Suspension Cable Forces Explained

A suspension cable is a flexible structural element used in various applications, most notably in suspension bridges. These cables are designed to carry loads by hanging between two support points.

Understanding Suspension Cable Behavior

When loads are applied to a suspension cable, gravity pulls these loads downwards. The cable, being flexible, takes a shape (often catenary-like, or parabolic if the loads are uniformly distributed horizontally) that allows it to transfer the downward forces from the loads to the support structures (like towers or anchors).

Consider the forces acting on the cable:

• The loads act downwards.
• The supports at the ends provide upward and inward reactions.
• The cable itself must resist the pulling forces exerted by the loads and the reactions from the supports.

Since the cable is hanging and transferring pulling forces along its length towards the supports, every section of the cable is being stretched or pulled apart. This internal force resisting the stretching is known as tension.

Let's look at why the other options are generally incorrect for a primary suspension cable:

• Compression: Compression is a force that pushes or squeezes a material. Flexible cables are not designed to withstand significant compression; they would buckle. Beams or columns typically handle compression.
• Bending: Bending involves forces that cause a material to curve or deflect across its cross-section. While there might be minor local bending effects at attachment points, the primary and dominant force acting throughout the length of a flexible suspension cable under load is tension, not bending. Rigid beams resist bending.
• Compression and bending: As explained, compression and significant bending are not the primary forces in a flexible suspension cable supporting loads.

Therefore, a suspension cable, by its nature and function of supporting loads by hanging, is predominantly under tension.