Tall Building Structures Marti Carrera Comamala Assignment 3:Earthquake design code in Spain 1436379 EARTHQUAKE DESIGN CODE IN SPAIN INTRODUCTION Spain is an area of low-to-moderate seismic activity,with relatively low seismic hazard in a worldwide frequent and destructive to endure in people's memory.For ause other natural hazard (heavy storms,floods)are more common in Spain,seismic hazard is deemed as a secondary issue,and consequently,earthquake protection and preparedness measurements are not as developed as in other countries with higher seismicity. The Basic directive of civil protection planning against seismic risk (Directriz basica de planificacion de proteccion civil ante el riesgo sismico,DBPPCRS,1995,updated in 2004)is the country-wide Spanish norm that sets up the general conditions under which emergency response plans for earthquake disasters ought to be carried out,as well as their general characteristics.Three action levels may be distinguished:general, egional and local.The general level is represented by the seismic hazard map of spain contained in DBPPCRS(2004).Regions displaying relatively high hazard on the map reported belov (lMsK>VI)must developspecalregionalplansforearthquakerisk.Thedevelopmentoftheseregionalplans,fromtsdesig to its eventual implementation,is responsibility ofthe respective regional authorities.However,all regional special plans must be accredited by the National Commission for Civil Protection,implying that they are granted official normative status.After accreditation,results of regional seismic risk studies are used to establish the municipalities with high hazard levels (typically Intensity2Vll)that have to develop and ergency response.Whilst the general plan s completed,special regional plansare under development and only a are terminated FRANCE SPAIN 14 8 6°
1 Tall Building Structures Martí Carrera Comamala Assignment 3: Earthquake design code in Spain 1436379 EARTHQUAKE DESIGN CODE IN SPAIN INTRODUCTION Spain is an area of low-to-moderate seismic activity, with relatively low seismic hazard in a worldwide perspective. Although several strong, damaging events have occurred in the recent past, they are not as frequent and destructive to endure in people’s memory. For this reason and because other natural hazards (heavy storms, floods) are more common in Spain, seismic hazard is deemed as a secondary issue, and consequently, earthquake protection and preparedness measurements are not as developed as in other countries with higher seismicity. The Basic directive of civil protection planning against seismic risk (Directriz básica de planificación de protección civil ante el riesgo sísmico, DBPPCRS, 1995, updated in 2004) is the country-wide Spanish norm that sets up the general conditions under which emergency response plans for earthquake disasters ought to be carried out, as well as their general characteristics. Three action levels may be distinguished: general, regional and local. The general level is represented by the seismic hazard map of Spain contained in DBPPCRS (2004). Regions displaying relatively high hazard on the map reported below (IMSK≥VI) must develop special regional plans for earthquake risk. The development of these regional plans, from its design to its eventual implementation, is responsibility of the respective regional authorities. However, all regional special plans must be accredited by the National Commission for Civil Protection, implying that they are granted official normative status. After accreditation, results of regional seismic risk studies are used to establish the municipalities with high hazard levels (typically Intensity≥VII) that have to develop and eventually bring to practice local plans for earthquake risk and emergency response. Whilst the general plan is completed, special regional plans are under development and only a few local plans are terminated
In the previous picture,it is the left the DBPPCRS seismichazard map of Spain(DBPPCRS 2004)and on the right the regions (in light grey)that must develop specific regional seismic risk studies (regions with already accredited plan are shown in dark grey). Seismic Zones in Spain: .Low seismicity zone:IMsK <VI .Intermediate seismicity zone:VI IMsK VIII ·High seismicity zone Iusk≥VIl Chronology of seismic requlations in Spain .1962:CODE MV-101(seismic actions for buildings),Ministry of Housing .1967:Instruction for large dams(IGP),Ministry of Public Works 1968:Code PGS-1*(experimental provisional norm)Inter-Ministry Commission for Seismic Normative 1974:Code PDS-1 Inter-Ministry Commission for Seismic Normative 194:Code NCSE-94 Permanent Commission for Seismic Normative 2002;Code NCSE-o2 Permanent Commission for Seismic Normative 2007:Code NCSP-07 for bridges Permanent Commission for Seismic Normative Due to the nor rs to the Euro ing with civil cdesign,the national zones and concerns Eurocode8(EN 1998,"Design of structures for earthquake resistance"),applies indeed to the design and construction of buildings and civil engineering works in seismic regions. tsseismicdesign shoud ments and be subjected to m entary verifications The objectives ofs icdesign in accordance with explicity stated.Its purpose istoensure that in the event of earthquakes human lives are protected ·damage important structuresfor civilprotection remain operational Fundamental requirements EN1998-1asks for a two level seismic design establishing explicitly the two following requirements: .No-collapse requiremen The structure shall be designed withstand the design seismicaction global collapse,thus retaining its structural integrity and a residual load bearing capacity after the seismic event.This performance requirement is associated with the Ultimate Limit State(ULS)since it deals with the safety of people or the whole structure. 2
