Tower structures, wind cables and ties

Tower structures

Towers carry the main cables over a saddle and transfer the vertical component of the cable pull to the foundation. They may be of steel or reinforced concrete and are built as:

• Portal frame towers: two legs connected by cross beams (struts) with diagonal bracing; used for road bridges, and stiff against wind across the bridge.
• Single-column or A-frame towers: for narrow bridges or where the cables are in one plane.
• Rocker towers (hinged at the base): allow the tower top to move with the cable.

Design points: the tower is designed mainly for axial compression. If the cable is clamped to the tower top, any difference between the horizontal pulls of the main span and the side span causes bending in the tower. To avoid this, the cable is usually seated on a saddle on rollers (or a sliding plate) at the top, so that the horizontal pulls on the two sides are equal and the tower carries essentially only vertical load.

     saddle
   ___|___|___        top strut
  |    X    |        cross bracing
  |   / \   |
  |  /   \  |        legs (compression)
  |_/_____\_|
   foundation

Wind cables and ties

Suspension bridges are light and flexible, so they tend to sway sideways and vibrate in the wind. Their lateral stability is improved by:

• Wind ties (lateral bracing): horizontal bracing between the two stiffening girders, or between the hangers in the plane of the deck, forming a horizontal truss that carries the wind load to the towers and abutments.
• Wind cables: additional cables placed horizontally on each side of the deck (or inclined from deck level to the piers or the ground) and anchored at the abutments; they limit the lateral sway of the deck.
• Inclined suspenders and stays: hangers arranged in a triangular pattern, or cross ties, to damp vertical oscillation.
• Streamlined box girders: reduce wind forces and flutter.

 plan view:   =====girder=====        wind
              \   /  \   /  \          ->
              wind ties (horizontal truss)
              =====girder=====