SIST EN 17121:2019

SLOVENSKI STANDARD
SIST EN 17121:2019
01-december-2019
Ohranjanje kulturne dediščine - Zgodovinske lesene konstrukcije - Smernice za
ocenjevanje nosilnih lesenih konstrukcij na kraju samem
Conservation of cultural heritage - Historic timber structures - Guidelines for the on-site
assessment of load-bearing timber structures
Erhaltung des kulturellen Erbes - Historische Holzkonstruktionen - Leitlinien für die
Bewertung vor Ort von tragenden Holzkonstruktionen
Conservation du patrimoine culturel - Structures en bois du patrimoine - Lignes
directrices relatives à l’évaluation sur site des stuctures porteuses en bois
Ta slovenski standard je istoveten z: EN 17121:2019
ICS:
91.080.20 Lesene konstrukcije Timber structures
97.195 Umetniški in obrtniški izdelki. Items of art and handicrafts.
Kulturne dobrine in kulturna Cultural property and
dediščina heritage
SIST EN 17121:2019 en,fr,de
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST EN 17121:2019

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SIST EN 17121:2019


EN 17121
EUROPEAN STANDARD

NORME EUROPÉENNE

September 2019
EUROPÄISCHE NORM
ICS 91.080.20; 97.195
English Version

Conservation of cultural heritage - Historic timber
structures - Guidelines for the on-site assessment of load-
bearing timber structures
Conservation du patrimoine culturel - Structures en Erhaltung des kulturellen Erbes - Historische
bois du patrimoine - Lignes directrices relatives à Holzkonstruktionen - Leitlinien für die Bewertung vor
l'évaluation sur site des stuctures porteuses en bois Ort von tragenden Holzkonstruktionen
This European Standard was approved by CEN on 1 July 2019.

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, Turkey 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
© 2019 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN 17121:2019 E
worldwide for CEN national Members.

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EN 17121:2019 (E)
Contents Page
European foreword . 3
Introduction . 4
1 Scope . 5
2 Normative references . 5
3 Terms and definitions . 5
4 Assessment procedure . 8
4.1 General . 8
4.2 Preliminary assessment . 9
4.3 Desk study . 11
4.4 Historical analysis . 11
4.5 Preliminary visual survey . 12
4.6 Measured survey . 12
4.7 Structural analysis . 13
4.8 Preliminary report . 14
5 Detailed survey . 15
5.1 Overview . 15
5.2 General . 15
5.3 Identification of wood species . 16
5.4 Estimating wood moisture content and moisture gradients . 16
5.5 Characterization of biological damage . 16
5.6 Strength assessment of timber . 17
5.7 Wood dating . 18
5.8 Detailed survey of timber joints . 19
5.9 Detailed structural analysis . 20
Annex A (informative) Tools for non-destructive analyses on timber structures . 21
Bibliography . 27

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European foreword
This document (EN 17121:2019) has been prepared by Technical Committee CEN/TC 346 “Conservation
of Cultural Heritage”, the secretariat of which is held by UNI.
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 March 2020, and conflicting national standards shall be
withdrawn at the latest by March 2020.
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.
According to the CEN-CENELEC Internal Regulations, the national standards organisations 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, Turkey and the United
Kingdom.
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Introduction
The purpose of the present document is to consider condition survey and diagnostic methods for
assessing heritage load-bearing timber structures with a view to establishing safe working loads or
determining the need for strengthening or repair in order to ensure their continuing use.
Heritage structures are important historic artefacts, which differ from other existing structures in that a
greater value is placed on their fabric because of their historical significance. It is necessary to gather data
in order to be able to assess the ability of such a structure to carry the required loads and continue to be
in use both now and for the foreseeable future, and to identify those areas of the structure that require
repair or strengthening. Both the conduct of the survey, assessment and any subsequent repair or
strengthening that could be necessary should involve minimum intervention. This could justify greater
expense both in the survey, diagnosis and assessment of the structure and in the consideration of the
repair methods that might be employed. The assessment of their condition and of their existing structural
characteristics is expected to be state of the art, which might require the use of more precise methods
than those that are used for other existing structures, with a cost that could not otherwise be justified.
Moreover intervention works (repair or strengthening) should only be carried out to a heritage structure
as a last resort and should have minimal impact on the building fabric (the original materials, structural
systems and techniques).
An important part of the work involves the documentation and understanding of the history of a
structure: loads, construction technology, and the period’s aesthetic details in order to be able to assess
the historical significance of either the overall structure or of any of its components. Historical
significance of a structure could relate to the history of the structure itself or that of the building of which
it is a part. Such understanding requires an interdisciplinary approach with any other profession that can
be helpful to the assessment procedure.
In some cases, where the problems are simple and obvious, the preliminary, visual survey might be all
1
that is required but in general a more detailed survey will be needed .

