ISO 21725-2:2021

INTERNATIONAL ISO
STANDARD 21725-2
First edition
2021-11
Simplified design of prestressed
concrete bridges —
Part 2:
Box-girder bridges
Conception simplifiée des ponts en béton précontraint —
Partie 2: Ponts à poutres caissons
Reference number
© ISO 2021
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Published in Switzerland
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Contents Page
Foreword .viii
Introduction .ix
1 Scope .1
2 Normative references .1
3 Terms and definitions .1
4 Symbols and abbreviated terms.6
5 Design and construction procedure .9
5.1 Procedure . 9
5.1.1 General . 9
5.2 Design documentation . 12
5.2.1 General .12
5.2.2 Calculation report .12
5.2.3 Geotechnical report . 12
5.2.4 Structural drawings .12
5.2.5 Specifications . 12
6 General provisions .13
6.1 Limitations . 13
6.1.1 General .13
6.1.2 Permitted use . 13
6.1.3 Maximum number of spans . 13
6.1.4 Recommended span length . 13
6.1.5 Maximum difference in span length . 13
6.1.6 Maximum cantilever length . 13
6.1.7 Maximum height of bridge . 13
6.1.8 Maximum number of lanes . 14
6.1.9 Width limitations . 14
6.1.10 Clearances . 14
6.1.11 Maximum skew angle . 15
6.1.12 Maximum bridge horizontal curvature . 15
6.1.13 Cross-section variation.15
6.1.14 Interaction between superstructure and substructure .15
6.2 Limit states . 15
6.3 Ultimate limit state design format . . 16
6.3.1 General . 16
6.3.2 Required factored loads. 16
6.3.3 Design strength . 17
6.4 Serviceability limit state design format . 17
7 Structural systems and layout .17
7.1 Description of the components of the structure . 17
7.1.1 General . 17
7.1.2 Superstructure system . 17
7.1.3 Substructure system. 17
7.1.4 Foundation . 18
7.2 General guide . 18
7.2.1 Architectural guide . 18
7.2.2 General structural guides for the project . 18
7.3 Structural layout . 19
7.3.1 General structural layout . 19
7.3.2 Vertical layout . 19
7.4 Feasibility under the guidelines . 20
8 Actions (Loads) .21
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8.1 General . 21
8.2 Dead loads . . 21
8.2.1 General . 21
8.2.2 Structural elements . 21
8.2.3 Non-structural elements . 21
8.3 Live loads .22
8.3.1 General .22
8.3.2 Design truck . 22
8.3.3 Design lane load . 22
8.3.4 Dynamic effect of live loads . 23
8.4 Longitudinal forces.23
8.5 Earth pressure .23
8.6 Wind loads . 24
8.7 Earthquake inertial forces . 24
8.7.1 General . 24
8.7.2 Seismic hazard . 24
8.7.3 No seismic hazard zones: . 24
8.7.4 Low seismic hazard zones: . 25
8.7.5 Intermediate seismic hazard zones: . 25
8.7.6 High seismic hazard zones: . 25
8.7.7 Soil profile types.29
8.7.8 Site effects .30
8.7.9 Design response spectral ordinates .30
8.7.10 Seismic equivalent uniformly distributed load . 31
8.7.11 Fundamental mode shape . 31
8.7.12 Lateral equivalent design forces . 32
8.8 Jacking and post-tensioning forces . 32
8.8.1 Jacking forces . 32
8.8.2 Forces for post-tensioning anchorage . 33
8.9 Thermal effects . 33
8.9.1 Seasonal variation . 33
8.9.2 Thermal coefficient . 33
8.9.3 Differential temperature . 33
8.10 Load combinations .33
8.10.1 Ultimate loads . 33
8.10.2 Service loads . 33
9 Structural analysis .34
9.1 General .34
9.1.1 B-region and D-region .34
9.1.2 Elastic behaviour .34
9.1.3 Small deflection theory .34
9.1.4 Secondary moments .34
9.1.5 Time-dependent properties of concrete.34
9.1.6 Geometric imperfections .34
9.1.7 Frame analysis .34
9.2 Longitudinal analysis .34
9.3 Transverse analysis . . 35
9.3.1 Empirical method . 35
9.3.2 Approximate method.35
9.3.3 Refined method . 36
10 Design requirements .37
10.1 General . 37
10.2 Box girder cross-section dimensions and details . 37
10.2.1 Minimum flange thickness . 37
10.2.2 Minimum web thickness . 37
10.2.3 Length of top flange cantilever . 37
10.2.4 Overall cross-section dimensions .38
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10.2.5 Longitudinal slope .38
10.3 Materials for structural concrete . 39
10.3.1 General .39
10.3.2 Cement . 39
10.3.3 Aggregates .39
10.3.4 Water . 39
10.3.5 Steel reinforcement .39
10.3.6 Prestressing steel .40
10.3.7 Post-tensioning anchorages and couplers .40
10.3.8 Ducts . 41
10.3.9 Admixtures. 41
10.3.10 .
