Chapter 4 Specifications

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FUNDAMENTALS OF BRIDGE DESIGN Chapter

3

Analysis and Design ! RC Bridges

SPECIFICATIONS FROM ERA BRIDGE DESIGN MANUALSLAB BRIDGESSingle span slab bridges are perhaps the most common bridges in Ethiopia. They canbe economical for spans from 1 m to 18 m. Above 15 m they should preferably be

ribbed as shown in gure below. nstead of ribs there are several types of prefabforms that can be used by contractors! if the designer has considered that particulartype.

Sections of Voided (Hollowed) Slab and Ribbed Slab Bridge Decks

"ormally the slab is made with a uniform depth over the whole bridge. Te re!"ireddesign de#t is "s"all$ %&% ' of te s#an lengt* due to the width of thecrac#s. f stressed reinforced concrete is used! the design depth shall be reducedto +&% of te s#an lengt&

The abutments at single or double span slab bridges should preferably be placedperpendicular to the bridge in order to avoid a s#ew in earth pressure! which maycause s#ew in the abutment front wall.

GIRDER BRIDGESA girder bridge is usually used for a single span bridge! or non$continuous girders fora multi$span bridge! in earth%ua#e areas. They shall be used for span lengthsbetween 1& $ &' m. (utside of earth%ua#e )ones! continuous girder bridges arepreferred. n this case the e*terior span length should be appro*imately '.8 times theinterior span. The +,- design method usually minimi)es materials used if thenumber of girders/beams is minimi)ed. The cantilever should preferably not e*ceed0' of the spacing of the girders! or &.8 m for a two$lane bridge.

The design depth of a normal girder bridge may vary between 2$1' of the spanlength depending on the number of beams used. f possible! a high stem of beam ispreferred to a certain e*tent! both technically and economically. -or constructionreasons however! the height should be minimi)ed. Esthetically a short bridge with a

high superstructure close to the water surface should be avoided. 3ere a slenderstructure 4slab is more appealing.

S#acing of Reinforce,ent

-ini,", S#acing of Reinforcing Bars

Cast-in-Place Concrete

"U # IT # Ci$il Engineering S%hl 1

SLABVOIDED SLAB

RIBBED SLAB


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-or cast$in$place concrete! the clear distance between parallel bars in a layer shall notbe less than6

• 1.5 times the nominal diameter of the bars!

• 1.5 times the ma*imum si)e of the coarse aggregate! or 7 8 mm.

Precast Concrete

-or precast concrete manufactured under plant control conditions! the clear distancebetween parallel bars in a layer shall not be less than6

• The nominal diameter of the bars!

• 1. times the ma*imum si)e of the coarse aggregate! or

• &5 mm.

Multi-Layers

E*cept in dec#s where parallel reinforcing is placed in two or more layers! with cleardistance between layers not e*ceeding 15' mm! the bars in the upper layers shall beplaced directly above those in the bottom layer! and the clear distance betweenlayers shall not be less than &5 mm or the nominal diameter of the bars.

./0.RETE,ecommended grade of concrete and corresponding specied strengths are shown in

Table 9$1 for both cylinder and cube strengths. :lasses of concrete corresponding tothese grades are shown in Table 9$&.

;rades of :oncrete :&5 :' :0' :5' :a &' &0 & 0' 08f c# 4&'' mm cubes! =>a &1 &5 0 0& 5'

f c# 415' mm cubes! =>a &5 ' 0' 5'


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and other properties. :oncrete with strengths below &' =>a at &8 days 415' mmcylinders should not be used in structural applications.

The specied compressive strength for prestressed concrete shall not be less than '=>a.

.oe5cient of Ter,al E6#ansionThe coefcient o thermal expansion should be determined by the laboratory tests onthe specic mi* to be used. n the absence of more precise data! the thermalcoe?cient of e*pansion shall be ta#en as6@ for normal density concrete6 27&8 6 27'9o.! and@ for low$density concrete6 9.' * 1'$


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,einforcement shall be deformed! e*cept that plain bars or plain wire may be used forspirals! hoops and wire fabric. Gars H I 1' mm should not be used for cast$in$placestructures.

The nominal yield strength shall be the minimum as specied for the grade of steelselected! e*cept that yield strengths in e*cess of 5&' =>a shall not be used for design

purposes. Gars with yield strengths less than &2' =>a shall be used only with theapproval of E,A. Tensile re%uirements are as indicated in Table 9$0.