1
The survey of existing buildings to determine their suitability for continuing use or for a change of use has been
considered by ISO 13822. Annex I of that standard considers heritage structures.
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1 Scope
This document gives guidelines on the criteria to be used for the on-site assessment of load-bearing
timber structures in heritage buildings. It is intended for all those concerned with the conservation of
heritage buildings which contain wooden elements, from the building owners or authorities who are
responsible for them to the professionals employed. It should also help decision-making regarding the
need for immediate measures. Its aim is to guarantee that condition survey and assessment provide the
necessary data for historical analysis, structural safety assessment and planning of intervention works.
This document is applicable to any kind of timber member and to any kind of historic timber structures.
It is not applicable to timber members made of engineered wood based panels and glued laminated
timber.
This document provides a comprehensive procedure for the on-site assessment. With a practical and
technical evaluation of the damage found and based on the responsibility of the involved professionals, a
sufficient assessment can also be made when not all the steps are followed.
In each different country, the document is expected to be applied in accordance with National legislation
and regulations.
2 Normative references
The following documents are referred to in the text in such a way that some or all of 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.
EN 335, Durability of wood and wood-based products — Use classes: definitions, application to solid wood
and wood-based products
EN 1912, Structural Timber — Strength classes — Assignment of visual grades and species
EN 1995-1-1, Eurocode 5: Design of timber structures — Part 1-1: General — Common rules and rules for
buildings
EN 14081-1:2016, Timber structures — Strength graded structural timber with rectangular cross
section — Part 1: General requirements
EN 16085, Conservation of Cultural property — Methodology for sampling from materials of cultural
property - General rules
EN 16096, Conservation of cultural property — Condition survey and report of built cultural heritage
3 Terms and definitions
For the purposes of this document, the terms and definitions given in EN 14081-1, EN 1912 and the
following apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— IEC Electropedia: available at http://www.electropedia.org/
— ISO Online browsing platform: available at http://www.iso.org/obp
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3.1
action
set of forces (loads) applied to the structure
[SOURCE: EN 1990:2002, 1.5.3.1, modified]
3.2
alteration
change in condition, beneficial or not, intentional or not
[SOURCE: EN 15898:2011, 3.2.4]
3.3
biological damage
damage caused by living organisms such as fungi and insects
Note 1 to entry: In load bearing timber structures, the damage is considered in terms of reduction of the
effective cross-section.
3.4
condition survey
inspection to assess the condition of the structure
[SOURCE: EN 15898:2011, 3.6.4, modified]
3.5
critical area/zone
part of a timber element over a length of 150 mm, or equal to the depth of the member, whichever is the
greater, which is considered to be relevant to the structural performance of the structure because of
strength reducing characteristics, position, state of preservation and also stress conditions as determined
by structural analysis
3.6
critical cross-section
cross-section which is representative of a critical zone; all the defects, alterations, damage and other
characteristics that are present in the critical zone and have an influence on its strength are attributed to
the critical section
3.7
damage
alteration that reduces significance or stability
[SOURCE: EN 15898:2011, 3.2.7]
3.8
defects
defects of wood
wood growth features which can negatively influence strength and stiffness, and/or the general
structural behaviour (e.g. the efficiency of joints) of timber members
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3.9
diagnosis
process of identifying the present condition of a structure and determining the nature and causes of any
change, as well as the conclusions drawn
[SOURCE: EN 15898:2011, 3.6.6, modified]
3.10
dowelled connection
connection made with a circular cylindrical rod of timber or steel, with or without a head, fitting tightly
in prebored holes and used for transferring loads perpendicular to the dowel axis
3.11
effective cross-section
part of the cross-section of a timber member which is assumed to be in a good state to resist stressors
3.12
in-situ load testing
testing of the structure or part of it by loading to evaluate its behaviour or mechanical properties, or to
predict its load-bearing capacity
3.13
rot
decomposition of wood by fungi resulting in softening, progressive loss of mass and strength, and often
a change of texture and colour
[SOURCE: EN 1001-1:2005, N. 385]
3.14
semi-destructive test
test that has a minor impact on a timber member and that influences its load-bearing capacity or its
appearance negligibly
3.15
non-destructive test
test that has no impact on a timber member and that does not influence its load-bearing capacity or
change its appearance
3.16
strength reducing characteristics
property or feature of a piece of timber that reduces its load-bearing capacity
[SOURCE: EN 14081-1:2016, 3.8, modified]
3.17
structural safety
evaluation that the structure or parts of the structure will not fail under the planned loading
3.18
serviceability assessment
evaluation that the structure or parts of the structure comply with specified limit states concerning
service conditions
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3.19
strength grading
procedure through which a single timber member can be allocated to a grade that corresponds to a
