Lecture 1-Introduction & Conceptual Bridge Design Consideration

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ACEN 3 DAY TECHNICAL TRAINING ON INTRODUCTION TO THE DESIGN OF SHORT SPAN BRIDGES

LECTURE 1

INTRODUCTION & CONCEPTUAL BRIDGE DESIGN

CONSIDERATION

OBJECTIVE

To provide overview of various bridge types and process involve in

selecting suitable bridge type for a design.

To identify the planning and conceptualization process of the

bridge design.

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1.0 INTRODUCTION

Bridge Engineering is an exciting field of Engineering. Bridges sometimes

offer the only solution for a transportation system to cross rivers, difficult

ground conditions, or reducing conflict points in transportation system by

carrying one mode/traffic over the other (flyovers), etc.

Bridge Engineering goes beyond mere design of the physical bridge

structures, such as deck, abutments, piers and foundations. It covers all

the topics that impact on the safe operation of the bridge throughout its

serviceable life span. The past century is known for the construction of

more bridges prior to the discovery of new technology, material

availability and advancement in bridge engineering. At this stage, bridge

design has evolved from early art forms to a sophisticated science.

Every bridge design must begin with the conceptualisation of the nature

of the bridge. This will involve the planning, obtaining economical span

length for the bridge, the nature of the bridge site / environment, the

bridge loading and as well as the type of bridge to be designed for.

Concrete bridges are undoubtedly the most predominant bridge

construction types in Nigeria today, probably due to the cost and ready

availability of materials for its construction. This means that bridge

designers in the country are constantly faced with the challenges of

designing concrete bridges. And a complete bridge design process

involves series of complex stages, each involving iterative processes of

analysis, synthesis and appraisal.

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1.1. Bridge and its Uses

A bridge is a route designed to provide continuous passage over a river,

valley or any obstacle. On a river, bridge allows traffic and water to flow

pass one another on different levels at the same geographical point, thus

resolving the most difficult problem a road would encounter on its route,

a bridge forms part of the road and owes its origin to the river or any

other obstacle.

Alternatively, a bridge is a continuation of a roadway/railroad, when the

latter takes off from the ground. Bridges are not only for crossing over

river but also over other types of flows, such as traffic routes (like in the

case of Flyovers), or other natural or artificial obstacle( In the case of

viaducts, i.e. bridge constructed over deep valleys which renders the

construction of embankment an expensive solution).

Other function of a bridge can be seen from it economical, social and

advancement of a community, by providing shortest effective

transportation route for commerce between communities. Also, an

aesthetically pleasing architectural bridge structure will serves as a

monumental edifice that will attract tourism, and this is a major concept

adopted in modern bridges today.

1.2. General Planning and Design Consideration

The following factors must not be ignored in bridge engineering:

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The site, as the choice of bridge site affects greatly the choice of

type of bridge structure and the economic span of the bridge.

The stream/river hydrology and hydraulics.

Environmental Impact:

Additional constraints have been added to the bridge planning and

design by increasing demands for an overall ecosystem approach –

environmental impact assessment. This means that in addition to

hydrology and hydraulics, consideration must be given to the

impact of a new bridge crossing on existing vegetation, terrestrial

and aquatic habitat, recreational areas and fishery resources.

Aesthetic, historical and archaeological impacts may also require

attention.

The environmental factors: wind, temperature, humidity, rainfall,

atmospheric pollution level.

The soil characteristics

Bridge loading

In river crossing, navigational restraints, the types of vessels using

the river.

Bridge design data

bridge construction sequence

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2.0 SELECTION OF BRIDGE TYPE

It is very important for every bridge designer to understand the various

bridge types and their attributes for effective selection of suitable bridge

type.

Four main factors are used in describing a bridge. By combining these

terms one may give a general description of most bridge types.

I. Span (simple, continuous, cantilever)

II. Material (stone, concrete, metal, etc.)

III. Placement of the travel surface in relation to the structure

IV. Form (beam, arch, truss, etc.).

Table 2.1 below can be useful as rough guide in selecting bridge deck type

based on economic span range.

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TABLE 2.1

Span Bridge Deck Type

Up to 20m In‐situ reinforced concrete.

In‐situ prestressed post‐tensioned concrete.

Prestressed pre‐tensioned inverted T beams with in‐situ fill.

16m to 30m In‐situ reinforced concrete voided slab.

In‐situ prestressed post‐tensioned concrete voided slab.

Prestressed pre‐tensioned M and I beams with in‐situ slab.

Prestressed pre‐tensioned box beams with in‐situ topping.

