A CRITICAL ANALYSIS

Abstract: This report is a detailed analysis of the Itchen Bridge. The bridge isconstruction that crosses the river Itchen as it passes through Southampton. after almost 100 years in the pipeline.loading and strength of the bridge.

Keywords: Concrete, Prestressed, Balanced Cantilever, Southampton.

1 Introduction

1.1 General Information

The Itchen Bridge is a road and pedestrian crossing over the River Itchen joining the areas of Woolston and Chapel, in Southampton. It carries the A3025, a major cross city route in Southampton.

The bridge is built from reinforced concrete and consists of five spans; three central spans of metres and two ‘end’ spans of around 80 metre

The bridge is not monolithic but each span is made up of two 45m cantilevers from each pier completed with a simply supported section of 35 m length.

The road across the bridge is two lanes wide, one dedicated to each direction and the traffic is controlled by lights and a toll booth at the Woolston End.

Pedestrians also have access to the bridge and is a well used route especially on Saturday afternoons when

Proceedings of Bridge Engineering 2 Conference 200823 April 2008, University of Bath, Bath, UK

A CRITICAL ANALYSIS OF THE ITCHEN BRIDGE, SOUTHAMPTON

H J T Richardson1

1Student – University of Bath

is a detailed analysis of the Itchen Bridge. The bridge is a reinforced concrete prestressed er Itchen as it passes through Southampton. The bridge was opened in 1977

after almost 100 years in the pipeline. This report critically analyses the aesthetics, construction method,

Concrete, Prestressed, Balanced Cantilever, Southampton.

The Itchen Bridge is a road and pedestrian crossing over the River Itchen joining the areas of Woolston and

It carries the A3025, a major

is built from reinforced concrete and three central spans of around 125

metres.The bridge is not monolithic but each span is made up

pier completed with a

The road across the bridge is two lanes wide, one dedicated to each direction and the traffic is controlled by lights and a toll booth at the Woolston End.

e bridge and is a well used route especially on Saturday afternoons when

Southampton Football Club providing an excellent link between the stadium and a large residential area of Southampton.

Figure 2: Location map of the Itchen Bridge

ge Engineering 2 Conference 2008

Figure 1: The Itchen Bridge

, SOUTHAMPTON

a reinforced concrete prestressed The bridge was opened in 1977

critically analyses the aesthetics, construction method,

Southampton Football Club has a home game, the bridge providing an excellent link between the stadium and a large residential area of Southampton.

ap of the Itchen Bridge

1.2 History

On 1st March 1977 Mrs. Edith Park became the first person to cross the new Itchen Bridge. The following day the bridge was officially opened to traffic after 94 years of sporadic development.

In 1883 the Itchen Bridge Company was formed with the intention of creating a crossing over the river Itchen. Their plans for a 17 span arch bridge with a central swinging section were stopped by the British Admiralty due to fears that access to the Itchen navigation would beblocked.

Instead of a Bridge they introduced a steam driven chain ferry to provide a crossing. This chain ferry remained operational, under many guises but always in the same place, until May 1977 just after the opening of Itchen Bridge. The Itchen ferry was a much loved Southampton institution and its replacement led to resentment of the new bridge.

Six schemes were suggested and in 1938, including another swing bridge, a transporter bridge and a vertical lift bridge. These schemes were well into development when the Second World War broke out in 1939 halting their progress. After the war there was no money to build the bridge.

In 1960 plans were again underway for a crossing over the Itchen. The government agreed to pay 75% of the cost of the bridge from their budget, the remainder coming from Southampton City Council. This deal fell through however and the plans were, once again, scrapped.

In 1970 Southampton City Council decided to go ahead with the 1960 design and to raise the £12.174million required for construction through loans to be paid off by placing a toll on use of the bridge.

Kier Ltd. won the tender for construction and workbegan on 22nd March 1974. Just under three years later the Itchen Bridge was finished ahead of schedule ready for Mrs. Edith Park’s first journey in 1977.

