 Ethiopia is one of a mountainous

country in Africa.
 Topographic condition: rough terrain,
deep gorges, rivers, …..

 Roads without constructing crossways

is difficult. Thus, Bridges are vital
structures required during roads
construction.
Bridge Construction in Ethiopia
1. On Blue Nile near Alata, Almeida Bridge,
thick log is placed. (The 1st Bridge)
2. Bridge between Gondar and Gojam-
Portugal Bridge (was constructed in 1625)
3. After 1667 many bridges were constructed in
Gondar and Lake Tana area.
4. For instance in Addis Ababa, Kebena and
Ras Mekonnen (1902) bridges were
constructed;
5. Abay #2 bridge was constructed in 1959 (13
span RC girder bridge – located in Bahir dar)
Bridge between Gonder and Gojam- Portugal Bridge (estimated to be built1625)
7
Bridge Construction in Ethiopia cont’d
• 1935-1945 (Italian occupation)

• During the last 30 years (many bridges were constructed)

• Slab ,T- and Box Girder bridges made using RC, are the
most commonly used bridges in Ethiopia.(almost 85%)

• Most of the bridges so far constructed in the country have

now been widened and replaced because their roadway
widths were too restrictive for the safety of
accommodating modern traffic.
Introduction
Bridges by year of construction (Bridges along Federal road
Network)
1200
1007
1000

800 731

600 556
507
391
400
286
200
200 115
10
0

Almost 40% of the bridges are found in fair & bad conditions
ERA (Ethiopian Roads Authority) Data ERA-BMS Software
9
 Bridge by Type ( Bridges along Federal road Network)

ERA (Ethiopian Roads Authority) Data ERA-BMS Software

10
 Major Bridges in Ethiopia
Name Type Length Year Remark
Tekeze RC Deck Girder 424 2014

Blue Nile RC Deck Girder and 355 2010 along Sherkole-Blue

Steel Bridge Nile road segment
(Asossa)
Beshilo RC Deck Girder 319 2002 7-span

Hidasse Extradose, PC 303 2009 3-span

Tekeze No.3 Steel Truss 280 2001 The longest steel

Bridge

Baro RC Deck 276 1981

Multi span bridge
Abay No.4 PC Box Girder 236 1992
Tekeze Bridge
12
Bile Nile Bridge
13
 Beshilo
Bridge (Wollo)

14
Abay Hidassie Bridge
15
Tekeze Bridge
16
Baro Bridge (Gambela)
17
Abay No. 1, 204m
18
Rigid frame bridge-Adaitu Bridge
19
 Chapter 2

INVESTIGATION FOR
BRIDGE SITE

20
2. INVESTIGATION FOR BRIDGE SITE
Bridge Site Selection
In locating a bridge crossing the following should be
considered
 The reach of the river should be straight
 The channel in the reach should be well defined
 The crossing site should be as narrow as possible
 The crossing site should have firm high banks which
are fairly inerodable.
 The site should be selected where skewness can be
avoided …..
There are two types of rivers namely alluvial
and incised.

Alluvial rivers are winding and they erode their

banks and scour their beds;
They are continually active, scouring and
depositing materials on the banks and
transporting sediments.
Incised rivers have a relatively stable banks and
arc generally narrower and deeper than alluvial
rivers.
Data Collection

 Once the engineer has identified a

likely site for the bridge, it is necessary
to obtain field information on the
catchment area and run off, local
terrain conditions and water levels, and
other clearance requirements.
River Survey

 High water marks can be obtained

from: the hydrologic characteristics of the
basin or watershed of the stream under
study are needed for any predictive methods
used to forecast flood flows. gauges or from
local people.
Soil Investigation
Once at the site it is easy and of great
value to sample for soil, rock, stone,
water, etc.
 Soil investigation is required to get soil
profile, engineering property of the
foundation material and foundation
level of the abutments and piers for
design of the foundation.
 boreholes, test pits or geophysical
surveying shall be conducted
Field Sketching and Photos

 It has proved very practical to make a simple

sketch of the bridge site with approximate
water shores, existing structures, scour
holes, main stream location, etc including
very rough dimensions with approximate
measurements
 Span Determination
Span determination is usually dictated by the hydraulic
requirement. However, there are conditions where
lengthen spans are chosen for the sake of road
alignment.
1. Economy
Structural types, span lengths, and materials shall be
selected with due consideration of projected cost. The cost
of future expenditures during the projected service life of
the bridge should be considered. Regional factors, such as
availability of material, fabrication, location, shipping, and
erection constraints, shall be considered.
(For a given span the most economical span is the length at
which superstructure cost equals to substructure cost.)
2. Hydraulic requirement (DFL,OFL,…)

 Bridges are designed to accommodate design

discharge at design flood. When a river has a
wide flood plain, the economical solution
may be using short span bridge with proper
scour and erosion protection for the
embankment, abutments and piers.

