EN ISO 19901-2:2026

SLOVENSKI STANDARD
01-september-2026
Posebne zahteve za naftne ploščadi - 2. del: Potresno projektiranje (ISO 19901-
2:2026)
Specific requirements for offshore structures - Part 2: Seismic design (ISO 19901-
2:2026)
Öl- und Gasindustrie einschließlich kohlenstoffarmer Energieträger - Spezielle
Anforderungen für Offshore-Anlagen - Teil 2: Seismische Auslegungsverfahren und -
kriterien (ISO 19901-2:2026)
Exigences spécifiques relatives aux structures en mer - Partie 2: Conception sismique
(ISO 19901-2:2026)
Ta slovenski standard je istoveten z: EN ISO 19901-2:2026
ICS:
75.180.10 Oprema za raziskovanje, Exploratory, drilling and
vrtanje in odkopavanje extraction equipment
91.120.25 Zaščita pred potresi in Seismic and vibration
vibracijami protection
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN ISO 19901-2
EUROPEAN STANDARD
NORME EUROPÉENNE
July 2026
EUROPÄISCHE NORM
ICS 75.180.10 Supersedes EN ISO 19901-2:2022
English Version
Specific requirements for offshore structures - Part 2:
Seismic design (ISO 19901-2:2026)
Exigences spécifiques relatives aux structures en mer - Öl- und Gasindustrie einschließlich kohlenstoffarmer
Partie 2: Conception sismique (ISO 19901-2:2026) Energieträger - Spezielle Anforderungen für Offshore-
Anlagen - Teil 2: Seismische Auslegungsverfahren und -
kriterien (ISO 19901-2:2026)
This European Standard was approved by CEN on 30 May 2026.

CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this
European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references
concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN
member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by
translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management
Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and
United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2026 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 19901-2:2026 E
worldwide for CEN national Members.

Contents Page
European foreword . 3

European foreword
This document (EN ISO 19901-2:2026) has been prepared by Technical Committee ISO/TC 67 "Oil and
gas industries including lower carbon energy" in collaboration with Technical Committee CEN/TC 12
“Oil and gas industries including lower carbon energy” the secretariat of which is held by NEN.
This European Standard shall be given the status of a national standard, either by publication of an
identical text or by endorsement, at the latest by January 2027, and conflicting national standards shall
be withdrawn at the latest by January 2027.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
This document supersedes EN ISO 19901-2:2022.
Any feedback and questions on this document should be directed to the users’ national standards
body/national committee. A complete listing of these bodies can be found on the CEN website.
According to the CEN-CENELEC Internal Regulations, the national standards organizations of the
following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria,
Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland,
Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of
North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and the
United Kingdom.
Endorsement notice
The text of ISO 19901-2:2026 has been approved by CEN as EN ISO 19901-2:2026 without any
modification.
International
Standard
ISO 19901-2
Fourth edition
Specific requirements for offshore
2026-07
structures —
Part 2:
Seismic design
Exigences spécifiques relatives aux structures en mer —
Partie 2: Conception sismique
Reference number
ISO 19901-2:2026(en) © ISO 2026

ISO 19901-2:2026(en)
© ISO 2026
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii
ISO 19901-2:2026(en)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and abbreviated terms.4
4.1 Symbols .4
4.2 Abbreviated terms .5
5 Seismic hazards . 6
6 Performance objectives and limit states. 6
6.1 General .6
6.2 Ultimate limit states .7
6.3 Performance objectives .8
6.4 Seismic risk category .9
6.5 ULS verification (using hazardous events with intensity = S ) .10
DL a,ELE
6.6 ULS verification (using hazardous events with intensity = S ) .10
NC a,ALE
6.7 Seismic design procedure .10
6.8 Methods for limit state verification .10
7 Analysis types for structural response .12
7.1 Response spectrum analysis . 12
7.2 Time history analysis . 13
7.3 Nonlinear pushover analysis.14
8 Simplified procedure for determining S and S . 14
a,ELE a,ALE
8.1 General .14
8.2 Spectral accelerations .14
8.3 Site class . 15
8.4 Site correction factor coefficients .16
8.5 1 000-year horizontal acceleration spectrum .17
8.6 1 000-year vertical acceleration spectrum .18
8.7 Damping adjustment .19
8.8 Determining S and S .19
a,ELE a,ALE
9 Detailed procedure for determining S and S .20
a,ELE a,ALE
9.1 Probabilistic seismic hazard analysis . 20
9.2 Deterministic seismic hazard analysis . 22
9.3 Determining C . 23
c
9.4 Determining S and S . 26
a,ELE a,ALE
9.5 Dynamic site response analysis .27
10 Floating structures .27
Annex A (informative) Additional information and guidance .28
Annex B (normative) Seismic maps with spectral accelerations for simplified action procedure . 41
Annex C (normative) Regional information .88
Bibliography .93

