion occurs more rapidly in moist environments, in, if not all western countries the air is

contaminated with impurities which means corrosion can occur at lower humidity. Again, similar

characteristics are also found in concrete. Corrosion will be enhanced if de-icing salts come into contact

with the steel.

Design should reduce this factor by designing a structure which will reduce moisture collecting. For

example a structure that does not produce damp corners. However this is difficult but a structure that is

breathable should be a consideration to reduce moisture. This is discussed in the crevices section below.

There are many diverse ways of preventing corrosion on an open steel structure like a bridge, however

the most common type of deterrence is the use of a coating. There a variety of different coating

methods available and over the last 30 years have seen significant improvements in durability, which

means maintenance is not needed as often as the coats life span has improved. Coatings will be

required during the bridges lifespan which may cause traffic problems because scaffolding or machinery

may be necessary to access the areas that need painting. A bridge made from I beams offers crevices

which have the potential to hold moisture and cause corrosion, therefore for this situation the box

girder is the preferred option.

2.2.2 Water corrosion

Water can contain many types of dissolved substances which can be harmful to the steel. A by-product

of transport is sulphur dioxide which forms with water to produce sulfuric acid which can be very

harmful to steel.

Prevention of water gathering in the structure is also down to design which is discussed in the crevices

section. The use of coatings is also discussed in 2.2.1.

2.2.3 Crevices

Crevices can be accidentally created during the design period of the structure, where in essence, parts of

the structure act as a trough which collects water and other harmful materials to steel. Corrosion often

occurs at the tip of crevice as the anode becomes localized causes high rates of corrosion within this

area.5.

The designers first thought should be to reduce the amount of crevices. Crevices most commonly occur

at the joints of the structures, this area may be difficult to manage, where welding the gaps or using a

form of filler may only be a temporary solution. Pre painting the surface or creating holes for water to

pass through is seen as more worthwhile.6.

Overall if steel was used primarily in a structure, design would ensure that it is protected to prevent

corrosion from occurring. One of the best bridge designs for protection would be the steel box girder as

the slab on top will protect the steel from a large portion of the elements. It provides easy access to

assess inside the girder itself and it can be designed as a trapezoidal shape which helps reduce the

potential area affected by the elements. If I beams where used instead of the box girder it would make it

more difficult to coat due to hard to reach places and it is genially an odd shape to paint compared to

the girder. They also offer more crevices where moisture can build up on the webs and joints of the

beam.

3 COLUMN POSITIONS EFFECTING SPANS

3.1 PEDESTRIAN AND BUILDINGS

The position of the columns need to offer the least amount of hindrance in the desired area. Pedestrian

walkways ought to be avoided as this would cause an obstacle for walkers and the adjacent buildings.

One of the specifications for the bridge is to allow sufficient space on the bridge deck for both cyclists

and pedestrians. This does not make walkways on ground level obsolete. Both bridge and pedestrian

walkways need to collaborate together in order to create a friendly environment and a greater

‘perceived safety’ for pedestrians.

Positioning columns (which in this case are of considerable size, 800 mm by 800 mm) may well cause

obstacles for pedestrians and on a busy street cause annoyance as it will affect the flow and speed of

pedestrians. In some situations columns and other obstacles may create choke points on walkways

where the latter could arise.

Columns placed near the edge of pavements adjacent to the road will have significant safety

complications. For example pedestrians wishing to cross the road may find their view obscured by a

column, making it difficult to see oncoming traffic; equally for the road user spotting pedestrians wishing

to cross may become more difficult. For this reason columns should be positioned in areas where

pedestrians are least likely to cross, which is difficult, but the area in question has a large division in the

road to separate oncoming traffic. Pedestrians are less likely to use this area for crossing where instead

there are designated crossings which are a lot safer.

Columns located near buildings may cause visual effects for the occupiers and even decrease income for

retail buildings. Even though there are no definitive studies that show such an event will cause a

negative consequence on a buildings income it is reasonable to conclude that this position is least

desired. Another reason

One noticeable effect of having the columns and therefore the bridge near or over an existing

pedestrian walkway is that it will provide cover from the elements such as rain. However overall the

negatives outweigh this issue, and for that reason column positions should avoid areas used for

pedestrians and where there are buildings are nearby. Therefore the most sought after area is the zone

used to segregate oncoming traffic. This however will have an effect on the span and the type of deck

used.

