english terms for cr 1-1-4

8/12/2019 English Terms for CR 1-1-4

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Basis of Structural Design

Course 10

Actions on structures:

Wind loads

Other loads

Course notes are available for download at

http://www.ct.upt.ro/users/AurelStratan/

Wind loading: normative references

Normative references

EN 1991-1-4: Eurocode 1: Actions on structures - Part 1-4:

General actions - Wind actions

CR 1-1-4/2012: Cod de proiectare. Evaluarea aciunii vntului

asupra construciilor.

Wind action is classified as variable fixed actionsaccording to EN 1990


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Nature of wind loading

Wind represents masses of air moving mainlyhorizontally (parallel to the ground) from areas of high

pressure to ones of low pressure

Wind generates pressures on external (and also internal)

surfaces of structures

The main effect of wind is a horizontal loading of

buildings (especially high-rise)

The effect of the wind on the structure (i.e. the response

of the structure), depends on the size, shape and

dynamic properties of the structure.

Basic value of mean wind velocity

The reference value of the wind velocity, vb, is the

characteristic 10 minutes mean wind velocity,

irrespective of wind direction and time of year, at 10 m

above ground level in open country terrain with low

vegetation such as grass and isolated obstacles with

separations of at least 20 obstacle heights.

Reference values of wind velocity are determined forannual probabilities of exceedence of 0.02, which is

equivalent to a mean return period of 50 years.

For design purposes, basic values of wind velocity are

obtained from maps and tables given in codes (CR 1-1-

4/2012).


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Reference wind pressure

Reference wind pressure qb is the wind pressurecorresponding to the reference value of the wind velocity

vb

where:

is the air density, which depends on altitude,temperature, latitude and season. The recommended

value for design is 1.25 kg/m3

For design purposes, reference wind pressure areobtained from maps and tables given in codes (CR 1-1-4 /

2012).

21

2b b

q v

Reference wind pressure


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Mean wind velocity: gradient height

The mean wind velocity at greatheights above the ground is

constant and it is called the

gradient wind speed.

Near the ground the mean wind

velocity is decreasing much due to

frictional forces caused by the

terrain, being equal with zero at the

ground level.

There is a boundary layer within

which the wind speed varies fromzero to the gradient wind speed

(mean wind velocity increases with

height).

Mean wind velocity: gradient height

The thickness of the boundary layer (gradient height)

depends on the ground roughness. Larger the

roughness, larger the gradient height.


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Mean wind velocity: terrain categories

Mean wind velocity: terrain categories


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Mean wind velocity: variation with height

The terrain factor kr(z0) is given by the relationship:

0,07

0

0 0,1890,05

r

zk z

Mean wind pressure: variation with height

The roughness factor cr(z) is

used to describe the variation of

wind pressure with height 2m r bq z c z q


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Wind turbulence

Wind velocity varies with time as shown in the figurebelow. This variation with respect to the mean wind

velocity is called turbulence and is generated by the

eddies caused by the wind blowing over obstacles

Wind turbulence

The turbulence intensity I(z) at height zis defined as the

standard deviation of the turbulence divided by the mean

wind velocity.

The turbulence intensity I(z) at height zcan be expressedas:

vv

m

I zv z

min max

0

min

min

200

2.5lnv

v

for z z z mz

I zz

for z zI z z


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Wind turbulence

Wind turbulence decreases with height above ground

Wind turbulence: gust factor

The gust factor cpq(z) is the ratio between the peak

pressure (due to wind turbulence) and mean pressure

(due to mean wind velocity)

The gust factor cpq(z) can be determined as:

where:g= 3.5 is the amplitude factor

Iv(z) is the turbulence intensity at height z

1 2 1 7pq v vc z g I z I z


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Wind turbulence: gust factor

Wind pressure at height z

Wind pressure at height zabove ground can be obtained

by considering the effects of mean wind velocity, wind

turbulence, and topography on the reference pressure qb(at the ground level)

Mean wind velocity increases with height above ground. The

effect of mean wind velocity on wind pressure profile is

accounted through the roughness factor cr(z)

Wind turbulence decreases with height above ground. The effectof wind turbulence on wind pressure at height zis accounted

through the gust factor cpq(z)

Isolated hills and other local topographical accidents can affect

the mean wind velocity. In design this effect is accounted through

the orography factor co. It need not be considered when the slope

is less than 5% (co=1.0).


