Structural Engineering & Statics
where the load goes
Concept

Actuator — where it appears

A powered device that pushes between two parts of a structure under a control law, such as the drive behind an active tuned mass. Its force is limited, it acts with a delay, and switching it on or off is a step in force applied at once to the floor that carries it.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

Also named here as stroke, transient — the same set of essays touches all of them, so they are one junction rather than several.

The moment the law lets go. The response of a structure of one mode with 1 per cent damping and a tuned mass of 2 per cent of its mass, its spring cancelled by an actuator held to 0.50 of the applied force and a stroke of 1.50 times the passive mass's, per unit static deflection, under a steady harmonic force at 0.90 of the structure's frequency, through a health check failing after sixty periods, when the actuator lets go and the mass is left on its own spring. With the law running the structure moves 4.63; left to the passive mass in a steady state it would move 6.97. In the cycles just after the switch it moves 8.78, more than either: the free mass's large stroke is handed to a spring that holds it at a frequency the forcing is not at, and the two beat before the passive state settles.

The switch that hands the mass back

An actuated tuned mass that cancels its own spring does far better than the passive mass, and the passive mass is always underneath it, so the obvious safeguard is a switch: when the stroke nears its limit or the controller fails, let go and leave the mass on its spring. The switch does not land on the passive mass. For several periods after it, the structure moves more than it would with the law or with the passive mass alone — up to the passive mass's own worst peak, at frequencies where the passive mass would have been comfortable. Letting go gradually is worse still, and a switch that is slow lets a growing storm through.

dynamics · Tuned mass damper Taking hold of a swinging mass throws it. The largest stroke of a tuned mass of 2 per cent, as a share of its limit, after the free-mass law takes hold of it while it swings on its own spring and damper in a steady storm at the structure's frequency, against the instant in the structure's period at which the law takes hold; dashed, the law waiting for the stroke to cross zero, and for the mass to come to rest relative to the structure. Depending on the instant the stroke reaches 1.39 to 3.12 times the limit; waiting for zero stroke, 2.81; waiting for rest, 2.27.

Taking hold is harder than letting go

A tuned mass driven by a law that cancels its spring is safe to let go of: the transient is the same whenever the switch falls, and the passive mass takes over. Switching the law back on is the reverse operation and behaves nothing like it. The mass the spring was holding is thrown up to three times past its stroke limit, by an amount that depends on the instant, and a switch that is allowed to come back in a storm near its threshold comes back hundreds of times.

dynamics · Tuned mass damper

Named alongside it

The objects these essays reach for when they reach for this one.

FeedbackResonanceStrokeTransientTuned mass damperBeating

All concepts