Prestressed post‐tensioned beams with in‐situ slab.

Steel beams with insitu slab.

30m to 40m Prestressed pre‐tensioned Y U beams with in‐situ slab.

Prestressed pre‐tensioned box beams with in‐situ topping.

Prestressed post‐tensioned I beams with in‐situ slab.

Steel beams with in‐situ slab.

30m to 250m Box girder bridges ‐ As the span increases the construction

tends to go from 'all concrete' to 'steel box / concrete deck'

to 'all steel'.

Truss bridges ‐ for spans up to 50m they are generally less

economic than plate girders.

150m to 350m Cable stayed bridges.

350m to? Suspension bridges.

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Environment (i.e. bridge site) also imposes great varying conditions on the

bridge structure, giving rise to a whole variety of bridge types and shapes.

Bridge designs differ in the way they support loads. These loads include

the weight of the bridges themselves, the weight of the material used to

build the bridges, and the weight and stresses of the vehicles crossing

them.

2.1. Bridge Types

There are basically eight common bridge designs: beam, cantilever, arch,

truss, suspension, cable-stayed, movable, and floating bridges. Each

design differs in appearance, construction methods and materials used,

and overall expense. Some designs are better for long spans. Beam

bridges typically span the shortest distances, while cable-stayed and

suspension bridges span the greatest distances.

Beam Bridges

Beam bridges represent the simplest of all bridge designs. A beam bridge

consists of a rigid horizontal member called a beam that is supported at

both ends, by vertical posts called piers or abutments. Beam bridges are

the most commonly used bridges in highway construction.

Cantilever Bridges

Cantilever bridges are a more complex version of the beam-bridge design.

In a cantilever design, a tower is built on each side of the obstacle to be

crossed, and the bridge is built outward, or cantilevered, from each

tower. The towers support the entire load of the cantilevered arms. The

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arms are spaced so that a small suspended span can be inserted between

them. The cantilevered arms support the suspended span, and the

downward force of the span is absorbed by the towers.

Arch Bridges

Arch bridges are characterized by their stability. In an arch, the force of

the load is carried outward from the top to the ends of the arch, where

abutments keep the arch ends from spreading apart. Arch bridges have

been constructed of stone, brick, timber, cast iron, steel, and reinforced

concrete.

Truss Bridges

Truss bridges utilize strong, rigid frameworks that support these bridges

over a span. Trusses are created by fastening beams together in a

triangular configuration. The truss framework distributes the load of the

bridge so that each beam shares a portion of the load. Beam, cantilever,

and arch bridges may be constructed of trusses.

Suspension Bridges

Suspension bridges consist of two large, or main, cables that are hung

(suspended) from towers. The main cables of a suspension bridge drape

over two towers, with the cable ends buried in enormous concrete blocks

known as anchorages. The roadway is suspended from smaller vertical

cables that hang down from the main cables. In some cases, diagonal

cables run from the towers to the roadway and add rigidity to the

structure. The main cables support the weight of the bridge and transfer

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the load to the anchorages and the towers. Suspension bridges are used

for the longest spans.

Cable-Stayed Bridges

Cable-stayed bridges represent a variation of the suspension bridge.

Cable-stayed bridges have tall towers like suspension bridges, but in a

cable-stayed bridge, the roadway is attached directly to the towers by a

series of diagonal cables. A cable-stayed bridge is constructed in much the

same way as a suspension bridge is, but without the main cables.

Movable Bridges

Movable bridges make up a class of bridge in which a portion of the

bridge moves up or swings out to provide additional clearance beneath

the bridge. Movable bridges are usually found over heavily travelled

waterways.

Floating Bridges

Floating bridges are formed by fastening together sealed, floating

containers called pontoons and placing a roadbed on top of them. A

pontoon typically contains many compartments so that if a leak occurs in

one compartment, the pontoon will not sink. Some floating bridges are

constructed using boats or other floating devices rather than pontoons.

Floating bridges were originally developed and are most widely used as

temporary structures for military operations.

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Combination Bridges

Combination bridges include crossings consisting of several types of

bridges or both bridges and tunnels.

3.0 CHOICE OF BRIDGE SITE

The bridge design of necessity begins with the selection of a site. Choice

of the right location is crucial for the designing and planning of a bridge.