2 In the previous picture, it is possible to see on the left the DBPPCRS seismic hazard map of Spain (DBPPCRS, 2004) and on the right the regions (in light grey) that must develop specific regional seismic risk studies (regions with already accredited plan are shown in dark grey). Seismic Zones in Spain: Low seismicity zone: 𝐼𝑀𝑆𝐾 < 𝑉𝐼 Intermediate seismicity zone: 𝑉𝐼 ≤ 𝐼𝑀𝑆𝐾 < 𝑉𝐼𝐼𝐼 High seismicity zone 𝐼𝑀𝑆𝐾 ≥ 𝑉𝐼𝐼𝐼 Chronology of seismic regulations in Spain 1962 : CODE MV-101 (seismic actions for buildings), Ministry of Housing 1967 : Instruction for large dams (IGP), Ministry of Public Works 1968 : Code PGS-1* (experimental provisional norm) Inter-Ministry Commission for Seismic Normative 1974 : Code PDS-1 Inter-Ministry Commission for Seismic Normative 1994 : Code NCSE-94 Permanent Commission for Seismic Normative 2002 ; Code NCSE-02 Permanent Commission for Seismic Normative 2007 : Code NCSP-07 for bridges Permanent Commission for Seismic Normative Due to the non-existence of a proper national code or regulation concerning the seismic design, the national DBPPCRS refers to the Eurocodes (in particular to the EC8) when dealing with civil constructions in seismic zones and concerns. Eurocode8 (EN 1998, “Design of structures for earthquake resistance”), applies indeed to the design and construction of buildings and civil engineering works in seismic regions. Its seismic design should satisfy additional requirements and be subjected to complementary verifications. The objectives of seismic design in accordance with Eurocode 8 are explicitly stated. Its purpose is to ensure that in the event of earthquakes: human lives are protected; damage is limited; important structures for civil protection remain operational. Fundamental requirements EN 1998-1 asks for a two level seismic design establishing explicitly the two following requirements: No-collapse requirement The structure shall be designed and constructed to withstand the design seismic action without local or global collapse, thus retaining its structural integrity and a residual load bearing capacity after the seismic event. This performance requirement is associated with the Ultimate Limit State (ULS) since it deals with the safety of people or the whole structure
Damage limitation requirement The structure shall be designed and constructed to withstand a seismic action having a larger probability of occurrence than the design seismic action,without the occurrence of damage and the associated limitations of use,the costs of which would be disproportionately high in comp ison with the costs of the strcture itself.The strct retain its original strength and stiffness and hence should not ne ed structural repa This performance requirement is associated with the Serviceability Limit State(SLS)since it deals with the use of the building,comfort of the occupants and economic losses. As indicated above,the two performance levels are to be checked against two different levels of the The extent of the protection that can be provided is a matter of optimal allocation of resources and is therefore expected to vary from country to country,depending on the relative importance of the seismic risk with respect to risks of other origin and on the global economic resources. In spite of this EN 1998-1 addresses the issue,starting with the case of ordinary structures,for which it mmends the following two levels: Design seismic action (for local collapse prevention,i.e no-collapse requirement)with 10% probability of exceedance in 5o years(recommended value)i.e.with a mean return period of 475 vears. Damage limitation seismic action with 10%probability of exceedance in 1o years(recommended value)i.e.with amean retur period of 5years The damage limitation seismic action is sometimes also referred to the Serviceability seismic action. Reliability differentiation The levels of the seismic action described above are meant to be applied to ordinary structures and are considered the reference seismic action(which is anchored to the reference peak ground acceleration aR). However,EN 1998-1 foresees the possibility to differentiate the target reliabilities (of fulfilling the no- collapse and ds requireme ts)for different types of buildings or other constructions, dependingcnitsimp rtance and consequences of failure So the target reliability requirement depends on consequences of failure,and its differentiation is implemented in two ways: By classifying structures into different importance classes.An importance factor Cu is assigned to ach impo s.Wherever feasible this factor should bed erived so as 404Pe的us1a540 pouad wn431a4中o3 nie Jamo0 period)as appropriate for the design of the specific category of structures; By modifying the hazard level considered for design,i.e.modifying the mean return period for the selection of the seismic action for desian. The different eves of reliability are obtained by multiplying the referenceseismicactio by this importance factor,which may be applied directly to the action effects obtained with the reference seismic action