iii
ISO 19901-2:2026(en)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
ISO technical committees. Each member body interested in a subject for which a technical committee
has been established has the right to be represented on that committee. International organizations,
governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely
with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO document should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 67, Oil and gas industries including lower
carbon energy, Subcommittee SC 7, Offshore structures, in collaboration with the European Committee for
Standardization (CEN) Technical Committee CEN/TC 12, Oil and gas industries including lower carbon energy,
in accordance with the Agreement on technical cooperation between ISO and CEN (Vienna Agreement).
This fourth edition cancels and replaces the third edition (ISO 19901-2:2022), which has been technically
revised.
The main changes are as follows:
— the scope has been expanded to cover offshore wind and other renewable energy offshore structures;
— requirements from common industry specifications (IOGP JIP 35) have been incorporated;
— the seismic hazard maps have been updated.
A list of all parts in the ISO 19901 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

iv
ISO 19901-2:2026(en)
Introduction
The International Standards for offshore structures prepared by ISO TC 67/SC 7 comprise:
— ISO 19900, the unifying International Standard for offshore structures;
— the ISO 19901 series, providing specific requirements for offshore structures;
— ISO 19902, ISO 19903, ISO 19904-1, the ISO 19905 series and ISO 19906, “structure type” standards.
Figure 1 illustrates the relationships between the International Standards for offshore structures prepared
by ISO TC 67/SC 7.
Figure 1 — Relationship of International Standards for offshore structures prepared by
ISO/TC 67/SC 7
Seismic conditions vary widely around the world, and the design criteria depend primarily on observations of
historical seismic events together with consideration of seismotectonics and local soil conditions. Additional
information and guidance are given in Annex A, where the clause numbering mirrors the normative clauses
to facilitate cross referencing.
Statements containing requirements (“shall” and “shall not”), recommendations (“should” and “should not”)
or permissions (“may”) are collectively referred to as provisions.

v
International Standard ISO 19901-2:2026(en)
Specific requirements for offshore structures —
Part 2:
Seismic design
1 Scope
This document contains requirements for seismic design and assessment of offshore structures.
This document includes recommendations for the effects of seismic events on floating structures.
This document addresses specifically the design and assessment of offshore structures subjected to
earthquake-induced ground motions. It also covers briefly other geologically induced hazards such as
liquefaction, slope instability, fault surface displacement, tsunamis, mud volcanoes and shock waves.
This document provides requirements for site-specific probabilistic seismic hazard analysis for offshore
structures in high seismic areas and for offshore structures with high consequence levels.
2 Normative references
The following documents are referred to in the text in such a way that some or all their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO 19900, Oil and gas industries including lower carbon energy — General requirements for offshore structures
ISO 19902, Petroleum and natural gas industries — Fixed steel offshore structures
IOGP S-631-11, Supplementary Specification for Fixed Steel Offshore Structures, International Oil & Gas
Producers Association, Version 2, 8 February 2021
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 19900 and the following apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
abnormal level earthquake
ALE
ground motion (3.6) due to a rare intense earthquake with a very low probability of occurrence during the
life of the structure
Note 1 to entry: Typically, having an annual probability of exceedance (3.10) of one in a few thousand years.
3.2
active fault
fault likely to have another earthquake in the future

