gravity load ppt

GRAVITY LOADS&

TORSIONAL EFFECTS

BySheth Devendra N.

(06MCL016)

LOADS

1/18

&

FAILURE

Whatis a load?

2/31

A load is a forcethat acts on a structure

Whatdo loads do?

3/31

Try to destabilize a structure

Try to break a structure

Try to deform a structure

Different Loads

Loads

DynamicStatic

Dead Loads(fixed)

Forces due to

Settlements, Thermal effects,

...

Live Loads(movable)

Continuous Impact

Self-WeightOf

Structure

FixedBuildingElements

Earthquakes

WindOccupancy Environmental(snow, ...)

Loads

Gravity

Wind Earthquake4/31

The gravity loads are mainly two types:-

Dead Loads• Permanent weight of structure

non-moveable partitions

built-ins, heavy equipment

Roof

WallsFloors

Equipment

6/28

Live Loads

• Furniture, Equipment, People, Snow• Moveable Partitions

May or may not be acting all the time

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Live Loads People move around

May get heavy concentrations

10/28

Reduction of live loads

Number of Floors Reduction in Total

Live Load

1

2

3

4

5 to 10

Over 10

0

10

20

30

40

50

As per IS 875 (part 2) – 1987 (clause3.2)

Impact Gravity loading

• An elevator being accelerated upward or

brought to rest on its way down.

• Increase of 100% of Static elevator Load

Construction loads

• Most severe load building has to withstand.• Load of Floor forms, newly placed slab• Climbing crane.

Torsional Effect

• Definition:-• Center of Stiffness:-

The point through which the resultants of the restoring forces of a system acts.

• Center of Mass:-

The point through which the resultants of the masses of the system acts.

What Torsion Means

• Torsion in Buildings is rotation of whole mass or group of masses about a point which is generally Centre of Stiffness

Definition Point of View :-

cmcs

eF

When the C.M.&C.S.of a 3D frame do not coincide or when point of application of loads is different from point of development of resistance, the eccentricity causes the frame to rotate and thus Torsion is induced.

Causes

The main causes of twisting in buildings to happen are Lateral Forces

Earth quake :-• It causes the base to vibrate in horizontal plane

creating storey shears on account of inertial forces.• These act at the respective centre of masses and

eccentricity with centre of stiffness causes the storey to rotate in its plane

Wind :-• It causes the building to vibrate in along-wind and

across wind direction

• Mass Unbalance:-If the mass of the floor of building is more on

one side then that side of the building moves more underground movement.

• Uneven column:-In building with unequal vertical members

also the floor twists about the vertical axis and displaced horizontally.

Unequal Vertical Member

• One side open ground story building twists during earthquake shaking

• This induces more damage in the columns and walls on the side that moves more.

• It is best to minimize this twist by ensuring that buildings have symmetry in plan (i.e., uniformly distributed mass and uniformly placed vertical members).

Vertical members of buildings that move more

horizontally sustain more damage.

Wind Induced Torsion

• Flow around a tall building :

Wind Induced Torsion

• Overall loading and dynamic response :

• Cross-wind vibrations are usually greater than along-wind vibrations for buildings of heights greater than 100m (330 feet)

• The vortex shedding and the uneven along & cross wind vibrations give rise to Wind Induced Torsion in the structure.

along wind

cross wind

Wind Induced Torsion

Why to consider Wind Induced Torsion ?

• Wind induced torsional vibration of tall buildings can enlarge the displacement and acceleration near the peripheries of cross-section of tall buildings; especially when the side faces of a rectangular tall building are wider.

• Its lowest torsional natural frequency approaches either of its lowest translational natural frequencies.

• Wind-induced torsional responses may become the main part of the total responses for the peripheral points of such a building.

• Meanwhile, habitants in a tall building are

more sensitive to torsional motion than translational motion.

• Therefore, wind-induced torsional responses should be taken into account in the design of tall buildings.

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