ISO 18953
(Main)Steel structures — Structural bolting — Test methods to determine loss of pretension from faying surface coatings
Steel structures — Structural bolting — Test methods to determine loss of pretension from faying surface coatings
Test method to determine the loss of bolt pretension (preload) caused by creep of coatings, including consideration of the specific coating, coating thickness, and other related factors.
Structures en acier — Boulonnerie de construction métallique — Méthodes d’essai pour déterminer la perte de précontrainte due aux revêtements de surface
General Information
Standards Content (Sample)
FINAL DRAFT
International
Standard
ISO/FDIS 18953
ISO/TC 167
Steel structures — Structural
Secretariat: SN
bolting — Test methods to
Voting begins on:
determine loss of pretension from
2025-09-09
faying surface coatings
Voting terminates on:
2025-11-04
Structures en acier — Boulonnerie de construction métallique —
Méthodes d’essai pour déterminer la perte de précontrainte due
aux revêtements de surface
RECIPIENTS OF THIS DRAFT ARE INVITED TO SUBMIT,
WITH THEIR COMMENTS, NOTIFICATION OF ANY
RELEVANT PATENT RIGHTS OF WHICH THEY ARE AWARE
AND TO PROVIDE SUPPOR TING DOCUMENTATION.
IN ADDITION TO THEIR EVALUATION AS
BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO
LOGICAL, COMMERCIAL AND USER PURPOSES, DRAFT
INTERNATIONAL STANDARDS MAY ON OCCASION HAVE
TO BE CONSIDERED IN THE LIGHT OF THEIR POTENTIAL
TO BECOME STAN DARDS TO WHICH REFERENCE MAY BE
MADE IN NATIONAL REGULATIONS.
Reference number
ISO/FDIS 18953:2025(en) © ISO 2025
FINAL DRAFT
ISO/FDIS 18953:2025(en)
International
Standard
ISO/FDIS 18953
ISO/TC 167
Steel structures — Structural
Secretariat: SN
bolting — Test methods to
Voting begins on:
determine loss of pretension from
faying surface coatings
Voting terminates on:
Structures en acier — Boulonnerie de construction métallique —
Méthodes d’essai pour déterminer la perte de précontrainte due
aux revêtements de surface
RECIPIENTS OF THIS DRAFT ARE INVITED TO SUBMIT,
WITH THEIR COMMENTS, NOTIFICATION OF ANY
RELEVANT PATENT RIGHTS OF WHICH THEY ARE AWARE
AND TO PROVIDE SUPPOR TING DOCUMENTATION.
© ISO 2025
IN ADDITION TO THEIR EVALUATION AS
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO
LOGICAL, COMMERCIAL AND USER PURPOSES, DRAFT
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
INTERNATIONAL STANDARDS MAY ON OCCASION HAVE
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
TO BE CONSIDERED IN THE LIGHT OF THEIR POTENTIAL
or ISO’s member body in the country of the requester.
TO BECOME STAN DARDS TO WHICH REFERENCE MAY BE
MADE IN NATIONAL REGULATIONS.
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 Reference number
ISO/FDIS 18953:2025(en) © ISO 2025
ii
ISO/FDIS 18953:2025(en)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 General . 2
4.1 Principles .2
4.2 Significant variables .2
4.3 Pretension loss over time .3
5 Test equipment and preparation . 3
5.1 Test instrumentation .3
5.1.1 General .3
5.1.2 Bolt strain gages .3
5.1.3 Donut load cell .4
5.2 Test plate preparation .4
5.3 Test execution . .5
5.4 Data acquisition .5
6 Test assemblies . 5
6.1 General .5
6.2 Test A (Single-bolt test assembly) . .5
6.2.1 General .5
6.2.2 Geometric configuration .6
6.2.3 Bolting requirements .7
6.3 Test B (Multiple bolt test assembly) .7
6.3.1 General .7
6.3.2 Geometric configuration .7
6.3.3 Bolt pretensioning . .8
6.4 Test results .8
6.5 Test report .9
Annex A (informative) Bolt pretension measurements . 10
Bibliography .11
iii
ISO/FDIS 18953:2025(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 167, Steel and aluminium structures.