3 Damage limitation requirement The structure shall be designed and constructed to withstand a seismic action having a larger probability of occurrence than the design seismic action, without the occurrence of damage and the associated limitations of use, the costs of which would be disproportionately high in comparison with the costs of the structure itself. The structure should not have permanent deformations and its elements should retain its original strength and stiffness and hence should not need structural repair. This performance requirement is associated with the Serviceability Limit State (SLS) since it deals with the use of the building, comfort of the occupants and economic losses. As indicated above, the two performance levels are to be checked against two different levels of the seismic action, interrelated by the seismicity of the region. The extent of the protection that can be provided is a matter of optimal allocation of resources and is therefore expected to vary from country to country, depending on the relative importance of the seismic risk with respect to risks of other origin and on the global economic resources. In spite of this EN 1998-1 addresses the issue, starting with the case of ordinary structures, for which it recommends the following two levels: Design seismic action (for local collapse prevention, i.e no-collapse requirement) with 10% probability of exceedance in 50 years (recommended value) i.e. with a mean return period of 475 years. Damage limitation seismic action with 10% probability of exceedance in 10 years (recommended value) i.e. with a mean return period of 95 years. The damage limitation seismic action is sometimes also referred to the Serviceability seismic action. Reliability differentiation The levels of the seismic action described above are meant to be applied to ordinary structures and are considered the reference seismic action (which is anchored to the reference peak ground acceleration agR). However, EN 1998-1 foresees the possibility to differentiate the target reliabilities (of fulfilling the nocollapse and damage limitation requirements) for different types of buildings or other constructions, depending on its importance and consequences of failure. So the target reliability requirement depends on consequences of failure, and its differentiation is implemented in two ways: By classifying structures into different importance classes. An importance factor CU is assigned to each importance class. Wherever feasible this factor should be derived so as to correspond to a higher or lower value of the return period of the seismic event (with regard to the reference return period) as appropriate for the design of the specific category of structures; By modifying the hazard level considered for design, i.e. modifying the mean return period for the selection of the seismic action for design. The different levels of reliability are obtained by multiplying the reference seismic action by this importance factor, which may be applied directly to the action effects obtained with the reference seismic action
Relationship between the Importance Factor and the return period Buildings in En1908-1 are classified in 4 importance classes. Starting from the importance class of a structure it is possible to get the retum period by using graphical procedures on plots reported in the text. implicit retum period is of the order years,whereas for Class structures it is of the order of oo to years.For Class IV structures the implicit return periods varies more widely,ranging from 1.1oo to 1.8oo years. Table 1.2.1 Importance classes and recommended values for importance factors for buildings Buildings 0.8 inary builings,not belonging in the other 1,0 12 g.scho 1,4 ns.pow nts,etc Compliance criteria EN1998-1 prescribes that in order to satisfy the fundamental requirements two limit states should be checked: Ultimate Limit States (UL S): .Damage Limitation States(associated with Serviceability Limit States-SLS). Ultimate limit state The no-collapse performance level is considered as the Ultimate Limit State.It does not require that the structure remains elastic under the design seismic action,but it allows the development of significant inelastic deformations in the structural members. The basic concept is the possible trade-off between resistance and ductility that is at the base of the introduction of Ductility Classes and the use of behaviour factors that is a main feature of En1908-1 This is explained in the ode as follows The resistance and energy-dissipation structure are related to the extent towhich its non-linear response is to be exploited.In operational terms such balance between resistance and energy- dissipation capacity is characterized by the values of the behaviour factor q and the associated ductility classification,which are given in the relevant Parts of EN1998.Asa limiting case,for the design of structures classified as low-dissipative,no account is taken of any hysteretic energy dissipation and the behaviour factor may not be taken,in general,as being greater than the value of1,5 considered to account for over strengths.For steel or composite steel concrete buildings,this limiting value of the q factor may be taken as being between1,5 and 2(see Note1 of Table 6.1or Note1of Table.1,respectively).For dissipative structures the behaviour factor is taken as being greater than these limiting valuesacoting for the hysteretic energy dissipation that mainly occurs in specifically designed zones,called dissipative zones orcritical regions