ISO 19901-2:2026(en)
3.3
attenuation
decay of seismic waves as they travel from the earthquake source to the site under consideration
3.4
deaggregation
separation of seismic hazard contributions from different faults and seismic source zones
3.5
extreme level earthquake
ELE
ground motion (3.6) due to an earthquake with a reasonable probability of occurring during the life of the
structure
Note 1 to entry: Typically having an annual probability of exceedance (3.10) of one in a few hundred years.
3.6
ground motion
accelerations, velocities or displacements of the ground produced by seismic waves radiating away from
earthquake sources
Note 1 to entry: A fixed offshore structure is founded in or on the seabed and consequently only seabed motions or
motions along the length of piles are of significance. The expression "ground motions" is used rather than "seabed
motions" for consistency of terminology with seismic design for onshore structures.
Note 2 to entry: Ground motions can be at a specific depth or over a specific region within the seabed.
3.7
liquefaction
fluidity of soil due to the increase in pore pressures caused by earthquake action under undrained conditions
3.8
mud volcano
diapiric intrusion of plastic clay causing high pressure gas-water seepages which carry mud, fragments of
rock (and occasionally oil) to the surface
Note 1 to entry: The surface expression of a mud volcano is a cone of mud with continuous or intermittent gas escaping
through the mud.
3.9
probabilistic seismic hazard analysis
PSHA
framework permitting the identification, quantification and rational combination of uncertainties in
earthquakes' intensity, location, rate of recurrence and variations in ground motion (3.6) characteristics
3.10
probability of exceedance
P
e
probability that a variable (or an event) exceeds a specified reference level within a given exposure period
EXAMPLE The annual probability of exceedance of a specified magnitude of ground acceleration, ground velocity
or ground displacement.
3.11
pushover analysis
application and incremental increase of a global static pattern of actions on a structure, including equivalent
dynamic inertial actions, until a global failure mechanism occurs
3.12
response spectrum
maximum responses of a series of single-degree-of freedom systems subjected to a given base motion,
plotted as a function of natural frequencies for specific values of damping

ISO 19901-2:2026(en)
3.13
safety system
system provided on the offshore structure to detect, control and mitigate hazardous situations
EXAMPLE Gas detection, emergency shutdown, fire protection, and their control systems.
3.14
seabed slide
failure of seabed slopes
Note 1 to entry: The seabed is defined as the soil material below the sea floor (ISO 19900:2019, 3.47).
Note 2 to entry: The effective seabed is defined in 8.3.2; typically, being the upper 30 m of seabed material.
3.15
seismic hazard curve
curve showing the annual probability of exceedance (3.10) against a measure of ground motion (3.6) or
response of the single degree of freedom oscillator
Note 1 to entry: The seismic measures can include parameters such as peak ground acceleration, spectral acceleration
(3.19), spectral velocity or spectral displacement.
3.16
seismic reserve capacity factor
C
r
factor indicating the structure’s ability to sustain ground motions (3.6) due to earthquakes beyond the level
of the extreme level earthquake (3.5)
3.17
seismic risk category
SRC
category defined from the consequence level and the intensity of seismic motions
3.18
site response analysis
wave propagation analysis permitting the evaluation of the effect of local geological and soil conditions on
the ground motions (3.6) as they propagate up from depth to the surface at the site
3.19
spectral acceleration
maximum acceleration response of a single degree of freedom oscillator subjected to ground motions (3.6)
due to an earthquake
3.20
tsunami
long period sea waves caused by rapid vertical movements of the sea floor
Note 1 to entry: The vertical movement of the sea floor is often associated with fault rupture during earthquakes or
with seabed slides (3.14).
3.21
zero pad
technique used in signal processing particularly when analysing earthquake recordings using the fast
Fourier transformation
ISO 19901-2:2026(en)
4 Symbols and abbreviated terms
4.1 Symbols
a tail slope of the seismic hazard curve
R
C correction factor for the acceleration part (shorter periods) of a response spectrum
a
C correction factor applied to the spectral acceleration to account for uncertainties not
c
captured in a seismic hazard curve
C seismic reserve capacity factor
r
C correction factor for the velocity part (longer periods) of a response spectrum
v
d thickness of soil layer
D scaling factor for damping
G initial (small strain) shear modulus of the soil
max
g acceleration due to gravity
H horizontal spectral accelerations
M magnitude of an earthquake measured by the energy released at its source
N scale factor for conversion of the site 1 000-year acceleration spectrum to the site ALE
ALE
acceleration spectrum
p atmospheric pressure
a
P annual probability of exceedance for the ALE event
ALE
P annual probability of exceedance for the ELE event
ELE
P annual probability of exceedance
e
annual probability of exceeding the ULS
P
DL
ULS
DL
P annual probability of exceeding the ULS
NC
ULS
NC
q cone penetration resistance of soil
c
normalized cone penetration resistance of soil
q
cl
average normalized cone penetration resistance of sand in the effective seabed
q
cl
R representative capacity
k
S (T) spectral acceleration (associated with single degree of freedom oscillator period, T)
a
S (T) ALE spectral acceleration
a,ALE
S (T ) ALE spectral acceleration at T
a,ALE dom dom
S (T) ELE spectral acceleration
a,ELE
S (T ) ELE spectral acceleration at T
a,ELE dom dom
S (T) 1 000-year bedrock spectral acceleration obtained from seismic maps
a,map
ISO 19901-2:2026(en)
S (T) site spectral acceleration corresponding to a return period of 1 000 years
a,site
spectral acceleration having an annual probability of exceedance (P ) obtained from a
ST