2 Aesthetics

Fritz Leonhardt’s 10 aspects of aesthetics are generally accepted as the definitive criteria for defining beauty in bridges. Here the Itchen Bridge has beenanalysed with a view to these rules.

From a distance the Itchen Bridge is a beautiful bridge. The clean lines and good proportions mean it is very easy for the eye to flow along the length.

The ability to run the eye across the Itchen Bridge is a result of two well used effects; order and the use of colour. The fascia of the Itchen Bridge has been made of a slightly lighter coloured concrete. This naturally draws the eye to the fascia which sweeps across the bridge. This can be seen in Fig. 2. If the fascia were a starker white, the effect may be more pronounced, perhaps improving the overall aesthetic.

In addition to the ‘smooth’ fascia there are no sudden depth changes or unnecessary lines that jar the eye. The underside of the bridge again has smooth curves between piers and there are no sudden changes in depth or any jarring corners at a first glance.

Figure 3: Fascia and expansion joints detail.

Upon closer inspection there are a series of dark lines that run down the deck which causes the eye to jar slightly, shown in Fig 3. These are caused by the placing of expansion joints where the central portions of eachspan were dropped into place. There is little that can be done to avoid this issue.

It is vital that a bridge exudes a sense of strength and safety to the public who will use it every day. From even a brief glance at the Itchen Bridge it is clearly a strong and stable bridge without looking overly heavy.

All the elements of the Itchen Bridge feel to be in theright proportion. The deck is not too deep or too thin, the piers neither too slender nor too thick and the curvature of the top deck is, again, just right being not too steep or too shallow.

It is generally thought that a bridge should have a constant ratio of width to depth for its spans. This is not the case on Itchen Bridge; instead the three main spans have a constant width despite varying heights. If the Itchen Bridge were designed for constant proportionalityit would have made very little difference to the look of the bridge. This is because the relatively flat profile of the spans.

The haunches, where the deck meets the piers could be criticised for being too deep. However due to the length of cantilevers there is little that can be done to reduce them as they must carry the large hogging moments over the piers.

From a closer perspective the Itchen Bridge loses the sense of beauty it has from a distance. This is because there are several design flaws that can only be seen when close or on the bridge.

The initial impression of a bridge from up close is defined by the textures used on the construction. Figure 3shows the two main textures on the Itchen Bridge. At the bottom of the picture is the ribbed concrete making up the majority of the ‘structural’ elements and the lighter area is the texture of the fascia.

Figure 4: Bridge texturesBoth of these textures are unpleasant and seriously

detract from the aesthetic of the bridge. If the bridge were rebuilt these textures should be replaced with smoother, more pleasant finishes.

This being said there are several interesting refinements of design that can only be seen when up close. The most interesting refinement is that the hand rails have been swept back such that they cannot be seen from ground level where they would interrupt the lines of the bridge. This can be seen in Fig. 5.

Figure 5: Hand rail refinement

Unfortunately there are many more bad elements on the Itchen Bridge than good when viewed from up close.Some of these poor features are shown in Fig 6. Some of these poor features are bad design but the vast majority seem to be either after thoughts, added after building or bad maintenance.

Figure 6: Poor design features

It is possible to get underneath the bridge around both abutments. Due to the height of the bridge it looms over houses and shopping areas that have developed. This is a serious problem with the bridge now but when it was built, in 1977, the areas of Woolston and Chapel were far more industrial than they are now. As a result of the development and urban regeneration of these areas the Itchen Bridge does not fit in with its environment and has created some very awkward and uncomfortable spaces. This can be seen in the top image of Figure 6

Figure 7: The Itchen Bridge from Woolston shore

Closer to the shorelines the bridge has enough height to avoid creating dark awkward spaces but is a surprisingly pleasant experience. Figure 7 was taken from a small housing development on the Woolston shoreline.

