Bridges & Forces

How Forces Affect Different Types of Bridges

Forces on a Beam Bridge

• Simplest design (girder bridge)• Compression on top of the beam• Tension on bottom of beam• Middle part not much of either forces

Tension & Compression

•If add enough weight the top surface of the beam would buckle•The bottom would snap•Add truss lattice to dissipate the tension and compression•The force spreads through the truss

Forces on an Arch Bridge• The arches allow the forces to Dissipate or transfer (Transferring force- the spread out evenly over a greater area)

• Design allows to move stress from an area of weakness to an area of strength

• Arch bridges are able to span greater distances than beam or suspension

Forces on an Arch Bridge•Tension and compression are present in all bridges•Buckling occurs when compression overcomes an object’s ability to endure that force•Snapping is what happens when tension surpasses an objects ability to handle the lengthening force

Forces on a Truss Bridge• A truss bridge is a beam bridge with a

triangular structure either above the bridge called Through Truss or below the bridge called Deck Truss

• Compression affects the top of the beam• Tension affects the bottom of the beam• A truss structure has the ability to dissipate a

load through the truss triangle’s rigid structure• Transfers the load from one point to wider area

Forces on a Suspension Bridge• In a suspension bridge, the roadway is suspended by cables from two tall towers

• The towers support the majority of the weight as compression pushes down on the suspension bridge’s deck and then travels up the cables

• Transfer compression to the towers

Forces on a Suspension Bridge

• The towers then dissipate the compression directly into the Earth• The supporting cables receive the bridge’s tension forces

Forces on a Suspension Bridge•The cables run horizontally between the two flung anchorages•Anchorages are solid rock or massive concrete blocks in which the bridge is grounded

•Tensional force passes to the anchorages and into the ground•Have a deck truss beneath the bridge which helps to stiffen the deck and reduce the tendency of the roadway to sway and ripple•Span 2,000-7,000ft (610-2,134m)

•anchorage

Forces on a Cable Stayed Bridge

• Cables attached from different points to a single point on the tower

• Basic design in 16th century• Europe- after WWII

Forces on a Cable Stayed Bridge

•Span– 500 – 2,800ft (152-853m)•Lower cost than suspension bridge•Less steel cable, faster to build, more precast concrete sections

Forces on a Cable Stayed Bridge

• Don’t require anchorages nor do they need two towers

• The cables run from the roadway up to a single tower that alone bears the weight

• It absorbs and deals with compressional forces

Torsion

• Especially in suspension bridges• Torsion occurs when strong winds cause the suspended roadway to rotate and twist like a rolling wave

• Washington’s Tacoma Narrows Bridge disaster 1940

• Arch and truss bridges are protected from this force

Torsion•Suspension bridge engineers use deck truss to protect the bridge from torsion•In long spans use aerodynamic truss structures and diagonal suspender cables to mitigate the effects of torsion

Shear & Resonance• Shear stress occurs when two fastened structures (or two parts of a single structure) are forced in opposite directions

• If unchecked can rip the bridge materials in half

Shear & Resonance•Resonance is the vibration as in a snowball rolling down a hill and becoming an avalanche•Begins small and grows big•A stimulus in harmony of natural vibration of bridge•Vibration can increase in the form of waves

Shear & Resonance•Example- Tacoma Narrows Bridge in Washington, 1940•Like singer shattering a glass•Engineers create dampeners in the design to interrupt the waves•Create sections overlapping which change the frequency of the waves and prevents waves from building up

Tacoma Narrows BridgeGalloping Girder