1 3 6 overview of eq design uk na for en1998 2 pd 6698

E Booth, J Lane, R Ko, D MacKenzie page 1

OVERVIEW OF EARTHQUAKE DESIGN AND DEVELOPMENT OF UK NA FOR EN1998-2 AND PD6698 Edmund Booth, Consultant, London, UK John Lane, RSSB, London, UK Ron Ko, Highways Agency, Dorking, UK David MacKenzie, Flint and Neill, London, UK

Abstract Damaging earthquakes are rare in the UK, though there are well recorded instances of them

occurring. This is recognised in the UK National Forewords to the various parts of EN 1998,

which state:

There are generally no requirements in the UK to consider seismic loading, and the whole of the UK may be considered an area of very low seismicity in which the

provisions of EN 1998 need not apply. However, certain types of structure, by reason

of their function, location or form, may warrant an explicit consideration of seismic

actions.

The introduction of EN1998-2 as a British Standard provided the necessity, and opportunity,

to set out more formally advice on the situations where seismic design should be considered

for bridges, and where needed, what the design procedures should be. The paper summarises

the requirements of BS EN1998-2 for seismic design of bridges in areas of low seismicity and

the supplementary guidance given in the UK National Annex to BS EN1998-2 and the BSI

Published Document PD 6698:2009. The basis of the guidance for UK bridges is explained

and the current statutory position is also described.

The possible implications for the design of major UK road and rail bridges are discussed; it is

recognised that the recommendations will need to be reviewed in the light of experience after

a suitable period of practical implementation.

Notation Gk characteristic value of a permanent action;

Pk characteristic value of prestressing after all losses;

AEd design seismic action;

Q1k characteristic value of the traffic load;

21 combination factor (quasi-permanent value) for traffic loads Q2 quasi-permanent value of actions of long duration

(e.g. earth pressure, buoyancy, currents etc.)

Introduction Bridges where loss of serviceability would have a major regional or national economic impact

are an example of the type of structure which, prima facie, might be thought to warrant

seismic design, and there are a number of precedents for doing so in the UK, dating back


many years (Cullen Wallace and Nissen[1]

, Mizon and Kitchener[2]

). The issue of seismic

design for structures in the UK raises difficult issues; although the low level of seismic

activity in the UK has caused well documented cases of significant damage to buildings over

many centuries and even a few deaths (Arup[3]), to the authors knowledge there have been no

recorded cases in the past hundred years of significant damage to well-built engineered

structures of steel or concrete. This in itself justifies the common sense view that in the

absence of special considerations, seismic actions need not be considered in the design of

components of the built environment in the UK.

However, there is a consensus among seismologists that the UK, despite its low seismicity,

may on rare occasions experience an earthquake of a magnitude (say M5) which is locally capable of producing potentially damaging motions. The probability of this occurring at any

particular point in the UK is very low; in the language of probability theory, the hazard lies in

the tail of the distribution (Figure 1). For facilities such as nuclear power plants or liquid

natural gas (LNG) storage tanks, where the consequences of failure could be very adverse,

there is general acceptance that the inclusion of seismic actions in the statutory requirements

for design is, in principle, reasonable.

Figure 1: Extreme value statics for earthquake and wind loading

The issue becomes much harder to decide for other key elements of the UK infrastructure,

such as bridges, forming vital communication links. What measures are reasonable to provide

protection against rare earthquakes? Might the general provisions for robustness on their own

provide a sufficient level of seismic protection? that is, a level of seismic resistance which protects society with a level of reliability comparable to that provided against accidental


conditions such as terrorist action? Could simple design measures be devised which might

reduce seismic vulnerability without significantly increasing design and construction costs,

while increasing more generally the robustness and resilience of the structure? Might the

variation in seismic hazard across the UK (well established in principle) and the differences in

inherent seismic resistance between different structural forms allow a classification scheme

which pinpointed particular cases where seismic design was warranted while exempting

others?