3.2 EXISTING SERVICES

In this project this topic will not be considered. But it is worth noting that such an issue should be well-

thought-out when considering the location of columns. Existing services cause substantial constraints for

foundations and therefore columns. Gas, electricity and water lines quite often run alongside the road

side near the pavement and if a column was to be positioned within areas of existing services then such

services will have to be diverted or meandered around the foundation itself. This should be avoided if

possible as it will decrease labor and material costs in diverting the services.

3.3 TYPE OF COLUMN

Piers provide vertical supports for spans at intermediate points and perform two main functions:

transferring superstructure vertical loads to the foundations and resisting horizontal forces acting on the

bridge. One of the most important factors for bridge columns is the impact load caused by an accidental

force such as a car crash. There are both steel and composite concretes that can be designed to

withstand such a force, both have their advantages and disadvantages. A damaged steel column is easier

to replace than a concrete one. However the steel may be more susceptible to the elements where the

concrete can provide greater cover for reinforcement and the concrete within the cover offers vertical

restraint. Concrete columns are also the most popular type of column used for highway bridges in the

UK. They also have a greater atheistically look, especially if the bridge deck itself is concrete. Therefore

the column should be a concrete one.

4 CONSIDERATION OF BRIDGE DECK TYPES FOR MAXIMUM SPANS

According to Figure 1, shown below, column positions require the decks span to be no less than 50

meters. Mainly due to the fact the bridge has to span over areas with large intersections. Therefore the

bridge deck needs to sustain a distance of 50 meters. The bridge deck selected needs to sustain such a

span.

Figure 1: Column Positions

Considering the biggest span of the bridge is 49 meters it is sensible to ignore the deck types spanning from 0 m > 40 m. it therefore follows that the deck types with the larger spans that will cover 50 meters need to be examined in more detail.

Span length Deck Type0 m> 20 m Insitu reinforced concrete

16 m > 30 m Insitu reinforced concrete slabSteel beams with insitu slab

12 m > 60m Beam and slab bridges30 m > 300 m Box girder bridges

Truss bridges 150 m > 1000 m Cable stayed bridges350 to? Suspension bridges

5 BEAM AND SLAB BRIDGES

Figure 2: http://www.concrete.org.uk/fingertips-nuggets.asp?cmd=display&id=295

5.1 INTRODUCTION

This type of bridge deck is a form of beam and slab bridges. It is one of the most common types of

bridge deck. This type of construction is good for spans reaching over areas such as railways and

waterways (precast beams insitu slab) where the span often ranges in-between 25-40 meters. However

in some situations bridges using this method have had spans as long as 60 metres.7. According to Nigel

R. Hewson spans over 40 meters are not as efficient as other types of bridge deck where a form of box

girder would be preferred.8. They are also less atheistically pleasing than other types. Even though this

may be the case other factors still have to be analyzed; this should not be seen as the leading factor. For

instance it may be easier to construct than some other types that are more efficient.

5.2 GENERAL ARRANGEMENT

Figure 3: ICE Prestressed Concrete Bridges, second edition

Beams are usually precast and the slab itself poured on site, typically has a depth of 200 mm < 300 mm.

in this case problems have been found when the slab itself was to thin and could not support the traffic

loads acted upon it.9. Figure 2 shows a variety of beams, they all differ in depth because of the ranges of

spans. It is taken that the span-to-depth ratios are usually 16:1 or 20:1.10. If the span taken is 50 meters

this means the depth of the deck would vary between 2.5 and 3.125 meters for this particular type of

bridge deck. If the ratio 20:1 was to be the case and a 50 meter span was used then the depth would be

greater than 3 meters which may cause atheistic issues as it won’t look as elegant as other methods.

5.3 CONSTRUCTABILITY

This type of bridge deck often encompasses a fully continuous arrangement.11. Making the slab

continuous and the beams simply supported. This makes the method of construction simpler and does

not require as many expansion joints. This stops leaking and makes inspections and maintenance a lot

easier. As discussed in the material section, less joints offer less access for aggressive substances.

Bridges which use bearings and other kinds of mechanisms need to be replaced over time due to wear

from the elements. The beam and slab method offers easier access to such mechanisms.

