Deck post footing at edge of a 45-degree hillside

[PSUengineer1]

Looking for recommendations on design of deck footing on edge of 45-degree hillside. References to IRC and specific design examples appreciated. Thank you.

 

[mjr6550]

You will not find any references in the IRC that will be applicable. If a concrete pier footing is used it will have to extend deep enough to stable soil and meet requirements for depth based upon the geographic area. Another option could be helical piers and perhaps a reinforced cantilevered spread footing.

I have a bit of practical experience and basic geotech knowledge, but I'm not an expert in this area.

 

[dhengr]

PSUengineer1:
I agree with what Mjr6550 has suggested. I would just add that this is probably a deck and terrain condition which does beg the need for a GeoTech guy’s involvement. On that steep a slope one of the things that can happen is a slope soil shear failure, starting behind and immediately below the footing bearing level. And, this soil failure condition gets more involved than what most structural engineers want to tackle. It can involve stabilizing the slope below the ftg. so that it can take the ftg. pressure without slope failure.

 

[jdgengineer]

Totally agree you need a geotech. In our area, this would almost certainly be supported on drilled piers.

As a rough rule of thumb some geotechs in our area assume supporting material is roughly based on the rule of 10. Measure out 10' and see how much grade you've lost. In your case this would be roughly 10'. Therefore, we would neglect the top 10' of the pier for passive and friction design and apply soil creep in this area. This would lead to a pier roughly 20-25' deep from the ground surface.

As others have mentioned global slope stability can also be an issue. 100% agree you need a geotechnical analysis. Helicals would often not be suggested in our area for these conditions due to the soil creep.