BS NA EN 1998-2: UK National Annex to Eurocode 8. Design ...

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BS NA EN 1998-2 (2005) (English): UK NationalAnnex to Eurocode 8. Design of structures forearthquake resistance. Bridges

NA to BS EN 1998-2:2005

UK National Annex to Eurocode 8: Design of structures for earthquake resistance -Part 2: Bridges

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NA to BS EN 1998-2:2005 NATIONAL ANNEX

Publishing and copyright information The BSI copyright notice displayed in this document indicates when the document was last issued.

© BSI 2009

ISBN 978058055091 1

ICS 91.120.25; 93.040

The following BSI references relate to the work on this standard: Comm ittee reference B/525/8 Draft for comment 08/30129892 DC

Publication history First published June 2009

Amendments issued since publication

Date Text affected

NATIONAL ANNEX

Contents Introduction 1 NA.1 Scope 1 NA.2 Nationally Determined Parameters

NA to BS EN 1998-2:2005

NA.3 Decisions on the status of informative annexes 8 NA.4 References to non-contradictory complementary information 8

Bibliography 9

List of tables Table NA.1 - UK values for Nationally Determined Parameters described in BS EN 1998-2:2005 2

Summary of pages This document comprises a front cover, an inside "front cover, pages i to ii, pages 1 to 10, an inside back cover and a back cover.

© BSI 2009 •


ii • © BSI 2009 This page deliberately left blank

NATIONAL ANNEX NA to BS EN 1998-2:2005

National Annex (informative) to BS EN 1998-2:2005, Eurocode 8: Design of structures for earthquake resistance -Part 2: Bridges

Introduction This National Annex has been prepared by BSI Subcommittee B/525/8, Structures in seismic regions. In the UK it is to be used in conjunction with BS EN 1998-2:2005.

NA.1 Scope This National Annex gives:

a) the UK decisions for the l\Iationally Determined Parameters described in the following subclauses of BS EN 1998-2:2005:

2.1(3)P 5.6.2(2)P b

2.1 (4)P 5.6.3.3(1)P b

2.1 (6) 6.2.1.4(1)P

2.2.2(5) 6.5.1(1)P

2.3.5.3(1) 6.6.2.3(3)

2.3.6.3(5) 6.6.3.2(1)P

2.3.7(1) 6.7.3(7)

3.2.2.3 7.4.1(1)P

3.3{1 )P 7.6.2(1)P

3.3(6) 7.6.2(5)

4.1.2(4)P 7.7.1 (2)

4.1.8(2) J.1 (2)

5.3(4) J.2{1 )

5.4{1 )

b) the UK decisions on the status of BS EN 1998-2:2005 informative annexes; and

c) references to non-contradictory complementary information.

NA.2 Nationally Determined Parameters UK decisions for the Nationally Determined Parameters described in BS EN 1998-2:2005 are given in Table NA.1,

© BSI 2009 • 1

N Table NA.1

@ Subclause OJ 2.1 (3)P ~ N 0 0 \.0

2.1 (4)P

2.1 (6)

2.2.2(5)

2.3.5.3(1)

UK values for Nationally Determined Parameters described in BS EN 1998-2:2005

Nationally Determined Parameter

Reference retu rn period T NCR of seismic action for the no-collapse requirement (or, equivalently, reference probability of exceedance in 50 years, PNCR)

Eurocode recommendation

TNCR = 475 years

PNCR = 10%

UK decision

In the absence of a project-specific assessment, adopt a return period TNCR of 2 500 years. Further guidance is given in PD 6698.

Importance In general road and railway bridges are considered to belong to importance class II The importance classes should be established on a project-specific basis. Further guidance is given in PD 6698.

classes for (average importance). bridges Importance class III comprises bridges of critical importance for maintaining

communications, especially in the immediate post-earthquake period, bridges where failure is associated with a large number of probable fatalities, and major bridges where a design life greater than normal is required.

