Structural Report for Debewoyni-Kelafo Gravel Road Project (Lot-2)
EAGLE GENERAL ENGINEERING CONSULTANT
STRUCTURAL REPORT FOR
DEBEWOYNI-KELAFO
GRAVEL ROAD PROJECT
(LOT-2)
October,2023
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Eagle General Engineering Consultant
October,2023
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Structural Report for Debewoyni-Kelafo Gravel Road Project (Lot-2)
Contents
1.Design of Structures ................................................................................................................ 1
1.1 Bridge Design Standards .................................................................................................. 1
1.2 Structure Type Selection .................................................................................................. 1
1.3 Design Methodology ........................................................................................................ 1
1.4 Design Criteria ................................................................................................................. 2
1.5 Structural Loading ........................................................................................................ 2
1.5.1 Permanent Loads ....................................................................................................... 2
1.5.2 Vehicular Live Load (LL) ......................................................................................... 3
1.6 Dimensional Clearances ................................................................................................... 4
1.7 Materials ........................................................................................................................... 5
3.Culvert Design Standards ....................................................................................................... 6
3.1 Culvert Sizes Shape and Layout....................................................................................... 6
3.3 Material Selection ............................................................................................................ 7
3.4 Culverts in the project ...................................................................................................... 8
3.5 Erosion Protection ............................................................................................................ 8
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Structural Report for Debewoyni-Kelafo Gravel Road Project (Lot-2)
List of Tables
Table 1: Loading Specification and Design Criteria.................................................................. 5
Table 23: Minor Structures on the project route (Lot-2) ........................................................... 8
List of Figures
Figure 1: Load Arrangement of HL-93 Truck Load .................................................................. 3
Figure 2: Load Arrangement of Tandem Loading ..................................................................... 3
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Structural Report for Debewoyni-Kelafo Gravel Road Project (Lot-2)
1.Design of Structures
1.1 Bridge Design Standards
The design of the Structures has been based on AASHTO LRFD Bridge Design Manual 2004
and the new Ethiopian Roads Authority Bridge Design Manual (ERA BDM-2013). As per ERA
BDM-2013, the design vehicle load shall be HL-93 for the design of new highway structures.
The new Vehicular load, HL-93, consists of “Design Truck” or “Design Tandem” loaded
simultaneously with lane load.
1.2 Structure Type Selection
The type of drainage structure specified for this project is determined based on economic,
topography, hydrologic and hydraulic considerations. The following can serve as a guide in the
selection of the type of structure.
Major drainage structures (Bridges and culverts above 6m span) are used where they are more
economical than minor drainage structures. They are also employed to satisfy land use
requirements, to mitigate environmental harm possible with minor drainage structures, to avoid
floodway or irrigation canal encroachments, and to accommodate large debris.
Minor drainage structures are used where major drainage structures are not hydraulically
required, where debris is tolerable, and where they are more economical than a bridge.
The type of drainage structure appropriate to each type of watercourse will be recommended.
Structures of less than 6m span will be designed as minor drainage structures and structures of
6m and above will be designed as major drainage structures.
1.3 Design Methodology
The Highway structures are designed for specified limit states to achieve the objectives of
constructability, safety, and serviceability, with due regard to issues of inspect ability,
economy, and aesthetics.
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Structural Report for Debewoyni-Kelafo Gravel Road Project (Lot-2)
The design is based on Load and Resistance Factor Design (LRFD), as presented in the both
Specifications. Load and Resistance Factor Design (LRFD) takes into account both the
statistical mean resistance and the statistical mean loads. The fundamental LRFD equation
includes a load modifier (η), load factors (γ), force effects (Q), a resistance factor (φ), a nominal
resistance (Rn), and a factored resistance (Rr = φRn). LRFD provides a more uniform level of
safety throughout the entire bridge, in which the measure of safety is a function of the
variability of the loads and the resistance.
The substructure is designed to sustain all the worst conditions imposed by the load multiplied
by the applicable load factors. Foundation is designed to support all loads in accordance with
general principles specified in the references for strength and stability.
1.4 Design Criteria
1.5 Structural Loading
Based on the requirements of the ERA Bridge Design Manual - 2013, the following permanent
and transient loads and forces are considered in the design of Highway structures
1.5.1 Permanent Loads
a) Dead load of structural components and non-structural attachments (DC).
As the structures are reinforced concrete, this load is dead load of concrete (unit Weight
24.5KN/m3)
b) Vertical pressure from dead load of earth pressure (EV).
Rolled Gravel or Hand Laid Rock Ballast as earth fill material is assumed (unit weight taken
is 19.5 KN/m3)
c) Horizontal Earth pressure Load (EH).
