how to assume drilled pier skin friction and lateral capacity for patio column. no geotech.

[delagina]

I friend is adding covered patio.
The patio column will be on drilled pier.
I can calculate the wind and all other forces that will be on top of drilled pier.
But how do I estimate the drilled pier skin friction, bearing and lateral capacity without geotech?
any other better option for foundation of column patio?
Is it cheaper to use DIY helical piles?

there is already have a 6" existing slab that we rather not on dowell into.
we'll put the drilled pier outside the existing slab.

 

[jayrod12]

Helical piles will have negligible lateral resistance.

Beyond that comment, I would recommend a geotech. You could look for ansi/asae shallow post foundation design for some guidelines. However, don't forget about frost jacking depending on the location.

 

[delagina]

no frost in location.

 

[BAretired]

But how do I estimate the drilled pier skin friction, bearing and lateral capacity without geotech?

You can't. You need to know the type and characteristics of the subsoil.

BA

 

[oldestguy]

If you have local laws requiring a building permit, it is likely the inspector will want to see something to prove the support is there. Otherwise there are many references available giving average soil properties, but you should know more about what is down there first.

 

[delagina]

also I'm not being paid here. more like friendly advice so I just give that.

I do have an idea about soil because I know the location

 

[jayrod12]

Friendly advice will bite you in the ass. Recommend a geotech.

 

[delagina]

this is just a small patio. he is not even using a contractor and will do it himself. I don't think he will do a geotech report.

Can I make the weight of drilled pier heavier than the tension. that should take care of this patio being blown away by hurricane

 

[delagina]

Also do you advice this patio beam connected to house ledger?
I see conflicting recommendations regarding this.
We can put 4 columns so the patio can be stable as detached and just use proper flashing for patio roof to house siding.

Based on google, more DIYer or contractor recommends connecting to ledger house regardless if I can make it stable as detached.

 

[Archie264]

Here is some prescriptive information from AWC for decks. A porch addition would be different, of course, but there should be enough similarity to some of the components that it might be of use.

http://www.awc.org/pdf/codes-standards/publications/dca/AWC-DCA62012-DeckGuide-1405.pdf

 

[delagina]

here is a section view.
he wants the patio column and purlins made of steel instead of wood.

should this be connected this to ledger of house?

I get conflicting opinions which is better.

 

[Lomarandil]

Call me cowboy, Jayrod, but I think there's room for some conservative assumptions here to allow delagina's friend to size his drilled shafts appropriately without needing a geotech. (Otherwise we'd have thousands of patio failures around the country, right? Or geotechs would be doing a lot more residential business than I observe).

For tension loading, absolutely, size the drilled shaft to be heavier than the uplift (and be able to transfer that load to the shaft, of course) and you're good to go.

For lateral and gravity load, you'll need to make reasonable observations and assumptions about the material. Hopefully you can lean on local experience or a basic soil mechanics class in university to figure out a basic soil classification. With those in hand, take a look at NAVFAC or VulcanHammer.net to find some general guidance.

For patio-sized drilled shafts, in reasonable material, I would suspect that end bearing will be more reliable than skin friction.

What kind of foundation does the rest of the structure use?

 

[TLHS]

Don't use skin friction for short posts. The top layers of soil are pretty useless for uplift. Think about the failure mechanism. If you're only a couple of feet deep, how much soil are you really going to pull up with you in an uplift scenario?

Use concrete weight.

For bearing, a practical footing size will likely give you a reasonable amount of capacity. Use a conservative bearing capacity. It's likely that your jurisdiction has some presumptive values you're allowed to use that are really conservative. Go with that.

For lateral capacity, use the post foundation equations in the IBC. There are presumptive values for allowable lateral capacities that can be used in that equation as well.

I agree that a geotech report is probably overkill for a reasonably low risk residential structure. The tradeoff for that, though, is that your assumptions need to be more conservative to account for the risk.

This guy should check with the local building department. I know that around here as soon as you start using steel for anything load bearing it's pretty much immediately assumed to require engineering. There are prescriptive designs available for wood, and it's reasonably reviewable by the building department, but there aren't generally for cold formed or rolled steel.

 

[Canuck65]

For something small like this we would specify something like a 'bigfoot' form (plastic mold for 24" to 36" diameter bell footing)for the base of the footing, combined with appropriate diameter sonotube. Both of these are homeowner do it yourself friendly, as long as they don't mind using a shovel. The extra footing area should reduce your bearing capacity significantly, plus also provide a bit more dead weight to resist uplift. In my area we often design for a bearing capacity of 75 kPa, which is relatively poor soil, however, if you design for 50 kPa, than even better. Rarely would the soil bearing capacity insitu become a problem with such a low assumed bearing capacity, however, it has been known to happen. Homeowner to verify.

For designing something like this, we never assume any skin friction on the pier, small residential is typically designed using only the bearing capacity at the base. Helical piles can be used as long as the gravity and lateral loads at top of the pile are provided to the installer.

 

[jayrod12]

Call me cowboy, Jayrod, but I think there's room for some conservative assumptions here to allow delagina's friend to size his drilled shafts appropriately without needing a geotech. (Otherwise we'd have thousands of patio failures around the country, right? Or geotechs would be doing a lot more residential business than I observe).

I did give the shallow post reference, which is giving him some conservative assumptions. That guide goes through everything.

I feel though that giving friends free advice over something like this is cause for concern. I'm more confident about a friend replacing a load bearing wall in their house with a beam than I am with them building a covered patio that won't fall down.

 

[CELinOttawa]

I agree with Jayrod, and I think we need to be more diligent about stopping this type of thing around here.

Before the pile-on, please give this a thought: When did you last get free advice from a Doctor? Lawyer? Free advice reduces the value of our knowledge and harms all of our fees.

An hour's work for an hour's pay. Where your buddy is doing something for you in kind, fine, but don't engineer for free.

 

[MotorCity]

You should not rely on skin friction for such a small application. Skin friction is only applied in deep foundations, not piers for a patio that extend a few feet below grade. (Geotechnical engineers typically recommend neglecting skin friction within one or two pier diameters below the surface.)

 

[canwesteng]

I get free advice from my lawyer friends often enough. And advice from my doctor is always free (in the soviet republic of canuckistan).

 

[jayrod12]

It's just too bad it takes three weeks to see said doctor for his "free" advice.

 

[CELinOttawa]

Free as in paid in advance is not free... lol