But above all, safety considerations that govern the technical, functional,

economic, efficiencies, and expeditiousness, and authentic requirements

are very important, it is necessary for a bridge and each of its components

to be safe, durable, reliable, and stable. This is usually checked by using

current specifications. But not all questions of durability, reliability, and

stability may be answered by analysis. Therefore in some cases it is

necessary to provide special measures such as testing the performance of

the structure and examining its behaviour under maximum loading on

site. Occasionally an unusual structural form may be subjected to wind

tunnel test. When in doubt seek a second opinion. The site for road or

rail bridge crossing will be restricted to certain reaches of the river

through constraints imposed by the alignment of the road or rail. The

choice of site must of necessity avoid costly river works.

The choice of site for a bridge must start with desk study using maps,

aerial photographs and Satellite imageries; followed by site inspection –

(reconnaissance survey). The bridge site is not chosen in isolation; it is

chosen to harmonize with the transportation-desired alignment.

Sometimes transportation alignment dictates and generally governs the

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selected site. However the following information should be taken into

consideration in the choice of bridge site:

The river channel regime, that is, whether it has a wide flood plain

or whether it is incised with little or no flood plain.

The stability of channel. It is necessary to determine whether the

river is statically stable, dynamically stable or unstable. A

meandering river may during flood change its course. This is

precisely the case with the Owena River Bridge at Okitipupa in

Ondo State during the 1964 flood. The river changed its course and

the bridge was no longer perpendicular to the river but in line with

the river flow. The road embankments were completely washed

off.

The river flow pattern. It is necessary to find out whether the

channel migration is active.

Range of water levels, low and high-water levels and frequency of

occurrence.

Range of discharges, particularly flood discharges and their

frequency of occurrence.

Width of waterways, width of flood plain, meander length and

width.

Type and grade of river bed material.

Type of material composing the river bands.

Location of any rock outcrops or other hard points in the channel

boundaries that may act like bridge substructures to cause a form

of local scour.

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3.1. Factors Affecting Design

Economical aspects in fixing spans of a bridge:

Let a = length of one span

n = total number of spans

L = total length of bridge

L = na ... ... (1.01)

Let F = cost of one pier

P = cost of superstructure per metre of span

(is proportional to length of span)

P = ka (k = cost coefficient)

Let Q = cost of superstructure per span

Q = Pa = ka x a

Q = ka² ... ... (1.02)

Let C = total cost of bridge excluding abutments

C = nQ+F (n-1) ... (1.03)

but n = L/a

Substituting for n in equation (1.03)

C = L/a [Q + F (L/a-1)]

Substituting for Q = ka² in equation (1.03)

C = L/a [ka² + F (L/a -1)]

= kLa + F (L/a-1) ... (1.04)

C is minimum when dc/da = 0

Differentiating equation (1.04)

dc/da = kL- FL/a² = 0 = KLa² – FL = 0

∴ F = ka²

From equation (1.02)

F = Q

The span is economical when the cost of pier equals the cost of one span

of the superstructure.

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4.0 HYDRAULIC CONSIDERATIONS

4.1. Basic Hydraulic Requirements

For a bridge crossing, there are two basic hydraulic requirements that the

bridge crossing must seek to satisfy:

A suitable location and alignment from the perspective of the river

characteristic, traffic and the environment.

The capability of the bridge opening and embankments to pass the

design discharge, and to withstand severe flood or debris

conditions without compromising the serviceability and structural

integrity of the bridge or adversely affecting the environment.

4.2. Bridge Location (Hydraulic/Environmental Aspects)

Where the location of a proposed bridge is not fixed by other

considerations, the following considerations should be used as guide in

selecting the location for the bridge and the route.

I. The choice of bridge location is generally determined by economics

balanced with environmental considerations. In crossing a flood

plain, the most economical solution that combines bridge opening

and the most economical embankment height must be determined.

II. Environmental factors cannot be ignored in the selection of bridge

location. The interaction of the route and bridge location must be

carefully evaluated in order not to adversely affect a fragile

ecosystem.

III. Location of existing utilities must not be ignored especially in the oil

producing areas. The integrity and normal operation of these

utilities must be ensured.

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IV. Stream characteristics and geomorphology sometimes vary within

the river under consideration for crossing sites. Stable river reaches

that do not require elaborate channel control works are preferred.

V. The alignment of the bridge and the location of the waterway

opening should fit so far as possible the existing river alignment and

the location of the principal channel. However in some situations

where the alignment route dictates, channel modification may be

necessary to provide a reasonable maintainable crossing. In the

case of meandering streams, they are sometimes modified by

channel diversion. Where streams are alternating single-and

multiple-channel reaches, single-channel locations are normally

preferable.

VI. Bridges are usually located on straight reaches of the river avoiding

sharp bends. However where the bends provide stable channels,

there is no reason why the bridge should not be located there; as it

may offer the shortest total length of the bridge. In unstable rivers,

the outer banks at bends are often particularly unstable to bank

erosion. If the bend location is adopted, the banks should be

stabilized.

