SIST EN 17121:2019

Conservation of cultural heritage - Historic timber structures - Guidelines for the on-site assessment of load-bearing timber structures

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.

Erhaltung des kulturellen Erbes - Historische Holzkonstruktionen - Leitlinien für die Bewertung vor Ort von tragenden Holzkonstruktionen

Dieses Dokument enthält Leitlinien zu den Kriterien für die Vor-Ort-Bewertung von tragenden Holzkonstruktionen in denkmalgeschützten Bauten. Es richtet sich an alle Anwender, die an der Erhaltung von Bauten des Kulturerbes mit hölzernen Bauelementen beteiligt sind, von den verantwortlichen Eigentümern oder Behörden bis hin zu den eingesetzten Fachleuten. Es sollte auch dabei helfen, Entscheidungen bezüglich des Bedarfs an Sofortmaßnahmen zu treffen. Sein Zweck ist sicherzustellen, dass die Zustandserhebung und -bewertung die notwendigen Daten für die historische Analyse, die Bewertung der Tragfähigkeit und die Planung von Eingriffen bereitstellen.
Dieses Dokument gilt für jede Art von Holzbauteil und für jede Art von historischer Holzkonstruktion. Es gilt nicht für Holzbauteile aus Holzwerkstoff und Brettschichtholz.
Dieses Dokument stellt ein umfassendes Verfahren für die Bewertung vor Ort bereit. Mit einer praktischen und technischen Bewertung des gefundenen Schadens und auf Grundlage der Verantwortung der einbezogenen Fachleute kann eine ausreichende Bewertung auch dann durchgeführt werden, wenn nicht sämtliche Schritte eingehalten werden.
Es wird erwartet, dass dieses Dokument in jedem einzelnen Land in Übereinstimmung mit nationalen Gesetzen und Vorschriften angewendet wird.

Conservation du patrimoine culturel - Structures en bois du patrimoine - Lignes directrices relatives à l’évaluation sur site des stuctures porteuses en bois

Le présent document fournit des lignes directrices relatives aux critères à utiliser pour l’évaluation sur site des structures porteuses en bois des bâtiments patrimoniaux. Il est destiné à toutes les personnes concernées par la conservation des bâtiments patrimoniaux qui contiennent des éléments en bois, des propriétaires des bâtiments ou des autorités qui en ont la responsabilité aux professionnels auxquels il est fait appel. Il convient qu’il aide également à la prise de décisions quant à la nécessité de mesures immédiates. Il vise à garantir que l’évaluation et le constat d’état fournissent les données nécessaires à l’étude historique, l’évaluation de la sécurité structurelle et la planification des travaux d’intervention.
Ce document s’applique à tout type d’élément en bois et à tout type de structure historique en bois. Il ne s’applique pas aux éléments en bois faits de panneaux de bois transformé et de lamellé-collé.
Ce document indique l’intégralité de la procédure à suivre pour l’évaluation sur site. Avec une évaluation pratique et technique des dommages constatés, et sous la responsabilité des professionnels concernés, il est possible d’effectuer une évaluation sans respecter la totalité des étapes.
Ce document doit normalement être appliqué dans chaque pays conformément à la législation et à la réglementation nationales.
Ce document doit normalement être appliqué dans chaque pays conformément à la législation et à la réglementation nationales.

Ohranjanje kulturne dediščine - Zgodovinske lesene konstrukcije - Smernice za ocenjevanje nosilnih lesenih konstrukcij na kraju samem

Ta standard podaja vodila glede meril, ki jih je treba uporabiti za ocenjevanje nosilnih lesenih konstrukcij v zgodovinskih lesenih zgradbah na kraju samem. Namenjen je vsem, ki se ukvarjajo z ohranjanjem zgradb kulturne dediščine, ki vsebujejo lesene elemente, od lastnikov zgradb ali državnih organov, ki so odgovorni zanje, do strokovnjakov, ki se jih zaposli. Naj bi bil v pomoč pri sprejemanju odločitev glede potrebe po takojšnjih ukrepih. Njegov cilj je zagotoviti, da raziskave in ocena stanja zagotovijo potrebne podatke za zgodovinsko analizo, oceno varnosti konstrukcije in načrtovanje intervencijskih del. Ta dokument se uporablja za vse vrste lesenih členov in za vse vrste zgodovinskih lesenih konstrukcij. Ne uporablja se za lesene člene, izdelane iz proizvedenih lesnih plošč in lepljenega laminiranega lesa. Ta dokument določa celovit postopek za oceno na kraju samem. S praktičnim in tehničnim vrednotenjem odkrite škode in na podlagi odgovornosti vpletenih strokovnjakov je mogoče ustrezno oceno podati tudi brez upoštevanja vseh korakov. Pričakovati je, da se bo v različnih državah dokument uporabljal v skladu z nacionalno zakonodajo in predpisi.