Visible in Fig. 8 are the artificial islands on which the Itchen Bridge sits. At the point at which the bridge crosses the Itchen the river is tidal and when the tide is in these islands have a very low profile however when the tide goes out these islands are very high out of the water. At such times the islands seriously detract from the aesthetic of the bridge.

Figure 8: Artificial islands when the tide is out

The Itchen Bridge fails to fulfil Leonhardt’s 10 criteria. According to Leonhardt therefore the Itchen Bridge should not be beautiful. This is certainly true from a close perspective due to the poor detailing and maintenance. However from a distance the Itchen Bridge is beautiful as a result of the clean lines and good proportions.

3 Vandalism

Aesthetics and Vandalism are very closely related. The effect of vandalism is to reduce the attractiveness of a building or object.

Figure 9: Graffiti at the Woolston abutment

Along the hand rails of the Itchen Bridge there are small amounts of graffiti but by far the worst amount of graffiti is around the Woolston abutment.

Other than graffiti there is no physical vandalism on the Itchen Bridge. This is because the bridge is made of hard materials.

4 Durability

Durability is defined in Ref. [1] as the “Capability of withstanding decay or wear.” The Itchen Bridge is made from concrete which is susceptible to numerous forms of deterioration.

After 31 years it would be expected that a concrete bridge would begin to deteriorate in numerous places.This is true of the Itchen Bridge.

Even with a quick and untrained eye there are several places on the Itchen Bridge where this deterioration has occurred. The worst example of this is where the concrete has begun to spall.

Spalling occurs when the concrete cracks allowing water and oxygen to reach the reinforcement. The reinforcement corrodes which causes expansion. This expansion forces the cracked concrete to break away.

Figure 10: Wear and tear

It would be reasonable to assume that of this kind of damage has occurred on the parapets of the bridge that similar damage could have occurred elsewhere on the bridge. This damage, if left unchecked, could have very serious consequences for the structural integrity of the Itchen Bridge.

It is for this reason that regular checks are undertaken on all bridges. It is assumed, and hoped, that any other potentially serious damage is known about on the bridge and steps are being taken to correct it.

5 Construction

The Itchen Bridge was built using cantilever construction, or balanced cantilever construction as it is often known. This is a common and well used method of construction now but in 1977 it was a relatively new technique.

5.1 Balanced cantilever construction

The technique was developed by German Engineers Dykerhoff and Widmann and was first used on Bendorf Bridge over the Rhine, completed 1964 and was referred to in Ref. [2] as “An outstanding example of ingenuity...”

The concept of this construction technique is very simple. In Ref. [3] the concept is described as building“...outwards in both directions so that balance is maintained over the pier...”

The construction process begins with the building of the piers for the bridge. After this the deck section above the pier is built.

The next stage is to build outwards from either pier in sections. The sections can either be cast in situ or precast. Either way the sections are constructed or simultaneously.

The Itchen Bridge was built using precast sections.This is obvious as on several points on the bridge,numbers can be seen written on the sections of the bridge. One example of this can be seen in Fig. 12

Figure 12: Itchen Bridge under construction

In this image the bridge is seen nearing completion. On the left of the image you can clearly see a crane which would be used to lift the precast sections into place.

The precast sections are craned into place; one either side of the central section and are glued into place. The glue used will be some form of epoxy resin and has advantages beyond the obvious. The glue will also prevent water infiltration between the sections, providing protection from corrosion.

This has the effect of keeping the system balanced.After this post tensioning is added to prestress the whole system. This process is repeated, building out from the centre.

Figure 13: Balanced cantilever construction

The addition of these additional segments creates additional hogging moments in the section over the pier. These hogging moments must be resisted through the prestressing. Figure 13 represents a bridge under construction; the white sections have been added to the existing grey construction. When this is done the only prestressing in place is in the, already constructed grey sections. This pre-stressing must therefore carry the additional weight.

W

l

Figure 11: The Itchen Bridge under construction

The prestress must be carefully chosen to restrict the final state stresses in the central section never exceeding zero. This is because the concrete must not be allowed to go into tension. In reality the concrete can go into a small amount of tension but this should be minimised.