4 Relationship between the Importance Factor and the return period Buildings in EN 1998-1 are classified in 4 importance classes. Starting from the importance class of a structure it is possible to get the return period by using graphical procedures on plots reported in the text. For Class I structures the implicit return period is of the order of 200 to 250 years, whereas for Class III structures it is of the order of 800 to 1.000 years. For Class IV structures the implicit return periods varies more widely, ranging from 1.100 to 1.800 years. Compliance criteria EN 1998-1 prescribes that in order to satisfy the fundamental requirements two limit states should be checked: Ultimate Limit States (ULS); Damage Limitation States (associated with Serviceability Limit States – SLS). Ultimate limit state The no-collapse performance level is considered as the Ultimate Limit State. It does not require that the structure remains elastic under the design seismic action, but it allows the development of significant inelastic deformations in the structural members. The basic concept is the possible trade-off between resistance and ductility that is at the base of the introduction of Ductility Classes and the use of behaviour factors that is a main feature of EN 1998-1. This is explained in the code as follows: The resistance and energy-dissipation capacity to be assigned to the structure are related to the extent to which its non-linear response is to be exploited. In operational terms such balance between resistance and energydissipation capacity is characterized by the values of the behaviour factor q and the associated ductility classification, which are given in the relevant Parts of EN 1998. As a limiting case, for the design of structures classified as low-dissipative, no account is taken of any hysteretic energy dissipation and the behaviour factor may not be taken, in general, as being greater than the value of 1,5 considered to account for over strengths. For steel or composite steel concrete buildings, this limiting value of the q factor may be taken as being between 1,5 and 2 (see Note 1 of Table 6.1 or Note 1 of Table 7.1, respectively). For dissipative structures the behaviour factor is taken as being greater than these limiting values accounting for the hysteretic energy dissipation that mainly occurs in specifically designed zones, called dissipative zones or critical regions
Damage limitation state The performance requirement associated with this Limit State requires the structure to support a relatively frequent earthquake without significant damage or loss of operationally. Damage is only expected in non-structural elements and its occurrence depends on the deformation that the structure,in response to the earthquake,imposes on such elements.The same essentially applies to the loss of operationally of parts of the structure. ad mtihmmthat dendo the characteristics of the non-structural elements. Specific measures EN 1998-1 provides implicitly the satisfaction of a third performance level that intends to prevent global collapse during a very strong and rare earthquake. This is not achieved by specific checks for a higher level of the design seismicaction but rather by imposing sures to be tak in consideration along the design process These specific measures,which aim at reducing the uncertainty of the structural response,indicate that To the extent possible,structures should have simple and regular forms both in plan and elevation. dissipative and ductile behaviour,failre or the premature formation of unstable mechanisms should be avoided.This is used to obtain a hierarchy of resistance of the various structural components and of the failure modes necessary for ensuring a suitable plastic mechanism and for avoiding brittle failure modes. Special care should be given in the design of the regions where nonlinear response is foreseeable since the seismic performance of a structure is largely dependent on the behaviour of these critical Theanyhodb baseqte structural models,which,when neessasoud take into account the influence of soil deformability and of non-structural elements. The stiffness of the foundations shall be adequate for transmitting the actions received from the superstructure to the ground as uniformly as possible. The design documents should be quite detailed and include all relevant info mat materials characteristics,sizes of al appropriate. The necessary quality control provisions should also be given in the design documents and the checking methods to be used should be specified.namely for the elements of special structural importance. In regions of high seismicity and in structures of special importance,formal quality system plans covering design,construction,and use,additional to the control procedures prescribed in the other relevant Eurocodes,should be used
