Basis of Structural Design EN 1990: Classification of loads

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Basis of Structural Design

Course 9

Actions on structures: permanent loads, imposed loads and snow loads

Course notes are available for download athttp://www.ct.upt.ro/users/AurelStratan/

EN 1990: Classification of loads

Actions are classified by their variation in time as follows:– permanent actions (G), e.g. self-weight of structures, fixed

equipment and road surfacing, and indirect actions caused by shrinkage and uneven settlements;

– variable actions (Q), e.g. imposed loads on building floors, beams and roofs, wind actions or snow loads;

– accidental actions (A), e.g. explosions, or impact from vehicles.

Actions can also be classified– by their origin, as direct or indirect,

– by their spatial variation, as fixed or free, or

– by their nature and/or the structural response, as static or dynamic.

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Permanent action is one that is likely to act throughout a given reference period and for which the variation in magnitude with time is negligible, or for which the variation is always in the same direction (monotonic) until the action attains a certain limit value

Variable action is one for which the variation in magnitude with time is neither negligible nor monotonic

Accidental action is usually of short duration but of significant magnitude, that is unlikely to occur on a given structure during the design working life


Certain actions, such as snow loads, may be considered as either accidental and/or variable actions, depending on the site location

Actions caused by water may be considered as permanent and/or variable actions depending on the variation of their magnitude with time

Direct action: a set of forces (loads) applied to the structure

Indirect action: a set of imposed deformations or accelerations caused for example, by temperature changes, moisture variation, uneven settlement or earthquakes

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A fixed action is one that has a fixed distribution and position over the structure or structural member such that the magnitude and direction of the action are determined unambiguously for the whole structure or structural member if this magnitude and direction are determined at one point on the structure or structural member

A free action is one that may have various spatial distributions over the structure

An action should be described by a model, its magnitude being represented in the most common cases by one scalarNOTE: For some actions and some verifications, a more complex representation of the magnitudes of some actions may be necessary.

Permanent actions: EN 1991-1-1

The self-weight of construction works is classified as a permanent fixed action

Permanent action is one which is likely to act throughout a given reference period and for which the variation in magnitude with time is negligible, or for which the variation is always in the same direction (monotonic) until the action attains a certain limit value

Examples of permanent actions: – self-weight (or dead load) of structures,

– fixed equipment and road surfacing,

– and indirect actions caused by shrinkage and uneven settlements

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Normative references: EN 1991-1-1: Eurocode 1: Actions on structures - Part 1-1: General actions - Densities, self-weight, imposed loads for buildings

The total self-weight of structural and non-structural members should be taken into account in combinations of actions as a single action.

The self-weight of new coatings and/or distribution conduits that are intended to be added after execution should be taken into account in design situations.

The source and moisture content of bulk materials should be considered in design situations of buildings used for storage purposes.


The self-weight of the construction works should be represented in most cases by a single characteristic value and be calculated on the basis of the nominal dimensions and the characteristic values of the densities.

The self weight of the construction works includes the structure and non-structural elements including fixed services as well as the weight of earth and ballast.

Non-structural elements include:– roofing;

– surfacing and coverings;

– partitions and linings;

– hand rails, safety barriers, parapets and kerbs;

– wall cladding;

– suspended ceilings;

– thermal insulation;

– fixed services.

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– roofing;

– surfacing and coverings;


– partitions and linings;

– hand rails, safety barriers, parapets and kerbs;

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– wall cladding;

– suspended ceilings;


– thermal insulation;

– fixed services

Fixed services include:– equipments for lifts and moving

stairways;

– heating, ventilating and air conditioning (HVAC) equipment;

– electrical equipment;

– pipes without their contents;

– cable trunking and conduits.

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Characteristic values of self-weight are determined using – nominal dimensions (from architectural plans and details) and

– characteristic values of densities (obtained from Annex A to EN 1991-1-1 or manufacturer)


For manufactured elements such as flooring systems, facades and ceilings, lifts and equipment for buildings, data may be provided by the manufacturer

For determining the effect of the self-weight due to movable partitions, an equivalent uniformly distributed load shall be used and added to the imposed load

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Self-weight: example

CARPET FLOOR

LEVELING MORTAR

RAISED FLOOR SYSTEM

REINFORCED CONCRETE SLAB

Thickness,mm

Specificweight,kN/m3

Weight,kN/m2

CARPET FLOOR ON RAISEDFLOOR SYSTEM

0.40

LEVELING MORTAR 30 21.0 0.63REINFORCED CONCRETE SLAB 150 25.0 3.75

TOTAL 4.78

Imposed loads on buildings - EN 1991-1-1

Imposed (or live) loads on buildings are those arising from occupancy, including: – normal use by persons;

– furniture and moveable objects (e.g. moveable partitions, storage, the contents of containers);

– vehicles;

– anticipating rare events, such as concentrations of persons or of furniture, or the moving or stacking of objects which may occur during reorganization or redecoration

Imposed loads shall be classified as variable free actions

The imposed loads are modelled by uniformly distributed loads, line loads or concentrated loads or combinations of these loads.