In Fig. 14 the stress state above the pier, caused by the addition the white segment (Fig. 13), is broken down into its constituent parts.

The additional hogging moment must be negated bythe prestressing. This will occur both as a direct compression and as a result of the eccentricity of the load.The prestress must be carefully selected such that no tension is added to the structure. This is a very powerful tool available to the engineer.

Once the sections have been built out from the pier there are, again, several options open to the engineerregarding the completion of the spans. On the Itchen Bridge the centres of each span are simply supported beams that have been dropped into place. This creates additional and large hogging moments at the piers, due toconcentrated loads at the end of the cantilever sections and as such the deck needs to be very deep at these points. This is why the haunches on the Itchen Bridge are so large.

5.2 Cofferdams

A cofferdam is an enclosure built to dam part of a waterway in order for construction to take place. These were used on the Itchen Bridge to allow the foundations and piers to be built.

Cofferdams used in bridge construction are usually built with sheet piles forming the sides and the area inside is excavated. To finish the dam any piles for the bridge are built and a concrete top is used to finish the foundations and to seal the base of the dam. After this is completed, pier construction begins from within the cofferdam.

On both sides of the river the Itchen cofferdams were built back to the shore to allow for ease of access to the construction site. The cofferdams can be seen in Fig. 8.

Usually once the piers have been completed the cofferdam is removed and the waters allowed back. On the Itchen Bridge however this was not done. The cofferdams were left in place throughout construction and only removed when the bridge was completed. This allows the area behind the dam to be used as a site area.

6 Loading

Bridges are designed using Ultimate Limit State (ULS) and Serviceability Limit State (SLS) according to BS5400-1:2006. There are five load types that must be considered these are dead loading, super-dead loading, live (traffic) loading, wind loading and temperature effects. The loads are determined as nominal loads which are then factored under different loading combinations.

6.1 Dead and Super-Dead Loading

Dead loading comprises the self weight of the structural elements of the bridge. The super-dead loading comprises the additional dead loading added after the structure is complete such as façade panels, road surfacing etc

Unfortunately it is impossible to determine what on the Itchen Bridge is to be considered dead load and what is to be considered super-dead. I will assume that approximately 15 percent of the total dead weight is super-dead.

Taking the weight of reinforced concrete to be 2400kg/m3 and an assumed cross section of 17.75m2 gives a total weight for the bridge of 24.5 million kg. This is equivalent to approximately 245,000kN, which is 426kN/m.

Assuming 15% of this is super-dead and the remainder is dead weight the dead weight is 208,000kN(360kN/m) and the super-dead 36,800kN (64kN/m).

6.2 Live Loading

Live loading on bridges is calculated using HA and HB loading. These are loading cases set up by the highways agency.6.2.1 Notional lanes

The number of lanes of traffic the bridge is intended to carry is ignored when it comes to working out the live loading on bridges. Instead the concept of notional lanes is used.

The carriageway on the Itchen Bridge is assumed to be 10m, excluding the pavement at either side which are dealt with in a different way. According to BS5400 there should therefore be 3 notional lanes, giving a notional lane width of 3.33m.

e

P

+ + =

Figure 14: Stress states above the pier

Loading Prestress Eccentric loading Hogging moment Final State P (Pe)y (Wl)y A I I

6.2.2 HA loadingHA loading represents supposedly normal loading on

the bridge. It is representative of the bridge being fully loaded with heavy fast moving trucks.

HA loading consists a uniform loading over a single notional lane with an additional knife edge load (KEL) positioned to give the most adverse effect.

The unfactored distributed load can be found using the following formula:

W = 151 (1 / L) 0.475 (1)

Where L is the total length of the bridge.Using eq. (1) and taking the length of the bridge to be

575m the uniform loading becomes 7.4kN/m. For notional lanes of 3.33m width this is equivalent to 2.22kN/m2.

Full HA loading is applied over two notional lanes with any other lanes taking one third of full loading.

