Simplified design of prestressed concrete bridges — Part 2: Box-girder bridges

This document provides information to perform the design of the prestressed concrete box girder bridge for road that complies with the limitations established in 6.1. The rules of design as set forth in the document are simplifications of more elaborate requirements. Among several erection methods of box girder bridges, the provisions of this document are mainly applicable to full staging method (FSM). Designs and details for new road bridges address structural integrity by considering the following: — the use of continuity and redundancy to provide one or more alternate paths; — structural members and bearing seat widths that are resistant to damage or instability; and — external protection systems to minimize the effects of reasonably conceived severe loads.

Conception simplifiée des ponts en béton précontraint — Partie 2: Ponts à poutres caissons

General Information

Status
Published
Publication Date
31-Oct-2021
Current Stage
6060 - International Standard published
Start Date
01-Nov-2021
Completion Date
01-Nov-2021
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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 21725-2:2021(E)
© ISO 2021
---------------------- Page: 1 ----------------------
ISO 21725-2:2021(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2021

All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may

be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on

the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below

or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
© ISO 2021 – All rights reserved
---------------------- Page: 2 ----------------------
ISO 21725-2:2021(E)
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

iii
© ISO 2021 – All rights reserved
---------------------- Page: 3 ----------------------
ISO 21725-2:2021(E)

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

© ISO 2021 – All rights reserved
---------------------- Page: 4 ----------------------
ISO 21725-2:2021(E)

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 .............................................................................................................................

...

FINAL
INTERNATIONAL ISO/FDIS
DRAFT
STANDARD 21725-2
ISO/TC 71/SC 5
Simplified design of prestressed
Secretariat: KATS
concrete bridges —
Voting begins on:
2021-07-12
Part 2:
Voting terminates on:
Box-girder bridges
2021-09-06
RECIPIENTS OF THIS DRAFT ARE INVITED TO
SUBMIT, WITH THEIR COMMENTS, NOTIFICATION
OF ANY RELEVANT PATENT RIGHTS OF WHICH
THEY ARE AWARE AND TO PROVIDE SUPPOR TING
DOCUMENTATION.
IN ADDITION TO THEIR EVALUATION AS
Reference number
BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO-
ISO/FDIS 21725-2:2021(E)
LOGICAL, COMMERCIAL AND USER PURPOSES,
DRAFT INTERNATIONAL STANDARDS MAY ON
OCCASION HAVE TO BE CONSIDERED IN THE
LIGHT OF THEIR POTENTIAL TO BECOME STAN-
DARDS TO WHICH REFERENCE MAY BE MADE IN
NATIONAL REGULATIONS. ISO 2021
---------------------- Page: 1 ----------------------
ISO/FDIS 21725-2:2021(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2021

All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may

be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting

on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address

below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2021 – All rights reserved
---------------------- Page: 2 ----------------------
ISO/FDIS 21725-2:2021(E)
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

© ISO 2021 – All rights reserved iii
---------------------- Page: 3 ----------------------
ISO/FDIS 21725-2:2021(E)

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

iv © ISO 2021 – All rights reserved
---------------------- Page: 4 ----------------------
ISO/FDIS 21725-2:2021(E)

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

12.2.1 Anchorage set ....
...

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