e
a,P
e
PSHA.
ST site-specific spectral acceleration at T having an annual probability P of

dom
ad,P om ULS
ULS DL
DL
exceeding the ULS
DL
ST site-specific spectral acceleration at T having an annual probability P of

dom
ad,P om ULS
ULS NC
NC
exceeding the ULS
NC
s undrained shear strength of the soil
u
s̅ average undrained shear strength of the soil in the effective seabed
u
T natural period of a simple, single degree of freedom oscillator
T dominant modal period of the structure
dom
T return period
return
V vertical spectral accelerations
representative shear wave velocity
v
s
v
average representative shear wave velocity in the effective seabed
s
ρ mass density of soil
η per cent of critical damping
σ logarithmic standard deviation of uncertainties not captured in a seismic hazard
LR
curve
σ′ in situ vertical effective stress of soil
v0
4.2 Abbreviated terms
DL damage limitation state (specified in 6.2.3 and 6.2.4)
DSHA deterministic seismic hazard analysis
DSRA dynamic site response analysis
L1, L2, L3 consequence levels (see ISO 19900)
NC near collapse state (specified in 6.2.1 and 6.2.2)
NPA nonlinear pushover analysis
NTHA nonlinear time history analysis
RSA response spectrum analysis
SDOFS single degree of freedom
SLS serviceability limit state
SPA static pushover analysis
ISO 19901-2:2026(en)
ULS ultimate limit state
5 Seismic hazards
5.1 Seismic design and assessment of offshore structures shall include the effect of ground motions due to
earthquakes.
5.2 In addition to the effect of ground motion, design and assessment of offshore structures shall also
include the effects of the following seismic hazardous events, as developed by specialists in geologic site
hazards:
a) soil liquefaction,
b) seabed slide,
c) fault surface displacement,
d) tsunamis,
e) mud volcanoes,
f) velocity pulse from directivity effects (earthquake-induced shock wave in the water column typically
associated with volcanic eruptions).
NOTE Provisions for seismic design and assessment of floating structures is covered in Clause 10.
6 Performance objectives and limit states
6.1 General
6.1.1 The risks, due to the structure being exposed to seismic hazardous events, shall be demonstrated to
be tolerable by stake holders in accordance with 6.1.2 to 6.1.4.
NOTE The requirements in this document ensure that the structure has sufficient strength and ductility such that
the risks (life-safety, environmental-pollution, and business-disruption) are tolerable when the structure is exposed
to seismic hazardous events, as defined by the seismic hazard curve for the site.
6.1.2 Demonstration that the structure meets the performance objectives shall be by limit state
verification in accordance with ISO 19900.
6.1.3 The performance objective for life-safety and environmental risks, typically associated with an ALE,
shall specify the maximum tolerable annual probability that the (damaged) state of structure can exceed
ULS .
NC
NOTE 1 Life-safety risk refers to the potential for injury or loss of life during an earthquake.
NOTE 2 ULS is described in 6.2.1. When the state of the structure exceeds ULS one or more components can
NC NC
have failed and the structure can be near to collapsing.
6.1.4 The performance objective for business-disruption risk, typically associated with an ELE, shall
specify the maximum tolerable annual probability that the (damaged) state of structure can exceed ULS .
DL
NOTE ULS is described in 6.2.3. When the state of the structure exceeds ULS , the structural system remains
DL DL
stable, but the lateral system of the structure (braces, joints, and piles) can have plastic strains that result in permanent
deformation that can affect the functionality of the structure. The permanent deformation is likely repairable and
thus the functionality of the facility can be restored.