A bridge may be classified as importance class I (less than average importance) when both the following conditions are met:

the bridge is not critical for communications, and

the adoption of either the reference probability of exceedance, PNCR, in 50 years for the design seismic action, or of the standard bridge design life of 50 years is not economically justified.

Importance classes I, II and III correspond roughly to consequences classes CC1, CC2 and CC3, respectively, defined in BS EN 1990:2002, B3.1.

Importance factors for bridges For importance class I, YI = 0,85

For importance class III, YI = 1,3

Where a value for the reference return period TNCR of 2 500 years has been adopted for CC3 bridges, YI=1 may be assumed. Where TNCR has been assessed on a project-specific basis, YI should also be chosen on a project-specific basis. Further guidance is given in PD 6698.

Conditions under which the seismic action may be considered as an accidental action, and the requirements of 2.2.2(3) and 2.2.2(4) may be relaxed

Expression for the length of plastic hinges

2.2.2(3) and 2.2.2(4) should apply when the reference period TNCR is approximately equal to 475 years.

Use the expression in Annex E.

In the absence of project-specific requirements, the requirements of 2.2.2(3) and 2.2.2(4) may be relaxed. Further guidance is given in PD 6698.

Expression E.19 in Annex E may be adopted, or other appropriate values may be used.

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Table NA.1 UK values for Nationally Determined Parameters described in BS EN 1998-2:2005 (continued)

Subclause

2.3.6.3(5)


PE and PT, fractions of design seismic and thermal displacements for checking non-critical structural elements

2.3.7(1) Note 1 I Selection of categories of bridge, ground type and seismic zone for which the provisions for low seismicity apply

2.3.7(1) Note 2

3.2.2.3(1)P

3.3(1)P

Simplified criteria for the design of bridges in cases of low seismicity

Definition of active fault

Length of continuous deck Llim

beyond which the spatial variability of seismic action might have to be taken into account where soil properties along the bridge are approximately uniform


PE = 0.4 (for the design seismic displacement)

PT = 0.5 (for the thermal movement)

The recommended definition of low seismicity is given in the Note to BS EN 1998-1 :2004, 3.2.1 (4).

The simple criteria should be based on a limited ductility/essentially elastic seismic behaviour of the bridge, for which no special ductility requirements are necessary.

A seismotectonic fault may be considered active when there is an average historic slip rate of at least 1 mm/year and topographic evidence of seismic activity within the Holocene times (past 11 000 years).

Lg/1,5 where Lg is defined in Table 3.1 N, which is contained in Note 1 to 3.3(6) and is reproduced in the row below.

UK decision

Use recommended values.

All categories of bridge, ground type and seismic zone may be designed using the provisions for low seismicity.

Further guidance is given in PD 6698.

For the purposes of this requirement, all faults may be considered as not active. Further guidance is given in PD 6698.

is given in the table below.

-Ground IA IB Ie ID IE type

1 See Note

1 See Note

1 See Note 1

200 1

333

NOTE Spatial variability of ground motions need not be considered for bridges with continuous decks, where the supports are founded on approximately uniform soils of type A, B or C.


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Subclause

3.3(6) Note 1

3.3(6) Note 2

4.1.2(4)P

4.1.8(2)

5.3(4)

5.4(1)

5.6.2(2)P b


Distance Lg beyond which the seismic ground motions can be considered as completely u ncorrelated

Factor f3r used in calculating differential displacements between piers

l/12,1 values for traffic loads assumed concurrent with the design seismic action

Upper limit for the value in the left-hand-side of expression 4.4 for the seismic behaviour of a bridge to be considered irregular

Value of overstrength factor Yo

Simplified methods for second order effects in linear analysis

Value of additional safety factor YBd1 on shear resistance


Table 3.1 N: Distance Lg beyond which ground motions may be considered uncorrelated

Ground I A I B I C I DIE type

Lg(m) I 600 I 500 I 400 I 300 I 500

f3r = 0,5 when the ground type is the same for the three piers

f3r = 1,0 when the ground type of one of the three piers is different

For road bridges l/12,1 = 0,2

For rail bridges l/12,1 = 0,3

Po = 2

For concrete members Yo = 1,35

For steel members Yo = 1,25

Assume the increase of bending moments fJ.M of the plastic hinge section due to second order effects is:

1+q ~M = --dEdNEd 2

where NEd is the axial force and d Ed is the relative transverse displacement of the ends of the considered ductile member, both in the design seismic situation.

