1 3 6 Overview of Eq Design UK NA for EN1998 2 PD 6698

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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 Pk AEd Q1k ψ21 Q2

characteristic value of a permanent action; characteristic value of prestressing after all losses; design seismic action; characteristic value of the traffic load; combination factor (quasi-permanent value) for traffic loads 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

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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 M≥5) 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‘

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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 NA‘s 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.

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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 projectspecific 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 Economic impact

Typical example Bridges where loss of serviceability would have a major regional or national economic impact Bridges where loss of serviceability could have a major impact on the rescue effort or on aid delivery Strengthening or upgrading of bridges which are an important part of the national heritage Bridges that carry more than one level of traffic Bridges with suspension systems supporting spans over 50 m (see Note)

Impact on post-earthquake relief

Historic or cultural importance Structural form (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])

Structure Category in accordance with BD 2

0&1

Consequence Class CC1 EN1990 Table B1 Importance Class IC I EN1998-2 clause 2.1(4)P Note

2

3

CC2

CC3

Comments Structure Categories are assumed to correspond to the Consequence Class as shown For a whole structure

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.

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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.

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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 bridge‘s 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"+"ψ21Q1k "+" 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; ψ21 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

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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.

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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 bridge‘s 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.

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References [1] [2]

[3] [4] [5] [6]

[7]

Cullen Wallace A A and Nissen J (1984). Kessock Bridge: Joint Engineers' role. ICE Proceedings, 76, Part 1, Paper 8745, pp.67-80, (February). Mizon D. H. and Kitchener J. N. (1997), Second Severn crossing—viaduct superstructure and piers. ICE Proceedings, Civ. Engng, Second Severn Crossing, 3548. Paper 11442. Arup (1993). Earthquake hazard and risk in the UK. Report for the Department of the Environment. HMSO, London. Booth E and Skipp B (2008). Establishing the necessity for seismic design in the UK. Research Report for the Institution of Civil Engineers, London. Musson R and Sargeant S (2008) Eurocode 8 seismic hazard zoning maps for the UK. British Geological Survey, Edinburgh. 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. 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).