What is vibration Serviceability?

When an opera singer shatters a glass this is because the Natural Frequency of her voice is the same as the Natural Frequency of the glass.

Most structures have a Natural Frequency.

As steel bridges (or steel and timber bridges) increase in length they become prone to this – typically in the middle to upper 20’s of metres.

An equation predicts this and it contains length, weight per metre and length.

As length increases it is necessary to increase the depth of the beams to increase the resistance to deflection (increased I Value). Increasing the depth usually also increases the weight per metre.

There comes a point when the natural frequency of the structure comes close to that imposed by a person walking across it. At this point very small inputs excite a disproportionate response in the bridge.

Your bridge designer has seen a dog crossing a sensitive bridge make the bridge and the dog jump violently about.

Cambering is useful but the maths are difficult to prove.

When the critical length is approached the designer usually turns to trusses or suspension bridges as a solution.