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Chapter 5Bridge Design Manual - 2002 Preliminary Design/Layout of Bridges And Culverts

Ethiopian Roads Authority Page 5-1

5 PRELIMINARY DESIGN/LAYOUT OF BRIDGES AND CULVERTS

5.1 GENERAL

Preliminary bridge design is a part of the road design. The site for a bridge is usuallygoverned by engineering, economic, social, environmental, aesthetic and safetyconsiderations.

The “best” preliminary bridge layout is not always the optimized solution. Because manyfactors are contradictory, the material, labor, and construction prices may vary from the timeof preliminary design to actual construction. Also competition between bidders may result incompletely new layouts. Sometimes bidders have their own falsework, moulding formwork,building methods and machines, which are not anticipated by ERA. This has to beconsidered at the bidding procedure.

Subsequent to the Planning Stage (see Chapter 4: Planning, Feasibility, and SiteInvestigation), the alignment of the road should have been selected from the differentproposed alternatives. The bridge designer may then propose the stations and approximatesize of the footings for a more detailed geotechnical survey. Then the type and approximatedepth of foundation shall be selected.

The layout will begin with an estimation of possible loads, such as traffic loads, earthpressure, earthquake loads, temperature movements, etc. (see Chapter 3: LoadRequirements). The second step will be to consider in what way these loads affect the bridge.Then, with the result from the soil investigation and in cooperation with the geotechnicalsurveyor, the static system can be selected.

Once the bridge layout has been determined, a cost estimation should be made and comparedwith that from the planning stage. The selected bridge type should then be evaluated in termsof economics, aesthetics, constructability, maintenance, and environment.

The bridge is normally shown on a layout drawing with plan, elevation and section. Themain dimensions should be given. Other necessary technical information shall be given inthe Preliminary Design Specification (PDS) for the particular bridge (see Chapter 16:Calculations, Drawings, and Specifications).

5.2 BASIC INFORMATION

5.2.1 TOPOGRAPHICAL MAP

For the preparing of the Preliminary Design Drawings, the part of the road plan covering thebridge site should be reproduced at scale 1:100 or 1:250. It should include levels withcontours at a one-meter interval from the field survey. It should also show:

Chapter 5Preliminary Design/Layout of Bridges And Culverts Bridge Design Manual - 2002

Page 5-2 Ethiopian Roads Authority

• Proposed roadway including stations.• The waterlines, and rivers with direction of flow indicated.• Direction of north and if possible the coordinates.• Islands or rock outcrops in the waterway.• Name of the streams and of the road• Other structures, which may affect the bridge such as buildings, walls, adjacent roads,

railways, electric power and telephone posts and drainage channels/ditches.• Placement, levels and dimensions for ducts and cables.

5.2.2 SURVEYING, BENCHMARKS, AND MEASUREMENT OF WATER DEPTHS

Normally the ground at the bridge site should be surveyed along the road centerline, at 25 mincrements, up to at least 20 m left and right from the alignment with 5 meters interval. Allbreak points shall be surveyed. At larger bridges this should be modified according to thebridge designer. The length of the bridge survey should cover at least 20 m beyond the high-water-mark (HWM), or 2 m vertically above the HWM.

At rivers the leveling should include profile and cross sections of the river according to theERA Drainage Design Manual-2002, Chapter 4: Hydrologic Survey, Section 4.4: Data onstreams, rivers, ponds, lakes and wetlands. The skew angle of the main stream should beindicated to analyze possible erosion problems. In rivers with deposits of silty loose material,the underlying harder strata should be measured if possible.

The water level at the time of survey shall be indicated to compare it with the theoreticallycalculated levels determined as per the ERA Drainage Design Manual-2002, Chapter 5:Hydrology. Visible traces of high water marks shall be surveyed and indicated.

A benchmark should be positioned as close to the bridge as possible. It shall however beplaced sufficiently far from the site such that it will remain undamaged during and after theconstruction work. The ERA Geometric Design Manual-2002, Chapter 4: SurveyRequirements, Section 4.7: Benchmarks gives an example of a benchmark. (figure 4.1).

At large rivers there should be one benchmark on each shore above the high-water mark. Thebenchmark shall be described on the Preliminary Design Drawing. If a local level system isused it shall be thoroughly described, and if possible tied to either the BMP, BMS or BMTTrigonometrical Stations of the Ethiopian Mapping Authority (see ERA Geometric DesignManual-2002, Chapter 4: Survey Requirements, Section 4.5: Field Surveys). Then the levelfor the calculation of the free board clearance can be considered.

Coordinates for the plan of bridge should if possible be according to the 1000 m UniversalTransverse Mercator Grid (UTM Zone 36) or any other current system approved by theEthiopian Mapping Authority.



5.2.3 RIVER DATA

To decide upon the placement of piers, span length and clearance height, as well asdetermining needs for realignment of the river and protection against scour/erosion, riverdata is required. The following information if appropriate should be given on the preliminarysketch:

• River basin (discharge area) and area of lakes in km2 (square kilometers).• Water flow quantities in m3/s (The statistical highest high water flow every 100 years,

design water flow, medium water flow, lowest low water flow and if possible also thenormal high-water flow and normal low water flow).

• The elevations of important areas (fields, etc) upstream the proposed bridge site.• Cross section of the waterway including river beds at least to 2-m above the high-water

mark.• Collecting of opening dimensions and levels of adjacent bridges.• Collection of data from adjacent irrigation projects/channels.

Sources of this information include the National Meteorological Services Agency, theEthiopian Mapping Agency, Water Power stations, and the Ministry of Water Resources.

5.2.4 SOIL INVESTIGATIONS

For all bridge types, at least two soil investigation points for each pier, abutment or spreadfooting should be made. In depth the borings shall be at least 3.0 m below the anticipatedfoundation level or the lower side of the footing. Settlement calculations on footings, infriction soil or over consolidated clay, should normally be made to the depth of 4 times theeffective width of the footing. See the ERA Soils and Materials Investigation Manual-2002for details.

5.2.5 BRIDGE REFERENCE NUMBER

Every bridge in Ethiopia is given a unique bridge reference number associated with the roadnumber (see ERA Geometric Design Manual-2002, Chapter 16: Appendices, Appendix A:Classification and Description of Roads) which can be obtained from the Bridge Branch ofthe Ethiopian Roads Authority (ERA). All drawings should refer to that number, i.e. “roadnumber” - "bridge number" – drawing number (A2-45-6).

5.2.6 MISCELLANEOUS

Other information to be collected in an early stage is:

• Type of roadway, type of ditch, typical section, traffic flow and velocity.• Clearance height and width requirements• Aesthetical (architectural) requirements• Environmental requirements• Type of curbing



• Type of railings − especially if a railing between the roadway and the walkway is to beused.

• Existing and planned cables and ducts• Material to be utilized for bridge slopes. This will give the maximum slope inclinations,

which in turn can give the total length of the bridge.

5.3 GEOMETRIC REQUIREMENTS

For geometric requirements, refer to Chapter 2: General Requirements.

5.4 LOAD ASSUMPTIONS

For load assumptions, including design vehicle load, traffic load, accidental loads and otherloads, see Chapter 2: General Requirements.

5.5 FOUNDATIONS

5.5.1 GENERAL

The most suitable way of founding a bridge will be determined from the geotechnical surveyin cooperation with the geotechnical engineer.

The foundations and their levels are dependent upon:

• Soil conditions, given the coefficients to calculate the bearing capacity of the soil,stability and settlement.

• Groundwater level or water level.• Bridge type.• Embankments of the adjacent road and the founding of them.• Construction method.• Proximity to existing structures.

The selection of foundation type and level depends heavily upon the impact of undergroundconditions as indicated by the results of the soil investigation. The demands on the foundingof bridges and access road embankments include a sufficient safety factor and minimalsettlement in order to achieve a maintenance-free and long-life structure. During bridgedesign, stability and settlement should always be investigated.

Foundations on rock will usually provide the simplest and most economically favorablesolution, since the size of the footing will be less than that on soil. If the rock is not visible,investigations should be made in all four corners of the footing. Sometimes investigation pitsare preferred. Bridges should not be placed on loose rock or sloping rock. It is recommendedin such case to remove loose rock and to level the rock surface by blasting or other rockexcavation and thereafter place the footing on compacted fill.



