Use of modification factors
The accuracy of determination of the second-order effects for concrete structures is dependent upon a reasonable estimation of the concrete long term properties. Consequently design codes can require that analysis stiffness adjustment factors are applied to concrete members, (as the appropriate properties to use in analysis are load and time dependent).
For non-concrete members it is also possible that you will want to apply an adjustment to properties for various other investigations. One suggested example is the assessment of steel structures subject to corrosion. Another classic requirement in this regard relates to torsion, it is common engineering practice to assume that if it will work without assuming any torsional strength, then torsion can be ignored.
In order to make these adjustments in Tekla Structural Designer you can apply modification factors from the Analysis Settings dialog.
Note that while modification factors affect analysis (and hence the forces considered in design), they have no effect on Tekla Structural Designer member design checks, including deflection calculations.
Use of modification factors in sway and drift checks:
If the design considers building analysis, (but not staged construction analysis), the building analysis results are used both for sway and drift checks AND for member design for all combinations at both strength and service levels. The default building analysis modification factors are set for determination of second-order effects and so reflect long term properties. This means that, by default, the building analysis displacements used for the drift checks for transient loading - Wind and Seismic (ELF) - would be overestimated. To obtain more realistic deflections and hence drift checks, it is necessary to run a second analysis process after adjusting the building analysis modification factor values to reflect short term properties (similar to the default values for Modal Analysis (and RSA)).
When the design considers staged construction analysis, this enables assessment of drift using short term properties and member design using long term properties in a single design process. For further details see: Staged construction modification factors.