I usually ignore friction on the base of a footing for sliding. I have a situation where the sliding force is slightly greater than the passive resistance on the footing edge and the key. Is it wise to include a component of the friction tan(phi) angle as part of the resistance? Is this commonly done?
I don’t want to increase the key depth if I don’t have to.
@dik I usually do essentially the opposite of what you are doing. I use the friction on the base and ignore the passive pressure as a first trial design for a shallow foundation. Maybe I am misunderstanding your question. In my experience it is very common to use friction on the base of the foundation to resist sliding.
@bootlegend Thanks… We have 6’ of frost protection in these environs… sliding is never an issue so just a thumbnail calc for passive resistance. The wall I’ve done only has 3’ and I’ve used a key. I’ve modified an old SMath program; if you want to review it, I’ll post it
For friction do use tan(phi) or tan(phi/2) for coefficient of friction?
@dik See the attached sheets. The values are based on tan (phi). To be clear, I am pretty conservative on my assumption of material beneath the footer. I typically use 0.35 as my friction factor unless I can be certain of something better and still require a factor of safety of 1.5 against sliding. For me that’s a lot quicker than figuring out the passive pressures, neglecting some top portion, etc. But I rarely have a foundation 6’ deep like you mention. On a couple retaining walls I have used a key and counted on the passive pressure on the key.
I am interested in seeing what you’ve done with the Smath sheet. I need some motivation to start working with the program.
@dik You are welcome and thank you for the calculation sheet. I will take a look at it closer when I have a few minutes. Looks very useful. I have recently been using the ASDIP Retain software for retaining walls and do like it for it’s simplicity compared to other programs, but the Smath sheet can be modified on the fly which is a serious advantage.