Reinforcement in pier?

[aenger]

Hi Guys,

I am designing footings for a car port(portal frame). I have one pier under each column to the depth of say 3m. So the pier is gonna act as post footing to resist moment. Just wondering if reinforcement is needed in the pier? I am not confident on this.
Thanks

 

[BridgeSmith]

It will depend primarily on 4 things -

1) The magnitude of the bending load applied
2) The stiffness of the post
3) The embedment depth of the post into the pier
4) The diameter of the pier

All 4 relate back to one factor - Does the design load exceed the rupture stress in the concrete?

If it was me, unless the post extends to the bottom of the pier, I would not consider leaving the pier unreinforced, and even then I would only consider it if the pier was significantly larger than the post (pier diameter 6 times the post diameter). Without doing analysis to determine the stresses on the concrete (which would likely take more time than it's worth), I'd take the cautious approach, especially with something as cheap as reinforcement.

 

[oldestguy]

Added info needed: What lateral load must be resisted? What is the soil type? Why so deep? Looks like a much larger foundation, than normally would be used for a car port. Also any wind uplift info?

 

[aenger]

Thanks for your replies.
Sorry the pier is to be 600mm Diameter. The reason why it is so deep is because of the deep fill.
The design M* is say 20kNm and N* is -10kN (uplift).
So instead of embed the post, I am thinking if it is okay to stop the post on the surface of pier so the pier is gonna be a deep pad footing?
With 600mm Dia pier, Area = 0.28m2, Z = 0.02m3, so f = N*/A + M*/Z = 1.04MPa < f'ct = 1.8MPa. so no reo is required? (what the structural behaviour is like for this when skin friction is involved?) Sorry these may sound like ridiculous but I have not designed this kind of deep pad footing before, really have no idea what should be the correct scenario.
(FYI, for the bearing capacity check, I am gonna use the skin friction as well)

Thanks again.

 

[BridgeSmith]

In your stress calculation, I don't think using the plastic section modulus, "Z" is valid for unreinforced concrete. I think you should be using "S"

I'm not clear on the reason for the 9m depth, but at that depth, a 600mm diameter pier seems unnecessary for such small loads. A concrete shaft half that big with 4 rebars would be far less expensive, have far less uncertainty with regard to strength, and require less embedment depth for the anchor bolts. With an unreinforced shaft, the anchor bolts would need to go deep enough to be developed below the point where the load is fully distributed to the shaft.

 

[SlideRuleEra]

So instead of embed the post, I am thinking if it is okay to stop the post on the surface of pier...
I have not designed this kind of deep pad footing before, really have no idea what should be the correct scenario.

Why not just auger the 3 meter deep hole, embed the post, and be done?
If the post needs to be "longer" to do this, perhaps you could consider something like an HP 8 x 36, a steel pipe, or a wood pole as the "post".
Probably less expensive than fiddling around constructing a pier, also.

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[aenger]

Sorry but it is 3m depth not 9m. With 3m depth the bearing capacity is say 400kPa. So do you think 600mm Diameter is enough if no friction is involved considering the stress that I calculated using 'Z' is around 1000kPa ? (I am a bit confused by how to comp the bearing capacity when friction is involved for a circle section at stress level. Could you give a brief computation using 'S' on this?)

The post is inclined and that's why I didn't extend the post. Not sure what steel pipe is, I will have a look but I guess the client won't choose this.

Thank you very much.

 

[SlideRuleEra]

1)The post is inclined...
2)...that's why I didn't extend the post.

1) If the pier is plumb, and post is inclined, design of the pier becomes more complex than you are assuming.

2) Inclined (battered) driven piling are used frequently. An inclined embedded post is practical, too.

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[BridgeSmith]

If I've done my conversions right, 400kPa allowable bearing pressure with a 300mm diameter base will give you 28.27 kN of end bearing capacity.

For our drilled shaft foundations, we don't combine skin friction and end bearing. Our typical materials (sand & gravel) mobilize resistances at such different values of movement (settlement) that when we have done the calculations per the method recommended in the spec (O'Neil & Reese 1999) we only get about 5% of one to combine with the full value of the other. We just choose one or the other.

