ISO 4355:2013
(Main)Bases for design of structures — Determination of snow loads on roofs
Bases for design of structures — Determination of snow loads on roofs
ISO 4355:2013 specifies methods for the determination of snow load on roofs. It can serve as a basis for the development of national codes for the determination of snow load on roofs. National codes should supply statistical data of the snow load on ground in the form of zone maps, tables, or formulae. The shape coefficients presented in ISO 4355:2013 are prepared for design application, and can thus be directly adopted for use in national codes, unless justification for other values is available. For determining the snow loads on roofs of unusual shapes or shapes not covered by ISO 4355:2013 or in national standards, it is advised that special studies be undertaken. These can include testing of scale models in a wind tunnel or water flume, especially equipped for reproducing accumulation phenomena, and should include methods of accounting for the local meteorological statistics. Examples of numerical methods, scale model studies, and accompanying statistical analysis methods are described in ISO 4355:2013 (Annex G). The annexes of ISO 4355:2013 describing methods for determining the characteristic snow load on the ground, exposure coefficient, thermal coefficient, and loads on snow fences are for information only as a consequence of the limited amount of documentation and available scientific results. In some regions, single winters with unusual weather conditions can cause severe load conditions not taken into account by ISO 4355:2013. Specification of standard procedures and instrumentation for measurements is not dealt with in ISo 4355:2013.
Bases du calcul des constructions — Détermination de la charge de neige sur les toitures
General Information
Relations
Standards Content (Sample)
INTERNATIONAL ISO
STANDARD 4355
Third edition
2013-12-01
Bases for design of structures —
Determination of snow loads on roofs
Bases du calcul des constructions — Détermination de la charge de
neige sur les toitures
Reference number
©
ISO 2013
© ISO 2013
All rights reserved. Unless otherwise specified, 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
Case postale 56 • CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail copyright@iso.org
Web www.iso.org
Published in Switzerland
ii © ISO 2013 – All rights reserved
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Snow loads on roofs . 3
4.1 General function describing intensity and distribution of the snow load on roofs . 3
4.2 Approximate formats for the determination of the snow load on roofs . 3
4.3 Partial loading due to melting, sliding, snow redistribution, and snow removal . 4
4.4 Ponding instability . 4
5 Characteristic snow load on the ground . 4
6 Snow load coefficients . 4
6.1 Exposure coefficient . 4
6.2 Thermal coefficient . 6
6.3 Surface material coefficient . 6
6.4 Shape coefficients . 6
Annex A (informative) Background on the determination of some snow parameters .8
Annex B (normative) Snow load distribution on selected types of roof .13
Annex C (informative) Determination of the exposure coefficient for small roofs .28
Annex D (informative) Determination of thermal coefficient .31
Annex E (informative) Roof snow retention devices .34
Annex F (informative) Snow loads on roof with snow control .36
Annex G (informative) Alternative methods to determine snow loads on roofs not covered by the
prescriptive methods in this International Standard .38
Bibliography .39
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 documents 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).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation on the meaning of ISO specific terms and expressions related to conformity
assessment, as well as information about ISO’s adherence to the WTO principles in the Technical Barriers
to Trade (TBT) see the following URL: Foreword - Supplementary information
The committee responsible for this document is ISO/TC 98, Bases for design of structures, Subcommittee
SC 3, Loads, forces and other actions.
This third edition cancels and replaces the second edition (ISO 4355: 1998), which has been technically
revised.
iv © ISO 2013 – All rights reserved
Introduction
The intensity and distribution of snow load on roofs can be described as functions of climate, topography,
shape of building, roof surface material, heat flow through the roof, and time. Only limited and local data
describing some of these functions are available. Consequently, for this International Standard it was
decided to treat the problem in a semi-probabilistic way.
The characteristic snow load on a roof area, or any other area above ground which is subject to snow
accumulation, is in this International Standard defined as a function of the characteristic snow load on
the ground, s , specified for the region considered, and a shape coefficient which is defined as a product
function, in which the various physical parameters are introduced as nominal coefficients.
The shape coefficients will depend on climate, especially the duration of the snow season, wind, local
topography, geometry of the building and surrounding buildings, roof surface material, building
insulation, etc. The snow can be redistributed as a result of wind action; melted water can flow into
local areas and refreeze; snow can slide or can be removed.
In order to apply this International Standard, each country will have to establish maps and/or other
information concerning the geographical distribution of snow load on ground in that country. Procedures
for a statistical treatment of meteorological data are described in Annex A.