General Information

• Status: Published
• Public Enquiry End Date: 13-Sep-2017
• Publication Date: 08-Oct-2019
• ICS: 91.080.20 - Timber structures
• ICS: 97.195 - Items of art and handicrafts. Cultural property and heritage
• Technical Committee: I13 - Imaginarni 13
• Current Stage: 6060 - National Implementation/Publication (Adopted Project)
• Start Date: 04-Oct-2019
• Due Date: 09-Dec-2019
• Completion Date: 09-Oct-2019
• Ref Project: EN 17121:2019 - Conservation of cultural heritage - Historic timber structures - Guidelines for the on-site assessment of load-bearing timber structures

Overview

EN 17121:2019 - published by CEN - provides clear, practical guidelines for the on-site assessment of load-bearing timber structures in the conservation of cultural heritage. The standard targets historic timber members and whole timber structural systems in heritage buildings to ensure condition survey and assessment deliver the data needed for historical analysis, structural safety evaluation and planning of interventions. It applies to all kinds of traditional timber members but explicitly excludes engineered wood-based panels and glued laminated timber (glulam). EN 17121 emphasises minimum intervention, professional responsibility and alignment with national legislation.

Key topics and requirements

• Assessment procedure: A two-phase approach - a preliminary assessment (desk study, preliminary visual survey, measured survey and preliminary structural analysis) followed by a detailed survey when required.
• Documentation & historical analysis: Collect load history, construction methods and conservation aims to inform decisions and interventions.
• Condition survey elements:
  • Identification of wood species
  • Estimation of wood moisture content and moisture gradients
  • Characterisation of biological damage (fungal rot, insect attack)
  • Detection and classification of defects and strength-reducing characteristics
  • Detailed survey of timber joints and connections
• Structural assessment: Determination of critical areas/cross-sections, effective cross-sections and strength grading for structural analysis and serviceability assessment.
• Testing methods: Hierarchy from non-destructive and semi-destructive tests to in-situ load testing as appropriate; Annex A lists tools for non-destructive analyses.
• Professional judgement: The standard allows a sufficient assessment via practical evaluation even when not all steps are possible, but places responsibility on involved professionals.
• Compliance: Apply in accordance with national legislation and other normative references.

Applications and users

EN 17121 is intended for:

• Conservation professionals (architects, conservators, surveyors)
• Structural engineers and timber specialists assessing load-bearing capacity
• Heritage building owners, managers and public authorities
• Contractors and inspectors involved in repair, strengthening or emergency stabilization

Practical uses:

• Prioritising immediate safety measures and emergency shoring
• Planning minimal-impact repairs or strengthening works
• Preparing technical documentation for interventions and permits
• Guiding non-destructive testing and sampling strategies for historic timber

Related standards

Normative and related references cited in EN 17121 include:

• EN 335 (durability/use classes)
• EN 1912 (strength classes/visual grades)
• EN 1995-1-1 (Eurocode 5) (timber design)
• EN 14081-1:2016 (strength graded timber)
• EN 16085 / EN 16096 (sampling and condition survey of cultural property)
• ISO 13822 (survey of existing structures - heritage considerations referenced)

Keywords: EN 17121, historic timber structures, conservation of cultural heritage, on-site assessment, load-bearing timber structures, timber inspection, condition survey, non-destructive testing, structural analysis, moisture content, biological damage, strength assessment.

Standards Content (Sample)

SLOVENSKI STANDARD
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
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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.

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

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.
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
that is required but in general a more detailed survey will be needed .