For the determination of the imposed loads, floor and roof areas in buildings should be sub-divided into categories according to their use.

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Imposed loads on buildings - EN 1991-1-1

Heavy equipment (e.g. in communal kitchens, radiology rooms, boiler rooms etc) are not included in the loads given in EN 1991-1-1. Loads for heavy equipment should be agreed between the client and/or the relevant Authority.

Generally, imposed loads are considered as uniformly distributed. To ensure a minimum local resistance of the floor structure a separate verification shall be performed with a concentrated load. The concentrated load shall be considered to act at any point on the floor (over an area with a shape which is appropriate to the use and form of the floor)

qk Qk

Imposed loads on buildings: Categories

Areas in residential, social, commercial and administration buildings are divided into categories according to their specific uses

Dynamic effects shall be considered where it is anticipated that the occupancy will cause significant dynamic effects

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Imposed loads on buildings: load values

Characteristic values qk for uniformly distributed load and Qk for concentrated load are assigned to each category. Recommended values are underlined.

Imposed loads on buildings: load values

Romanian National Annex to SR EN 1991-1-1:2004

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Imposed loads on buildings

Where necessary qk and Qk should be increased in the design (e.g. for stairs and balconies depending on the occupancy and on dimensions). Where no value is specified in the code, informatively, the loads on stairs and balconies can be increased by 1.0 kN/m2.

Imposed loads on buildings: movable partitions

Provided that a floor allows a lateral distribution of loads, the self-weight of movable partitions may be taken into account by a uniformly distributed load qk which should be added to the imposed loads of floors. This defined uniformly distributed load is dependent on the self-weight of the partitions as follows:– for movable partitions with a self-weight ≤ 1.0 kN/m wall length: qk =0.5 kN/m2

– for movable partitions with a self-weight ≤ 2.0 kN/m wall length: qk =0.8 kN/m2;

– for movable partitions with a self-weight ≤ 3.0 kN/m wall length: qk =1.2 kN/m2

Heavier partitions should be considered in the design taking account of:– the locations and directions of the partitions;

– the structural form of the floors

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Imposed loads are free actions: – the most unfavourable spatial distribution shall be considered

– in practice, several "chessboard" distributions are considered in addition to the uniform distribution

uniform distribution chessboard distribution 1 chessboard distribution 2


EN 1991-1-1 contain provisions for calculation of characteristic values of loads for the following types of use of buildings:– Residential, social, commercial and administration areas

– Areas for storage and industrial activities (including actions induced by forklifts, actions induced by transport vehicles)

– Garages and vehicle traffic areas (excluding bridges)

– Roofs

Additionally, horizontal loads on parapets and partition walls acting as barriers need to be considered in design.

Normative references: EN 1991-1-1: Eurocode 1: Actions on structures - Part 1-1: General actions - Densities, self-weight, imposed loads for buildings

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Snow load: normative references

Normative references– EN 1991-1-3: Eurocode 1 - Actions on structures -

Part 1-3: General actions - Snow loads

– CR 1-1-3/2012: Cod de proiectare. Evaluarea acţiunii zăpezii asupra construcţiilor

EN 1991-1-3 and CR 1-1-3-2012 give guidance to determine the values of loads due to snow to be used for the structural design of buildings and civil engineering works

Snow load: special cases

The two codes does NOT give guidance on specialist aspects of snow loading, for example:– impact snow loads resulting from snow sliding off or falling from

a higher roof;

– the additional wind loads which could result from changes in shape or size of the construction works due to the presence of snow or the accumulation of ice;

– loads in areas where snow is present all year round;

– ice loading;

– lateral loading due to snow (e.g. lateral loads exerted by drifts);

– snow loads on bridges.

In regions with possible rainfalls on the snow and consecutive melting and freezing, snow loads on roofs should be increased, especially in cases where snow and ice can block the drainage system of the roof

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Snow load: classification and characteristics

Generally, for the purpose of applying EN 1990, snow loads are classified as variable, fixed, and static actions.