The KEL is always taken to be 120kN per notional land and perpendicular to the flow of traffic. For notional lanes of 3.33m width this is equivalent to 36kN/m.6.2.2 HB loading

HB loading represents abnormal loading on bridges, for example a long heavy objet under transport.

HB loading consists of a set of point loads, representing the wheels of the truck, each loaded with 2.5kN per unit of loading. Full loading is considered to be 45 units of loading, which gives each wheel a total load of 112.5kN. The spacing of the wheels is shown in Fig 15.

Figure 15: HB loading spacing

The central spacing, marked with an arrow in Fig 14, is not a fixed distance. It can either be taken as 6m, 11m, 16m, 21m or 36m dependent on which will give the most severe effect.

When HB loading is applied to notional lanes with a width of less than 3.5m, as in this case, the HB loading will be spread over two lanes, with any remaining lanes taking one third of HA loading. Either side of the HB loading there is a 25m zone with no loading followed by full HA loading.6.2.3 Additional live loading

There are several additional live loads that must be considered. These loadings represent the effects of skidding, centrifuge, longitudinal and collision loads.

Each of these loads consists of a secondary loading effect and the associated primary loading.

Centrifugal loading needs to be considered where the bridge is curved along its plan. This is not the case for the Itchen Bridge and so can be ignored.

Longitudinal loading of 8kN/m on a single notional lane plus an associated 200kN load acting vertically is

applied to represent braking of trucks. In addition, under HB loading, 25% of the total HB load must be applied over 2 axles acting longitudinally.

Skidding is modelled as a 250kN horizontal point load acting anywhere on the bridge, in any direction, with the associated HA loading.

The parapet must be designed to withstand 25 units of HB loading acting horizontally.6.2.4 Collision loading

Collision with the piers must also be considered when designing a bridge. The piers must withstand a load representing a large truck travelling at high speeds. This is irrelevant to the Itchen Bridge due to the fact that the piers are in a river.

It would be reasonable to assume that instead of a fast moving truck a load representing a ship impact would be used. If this were done however the piers on this bridge would have to be enormous. Figure 16 shows a typical ship that passes under the Itchen Bridge.

Figure 16: Typical ship passing under the Itchen Bridge

This boat, Sand Harrier, has an unloaded deadweight of 5916 tons, equivalent to approximately 59,000kN. Unlike road vehicles, ships are not designed to buckle upon impact and as such even at low speeds the impact forces would be immense. Clearly if the piers were designed to withstand loads in this order they would be massive.

To avoid this issue the underwater foundations of the Itchen Bridge are very large and have a profile designed to push ships away from the piers and thus avoid the large collision loads.

In addition to this Southampton VTS will provide a pilot boat for any long or heavy ships in the Itchen waterway to guide ships under the Itchen Bridge. For ships that do not require piloting there is a marker on the central span of the bridge to mark the centreline of the deep channel under the bridge.

Figure 17: Central channel marker

6.3 Wind Loading

The following calculations have been prepared from BS5400 to provide a simple analysis of the wind loading on the Itchen Bridge.6.3.1 Maximum wind gust

The maximum wind gust striking the bridge can be calculated using the following formula:

Vc = K1 S1 S2. (2)

Where v is the mean hourly wind speed, K1 is a windcoefficient, S1 is a funnelling factor and S2 is the gust factor.

From BS5400 the mean hourly wind speed is 27ms-1, the gust factor for 600m loaded length and 40m height is 1.40, and the tunnelling factor is 1.00 as tunnelling is unlikely for the Itchen Bridge. Using Eqn (2), Vc =38ms-1.

6.3.2 Horizontal wind loadThe horizontal wind load, acting at the centroid of the

part of the bridge under consideration is calculated using the following formula:

Pt = q A1 CD. (3)

Where q, the dynamic pressure head, is;

q = 0.613 Vc2 (4)

A1 is the solid projected area in m2 which assuming a an average deck depth of 7.5m and the length 575m the projected area is 4313m2. CD is the drag coefficient, taken from BS5400 and is dependent on the width to depth ratio of the bridge. Taking the average deck depth to be 6.5 metres and the width as 15 metres gives a ratio of 2.3 and from BS5400. This corresponds to a drag coefficient of 1.5.

q = 885 N/m2.