If the soil consists of thick layers of clay or loose silt, investigations may indicate that thesoil strata are not able to carry the load of the footing. If such layers are less than 3-4 m inthickness, it is advisable to remove the layers and exchange them with compacted stonefill.

For conditions worse than this, piling is advisable.

Regarding longitudinal settlement, the calculated difference in settlement between twosupports should preferably not exceed 1/500 of the span length. Sometimes the transversalsettlement also has to be considered in the pre-design stage.

The construction method also affects the selection of foundation type. Temporary sheetpiling shall be needed to achieve a certain foundation type or to build foundations close toexisting structures such as railways. Generally the construction method should be consideredbefore deciding upon foundation types, the placing of the piers, and levels.

5.5.2 SPREAD FOOTINGS ON ROCK

The rock should usually be as horizontal as possible. Where this is not the case, it should beblasted to achieve a horizontal plane. For sloping rock along a pier it shall be advantageousto blast the rock in steps similar to a staircase.

A footing directly on rock will result in a very stiff support. In some cases it shall beadvantageous to place the footing on 0.5 m compacted gravel fill in order to have a moreelastic founding. This applies especially to open frame structures with short legs/frontwalls.

It is often not possible to give an exact level of the bottom of the foundation on rocks.Therefore often the level of the upper side of the footing is given on the “Preliminary LayoutDrawing”

5.5.3 SPREAD FOOTING ON SOIL

The foundation level is dependent on soil bearing capacity, settlement, scour/erosion and themethod of construction. Normally the bottom of the footing shall be given on the“Preliminary Layout Drawing”.

It has proved practical to extend the footing 0.5 m at concrete piers and 0.3 m at masonrypiers as a cantilever from the face of the pier and 0.3 m at the sides for both types. At workinside sheet piling it shall be necessary to increase these dimensions. If existing structuresare close to the footing or if the soil is very poor, it shall be necessary to make a roughcalculation of the size of the footing before it is drawn on the “Preliminary LayoutDrawing.”

In case of cohesive type soil or very soft friction soil, the footing should be placed on a bedof at least 0.3 m compacted gravel fill. If the soil is very soft it is sometimes suitable tocompact the soil in a several meters thick layer with a heavy falling weight (deepcompaction) combined with normal surface compaction. The foundation level shall be raised



several meters above the water level, as a foundation level above the water is often cheaperthan under the water. If the water surface is close to the foundation level, considerationshould be given whether it is possible to lower the groundwater level temporarily bypumping to some 0.5 m under the bottom of the footing.

If the foundation level is less than 2 m under the water level it shall be advantageous to castthe footing above the water. This can be accomplished using watertight sheet piling and anunderwater cast concrete slab heavy enough to resist the buoyancy when the water inside thepiling is pumped dry, before casting the footing itself.

5.5.4 FOOTINGS ON COMPACTED FILL

Sometimes it shall be suitable to raise the footing above the hard soil layer. This consists ofexcavating the soft material and replacing it with compacted well-graded stone fill. Thestone size should preferably be 0 - 100 mm (d50 ≥ 70 mm). In that case intensive compactionshould be made in 0.3-0.6 m thick horizontal layers.

Figure 5-1 below shows the normal method of constructing a fill with side supports made outof quarried stones or similar.

Distances in meters

Figure 5-1 Compacted Fill Under a Bridge Footing



5.5.5 PILE FOUNDATIONS

Piling should be considered when footings cannot be founded on rock, stiff cohesive, orgranular foundation material at a reasonable expense. At locations where soil conditionswould normally permit the use of spread footings, but the potential for erosion exists, pilesshall be used as a protection against scour.

Piling can be made either with in situ bored piles or with prefabricated (prefab) RC piles. Atypical prefab concrete pile with the dimensions 290x290 mm reinforced with 4ø16 barsmay typically carry 450 - 600 kN in the service limit state. If the soil investigation showssulfuric soils, it should be indicated on the “Preliminary Design Drawing” that the concretecover of the prefab piles should be at least 45 mm. Cast-in-place concrete piles include pilescast in driven steel shells that remain in place and piles cast in unlined drilled holes or shafts.

Should the soil investigation show many boulders in the soil, steel piles should beconsidered. In some cases with very deep water it shall be suitable to use RC filled steelpipes with a diameter of 0.6 m. Wooden piles should normally be avoided due todeterioration and insect problems.

The soil investigation report must clearly indicate:• if the piles are to be of end bearing type or skin friction type (the latter used where the

hard soil strata is deeper than some 30 m),• expected overall length of piles,• the grade of difficulty to perform the piling, especially in water,• necessary soil parameters for the design

Pile foundations with simultaneous large horizontal load and small vertical load such asretaining walls, will be large and should therefore be avoided. Often it will then be moreeconomical to increase the span length (to make the retaining wall smaller) or change thetype of bridge (to one without any retaining wall at all).

5.6 SCOUR/EROSION, RIPRAP, SHEET PILING, RIVER TRAINING, ETC

Scour protection is usually required where a bridge is built across a meandering stream,when the natural stone protection is removed or when some restriction to the flow of thedesign flood occurs at a bridge. Protection measures can take the form of:

• riprap on slopes or river bed• gabion or Reno mattress aprons or revetments• sheetpiled walls• vegetation with deep roots• river training works

Riprap riverbed protection consists of a carpet of loose stones, heavy enough to resist beingwashed away by maximum water velocities during a flood. If the velocity exceeds 1.0 m/s



this protection should not be installed in a manner which reduces the area of the waterway.The main advantages of riprap are:

• low construction cost• material often available close to the site• a flexible protection• easy to install and repair with appropriate lifting gear such as tripod, ginpole or tractor-

crane.

The stones should be well-graded. Durable and heavy stones with a cubic shape arepreferred. Flaky stones should be avoided. The thickness of the blanket should be at leastthe length of the largest stones and about the nominal mean diameter (d50). If no separatecalculations of scour have been performed, scour protection should always be applied atleast 3 m around piers and abutments with riprap of a minimum stone size according toTable 5-1 below.

Max. Velocityat Riverbed

(m/s)

Max. Velocityat stone slopes

1,5:1 (m/s)

Max. Velocityat gravel slopes

2:1 (m/s)

No. ofLayers

d50

mmCrushed

StoneFraction

(mm)1,3 0,9 0,8 1 ≥70 0-1002,2 1,5 1,4 1 ≥200 0-3003,0 2,0 1,8 2 ≥350 0-5003,2 2,2 2,0 2 ≥400 0-600

Table 5-1 Maximum Design Water Velocity at Different Scour Protections

Gabions are rectangular baskets made of steel wire mesh with internal tie wires at everyhalf-meter. They are normally filled at the site with natural or quarried stone. The gabionstructure is more stable and durable if the stones are packed by hand. Standard sizes are 2, 3and 4 m long by 1 m wide by 0.5 or 1 m high.

Filter blankets should be applied to the back of the gabions or beneath the riprap if theriverbank consists of fine, non-cohesive material, to prevent such material from beingwashed away through the voids in the riprap or gabion lining. The filter blanket can consistof a 0.5 mm thick polyester non- woven textile carpet (minimum weight 250 g/m2) ormultiple layers of stones with the finest layer closest to the river bank and the coarsest layertowards the water. The polyester carpet should be protected from sunshine, and should beplaced, overlapped and anchored according to the manufacturer.

Sheet piling of prefabricated RC or steel shall be driven to form a continuous wall. Theresulting wall is less flexible than gabions or riprap and may fail due to movements in theground. To withstand the earth pressure it should be designed through calculations. Onlytemporary sheet piling shall be made out of wood.



Vegetation can be used to protect riverbanks. The most successful plants are those foundgrowing naturally along the river. Plants with deep roots are preferred.

River training works require extensive experience. It is useful to examine other structures inthe area, observe the flow during the rainy season or at flood, and to examine plans and mapsshowing how the river shape has progressed, in order to understand where bankstrengthening and guide walls are needed (Ref. 1). For further guidance on river training,refer to the ERA Drainage Design Manual-2002.

