1、 ISO 2013 Bases for design of structures Determination of snow loads on roofs Bases du calcul des constructions Dtermination de la charge de neige sur les toitures INTERNATIONAL STANDARD ISO 4355 Third edition 2013-12-01 Reference number ISO 4355:2013(E) ISO 4355:2013(E)ii ISO 2013 All rights reserv
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4、 4355:2013(E) ISO 2013 All rights reserved iii Contents Page Foreword iv Introduction v 1 Scope . 1 2 Normative references 1 3 T erms 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
5、 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 Sno w load c oefficients 4 6.1 Exposure coefficient . 4 6.2 Thermal coefficient . 6 6.3 Surface m
6、aterial 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) Det ermination of the e xposur e c oefficient for small r oofs .28 Annex D
7、 (informative) Det ermination of thermal c oefficient .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 Int
8、ernational Standard .38 Bibliography .39 ISO 4355:2013(E) 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 committe
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14、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 t
15、echnically revised.iv ISO 2013 All rights reserved ISO 4355:2013(E) 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 desc
16、ribing 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 Sta
17、ndard defined as a function of the characteristic snow load on the ground, s 0 , 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
18、 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 s
19、lide 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 Ann
20、ex A. ISO 2013 All rights reserved v 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
21、 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 j
22、ustification 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 f
23、lume, 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 metho
24、ds 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 c
25、onditions 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 norma
26、tively 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. ISO 2394 1) , General principles on reliability for structures
27、 3 T erms a nd definiti ons For the purposes of this document, the following terms and definitions apply. 3.1 characteristic value of snow load on the ground s 0 load with a specified annual exceedance probability Note 1 to entry: It is expressed in kilonewton per square metre (kN/m 2 ). Note 2 to e
28、ntry: In meteorology, the term “weight of the ground snow cover” is also used. 1) In process of revision. INTERNATIONAL ST ANDARD ISO 4355:2013(E) ISO 2013 All rights reserved 1 ISO 4355:2013(E) 3.2 s h ap e c o e f f ic ie nt coefficient which defines the amount and distribution of the snow load on
29、 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 0 , and appropriate shape coefficients Note 1 to entry: The value of s is also dependent
30、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 2 ). 3.4 b a s i c l o a d c o e f f i c i e n t b coefficient defining the red
31、uction of the snow load on the roof due to a slope of the roof, , and the surface material coefficient, C m 3.5 d r i f t l o a d c o e f f i c i e n t 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 r
32、oof to wind, C e , and the geometrical configurations of the roof 3.6 sl i d e l o a d c o e f f i c i e n t 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 e x p o s u r e c o e f f i c i e n t C e coefficient which accounts
33、for the effects of the roofs exposure to wind 3.8 e x p o s u r e c o e f f i c i e n t f o r s m a l l r o o f s 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 func
34、tion of roof dimensions 3.10 t her m a l c o ef f ic ient 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 t , in some cases, can be greater than 1,0. Further guidance is given in 6.2 and Annex D.2 ISO 2013 All rights res
35、erved ISO 4355:2013(E) 3.11 s u r f a c e m a t e r i a l c o e f f i c i e n t 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 e density for calculating the annual maximum snow load from annual m
36、aximum snow depth 3.13 snow density 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: (1) where the symbols are as
37、defined in Clause 3. While C e , C t , and C mare assumed constant for a roof or a roof surface, b , d , and sgenerally vary 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
38、a basic load part, s b , a drift load part, s d , and a slide load part, s s . Thus, for the most unfavourable condition (lower roof on leeward side): ss ss (2) where “+” implies “to be combined with”. Effects of the various parameters are simplified by the introduction of product functions. (3) ss
39、(4) ss (5) The basic roof snow load, s b , is uniformly distributed in all cases, 12except for curved roofs, where the distribution varies with the slope, (see B.4). The basic load defines the load on a horizontal roof, and the load on the windward side of a pitched roof. Since any direction can be
40、the wind direction, the basic load is treated as a symmetrical load on a symmetrical roof, thus defining a major part of the total load on the leeward side as well. ISO 2013 All rights reserved 3 ISO 4355:2013(E) The drift load is the additional load that can accumulate on the leeward side due to dr
41、ifting. The slide load is the load that can slide from an upper roof onto a lower roof, or a lower part of a roof. 4.3 Partial loading due to melting, sliding, snow redistribution, and snow removal A load corresponding to severe imbalances resulting from snow removal, redistribution, sliding, meltin
42、g, etc. (e.g. zero snow load on specific parts of the roof) should always be considered. Such considerations are particularly important for structures which are sensitive to unbalanced loading (e.g. curved roofs, arches, domes, collar beam roofs, continuous beam systems) which are addressed in other
43、 clauses of this International Standard. 4.4 Ponding instability Roofs shall be designed to preclude ponding instability. For flat roofs (or with a small slope), roof deflections caused by snow loads shall be investigated when determining the likelihood of ponding instability from rain-on-snow or fr
44、om snow meltwater. 5 Characteristic snow load on the ground The characteristic snow load on the ground, s 0 , is determined by statistical treatment of snow data. Snow load measurements on the ground should be taken in an undisturbed area not subject to localized drifting. Methods for the determinat
45、ion of the characteristic snow load on the ground, s 0 , are described in Annex A. For practical application, the characteristic snow load on the ground will be defined in standard step values, which will yield basic values for the preparation of zone maps as described in Annex A. 6 Sno w lo ad c oe
46、fficie nts 6.1 Exposur e c oefficie nt The exposure coefficient, C e , should be used for determining the snow load on the roof. The choice of C eshould consider the future development around the site. For regions where there are no sufficient winter climatological data available, it is recommended
47、to set C e= 1,0. For most cases, the exposure coefficient, C e , is equal to the exposure coefficient for small roofs, C e0 . However, for very large flat roofs, wind is less effective in removing snow from the whole roof. To compensate for this, the exposure coefficient for large roofs is higher th
48、an for smaller roofs. (6) wherel c is the effective roof length equal to 2 2in metres;C e0 is the exposure coefficient for small roofs. Methods for the determination of C e0are given in Annex C.4 ISO 2013 All rights reserved ISO 4355:2013(E) In the expression for l c , W is the length of the shorter
49、 side of the roof and L is the length of the longer side (see Figure 1). W L Figure 1 Rectangular roof dimensions For non-rectangular roofs, W and L can be taken as the shorter and longer side of the roofs major dimensions along two orthogonal axes. For example, for an elliptical shape, W is measured along the short axis and L along the long axis. A
