Bridges & Forces

1. Bridges & Forces

2. How Forces Affect Different Types of Bridges

3. 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

4. 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

5. 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

6. 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

7. 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

9. 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

10. Forces on a Suspension Bridge • The towers then dissipate the compression directly into the Earth • The supporting cables receive the bridge’s tension forces

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. Tacoma Narrows BridgeGalloping Girder