Lecture 4 Stability

8/3/2019 Lecture 4 Stability

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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

Lecture 4 Stability

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