VII. The hydraulic effects of existing nearby bridges and other

infrastructure should be carefully considered especially the effect

on the proposed bridge hydraulics. In the case of complex

combinations, numerical or physical hydraulic modelling is

sometimes used to evaluate current patterns.

VIII. When planning a major new bridge, overall watershed-planning

issues should be addressed through consultation with all

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stakeholders involved in the basin. Future plan for Dam

developments or other works could affect the choice of the

location. Consideration should be given to factors such as climate

change and land-use change that might result in increased flows,

increased settlement yield, and morphologic changes to the river.

4.3. Design Discharge and High Water Levels

Criteria

The fixing of the design discharge and design high water ideally should be

based on the cycle cost of the project, i.e. the initial capital cost and the

operational maintenance costs (which should include costs associated

with road closures due to flooding). There has to be appropriate balance

between capital cost and operating/maintenance costs depending greatly

on the scale of the planned structure and integrity of use.

The design discharge and design high water levels depend on the flood

return period T.

Various countries fixed the flood return period for estimating design

discharge and design high water levels. The British mostly consider 100-

year return period. The U.S. guidelines (Richardson and Davis 1995)

recommend that the return period of the super-flood should be in the

order of 500 years. In the absence of reliable estimate for the return

period, a discharge equal to 1.7 times the 100-year is suggested.

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Design Discharge

The design discharge corresponding to the return period T is computed

based on statistical frequency analysis of stream flow data. However the

result of the frequency analysis is not necessarily the final value of the

design discharge. It is sometimes advisable to review or adjust T-year

value as derive from frequency analysis, or use other methods of analysis,

in order to take account of the following factors:

Climate change

Land-use changes in the catchment area (past and potential future)

Possible changes to the operating rules for existing reservoir

Planned upstream or downstream reservoirs

Possible flow diversions into or out of the river

Possible influences on other riparian water users

Criteria used for existing nearby bridges

Past occurrence of catastrophic flood.

Design High Water Level

For open-water river conditions, the high water level corresponding to the

design discharge may be estimated from documented stage-discharge

relationships or from open channel hydraulics computations.

In some cases, discharge data may be scarce but a record of annual

maximum water levels (stages) may be available. In such a case, design

high water level may be derived from frequency analysis of the stage

data. If the data are for open-water floods only, the corresponding design

discharge may be estimated from hydraulic computations.

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4.4. Height of Bridge

Sometimes the route profile may control the height of the bridge. But for

most bridges, especially major bridges, the elevation of the underside of

the superstructure is normally determined by the design high water level

plus appropriate freeboard or clearance. The following factors should be

taken into account when considering clearance:

The maximum expected height of waves, where not included in the

design high water level

Wind set up and storm surge in coastal waters, lakes and reservoirs,

where not included in the design high water level

Elevation of the water at light bends in high velocity streams

Projection of logs and other floating debris

Statutory navigational requirements

Consideration should also be given to the susceptibility of the

superstructure to the impact by debris and to the hydraulic forces

that would be generated.

4.5. Hydraulic Design Procedures

Before sealing the preliminary design and estimate for a new bridge and

approach roads, it is advisable to go through a systematic process for

checking the various items of hydraulic design as discussed above. Figure

4:1 sets out this process in the form of a linear flow chart. At various

stages the consequences of a choice can be assessed against previous

choices and adjusted if necessary.

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FIG. 4.1: Flow chart with suggested procedure for bridge hydraulic studies

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1. Initial Conception of Project

Hydraulic factors should be given priority consideration when a road

project is conceived, because they have important bearing on initial

design and route location. For example the relative cost of alternative

routes may depend on the cost of drainage structures. Also the

environment traversed by the route may greatly influence choice. For

example, one route might cross an environmentally sensitive wetland

while the other may pass through a favourable and stable environment

2. Basic Data Collection

After the choice of alternative routes, all available information pertaining

to the hydraulic study should be assembled and reviewed and

arrangements should be made for collection of additional information if

necessary.

3. Selection of Design Values

The hydraulic capacities of existing structures and channels should be

evaluated using assembled office and field information. Using hydraulic

analysis and other considerations, values may then be determined for

design discharge and high water level etc. as discussed above. Other

constants such as permissible backwater effects, clearance and statutory

navigational requirements should be examined.

4. Selection of Bridge Location and Waterway Opening

The selection of bridge location is not only controlled by hydraulic factors

but by such other factors as road alignment, slope stability, nature of

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foundation materials, and assessment of economic and other

consequences of alternative choices.