Casting yards found in the UK that produce precast beams only or most likely create beams ‘M’ ‘Y’ and

‘SY’. These types of beams will not cover the span required for this type of bridge, they may produce

one off productions such as the USA T beam. However this seems unlikely and if possible a one off

production for such a beam that will have a span 50 meters will be more costly as the manufactures will

have to produce new formwork which takes time and causes the costs to rise. Therefore if this type of

bridge deck was too carried out a beam like the USA T beam would have to be cast on site.

Insitu beams for longer spans with wide decks are often used on sites which have a big enough area to

create a casting zone. The most common type of bridge slab to be casted in this situation is the ladder

beam arrangement.12. Falsework needs to be erected which means good access is needed as well with

good ground conditions. This method is time consuming and labour intensive. For the given situation the

construction process that takes the least amount of time should be used unless other factors force the

designer to consider otherwise. ICE specified this method of falsework is best suited for wide decks. The

bridge specifications state the design of the bridge deck is to be 4 meters, which is not classed as a ‘wide

deck’. Giving the indication that a more efficient method should be used.

The most common form of placement for pre-cast beams is done by crane however in some situations

for long spans where the beam is too heavy in some countries two cranes are used, but in the UK is

often least preferred due to the increased safety risks.13. However a precast footbridge beam could be

lifted by a crane and the site does allow enough freedom of space for this to be carried out. Pre cast

beams can be placed directly onto the columns. In case of a continuous structure beams are positioned

onto temporary supports so the in situ connection is fashioned. The beams rest directly on top of

bearings, which makes it easier to access for future replacements. The beams are often held in place by

temporary frames until the slab is poured to prevent movement. Transportation of a precast beam

would be extremely difficult due to the weight and length, for this reason an insitu beam would be

preferred therefore requiring falsework and formwork.

Closely spaced beams means less formwork is needed than beams spaced wider apart.14. Formwork is

needed to support the construction of the slab, so good access is needed. The site does offer good

access, however a precast system craned into position will be faster than the insitu arrangement.

5.4 TRANSPORTATION

Transportation is needed for the precast units from the casting yard to the site. Special U beams which

are designed for longer spans can exceed weights as much as 150 tonnes. This requires special

transportation equipment and cranes capable of lifting such a weight. Beams spanning 50 meters would

be very difficult to transport and costly. Bradford has a lot of sharp corners and it seems highly unlikely

that a vehicle spanning such a distance will be able to get round the corners.

5.5 BEAM AND SLAB SUMMERY

The beam and slab bridge deck is a simply construction method and does reach the required span

length, however with some uncertainty. As stated before it is possible for some UK beams (most likely

SY beams) reaching spans up to 60 meters. For a higher level of certainty the USA bulb T beam should be

used to reach the desired span of 50 meters. However the precasting yards would ask for a higher

production cost, as they will most likely have to construct new formwork and methods. They are not as

efficient over this span as the box girder and perhaps not as atheistically pleasing. Because they are not

as efficient for this span it is also not a sustainable method of construction and can be seen as wasteful.

If the bridge deck was to be simply supported bearing checks and movement joint checks need to be

carried out more regularly as they are more exposed. However the structure can be made continuous

which reduces the amount of expansion joints and bearings needed, but resulting in a slower

construction process. Because the concrete is cast around the reinforcement it would make it difficult to

dismantle and separate the two materials for future recycling, especially if the structure is continuous.

Constructability Sustainability Aesthetics Durability DemolitionGood access needed.Simple design.Precast beams too big to transport. Falsework and formwork needed.Slow construction.

Large span for UK beams- more efficient methods available. If carried out large amounts of concrete and rebar needed. Therefore not v.sustainable.

Very low aesthetical qualities, basic design.

Insitu allows for continuous construction, less bearing and joints needed. Therefore good durability. Precast beams create joint problems – low durability.

Large amounts of conc and rebar to be dismantled. Poor demolition traits.

6 SINGLE-CELL BOX GIRDER BRIDGES

Figure 4: http://bridges.transportation.org/Pages/Virginia.aspx

6.1 INTRODUCTION

This type of bridge deck is very versatile it can produce ranges from 40 to 300 meters. It is also seen as

an aesthetically pleasing option.15. The method is efficient and makes good use of concrete and steel

compared to other solutions, however it is labour intensive for insitu designs.

