gravity load ppt
TRANSCRIPT
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GRAVITY LOADS&
TORSIONAL EFFECTS
BySheth Devendra N.
(06MCL016)
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LOADS
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&
FAILURE
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Whatis a load?
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A load is a forcethat acts on a structure
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Whatdo loads do?
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Try to destabilize a structure
Try to break a structure
Try to deform a structure
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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, ...)
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Loads
Gravity
Wind Earthquake4/31
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The gravity loads are mainly two types:-
Dead Loads• Permanent weight of structure
non-moveable partitions
built-ins, heavy equipment
Roof
WallsFloors
Equipment
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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
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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)
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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.
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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.
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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.
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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
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• 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.
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• Uneven column:-In building with unequal vertical members
also the floor twists about the vertical axis and displaced horizontally.
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Unequal Vertical Member
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• One side open ground story building twists during earthquake shaking
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• 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.
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Wind Induced Torsion
• Flow around a tall building :
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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
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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.
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• 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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THANK YOU