5.7 SUBSTRUCTURE (ABUTMENTS, PIERS, WINGWALLS, AND RETAING WALLS)

Every bridge is divided into substructure and superstructure. The division is normally madeat the bearings. For frame bridges the limit between substructure and superstructure is at thejoint between the top of the footing and the bottom of the front wall.

An Abutment is a free, independent end of a bridge. It normally carries both the load from thesuperstructure and the load from the adjacent road embankment. Wing-walls are sometimesplaced parallel to the roadway in order to minimize the overturning moment of the abutment(see Figure 5-2).

Many existing abutments and piers in Ethiopia are made of stone masonry or mass concrete.

If an abutment is higher than 8 m, it should preferably be of an open type without a frontwallunder the level 1.5 m below the slope (see Figure 5-2 below). This configuration will reducehorizontal forces from earth pressure and the traffic load on the embankment. This type ofabutment saves material and therefore more economical than one with a solid front wall.

Piers are normally designed as a wall or as two braced columns. At large bridges a box typeis sometimes used. Piers in high velocity streams should preferably be made as one singlecolumn or as a wall under the normal high water level (HWL), due to increased scour. Forthe same reason, such pier walls should if possible be parallel to the main stream flow. Theminimum thickness of piers in water should not be less than 0.6 m or if the design watervelocity (DWV) exceeds 1.5 m/s not less than 0.8 m and if pier height at the same timeexceeds 8 m not less than 1.0 m. Edges should always be rounded at DWV exceeding 1.5m/s or where debris is expected.

Elevation: Front Wall Section: Open Type

Figure 5-2 Two Types of Abutments



Wingwalls are made to take the difference in height at the abutments. In wingwalls attachedto the abutment the size of the horizontal main reinforcement in wingwalls should be assmall as possible (≤ ∅ 12 mm) and preferably not exceed ∅ 16 mm, to minimize the widthof the cracks in the face of the wall towards the soil. The front wall should always be made0.1 m thicker than the wingwall, unless a FEM (Finite Element Modeling) analysis or similarproves otherwise (See Figure 5-3 below).

The shortest length of a wingwall in a slope is obtained by directing the wingwall to thebisector (half of the skew-angle, see Figure 5-3). In such case the bottom side of thewingwall should be parallel to the slope and at least 1.0 m deeper than the slope, measuredperpendicular to the slope surface. If sheet piling is used for the footing, the wingwall willinterfere with it and hence a wingwall parallel to the roadway is preferred.

Figure 5-3 Typical Parallel and 45° Wingwalls Attached to the Abutment

Aesthetically the length of the wingwalls should never exceed the overall width or the spanlength. If the design length of the wingwall exceeds 5.0 m, the use of a retaining wallusually saves material and is therefore more economical (see Figure 5-4). In that case theattached wingwall should not exceed 3.0 m due to the deflection from earth pressure, whichshould match the deflection of the retaining wall. If the wingwall is too short the twofootings may interfere with each other. To minimize the height of the retaining wall it can beraised to a higher level than the abutment footing by placing it on a compacted stonefill.Piled retaining walls in slopes should be avoided due to the large horizontal loads, whichmakes them very expensive.



Figure 5-4 Typical Combined Attached Wingwall and Retaining Wall

Examples of retaining wall design are given in the appendix RW: Retaining Wall Design.

5.8 LOW LEVEL WATER CROSSINGS (FORDS, IRISH CROSSINGS, VENTED CAUSEWAYS, ETC)

In favorable conditions, low level water crossings can provide economical and relativelysimple alternatives to conventional bridges. There are three basic types of low levelcrossing:

- Fords (also called Irish crossings) and bed-level causeways, which are in essencereinforced roadways on the bottom of the stream.

- Vented causeways, where low flow is handled by openings under the roadway level- Submersible bridges, which are temporarily submersed low bridges.

All types are appropriate for roads with low traffic volumes or where a reasonably shortdetour provides access to an all-weather bridge. The crossing should be designed such thatfor most of the year the maximum depth of water over the crossing is less than 0.15 m. Theservice life of the structure will depend considerably on its hydraulic design as outlined inthe ERA Drainage Design Manual-2002, Chapter 8: Bridges.

Fords and bed-level causeways, like conventional bridges, shall be constructed so that theycause little interference with the design flood. Since all water flowing in the river channelovertops fords and bed-level causeways, there is no reason to raise the road surface morethan 0.1 m above the streambed.



Fords (unpaved) are the simplest form of river crossing. They generally are placed where thestream is wide, shallow and slow, the approach gentle, and the surface firm. Improvementsto the approaches are usually confined to reducing the gradient. The running surface in thestream can be strengthened and made more driveable by using stones imported and buriedjust below the surface (See Refs. 3 and 4). A more durable improvement shall be made to therunning surface by replacing the stones with gabions or reno mattresses. The gabions shouldnot rise more than 0.10 m above the natural bed level of the river, otherwise they may causeheavy scour downstream of the crossing.

Bed level causeways (paved) shall be used where the traffic composition or the lack of anearby all-weather crossing justifies the expense; a pavement shall be laid on the riverbed.A bed-level causeway is also called a paved ford, drift, paved dip or Irish bridge (Ref. 5).Figure 5-5 below illustrates three common designs. Further detail for low-level watercrossings is presented in the Standard Detail Drawings-2002, Chapter 7: Bridge Drainage,drawing BR-36.

Vented causeways and submersible bridges inevitably disrupt river flow, and so are liable tosustain damage or indirectly cause scour to the riverbed or banks, which in turn may affectthe road approaches to the crossing. These bridge types usually present a dry roadway forordinary flows and are designed to be overtopped at less than an annual flood, or near thedesign flood as determined using the ERA Drainage Design Manual-2002, Chapter 2:Standards and Departures from Standard, Section 2.1: Design Storm/Flood..

Vented causeways are built where the river flow is too great for too many days in the year toallow the traffic to cross a ford or bed level causeway without significant disruptions.Structures include multiple pipe culverts for low flow and should be designed following theguidelines in the ERA Drainage Design Manual-2002, Chapter 2: Standards andDepartures from Standard, Section 2.4: Hydraulic Design Elements. However, the designflood used to calculate the vent/culvert sizes will be less than the annual flood (see ERADrainage Design Manual-2002, Chapter 2: Standards and Departures from Standard,Section 2.1: Design Storm/Flood), provided it is acceptable that the roadway shall beovertopped for a few days each year during the annual high flood.

Submersible bridges shall be used where the traffic density justifies a dry crossing of asubstantial ordinary flood, but the annual high flood is much greater than this. A submersiblebridge designed to pass the ordinary flood but to be overtopped by the high flood will beconsiderably cheaper than a high level, all weather bridge. Submersible bridges have theadvantage of being able to pass a larger flow than the vents of a causeway of the sameheight, but are more susceptible to damage by the river. Since the flood horizontal forces onthe piers and bridge decks are quite substantial, submersible bridges should preferably bedesigned as multiple box culverts. Because of these difficulties submersible bridges are notrecommended above any foundation other than rock, and even then a vented causeway or aconventional bridge is likely to be a more durable alternative.

To protect the pavement from scour damage, curtain walls are usually required on both sidesof the roadway and these must continue up the approaches to the height of the design flood.



It is recommended that curtain walls should be to the minimal depths upstream anddownstream as indicated in Figure 5-5, unless rock is reached before that depth. If the bed isinerodible, the causeway need not have curtain walls but the bed on both the upstream anddownstream sides of the crossing should be trimmed flat to reduce turbulence.

Figure 5-5 Bed Level Causeways, Common Types

2.01.0



Figure 5-5 type a) shows a section through a basic bed level causeway suitable for lighttraffic and maximum water velocity below 2.0 m/sec. The crossing shown in Figure 5-5 typeb) requires good concrete technology and may sustain damage to the apron that is difficult torepair. Figure 5-5 type c) shows a design employing a good combination of concretepavement with flexible protection. Generally, lean concrete is used and slabs are jointedusing crack inducers every 5.0-m.

All low level crossings should have guideposts and a depth gauge to alert the driver to theplacement of the edges of the crossing and the water depth.

