lecture 4 stability
TRANSCRIPT
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Stability of Structures
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Loads
Loads forces which are applied to a
structure.- may be described according to its
a. types
b. distribution
c. time
d. location and method ofapplication
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Types of Loads1.) Dead Loads loads that are relatively
constant throughout the structures life.a. selfweight of structural components
b. floor finishes, roof finishes
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Types of Loads2.) Live Loads - variable within the object's
normal operation cycle not includingconstruction or environmental loads
a. ES 10 students
b. What are other live loads in this room?
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Types of Loads3.) Wind Load - force exerted by the wind
on a structure or part of a structuresailboat billboards
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Glasgow Towersrc: wikipedia
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Types of Loads4.) Earthquake Load - force exerted due to
lateral or vertical movement of theearth.
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Glimpse of Metro Manila
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Types of Loads5.) Snow Load we dont have it here in the
Philippines.
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Distribution of Loads
1.) Concentrated Load load applied at a point.
Sometimes, referred to as point load.
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Distribution of Loads2.) Distributed Load load distributed along a
length or over an area.
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Wind Load
Generally, for the same load
intensity, a bigger surface area
means greater force.
Glasgow Towersrc: wikipedia
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Effect of a Distributed Load- The analysis is the same as determining the
center of gravity of the distributed load.
iirxWWxR =
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Reactions Motion resisting forces that develop at
the supports of an object. Different supports exert different
reactions
The magnitudes of the reactions dependalso on the loads.
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Reactions
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Normalforce
Some types of Reactions1. ) Normal - A reaction
perpendicular to thecontact surface.
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Some types of Reactions2.) Hinge - A smooth
pin resists horizontaland vertical motionbut not rotation
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Some types of Reactions3.) Cantilever -When
load is applied on acantilever, it does notmove and rotate.
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Friction
Friction is a reactionthat develops betweensliding bodies.
bodies that tend to slideover one another (static)
sliding bodies (dynamic)
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Types of Friction Static friction - develops between objects
that tend to slide.
Kinetic friction - develops between sliding
objects.
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State of Impending Motion The state of motion when the object is
about to slide.
In this state, the static friction is directly
proportional to the: normal force (reaction) on the same surface
of the object
Relative roughness of the surfaces involved
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FrictionFriction,f=N
=coefficientoffrictionN=NormalForce
f
P
Equilib Motion
f
P
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N
W Static
= 0Fr
fs
T Static
= 0Fr
Static(impendingmotion)
= 0Fr
fsmax
Moving
fk
Nature of Friction
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Stability
The property of a bodythat causes it to develop
actions that restore the
original condition whendisturbed from a condition
of equilibrium.
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Is this object in equilibrium?
Will it remain stationary?
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Stable Equilibrium The response to a small
disturbance is actionsthat tend to restore the
equilibrium.
The small disturbance
causes the CG to move
up.
Restoringmoment
cgcg
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Stable Equilibrium
Impending(still stable)
cgcg
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Stable Equilibrium
cgcg
Overturning
moment
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Is this object in equilibrium?
Will it remain stationary?
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Unstable Equilibrium If the body is
displaced an
arbitrarily smalldistance fromequilibrium,overturning forces
develop. The small
disturbance causes
the CG to movedown.
Overturningmoment
cg cg
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Marginally Stable Equilibrium If the body is displaced a small amount, it
will stay in the new position.
cg
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Stability A stable object has a wide base, low CG,
or both a wide base and low CG.
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Example
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Building Imposed with Gravity Loads The weight of the
building is balanced
by the reaction of
the ground.
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Soft soil
Building Imposed with Gravity Loads When the ground
can not develop
enough force to
counteract the
buildings weight, the
building will settle.
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Wind or earthquake
can cause
redistribution of the
ground reaction.
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Building Imposed with Gravity Loads Adjacent excavation
can weaken the soil
and cause
redistribution of the
ground reaction.
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Instability and Disaster
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Person
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Bird Balance
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Judo