Multistorey plane frames

Stiffnesses K and stiffness factors k of the columns composing the one-storey frames of the previous paragraphs, reflect the actual ones, as depending only on the geometry and not on the magnitude of the forces or their distribution along the height of the building, since the structures were only single storey. These stiffnesses were the absolute ones, but at the same time they were also the relative ones as there was only one storey.

In multistorey buildings, considered in this paragraph, the seismic forces are applied along the structure height and therefore another factor is involved, namely the distribution of horizontal seismic forces (orthogonal, triangular and random). Consequently, stiffnesses are independent ofthe force magnitude, but dependent on the horizontal seismic force distribution. Eventually, both stiffnesses K and stiffness factors k derived will be the apparent values.

Assuming that the horizontal forces act simultaneously at the floors, the apparent floor stiffness is meant to be relative with respect to its lower one.

The actual stiffnesses of the multistorey frames are discussed in Appendix B and the conclusion is that the values of the apparent and the relative actual stiffnesses do not differ significantly.