Storey drifts

Drifts are used to evaluate the relative horizontal displacements of a structure, in particular to check storey drifts under seismic actions.

In LIRA-FEM, drift is determined for a specified pair of nodes as the ratio of the difference between their horizontal displacement vectors to the distance between the nodes. Node pairs can be defined in advance or added directly when reviewing the calculation results.

Drift calculation

To define drifts, specify nodes at characteristic horizontal levels of the structure. For the specified pairs, LIRA-FEM determines the vertical distance between the nodes dZ.

After the calculation is performed, the following value is displayed in the table:

where qd is the behaviour factor for displacements by which the calculated drift is multiplied. The default value of the factor is 1.

The calculation results can also be displayed directly on the design model as a coloured drift mosaic.

Storey drift limitation

LIRA-FEM also provides a separate storey drift limitation check intended to ensure the integrity of cladding and other non-structural elements.

For this check, the horizontal storey drift d_rs is determined taking into account the possible deformation mode of the storey: shear, flexural, or combined flexural-shear deformation.

The calculation uses the difference between the average horizontal displacements of the upper and lower floor levels, the storey height h, and the storey rotation angle in the vertical plane α.

The calculation result is displayed in the table as:

where ν is the reduction factor that takes into account the lower return period of the seismic action considered for the damage limitation requirement.

Why d_rs may be equal to 0

If zero values are displayed in the d_rs×qd×ν×1000/h column even though regular drifts have been calculated for these node pairs, check whether additional nodes have been specified to define the horizontal plane of the lower storey level.

Fig. 1. Example: drifts are calculated for the node pairs, but d_rs values are equal to zero

To determine a regular drift, it is sufficient to specify a pair of nodes. To calculate d_rs, the horizontal plane of the lower storey level must additionally be defined.

For this purpose, add the following to the corresponding pair:

• one additional node - for a plane frame;
• two additional nodes - for a spatial design model.

After the additional nodes are specified, LIRA-FEM calculates d_rs and the angle α. The additional nodes are displayed in parentheses in the table after the main node pair.

Storey drift mosaic

The calculation results can be displayed directly on the design model using the "Show storey drift mosaic" command.

The d_rs×qd×ν×1000/h values are displayed as coloured lines connecting the specified node pairs. This makes it possible to quickly evaluate the distribution of storey drifts over the height of the structure and identify areas that require additional attention.