3. Location of the bridge site

 Selection of Feasible site for Bridges
To select an optimum and the most feasible site , different
options should be considered. The factors include:
 Geometry of Approach Road
 Approach Road Earth work volume and its impact
 Opening size
 ROW (Right of Way) and others
 The Right of way along the Bridge crossing option consists of
houses, fences and service lines.

Location of Bridge’s Crossing Options

 In order to select the best crossing, the relevant geometric
design parameters of the approach road to the bridge crossing
have to be considered in line with an appropriate Geometric
Design manual.
29
 Example

Options Cost for Cost for Cost for Cost for Cost for Other Total
Earth Embankment Retaining Approach Bridge costs Cost
works wall Road structure
Pavement
1
2
3

30
 4. Location of Piers
 Piers should be located in such a manner that they can
provide the required lineal waterway and navigational
clearance.
 The alignment of piers and abutments should, if
possible, be set parallel to the direction of flow during
maximum flood.
5. Grade requirement of the road
Often in mountainous areas the road way grade is
governed by the capacity of heaviest vehicle to climb,
vertical curve and sight distance. These requirements
may increase the span beyond the hydraulic
requirement.
 6. Free board: 0.3 – 1.2m fn(Q)
 The waterway below the superstructure must be
designed to pass the design flood and the floating
debris carried on it.

 These clearance measurements should be increased

for backwater effects when the flow is restricted by
short span bridge.
 Chapter 3

Types of Bridges and

their Selection
 Classification of Bridges
Bridges can be classified on the basis of the following
Characteristics

 Functionality as Road bridge, Railway bridge,

Pedestrian bridge
 Construction material as Steel, Concrete, Timber or
combination of any two or more.
 Span length as short, medium or large.
 Structural forms as Slab, T-Girder, Box Girder, Arch,
Rigid, Suspension etc.
 Classification of Bridges

Bridges can be classified on the basis of the following

Characteristics

 Span type as single or multi-span, cantilever.

 Movements as movable or fixed Bridges
 Arrangement as curved, skewed, Inclined, straight, ..
 Life span as Temporary, Permanent
Pedestrian Bridge
Men pull each other across Blue Nile River by
rope prior to the building of a new bridge

37
38
39
40
41
 Span Length

 - L ≤ 6m (Culvert)
 - 7m < L ≤ 15m (Small span bridges)
 - 16 ≤ L ≤ 50m (Medium span Bridges)
 - 50 ≤ L≤ 150m (Large Span Bridges)
 - L≥150m (Extra Large Span Bridges)
 Structural Forms
 - Slab Bridges
 - Girder (Deck girder Bridges)
 - Box Girder
 - Arch Bridges
 - Truss Bridges
 - Rigid Bridges
 - Plate Girder Bridges
 - Cable Stayed Bridges
 - Suspension Bridges
 - Box Cell/ Box culvert
Slab Bridges
 Girder (Deck girder Bridges)

 Chiro-Majete
Holeta Bridge

46
47
Box Girder
Arch Bridges
Truss Bridge
Rigid Frame Bridge

Rigid frame bridge- Adaitu Bridge

52
53
Cable Stayed Bridge
 Suspension Bridge

pedestrian cable bridge over the Blue Nile in Ethiopia

56
57
58
Cantilever bridge-Awash Bridge (Tulubolo)

59
Box Cell/ Box culvert
Span Support
This is on the basis of the Bridge’s support.

 If all the spans of the bridge are simply supported (a

pinned support and a roller support )- Simple Span
Bridge

 A bridge supported by combinations of more than two

pinned and roller supports - Continuous Span Bridge

 A bridge that is built-in at one end and free at the other -

Cantilever Span Bridge
Multiple simple span bridge- Woama Bridge (Afar)
Continuous bridge- Modjo Bridge
Simple Span Bridge
 Particular Problem of Selection
 Different Manuals recommend different
span lengths to be criteria of selection of
bridge types.