5 Damage limitation state The performance requirement associated with this Limit State requires the structure to support a relatively frequent earthquake without significant damage or loss of operationally. Damage is only expected in non-structural elements and its occurrence depends on the deformation that the structure, in response to the earthquake, imposes on such elements. The same essentially applies to the loss of operationally of parts of the structure. Accordingly an adequate degree of reliability against unacceptable damage is needed and checks have to be made on the deformation of the structure and its comparison with deformation limits that depend on the characteristics of the non-structural elements. Specific measures EN 1998-1 provides implicitly the satisfaction of a third performance level that intends to prevent global collapse during a very strong and rare earthquake. This is not achieved by specific checks for a higher level of the design seismic action but rather by imposing some so-called specific measures to be taken in consideration along the design process. These specific measures, which aim at reducing the uncertainty of the structural response, indicate that: To the extent possible, structures should have simple and regular forms both in plan and elevation. In order to ensure an overall dissipative and ductile behaviour, brittle failure or the premature formation of unstable mechanisms should be avoided. This is used to obtain a hierarchy of resistance of the various structural components and of the failure modes necessary for ensuring a suitable plastic mechanism and for avoiding brittle failure modes. Special care should be given in the design of the regions where nonlinear response is foreseeable since the seismic performance of a structure is largely dependent on the behaviour of these critical regions or elements. The analysis should be based on adequate structural models, which, when necessary, should take into account the influence of soil deformability and of non-structural elements. The stiffness of the foundations shall be adequate for transmitting the actions received from the superstructure to the ground as uniformly as possible. The design documents should be quite detailed and include all relevant information regarding materials characteristics, sizes of all members, details and special devices to be applied, if appropriate. The necessary quality control provisions should also be given in the design documents and the checking methods to be used should be specified, namely for the elements of special structural importance. In regions of high seismicity and in structures of special importance, formal quality system plans, covering design, construction, and use, additional to the control procedures prescribed in the other relevant Eurocodes, should be used
GROUND CONDITIONS The earthquake vibration at the surface is strongly influenced by the underlying ground conditions and correspondingly the ground characteristics very much influence the seismic response of structures. The importance of such influence is taken in consideration in EN 1998-1 that requires that appropriate investigations(insitu or in the laboratory)must be carried out in order to identify the ground conditions. This gro Toallow the classification of the soil profile,in view of defining the ground motion appropriate to the site (i.e.allowing the selection of the relevant spectral shape,among various different possibilities). Toidentify the possible occurrence ofasoil behavior during an earthquake,harmfulto the response of the structure Inrelation to the latter aspect,the construction site and the nature of the supporting ground should normally be free from risks of ground rupture,slope instability and permanent settlements caused by liquefaction or densification in the event of an earthquake. If the ground investigation shows that such risks do exist,measures should be taken to mitigate its negative effects on the structure or the location should be reconsidered. In what conc the classification of the soil,EN998-provides five ground profiles,denoted Ground Three parameters are used in the classification for a quantitative definition of the soil profile The value of the average shear wave velocity,vs,3o(leading parameter) The number of blows in the standard penetration test(NSPT) The undrained cohesive resistance(cu) able 1.2.3 Ground Types Description of stratigraphic profile Parameters (m/s)m ckPa】 A Rock or other >800 360-800 >50 >250 180-360 15-50 70-250 D 180 <15 se 10-20 y m conten 6
6 GROUND CONDITIONS The earthquake vibration at the surface is strongly influenced by the underlying ground conditions and correspondingly the ground characteristics very much influence the seismic response of structures. The importance of such influence is taken in consideration in EN 1998-1 that requires that appropriate investigations (in situ or in the laboratory) must be carried out in order to identify the ground conditions. This ground investigation has two main objectives: To allow the classification of the soil profile, in view of defining the ground motion appropriate to the site (i.e. allowing the selection of the relevant spectral shape, among various different possibilities). To identify the possible occurrence of a soil behavior during an earthquake, harmfulto the response of the structure. In relation to the latter aspect, the construction site and the nature of the supporting ground should normally be free from risks of ground rupture, slope instability and permanent settlements caused by liquefaction or densification in the event of an earthquake. If the ground investigation shows that such risks do exist, measures should be taken to mitigate its negative effects on the structure or the location should be reconsidered. In what concerns the classification of the soil, EN 1998-1 provides five ground profiles, denoted Ground types A, B, C, D, and E. Three parameters are used in the classification for a quantitative definition of the soil profile: The value of the average shear wave velocity, vs,30 (leading parameter) The number of blows in the standard penetration test (NSPT) The undrained cohesive resistance (cu)