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Effect of topography

Wind pressure at height zcan be obtained as:

The product between the gust factor, the roughness

factor and the topographical factor is called the exposure

factor, and is denoted by ce(z):

p e bq z c z q

2 2e o r pqc z c c z c z



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Wind pressure on surfaces

Wind pressure w(z) on rigid exterior and interior surfacesof the structure at height zabove ground are obtained as:

where:

Iw the importance factorqp(ze) peak wind pressure at level zeze reference height for external pressure.

cp aerodynamic pressure coefficient (cpe for exterior

surfaces; cpifor internal surfaces)

Pressures are considered positive (+) Suction is considered negative (-)

The total pressure on a structural element is obtained as

the algebraic sum of pressures on one side and suction

on the other side

e Iw pe p ew c q z i Iw pi p iw c q z

Wind pressure on surfaces

Wind pressure w(z) on rigid exterior and interior surfaces

of the structure at height zabove ground are obtained as:

e Iw pe p ew c q z i Iw pi p iw c q z


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Aerodynamic pressure coefficients

Aerodynamic pressure coefficients depend on: geometry of the structure/element

size of the structure/element

terrain roughness

wind direction with respect to the structure

Reynolds number

etc.

Pressure coefficients: loaded area

Aerodynamic pressure coefficients cpe for buildings and

parts of buildings depend on the size of the loaded area

A, which is the area of the structure, that produces the

wind action in the section to be calculated

Values for cpe,1 are intended for the design of small elements and

fixings with an area per element of 1 m2 or less such as cladding

elements and roofing elements. Values for cpe,10may be used for

the design of the overall load bearing structure of buildings. Due to non-uniform

action of wind, peak

pressure on a small

area is higher than

the peak overall

pressure on a large

area (for which

some portions

are loaded less)


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Press. coeff.: vertical walls of rect. plan buildings

The reference heights, ze, for rectangular plan buildingsdepend on the aspect ratio h/b and are always the upper

heights of the different parts of the walls

Reference heights are used to compute the exposure

factor ce(z)

Three cases:

A building, whose height h is less than b should be considered to

be one part.


A building, whose height h is greater than b, but less than 2b, may

be considered to be two parts, comprising: a lower part extending

upwards from the ground by a height equal to b and an upper part

consisting of the remainder.


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A building, whose height h is greater than 2b may be consideredto be in multiple parts, comprising: a lower part extending

upwards from the ground by a height equal to b; an upper part

extending downwards from the top by a height equal to b and a

middle region, between the upper and lower parts, which may be

divided into horizontal strips with a height hstrip (max hstrip = b)


Depending on geometry and position with respect to wind

direction, different regions of vertical walls are assigned

different names, with corresponding values of pressure

coefficients cp


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Depending on geometry and position with respect to winddirection, different regions of vertical walls are assigned

different names, with corresponding values of pressure

coefficients cp

Pressure coefficients

Similar procedure are specified in the code for roofs of

buildings (of different geometry), canopies, isolated

vertical walls, fences etc.


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Wind forces method

For structures like signboards, lattice structures andscaffoldings, flags, etc. wind actions is modelled as a

resultant force

where:

Iw the importance factorqp(ze) peak wind pressure at level zeze reference height for external pressure.

cf- wind force coefficientcd- dynamic response coefficient

Aref- reference area perpendicular on wind direction

w Iw d f p e ref F c c q z A

Other loads: traffic loads on bridges

In practice a highway bridge is loaded in a

very complex way by vehicles of varying

sizes and groupings.