AAS3T( =1 = ;rade ;rade '' ;rade 0&' ;rade 5&'

E%uiv. European bars G5''G Jsrecast >restressed >iles 0' 5' n#rotected -ain Reinforcing Steel (,,)

The modulus of elasticity! Ec! of bars and un$deformed wires shall be assumed as &''''' =>a.



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=inimum cover to main bars! including bars protected by epo*y coating! shall be &5mm. :over to ties and stirrups shall be 1& mm less than the values specied in Table9$5 for main bars! but shall not be less than 25 mm.>recast so?t form panels are not considered as main bars.Concrete Cover for unprotected prestressing and reinforcing steel for the actual

water$cement ratio shall not be less than as specied in Table 9$5 above! unlessotherwise specied herein. :oncrete cover and placing tolerances shall be shown inthe contract documents and/or at the detail drawings.:over for pretensioned prestressing strand! anchorage hardware and mechanicalconnections for reinforcing bars or post$tensioned prestressing strands shall be thesame as for reinforcing steel.:over for metal ducts for post-tensioned tendons shall not be less than6

• that specied for main reinforcing steel!

• one$half the diameter of the duct! or

• that specied in Table 9$5.

Protective Coatings: >rotection against chloride$induced corrosion shall be providedby epo*y coating or galvani)ing of reinforcing steel! post$tensioning duct andanchorage hardware and epo*y coating of prestressing strand.

=le6"ral Reinforce,ent

E*cept at supports of simple$spans and at the free ends of cantilevers! reinforcementshall be e*tended beyond the point at which it is no longer re%uired to resist Ne*urefor a distance not less than6

• the eCective depth of the member!

• 15 times the nominal diameter of bar! or

• 1/&' of the clear span.:ontinuing reinforcement shall e*tend not less than the development length! ld!beyond the point where bent or terminated tension reinforcement is no longer

re%uired to resist Ne*ure."o more than %7 of the reinforcement shall be ter,inated at an$ section! andadBacent bars shall not be terminated in the same section.

-ini,", Reinforce,ent

-or components containing no prestressing steel! the minimum reinforcementprovision herein shall be considered satised if6

ρmin ≥ '.' f ′c/ f y 49.1

where6 ρmin ratio of tension steel to gross area

f ′c specied concrete strength 4=>af y yield strength of tension steel 4=>a

n T$beams where the web is in tension! the determination of the actual mild steel

ratio! ρ! for comparison with the re%uirement of E%uation 9.1 shall be based on thewidth of the web.



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.ontrol of .racking b$ Distrib"tion of Reinforce,ent

>rovisions specied! herein! are used for the distribution of tension reinforcement tocontrol Ne*ural crac#ing in beams.-rom the standpoint of appearance! many ne crac#s are preferable to a few widecrac#s. The best crac# control is obtained when the steel reinforcement is well

distributed over the )one of ma*imum concrete tension. Several bars at moderatespacing are more eCective in controlling crac#ing than one or two larger bars of e%uivalent area.

:omponents shall be so proportioned that the tensile stress in the mild steelreinforcement at the service limit state! f s does not e*ceed6

f s O ≤ '.< f y 49.10

4dc A1/

where6 dc depth of concrete measured from e*treme tension ber to center of bar

or wire located closest thereto for calculation purposes! the thic#ness of clear cover used to compute dc shall not be ta#en to be greater than 5'mm

A area of concrete having the same centroid as the principal tensilereinforcement and bounded by the surfaces of the cross$section and astraight line parallel to the neutral a*is! divided by the number of bars orwires 4mm& for calculation purposes! the thic#ness of clear concrete coverused to compute A shall not be ta#en to be greater than 5' mm

O crac# width parameter 4"/mm

E*cept for cast$in$place reinforced concrete bo* culverts! the %uantity O in E%uation9.10 shall not e*ceed ' #"/mm for members in moderate e*posure conditions! &

#"/mm for members in severe e*posure conditions! and 12.5 #"/mm for buriedstructures.

Srinkage and Te,#erat"re Reinforce,ent

,einforcement for shrinage and temperature stresses shall be provided nearsurfaces of concrete e*posed to daily temperature changes and in structural massconcrete. Temperature and shrin#age reinforcement shall be added! so that the totalreinforcement on e*posed surfaces is not less than that specied herein.Components less than !2"" mm thic: ,einforcement for shrin#age and temperatureshall be in the form of bars! welded wire fabric or prestressing tendons. -or bars orwelded wire fabric! the area of reinforcement in each direction shall not be less than6

As ≥ '.25 Ag/f y 49.12

Fhere6 Ag gross area of section 4mm&f y Specied yield strength of reinforcing bars 4=>a

Shrin#age and temperature reinforcement shall not be spaced farther apart thaneither .' times the component thic#ness or 05' mm.