known level of mechanical performance
3.20
structural system
load-bearing members of a building or civil engineering works and the way in which these members
function together
[SOURCE: EN 1990:2002, 1.5.1.9]
3.21
structural member type
classification of timber members, through structural analysis, according to the type of their loading
condition (compression, tension, bending, shear, or combination of these), necessary for their strength
grading
3.22
survey
inspection of a structure to determine its geometry, its detailing and its existing condition
3.23
wane
original rounded surface of a log, with or without bark, on any face or edge of sawn timber
[SOURCE: EN 844-3:1998, 3.6]
3.24
wood moisture content
mass of moisture in wood expressed as a percentage of its oven-dry mass
[SOURCE: EN 844-4:1997, 4.1]
4 Assessment procedure
4.1 General
The procedure required for the on-site examination and assessment of an historic timber structure is
briefly as follows:
a) First phase, preliminary assessment dealt with in detail in Clause 4 of this standard. This includes:
1) a desk study, that should provide information and documentation that will help in determining
the history of the structure and if possible the corresponding load history. This should also
clearly indicate the intentions of the building owner so that the intended ultimate load and
environmental conditions are known.
2) a visual survey. This is to obtain an overview of the structure that is sufficient to plan the next
stage, identifying also what provisions need to be made to gain access to the timbers.
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3) a measured survey to determine the overall disposition of the structural members and locate
main problems. This survey should include principal dimensions and the nominal sizes of the
members that are critical for the preliminary structural assessment. It should also note any
obvious signs of damage, decay or structural problems, which will need to be investigated in
more detail at a subsequent stage.
4) a preliminary structural analysis to determine the overall forces, the structural type of each
member and general levels of stress and deflections/deformations within the structure.
5) a preliminary report shall be prepared that includes a general description of the structure and
its existing condition, with a note of areas of concern that require more detailed study and which
possibly require immediate safety measures and actions. It shall include also a description of the
pathology, the service conditions and the structural behaviour. Vulnerable areas shall be
identified, together with those members and connections that are carrying the larger stresses
and/or deformations; any problems and/or strength reducing characteristic within the
structure shall be noted together with a general preliminary assessment of the present condition
of the structure and guidelines/proposals for the continuation of the investigation. These
guidelines will specify any additional survey work that may be necessary. This would indicate
any aspects of the structure, which require further investigation and the methods recommended.
b) Second phase, detailed survey dealt with in detail in Clause 5.
1) A detailed survey as indicated in the preliminary report. This should include the measurement
of areas of biological attack and damage, the assessment of timber grades and the results of non-
destructive methods where appropriate and will also consider the adequacy of joints.
2) A diagnostic report on the condition of the structure and causes of distress with proposals for
remedial measures where necessary. This may imply a new structural analysis considering the
data gathered in the detailed survey.
Although the list above suggests a linear process it is essential to recognize that conservation work
frequently involves iterations. For example, the preliminary visual survey might raise questions that
could be answered by a more thorough desk study, able to document changes that have been observed
in the structure. Iterations between structural analysis and repair strategy stages are also often required.
A holistic approach is always required, considering and assessing the structure as a whole, rather than
just the individual members and joints.
4.2 Preliminary assessment
4.2.1 Need for the preliminary assessment
The purpose of the assessment is to determine whether the structure is at present adequate to carry the
loads that it is required to carry and will continue to be adequate for the foreseeable future. The work
should be carried out by professionals, which should have knowledge on timber structures. Note that
structures that have proved to be adequate in the past will continue to be adequate requiring no detailed
assessment of timber mechanical properties, except in the following circumstances:
a) there is a possible change in loads (due to e.g. a change in use of the structure);
b) there has been significant rot or/and insect attack to the timbers, or the structure has suffered
damage, e.g. due to fire;
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c) there has been mechanical damage, failures or excessive deflection indicating overloading of the
timbers in the past, the materials used are of poor quality, there was inferior initial design or/and
workmanship. The evaluation depends on the type of structure, its historic importance and on
national regulation;
d) there have been alterations or interventions to the structure during its lifetime, that have resulted in
a reduction of its structural capacity or changes to the original structural scheme;
e) to ensure that the structure is adequate to withstand extreme climatic or loading events (earthquake,
wind, ground sliding, etc.);
f) there has been a variation of the thermo-hydrometric conditions, that have changed the service
conditions.