Therefore,

Pt = 5723 kN.

6.3.3 Vertical wind loadVertical wind loading is calculated using the

following formula. Vertical wind loading can either be taken as a downward pressure or as an uplifting force.

Pv = q A3 CL. (5)

Again, q is the dynamic pressure head given by Eq. (4). A3 is the deck plan area, 8625m2, and CL is a lift coefficient and is dependent on the width to depth ratio of the deck, again 2.3, corresponding to a CL of 0.4. Using Eqn. (5), Pv =3058 kN.

6.4 Temperature loading

Temperature effects are important when considering stresses in bridges. Due to their length the temperature induced expansions can be very large. In buildings the expansion is mostly vertical whereas on bridges the

expansion will be horizontal. If the bridge is restrained horizontally large stresses can build up which can result in failure.

The following calculations are very simplified but will still give an indication of the expansion and stresses that can result from temperature effects.

To calculate the thermal expansion the following formulae are used:

δ = εl.. (6)

Where,

ε = ∆Tα. (7)

The Itchen Bridge is constructed from reinforced concrete so the coefficient of thermal expansion (α) will be 12x10-6. Assuming a change in temperature of 25oCand the length of the Itchen Bridge to be 575m Eqn. (6) gives ε = 300με and therefore Eqn. (7) gives δ = 172.5mm.

This large expansion must be accommodated within the structure. This is done by putting expansion joints within the structure. On the Itchen Bridge there are 8 expansion joints along the length of the bridge.

Figure 18 shows the one of the expansion joints on the Itchen Bridge. These expansion joints would be easily capable of taking the required expansions. However, as can be seen, they have become partially clogged and as such will not fully function.

Figure 18: Clogged expansion joints.

If expansion joints are unable to function the expansion cannot be accommodated and as such stresses will build up within the structure. The relationship between expansion and stress is well known and as such the calculation of these stresses is simple.

σ = ε E. (8)

Using Young’s Modulus, E, for concrete (30,000 N/mm2) and the strain value calculated from Eqn. (7), Eqn. (8) gives σ = 9N/mm2.

This is a large stress and would seriously affect the useful capacity of the bridge. Concrete has an approximate capacity of 14 N/mm2 and with a potential 9N/mm2 of stresses resulting from the temperature effectsalmost two thirds of this capacity will be wasted.

Without expansion joints the bridge deck would need to be very deep in order to carry the loads in addition to the temperature induced stresses.

6.5 Load Combinations

There are five principal load combinations defined in BS5400 that must be applied to the bridge under different load factors. These five load combinations are detailed below.

1. Dead + Super-dead + Primary (vertical) Live.2. Dead + Super-dead + Live + Wind.3. Dead + Super-dead + Live + Temperature.4. Dead + Super-dead + Live + Secondary Live.5. Dead + Super-dead + Loads due to Friction.

The load factors change for each combination and can be found in BS5400-2 Table 1. The nominal loads calculated above must be multiplied by two factors before application to the bridge; γfl, the partial loading factor and γf3, an additional factor to take into account any inaccuracies in the analysis.

Load combinations 3 and 5 can be ignored for the Itchen Bridge as temperature effects can be ignored due to the presence of expansion joints along the length of the bridge and loads due to friction at the piers can also be ignored. This is because the piers are monolithic with the cantilevered sections.

7 Load Application

Loading is applied in order to produce the worst possible effect for both sagging and hogging. Here the worst hogging moment will be calculated for the cantilever sections and the worst sagging moment for the simply supported sections.

The analysis below has been completed to SLS loading as the bridge has already been constructed and as such γf3 is 1.00.