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Strengt Li,it State

-actored resistance shall be the product of nominal resistance! and the resistancefactor in

Table 9$2 below6

#esistance $actors! :onventional :onstruction6 ,esistancefactor ϕ6

• -or Ne*ure and tension of reinforced concrete .........'.9'

• -or Ne*ure and tension of prestressed concrete........1.''

• -or shear and torsion6

"ormal density concrete............................................'.9' +ow$density concrete...............................................'.2'

• -or a*ial compression with spirals or ties..................'.25

• -or bearing on concrete ............................................'.2'

• -or compression in strut$and$tie models ..................'.2'

• -or compression in anchorage )ones6"ormal density concrete..........................................'.8'+ow$density concrete................................................'.


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41&.01

Fhere6 E e%uivalent width 4mm+1 modied span length ta#en e%ual to the lesser of the actual span or 18

''' 4mm

F1 modied edge$to$edge width of bridge ta#en e%ual to the lesser of theactual width or 18 ''' mm for multi$lane loading! or 9''' mm for singlelane loading 4mm

F physical edge$to$edge width of bridge 4mm"+ number of design lanes as specied in section %&: (ravity )oads:

*ehicular )ive )oad&

Eui!alent stri" #i$t% &or o!er%an( "art o& (ir$er 'ri$(e@E 110' P '.8Q

Fhere Qdistance from load to point of support 4mm

Girders (Si,#le S#an* .ontin"o"s* Bo6)

Some recommended dimensions for cast$in$place girders! bo* and T$beams are6 The thic#ness of to# anges serving as dec# slab6

• same as for bridge dec#s

• not less than 5 of the clear span between llets! haunches! or webs! unlesstransverse ribs at a spacing e%ual to the clear span are used.

And for the botto, ange thic#ness not less than either6

•10' mm!

•1/1< of the distance between llets or webs of non prestressed girders and beams!

or

•1/'th of the clear span between llets! haunches or webs for prestressed girders!

unless transverse ribs at a spacing e%ual to the clear span are used.

The thic#ness of webs shall be determined by re%uirements for shear! torsion!concrete cover and placement of concrete. -or ade%uate eld placement andconsolidation of concrete! usually a minimum web thic#ness of &'' mm is needed forwebs without prestressing ducts. -or girders over about &.0 m in depth! the abovedimensions should be increased to compensate for the increased di?culty of concreteplacement. :hanges in girder web thic#ness shall be tapered for a minimum distanceof 1&.' times the diCerence in web thic#ness.

Reinforce,ent for .ast'in'#lace Girder* Bo6 and T'bea,s

The reinforcement in the deck slab of cast'in'#lace T'bea,s and bo* girders shallbe determined by either the traditional or by empirical design methods. Fhere thedec# slab does not e*tend beyond the e*terior web! at least one'tird of the bottomlayer of the transverse reinforcement in the dec# slab shall be e*tended into the

e*terior face of the outside web and anchored by a standard 17 ook . f the slabe*tends beyond the e*terior web! at least one$third of the bottom layer of thetransverse reinforcement shall be e*tended into the slab overhang and shall have ananchorage beyond the e*terior face of the web not less in resistance than thatprovided by a standard hoo#.



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+nterior ,eams ith Concrete .ecs: The live load Ne*ural moment for interior beamswith concrete dec#s shall be determined by applying the lane fraction specied inChapter !%: /pproximate 0ethods o /nalysis& -or preliminary design! the terms J g/4+7ts and /R shall be ta#en as 1.'.

=orce ECect L (,,)

>ositive =oment The length of the span forwhich moment is beingcalculated

"egative =oment "ear interiorsupports of continuous spans frompoint of contra Ne*ure to point of contra Ne*ure under a uniform load onall spans

The average length of the twoadBacent spans

"egative =oment (ther than near

interior supports of continuous spans

The length of the span for

which moment is beingcalculatedShear The length of the span for

which shear is being calculatedE*terior ,eaction The length of the e*terior span

nterior ,eaction of :ontinuous Span The average length of the twoadBacent spans

Table 2


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Fi(ure )*-+ Co,,on Dec Su"erstructures



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T$#e of

S"#erstr"ct"re

A##licable

.ross'sectionfro, =ig"re

2


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T$#e of S"#erstr"ct"re

A##licable.ross'

section fro,=ig"re 2

Chapter 4 Specifications

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