During the period leading up to the introduction of the various parts of BS EN1998 as British

Standards, a literature search and a wide consultation exercise, funded by the Institution of

Civil Engineers and others, considered these issues and produced recommendations (Booth

and Skipp[4]

). A parallel exercise by the British Geological Survey in Edinburgh, reviewed

the latest data on the level and spatial variation of seismic hazard across the United Kingdom

(Musson & Sargeant[5]

); this is described more fully in a companion paper (Lane et al[6]

).

These two exercises formed the key inputs to the first drafts of the UK NAs to the various parts of BS EN 1998 and of the BSI Published Document PD6698. The rest of this paper

outlines the advice provided on whether or not seismic actions need consideration for the

design of UK bridges and, for situations where they are, summarises the consequences for

design and detailing. Finally, the wider implications for the design of major UK bridges is

discussed.

Situations Where Seismic Design Is Warranted for UK Bridges As quoted above, the National Foreword to BS EN 1998-2 states that certain types of structure, by reason of their function, location or form, may warrant an explicit consideration

of seismic actions (italics added). PD 6698 discusses these three factors (namely function, location and form) as they apply generally to important structures in the following terms.

Influence of function In some cases the function of a structure is such that failure due to very low probability

events, including earthquakes, might need to be considered. At least four such categories of

structure can be distinguished, as follows.

1) Structures where failure poses a large threat of death or injury to the population.

Examples include nuclear power plants and major dams (both of which are explicitly

outside the scope of BS EN 1998) and certain petrochemical installations, such as

liquid natural gas (LNG) storage tanks and high pressure gas pipelines (which are

within the scope of BS EN 1998).

2) Structures which form part of the national infrastructure and the loss of which

would have large economic consequences. An example is a major bridge forming a

transportation link vital to the national economy.

3) Structures whose failure would impede the regional and national ability to deal with

a disaster caused by a major damaging earthquake.

4) Strengthening or upgrading of historic structures forming an important part of the

national heritage.


In many cases, structures may fall into more than one category; for example, the seismic

failure of a busy estuarial bridge might cause extensive human casualties, affect the regional

or national economy and also impede the flow of disaster relief into the area affected by the

earthquake.

PD 6698 advises that there is no need to consider seismic actions for the design of bridges in

Consequence Classes CC1 and CC2 (according to BS EN1990). For bridges in Consequence

Class CC3, the need to design bridges for seismic actions should be considered on a project-

specific basis. Factors to be considered include the safety, economic, social and

environmental consequences of failure. Examples of bridges where the consequences of

failure might be high enough for a seismic design to be considered are shown in Table 1. Such

bridges do not necessarily require explicit seismic design, but should nevertheless be assessed

to see if that need applies.

Table 1 Examples of bridges with high consequence of failure where seismic design might need to be considered (from PD6698)

Factor influencing decision Typical example

Economic impact

Bridges where loss of serviceability would

have a major regional or national economic

impact

Impact on post-earthquake relief Bridges where loss of serviceability could

have a major impact on the rescue effort or

on aid delivery

Historic or cultural importance Strengthening or upgrading of bridges which

are an important part of the national heritage

Structural form (see Note) Bridges that carry more than one level of

traffic

Bridges with suspension systems supporting

spans over 50 m (see Note) NOTE Certain types of bridge, including suspension bridges and historic bridges, are not included in the

scope of BS EN 1998-2, so other sources of standards would be needed for their design.

The relationship between Consequence Class, Importance Class and Structure Category, is

shown in Table 2, based on Interim Advice Note IAN 124/10[7]

.

Table 2. - Importance Classes of Highways Structures (from IAN 124/10[7])

Comments

Structure Category in

accordance with BD 2

0 & 1 2 3 Structure Categories are assumed to

correspond to the Consequence

Class as shown

Consequence Class

EN1990 Table B1

CC1 CC2 CC3 For a whole structure

Importance Class

EN1998-2 clause

2.1(4)P Note

IC I IC II IC II or

IC III as

agreed

by TAA

Seismic design need not be

considered for IC I and II. Technical

Approval Authority is defined in

BD 2.