5. Waterway Design Proposal

Key information on hydraulic parameters and proposed waterway

geometry should be summarized for use by the structural engineers. Such

information may include length and height requirements, critical water

levels, estimated depth of scour, arrangement of channel control works

and approaches, and other relevant data such as borings, and wave

lengths. Any hydraulic constraints on span layout and on details of piers,

etc should be noted.

6. Selection of Span Arrangement and Foundation Type

The economic span of a bridge at any location is dictated by the

foundation materials and they are governed by structural and foundation

considerations. There has to be close collaboration between the

hydraulic engineer and the structural engineer in arriving at a compromise

design if necessary. This may involve modifying the waterway design

proposal, since choice of span lengths and foundation geometry to meet

other conditions may affect allowable scour and the size of the waterway

opening. In some cases potential for debris blockage may be an

important consideration when selecting span arrangements.

Consideration should also be given to the potential effects of likely

construction works and procedures on scour, backwater and channel

processes in the general vicinity of the bridge. As a rough guideline, the

general vicinity refers to a length of at least 10 channel widths on either

side of the bridge.

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7. Preliminary Drawings

General arrangement plans and profile of the river and the selected road

alignment should show sufficient length of the river to enable an

appreciation of the river’s silent features and their relationship to the

bridge. Air photo-mosaic plans or satellite imagery are most useful in this

respect: Critical water bed and subsoil levels should be indicated on the

drawings and an accompanying report.

8. Review, Approval and Adoption of Project Proposal

The complete project design proposal (which may involve alternatives)

should be checked for compliance with the basic hydraulic requirements

discussed above. Environmental Impact assessment should be carried out

at this stage, seeking approval from the appropriate authorities and

localities.

9. Performance Evaluation and Feedback

The hydraulic performance of bridges in service should be reviewed

periodically. Items to be considered include flood levels, scour problem,

damage or problems related to debris, erosion of riverbanks and channel

control works, environmental consequences and changing of river

morphology and behaviour. If changes to the catchment area are

apparent, such as increase urbanization, deforestation, mass wasting,

etc., it may be prudent to review the hydrology and hydraulics to ensure

that an acceptable level of safety and reliability persists.

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5.0 LOADING CONSIDERATIONS

Bridges unlike buildings are designed to carry moving loads and are

sometime overloaded. Bridges are designed to interact with the

environment, with the river which they cross, the soil which carries them,

their self-weight, imposed dead weights and the moving live loads (see

Fig. 5:1).

FIG. 5.1: Bridge loads and load distribution

The predominant loads on bridges are gravity loads due to self weight and

that of moving traffic using the bridge and its dynamic effects. Others

loads includes wind, earthquakes, snow, temperature etc.

Bridge loading and their applications in bridge design will be

demonstrated in the subsequent lectures.

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6.0 CONSTRUCTION SEQUENCE

To produce a buildable bridge, the designer must have a good

appreciation of the site conditions and of modern construction

technology. The way a bridge is built affects to a greater extent the

moments and shears generated in the structure.

The construction method assumed in the design should always be on the

drawings while any special constraints which the design would impose on

alternative methods of construction clearly stated in the drawings

6.1. Method of Construction

To a large extent, the method of construction will be dictated by the

nature of obstacle to be crossed.

Below is an example of typical bridge stages of construction

Stage 1: Piling

Stage 2: Casting of pile cap

Stage 3: Casting of abutment walls, pier columns

Stage 4: Casting of part of capping beam, plinth, laying of bearings

and precast beams

Stage 5: Laying of precast beams and precast planks

Stage 6: Casting of part of capping beam up to top of precast beam

Stage 7: Casting of In-situ concrete slab

Stage 8: Compaction of backfilling behind abutment

Stage 9: Placement of precast parapet & casting of inner kerb and

approach slab

Stage 10: Laying of walkway pipe, casting of lean concrete

Stage 11: Laying of bituminous surfacing within carriageway

Stage 12: Stone pitching around abutment

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Discussion

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REFERENCE

I. Etteh.E.I.I, (Unpublished); Introduction to Bridge Design Volume 3.

II. Parke and Hewson,(2008); ICE Manual of Bridge Engineering, Second

Edition,

III. Ryall, M.J. et al (Edited), 2000; Manual of Bridge Engineering, Institution

of Civil Engineers, UK

Lecture 1-Introduction & Conceptual Bridge Design Consideration

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Transcript of Lecture 1-Introduction & Conceptual Bridge Design Consideration