Depth gauges should indicate the depth of water at the lowest point of the crossing. Simpleblack and white markings at every 0,1 m are best - with an indication of the units used.Posts should be of concrete 0.3 m in diameter or square, placed within easy vision of theapproach but well away from possible impact damage by vehicles.

Guideposts should be set each side of the roadway between 2 and 4 m apart, depending onthe likelihood of catching floating debris. They should be sufficiently high to be visibleduring the highest expected floods and be made of concrete. An additional guide for vehiclesshall be provided by building a ridge down the center of concrete causeways, as shown inFigure 5-5 type b). This ridge also offers restraint against sideways drifting of vehicles instrong currents.

5.9 FRAME BRIDGES

A reinforced concrete frame bridge is a simple and economical type of bridge. Its mainfeature is a low design height, which is shown in Figure 5-6 below. It can be made in one orseveral spans, several spans being a common configuration. The walls should preferably beof an equal height in order to achieve a balance between the earth pressures. If piled footingsare necessary for an open single span frame bridge (without bottom slab) it is in most casesfavorable to add some beams between the footings to eliminate the horizontal forces fromthe earth pressure on the piles.

The most economical span/opening for a single span frame bridge is 6 − 20 m provided thatthe height of the walls are more than ¼ of the span length and that the soil is sufficientlystable, otherwise a slab or a girder bridge is preferred.

The span length of a multiple span frame bridge, such as a double or triple box culvert,should not exceed 8 − 10 m for each span due to the required footing thickness and becauselarger structures are sensitive to movements from earthquakes. Multiple open span framebridges (without bottom slabs) should be avoided because they are usually more expensivethan continuous slabs or girders, unless the latter employs a very expensive type of bearings.

The standard Single Box Culvert may also be used as underpasses for pedestrians (internalheight 3.0 m), cattle (internal height 3.0 – 4.0 m) or for traffic (internal height 5.0 m).Skewed single open frame bridges are quite difficult both to design and construct; therefore3-span slab bridges are more preferable. For a one lane bridge with a skew angle exceeding



20° or a two-lane bridge with a skew angle exceeding 30° the design criteria according toFigure 5-7 below should be checked. Otherwise the earth-pressure might cause the bridge to“rotate” horizontally due to sliding and the sharp corners might have resulting uplift forces.In such cases the bridge and especially the deck should be designed with a refined methodsuch as FEM-analysis or the finite strip method.

A Girder Frame Bridge is an open single span frame bridge with girders under the bridgedeck. It is economical for span lengths between 18 − 25 m in non-earthquake areas, underthe same provisions as the single span frame bridge above.

Figure 5-6 Design Heights of Open Single Span Frame Bridges (H= Height of wall)



Figure 5-7 Requirements for Skewed Frame Bridges

Regarding the detailed design of frame bridges see section 12.7.

5.10 SLAB BRIDGES

Single span slab bridges are perhaps the most common bridges in Ethiopia. They can beeconomical for spans from 1 m to 18 m. Above 15 m they should preferably be ribbed asshown in Figure 5-8 below. Instead of ribs there are several types of prefab forms (Texaspan,etc) that can be used by contractors, if the designer has considered that particular type.

Figure 5-8 Sections of Voided (Hollowed) Slab and Ribbed Slab Bridge Decks

Normally the slab is made with a uniform depth over the whole bridge. The required designdepth is usually 5.5 - 6 % of the span length, due to the width of the cracks. If stressedreinforced concrete is used, the design depth shall be reduced to 4.5 % of the span length.

The abutments at single or double span slab bridges should preferably be placedperpendicular to the bridge in order to avoid a skew in earth pressure, which may cause skewin the abutment front wall.

ab

θ

θ = skew angleb = length of front walll = length of edge beam

a > 0.3 * bl

SLABVOIDED SLAB

RIBBED SLAB



The abutments shall be designed as simple walls if an end-wall with at least 1.5-m height isprovided at each end of the slab. The end-wall must be designed for all the longitudinalforces from the superstructure as well as part of the earth pressure, and the wingwalls arefixed, supported in the end-wall. If the Slab Bridge is continuous and long, the active earthpressure on the end-wall may turn into a passive earth pressure, where the much larger kp-coefficient should be used in the design of the end-wall. Recent results show that raised piledfootings “hidden” behind a rather high (approx. 3 - 3.5 m) end-wall, deleting all horizontalforces on the footing, has proved to be economical (see Figure 5-9).

The piers are usually designed as walls. At skewed slab bridges it is however sometimesfavorable to use one thick column if the width of the bridge is not more than 8 m. In suchcases a check for punching of the slab above the column should always be made. Sometimesthe top of the column can be tapered to avoid or minimize the punching reinforcement.

Figure 5-9 Typical Elevated Footing with Endwalls

5.11 GIRDER BRIDGES

A girder bridge is usually used for a single span bridge, or non-continuous girders for amulti-span bridge, in earthquake areas. They shall be used for span lengths between 12 - 20m. Outside of earthquake zones, continuous girder bridges are preferred. In this case theexterior span length should be approximately 0.8 times the interior span. The LRFD designmethod usually minimizes materials used if the number of girders/beams is minimized. Thecantilever should preferably not exceed 40 % of the spacing of the girders, or 2.8 m for atwo-lane bridge.



The design depth of a normal girder bridge may vary between 7-10% of the span lengthdepending on the number of beams used. If possible, a high stem of beam is preferred to acertain extent, both technically and economically. For construction reasons however, theheight should be minimized. Esthetically a short bridge with a high superstructure close tothe water surface should be avoided. Here a slender structure (slab) is more appealing.

Regarding endwalls, the same restraints mentioned for slab bridges applies to girder bridges,as long as the total length of the continuous superstructure does not exceed 70-90 m.

RC Box girder bridges are sometimes used for span lengths of between 30 - 90 m, especiallyif a slender structure is desired or for curved bridges with small horizontal curves where agreat resistance to torsion is required. The interior height should not be less than 1.0 m and a0.6-m wide manhole is required for maintenance reasons.

Steel girder bridges are most favorable over deep or muddy waters since scaffolding fromthe ground is not necessary. The scaffolding is braced from the bottom flanges of the steelbeams. Cost competitiveness versus a concrete bridge depends almost solely on the steelprice compared to the concrete price, but also on the availability of competent welders, sizesof workshops and similar.

Today rolled beams of 1.1-m height made of high quality steel shall be obtained from someof the largest steel plants. Such beams may span some 20m, and for composite bridges theymay span 24m. However the most economical construction would be to import high tensilesteel plates with a thickness between 10 mm (webs) and 50 mm (flanges) and fabricate thebeams in local workshops. This however requires highly trained and licensed welders. Thelimited transportation facilities in the country make it practical to fabricate relatively shortbridge pieces in the workshop and assemble them at the site.

Steel box girders are quite complicated sections and should be considered only if most of therequirements for steel girder bridges above are fulfilled.

Composite steel girder bridges will be used more in the future due to new research results onthe interaction between the steel beams and the concrete bridge decks through shearconnectors (studs). These are some 200 mm high with diameter ∅ 19, 22 or 25 mm weldedto the top of the upper flange, which should be at least 20 mm thick. The reduction inmaterial use is quite remarkable and makes steel bridges more competitive, although thedesign calculations are somewhat more difficult. The design depth of the superstructure isreduced to some 4 - 6 % of the span length. Since the weight is much less, this type of bridgecan be used to replace an old concrete bridge, especially if the live load is increased. If anend-wall type of bridge is used, the overall length of the continuous bridge deck should notexceed 80 - 100 m due to temperature movements, which create a large passive earthpressure at the end-walls.

The most common construction methods are either to lift the steel beams with one or morecranes from one or both the river shores, or to slide the steel structure on temporary slidingbearings from one abutment. The launching forces should be considered as Constructionload.



5.12 MASONRY AND CONCRETE ARCH BRIDGES

Stone masonry arch bridges were dominant in Ethiopia prior to the 1950s. If made fromgranite or similar hard stone they may withstand any standard highway loading. In Europeseveral 2000 year-old bridges are still in use. This type of bridge may still be preferredwhere weather-resistant rocks shall be obtained, provided that the ground will not allow anyhorizontal gliding between the footing and the soil, and that skilled masons are to be found.