Influence of location The location of a structure affects the regional seismic hazard, which varies significantly

across the UK, (Musson and Sargeant[3]

); that is, the earthquake ground motions for a given

annual probability of exceedence are significantly greater in some parts of the UK than others,

although everywhere the hazard is very low by international standards. As discussed in the

companion paper (Lane et al [3]

), this may be allowed for by use of the seismic hazard map

provided in Figure 2 of PD6698. Alternatively, site specific seismic hazard assessment may

be carried out, in which case a return period for the ground motions may be chosen which is

commensurate with the consequences of failure of the bridge in question, instead of the

default value of 2,500 years which applies to the PD6698 map. Also, the site-specific

assessment would account for the influence of local faults on the seismic hazard, which the

PD6698 map would not. A site specific assessment may be the most appropriate choice for a

major UK bridge in an area of higher than average seismicity.

Location also affects the local influences on seismic hazard and in particular, the effect of

superficial soil deposits in modifying the seismic ground motions. As discussed in the

companion paper (Lane et al [3]

) the seismic response spectra provided in BS EN 1998-1

depend on the profile of the foundation soils involved, and thereby introduce an allowance for

this effect.

Influence of structural form

All structures possess some degree of earthquake resistance, and this is greatly enhanced by

the regulatory requirements to provide measures enhancing robustness, such as peripheral ties

in buildings, detailing to increase ductility, and by the provision of wind and impact

resistance. In many cases, these measures are considered to provide sufficient protection

against seismic actions in the UK. In the context of bridge design, additional shear links,

staggered splices, good tying in of steel, adequate bearing shelves, and similar measures can

significantly improve structural performance in earthquakes for little additional cost.

By contrast, certain features can result in designs that are satisfactory for resisting wind or

impact, but are vulnerable to seismic loading. Examples of such seismically unsatisfactory

features in building structures are open and relatively weak ground storeys (soft storeys), very heavy roof masses and, large eccentricities between centres of mass and stiffness.

Examples for bridges are bridge decks on bearings which provide poor lateral restraint and

concrete bridge piers which are poorly confined by transverse reinforcement.

Decision on the need for seismic design of bridges In the case of buildings, Booth and Skipp

[3] propose a screening process for deciding whether

or not seismic design is warranted; the screening process is not included in PD6698, nor the

UK NA to EN 1998-1, but PD6698 does refer to it. It involves assessing three aspects of the

seismic vulnerability of Importance Category 3 buildings, namely the level of the regional

seismic hazard in comparison to the UK average, the presence or otherwise of particularly

unfavourable structural features such as soft storeys, and the presence or otherwise of

unfavourable soils such as soft soils. Where at least two of these three features are present, a

seismic design is recommended, but where only one applies (for example above average

seismicity but with good structural features and foundations soils) then it is suggested that an

explicit seismic design is not warranted.


To the authors knowledge, no such screening process has been proposed for bridges in the UK or indeed any other area of low seismicity. However, a similar process may be found

helpful for UK bridges, weighing the influence of the bridges location (particularly regional seismicity and local soils), its structural form and finally the function of the bridge in terms of

the regional and national consequences of failure and the cost and time of repair. As

experience develops in the application of EN 1998-2 to UK bridges, it would be valuable to

develop further guidance on deciding on the need for seismic design, perhaps in collaboration

with bridge engineers in other low seismicity areas of northern Europe. The current

regulatory position in the UK is outlined in the section Design requirements for seismic action in the UK below.

Load Combinations for the Seismic Loadcase The design value Ed of the effects of actions in the seismic design situation in EN 1998-2 is

given by equation 1.

Ed = Gk "+"Pk "+"AEd"+"21Q1k "+" Q2 (1) where:

+ implies to be combined with; Gk are the permanent actions with their characteristic values;

Pk is the characteristic value of prestressing after all losses;

AEd is the design seismic action;

Q1k is the characteristic value of the traffic load;

21 is the combination factor (quasi-permanent value) for traffic loads Q2 is the quasi-permanent value of actions of long duration (e.g. earth pressure,

buoyancy, currents etc.) Actions of long duration are considered to be concurrent with

the design seismic action.