 No clear demarcations on the selection of

spans for slab and T- Girder bridges if the
span lies with in the intervals on the basis of
economy. This is a serious design problem
that Engineers face during selection of
bridge type.
 Selection of Bridge Type

 Typically there are three to four viable structure

types for each span length. Criteria to select bridge
include:
 Geometric Condition of the Site
 Subsurface Conditions of the Site
 Functional Requirements
Economy and
ease of maintenance,
aesthetics, etc
 Selection of Bridge Type
Economy
 A general rule is that the bridge with minimum
number of spans, fewest deck joints and widest
spacing of girders will be the most economical.
 By reducing the number of spans, the construction
cost of one pier is eliminated.
Constructability
Construction and erection considerations:
 In general, the larger the prefabricated or precast member, the
shorter the construction time.
 However, the larger the members, the more difficult they are to
transport and lift into place.
 The availability of skilled labour and specified materials will
also influence the choice of a particular bridge type.

Legal Considerations:
 Applicable laws like environmental laws also govern the
type of bridge.
Criteria used for Determination of Location of a Bridge
(Span and span arrangement)

 Geometric requirements for bridges:

These are the requirements of the road alignment such
as vertical and horizontal curves including grade.

 Environmental concern - bridge and its associated

works should not have an adverse impact on adjoining
land or buildings, or the bridge itself be susceptible to
damage from the local environment.

69
 . . . cont’d
 Supporting sub-soils must be strong enough to
ensure the stability of the structure;
 Road safety
 Economic considerations
 Hydrology/Hydraulics shall play vital role in the
selection of the span and span arrangement.

The choice of location of a bridge:-based on economic,

engineering, social and environmental concerns as well as
costs of operation involving maintenance associated
activities.

70
 CHAPTER 4
Bridge Loadings
 Type of Loads
 Permanent Loads: Dead and Earth Loads
 Transient Load: Live, Water, Wind
 Dynamic Loads: Earthquake Loads
 Force effects due to superimposed deformations
(temp gradient, shrinkage, creep, settlement, . .)
 Friction Forces
 Vessel Collision
 Other stresses
 Dead Load

 Dead load shall include the weight of all

components of the structure, appurtenances
and utilities, earth cover, wearing surface,
future overlays, and planned widening. In the
absence of more precise information, the unit
weights, specified in AASHTO, Table 3.5.1-1,
may be used for the computation of dead loads.
Dead Load
 Vehicular Live Loads
Vehicular live load is designated as HL-93 and
shall consist of a combination of the followings:
• Design truck or design tandem, and
• Design lane load

Except as modified in Article 3.6.1.3.1 of ERA

Bridge Design Manual, each design lane under
consideration shall be occupied by either the design
truck or tandem, coincident with the lane load, where
applicable. The loads shall be assumed to occupy
3000mm transversely within a design lane
Vehicular Live Loads
Design truck :-The weights and spacing of axles and wheels
for the design truck shall be as specified in Figure 1 (HS-20
Loading). A dynamic load allowance shall be considered.
Except as specified in Articles 3.6.1.3.1 and 3.6.1.4.1, of
ERA Bridge Design Manual, the spacing between the two
145000- N axles shall be varied between 4300 and 9000mm
to produce extreme force effects.
Design tandem:- shall consist of a pair of 110kN axles
spaced 1200mm apart. The transverse spacing of wheels
shall be taken as 1800mm. A dynamic load allowance
shall be considered as specified in AASHTO, Article
3.6.2.
Design Truck Load

Fig.1 Design Truck Load

Design Tandem Load

Fig.2 Design Tandem Load

 Design Lane Load:
 The design lane load shall consist of a load of 9.3kN/m,
uniformly distributed in the longitudinal direction.
 Transversely, the design lane load shall be assumed to
be uniformly distributed over a 3.0m width.
 The force effects from the design lane load shall not be
subject to a dynamic load allowance. (AASHTO, ERA Design
manual)
 Dynamic Load Allowance
(IM = Vehicular Dynamic Load Allowance): Dynamic
effects due to moving vehicles shall be attributed to two
sources:
 Hammering effect is the dynamic response of the
wheel assembly
 Dynamic response of the bridge as a whole
Dynamic load allowance need not be applied to:
 Retaining walls not subject to vertical reactions from
the superstructure, and
 Foundation components that are entirely below
ground level.
 Dynamic Load Allowance……

 The dynamic load allowance shall not be applied

to pedestrian loads or to the design lane load.
 The factor to be applied to the static load shall be
taken as: (1 + IM/100).
 Dynamic Load Allowance…..

