When to account for horizontal loading in foundation design

[Paul Beetham]

Hi Andrew / everyone,

I am trying to establish if there is a clause within the Eurocode suite which states at what point horizontal loads imposed upon / by a building need to accounted for in the substructure design.

Thinking back to my uni days - As an exercise we were tasked with undertaking building action and pile design calcs, using the draft versions of EC-1 and EC7, for a fictional 7 storey building. We went through the process of calculating self / wind / snow / imposed actions, to determine the characteristic action transmitted to the pile group. When it came to consideration of whether the piles needed to be designed for lateral forces, we accepted the guidance of our Professor, that as the horizontal actions were less than 25% of the vertical actions, then they could be discounted. So, glad of something less to do, we did! Without thinking too hard about where the 25% rule came from.

Fast forward today and I find myself needing to know where does (if indeed it does) the EC suite state that horizontal actions can be ignored for sub structure design, if they are less than x% of vertical action?

Is there a clause tucked away somewhere that alludes to it, or is it Engineering judgment? If engineering judgement then surely a 'one liner' with a literature reference of some description would be in order to satisfy horizontal loading as a limit state?

I have had a quick scan through various parts of EC1 and EC7-1 and drawn a blank. I've also failed to find any information on this in 'decoding EC7' (apologies if it is in there!), although it is more of an EC1 question, so probably beyond the scope of the book?

Any guidance would be gratefully received.

Kind regards Paul

[Andrew Bond]

Paul

You raise a good question, to which I don't have an answer I'm afraid. I'm not aware of any clauses in the Eurocodes that cover this issue.

The topic is of interest to me for another reason - with some of my other colleagues on the EC7 committee, I am currently looking into the application of the EQU limit state for problems in which there is purely balanced, compression loading. An example might be an elevated highway supported on a monopile.

The question is: what horizontal force and/or moment should the foundation be designed for, when there is no out-of-balance load from the superstructure? In existing BS practice, I believe a minimum horizontal load is applied, which then also produces a moment. The Eurocodes do not specify such a mimimum force.

If I find out anything more I will report back here.

Andrew

[Paul Beetham]

Hi Andrew,

Many thanks for the reply and the confirmation that there is no (obvious) Eurocode ruling.

For the time being I can only conclude that the 25% rule was 'guidance only' from a worldly wise and experienced mind.

My personal interpretation is that the 25% value may relate to a value that 'by inspection' will be acceptable in view of the frictional / passive resistance and self weight of the superstructure. At least for the ULS.

As with any guidance, however, I'm sure there will be exceptions and occasions where a detailed assessment of the lateral forces may be required, regardless of how low the horizontal load may appear in percentage terms, especially for the SLS.


Thanks Paul