Granites can be found mainly west of Nakemte, in Tigray, in the southwestern regions, andsouth of Dodola where however there are very few bridges. For most small and mediumsized bridges (below 50 m length) the hardest types of basalt stone, sandstone and limestonecan also be used, and therefore nearly the whole country could make use of stone masonrybridges. Hard burned clay stones (called clinker) could also be utilized for this type ofbridge.

At span lengths over 20 - 25 m it is often advantageous to apply secondary arches on top ofthe main arch barrel. This requires both skilled and experienced designers and contractors.

Regarding the structural design of this type of bridge it should be noted that temperatureforces and settlement might cause pressure on the under side of the arch barrel at the crownand pressure on the topside of the arch barrel at the abutments.

The preliminary main dimensions of the arches should be as in Table 5-2 below:

Span opening (m) 8 9 10 12 15 20

Thickness at crown (top) mThickness at abutment m

0.550.80

0.600.90

0.701.00

0.751.00

0.861.20

0.901.35

Table 5-2 Normal Thickness of Arch Barrel (Arch Ring)

Skewed arch bridges are very complicated both to construct and design (with Finite ElementModeling, FEM-analysis) and should therefore be avoided. For detailed design seesubchapter 12.7.

Concrete arch bridges should preferably be designed with 3 hinges (joints) in order tominimize stresses in the arch barrel. This applies especially for earthquake zones.

The design and construction of the hinges requires special skill and experience. Thesimplest type is the cross-reinforced hinge shown in Figure 5-10 at right. The highestaccuracy is needed in placing the bars. The joint must be waterproofed and sealed to avoidcorrosion.



Figure 5-10 Cross-Reinforced Hinge

5.13 PREFABRICATED BRIDGES

Prefabricated bridges usually have the following advantages:• Falsework is not needed, e. g. crossing waterways or deep gorges.• The erection time is reduced, which could be useful if there is no other place to cross the

river.• The same formwork made out of steel plate can be used over and over again, which

reduces the need for timber formwork.• Usually quality is high since the personnel in a pre-casting factory perform the same type

of work repeatedly, compared to temporary laborers at a bridge site.

Due to transport and lifting difficulties the weight of each panel should not exceed 20 tons(200 kN) and the length should be less than 12 -20 m unless adequate hauling devices areprevalent. Prestressed RC beams up to some 35 m length may however be used for bridges.The available design depth should be at least 5% of the span length.

Sometimes prefabricated bridge deck panels are used together with composite steel or RCgirders. In such cases the transversal joints should be made to interact with each other.Recent research has shown that high tensile steel dowels are most suitable to withstand thefatigue load.

Prefab slab bridges shall be used for culverts up to 6-m spans. See Figure 5-11.



Distances in millimeters

Figure 5-11 Section of a Typical Slab Panel

Prefab Girder bridges could be made of U-shaped panels as shown in Figure 5-12 or of T-shaped panels of stressed RC according to Figure 5-13. Another very common shape is the I-shaped beam panel shown in Figure 5-14. This type may also be used for pedestrian bridgeswith a slight alteration, as in Figure 5-15.


Figure 5-12 Section of U-Shaped Type of RC Girder Panels

T1 and T2 vary depending onthe span length




Figure 5-13 Section of a Normal T-Shaped Type of Stressed RC Girder Panel

Figure 5-14 Section of Stressed I-ShapedType of RC Girder Panels

Figure 5-15 Section of I-Shaped Type ofRC Girder Panel Bridge for Pedestrians

Chapter 5Bridge Design Manual-2002 Preliminary Design/Layout of Bridges And Culverts


5.14 PEDESTRIAN FOOTBRIDGES

Pedestrian footbridges are different from other bridges only regarding live loads and widths.

5.15 CATTLE UNDERPASSES

The dimensional requirements of cattle underpasses are given in subchapter 2.5: MinimumClearance above Waterways, Roads, and Railways. In all other respects, they are the sameas standard single box or slab culverts, with the most favorable design including fill withgravel to a depth of 0.2m on the bottom slab. Sometimes guiding cattle fences are need atboth ends of the culvert for road safety. Drainage by open channels is required from thelowest point.

5.16 OTHER TYPES OF BRIDGES (CABLE STAY, SUSPENSION, TRUSS, RAILWAY)

Cable stayed bridges contain three main parts: cables, pylons (cable towers), and horizontalbracing, usually the bridge deck. The cables shall be arranged either parallel or fan shaped.This type of bridge is usually more economical than trusses, arches and suspension bridgesfor span lengths 100 - 500 m. The main advantage is that it can be built without falsework. Itrequires extraordinary experience both from designers and constructors apart from specialtypes of cranes. During erection it is sensitive to strong sideways wind if it is not anchoredhorizontally with temporary stays.

Suspension bridges contain the same parts as cable bridges and in addition, an anchor-blockfor the cables. The anchor-block may preferably be anchored in the rock itself. Thesuspension bridge is well suited for span lengths above 500 m, which are not common atpresent in Ethiopia.

Truss bridges are usually made of steel. They are more economical than steel girder bridgesin this country. Another advantage is that they are quite lightweight which makes themsuited as temporary bridges during the time of construction, since they are easier to transportthan a girder bridge. This type of bridge is especially suited to pedestrian bridges acrosshighways, since they can be erected without interfering with the traffic on the roadway. Themounting of the superstructure can be made during nighttime when the traffic is low. Thebridge deck can be made of some kind of steel plate, special impregnated wood or plasticplanks. The free clearance above the roadway should be at least 5.3 m due to the risk ofcollision with overloaded trucks. They require a significant amount of welding.

Railway bridges differ from highway bridges in the increased live load and dynamicallowance. Load induced fatigue strength state sometimes govern, which is almost never thecase for highway bridges. At the transition between the abutment and the railwayembankment the skew angle should not exceed 20 degrees and the joint should be square(90°). Some typical sections together with suitable span lengths are shown in Figure 5-16.



5.17 TEMPORARY BRIDGES

Temporary bridge structures are intended to provide quick solutions to short-term accessproblems. Decisions relative to such construction include the high expense of a quicksolution and the likelihood that benefits will accrue for only a limited period, hence the needto keep costs low. The principal reasons for requiring a temporary crossing are:

• damage to an existing bridge results in the need for a temporary structure on a nearby sitewhile the disabled bridge is being repaired or replaced. (This could be an opportune timeto consider upgrading the existing structure in terms of width, traffic capacity, clearanceheight above the water and durability.)

• upgrading the road requires a wider, higher or stronger bridge and a temporary crossing isneeded for a detour to carry traffic while the new road and bridge are built on the oldalignment.

• a river crossing is required urgently and for a short term by an Organization other than aroads department, e.g. an aid organization concerned with the delivery of emergencyrelief supplies

• temporary access to a construction site is needed in advance of the construction of apermanent roadway.

The time factor is central to the decisions relative to such a crossing, influencing in particularthe type of structure to be used. For example, a washed-out bridge on a strategic road willrequire substitution at the earliest possible moment, whereas a detour for a road upgradingproject can be planned well in advance, enabling the most economic solution to beemployed.

The length of time that the temporary structure will be in service also influences its designand cost. A low-level structure shall be adequate for a short period, but if it is likely to beneeded beyond the end of the dry season, a ford or culvert may not suffice and a temporarybridge with associated bank protection shall be required. A realistic estimate of the timeneeded to build a permanent replacement is necessary, together with a worst-case view of theweather and the projected traffic loading, before a firm decision can be made regarding thetype of temporary structure to be built.



Figure 5-16 Typical Sections of Railway Bridges and their Suitable Span Lengths

Generally, the order of preference for the type of structure will follow the order of increasingcost, unless specific skills or materials are unavailable. This order of preference is likely tobe:• fords• causeways• temporary beam bridges• prefabricated decks.



A ford shall be no more than a prepared descent to and exit from a river bed, taking intoaccount the traction requirements of known traffic on the slopes (See “Low Level WaterCrossings” above).

A vented earth causeway can be built using pipes stocked in most state road departmentdepots, taking measures to prevent erosion of the fill around the pipes. A temporarysurfacing may also be required.