Snow load may be treated as accidental in two cases: – In particular situation of a snow fall which has an exceptionally

infrequent likelihood of occurring

– In particular situation of a snow deposition pattern which has an exceptionally infrequent likelihood of occurring

Snow action is modelled as a gravity (vertical) load applied on roofs of buildings, acting per unit area of horizontal projection

Snow load on the ground

The characteristic value of snow load on the ground (sk) is based upon the probability of 0.02 being exceeded for a reference period of one year. This is equivalent to a mean return period of 50 years.

CR 1-1-3-2013 gives ground snow load map of Romania, representing characteristic values of snow load on ground, for altitudes below 1000 m

For higher altitudes, the following relations can be used to obtain characteristic values of snow load on ground:

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Snow load on the ground

Snow load: nature of load

Snow can be deposited on a roof in many different patterns. Properties of a roof or other factors causing different patterns can include:– the shape of the roof;

– its thermal properties;

– the roughness of its surface;

– the amount of heat generated under the roof;

– the proximity of nearby buildings;

– the surrounding terrain;

– the local meteorological climate, in particular its windiness, temperature

– variations, and likelihood of precipitation (either as rain or as snow).

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Snow load

Two primary load arrangements should be considered when modelling snow action:– undrifted snow load on the roof:

load arrangement which describes the uniformly distributed snow load on the roof, affected only by the shape of the roof, before any redistribution of snow due to other climatic actions.

– drifted snow load on the roof: load arrangement which describes the snow load distribution resulting from snow having been moved from one location to another location on a roof, e.g. by the action of the wind.

undriftedsnow

driftedsnow

Snow load: code procedure

Snow load on the roof in the persistent/transient design situation is determined as follows:s = Is i Ce Ct sk

Is is the importance – exposure factor for snow load

i is the snow load shape coefficient, depending on the shape of the roof

sk is the characteristic value of snow load on the ground, depending on geographic location of the building and on altitude

Ce is the exposure coefficient, accounting for the degree in which wind sweeps the snow from the roof

Ct is the thermal coefficient, defining the reduction of snow load on roofs as a function of the heat flux through the roof, causing snow melting

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s = Is i Ce Ct sk Is is the importance – exposure factor for snow load


s = Is i Ce Ct sk sk is the characteristic value of snow load on the ground,

depending on geographic location of the building and on altitude

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s = Is i Ce Ct sk

The thermal coefficient Ct is used to account for the reduction of snow loads on roofs with high thermal transmittance (> 1 W/m2K), in particular for some glass covered roofs, because of melting caused by heat loss

For most building structures, the roofs do not fit the above condition, having a lower thermal transmittance, and, therefore, Ct = 1.0


s = Is i Ce Ct sk Ce is the exposure coefficient, accounting for the degree

in which wind sweeps the snow from the roof, and depends on the topography at the building site

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s = Is i Ce Ct sk i is the snow load shape coefficient,

depending on the shape of the roof

Roof shape coefficients are available for undrifted and drifted snow

Example: monopitch roofs– Values for roof shape coefficients apply

when the snow is not prevented from sliding off the roof.

– Where snow fences or other obstructions exist or where the lower edge of the roof is terminated with a parapet, then the snow load shape coefficient should not be reduced below 0.8


Example: pitched roofs– case (i): undrifted snow

– case (ii): drifted snow

– case (iii): drifted snow

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Example: multi-span roofs– case (i): undrifted snow

– case (ii): drifted snow


Further guidance is available in codes for roof shape coefficients for:

– Cylindrical roofs

– Roof abutting and close to taller construction works

• s – snow load shape coefficient due to sliding of snow from the upper roof

• w – the snow load shape coefficient due to wind

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Roof shape coefficients are also specified for local effects: – drifting at projections and obstructions;

– the edge of the roof;

– snow fences

Drifting at projections and obstructions: – in windy conditions drifting of snow can occur on any roof which

has obstructions as these cause areas of aerodynamic shade in which snow accumulates

– accumulation of snow due to parapets at roof edges can be modeled using this procedure


Snow overhanging the edge of a roof: the design of those parts of a roof cantilevered out beyond the walls should take account of snow overhanging the edge of the roof, in addition to the load on that part of the roof

Snow loads on snowguards and other obstacles: under certain conditions snow may slide down a pitched or curved roof. The sliding mass of snow need to be considered for the design of the obstacles preventing this movement.

Documents

Basis of Structural Design EN 1990: Classification of loads