UK decision



In the absence of project-specific requirements, the recommended values may be adopted.

Use recommended value.




YBd1 = 1,25 I Use recommended value.

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Subclause Nationally Determined Parameter Eurocode recommendation UK decision

S.6.3.3(1}P b Choice of alternative methods of Alternative 2 is more conservative Either method may be used. calculating YBd

6.2.1.4(1}P

6.S.1(1}P

6.6.2.3(3}

6.6.3.2(1}P

Type of confinement reinforcement

Simplified verification rules for bridges of limited ductile behaviour in low seismicity cases

Allowable extent of damage of elastomeric bearings in bridges where the seismic action is considered as accidental action, but is not resisted entirely by elastomeric bearings

Percentage PH of the compressive (downward) reaction due to the permanent load that is exceeded by the total vertical reaction on a support due to the design seismic action for holding-down devices to be required.

All types are acceptable

In areas of low seismicity, the same verification rules should apply to the design of structures of limited ductile behaviour as those applicable in areas of moderate or high seismicity.

[None]

PH = 80% in bridges of ductile behaviour, where the vertical reaction due to the design seismic action is determined as a capacity design effect.

PH 50% in bridges of limited ductile behaviour, where the vertical reaction due to the design seismic action is determined from the analysis under the design seismic action alone (including the contribution of the vertical seismic component).

All types are acceptable, provided limited ductility or elastic behaviour is achieved.

Use the recommended procedure (i.e. the provisions of BS 1998-2:2005, 6.5, should be observed in bridges designed for limited ductile behaviour).

In the absence of a project-specific requirement, the extent of damage should be limited to enable the bearings to sustain Load Models 1 and 2 loading in accordance with BS EN 1991-2:2003. Further guidance is given in PD 6698.


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m Table NA.1 UK values for Nationally Determined Parameters described in BS EN 1998-2:2005 (continued)

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Subclause

6.7.3{7}

7.4.1 (1}P

7.6.2{1}P

7.6.2{5}

7.7.1{2}

J.1 {2}

,


Upper value of design seismic displacement d lim for monolithic connection of abutment to the deck to limit damage of the soil or embankment behind the abutments.

Value of control period TD for the design spectrum of bridges with seismic isolation

Value of amplification factor YIS

on design displacement of isolator units

Value of Ym for elastomeric bearings

Ow and Dd, numerical coefficients expressing appropriate fractions of Wd and da,max respectively

Tmin,b, minimum design temperature for determining upper and lower bound design properties of the isolator

Eurocode recommendation UK decision

Recommended limit value of design In the absence of project-specific requirements, use seismic displacement at abutments rigidly the recommended values. connected to the deck

Bridge importance Displacement class limit d lim {mm}

III 30

II 60

I No limitation

A value of TD greater than that specified In the absence of a project-specific requirement, in BS EN 1998-1 :2004, 3.2.2.5.8{P), may be a value of TD greater than the period of the specified. fundamental mode of vibration of the bridge T

should be adopted.


YIS = 1,5 Use recommended value.

Ym = 1,15 Ym = 1,0

Ow 0,015 In the absence of a project-specific requirement, use

Dd = 0,5 the recommended values. Further guidance is given in PD 6698.