Though a ford or causeway may not be serviceable for use by general traffic at times of highwater, it might be acceptable in the short term; an existing crossing of this type might evenbe found on a nearby older alignment.

A Timber Beam Bridge shall be the best solution if material is readily available in thelocality The design of timber bridge decks is detailed in Section 12.10. Timber beam bridgesdo not require preservative treatment for temporary duty. A manufacturer of timber girderbridges in presented in Table 5-3 below.

Timber truss decks of the trestle type require special skills and it is no longer easy build onequickly.

If steel or concrete beams are available for short-term use, it is necessary to ascertain theirbending and shear characteristics and to employ them in a similar manner to timber beamswith timber decking.

Steel panel bridges such as the Bailey Bridge have been used worldwide for 50 years. Thesteel panels to make bridges of different lengths, widths and load capacity are assembled in acouple of days. Other well-known versions of unitary construction bridges are the AcrowPanel Bridge and the Callender Hamilton designs. Suppliers of these bridges are listed inTable 5-3. They issue detailed design and assembly instructions and provide a design andadvisory service for customers. Most also have websites.

Table 5-3 Suppliers of Premade Bridges

Bailey Bridges Mabey and Johnson Ltd. Floral Mile, Twyford, Reading RGIO 9SQ,England.Bailey Bridges, Inc., Route 4 Box 188 Greenville, Alabama, USA36037

Steel Panel Bridges Thos Storey (Engineers) Ltd. 52 Queens Road, Weybridge KT13OAN, England.The Callender Hamilton Bridge: Balfour Beatty Power ConstructionLtd. 7 Mayday Road, Thornton Heath CR7 7XA, England.

Pedestrian SteelTruss

BILTOLAST Products, Inc. 119 40th Street NE, Fort Payne,Alabama, USA.

Timber Girder Western Wood Structures, Inc., P. O. Box 130, Tualatin, Oregon,USA 87062.

Parts of old Bailey bridges can be found in most countries, but great care should be taken toidentify these parts correctly. Detail design modifications for greater load-carrying capacities



have been made over the years. New and old panels should not be assembled in the samestructure. Old components must be used only in accordance with the corresponding manuals.

Prefabricated steel bridges are unlikely to offer an economical solution to the need fortemporary bridging, unless their facility for being dismantled and re-used can be properlyexploited. However, their component parts are so easily transported that a stock heldcentrally by the Ethiopian Roads Authority could be made available at short noticenationwide in most instances.

Abutments and Piers are often not needed since most temporary bridge decks can beassembled on existing or temporary abutments. Existing abutments, if they are sound andlocated on an acceptable alignment, have three important advantages:

• they have a proven record of sustaining the dynamic and static applied loads• they have shown satisfactory resistance to attack by the river• their use avoids the time and expense of building new abutments.

Temporary abutments shall be made from gabions (see Figure 5-17), temporary steel sheetpiling or logs. However, they are highly susceptible to scour and erosion, and should beconstructed with great care using ties and anchors where possible, because they can bedestroyed by a single flood.

Figure 5-17 Temporary Bridge on Gabion Abutments



Where water flow is low, timber piled abutments and piers have proved successful. The useof piers reduces the section of the road bearing beams, and a whole bridge can be built withtimber no larger than 0.3 m in diameter.

If the engineer is satisfied that there is material of sufficient strength at bed level or a littlebelow, open caisson piers shall be constructed using pre-cast concrete rings (See Figure 5-18below). The first ring is placed in position and excavation takes place from inside it. Ringsare added as the first progresses downwards until a firm base material is reached, then morerings are added until the required deck height is achieved. Lean concrete can be used to fillthe caisson and stronger concrete is used at the top to take anchor bolts for the transoms. Aheight to diameter ratio of 3:1 should not be exceeded without careful calculations. Thecaisson pier type will also obstruct the water-flow, which could increase the scour.

Temporary bridges must nonetheless offer reliable service over the required period of use. Aguidance to the selection of the design flood and flow characteristics for smaller temporarystructures is given in the ERA Drainage Design Manual-2002, Chapter 5: Hydrology. Thetwo key aspects of durability in temporary bridging are:

• load carrying adequacy• protection from water damage.

Figure 5-18 Bridge Pier made of Concrete Rings



A temporary bridge may not be completely adequate for all vehicles that normally use theroad. If the temporary structure has any limitation in load capacity, width or height, thismust be clearly marked at the entrances to the road on which the bridge is located andrepeated on the approaches to the bridge. It shall be possible to divert large vehicles to aprepared ford, while small ones are permitted to use the bridge. If possible, physical barriersshould be erected to prevent drivers of large vehicles from infringing the temporaryregulations in areas where drivers are known to ignore warning signs.

For most applications the two main precautions to be taken are:• to allow adequate clearance between high water level and the temporary deck• to build the sub-structure so that there is a minimum of interference to the flow.

5.18 BACKWATER

Regarding a bridge over a stream, the opening must be large enough not to cause anydamage due to backwater. Sometimes it shall be necessary to compensate for the backwaterby means of training or relining the stream. If the local populace and/or livestock normallywander along the shores, sometimes the bridge opening needs to be widened to provide forsuch passage under the bridge at normal water levels.

Calculation of backwater should always be made if the Design Water Velocity exceeds 1.0m/s. Examples are shown in the ERA Drainage Design Manual-2002, Chapter 8: Bridges,Section 8.7: Examples.

5.19 SELECTION OF BRIDGE TYPE

5.19.1 SKEWED CROSSINGS

Generally skewed crossings should be avoided, because skewed bridges are more difficult tocalculate, are longer, and need more reinforcement, which means they are more costly.

One lane open, framed, skewed bridges should be avoided due to the eccentric earth pressureon each of the frontwalls that may cause the whole structure to rotate. The moment fromrotation has to be taken in consideration when the slide between the footing and the groundis calculated.

However the bridge shall be made perpendicular even if the crossing is skewed as shown atFigure 5-19.



Figure 5-19 Skewed (Bottom) and Perpendicular (Top) Bridge

5.19.2 ECONOMICAL ASPECTS

The most economic overall bridge length and span length depends to a large extent upon thefoundation costs. If the piers have to be founded deep under the water or if piling is needed,then longer spans up to a certain limit shall be more economical. At larger bridges anddifficult soil conditions the most favorable locations of the piers should be sought. If no suchlocations can be located it may in some cases be more economical to realign the road to amore suitable bridge site, although all aspects should be re-investigated.

The cost of the superstructure mainly depends on the span length and the available designdepth.

To compare the construction costs of different bridges it is practical to use the specific costper square meter of bridge deck area. The construction costs from recently constructedbridges are valuable and therefore should be collected. If possible the costs should excludecosts for extraordinary scour protection, realignment of the river, etc, which do not form aportion of the bridge structure cost itself. Hence different bridge type alternatives can becompared at the same bridge site.

As mentioned before, the geotechnical/soil conditions are very important in the total cost ofthe structure. If the soil conditions for the adjacent road embankment are very poor andrequire piling or a pile deck, this should be compared to the cost of a longer bridge. The



same applies for very high embankments, where a longer bridge sometimes might be moreeconomic due to the savings in earthworks

5.19.3 ARCHITECTURAL AND SCULPTURAL ASPECTS

Close to cities or large towns the architectural and sculptural aspects of a bridge should beconsidered. If a pleasing bridge is desired a competition between architects shall be arrangedbefore or parallel with the actual pre-design of the bridge. Preferably the winning sketchshould be converted and elaborated into “photographs” of the proposed bridge by means ofsuitable computer software. The architect's viewpoints must be “translated” into thestructural limitations available for the particular bridge. To avoid misunderstandings both thesketch and the actual bridge layout drawings should be submitted to the bidder. The winnermay be invited to participate in the final design of the bridge and its details.

5.19.4 APPEARANCE AND SAFETY

There are however some basic facts regarding the appearance of the bridge that the structuraldesigner and the road engineer should consider:

• Normally the form of the bridge should coincide with the road both horizontally andvertically.

• If the bridge is close to a horizontal curve the skew transition should preferably be movedfrom the bridge deck.