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/FDIS 18953:2025(en)
Introduction
This document has been prepared to provide provisions and guidance in the determination of the loss of
pretension of bolts in the presence of coatings on the faying surface of a joint. For pretensioned bolted joints,
the potential loss of pretension can be checked when the coatings applied to any of the individual coated
faying surfaces are thick or consist of a creep-prone material. In this context, coatings exclude adherent mill
scale, and include liquid applied coatings as well as coatings that are the result of a galvanizing or metallizing
process. Examples of coatings include duplex coating and thermal spray coating. Loss of pretension due to
other material, such as insulation, in the grip of a bolt is not considered in this document.
This document provides provisions and guidance on the measurement of pretension loss, considering that
the suitability of the protective coatings or coating systems for use in pretensioned bolted joints depends
on the extent of the possible system reserves due to the tightening procedure, the target of pretensioning,
and the losses over the service life of a structure. The test methods in this document can be also applied to
uncoated faying surfaces.
v
FINAL DRAFT International Standard ISO/FDIS 18953:2025(en)
Steel structures — Structural bolting — Test methods to
determine loss of pretension from faying surface coatings
1 Scope
This document specifies test methods to determine loss of pretension in high-strength bolts due to the
presence of coatings on the faying surface(s) of a bolted joint to be used in structural steelwork, when any
of the coatings are thick enough to affect the pretension in a bolt in the short term, or can show significant
deformation over time under sustained loads (creep-prone materials).
The presence within the grip of the bolt of other materials having considerably smaller stiffness than steel,
such as insulation, is not included in this test method.
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.
ISO 17607-1, Steel structures — Execution of structural steelwork — Part 1: General requirements and terms
and definitions
ISO 17607-6:2023, Steel structures — Execution of structural steelwork — Part 6: Bolting
ISO 376, Metallic materials — Calibration of force-proving instruments used for the verification of uniaxial
testing machines
ISO 8503-1, Preparation of steel substrates before application of paints and related products — Surface
roughness characteristics of blast-cleaned steel substrates — Part 1: Specifications and definitions for ISO
surface profile comparato
...
ISO/FDIS 18953:2025(en)
ISO/TC 167
Secretariat: SN
Date: 2025-06-26
Steel structures – — Structural bolting –— Test methods to
determine loss of pretension from faying surface coatings
Structures en acier – — Boulonnerie de construction métallique -— Méthodes d’essai pour déterminer la perte
de précontrainte due aux revêtements de surface
FDIS stage
ISO/FDIS 18953:2025(en)
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. Dede Blandonnet 8
CH-1214 Vernier, Geneva
Phone: + 41 22 749 01 11
Fax: +41 22 749 09 47
Email: E-mail: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii
ISO/FDIS 18953:2025(en)
Contents
Foreword . iv
Introduction . v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 General . 2
4.1 Principles . 2
4.2 Significant variables . 2
4.3 Pretension loss over time . 3
5 Test equipment and preparation . 4
5.1 Test instrumentation . 4
5.2 Test plate preparation . 5
5.3 Test execution . 5
5.4 Data acquisition . 6
6 Test assemblies. 6
6.1 General . 6
6.2 Test A (Single-bolt test assembly) . 6
6.3 Test B (Multiple bolt test assembly) . 8
6.4 Test results . 10
6.5 Test report . 10
Annex A (informative) Bolt pretension measurements . 11
Bibliography . 12
iii
ISO/FDIS 18953:2025(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 167, Steel and aluminium structures.
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/FDIS 18953:2025(en)
Introduction
This document has been prepared to provide provisions and guidance in the determination of the loss of
pretension of bolts in the presence of coatings on the faying surface of a joint. For pretensioned bolted joints,
the potential loss of pretension shouldcan be checked when the coatings applied to any of the individual coated
faying surfaces are thick or consist of a creep-prone material. In this context, coatings exclude adherent mill
scale, and include liquid applied coatings as well as coatings that are the result of a galvanizing or metallizing
process. Examples of coatings include duplex coating and thermal spray coating. Loss of pretension due to
other material, such as insulation, in the grip of a bolt is not considered in this document.
This document provides provisions and guidance on the measurement of pretension loss, considering that the
suitability of the protective coatings or coating systems for use in pretensioned bolted joints depends on the
extent of the possible system reserves due to the tightening procedure, the target of pretensioning, and the
losses over the service life of a structure. The test methods in this document can be also applied to uncoated
faying surfaces.
v
ISO/FDIS 18953:2025(en)
Steel structures – — Structural bolting –— Test methods to determine
loss of pretension from faying surface coatings
1 Scope
This document specifies test methods to determine loss of pretension in high-strength bolts due to the
presence of coatings on the faying surface(s) of a bolted joint to be used in structural steelwork, when any of
the coatings are thick enough to affect the pretension in a bolt in the short term, or mightcan show significant
deformation over time under sustained loads (creep-prone materials).
