Typically a four-bolt column base is usually considered to be a nominally pinned support in structural analysis. However, benefit can be taken from the stiffness of nominally pinned bases to reduce frame deflections and to reduce the effects of the deformed geometry.
The traditional, conservative design approach classifies the below typical base plate joint as nominally pinned and models the structure with fully pinned column bases during analysis.
Some design guides and international standards, such as British Standard 5950 (cl 188.8.131.52) or SCI P397: Elastic Design of Single-span Steel Portal Frame Building to Eurocode 3 (Section 6.4), advice you to utilise the stiffness of the nominally pinned bases in the SLS design situation or when assessing the frame stability.
They proposed, that under working loads, the base stiffness can be assumed as a proportion of the column stiffness:
- 10% fixity for frame stability (calculating αcr)
- 20% fixity for deflection calculation in SLS
In the following example, we will show how the deflection results change using fully pinned and 20% partially fixed column bases.
We have a simple goal post arrangement with the following dimensions and loadings on it. The loading represents an SLS load case.
Comparing the analysis results from the two model variations, we see that if we use the proposed 20% base stiffness (fixity) for our frame deflection calculation, the horizontal movement of the frame eaves decreased by almost 40%. However, the mid-span deflection didn’t change significantly (less than 5%).
The following video demonstrates how to apply different base stiffnesses to ULS & SLS load cases on the above model in MasterSeries.
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