In order to simplify the design process this

real loading is typically simulated by two

basic imposed loads - a uniformly

distributed load and a knife edge load -

representing an extreme condition of

normal usage.

The design is then checked for a further

load arrangement representing the

passage of an abnormal load.

The magnitudes of all these loads are

generally related to the road classification,

the highway authority's requirements and

the loaded length of the bridge.


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Other loads: traffic loads on bridges

Railway bridge design must take account of static loading andforces associated with the movement of vehicles.

As for highway bridges, two models of loading are specified for

consideration as separate load cases. They represent ordinary

traffic on mainline railways and, where appropriate, abnormal

heavy loads. They are expressed as static loads due to stationary

vehicles and are factored to allow for dynamic effects associated

with train speeds up to 300km/h.

Eurocode 1 also gives guidance on the distribution of loads and

their effects and specifies horizontal forces due to vehicle motion.

Centrifugal forces associated with the movement around curves,

lateral forces due to oscillation of vehicles (nosing) and

longitudinal forces due to traction and braking are included. Other aspects of bridge loading which need to be considered

include accidental loads and the possibility of premature failure

due to fatigue under traffic loading.

Other loads: crane loads

For buildings fitted with travelling overhead cranes, the loads due

to the crane itself and the lifted load are considered separately.

The self weight of the crane installation is generally readily

available from the manufacturer, and the load lifted corresponds

to the maximum lifting capacity of the crane.

When a load is lifted from rest, there is an associated acceleration

in the vertical direction, which causes an additional force. This

force is in addition to the normal force due to gravity, and is

generally allowed for by factoring the normal static crane loads.

Movements of the crane, both

along the length and across the

width of the building, are also

associated with accelerations

and retardations, this time in

the horizontal plane. The

associated horizontal forces

must be taken into account

in the design of the

supporting structure.


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Other loads: wave loading

For offshore structures in deep waters, wave loads can beparticularly severe. The loads arise due to movement of water

associated with wave action. These movements can be described

mathematically to relate forces to physical wave characteristics

such as height and wavelength.

The treatment is therefore

similar to wind loads in

that these physical

characteristics are

predicted and

corresponding forces on

the particular structural

arrangement thencalculated. These

calculation procedures

are, however, very

complicated and must

realistically be performed

on a computer.

Other loads: temperature effects

Exposed structures such as bridges may be subject to

significant temperature variation which must be taken

into account in the design.

If it is not provided for in terms of allowing for expansion,

significant forces may develop and must be included in

the design calculations. In addition, differential

temperatures, e.g. between the concrete deck and steelgirders of a composite bridge, can induce a stress

distribution which must be considered by the designer.


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Other loads: retained material

Structures for retaining and containing material (granularor liquid) will be subject to a lateral pressure.

For liquids it is simply the hydrostatic pressure. For

granular material a similar approach can be adopted, but

with a reduction in pressure depending on the ability of

the material to maintain a stable slope - this is the

Rankine approach.

Ponding of water on

flat roofs should be

avoided by ensuring

adequate falls(1:60 or more) to gutters.

Other loads: seismic loads

Seismic actions on structures are due to strong ground

motion.

They are a function of the ground motion itself and of the

dynamic characteristics of the structure.

Strong ground motion can be measured by one of its

parameters, the peak ground acceleration being the

parameter most usually adopted for engineeringpurposes.


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Other loads: accidental loads

Accidental actions may occur as a result of accidentalsituations. The situations include fire, impact or

explosion. It is very difficult to quantify these effects.

In many cases it may be preferable to avoid the problem,

for instance by providing crash barriers to avoid collision

from vehicles or roof vents to dissipate pressures from

explosions.

Where structures such as crash barriers for vehicles and

crowds must be designed for 'impact' the loading is

treated as an equivalent static load.

english terms for cr 1-1-4

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