.Ground types A to D range from rock or other rock-like formations to loose cohesion less soils or soft cohesive soils. Ground Type E is essentially characterized by a sharp stiffness contrast between a(soft or loose) surface layer(thickness varying)and the underlying much stiffer formation. Two additional soil profiles(S1 and S2)are also exist,for sites with ground conditions matching either one of these ground types,special studies for the definition of the seismic action are required. SEISMIC ACTION The seismic actionto be considered for design purposes should be based on the estimation of theground motion expected at each location in the future,ie.it should be based on the hazard assessment Seismic hazard is normally represented by hazard curves that depict the exceedance probability of a certain seismologic parameter(for instance the peak ground acceleration,velocity or displacement)for a given period of exposure,at a certain location(normally assuming a rock ground condition). In eN1g08-1 the seismic hazard is described only by the value of the reference peak ground acceleration ndtypeA,(agR). For this purpose the national territories should be subdivided into seismic zones,depending on the loca hazard.The Seismic zonation map referred to the Spanish territory as been shown in the very first pages of this report.By definition(in the context of EN1998-1)the hazard withineach zone is assumed to be constant i.e.the reference peak ground acceleration is constant. The reference peak grou nd acceleration (agR),for each seismiczone, onds to the reference re period TNCR,chosen by the Nation al Authorities for the seismi action for the no-collapse requirement (it is recalled that,as indicated above,the recommended value is TNCR 475 years). Horizontal elastic spectra spons The basic shape of the horizontal elastic response spectrum (normalized by)is shown in the following picture. Fig.1.2.3 Basic shape of the elastic response spectrum in EN 1998-1
7 Ground types A to D range from rock or other rock-like formations to loose cohesion less soils or soft cohesive soils. Ground Type E is essentially characterized by a sharp stiffness contrast between a (soft or loose) surface layer (thickness varying between 5 to 20 m) and the underlying much stiffer formation. Two additional soil profiles (S1 and S2) are also exist, for sites with ground conditions matching either one of these ground types, special studies for the definition of the seismic action are required. SEISMIC ACTION The seismic action to be considered for design purposes should be based on the estimation of the ground motion expected at each location in the future, i.e. it should be based on the hazard assessment. Seismic hazard is normally represented by hazard curves that depict the exceedance probability of a certain seismologic parameter (for instance the peak ground acceleration, velocity or displacement) for a given period of exposure, at a certain location (normally assuming a rock ground condition). In EN1998-1 the seismic hazard is described only by the value of the reference peak ground acceleration on ground type A, (agR). For each country, the seismic hazard is described by a zonation map defined by the National Authorities. For this purpose the national territories should be subdivided into seismic zones, depending on the local hazard. The Seismic zonation map referred to the Spanish territory as been shown in the very first pages of this report. By definition (in the context of EN1998-1) the hazard within each zone is assumed to be constant i.e. the reference peak ground acceleration is constant. The reference peak ground acceleration (agR), for each seismic zone, corresponds to the reference return period TNCR, chosen by the National Authorities for the seismic action for the no-collapse requirement (it is recalled that, as indicated above, the recommended value is TNCR = 475 years). Horizontal elastic spectra The ground motion is described in EN1998-1 by the elastic ground acceleration response spectrum Se, denoted as the “elastic response spectrum”. The basic shape of the horizontal elastic response spectrum (normalized by ag)is shown in the following picture
