Storey drift check
Storey drift is the relative lateral displacement between two consecutive floors, divided by the storey height. sazeh checks it for every storey, in each direction, under every seismic combination, and prints the joint and the combination that govern.
What is compared with what
The code compares the inelastic drift with a limit. sazeh obtains the inelastic drift by multiplying the elastic drift by the deflection amplification factor Cd of the structural system, so the drift row depends on the system you chose.
| Item | Value |
|---|---|
| Inelastic drift | Cd times the elastic drift |
| Limit, building up to 5 storeys | 0.025 of the storey height |
| Limit, building over 5 storeys | 0.02 of the storey height |
| Where it is read | At the corner of the diaphragm |
These values are read from the tables of the seismic code and are printed on the sheet together with the edition. The clauses are 3-5-1 and 3-5-2.
Drift is a difference of two displacements at the same point
The drift of a storey is the displacement of a point on the floor minus the displacement of the same plan point on the floor below. A floor with a setback, a penthouse or an extra joint has no joint at the same offset as the floor below, so the point is read off the lower diaphragm instead of being matched by the offset of a corner. A match by offset has nothing to find in such a floor and falls back to zero, and the drift then becomes the whole displacement from the ground. On a real model with a penthouse this overstated the drift of that storey by more than a factor of two before it was corrected.
Cracked stiffness changes the answer
The drift depends on the stiffness assumed for cracked beams and columns, and two bases are in use. The one used by default is the base of the concrete code, with 0.35 for beams and 0.7 for columns. The other is the base that the seismic code uses for the period, with 0.5 and 1.0. The two bases have the same ratio of beam to column stiffness, so the forces barely change, but the drift does: on one four-storey test grid it differed by 42%, and the number of failing drift rows went from 0 to 32.
The sheet therefore reports whether the drift verdict would change if the other basis had been chosen. A file saved before the default changed keeps its own factors and is not stiffened silently.
Instead of a spreadsheet
A drift check is often done by exporting the displacements of the diaphragm joints from ETABS to a spreadsheet and applying Cd and the limit by hand. sazeh does it from the .e2k file and prints the joint and the combination that govern. It reads ETABS files only: a STAAD model cannot be uploaded.
What a failed drift row means
Drift can fail because the sections are small, or because the lateral system is too flexible for the building. A larger section helps in the first case only. When sazeh builds a corrected model, it enlarges the columns for the corner-drift clause only. Other storey-level clauses, such as wall flexure or modal mass, do not make it enlarge every column.
If the corrected model still fails drift, the row stays "not met" and the sheet states that a change of section does not close it.
Related pages
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