Cables

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

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

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

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

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

About cables Equal angles = equal forces What about unequal forces?

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

Cables with a Single Concentrated Load W C B A T T TyTy TxTx TxTx TyTy l h l = 24’ h = 3’ 3’ 12’ L = 24.8’

Think – Pair - Share W C B A T T TyTy TxTx TxTx TyTy l h 12’ 3’ l = 6’ h = 12’

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 W l = L T T

Cable – Stayed Bridges RadialParallel 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.

Trusses

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

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

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

Roadway – under or over

Roof Trusses

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