The presence within the grip of the bolt of other materials having considerably smaller stiffness than steel,
such as insulation, is not included in this test method.
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.
ISO 17607--1, Steel structures — Execution of structural steelwork — Part 1: General requirements and terms
and definitions
ISO 17607--6:2023, Steel structures — Execution of structural steelwork — Part 6: Bolting
ISO 376, Metallic materials — Calibration of force-proving instruments used for the verification of uniaxial
testing machines
ISO 8503--1, Preparation of steel substrates before application of paints and related products — Surface
roughness characteristics of blast-cleaned steel substrates — Part 1: Specifications and definitions for ISO surface
profile comparators for the assessment of abrasive blast-cleaned surfaces
ASTM B659, Standard Guide for Measuring Thickness of Metallic and Inorganic Coatings
ASTM E4, Standard Practices for Force Calibration and Verification of Testing Machines
SSPC-PA 2, Procedure for Determining Conformance to Dry Coating Thickness Requirements
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 17607-1 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 3.1
duplex coating
combination of two layers of different materials to give higher corrosion resistance
Note 1 to entry: The layers can be two layers of electroplated metals or can be a plated metal followed by an organic layer.
[SOURCE: ISO 2080:2022, 3.2.109]
ISO/FDIS 18953:2025(en)
3.2 3.2
thermal spray coating
high-temperature process by which finely divided metallic or non-metallicnonmetallic materials are
deposited in a molten or semi-molten condition to form a coating on a surface when cooled
[SOURCE: ISO 17945:2015, 3.12]
4 General
4.1 Principles
Steel surfaces of pretensioned bolted connections can be coated to protect them against corrosion, to change
the appearance of the surfaces, or to achieve an increased slip resistance.
The suitability for use of the chosen coatings or coating systems in pretensioned bolted connections depends
on the extent of the possible system reserves due to the tightening procedure used (i.e. the actual pretension
level in relation to the nominal minimum pretension force) and on the pretension losses over the service life
of a structure.
EXAMPLE If pretensioning is performed by the combined method (see ISO 17607-6:2023, Annex J) or turn-of-nut
method (see ISO 17607-6:2023, Annex M), a system reserve of approximately 30 % (1,3 × nominal minimum pretension)
can be expected.
The test method relates the loss of pretension to the thickness and characteristics of the coating. This test
method does not assess the effects of the thickness and characteristics of the coating on the friction coefficient
of the faying surfaces of slip-resistant connections.
For pretensioned bolted connections, the potential loss of pretension should be checked when the coatings
applied to any of the faying surfaces are thicker than 100 μm or consist of a creep-prone material.
4.2 Significant variables
The applicability of the test results is limited to cases where all significant variables are consistent with the
test specimens. The following variables shall be considered as significant for the test results:
a) a) the surface treatment of the faying surface(s);
b) b) the composition of the coating;
c) c) the treatment of each coating layer in the case of multi-layer systems;
d) d) the thickness of the coating, or of the coating layers in the case of multi-layer systems for each
coated surface;
e) e) the curing procedure and parameters;
f) f) the minimum time interval between application of the coating and the pretensioning of the
bolts in the connection;
g) g) the dimensions, coating characteristics, and property class of the bolt, the nut, and the
washer(s);
h) h) the number and configuration of washers, if needed;
i) i) the gage and spacing of the bolts in the connection, as applicable;
ISO/FDIS 18953:2025(en)
j) j) the pretension level from initial tensioning until completion of test (see 5.45.4 for frequency of
measurement);
k) k) the pretensioning sequence used, if applicable.
NOTE In some cases, the contact surface under either the head or the nut, or both, can vary, affecting the pressure
intensity and distribution.
4.3 Pretension loss over time
The test methods described in this standard result, as a minimum, in the establishment of the pretension loss
vs. logarithm of time diagram that can be extrapolated to the expected lifespan of the joint, as shown in
Figure 1Figure 1. The minimum duration of a test is set at 14 days, after which an extrapolation to the design
life of the structure can be permitted.
Key
X time (log time)
Y loss of pretension
1 measurement data tLd design life of
...
Questions, Comments and Discussion
Ask us and Technical Secretary will try to provide an answer. You can facilitate discussion about the standard in here.