Forces on Bridges

Page 1

Tension

Stretching force

The diameter of a tension member determines how much force it can take

Compression

Squeezing force

Length of a compression member determines how much force it can take

Page 2

Torque

Twisting force - wrench uses torque to tighten a bolt

Shear

cause materials to slide past one another, even if it is the same piece of material

scissors for example, use shear to cut

Page 3

Equilibrium of Forces

All forces in a bridge must be in equilibrium (the sums equaling 0)

If the force on the bridge is greater than the force the bridge or foundation can exert, it will collapse.

If the force a bridge or foundation exerts is higher, the bridge will fly off into space.

How the force is distributed is a ratio of the distance

between where the force is placed and where the

support is.

Page 4

-Hooke's law-

The amount of deflection on an object is proportional to the force applied

F=ku

F=force

u=amount of deflection

k=spring contact

-Stress-

measure of how much the material is being compressed or pulled at the molecular level

S=P/A stress=load/cross sectional area

-Strain-

amount of distortion that a force causes on the length of an object

e=l/L

e=strain

l=change in length

L=original length

Young's Modulus

ratio of stress and strain

Stress/strain

s/e=E

Young’s Modulus is constant for constant for each material

Page 5

Loads - Any effect that creates the need for resistive force

Types of loads

Static: a motionless or slowly changing load

Dynamic: loads that move or change rapidly - impact loads/resonant loads

Dead load: weight of the structural members

Live loads: loads that move and change over time - Occupant loads (people) /Natural elements(snow, earthquakes, etc)

Wood and wood joints

- Wood has a grain that runs the length of a board

- The grain is like a little straw that will suck up liquid- like glue

-Wood is stronger when force is applied with the grain with the grain, rather than perpendicular to it

-A butt joint is when two pieces of wood are just tuck next to each other an joined in some way (DO NOT USE BUTT JOINTS FOR PROJECT)

-A lap joint is when there is overlap between the tow pieces of wood (USE THESE FOR PROJECT)

-An edge point is when the edges of a board are joined together.

Page 6

6 inch simple beam