ISO 19901-2:2026(en)
Key
a
SRC 2 structures can alternatively be designed using the detailed seismic action procedure similar to SRC 3
and SRC 4 structures (see Table 4).
Figure 2 — Steps for limit state verification
6.2 Ultimate limit states
6.2.1 The state of the structure at the ULS , typically due to hazardous events with intensity S (T),
NC a,ALE
shall not include:
a) collapse of the structure’s gravity load resisting system (i.e. legs, pile to leg joints, pile-soil resistance) or
collapse of the gravity sub-system that supports the living quarters;
b) damage to safety systems, escape routes, and evacuation systems that prevent their functionality;

ISO 19901-2:2026(en)
c) loss of supports for critical hydrocarbon equipment that could lead to escalation by loss of process
containment;
d) collapse of the living quarters, masts, derricks, flare structures and other safety critical structures.
6.2.2 The state of the structure at the ULS may include damage to the structure’s lateral system (i.e.
NC
braces and their joints, and pile lateral deformation) as follows:
a) severed braces (by fracture or low-cycle fatigue, with or without strain ratcheting);
b) plastic local buckling of braces or global buckling of braces;
c) permanent deformation of joints, and collapse or fracture of joints;
d) permanent lateral deformation of piles;
e) spalling of reinforced concrete.
6.2.3 The state of the structure at the ULS , typically due to hazardous events with intensity S , shall
DL a,ELE
not include:
a) permanent deformation of the components comprising the structure’s gravity load resisting system (i.e.
legs, pile to leg joints, and pile-soil resistance);
b) permanent deformation to the components comprising the structure’s lateral system (i.e. braces and
their joints, and pile lateral deformation);
c) damage to safety systems, escape routes, and evacuation systems that prevent their functionality;
d) damage to pipelines, conductors, risers, and other safety-critical components due to displacements at
mudline elevation of the structure;
e) damage due to toppling of topsides equipment and cable trays;
f) damage to masts, derricks, and flare structures;
g) damage to sliding supports that prevent their functionality;
h) damage to piping systems that prevents their functionality due to differential displacement of supports.
6.2.4 The state of the structure at the ULS should not include damage that will result in dropped objects.
DL
6.3 Performance objectives
6.3.1 ULS verification shall be demonstrated for hazardous events with intensity S in accordance
NC a,ALE
with 6.3.2.
6.3.2 The annual probability of the structure exceeding the ULS (defined in 6.2.1) shall not be larger
NC
than the value of P listed in Table 1.
f
Table 1 — Maximum tolerable annual probability of exceeding P
f
Consequence level P
f
-4
L1 4 × 10 = 1/2 500
-3
L2 1 × 10 = 1/1 000
-3
L3 2,5 × 10 = 1/400
ISO 19901-2:2026(en)
NOTE 1 P values in Table 1 account for epistemic uncertainty due to the use of the mean hazard curve and the
f
mean fragility curve in Clause 9.
NOTE 2 Reference [22] describes the calibration of the simplified procedure in Clause 8 to the probabilities listed in
Table 1.
6.3.3 The maximum tolerable annual probability of exceeding the ULS may be less than listed in Table 1
NC
if specified by the operator or regulator, provided the structural response with probability of exceedance in
Table 1 is also demonstrated to be acceptable.
NOTE The lower probability event with its higher motions can result in nonlinear soil response which can