6.2 GENERAL ARRANGEMENT

Spans crossing up to 60 meters the depth/span ratio used is often 1:20. For a 50 meter span the depth

of the girder would be roughly 2.5 metres.16 which is a slenderer and more elegant option than the

previously discussed. Quite frequently the box girders proportion for this arrangement has a constant

depth throughout the entire span, this makes the method of construction much easier.17. Box girders

often have cantilevers which extend outwards up to 3.5 meters where any further the girder becomes

ineffective. The webs are often sloped mainly for atheistic purposes.18. Single cell boxes have widths

6<16 meters. Anything below 6 meters makes construction harder.19. The design specifications state that

the bridge deck is to be no greater than 5 meters which means if this statement is true construction will

be difficult, mainly due to the limited access inside the box girder that is needed for the tendons.

Tendons are used in the longitudinal axis to provide prestressing to the structure. These tendons are

most commonly stored inside the duct so inspection is possible, they tend to be made of grouped wires

or single bars as well. This type of bridge deck can either be built into the piers or supported on

bearings.

6.3 CONSTRUCTABILITY

For insitu construction good access is also needed to provide falsework and formwork for the creation of

the box girder. Clearly the higher the pier the higher the costs of the scaffolding will be, where in this

case a temporary truss is used to support the construction of the girders, trusses are quicker to erect

and dismantle than scaffolding. However if falsework was to be used for the given circumstances, time

taken to dismantle will not take long because the height of the columns will be no greater than 6

meters. If the entire span is supported by the scaffolding it is possible for construction to take place at

different sections of the span which speeds up the rate of erection.20. For example formwork and

reinforcement can be installed on one span whilst on the other concreting and prestressing can take

place. Another method is to tension the precast elements at ground level and crane them into position,

this could be quicker than other methods if there is enough space and there is a crane big enough to lift

such a structure. For insitu prestressed box girders span by span construction is a fast method, however

great amounts of falsework is needed along with other resources such as formwork and high skilled

labour to achieve fast construction.21. One advantage with using a concrete box girder is that the slab

can be precast with the girder itself, making it one object. This eliminates the slab construction phase on

site, which will save time greatly. Because the girders are arriving in sections there will be a higher risk of

corrosion due to the greater number of joints, this means a waterproofing layer will be needed before

asphalt is laid. As discussed in the material section the consequences are high, so careful and skilled

work would be needed to ensure all joints are sealed correctly.

6.3.1 Pre-tensioned Method

There are two types of prestressing, post tensioning and pre tensioning. Pre tensioning is where strands,

tendons or wires are first tensioned in a position eccentric to the beam/girders axis. This is done using

anchorages found at the ends of the beam, concrete is then poured into the mould, and once the

concrete has gained enough strength the anchors are released. This causes the prestressed force to be

exerted into the concrete. The ends are then cut away. This type of prestressing is best carried out in

factory conditions.22. This method of pre-tensioning would be very problematic for curved shapes to

arrange the tendons.23.

6.3.2 Post-tensioned Method

This is where the steel tendons are also tensioned by, but in this case with the use of a jacking system.

The jack is held into position and supported by an already existing beam at the ends of the span sat

upon or often connected with an existing formwork base. A large percentage of insitu on site products

are created this way. The tendons are passed through a precast concrete member – through the

member’s ducts and in built anchorages. The jacking force is transferred to the anchorages, which in

turn is transferred to the concrete.24.

6.3.3 Selected method

Because of the limited time allocated to accomplish the project a combined method of the above shall

be used. The box girder is to be pre cast and then delivered to site. Once delivered they are laid out on

site and immediately tensioned. The girders need to be laid out on some falsework to allow for access

for grouting the sections together. This method would require a large crane ~750 tonnes. The whole

span could not be tensioned and delivered because the span is too big to transport.