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.
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
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
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
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.
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;
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.
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.
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 survey and to note what provisions are required to provide adequate
access to the structure. It shall note additional measures needed to provide, lighting and any cleaning or
opening up of the structure.
4.6 Measured survey
4.6.1 General
The assessment of the structure requires as a minimum a measured survey that shows the general
arrangement of the structural members, various phases of execution and modification of the structure,
the principal dimensions, the sections and the sizes of the structural members and their joints that are
critical for the preliminary structural assessment.
4.6.2 Geometric survey
The geometric survey shall comprise
a) a drawn survey of the structure, fully dimensioned. This survey should also include non-structural
members and note possible interactions with other structures;
b) dimensions of the principal structural members;
c) a note of the joint types, materials of the fasteners used, number and typical dimensions, their
spacing, end and edge distances, dimensions of the cuttings, where these could be clearly seen;
d) some timber structures, principally roofs, may be supported by masonry structures and in such cases
the way in which the masonry interacts with the timber needs to be indicated.
In some cases, simple 2-dimensional drawings may need to be supplemented with 3-dimensional
sketches or dimensioned axonometric drawings to explain the structure.
4.6.3 Diagnostic survey
This shall comprise:
a) Indication of wood species. Initially this may be conducted through a quick visual evaluation, at least
placing the timbers within a clear group of species, if only to distinguish between softwood and
hardwood.
NOTE 1 Structures sometimes have a mixture of species, occasionally because there is a difference in species
used for primary and secondary members.
b) Determination of wood moisture content. Moisture content and temperature of ambient air and
indication of the weather conditions shall be recorded. Initially measurements are taken only in
specific locations of the structure, where the local conditions suggest the presence of higher moisture
content. However, measurements have also to be taken in other locations to determine the general
equilibrium moisture content of the timber at the time of the condition survey. Measurements shall
be made by electrical resistance method in accordance with EN 16682.
c) Determination of environmental conditions. Service Classes defined in EN 1995-1-1 (relevant for
mechanical properties) and Use Classes defined in EN 335 (related to biological hazard) to which the
timber member is exposed shall be identified.
NOTE 2 Different parts or members of a timber structure can belong to different use classes. If differences
between transitory (previous to or during a possible intervention) and target (final) conditions are important, both
situations will be identified.
d) Recording of pathology. Strength reducing characteristics and damage survey should comprise:
1) a record of the location and extension of any mechanical, biological or fire damage. The agent
responsible for biological damage shall be identified and a note made of whether or not it is
active;
2) type, location and extension of main natural strength reducing characteristics (knots and slope
of grain);
3) a note of any timbers missing from the structure;
4) a note of the kind and degree of restraint provided by supports;
5) a note of obvious changes or damage, missing fasteners or deviations from the ideal joint form;
6) a note of any indications of changes made to the structure during its life. These might have been
made to accommodate changes in use, changes in the plan or external form of the building or
previous repairs;
7) a note of any places where timbers could not be clearly seen and where opening up of the
structure might be required;
8) a note of any structural distress in the timbers, such as deep timber cracks, structural failure of
members and joints or excessive deflections;
9) a note of any distress in supporting structures or members and in non-load-bearing timber
elements that interact with the timber structure.
The last three cases need to be guided by the structural analysis that will help locating the critical
areas/zones of the timber structures.
4.7 Structural analysis
Following the survey, the person responsible for the structural analysis should carry out a preliminary
analysis of the structure. This shall describe the structural system i.e. shall show how the loads are
brought to the ground. The type and the approximate stress levels in the principal structural members
should also be determined.
The main aim of this structural analysis is the identification of highly stressed areas of the structure
where more attention is required, particularly for the critical areas/zones identified during condition
survey. This should also indicate the safety level of the structure.
NOTE 1 The definition of a critical area is given under Clause 3, Terms and Definitions.
NOTE 2 The identification of the highly stressed areas is important because in some cases the assessment of all
members and joints is impossible or not necessary.
In some cases, a simple and conservative assessment of the structural system, of the member properties
and their effective cross-sections, as well as joint performance, may be sufficient. Alternatively, the
preliminary assessment shall indicate the need for a more detailed structural analysis. In the case of
spatial load bearing systems, numerical analysis with 3-dimensional models may be necessary.
Because the numerical analysis of an historic structure is a difficult process with many uncertainties, it is
not always possible to measure the strength and stiffness of structural members and joints, the load
history and the boundary conditions, but they shall be estimated. A common mistake in numerical
analysis is to assume that the joints are capable of transferring forces which they cannot in fact. It may
also be necessary to allow for members that have become sufficiently decayed to have shed significant
loads to other members. Therefore one may well have to consider more than one load path, possibly
reviewing the structure itself to choose between them.
It may also be necessary to allow for members that have become sufficiently rotten to have shed
significant loads to other members. Therefore, one may well have to consider more than one load path,
possibly reviewing the structure itself to choose between them.
The analysis shall consider the loads appropriate for the proposed use of the building and the region (e.g.
live, seismic, snow and wind loads, if applicable).
In a conservative approach, EN 1990 (intended for new structures regarding the actions and the
combinations of actions to take into account) and mainly the new under revision edition of EN 1998-3,
that incorporated a new chapter for timber existing structures can be followed. In a more detailed
