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GRAVITY LOADS & TORSIONAL EFFECTS

By Sheth Devendra N. (06MCL016)

LOADS &

FAILURE1/18

What is a load?

A load is a force that acts on a structure

2/31

What do loads do?

Try to destabilize a structure Try to break a structure Try to deform a structure3/31

Different LoadsLoads

Static

Dynamic

Forces due to Settlements, Thermal effects, ...

Dead Loads (fixed)

Live Loads (movable)

Continuous

Impact

Earthquakes

Self-Weight Of Structure

Fixed Building Elements

Occupancy

Environmental (snow, ...)

Wind

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 Walls Floors Equipment

6/28

Live Loads Furniture, Equipment, People, Snow Moveable Partitions May or may not be acting all the time

9/28

Live LoadsPeople move around

May get heavy concentrations10/28

Reduction of live loadsAs per IS 875 (part 2) 1987 (clause3.2)

Number of Floors 1 2 3 4 5 to 10 Over 10

Reduction in Total Live Load 0 10 20 30 40 50

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 :-

cm F

e

cs

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.

CausesThe 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

Vertical members of buildings that move more horizontally sustain more damage. 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).

Wind Induced Torsion Flow around a tall building :

Wind Induced Torsion Overall loading and dynamic response :cross wind along wind

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.

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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