Seismic action effects need not be combined with action effects due to imposed deformations

(caused by temperature, shrinkage, settlements of supports, residual ground movements due to

seismic faulting). An exception is the case of bridges in which the seismic action is resisted

by elastomeric laminated bearings, where elastic behaviour of the system should be assumed

and the action effects due to imposed deformations should be accounted for. Note that the

displacement due to creep does not normally induce additional stresses to the system and can

therefore be neglected. Creep also reduces the effective stresses induced in the structure by

long-term imposed deformations (e.g. by shrinkage). Note also that wind and snow actions

are not included with the seismic design situation.

Recommendations for Seismic Design and Detailing of UK Bridges In cases where an explicit seismic design is considered necessary for Consequence Class CC3

bridges in the UK, the principal requirement is to carry out a seismic analysis and use its

results to provide sufficient lateral resistance and deformation capacity. For bridges with a

low fundamental lateral period of vibration, the lateral forces may be a substantial proportion

of the structural mass (see Figure 2 in the companion paper by Lane et al, [3]), but for more

flexible bridges relatively lower forces will apply. Simple equivalent static force analysis

(fundamental mode analysis) may be sufficient where wind action effects comfortably exceed

seismic ones, but more complex analysis methods (for example response spectrum, time

history or non-linear static) are likely to be needed in other cases. The analysis requirements

for cases not covered by BS EN 1998-2, in particular suspension bridges, would need


particular attention, but generally the considerations for defining design motions and load

combinations that apply to bridges within the scope of BS EN 1998-2 will apply.

For bridges in areas of moderate to high seismicity, providing sufficient lateral strength and

deformation restraint capacity to decks is only one aspect of seismic design. An equally

important aspect is to ensure adequate detailing. A crucial need is to identify the regions of

the structure designed to yield during a severe earthquake, and to ensure that they are

sufficiently ductile for the plastic deformation demands to which they may be subjected.

Bridges in areas of low seismicity are generally exempt from such considerations, because

they are designed as limited ductility structures where significant yielding is not expected

under actions due to the design earthquake. This greatly simplifies the design and detailing

process, and is expected to be the adopted option for UK bridges. However, some simple

measures can significantly increase ductility with relatively low impact on design effort and

construction cost, and such measures provide a reserve of capacity in cases where seismic

demands are greater than anticipated in design. BS EN 1998-2 requires a minimum set of

such measures, which are endorsed by the UK NA to BS EN1998-2; they are outlined below.

A possible way forward for the UK would be develop these minimum detailing rules to the

extent where an explicit seismic analysis was necessary only in exceptional cases; if such

simple rules were shown to have sufficiently low impact on cost and design effort, they might

be extended to most or all Consequence Class 3 bridges in the UK, simplifying the decision

making process for seismic design.

The minimum design and detailing rules in BS EN 1998-2, and the only ones additional to a

seismic analysis that are required by the UK NA to BS EN1998-2, are as follows.

1. Shear strength of elements is provided assuming seismic actions corresponding to a behaviour factor of q=1 (instead of the more favourable value of 1.25 to 1.5 usually

applying to the rest of the superstructure design) and the normal design shear

resistance reduced by 1.25. This is to suppress shear failures from occurring before

more ductile flexural failures. As discussed by Lane et al[3], the behaviour factor q is

applied as a reduction factor to the calculated elastic seismic response, allowing for

the reduction in response after the structure has yielded.

2. Foundations are designed for q=1, and the resistance calculated from the provisions of BS EN 1998-5. This is to suppress foundation failures in favour of yielding in the

superstructure. Foundation failures may be difficult to detect and repair. They may

also give rise to gross lack of alignment between piers and bridge deck, rendering the

bridge unserviceable.

