why structures fail

WHY STRUCTURES

FAILAn Introduction to engineering

analysis

BUILDING A TOWER Each team will design a tower made

of balsa sticks

Each team will do a structural analysis of their tower to predict where it will fail

Each tower will under go ‘destructive testing’

THE LANGUAGE OF ENGINEERS

MassVelocity

Force Pressure

Stress

Deformation System

Failure

Acceleration

Position

MASS A measure of how much matter an object is made of

Does not change with environment

Metric Unit:?

English Unit:Pounds (lb or lbm)

Grams (g)

90 kg200 lbm

90 kg200 lbm

Weight: (in pounds force)200 lbf

Weight:33 lbf

POSITION Position fully defines an object’s

location in spaceMetric Unit:Meters (m)

English Unit:Feet (ft)

Z

X

Y

(X, Y, Z) = (.2 m, .5 m, .6 m)

(X, Y, Z) = (.5 m, .1 m, .2 m)

(X, Y, Z) = (1 m, .3 m, .4 m)

VELOCITY Velocity describes the rate of change

of an object’s positionMetric Unit:Meters per second (m/s)

English Unit:Feet per second (ft/s)10 m/s

30 m/s

Can also be expressed as a ‘vector’(x,y) = (3 m/s, 4 m/s) 4 m/s

3 m/s

Includes both a magnitude and direction

30 m/s

Acceleration describes the rate of change of an object’s velocity

ACCELERATION

Metric Unit:m/s/s (m/s2)

English Unit:ft/s2

V = 10 m/sa = 0 m/s2

0m 10m

20m

30m

40m

50m

1 sec

0 sec

2 sec

3 sec

4 sec 5

sec

V = 10 m/sa = 2 m/s2

1 sec

2 sec

3 sec

0 sec

V = ?16 m/s

V = 10 m/s

FORCE Any influence that causes an object to

change speed, direction, or shape

Force = mass * acceleration F = m*a

Metric Unit:Newton (N)

English Unit:Pounds Force (lbf)

m = 90 kga = 9.81 m/s2

m*a = 883 N

100 kg

F = 500 N

a = 5 m/s2

PRESSURE When a force is exerted over a defined

area

Pressure = Force / AreaP = F / A

Metric Unit:Pascals (Pa)

English Unit:Pounds per square inch (PSI)

24 inches

24 inches

Force = 500 lbf

Area = 24 in x 24 in = 576 in2

P = 500 lbs / 576 in2 = .87 PSI

STRESS A measure of the internal forces acting

on a deformable structureMetric Unit:Pascals (Pa)

English Unit:Pounds per square inch (PSI)

20 m

20 m

20 m * 20 m = 400 m2

F = 883 N

Stress = P = 883 N / 400 m2 = 2.2 Pascals

F = 5000 kg * 9.81 m/s2 = 49050 N

Stress = P = 49050 N / 400 m2 = 122 Pascals

STRESS Types of stresses

Compression

Tension

Shear

DEFORMATION

Occurs when the stress in an object exceeds the yield strength of the material

Material:Purple Block

Yield Strength:1000 kPa

Deformation!

500 kPa

1200 kPa

Stressed

kPa = kilo-pascalOr 1000 pascals

SYSTEM FAILURE Occurs when the stress in an object

exceeds its ultimate strength (Compressive or Tensile)

Stressed

Deformation

System Failure!

Material:Purple Block

Yield Strength:1000 PSI

Ultimate Strength:2000 PSI

500 PSI

1200 PSI

2500 PSI

TOWER FAILURE

Next Week: Where will your tower fail?

Boom!