6.4 SINGLE CELL CONCRETE BOX GIRDER SUMMERY

The facts presented above show that the concrete box girder is a very efficient way of using concrete,

and therefore more sustainable than the slab beam method. Due to the construction method selected

the process is much faster than the previous bridge deck. Time will be saved as a lot of the work is done

off site, the selected method means there is no need for falsework which takes time to construct. It also

decreases the amount of workforce needed resulting in a safer working environment as safety measures

can be fully implemented and managed. Because the elements are precast standards will be high and

the construction process overall will be less noisy less dusty and less time taken. The cost may be higher

because of the precast girders and the hiring of the crane will be needed for a large portion of the

construction process. In the specific environment such a cost may be explainable to reduce the amount

of time taken to construct and thus delays to the public. Due to the efficiently of the design less

concrete and steel is needed to construct making it more of a sustainable design than the beam and slab

strategy. The steel tendons are not encased in the concrete but connected by anchorages inside the box

girder. Making demolition easier as less reinforcement is encased in the concrete itself.

Constructability Sustainability Aesthetics Durability DemolitionGood access needed.Complicated formation.Fast.Crane neededSmaller but high workforce needed.Cleaner/less noise/not as dusty.

Precast units produce little waste resulting in minimum concrete needed. Design offers efficient use of materials for the given span, therefore more sustainable than first option.

Good – capable of creating curves and free flowing lines. Joints can be seen however.

If trapezoid shape is used splashback from road surfaces is reduced. Steel tendons protected and easy to access. Joints may cause durability problems

Very easy demolition, little reinforcement in the concrete, makes it easier to dismantle.

7 COMPOSITE STEEL BOX GIRDER BRIDGES

Figure 5: http://www.idaengineers.com/major-projects.html

7.1 INTRODUCTION

Steel box girders are one of the most common types of bridge deck used, they have reputation for

creating modern clean designs. Due to the nature and properties of steel the box shape aids torsional

stiffness to the bridge, this advantage is good for bridges with a curved shape.25. If designed correctly more than half of their surface area is protected from the weather, which means less maintenance is

needed for such a bridge deck.26.

7.2 GENERAL ARRANGEMENT

Steel box girders can span large distances similar to concrete box girders. The depth/span ratio used is

often 0.65 to 1 ratio.27. This means for the 50 meter span the depth of the girder would be 3.25 meters.

Short end spans are least desirable because the length causes uplift and hogging effects.28. Box design

results in boxes of a narrow shape where the height is greater than width. This creates greater

resistance to buckling, torsion and distortion.29.

7.3 CONSTRUCTABILITY

The method of construction uses similar approaches used for prestressed girders. Because of the span

the girder will need to be separated into sections and then welded together on site. Following the same

approach as the prestressed concrete girder it can either be supported by falsework and then welded, or

welded at ground level and craned into position. A 3rd option is use hydraulic pulling equipment. Once

the deck has been fabricated and delivered to site it is lined up with the bridge columns and either

pushed or pull into position using hydraulic equipment. In this design it would be necessary for part of

the deck to be supported on some form of scaffolding so that it is level with the columns. This will give a

launching pad from where the deck can be pushed from. Curved bridges may find this method difficult

to accomplish due to the geometry of the bridge deck, where lifting of the sections would be a preferred

option. Once the bridge deck is in position formwork and reinforcement are constructed and the

concrete poured. Quite often the concrete is poured onto a corrugated steel sheet, the edges need to

be designed so they are not subject to the elements. Unless the slab is precast, if so the slab and the

steel beams are connect with long bolts protruding down through the slab into the steel beam. Again

care needs to be taken so that road surface water does not leak through the bolts. Therefore a

waterproofing layer will be needed.

One of the most expensive stages of construction is the fabrication process. The cost of steel is high and

requires a skilled workforce to weld the parts of the box section together. The longer the individual

girder the less welding or bolting of box girders together is needed, however large amounts of welding is

still needed if stiffeners need to be added. This has to be balanced with site access and transport.

7.4 TRANSPORT

Box girders are constructed to the biggest size possible that can reasonably transported to site. Steel

box girders are delivered in a similar fashion to the concrete box girders. However steel box girders are

lighter than precast members meaning lighter cranes are needed for steel.

7.5 SUMMARY OF STEEL BOX GIRDER BRIDGES

Due to the limited time available to construct the bridge the slab should be precast and formed with the

box girder, welded then lifted into place. The preferred shape would be trapezoid to limit splash back on

the bridge deck. This type of bridge deck also has areas where aggressive species can enter and effect

the overall structural integrity of the bridge. The ends of the metal sheeting upon which the slab is

rested, the shear connectors between the slab and girder all give the opportunity for aggressive species

to enter. A high skilled workforce for welding the sections together will be needed, no gaps can be let as

discussed in the materials section where crevices can form.