The horizontal seismic action is described by two orthogonal components,assumed as independent and being represented by the same response spectrum. The basic spectral shape is composed by four parts: Very low period branch,from peak ground acceleration to the constant acceleration branch Constant acceleration .Constant velocity .Constant displacement These bra ,TCand TD whichare Nationally Detemined ameter s (NDPs all wing the adjustment of the spectral shape to the seismo-genetic specificities of each country. EN1998-1 foresees also the possibility of using more than one spectral shape for the definition of the seismic action.This is appropriate when the earthquakes affecting a site are generated by widely differing sources (for instance in terms of Magnitudes and Distances).In such cases the possibility of using more than one ape for th spect sh co the maction to be adeuateh represen ed.The each type of spectrum ar earthquake(i.e.more than one zonation map isrequired). In order to enable a wider choice to National Authorities,EN 1998-1 includes,as recommended spectral shapes,two types of earthquakes:Type 1and Type 2. n general Type1should be used.However,if the earthquakes that contribute most to the seismic hazard ed for the site hav a surface reater than5,5,then ype2 is The underlying ground conditions at a site strongly influence the earthquake vibration at the surface and correspondingly the peak ground acceleration and the response spectrum shape. In EN1998-1 this is acknowledged by the use of a soil factor S,also a NDP,that multiplies the design ground acceleration()derived from the zonation map. Itisworth recalling atthis that should be taken from theonation map that s established for rock type ground conditio for the reference retm chosen by the Nation Authorities for the No-collapse requirement for ordinary structures Furthermore,in EN 1998-1 the ground conditions influence the values of the corner periods TB,TC and TD and correspondingly the spectral shape. nENthe spectral amplification(from peak ground acceleration to the acceleration at the constant acceleration branch)isf ed at 2,5 and is consistent with 5%viscou damping.It ishowever anticipated tha the spectral shape may be adjusted for other damping values with the correction factorgiven by: n=√10/5+万≥0,55 where is the viscous damping ratio of the structure,expressed as a percentage.The correction factor is depicted in the following picture:
8 The horizontal seismic action is described by two orthogonal components, assumed as independent and being represented by the same response spectrum. The basic spectral shape is composed by four parts: Very low period branch, from peak ground acceleration to the constant acceleration branch Constant acceleration Constant velocity Constant displacement These branches are separated by three “corner” periods: TB, TC and TD which are Nationally Determined Parameters (NDPs), allowing the adjustment of the spectral shape to the seismo-genetic specificities of each country. EN 1998-1 foresees also the possibility of using more than one spectral shape for the definition of the seismic action. This is appropriate when the earthquakes affecting a site are generated by widely differing sources (for instance in terms of Magnitudes and Distances). In such cases the possibility of using more than one shape for the spectra should be considered to enable the design seismic action to be adequately represented. Then, different values of ag shall normally be required for each type of spectrum and earthquake (i.e. more than one zonation map is required). In order to enable a wider choice to National Authorities, EN 1998-1 includes, as recommended spectral shapes, two types of earthquakes: Type 1 and Type 2. In general Type 1 should be used. However, if the earthquakes that contribute most to the seismic hazard defined for the site have a surface-wave magnitude, Ms, not greater than 5,5, then Type 2 is recommended. The underlying ground conditions at a site strongly influence the earthquake vibration at the surface and correspondingly the peak ground acceleration and the response spectrum shape. In EN 1998-1 this is acknowledged by the use of a soil factor S, also a NDP, that multiplies the design ground acceleration (ag) derived from the zonation map. It is worth recalling at this point that 𝑎𝑔 = 𝑎𝑔𝑅 ∙ 𝛾𝐼 and that 𝑎𝑔𝑅 should be taken from the zonation map that is established for rock type ground conditions and for the reference return period chosen by the National Authorities for the No-collapse requirement for ordinary structures. Furthermore, in EN 1998-1 the ground conditions influence the values of the corner periods TB, TC and TD and correspondingly the spectral shape. In EN 1998-1 the spectral amplification (from peak ground acceleration to the acceleration at the constant acceleration branch) is fixed at 2,5 and is consistent with 5% viscous damping. It is however anticipated that the spectral shape may be adjusted for other damping values with the correction factor η given by: 𝜂 = √10/(5 + 𝜉) ≥ 0,55 where 𝜉 is the viscous damping ratio of the structure, expressed as a percentage. The correction factor is depicted in the following picture:
Fig.1.2.10 Spectral or This cor rrection factor is applied directly to the spectral ordinates(for the refere nce value of 5%damping) for TzTB For the first branch of the spectrum,i.e.if os T<TB,the application of the damping correction factoris made in such a way that for T=o there is no correction and for T=TB the correction is applied fully.This is to ensure that at T=o,where the spectral ordinate represents the peak ground acceleration,there is no effect of the damping value. Vertical elastic spectra The vertical component of the ground motion is described in EN98-by an elastic ground acceleration response spectrum S.denoted as the "vertical elastic response spectrum" The spectrum is anchored to the value of the peak vertical acceleration avg.For each seismic zone this vertical acceleration is given by the ratio which is a NDP,to be defined by the National Authorities. The basic shape of the spectrum for the vertical component is similar to the one recommended for the horizontal c pecific。 