T min,b = 1/J2 T min + Ll T1 T min,b = Tave - 1/J2 {Tave T min} + LlT1

where: where:

T min is the value of the minimum Tave is the average shade air temperature at shade air temperature at the bridge the bridge location throughout the year. In the location having an annual probability absence of more precise data, Tave may be taken as of (negative) exceedance of 0,02, in Tave = (T min + T max}/2. accordance with BS EN 1991-1-5:2003, 6.1.3.2.

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Subclause

J.1 (2) (continued)

J.2{1 )


Values of A-factors for commonly used isolators


1/J2 = 0,50 is the combination factor for thermal actions for seismic design situation, in accordance with BS EN 1990:2002, Annex A2.

L1T1 takes the following values depending on the material of the bridge deck, in accordance with BS EN 1991-1-5:2003, Figure 6."

Value of IlT1 for the determination of minimum isolator temperature

Deck Concrete Composite Steel

7,5 5,0 (OC)

Use values and guidance in Annex JJ.

UK decision

is the shade air temperature at the bridge location with an annual probability of 0,0083 (120 year return) that a lower temperature is recorded, in accordance with BS EN 1991-1-5:2003, 6.1.3.2, adjusted by Annex A and its National Annex. (Note that a different annual probability may be specified for an individual project.)

T max is the corresponding shade air temperature at the bridge location with an annual probability of 0,0083 (120 year return) that a higher temperature is recorded.

1/J2 = 0,50 is the combination factor for thermal actions for seismic design situation, in accordance with BS EN 1990:2002, Annex A2.

L1 T1 takes the following values depending on the bridge deck type, as defined in BS EN 1991-1-5:2003,6.1.1.

Value of IlT1 for the determination of minimum isolator temperature

Deck Concrete Composite Steel

L1T1 7,5 5,0 -2,5 (OC)


Values of A-factors should be determined by tests on prototype isolators, particularly in the case of elastomeric isolators.

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8 • © BSI 2009

NA.3 Decisions on the status of informative annexes BS EN 1998-2:2005 informative Annex A: Probabilities related to the reference seismic action. Guidance for the selection of design action during the construction phase is not appropriate for use in the UK as an informative annex. See also PD 6698.

BS EN 1998-2:2005 informative Annex B: Relationship between displacement ductility and curvature ductility factors of plastic hinges in concrete piers may be used in the UK as an informative annex. See also PD 6698.

BS EN 1998-2:2005 informative Annex C: Estimation of the effective stiffness of reinforced concrete ductile members may be used in the UK as an informative annex. See also PD 6698.

BS EN 1998-2:2005 informative Annex D: Spatial variability of earthquake ground motion: model and methods of analysis may be used in the UK as an informative annex. See also PD 6698.

BS EN 1998-2:2005 informative Annex E: Probable material properties and plastic hinge deformation capacities for non-linear analyses may be used in the UK as an informative annex.

BS EN 1998-2:2005 informative Annex F: Added mass of entrained water for immersed piers may be used in the UK as an informative annex.

BS EN 1998-2:2005 informative Annex H: Static non-linear analysis (pushover) may be used in the UK as an informative annex.

BS EN 1998-2:2005 informative Annex JJ: A-factors for common isolator types may be used in the UK as an informative annex.

BS EN 1998-2:2005 informative Annex K: Tests for validation of design properties of seismic isolator units may be used in the UK as an informative annex pending the publication of BS EN 15129.

NA.4 References to non-contradictory complementary information The following is a list of references that contain non-contradictory complementary information for use with BS EN 1998-2.

• PD 6698:2008, Recommendations for the design of structures for earthquake resistance to BS EN 1998.

NATIONAL ANNEX NA to 85 EN 1998-2:2005

Bibliography

Standards publications

For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

BS EN 1990:2002, Eurocode - Basis of structural design

BS EN 1991-1-5:2003, Eurocode 1: Actions on structures - Part 1-5: General actions - Thermal actions

BS EN 1991-2:2003, Eurocode 1: Actions on structures - Part 2: Traffic loads on bridges

© B512009 • 9


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