Constructability Sustainability Aesthetics Durability DemolitionGood access needed.Simple design.Precast beams too big to transport. Falsework and formwork needed.Relatively quick construction.

Quality control assured in factory therefore very little steel wasted. Efficient design for this type of span therefore it has sustainable qualities.

Similar attributes to the concrete box girder however looks out of place with the use of a concrete column.

Trapezoid shape offers safety from elements

Little crevices

Strong concrete needed for slab.

Steel box girder easily recyclable.Slab may be harder due to reinforcement encased in concrete.

8 OTHER BRIDGE TYPES

Figure 6: http://happypontist.blogspot.co.uk/2012/12/lune-millennium-bridge-lancaster.html

Other bridge types such as truss, cable stayed and suspension shan’t be evaluated due to the fact that

these types of bridges have high supporting columns and generally their geometry and design will look

demanding and have an encroaching feel on neighboring buildings. For example one cable stayed bridge

in Lancaster has a total height of 40 meters which supports a span of 70 ~ 100 meters (minus

gangway).30. In this situation such an imposing structure would not suit Bradford’s city centre, where in

Lancaster’s situation the bridge spanned over a river resulting in the structure not impeding on nearby

buildings.

9 SELECTED BRIDGE DECK The best bridge deck for the given specification is the Pre-cast single-cell box girder. It offers the

quickest construction period, and looks the most atheistically pleasing. Beam and slab bridge decks are

not as efficient as girder design. Composite girders require higher skilled workforce and fabrications

costs will be high. Demolition of the concrete deck would be easier than the composite deck due to little

reinforcement. Concrete also needs less protective coatings than steel and has less mechanisms to go

wrong and service.

10 REFERENCES1. Durability of Concrete Structures Investigation, repair, protection, by Geoff Mays. Chapter: The

behaviour of concrete2. Durability of Concrete Structures Investigation, repair, protection, by Geoff Mays. Chapter: 1.3

Mechanisms and causes of failure3. Prestressed Concrete Design. Second Edition. Author: M.K. Hurst. Page 64. Durability of Materials and Structures in Building and Civil Engineering, by David H

Deacon. Page 1285. http://www.steelconstruction.info/Influence_of_design_on_corrosion 6. Durability of Materials and Structures in Building and Civil Engineering, by David H

Deacon. Page 1377. Prestressed Concrete Bridges: Design and Construction. By Nigel R. Hewson. Page 1588. Prestressed Concrete Bridges: Design and Construction. By Nigel R. Hewson. Page 1599. ICE Prestressed Concrete Bridges, second edition. Page 17610. ICE Prestressed Concrete Bridges, second edition. Page 17611. ICE Prestressed Concrete Bridges, second edition. Page 17712. ICE Prestressed Concrete Bridges, second edition. Page 17613. ICE Prestressed Concrete Bridges, second edition. Page 18614. ICE Prestressed Concrete Bridges, second edition. Page 18715. ICE Prestressed Concrete Bridges, second edition. Page 20916. ICE Prestressed Concrete Bridges, second edition. Page 20917. ICE Prestressed Concrete Bridges, second edition. Page 20918. ICE Prestressed Concrete Bridges, second edition. Page 21019. ICE Prestressed Concrete Bridges, second edition. Page 21120. ICE Prestressed Concrete Bridges, second edition. Page 21321. ICE Prestressed Concrete Bridges, second edition. Page 21322. Prestressed Concrete Design. Second Edition. Author: M.K. Hurst. Page 723. Prestressed Concrete Design. Second Edition. Author: M.K. Hurst. Page 824. Prestressed Concrete Design. Second Edition. Author: M.K. Hurst. Page 825. www.tatasteelconstruction.com/en/reference/teaching-resources/steel-bridge-resources/21st-

century-bridges/box-girder 26. http://www.bridgedesign.org.uk/parts/box.html 27. Bridge Engineering Handbook, Second Edition. Wai-Fah Chen and Lian Duan. Page 22928. Bridge Engineering Handbook, Second Edition. Wai-Fah Chen and Lian Duan. Page 22929. Bridge Engineering Handbook, Second Edition. Wai-Fah Chen and Lian Duan. Page 22930. http://www.lunemillenniumbridge.info/