basis of structural design - ct.upt.ro · romanian national annex to sr en 1991-1-1:2004. imposed...
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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 athttps://www.ct.upt.ro/studenti/cursuri/stratan/bsd.htm
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.
EN 1990: Classification of loads
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
EN 1990: Classification of loads
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
EN 1990: Classification of loads
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
Permanent actions: EN 1991-1-1
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.
Permanent actions: EN 1991-1-1
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.
Permanent actions: EN 1991-1-1
– partitions and linings;
– hand rails, safety barriers, parapets and kerbs;
Permanent actions: EN 1991-1-1
– 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.
Permanent actions: EN 1991-1-1
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)
Permanent actions: EN 1991-1-1
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
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.
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
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
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
Imposed loads on buildings
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
Imposed loads on buildings
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
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
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:
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).
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
Snow load: code procedure
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
Snow load: code procedure
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
Snow load: code procedure
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
Snow load: code procedure
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
Snow load: code procedure
Example: pitched roofs– case (i): undrifted snow
– case (ii): drifted snow
– case (iii): drifted snow
Snow load: code procedure
Example: multi-span roofs– case (i): undrifted snow
– case (ii): drifted snow
Snow load: code procedure
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
Snow load: code procedure
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 load: code 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.