analysis, if uncertainties about load history and material properties can be reduced, other combinations
of actions may be considered providing suitable safety level is guaranteed.
Although verification of serviceability limit states may also be important in certain cases, higher limits
(namely for deflection and vibrations) than those defined in new codes may be acceptable for heritage
structures.
The overall stability of the structure shall be assessed as well as the behaviour of individual members or
individual frames. Therefore, the adequacy of braces needs to be considered as well as that of the
principal structural members.
One purpose of the analysis is to identify timber members and joints that are carrying excessive load or
where the condition of the timber and hence its load carrying capacity is critical to the behaviour of the
structure as a whole. The identification of such areas is to be used for the detailed survey of the structure.
Some carpentry joints may be able to transfer moments to some extent, as well as shear and compression
loads. Only where suitable data about their moment-rotation capacity is available may their contribution
to the overall structural performance be taken into account instead of assuming them to be pinned joints.
It is important that the results of the structural analysis are compatible with the recorded pathology of
the structures, such as failure of members or joints, excessive deflections, etc.
4.8 Preliminary report
The preliminary report shall include:
a) dimensioned drawings of the whole structure and technical notes describing and the structural
system and its principal features, as listed in Clause 4.6.2;
b) documentation of problems and pathology, as listed in Clause 4.6.3;
c) a note or map of those timbers and joints that have not been inspected and why;
d) Service Classes and Use Classes;
e) structural assessment showing the assumptions for the calculations, the corresponding stresses and
deformations;
1) identification of the different structural member types according to the type of stresses;
2) location of critical areas, either from the point of view of the level of stresses or due to the
presence of timber strength reducing characteristics or biological damage;
f) existing wood treatments;
g) interpretation of the causes of damage and the general behaviour of the structure (diagnosis).
It shall identify subsequent survey work required, fully justifying the need for any recommended work
and noting any places where there are needs to open up the structure to enable the detailed survey work
to be carried out.
5 Detailed survey
5.1 Overview
The preliminary structural analysis should be used to identify areas of the structure subject to high
stresses where a careful inspection to determine the extent of any damage or deterioration and hence the
residual capacity of the member.
5.2 General
The aim of the survey is to obtain the following information for all members and structural joints of the
structure in greater detail:
— wood species;
— moisture content;
— strength grade or strength values;
— characterization of biological damage.
These are always particularly important for critical areas of the structure and will enable stress and
deformation levels to be recalculated to support the final diagnostic report that follows.
The survey should include dimensioned drawings of all significant structural timbers, noting also features
such as tool marks and marks made by the carpenters in setting out the framing. A note of the method of
conversion, identifying sawn faces and axe hewn or adzed faces is also significant for the historic and
structural contextualisation of the members and the structure as a whole.
The diagnostic report shall contain all the detailed information concerning the points previously listed.
As with the preliminary assessment, the detailed survey requires the conditions described in 4.2.1 for
safety, accessibility, visibility, lighting and cleaning.
5.3 Identification of wood species
This may simply be by visual evaluation. In some cases, small samples for laboratory identification of
timber (by microscopic analysis) may be required. The method used shall be clearly stated.
NOTE A detailed/full identification of the species is not always possible but it can be sufficient to identify the
genus.
Even where historic records provide information on the timber used, this shall be confirmed.
5.4 Estimating wood moisture content and moisture gradients
Estimations shall be made for all elements where there is higher risk of biological attack, such as close to
the wall supports on floor beams and on roof edges and corners. Estimations are to be made by electrical
resistance methods following suggestions and recommendations in EN 16682 and in A.2 of this standard.
Estimations should be repeated during different periods of the year and during different climate
conditions.
In the presence of fungal attack or certain insect attack, such as subterranean termite infestation, which
attack is favoured by high moisture content of timber, moisture gradients should be evaluated through
moisture content readings to be taken at several distances from the ends of the element and at various
depths from the surface using appropriate non-destructive methods such as insulated electrodes. The
equipment used for that purpose shall be calibrated for the specific species or group of species and
readings corrected for the environmental conditions, according to the equipment manufacturer’s
specifications.
NOTE Moisture content estimations will help to determine the environmental conditions and the biological
risk level. This can well vary along the length of a member according to the local environments. This data will also
help to establish the potential for further progress of biological deterioration and to plan subsequent interventions
aimed at reducing the risk.
5.5 Characterization of biological damage
5.5.1 Identification of biological damage
The specific cause of biological damage shall be identified: fungal, rot or insect attack.
Possible interaction between this and environmental conditions shall be noted.
Therefore, particular attention shall be given to the determination of moisture conditions of other
materials (masonry, metal, concrete etc.) in direct contact with timbers.
5.5.2 Assessment of effective cross-section
For sections with a biological attack, or where insect attack is confined to a well-defined area of the cross-
section (normally the sapwood of a durable species), the effective (residual) cross-section shall be
measured and used to calculate the strength. The estimation of the effective cross-section is normally
difficult (a safety margin should be used around the attacked zone). Non-destructive or low-destructive
techniques may be used to help establish the extension of the alteration caused by the attack and the
method used shall be recorded.
The strength of wood attacked by fungal rot is always equal to zero.
Insect attack on the surface of large cross-section members may be ignored.
5.6 Strength assessment of timber
5.6.1 General
Members judged to be important in the preliminary report (4.8 e) for the strength, serviceability and
stability of the structure shall be strength graded according to the provisions of this standard.
It shall be noted that the strength g
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