effectively reduce seismic demands on the structure.
−4
EXAMPLE 1 × 10 p.a. (1/10 000) for L1 facilities.
6.3.4 ULS verification shall be demonstrated for hazardous events with intensity S in accordance
DL a,ELE
with 6.3.5.
6.3.5 The annual probability of the structure exceeding the ULS , as defined in 6.2.3, shall not be larger
DL
than P or P as listed in Table 2.
ELE
ULS
DL
Table 2 — Maximum tolerable annual probability of exceeding ULS
DL
Consequence level
P
P
ULS ELE
DL
-3
L1 5,0 × 10 = 1/200 P is the annual probability of
ELE
exceedance of the spectral accel-
-2
L2 1,0 × 10 = 1/100
eration value, S given by the
a,ELE
-2
L3 2,0 × 10 = 1/50
hazard curve in Figure 6.
6.4 Seismic risk category
6.4.1 The facility shall be assigned a seismic risk category (SRC) in accordance with Table 3.
6.4.2 Site seismic zone shall be determined from the 1,0 s horizontal spectral acceleration maps in Annex B
or from a site-specific seismic hazard study (PSHA).
Table 3 — Seismic risk category
Consequence level
S (1,0)
Site seismic
a,map
(see ISO 19900)
zone
(see Annex B)
L1 L2 L3
<0,03 g 0 SRC 1 SRC 1 SRC 1
0,03 g to <0,10 g 1 SRC 3 SRC 2 SRC 2
0,10 g to <0,25 g 2 SRC 4 SRC 2 SRC 2
0,25 g to <0,45 g 3 SRC 4 SRC 3 SRC 2
≥0,45 g 4 SRC 4 SRC 4 SRC 3
6.4.3 For seismic design situations, a facility with environmental consequence level greater than L3, shall
not be classified as consequence level L2 unless it is normally unoccupied (e.g. limited to inspection and
maintenance visits).
NOTE No reliable forewarning of seismic actions is feasible and, consequently, it is not possible to evacuate prior
to an earthquake.
ISO 19901-2:2026(en)
6.5 ULS verification (using hazardous events with intensity = S )
DL a,ELE
6.5.1 S shall be determined in accordance with Clause 8 or Clause 9, depending on the SRC (described
a,ELE
in 6.4).
6.5.2 ULS limit state verification shall be performed using:
DL
a) actions arising from ground motion time histories where each ground motion has a spectral acceleration,
at the first lateral sway period of the structure, defined by S , see Clause 7;
a,ELE
b) representative capacities of members, joints, piles and soil, as defined in ISO 19900.
NOTE For fixed steel structures, ULS verification of the structure is demonstrated if ISO 19902:2020,
DL
Formulae (11.5-1) and (11.5-2) are satisfied.
6.6 ULS verification (using hazardous events with intensity = S )
NC a,ALE
6.6.1 S shall be determined in accordance with Clause 8 or Clause 9, depending on the SRC (described
a,ALE
in 6.4).
6.6.2 ULS limit state verification shall be performed using:
NC
a) actions arising from ground motion time histories where each ground motion has a spectral acceleration,
at the sway period of the structure, defined by S ;
a,ALE
b) mean capacities of members, joints, piles, and soil (by use of mean yield stress, mean component
capacities and expected soil capacity).
NOTE Clause 7 provides analysis types and requirements to determine the above actions.
6.7 Seismic design procedure
Seismic design shall follow the limit state verification methods in 6.8 and be based on one of the following
design strategies:
a) a linear strength-based design strategy (with implied ductility);
b) a nonlinear (explicit) ductility-based design strategy.
NOTE A.6.7 provides detailed descriptions of the above strategies.
6.8 Methods for limit state verification
6.8.1 Limit state verification methods depend on the SRC as described in 6.8.2 to 6.8.6.
NOTE The provisions in this subclause are summarised in Table 4.