7.1 Simply Supported Sections

Table 1 shows the factored loads to be applied. For maximum bending load combination two should be applied with a downward wind pressure.

For maximum bending from HB loading 45 units of loading should be applied.

Table 1: Factored loading

1-Factored LoadingLoad Nominal γfl FactoredDead 360kN/m 1.00 360kN/m

Super-dead 64kN/m 1.75 112kN/mHA 17.3kN/m

300kN1.20 20.8kN/m

360kNHB 2.5kN/m

400kN1.10 2.75kN/m

440kNWind 5.3kN/m 1.10 5.8kN/m

Figure 19 shows the loading and bending moment diagram for the simple supported sections of the Itchen Bridge.

Figure 19: Loading on simply supported span

7.2 Cantilever sections

The cantilever sections will experience both vertical and horizontal bending. This bending will always be a hogging moment and will always be at a maximum at the support.7.2.1 Vertical loading

The loads from Table 1 can also be used for the loading on the cantlever section. To achieve maximum hogging at the support, maximum loading must be applied to the cantilever.

The loads from Table 1 can also be used on the cantilevered sections of the Itchen Bridge. No changes are required to any of the load values but where they are placed may need to be changed to achieve maximum effect.

The positions of the point loads must be placed for maximum effect. This means they musy be placed as far from the support as possible. Again, the minimum spacing between axles must be selected for HB loading.

Loading from the simply supported section must also be taken into account. This will act as a point load at the very end of the cantilever section and at its maximum value will be a load of 8.4MN.

Assuming the prestressing has been designed to remove the hogging moments caused by the dead and super-dead loading, these can be ignored when considering the total moment on the cantilever sections.

Dead

Super-dead

Live HA

Live HB

Wind

Bending Moment Diagram

35m

12.7 1.8 6 1.8 12.7

360kN/m

112kN/m

20.8kN/m360kN

2.75kN/m440kN each

5.8kN/m

HA: Mmax: 82 MNmHB: Mmax: 88 MNm

Figure 20: Vertical loading on cantilevers

7.2.2 Horizontal loadingThe maximum horizontal loading that will occur on

the cantilever sections of the deck will occur under load combination four.

The load to be considered is the 250kN horizontal skidding load to be applied anywhere on the section. In this case the load should be applied to the very end of the cantilever so that the moment at the support is maximised.

Again the loading must be factored before it is applied. γfl for the skidding load is 1.00 making the factored load 250kN

Figure 21: Horizonal loading on cantilevers

7.3 Piers

The piers are monolithic with the cantilever sections of the deck on the Itchen Bridge, so must be designed to take any moments that are passed into them.

To feel bending there must be uneven loading either on side of the column. In order for this to happen the loading factors must be reduced for one of the spans.

The piers will feel bending both laterally and longitunally depending on how the cantilevers are loaded.

For maximum bending maximum loading must be applied on one cantilever and minimum loading on the other.7.3.1 Longitudinal bending

The minimum vertical loading will occur under loading combination one. To minimise the loads it is possible to ignore any live and super-dead loads and to take an unfactored dead load.

This gives a total loading of only 360kN/m acting along the cantilever. This gives a bending moment of 721MNm.

The maximum longitudinal bending moment in the pier is the differnce between the maximum bending moment and minimum bending moment in the cantilevers. This is 239MN.7.3.2 Lateral bending

Again the skidding load on either cantilever can be ignored as to maximise the bending in the pier. The maximum lateral bending within the pier is therefore 11MN.7.3.3 Axial loading

Each pier will feel an axial load equal to half the total loading on two cantilever sections and the half of the simply supported sections they themselves support. The maximum axial loading will occur when the loading on the cantilevers and simply supported sections is also at a maximum.

The maximum loading on the cantilevers is 23.4MN and the maximum loading from the simply supported spans is 8.4MN. The total load is therefore 31.8MN axially. This is assumed to be acting through the centroid of the pier and as such any effects of eccentric loading have been ignored.

