Pier with Footing Overturning Moment

[Renderuru]

I have a concrete pier with a footing sitting on bedrock below ground level. There is a moment applied on the pier that I want to resist by increasing the footing size. I wish to resist this moment with the weight of the fill applied to the footing. Can I just use the weight of the fill directly above the footing? I wonder if the fill 'fluidity' (for lack of a better term) makes this assumption incorrect.

I also plan to embed rebar into the rock but I don't want to include that in this calculation.

Thanks.

 

[oldestguy]

I'd use that fill to help, but I'd consider the volume to be within an inverted truncated prism with sides about 20 degrees from the vertical. If not where the ground water table will be near the surface, with an assumed density for ordinary soil at 115 pounds per cubic foot would be Ok for most soil. For rods GROUTED into rock, consider the weight of rock within an inverted cone surrounding the rod with sides at 30 degrees to vertical and density at 145 #/cf. Very little safety factor is assumed fr these numbers. Where water table would be above these zones, take the submerged density way down, That rakes some computation usually. Same goes for density of the footing.

 

[Renderuru]

Thanks oldestguy. I will be increasing my footing so the actual weight of the footing concrete plus the weight of the fill above is FULLY countering the moment. I would like to rely minimally on the rock anchors. Reasons being that I don't want to do geotechnical, and I don't know the type/depth of bedrock..... I appreciate the input on the inverted truncated prism suggestion. I've googled it and I think I understand

 

[BridgeSmith]

Is this an existing footing you're going to enlarge, or a new foundation?

 

[Renderuru]

This is a new deck pier with footing (being built in May). I've designed it to be buried 6FT, and resisting the small moment from the deck lateral loads. But I now realize that I could hit bedrock (I see bedrock in my backyard). If for some reason I hit bedrock early I need a plan of action for the contractor.

 

[oldestguy]

A little more explanation of the site and where the lateral load is coming from as well as why so deep will help. Right now we could be discussing something maybe not best related to the actual job.
The more the info the better the answers.

 

[Renderuru]

Deck surface area = 20FT x 28FT
Deck fully supportd by six piers - 12" concrete piers
Actual lateral(unfactored) load per pier is 500lbs
Pier design depth is 6FT.

 

[CANPRO]

Is the overturning always in the same direction? If it is, you could look in to offsetting the pier from the footing so gravity load counters the overturning.

 

[oldestguy]

Still not fully clear as to what this deck is. A dock where wind or boat impacts expected? Material for the "deck": wood, concrete slab? Loads expected on the deck? Trucks, cars? Any chance of tying in the deck to the piers with a moment resisting connection, such as the situation with a kitchen table with legs secured to the top? With that depth for footing there may be plenty of lateral resistance of soil against the piers. It's assumed your pier to footing connection is designed for that overturning moment.

As to this quote, what do you think some of us might be? Reasons being that I don't want to do geotechnical

A little local help may save a lot of unneeded grief and expense.

 

[BridgeSmith]

"A little local help may save a lot of unneeded grief and expense."

I'll second that suggestion, and add that with help from someone with knowledge of the local soils and bedrock, there may be an opportunity to save a significant amount if a drilled pier/drilled shaft foundation can be employed. With that type of foundation, if the bedrock is shallow, so much the better. Drilling 12" holes a few feet into the rock and avoiding the excavation altogether could have significant benefits, if you can access the site with suitable rig.

 

[Renderuru]

Thank you

 

[oldestguy]

Consider this. Dig to rock or your 6 feet proposed. Pour a concrete pad at each location sized in area and thickness for what goes on top and at same or 8" step top elevations. On top build a pier at each location with concrete block. I'd look at maybe 32" x 32" in plan view. If you want to fill the inside with concrete, but probably not necessary. Likely these piers will handle all lateral loading due to there plan view size,deck weight and with some backfilling. No re-bar needed in footing or "pier"., unless you want to "gild the Lilly".

 

[BridgeSmith]

Since it's for your own project, all the 'geotechnical' you may need for a drilled pier is to identify the type of bedrock and use a low-ball estimate of the subgrade modulus (k) for the soil and rock that you have in a lateral pile analysis program such as Allpile or Lpile, or even a hand method such as Broms'.

See section 16.6, Page 11 - Link

 

[oldestguy]

Some more from OG. Looking at my house and the WF beams sitting on pipe posts. Consider securing that deck to the piers, with them at least partly filled with concrete. Leave threaded rods projecting from that concrete say 3/4" dia. Then mount the WF beams so the threaded bars secure the lower flange. On top of the WF lay a 2 by and bolt it to the top flange. Then the floor joists can be secured to the 2 by with the sheet metal fasteners readily available for tying down roofs.