 The dynamic load allowance for culverts and other

buried structures, in %, shall be taken as:

IM = 33 (1.0 - 4.l*10-4 DE) > 0%

Where: DE = the minimum depth of earth

cover above the structure (mm)
 Number of Design Lanes
 Generally, the number of design lanes should be
determined by taking the integer part of the ratio
w/3600
Where: w is the clear roadway width in mm
between curbs and/or barriers.
 Multiple Presence of Live Load:
 Trucks will be present in adjacent lanes on roadways with
multiple design lanes but this is unlikely that all adjacent
lanes will be loaded simultaneously. This will be considered
by the multiple presence factors.

 When the loading condition includes the pedestrian loads

combined with one or more lanes of the vehicular live load,
the pedestrian loads shall be taken to be one loaded lane.
Centrifugal Forces (CE= Vehicular Centrifugal Force):
 Centrifugal force is due to inertia force of vehicles on
curved bridges at speed. Centrifugal forces shall be
applied horizontally at a distance 1.8 m above the
roadway surface.
 Centrifugal forces shall be taken as the product of the
axle weights of the design truck or tandem and the
factor C, taken as:
 Breaking Force (BR= Vehicular Braking Force):
 From AASHTO Commentary 3.6.4 Based on energy
principles, and assuming uniform deceleration
(retardation), the braking force determined as a fraction
"b" of vehicle weight.

 From AASHTO Article 3.6.4 Braking forces shall be taken as

25 % of the axle weights of the design truck or tandem per
lane placed in all design lanes headed in the same direction.
 These forces shall be assumed to act horizontally at a
distance of 1800 mm above the roadway surface in either
longitudinal direction to cause extreme force effects.
Vehicular Collision (CT= Vehicular Collision Force):
 Unless protections are provided a horizontal force of
1800kN applied at 1.2m above the ground should be
considered.

Pedestrian Loads
 A pedestrian load of 3.6kPa shall be applied to all
sidewalks wider than 0.6m and considered
simultaneously with the vehicular design live load.
 Water Loads

 Water Loads (WA= Water Load and Stream Pressure)

 Static Pressure: Static pressure of water shall be
assumed to act perpendicular to the surface that is
retaining the water.
 P=γ g h
 Stream Pressure
 Longitudinal: The longitudinal drag force shall be
taken as the product of longitudinal stream pressure
and the projected surface exposed thereto.
 Lateral: The lateral, uniformly distributed pressure
on substructure due to water flowing at an angle,θ, to
the longitudinal axis of the pier
 Wind Loads
 Wind Pressure on Structures, (WS): For small and
medium sized concrete bridges below 50m length the
wind load on structures shall be neglected.
 In the absence of more precise data, design wind
pressure, PD in kPa, shall be determined as:
 Earthquake Effects (EQ= Earthquake)
 Earthquake loads are given by the product of the
elastic seismic response coefficient Csm and the
equivalent weight of the superstructure.
 Earth Pressure
 Earth Pressure (EH = Horizontal Earth Pressure; ES = Earth
Surcharge; LS = Live Load Surcharge; DD = Down drag)
Earth pressure shall be considered as a function of the:
 Type and density of earth,
 Water content,
 Soil creep characteristics,
 Degree of compaction
 Location of groundwater table,
 Earth-structure interaction,
 Amount of surcharge, and
 Earthquake effects.
 EH = Horizontal Earth Pressure
Active and Passive Earth Pressures
 Coulomb theory is recommended by AASHTO for
masonry and RC abutment since this theory holds better
for the actual situation.
kh=coefficient of lateral earth pressure taken as ko
ES = Earth Surcharge; LS = Live Load Surcharge
 The “Wall Height” shall be taken as the distance between
the surface of the backfill and the bottom of the footing.

Down Drag (DD):

 When soil surrounding piles settle, it applies a downward
force. In this case, the force should be considered.
 Force Effects Due to Superimposed
Deformations: TU, TG, SH, CR, SE
 Uniform temperature, (TU): it is used to
calculate thermal deformation effects.
 Temperature Gradient, (TG): Temperature rise
can differ on the top and bottom surfaces of a
bridge.
 Differential Shrinkage, (SH):
 Creep, (CR): dependence on time and changes in
compressive stresses shall be taken into account.
 Settlement, (SE): This will cause internal forces in
continues structures.
 Design Philosophy
 In Engineering design the general principle is that
the resistance of a cross section has to exceed the
effects come from the applied loads. That is

Resistance ≥Effect of Loads

 Design Objectives
The objectives in a bridge design are safety,
serviceability, economy, constructability and
aesthetics.

Minimum requirements are provided for

clearances, environmental protection ,geological
studies, rideability, durability, inspectabilty, and
maintainability.