Modal mass — where it appears
Named by 3 essays across one field — each of them below, with the objects they name alongside it.
The period nobody chose
Every structure has a natural period, it decides the answer to every question in this field, and no drawing anywhere records it. It is a consequence of a mass picked for one reason and a stiffness picked for another — and it has already been computed, by the serviceability check.
Most of the mass moves together
A sixteen-storey frame has sixteen modes, and the first one carries eighty-four per cent of the mass. That is why a hand calculation on one mode gets the base shear right to within a tenth — and why the same calculation gets the acceleration at roof level wrong by a factor of two.
The floor that is strong and unusable
A floor can satisfy every strength check, deflect less than the limit, and still be rejected by the people who work on it — because somebody walking across it at two steps a second happens to be exciting it at exactly the rate it likes to move.
Named alongside it
The objects these essays reach for when they reach for this one.
Mode shapeNatural frequencyBase shearCharacteristic equationDampingDeflectionFloor accelerationFloor vibrationFootfallModal analysisNatural periodParticipation factor