8 Strength

In order to determine whether the Itchen Bridge is strong enough to withstand loading the loading capacities and moment capacities must be determined.

In order to do this several large assumptions must be made. Figure 22 shows the assumed cross section of the deck.

Figure 22: Assumed cross section

7500

6500

12000

5250 5250

Skidding

Bending Moment Diagram

45m

250kN

Mmax: 11MNm

Dead

Superdead

Live HA

Live HB

Wind

Simply Supported Load

Bending Moment Diagram

45m

35.4 1.8 6 1.8

0kN/m

0kN/m

20.8kN/m360kN

2.75kN/m440kN each

5.8kN/m

8.4MN

HA: Mmax: 421 MNmHB: Mmax: 458 MNm

Assuming the section acts as a simple concrete beam and that there is only reinforcement in the bottom of the section, a simple analysis can be made to determine the amount of steel reinforcement required to make the section able to resist the forces acting upon it.

Assuming the depth to the reinforcement of 7250mm, fcu is 30N/mm2 and fy is 460 N/mm2 simple analysis can be made to calculate values for k and the lever arm, z, using the following formulae:

k = M (9)fcu bd2

z = d[ 0.5 + √(0.25 – k/0.9) (10)

After this the amount of steel required can be calculated using the following formula:

As = M (11)0.95 fy z

Finally the capacity is checked using Eqns (12) (13) (14) and (15)

T = 0.95 fy As (12)

C = 0.67 fcu/γm (13)

x = C/T (14)

Mc = T (d – 0.45x) (15)

8.1 Simply supported sections

Table 2 shows the values calculated from the analysis described for the simply supported sections of the Itchen Bridge.

Table 2: Simply supported moment capacity

1-Simply supported moment capacityk 0.16z 7199 mm

As 28300 mm2

T 13 MNC 0.14x MNx 93

Mc 94 MNm

8.2 Cantilever sections

Table 3 shows the values calculated from the analysis described for cantilever sections of the Itchen Bridge.

Table 3: Cantilever moment capacity

3-Cantilever moment capacityk 0.16z 7119 mm

As 151200 mm2

T 66MNC 0.14x MNx 471

Mc 464 MNm

8.2 Piers

8.2.1 Moment capacityTable 4 shows the values calculated from the analysis

described for the piers. Assuming the dimensions of the piers are h = 12000mm, b = 8000 and depth the reinforcement is 11000mm

Table 4: Pier moment capacity

4-Pier moment capacityk 0.003z 10996 mm

As 28800 mm2

T 13 MNC 0.09x MNx 144

Mc 142 MNm

8.2.2 Axial capacityThe axial capacity of the piers can be found using

Eqn (16).

Pmax = σmax / A (16)

This gives a total axial capacity of 11000MN.

8.2 Capacity

Table 5 gives the calculated capacities and the loadings on the components of the bridge.

Table 5: Capacity and Load comparison

5-Capacity and Load comparisonSection Load Capacity

Simply supported 88 MNm 94 MNmCantilevers 458 MNm 363 MNm

Pier: moment Axial

11 MNm31.8 MN

142 MNm11,000MN

The data in Table 5 shows that the Itchen Bridge is strong enough to withstand the loading assuming there is at least the assumed amount of steel is present in the cross section. In reality it is likely that the amount of steel is greater than what has been calculated for this analysis.

9 Conclusions

The Itchen Bridge is not a beautiful bridge nor really and ugly bridge and it is not at the cutting edge of design nor is it the simplest bridge ever. It is however an excellent example of the average bridge.

10 References

[1] Oxford English Dictionary [online]., 2000. Oxford University Press, Oxford. Available from:www.oed.com [Accessed 7 April 2008].

[2] Hopkins, A., 1970. A Span of Bridges, An Illustrated History. David & Charles (Publishers) Limited, Newton Abbot.

[3] Ibell, T.J., 2007. Bridge Engineering Course Notes., Department of Architecture and Civil Engineering, University of Bath