3. Non-ductile structural components, such as fixed bearings, sockets and anchorages for cables and stays and other non-ductile connections are also designed for q=1. This

check may be omitted if it can be shown that the integrity of the structure is not

affected by failure of such connections. The seismic design should also address the

possibility of sequential failure, such as may occur in the stays of cable stayed bridges.

4. Minimum amounts of spiral or rectangular confining steel are required at potential plastic hinging points, defined as where the calculated bending demand is greater than

the bending resistance divided by 1.3.


Other measures might also be considered to improve seismic performance. These include

adding lockup devices at bearings which allow thermal and other long term deformations but provide restraint to seismic movements. An option for suspension bridges is to release the

end restraints at the towers, for example by providing either seismic fuses which prevent excessive seismic loads being applied to the towers, or alternatively damped lateral buffer

restraints to control seismic movements.

Design Requirements for Seismic Action in the UK The Highways Agency intends to publish an Interim Advice Note IAN 124/10 which would

state that the whole of the UK would be considered an area of very low seismicity. No formal

advice has been published for the design of railway bridges to resist seismic actions.

Therefore the provisions of BS EN1998 need not apply for the design of bridges, unless

otherwise specified by the Technical Approval Authority. Any site-specific seismic

requirements (see PD6698), should be considered for the individual structure, where

appropriate.

Implications for the Design of UK Bridges In the great majority of cases, there will be no impact from the introduction of BS EN 1998-2

in the UK, since all Consequence Class CC1 and CC2 bridges and at least some (possibly

most) Consequence Class CC3 bridges will not require any explicit seismic design.

Possible implications for bridges which do warrant seismic design might be as follows.

1. The significance of seismicity will tend to increase in relation to the ratio of the structural weight to the wind load. Generally, bridges with concrete and composite

decks will be more greatly affected than those with steel decks.

2. Seismic loading may govern lateral strength requirements for foundations, particularly where piling through very soft materials.

3. Seismic loading may govern the design of restraint and displacement capacity at deck bearings.

Conclusions The introduction of EN 1998-2 as a British Standard will not impact on the design of most

UK bridges, since Consequence Class CC1 and CC2 bridges are not recommended as needing

seismic design. PD6698 and IAN 124/10 provide some guidance on which Consequence

Class CC3 bridges should be considered for an explicit seismic design. However, judgement

will still be required, based on the severity of the consequences of failure of a particularly

bridge, the local and regional level of seismicity and the inherent seismic resistance of the

bridges structural form. Developing further advice on these matters would be desirable and should be possible in the light of experience gained from the use of BS EN 1998-2.

For bridges that do warrant an explicit seismic design, a seismic analysis is required and some

minimal level of seismic detailing. These are likely to have a particular impact on bridges

where the ratio of structural weight to wind load is high, and in the presence of soft

foundation soils. Seismic loading may also govern the design of restraint and displacement

capacity at deck bearings.


References [1] Cullen Wallace A A and Nissen J (1984). Kessock Bridge: Joint Engineers' role. ICE

Proceedings, 76, Part 1, Paper 8745, pp.67-80, (February).

[2] Mizon D. H. and Kitchener J. N. (1997), Second Severn crossingviaduct superstructure and piers. ICE Proceedings, Civ. Engng, Second Severn Crossing, 35-

48. Paper 11442.

[3] Arup (1993). Earthquake hazard and risk in the UK. Report for the Department of

the Environment. HMSO, London.

[4] Booth E and Skipp B (2008). Establishing the necessity for seismic design in the UK.

Research Report for the Institution of Civil Engineers, London.

[5] Musson R and Sargeant S (2008) Eurocode 8 seismic hazard zoning maps for the UK.

British Geological Survey, Edinburgh.

[6] Lane J, Booth E, Cooper D, Harris A and Gulvanessian H (2010). Overview of actions

in EN1991 and EN1998 for bridge design. Bridge Design to Eurocodes Conference,

Institution of Civil Engineers, London.

[7] IAN 124/10 Interim requirements for the use of Eurocodes for the design of highway

structures (under preparation by Highways Agency at the time of preparation of this

paper).

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