Spring Stiffness for portal frame base

[Italo01]

Hello,

The SCI(Steel Construction Institute) from UK has a document called "Elastic Design of Single Span Steel Portal Frame Buildings to Eurocode 3".

This document has the following recommendations for modelling of columns bases:

- For ULS, consider the base pinned even if you use a four bolt pattern with the bolts outside the flanges;

- For Stability assessment, consider a Spring Stiffness equal to 0.4EI_column/L_column;

- For SLS , consider a Spring Stiffness equal to 0.8EI_column/L_column;

I think that i understand the logic behind this approach, which i will describe and if i'm wrong, please someone corrects me:

- For ULS, the bending moment at the base increases significantly the cost of base plates and foundation while it doesn't reduce significantly the bending moments on the rafter;

- Although the foundation and base plate aren't designed for the bending moment, they will resist some and this will be enough to reduce the deflections considerably. ( When i analyze the portal frame with the spring, the bending moments at the base are very small compared to the ones obtained for rigid base)

But although i undersant the reason for the approach, i cannot understand how the base stiffness will be independent on the soil at which the foundation is supported. How can i use the same spring stiffness for a pile cap over sand and for a footing over a soft clay?

Thanks for your help.

 

[GeorgeTheCivilEngineer]

I've understood it as the mass of the concrete pad, internal slab and overlying soil would all contribute to reducing the deflections, not just the rotation of the foundation (necessitating a reaction from the soil). But I believe it is more empirical guidance than that as the stiffness given only relies on the stiffness of the beam and column.

However, I'm sure that if the soil is competent for the ULS vertical condition, it should be OK for the small spring-SLS moments. You could work this through to satisfy yourself that this is the case.

 

[Italo01]

Thanks George,

I think you are correct that is an empirical guidance. I've already use the recommendation and i'm not unconfortable with it because i see that the moments are very small and i know that even if i consider the base pinned, the fact that i'm using four bolts pattern will produce some stiffness. I asked just because i like when i know where the recommendations come from.