• Curves and especially reverse-curves on a bridge should be avoided for safety reasonssince the railing always reduces the sight length in the curve.

• A sag vertical curve should have the lowest point at the adjacent road embankment, not atthe bridge, to achieve sufficient drainage of the bridge deck.

• A crest vertical curve with the highest point at the bridge is suitable as long as the sightlength is sufficient. At longer bridges with esthetical demands it is advisable to arrange ahorizontal curve as well. Then the bridge and its main superstructure shall be viewed fromthe side before entering the bridge itself. If the bridge is attractive, a parking facility shallbe provided before the bridge to allow people to view the bridge and the water.

5.20 RAILINGS, AND PARAPETS

Railings are designed as a part of the overall bridge design. For details, refer to Chapter 12:Detail Design of Bridges; the appendix for worked examples; and the ERA Standard DetailDrawings-2002, Chapter 2: Guardrail Drawings for design.

5.21 CHECKLIST FOR THE PRELIMINARY DESIGN DRAWING(S)

Information on the layout and sizing of the drawings is given in Chapter 16: Calculations,Drawings, and Specifications. The following information are required in the preliminarydesign stage:



General Information

• the name and direction of the nearest town and/or the beginning and end of project,• bench mark used and its location and elevation (if a local Bench mark is used it shall

be described in detail and connected to the National Level Benchmark Network),• coordinates, coordinate system used and north arrow,• water / sewerage pipes• electrical cables or lines• fixed and expanding bearings and type of bearings• type of expansion joints if any, type of hinges• type of drainage outlets of bridge deck if any,• type of fill behind the abutments (at frame structures the filling should be made at

the same time at the same level behind both abutments, at abutments not founded onrock the backfill shall be made before the measure between the superstructure andthe abutment is decided upon),

• which Loading Specifications and accidental loads have been used,• which pavement type is to be used for the bridge deck,• which guardrail / pedestrian railing / parapets are to be used,• scale shown at each figure,• run-on-slabs for roadway, cables, water-pipes, etc.,• protection pipe for water-pipe,• Standards to be used for the detail design,• Specifications to be used for the construction,• Title block information, name of the bridge, scale used, bridge number• plan with the contours or spot levels of the river bed and the surroundings.

Foundations

Provided details of:• founding method for bridge supports,• proposed elevations of footings,• replacement of soft layer with compacted fill,• piling and length of piles,• type of piles (cast-in-place etc.),• proposed level of pile cap,• embankment reinforcement, i.e. piling, replacement with light weight material, etc.,• compaction methodology, requirements for fill,• scour protection, type, material, thickness, protected area,• sub water excavation, sub water casting of lean concrete slab under foundation,• types of soil at every support,• sheetpiling

Soil Conditions

Provide the following:• reference to the Soil Investigation Report for the bridge,• inclination of embankment slopes• cross sections in the alignment as well as 5 m up-/downstream at elevation,



• the levels of different soil layers, level of bedrock• the type of soil at every proposed support,• ground water level with the date it was observed,

Bridge Dimensions and Geometry

• overall length of bridge, span lengths,• width of road, width of bridge,• grade of bridge,• angle of skew, angle of abutments and piers,• width of columns (minimum thickness requirements),• lowest allowable level of superstructure• length of wingwalls adequate,• type of supports, i.e. masonry or concrete, rounded piers, etc• crossfall of bridge deck and/or superelevation,• depth of substructure, chamfers to be utilized,• retaining walls,• type of bearings (fixed/expansion),• the stations of the piers and abutments,• profile of the road - schematic

Information on the Stream

• name of the waterway,• direction of flow, design water speed (=water speed at design water flow),• quantities of flow, design water flow, normal and lowest water flow,• design water level, normal and lowest water level,• clearance heights,• cross sections of the waterway in the alignment

A sample preliminary design drawing is given in Figure 5-20.

5.22 CHECKLIST FOR THE PRELIMINARY DESIGN STANDARDS (PDS)

The contractor shall follow these requirements for the design of the bridge:

• General Requirements- Main tender (Preliminary Design Drawing No., minimum dimension requirements)- Contractors alternative Preliminary Design (requirements of width, height, radius,

grade, other given dimensions, skew angle, bridge cones, slope and placement,profile)

- Water levels and ground water levels (usually from the Drainage Investigation)- Formwork requirements- Traffic conditions during the Construction time (ADT; required width, height and

speed; provisional bridge/load; fencing, etc.)



• Design requirements- Earth Pressure (backfill material)- Piling (tension forces, drag forces, etc.)- Buoyancy (Design Water Level shall be indicated on the drawing, level of pumping,

etc.)- Settlement differences (if different from the LRFD Code)- Allowable creep of concrete (applicable only for unusual designs)- Allowable deformations and frequency (only if different from the LRFD Code)- Expansive soils (if the soil investigation indicates it)- Clearance requirements (over Design Water Level, roadway, walkway, railway, etc.)- Design Life requirements (according to the LRFD Code)

• Loading- General (Traffic load, if different from the LRFD Code)- Permanent loads (launching forces, dead load of unusual materials, displacement

loads)- Live Loads (emergency traffic load on pedestrian bridges, load on walkway intended

to be used as a traffic lane in the future, fatigue load, measured wind load for specialbridges, stream pressure/drag, etc.)

- Accidental Loads (level of collision load, etc.)- Loading combinations (if different from the LRFD Code)

• Foundation works- Soil Investigation used (dated, by whom)- Construction method proposed- Excavation works

Excavation and casting of concrete above waterPumping of ground water (assumed method, 2 000 l/min. normally)Sheetpiling (underwater excavation and casting of lean concrete under thefooting)Reporting to the Engineer at least 5 days before fill, casting of footings, etc.)

- Fill works (level of fill, if different from Bridge Specifications)- Embankment piling (usually the embankment piling shall be made before the

abutment piling)- Footings- Piling works

prefab piles or cast-in place piles, concrete cover of pilestip bearing or skin friction piles (results of test piling already made, designlength of piles at different supports, required number of test piles at eachsupport, etc.)checking of foundation work (highest allowable groundwater level under theexcavation level - normally 0.5 m; required additional checking of piling)

- Protection works (erosion protection, sheet piling, scour protection)



• Concrete works- Superstructure (edgebeam type, larger concrete cover than required in the LRFD

Code, if the slab shall be assumed continuous over supports, if expansion joints arenecessary over supports, maximum allowable crossfall/superelevation)

- Construction (if special methods are required, detail design drawings will beprovided later)

- Substructure- Footing (assumed bottom levels, if footing is allowed to be cast underwater)- Pier, Abutments, Retaining Walls and Cones (filling material, cone material,

minimum dimension of pier/abutment if different from the LRFD Code)

• Steel and Timber works- Superstructure (coating, construction methods)

• Bridge Details- Drainage (only for curbs: type, outlet; drainage of box piers and girders)- Pipes and Ducts (dimensions, nos., placing and dead load of content)- Paving (water insulation under the surface coat, pavement type, coat thickness,

material, traffic lane/pedestrian lane, islands)- Edge beam (type, standard or special)- Bearings (type, brand - only if required by ERA; lifting requirements at exchange of

bearings; if bearings are excluded from the tender/delivered by ERA)- Expansion joints (type - open or waterproof, required brand - only if required by

ERA)- Railing (type, parapet type, length, painting of railing and parapet)

• Miscellaneous- Inspection devices (manholes, inspection platforms, ladders)- Test loading (test program, calculations and evaluation to be made by the

Contractor/by ERA)- Built-as specifications and drawings (only additional requirements not stated in the

LRFD Code)- If military load should be applied or not.

All drawings and specifications supplied shall be signed and dated.

5.23 SAMPLE PRELIMINARY DESIGN SPECIFICATIONS

Sample preliminary design specifications for a bridge are given in the next example (figure5.20).



