Stability of a Foundation

[newbuilder]

If I design a portal frame with a footing under each column, then I can check the global stability against overturning by providing a suitable footing mass under each column.

The same frame is designed as being fixed at the base, i.e. a moment is transferred to each footing. Does this footing also need to resist the overturning induced on it by the fixed end moment from the column? Do I need a factor of safety of 1.5 against global stability (described in para 1) and also individual stability.

Please advise.

 

[Focht3]

A F.S. of 1.5 is, indeed, a minimum for this type of foundation assuming that the principal loads come from wind. (Earthquakes are a different matter.) If the portal frame is in a public area, I'd think about increasing it to 2 since there is no real redundancy in the design that could reduce the risk of a catastrophic failure. "Stand-alone" structures can often pose a greater risk -

Be sure that your geotechnical consultant looks at the issues of bearing capacity and progressive "toe" settlement under repeated wind loading. The settlement issue is important if you expect the portal frame to be exposed to "significant" repeated load cycles during its' design life.



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[newbuilder]

Thank you for your reply

 

[Lutfi]

I would never design a footing with less than 1.5 safety factor. This applies to uplift and overturning.

You will never go wrong doing so plus all codes that I am familiar with mandate such cushion.

Good luck.