NOTE 1 In some cases, rather than adopt extensive strengthening measures, safety can be ensured by limiting
the allowable loads. This can involve limiting public access.
NOTE 2 The deformation/deflection limits for historic structures can be different and less strict from the ones
that the modern codes specify (serviceability limit state design) for a new timber structure.
The preliminary assessment may include:
g) retrieval of possible historical documentation, to date the structure and identify the sequence of
changes made (historical phases);
h) a written description of the structure and a photographic documentation;
i) a survey of the structure and a graphic representation of its geometrical and constructional features
in 2D and 3D (isometric or axonometric) drawings;
j) identification of the species used for the structure (load-bearing and non-load-bearing members);
k) a description of the structural system. Note that this may need to include some description of the
supporting system (foundation, masonry or other type of structure);
l) a description of the environmental service conditions (usual values and variations of hydro-thermal
conditions) and the consequent moisture content of wood and possible moisture gradients;
m) a recording of the pathology and the identification of any areas of biological attack or mechanical
damage together with a first description of the possible causes of the damages (diagnosis).
4.2.2 Principles
According to EN 16096, all surveys and diagnostic methods used for the assessment of heritage
structures shall be semi or non-destructive so that no part of the structure shall be removed for
laboratory testing except for small samples used for identification of physical, mechanical properties,
timber species, biological agents and dendrochronology. In any case the sampling shall be according to
EN 16085 and shall not modify the load bearing capacity of the elements or have an impact on the other
properties (e.g. aesthetical or historical) of the timber structure.
4.2.3 Time dependant results
Note that the assessment results pertain to the moment of assessment since degradation is generally a
continuing process and further deterioration shall be anticipated until suitable remedial measures have
been adopted and become effective.
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4.2.4 Preliminary conditions for the survey
The following conditions shall be met:
a) Structural safety: the timber structure shall provide a reasonable safety level, otherwise propping or
shoring is necessary. If harmful wood preservatives are present, suitable measures for self-
protection shall be taken.
b) Health safety: it should be assessed if the preservation area is contaminated by timber preservatives
or mould spores (in case of fungi attacks) which would be harmful to health during further
assessments and conservation. This allows the definition of personal protective equipment
c) Accessibility/visibility: the timber members shall be made sufficiently accessible to allow for the
procedures to be carried out correctly. There are two aspects to this:
I) the ability to reach the timbers for inspection for which access will depend upon the nature of
the structure; it may be as basic as simple ladder or may require full scaffolding. Accessibility
has a great influence on the choice of the inspection technique;
II) the ability to see timbers which may be hidden by other elements of construction for which some
opening up of the fabric may be required. Such opening up might need to be agreed with
conservation authorities.
d) Lighting: proper light (quality and intensity) shall be used to permit a correct visual examination of
the timber members as a whole, the wood surface details, the joints, and the supports.
e) Cleaning: the surface of the wood shall not be covered or concealed in any way by debris, dirt and
dust. The surveyor shall be prepared to clean areas of the timber as required by using dry processes
(brushing, vacuum cleaning, air pressure.).
NOTE 1 In the case of decorated (painted) or covered timber members, wood surface accessibility and visibility
cannot be fully possible. In such cases, the condition survey report will detail what information was not obtained
and explain why.
NOTE 2 It is particularly important to determine the condition of the timber in highly stressed points of the
structure, near joints and at the supports. In all parts of the structure where a regular simple visual survey is not
possible, such as wall plates, the upper surfaces of purlins or the heads of beams inserted into supporting walls,
alternative condition survey and assessment methods can be required.
4.3 Desk study
A desk study is an integral part of any conservation process. Its purpose is to gather documents and other
sources of information (e.g. drawings, photos, oral testimonies) relating to the structure, which will
provide information on the historical aspects of the structure, the associated load history and any other
aspect which is related to its present status as a heritage building. Any available information regarding
previous interventions or restorations should be included.
4.4 Historical analysis
The purpose of this historical analysis is to describe the process and phases of building and identify any
subsequent events that had a significant influence on the behaviour of the structure. Such events could
be past changes of use, accidents, natural phenomena loads or deliberate interventions. If a more
thorough analysis is to be made, one should follow EN 16096.
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4.5 Preliminary visual survey
A preliminary visual survey is required to identify any obvious damage and susceptible and dangerous
zones of the structure, to set the assessment strategy, and to identify the possible need for any immediate
propping or safety measures, and/or restriction of access. The purpose of the preliminary survey is also
to plan the measured and detailed surv
...