ISO 19901-2:2026(en)
Table 4 — Summary of requirements for ULS verification
NC
Procedure for determining
Structural analysis
SRC Hazard analysis
S and S
RSA, NTHA or NPA
aE, LE aA, LE
1 None None required None required
ISO maps or regional information
RSA recommended Simplified (Clause 8) recommended
recommended
Site-specific PSHA permitted RSA recommended Simplified (Clause 8) recommended
complementary DSHA permitted NTHA or NPA permitted Detailed (Clause 9) permitted
Site-specific PSHA required NTHA recommended
3 Detailed (Clause 9) required
complementary DSHA permitted NPA permitted
Site-specific PSHA required
4 NTHA required Detailed (Clause 9) required
complementary DSHA permitted
6.8.2 Limit state verification of facilities classed as SRC 4:
a) shall perform the hazard analysis using PSHA in accordance with 9.1;
b) may perform a complementary hazard analysis using DSHA in accordance with 9.2;
c) shall determine S for ULS verification using the C method in accordance with 9.4;
a,ALE NC
c
d) shall perform the structural response analysis for the ULS by NTHA in accordance with 7.2;
NC
e) shall determine the structural response for verification of the ULS by either:
DL
1) linear time history analysis in accordance with 7.2; or
2) RSA in accordance with 7.1.
6.8.3 Limit state verification of facilities classed as SRC 3:
a) shall perform the hazard analysis using PSHA in accordance with 9.1;
b) may perform a complementary hazard analysis using DSHA in accordance with 9.2;
c) shall determine S for ULS verification using C in accordance with 9.4;
a,ALE NC
c
d) should perform the structural response analysis for the ULS by NTHA in accordance with 7.2;
NC
e) may perform structural response analysis for the ULS by NPA in accordance with 7.3;
NC
NOTE NTHA better simulates double hinging of piles in soft soils compared to NPA.
f) shall determine the structural response for limit state verification of the ULS by either:
DL
1) linear time history analysis in accordance with 7.2; or
2) response spectrum analysis in accordance with 7.1.
6.8.4 Limit state verification of facilities classed as SRC 2:
a) should perform the hazard analysis with the seismic maps and site correction factors in accordance
with 8.2 to 8.4;
b) may perform the hazard analysis with a PSHA in accordance with 9.1;
NOTE The simplified seismic action procedure is typically more conservative than the equivalent detailed
seismic action procedure.
c) may perform a complementary hazard analysis using DSHA in accordance with 9.2;

ISO 19901-2:2026(en)
d) shall determine S for verification (for ULS ) by either:
a,ALE NC
1) the C method in accordance with 9.4; or
c
2) the N scale factor method in accordance with 8.5 to 8.8;
ALE
NOTE A 1 000-year uniform hazard spectrum can be produced in accordance with 9.1 rather than the 1 000-
year design response spectrum produced in accordance with 8.5 to 8.8.
e) shall determine the structural response for verification of the ULS by either:
DL
1) linear time history analysis in accordance with 7.2; or
2) RSA in accordance with 7.1.
f) shall determine the structural response for verification of the ULS by any of the following:
NC
1) RSA in accordance with 7.1;
2) NTHA in accordance with 7.2;
3) NPA in accordance with 7.3.
6.8.5 Limit state verification of facilities classed as SRC 1 may be performed but is not required.
6.8.6 For facilities classed as SRC >1, the risks arising due to collapse of the facility in seismic hazardous
events shall be minimised in accordance with ISO 19902 for ductile design of the primary structure and
joints.
7 Analysis types for structural response
7.1 Response spectrum analysis
7.1.1 Combination of modal responses shall be performed by the complete quadratic combination.
7.1.2 Directional responses for both horizontal directions and the vertical direction shall be determined.
7.1.3 Combination of directional responses:
a) should be performed by the square root of the sum of the squares;
b) may be performed by linear combination.
NOTE 1 Linear combination can be applied by taking one component at its maximum and the other two components
at 40 % of their respective maximum values (with the sign of each response parameter selected to maximise the
response combination) - see Reference [23].
[24]
NOTE 2 Linear combination is always more conservative than the square root of the sum of the squares .
7.1.4 The number of modes used in a RSA shall be the number of modes to achieve at least 90 % mass
participation in each horizontal direction.
[19]
NOTE ASCE 7-22 provides guidance on achieving 90 % mass participation.
7.1.5 R
...