INTERNATIONAL STANDARD ISO 4355:2013(E)
Bases for design of structures — Determination of snow
loads on roofs
1 Scope
This International Standard specifies methods for the determination of snow load on roofs.
It can serve as a basis for the development of national codes for the determination of snow load on roofs.
National codes should supply statistical data of the snow load on ground in the form of zone maps,
tables, or formulae.
The shape coefficients presented in this International Standard are prepared for design application, and
can thus be directly adopted for use in national codes, unless justification for other values is available.
For determining the snow loads on roofs of unusual shapes or shapes not covered by this International
Standard or in national standards, it is advised that special studies be undertaken. These can include
testing of scale models in a wind tunnel or water flume, especially equipped for reproducing accumulation
phenomena, and should include methods of accounting for the local meteorological statistics. Examples
of numerical methods, scale model studies, and accompanying statistical analysis methods are described
in Annex G.
The annexes describing methods for determining the characteristic snow load on the ground, exposure
coefficient, thermal coefficient, and loads on snow fences are for information only as a consequence of
the limited amount of documentation and available scientific results.
In some regions, single winters with unusual weather conditions can cause severe load conditions not
taken into account by this International Standard.
Specification of standard procedures and instrumentation for measurements is not dealt with in this
International Standard.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. For dated references, only the edition cited applies. For undated
references, the latest edition of the referenced document (including any amendments) applies.
1)
ISO 2394 , General principles on reliability for structures
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
characteristic value of snow load on the ground
s
load with a specified annual exceedance probability
Note 1 to entry: It is expressed in kilonewton per square metre (kN/m ).
Note 2 to entry: In meteorology, the term “weight of the ground snow cover” is also used.
1) In process of revision.
3.2
shape coefficient
μ
coefficient which defines the amount and distribution of the snow load on the roof over a cross section
of the building complex and primarily depends on the geometrical properties of the roof
3.3
value of snow load on roofs
s
function of the characteristic snow load on the ground, s , and appropriate shape coefficients
Note 1 to entry: The value of s is also dependent on the exposure of the roof and the thermal conditions of the
building.
Note 2 to entry: It refers to a horizontal projection of the area of the roof.
Note 3 to entry: It is expressed in kilonewton per square metre (kN/m ).
3.4
basic load coefficient
µ
b
coefficient defining the reduction of the snow load on the roof due to a slope of the roof, β, and the
surface material coefficient, C
m
3.5
drift load coefficient
µ
d
coefficient which defines the amount and redistribution of additional load on a leeward side or part of
a roof, depending on the exposure of the roof to wind, C , and the geometrical configurations of the roof
e
3.6
slide load coefficient
µ
s
coefficient defining the amount and distribution of the slide load on a lower part of a roof, or a lower
level roof
3.7
exposure coefficient
C
e
coefficient which accounts for the effects of the roof’s exposure to wind
3.8
exposure coefficient for small roofs
C
e0
exposure coefficient for small roofs with effective roof length shorter than 50 m
3.9
effective roof length
l
c
length of the roof influenced by exposure coefficient given as a function of roof dimensions
3.10
thermal coefficient
C
t
coefficient defining the change in snow load on the roof as a function of the heat flux through the roof
Note 1 to entry: C , in some cases, can be greater than 1,0. Further guidance is given in 6.2 and Annex D.
t
2 © ISO 2013 – All rights reserved
3.11
surface material coefficient
C
m
coefficient defining a reduction of the snow load on sloped roofs made of surface materials with low
surface roughness
3.12
equivalent snow density
ρ
e
density for calculating the annual maximum snow load from annual maximum snow depth
3.13
snow density
ρ
ratio between snow load and snow depth
4 Snow loads on roofs
4.1 General function describing intensity and distribution of the snow load on roofs
Formally, the snow load on roofs can be defined as a function, F, of several parameters:
sF= sC,,CC,, μμ,, μ
()
0 et mb ds
(1)
where the symbols are as defined in Clause 3.
While C , C , and C are assumed constant for a roof or a roof surface, µ , µ , and µ generally vary
e t m b d s
throughout the roof.
4.2 Approximate formats for the determination of the snow load on roofs
This International Standard defines the snow load on the roof as a combination of a basic load part, s ,
b
a drift load part, s , and a slide load part, s . Thus, for the most unfavourable condition (lower roof on
d s
leeward side):
ss=+"" ss""+
bd s
(2)
where “+” implies “to be
...
Questions, Comments and Discussion
Ask us and Technical Secretary will try to provide an answer. You can facilitate discussion about the standard in here.