Storage of materials . 41
10.3.11 .
Minimum and maximum reinforcement bar diameter . 42
10.4 Concrete mixture proportioning . 42
10.4.1 General . 42
10.4.2 Durability requirements . 42
10.4.3 Required average compressive strength . 43
10.4.4 Proportioning of the concrete mixture . 43
10.5 Concrete cover of reinforcement .44
10.5.1 Minimum concrete cover .44
10.5.2 Special corrosion protection . 45
10.6 Minimum reinforcement bend diameter . 45
10.7 Standard hook dimensions . 45
10.8 Bar spacing and maximum aggregate size . 45
10.8.1 General . 45
10.8.2 Maximum nominal coarse aggregate size . 45
10.8.3 Minimum clear spacing between parallel bars in a layer .46
10.8.4 Minimum clear spacing between parallel layers of reinforcement .46
10.8.5 Clear spacing between parallel lap splices .46
10.8.6 Maximum flexural reinforcement spacing in solid slabs. 47
10.8.7 Maximum shrinkage and temperature reinforcement spacing in solid slabs . 47
10.8.8 Maximum reinforcement spacing in structural concrete walls . 47
10.8.9 Minimum spacing of prestressing tendons and ducts .48
10.8.10 .
Maximum spacing of prestressing tendons in slabs .49
10.8.11 .
Couplers in post-tensioning tendons .49
10.9 Development length, lap splicing and anchorage of reinforcement .49
10.9.1 Development length .49
10.9.2 Lap splice dimensions .50
10.10 Limits for longitudinal reinforcement . 51
10.10.1 General . 51
10.10.2 .
Shrinkage and temperature reinforcement . 51
10.10.3 .
Minimum area of tension flexural reinforcement . 52
11 Stress limitations .52
11.1 Stress limitations for prestressing tendons . 52
11.2 Stress limitations for concrete .53
11.2.1 For temporary stresses before losses-fully prestressed components .53
11.2.2 For stresses at serviceability limit state after losses-fully prestressed
components .54
12 Loss of prestress .55
12.1 Total loss of prestress.55
12.2 Instantaneous losses . 56
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12.2.1 Anchorage set .56
12.2.2 Friction .56
12.2.3 Elastic shortening . 57
12.3 Approximate estimate of time-dependent losses . 57
13 Details of tendon .60
13.1 Tendon confinement .60
13.1.1 General .60
13.1.2 Effects of curved tendons .60
13.2 External tendon supports . 61
13.3 Post-tensioned anchorage zones . 61
13.3.1 General . 61
13.3.2 General zone and local zone . 61
13.3.3 Design of general zone. 62
13.3.4 Design of local zone.64
14 Superstructure .65
14.1 Strength of members subjected to flexural moments .65
14.1.1 General .65
14.1.2 Factored flexural moment at section .65
14.1.3 Minimum design flexural moment strength .65
14.1.4 Nominal moment strength of PSC box girder .65
14.2 Strength of members subjected to shear stresses . 69
14.2.1 General .69
14.2.2 Factored shear . 69
14.2.3 Design shear strength .69
14.2.4 Design of shear reinforcement . 69
14.3 Girders, beams, joists . 70
14.4 Details of box girder . 70
14.4.1 Flange and web thickness . 70
14.4.2 Reinforcement . 70
14.5 Diaphragms . 71
14.6 Opening . 71
14.7 Railings .73
14.8 Splices of strand .73
14.9 Continuous structure .73
14.9.1 Scope of application .73
14.9.2 General design .73
14.9.3 Structural analysis .73
14.9.4 Fixed support . 75
14.9.5 Intermediate support . 76
15 Substructure .77
15.1 Girders that are part of a frame .77
15.2 Strength of members subjected to axial loads with or without flexure .77
15.3 Torsion .77
15.4 Bearing strength .77
15.5 Columns and piers .77
15.6 Concrete walls . .77
16 Foundations .77
16.1 Foundation type and capacity .77
16.2 Subsurface exploration and testing programs .77
16.3 Dimensioning of the foundation elements .77
16.4 Footings . 78
16.5 Foundation mats . 78
16.6 Footings on piles .78
16.7 Foundation beams . 78
16.8 Retaining walls . . 78
17 Lateral load resisting system .78
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17.1 General . 78
17.2 Specified lateral forces .78
17.3 Lateral force resisting structural system . 78
17.4 Minimum amount of structural concrete walls . 78
17.5 Special reinforcement details for seismic zones. 78
18 Bearings .78
18.1 General . 78
18.2 Multiple roller bearings . 78
18.3 Elastomeric bearings . 78
18.4 Anchorage . 79
18.5 Design forces for supporting structure . 79
Annex A (informative) Equivalent formulae for material factors .80
Bibliography .82
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Foreword
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