Cables

1. Cables

2. Principal Elements for practical suspension systems • Vertical supports or towers • Main cables • Anchorages • Stabilizers

3. Vertical Supports or Towers • Provide essential reactions that keep the cable system above the ground • May be simple vertical or sloping piers or masts, diagonal struts, or a wall. • Ideally, the axes of the supports should bisect the angle between the cables that pas over them

4. Main Cables • Primary tensile elements • Carry roof with a minimum of material • Steel used in cable structures has breaking stresses that exceed 200,000 psi

5. Anchorages • Because the cables are not vertical only, horizontal force resistance is required. • In suspension bridges, the massive concrete abutments provide the horizontal reaction force

6. Stabilizers • Lightweight roof or bridge systems are susceptible to pronounced undulation or fluttering when acted upon by wind forces. • Cables resists load through tension. • The destructive force is vibration or flutter

7. About cables • Equal angles = equal forces • What about unequal forces?

8. l B A h W Cable Geometry and Characteristics l = cable span L = AC + CB = cable length h = sag r = h/l = sag to span ratio

9. Cables with a Single Concentrated Load l = 24’ h = 3’ l Ty T Ty T h B A Tx Tx 12’ C 3’ W L = 24.8’

10. Think – Pair - Share l =6’ h = 12’ l T T Ty Ty Tx A B Tx 3’ h 12’ C W

11. Can a cable system support a load if the cable is completely horizontal? There is no force, however great, Can stretch a cord, however fine, Into a horizontal line That shall be absolutely straight. -Lord Kelvin l = L T T W

12. Cable – Stayed Bridges • Radial Parallel • Look similar to suspensions bridges • both have roadways that hang from cables • both have towers • Support the load of the roadway in very different ways. • Difference - how the cables are connected to the towers. • Suspension bridges - cables ride freely across the towers, transmitting the load to the anchorages at either end. • Cable-stayed bridges, the cables are attached to the towers, which alone bear the load.

26. Trusses

27. Truss • A truss is a triangulated assembly that distributes loads to supports through a combination of pin-connected members arranged in triangles • Ideally all members are in either pure compression or tension (no bending or shear) • All thrust force are resolved internally

28. Triangle • The triangle is the basic geometric unit of the truss • The triangle - shape cannot change without changing the length of it’s sides even when the joints are hinged • All other hinged polygons are unstable

29. Truss terminology • Top truss member – top chord • Bottom truss member – bottom chord • All members in between are web members • Plane trusses – all members in a single plane • Space trusses – 3D configuration

30. Roadway – under or over

34. Roof Trusses

38. Wrap-up • Cables are in tension • For a cable-stayed bridge, the weight of the bridge is supported by the towers • The triangle is the most stable shape • Trusses are comprised of triangles • Members of trusses are in pure compression or pure tension