Figure 5.20 EXAMPLE PRELIMINARY BRIDGE SPECIFICATIONS (PBS)

ETHIOPIAN ROADS AUTHORITY BRIDGE No. A1-123OROMIYA REGIONADDIS ABABA − DJIBOUTI HIGHWAY A1

EXAMPLEFor the Construction of the

MOJO RIVER BRIDGE1 KM NW OF MOJO VILLAGEAt Sta. 7+090, in the Oromiya Region

Addis Ababa, 28 Jan 1999SABA Engineering, PLC 794509-72



I GENERAL

I.1 General Requirements

Main Tender

In the Main Tender, the Bridge shall be built according to Preliminary Design Drawing No.A1-123: 01 (Figure 5-21).

Dimensions shall be according to the requirements of Chapter 16: Calculations, Drawings,and Specifications.

Contractors Alternative Tender

If an alternative tender is given by the Contractor, the bridge shall be constructed with thesame width, height, radius, grade, skew angle and other dimensions given on PreliminaryDesign Drawing No. A1-123: 01 (Figure 5-21).Bridge Cones shall not be steeper, or be placed closer to the water, than stated on thePreliminary Design Drawing No. A1-123: 01 (Figure 5-21).The profile of the road may not be altered.

I.2 Construction Requirements

Water Levels and Ground Water Levels

Water levels shown have been calculated according to the ERA Drainage Design Manual-2002, and ground water levels shall be as shown in the "Soil Investigation Report for MojoRiver Bridge," dated 25 Dec 1998. The normal (mean) water level has been estimated afterseveral field level measurements.

Formwork Requirements

Formwork requirements shall be as given on Preliminary Design Drawing No. A1-123:01.

I.3 Design Requirements

Earth Pressure

Backfill material shall be granular soil graded in accordance with the TechnicalSpecifications.

Piled Footings

All soil parameters which are not given on Preliminary Design Drawing No. A1-123: 01,shall be according to "Soil Investigation Report for Mojo River Bridge," dated 25 Dec1998.



Tension forces in each pile shall not exceed 50 kN. Downdrag need not be considered.

Buoyancy

When designing the buoyancy for the pile slabs, the water level +2062.50 shall be used.

Clearance Requirements

Clearance above the Design Water Level (DWL) shall not be less than 0.9 m.

II LOADINGS (NOT APPLICABLE)

III FOUNDATIONS

Soil Conditions

The soil parameters shall be according to the "Soil Investigation Report for Mojo RiverBridge," dated 25 Dec 1998.

Construction Work

Excavation and Casting of Footings for the supports shall be assumed above the watersurface. The excavation shall be according to the "Soil Investigation Report for MojoRiver Bridge," dated 25 Dec 1998 and Preliminary Design Drawing No. A1-123: 01.Excavation and Casting of Footings above water shall include pumping of ground water.At least 5 days before the casting of the lean concrete under the footing ERA shall benotified, in order to inspect the compaction of the fill under the footing.

Tip Bearing Piles

The length of the piles shall be determined by the contractor by 2 nos. of testpiles at eachabutment. The Contractor is responsible for using the right length of the piles. The designlength at each abutment given on Preliminary Design Drawing No. A1-123: 01 is to beused only for the bidding.

Protection for Scour

Temporary Protection for scour shall be included in the Bridge Construction work.Protection for scour shall be made of a 0.1-m layer of natural stones on top of 0.5 m layerof stones with the size 0-100 mm (d50≥ 70 mm) up to 3.0 m from the bridge abutmentedge according to Preliminary Design Drawing no. A1-123: 01.Bridge Cones surface shall be protected with a 0.3-m layer of stones sized 16-32 mm.



IV CONCRETE WORK

IV.1 Superstructure

Edge Beams

Edge beams shall be developed under the deck surface.

IV.2 Substructure

Footings

The elevations of the bottom side of the footings shall not exceed the levels given onPreliminary Design Drawing No. A1-123: 01.

Piers, Abutments, Retaining walls

Fill shall be made with material with equal parameters as assumed in the design and stated insection 1.31 above. Fill shall be as shown on Preliminary Design Drawing No. A1-123: 01.

V BRIDGE DETAILS

V.1 Drainage

The drainage of the bridge deck includes the delivery and casting in place of one no. ofStandard Scupper Drain Type D1 according to Standard Detail Drawing No. DR-01.

V.2 Bearings

The bearings shall be of the steel reinforced elastomeric type, of a brand approved by ERA.

When lifting the superstructure for the replacement of the bearings it shall be assumed thatthe jacks are 500 mm high and placed 600 mm inside the centerline of the bearings, asshown on Preliminary Design Drawing No. A1-123: 01.

V.3 Railings

Guardrail

Guardrail shall be made of concrete according to Standard Detail Drawing No. GR-1 as faras shown on Preliminary Design Drawing No. A1-123: 01.The adjacent four nos. of RC Parapet Panels are not included in the Bridge Constructionwork.

Addis Ababa, 28 Jan 1999

SAVA Engineering, PLC

Tsahai Bekila (Bridge Engineer, B.Sc.)



REFERENCES

1. Farraday and Charlton, Hydraulic Factors in Bridge Design, Wallingford, HydraulicResearch Station Ltd., 1983.

2. Parry J D, 1981. The Kenyan Low Cost Modular Timber Bridge. TRRL Laboratory Report1970. Crowthorne: Transport and Road Research Laboratory, England.

3. Bingham J, 1979. Low Water Crossings. Compendium 4. Washington: TransportationResearch Board.

4. Hindson J, 1983. Earth Roads - A practical guide to earth road construction andmaintenance. London: Intermediate Technology Publications.

5. Roberts P, 1986. The Irish Bridge - a low cost river crossing. Southampton: University ofSouthampton, Department of Civil Engineering.

6. TRRL Overseas Road Note no 9, “A Design Manual for Small Bridges”, Transport andRoad Research Laboratory, Crowthorne Berkshire UK, 1992.

7. Brokonstruktion - en handbok (Preliminary Bridge Design − A Handbook), Publication no.1996:63, Vagverket (Swedish Roads Authority), Borlange, Sweden, 1996. In Swedish.

8. "Design Standard", Ethiopian Roads Authority, compiled May 1993 (1961 - 1989)9. “Brobygging – I. Jernbeton, sten og trae” (Bridge Design – part I Reinforced Concrete, stone

and tree) in danish, Prof Anker Engelund, Copenhagen 1934.10. TRRL Overseas Road Note no 7, Vol. 2, “Bridge Inspectors Handbook", Crowtorne

Berkshire UK, Transport and Road Research Laboratory, 1988.11. “RTIM3 - Road Transport Investment Model", TRRL Overseas Centre Transport and

Research Laboratory, TRRL, Berkshire UK, 1988.12. Design manual for roads and bridges - Vol. 1: "Highway structures: approval procedures and

general design", The Stationary Office Ltd., London, 1998.13. Design manual for roads and bridges - Vol. 3: "Highway structures: inspection and

maintenance", The Stationary Office Ltd., London, 1998.14. Ethiopian Building Code Standard (EBCS), Volume 1 "Basis of Design & Actions on

Structures", 1995.15. Ethiopian Building Code Standard (EBCS), Volume 2 "Structural Use of Concrete", 1995.16. Ethiopian Building Code Standard (EBCS), Volume 3 "Design of Steel Structures", 1995.17. Ethiopian Building Code Standard (EBCS), Volume 5 "Utilization of Timber", 1995.18. Ethiopian Building Code Standard (EBCS), Volume 7 "Foundations", 1995.19. Ethiopian Building Code Standard (EBCS), Volume 8 "Design of Structures for Earthquake

Resistance", 1995.20. Eurocode 1 "Basis of Design and Actions on Structures - Part 3 Traffic Loads on Bridges",

European Prestandard ENV 1991-3, March 1995.21. Eurocode 2 "Design of Concrete Structures", European Prestandard ENV 1992.22. Eurocode 3 "Design of Steel Structures", European Prestandard ENV 1993.23. "BRO 94 - Brokonstruktionsbestammelser" (Bridge Design Code), Publication no. 1994:1-8,

57-1998, Vagverket (Swedish Roads Authority, in Swedish.), Borlange, Sweden, 1998.24. "Guide Design Specifications for Bridge Temporary Works", American Association of State

Highway and Transportation Officials, Washington, 1995.

05+-+Chapter05

Documents

ethiopian

finite element

ethiopian

preliminary

preliminary

swedish roads

soil investigation

bridge design