Lateral earth pressure is assumed for walls backfilled with earth. The walls are reviewed and
designed assuming that they can move away from the soil mass. The coefficient of lateral earth
pressure will be determined from the permitted backfill materials.
ii)
Transient loads
As per ERA BDM-2013, and ASHTO LRFD 2004 the design vehicle load for the design of
new highway structures shall be HL-93 for. The new Vehicular load, HL-93, consists of
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Structural Report for Debewoyni-Kelafo Gravel Road Project (Lot-2)
“Design Truck” or “Design Tandem” simultaneously loaded with a lane load. Whichever
design loadings creating the maximum effect would need to be considered in the design of the
highway structures.
1.5.2 Vehicular Live Load (LL)
a) Designs truck Load.
Total of design truck is 325 KN distributed between 3 axles. Front axle loading is 35KN and
two rear axles loading is 145 KN each. Distance between front and first rear axle is 4.3m. The
distance between two rear axles varies from 4.3 to 9m to produce extreme force effects. These
weights and spacing of axles are as specified in Figure 3-4 of ERA Bridge design Manual 2013.
Figure 1: Load Arrangement of HL-93 Truck Load
b) Design tandem load.
Design tandem load is exceptional loading and shall be applied for military vehicle loading
and for strategic bridges. Design tandem comprised of a pair of 110KN axles spaced at 1.2m.
Each axle comprised of two wheels spaced at 1.8m transversely.
Figure 2: Load Arrangement of Tandem Loading
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Structural Report for Debewoyni-Kelafo Gravel Road Project (Lot-2)
c) Design Lane Load.
Design lane load consist of 9.3 KN/m, uniformly distributed longitudinally. Transversely, the
load is assumed to occupy a 3.0m width.
d) Pedestrian load.
A pedestrian load of 4 KN/m2 is applied to all walkways wider than 0.6m. They are
considered to act simultaneously with the vehicular design live load.
iii)
Environmental Effects
Environmental effects are those resulting from natural effects such as wind, temperature and
seismic events.
a) Wind
It has been assumed that the base wind speed for structures (VB) is 165 km/h.
b) Temperature
A temperature range from 5o to 35oC has been assumed in the design of critical elements. For
the calculation of effects due to temperature gradients through superstructures the appropriate
values are:
T1 = 35oC, T2 = 10oC, T3 = 5oC
c) Seismic Effects
The Structures under project road are located in areas of the country defined as Zone 0. Hence,
Seismic force effect has not been considered in accordance with EBCS EN 1998-1:2013. In
addition as per Article 3.10.10.2 of ERA BDM structural seismic consideration is not required
for the single (dis-continuous) span structures regardless of the seismic zone.
1.6 Dimensional Clearances
The geometric requirements of structures, particularly bridges, are based on considerations of
topography, hydrology, bridge site economics safety and road standard.
For structures over waterways (such as those on this project) the overall length and the span
length will be determined form consideration of the hydraulic requirements and topographic
condition.
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Structural Report for Debewoyni-Kelafo Gravel Road Project (Lot-2)
1.7 Materials
The structural materials generally used for design of highway structures are reinforced
concrete and stone masonry.
i)
Reinforced Concrete
Design of concrete materials is based on the properties given below.
a) Concrète
Adopted grade of concrete and corresponding specified strength are shown below
Grade of concrete; - C30, Fck (150mm cubes) = 30MPa at 28 days.
; - C15, Fck (150mm cubes) = 15MPa at 28 days.
Other design properties of concrete are used as stated on ERA bridge design Manual -2013
(chapter -9: reinforced concrete)
b) Renforcement Steel
Reinforcing bars shall be deformed except for spirals, hoops and wire fabric. The minimum
nominal yield strength of deformed bars is as stated below,
AASHTO M31 M Grade 300: - 300 MPa
AASHTO M31 M Grade 420: - 420 MPa
c) Stone Masnory
According to ERA standard technical specification-2013, stone masonry is divided into three
classes. However; the consultant will use Class A stone masonry for the construction of major
watercourse crossing structures and Class B stone masonry for minor watercourse crossing
structures (culverts). This grade of stone masonry shall conform to the requirement of ERA
standard technical specification – 2013.
The following Table summarizes the specifications and design criteria adopted in the design
of the Structures.
Table 1: Loading Specification and Design Criteria
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Structural Report for Debewoyni-Kelafo Gravel Road Project (Lot-2)
Design Specifications
Design Method:
Live Loading:
Other Loadings:
Concrete (C-30):
(C-20):
Reinforcement steel:
Software:
Unit weight of concrete:
Wearing surface:
Exposure condition:
Typical cross section:
Bridge railing:
Deck drainage:
Bridge bearings:
Freeboard:
ERA Bridge Design Manual, BDM-2013
Load and Resistance Factor Design (LRFD)
HL-93
According to ERA, BDM-2013
F’c = 30 MPa on 150mm Cube sample.
F’c = 20 MPa on 200mm Cube sample.