or th vertical action).However,in this case,the spectral amplification f toris3,instead of the value2,5adopted for the horizontal spectra. Similarly to the horizontal components,two spectral shapes are recommended in EN1998-1 for the vertical components,one for Type 1 and another for Type 2 earthquakes. ,45 for seismic action Type 2(small Magnitude. Furthermore,it should be mentioned that,contrary to what is indicated for the horizontal components,it is considered that the vertical ground motion is not very much affected by the underlying ground conditions and so no use of the soil factor S is made. Design spectra for elastic analysis As indicated before,seismic design accordingto EN8-relies on the(stable)energy dissipation capacity of the struct re;and ino perational erms possibl rade-off between res ance and du y is reflec by the use of behaviour factors for the establishment of Design Spectra suitable for an elastic analysis The ordinates of these Design Spectra are reduced in comparison with the corresponding elastic spectra (which essentially are intended to represent the actual ground vibration)and such reduction is made by the behaviour factor. In the context of EN1998-1 the behaviour factor g is taken as: 9
9 This correction factor is applied directly to the spectral ordinates (for the reference value of 5% damping) for T ≥ TB. For the first branch of the spectrum, i.e. if 0 ≤ T < TB, the application of the damping correction factor η is made in such a way that for T =0 there is no correction and for T = TB the correction is applied fully. This is to ensure that at T = 0, where the spectral ordinate represents the peak ground acceleration, there is no effect of the damping value. Vertical elastic spectra The vertical component of the ground motion is described in EN1998-1 by an elastic ground acceleration response spectrum Sve, denoted as the “vertical elastic response spectrum”. The spectrum is anchored to the value of the peak vertical acceleration avg. For each seismic zone this vertical acceleration is given by the ratio 𝑎𝑣𝑔/𝑎𝑔 which is a NDP, to be defined by the National Authorities. The basic shape of the spectrum for the vertical component is similar to the one recommended for the horizontal components, including four branches (limited by the corner periods TB, TC and TD, specific of the vertical action). However, in this case, the spectral amplification factor is 3,0 instead of the value 2,5 adopted for the horizontal spectra. Similarly to the horizontal components, two spectral shapes are recommended in EN 1998-1 for the vertical components, one for Type 1 and another for Type 2 earthquakes. The recommended values for 𝑎𝑣𝑔/𝑎𝑔 are 𝑎𝑣𝑔 𝑎𝑔 = 0,9 for seismic action Type 1 (large Magnitude) and 𝑎𝑣𝑔 𝑎𝑔 = 0,45 for seismic action Type 2 (small Magnitude. Furthermore, it should be mentioned that, contrary to what is indicated for the horizontal components, it is considered that the vertical ground motion is not very much affected by the underlying ground conditions and so no use of the soil factor S is made. Design spectra for elastic analysis As indicated before, seismic design according to EN 1998-1 relies on the (stable) energy dissipation capacity of the structure; and in operational terms such possible trade-off between resistance and ductility is reflect by the use of behaviour factors for the establishment of Design Spectra suitable for an elastic analysis. The ordinates of these Design Spectra are reduced in comparison with the corresponding elastic spectra (which essentially are intended to represent the actual ground vibration) and such reduction is made by the behaviour factor. In the context of EN 1998-1 the behaviour factor q is taken as:
"an approximation of the ratio of the seismic forces that the structure would experience if its response was completely elastic with viscous damping,to the seismic forces that may be used in the design,with a conventional elastic analysis model.still ensuring a satisfactory response of the structure" The values of the behaviour factor,which also account for the influence of the viscous ifferentfromrivn forvarious materalsand structura systeminto theree classes in the various Parts of EN 1998. Hence EN1998-1 presents the so called Design Spectra for Elastic Analysis.In most of the period range,the ratio between the elastic spectrum and the corresponding design spectrum is simply the value of the behaviour factor g as indicated above
10 “an approximation of the ratio of the seismic forces that the structure would experience if its response was completely elastic with 5% viscous damping, to the seismic forces that may be used in the design, with a conventional elastic analysis model, still ensuring a satisfactory response of the structure”. The values of the behaviour factor q, which also account for the influence of the viscous damping being different from 5%, are given for various materials and structural systems according to the relevant ductility classes in the various Parts of EN 1998. Hence EN 1998-1 presents the so called Design Spectra for Elastic Analysis. In most of the period range, the ratio between the elastic spectrum and the corresponding design spectrum is simply the value of the behaviour factor q as indicated above