Grade 300 and Grade 420
Math CAD and Excel Spread Sheets,
24.50 kN/m³
Moderate – no aggressive atmosphere
0.8 m + 7.30m + 0.8 m
Concrete post and railing
Ø 100mm PVC pipe.
Steel Reinforced Elastomeric Bearing
Minimum freeboard for various discharge,
Discharge (0-3 m3/s) = 0.30m
Discharge (3-30 m3/s) = 0.60m
Discharge (30-300 m3/s) = 0.90m
Discharge (> 300 m3/s) = 1.50m
3.Culvert Design Standards
Minor Drainage structures can be concrete box culverts, Slab Culverts and reinforced concrete
pipe culverts. Basically, standardization of design has to be adopted for minor drainage
structures. The standard drawings will be prepared based on the standard drawings provided in
ERASDD 2013 Standard detail drawings.
Concrete box culverts are constructed with a square or rectangular opening, and with wing
walls at both ends. They are usually specified for larger flows, where the area of the opening
is larger than that available for manufactured concrete pipe culverts. They may also be used
where the cost estimate indicates that concrete box culverts constructed on site are less
expensive than manufactured and/or imported pipe culverts. Concrete box culverts are also
recommended where the bearing capacity of the ground is weak to accommodate slab culverts.
3.1 Culvert Sizes Shape and Layout
The culvert size and shape selected is to be based on Hydraulic and economic criteria related
to site conditions.
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Structural Report for Debewoyni-Kelafo Gravel Road Project (Lot-2)
Culvert alignment requirements can dictate larger or different barrel geometry than required
for hydraulic considerations, and use arch or oval shapes only if required by hydraulic
limitations, site characteristics, structural criteria, or environmental criteria.
Wing walls are used to retain the roadway embankment to avoid a projecting culvert barrel.
They are also used where the side slopes of the channel are unstable, and where the culvert is
skewed to the normal channel flow. They can affect hydraulic efficiency if the flare angle is <
30 or > 60.
The selection of end structure for pipe culverts is generally dependent on factors such as the
condition of the approaching channel, slope and stability of the outlet channel. The culvert inlet
type shall be selected from standard details from the Standard drawing prepared by the
consultant based on ERA’s Standard Detail Drawings.
Culvert Skew angle shall not exceed 45 degrees as measured from a line perpendicular to the
roadway centerline. Aprons are used to reduce scour from high headwater depths or from
approach velocity in the channel. They should extend at least 1.5 pipe diameters upstream, and
should not protrude above the normal streambed elevation. Outlet protection (dissipaters) for
the selected culvert design flood shall be provided where the outlet scour is anticipated:
➢ The scour hole will undermine the culvert outlet,
➢ The expected scour hole may cause costly property damage,
➢ The scour hole causes a nuisance effect (most common in urban areas)
Minor drainage ends sometimes present a hazard to traffic that runs off the road increasing the
severity of the accident and injuries. In this case barriers will be provided up to safe zones on
both ends.
3.3 Material Selection
Concrete and masonry are the preferred material for construction of culverts; however, other
materials may be more suitable for a particular location, hydraulic roughness, bedding
condition, or project. In evaluating the suitability of alternate materials, the selection process
shall be based on a comparison of the total cost of alternate materials over the design life of the
structure that is dependent upon the following:
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Structural Report for Debewoyni-Kelafo Gravel Road Project (Lot-2)
➢ Durability (service life),
➢ Cost
➢ Availability
➢ Construction and maintenance ease
➢ Structural strength,
➢ Traffic delays and
➢ Water tightness requirements.
3.4 Culverts in the project
The consultant has carried out site visit to assess the possible crossing location. However, the
visit implied that there exist no minor crossing drainages in the project route. Thus, all the
recommendations of the hydraulic study will be new minor crossing. The following tables
forwards the recommendations of minor structures for Lot-2 of the project route.
Table 22: Minor Structures on the project route (Lot-2)
S/No.
Type
Number
1
2
Pipe culvert
Slab Culvert
36
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3.5 Erosion Protection
Hence, in order to protect the road way and the drainage structures against the detriments of
erosion, the following protection measures are intended to be provided as counter measure
against erosion problems, Locations and appropriate measures will be determined depending
on the site condition.
•
Construction of grouted stone/concrete pitched water way in the apron and toe wall at
the ends of inlets and outlets of culverts
•
Gabion boxes and gabion mattresses where massive erosion has taken place at river
beds /river banks at big river crossings/streams, heavily eroded side drains and some
culverts
•
Retaining walls, dumped/packed ripraps and loose stone pitching and etc.
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Structural Report for Debewoyni-Kelafo Gravel Road Project (Lot-2)
The type and extent of erosion protection measures at specific location will be selected from
the point of view of stream characteristics, functionality, construction simplicity, maintenance
requirements, and associated costs.
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