In the context I assumed, 'Z' is the plastic section modulus, equal to d^3 (d cubed) / 6, while 'S' is the elastic section modulus of a circular section, equal to pi * d^3 / 32. 'S' is less than 60% of the 'Z' value, making the stress in the concrete very close to the tension limit you mentioned. I wouldn't be comfortable relying on the typically unpredictable tension strength concrete to be as predicted.

 

[Ingenuity]

I don't think using the plastic section modulus, "Z" is valid for unreinforced concrete. I think you should be using "S"

Outside Norrh America, and specifically where the OP is located (AU) “Z” refers to the elastic modulus and “S” is the plastic modulus.

Weird but true. Quirky like the use of the metric/imperial units system.

 

[BridgeSmith]

Ah, thanks Ingenuity.

aenger, please disregard my comments on 'S' vs. 'Z'. It seems there's more that we do opposite besides which side of the road we drive on.

 

[dik]

Thanks Ingenuity... didn't know that.

Dik

 

[dik]

Can you use a 24"dia cardboard sonotube for the pier form on a footing. Cast the concrete to within 1.5' of the top with the rebar (6#5's or whatever) extending to 2" from top. Once the concrete has hardened, place a Simpson post anchor and cast the balance of the concrete?

or drill a 2'dia friction pile and sleeve the top 1.5' with sonotube, and cast the friction pile to the u/s of the sonotube, place a Simpson post anchor and cast the balance of the concrete. This allows you to accurately place the post anchors, independent of the pier or friction pile.

Dik

 

[aenger]

Thanks for your replies again.

To SlideRuleEra: The builder doesn't prefer embed the post though.

To Ingenuity and HotRod10: Yeah by 'Z' I meant elastic modulus. With 400kPa capactity 300mm diameter pier does have 28kN capacity and 113kN for 600mm Dia pier. But as momemnt is involved, Don't we need to calculate the max stress and compare it with 400kPa like we do for shallow pad footing? (as my previous computations shows, it would fail as the max stress would be 1000kPa, which is greater than capacity (400kPa). Not sure how much the skin friction will help for it though)

To dik: I will use the anchor. I am more concerned about the pier itself though.


So in short, with 600mm Dia pier 3m depth, 400kpa soil bearing capacity, do you think it is safe for such loads? (M*= 20kNm and N=-10kN)?

 

[SlideRuleEra]

aenger - Ok, thanks.

If the pier is plumb but the post is inclined, soil will have to provide resistance to the horizontal component of the post load. Also, this will have to be considered for design of the pier itself. How much of a consideration this is depends on the slope of the post. For a slightly inclined post... not much concern. The the more the post slopes, the more horizontal load to be considered.

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[BridgeSmith]

"Don't we need to calculate the max stress and compare it with 400kPa like we do for shallow pad footing?"

No. This would be considered a deep foundation. The moment is resisted internally and by lateral soil resistance. Only the axial load is seen by the end of the shaft against the soil below. If your axial bearing load does not exceed 28kN, then you should be good for bearing with the 300mm shaft. You'll need enough skin friction to resist the uplift in excess of the weight of the concrete pile, which, if I've converted units properly, shouldn't be difficult for the 300mm shaft (the 600mm shaft weighs more than the uplift value, if I've done my math and metric conversions correctly), since the required skin friction value is about 1/10 of the lowest value I've seen from our geologists.

That just leaves the matter of the lateral bending resistance of the soil and the structural resistance of the shaft, which based on some assumptions I made of the soil properties and shear at the top of the pile, should work out with a lightly reinforced 300mm shaft. We use Allpile and LPile, both of which utilize P-y curves to analyze this type of foundation. Hopefully, you have access to something similar (several programs of this type have trial versions online). There are also hand methods, such as Broms.

If you decide to stick with the 600mm shaft, it would seem to be more than adequate for stability. I would still recommend against leaving it as an unreinforced shaft, regardless of whether the theoretical concrete tension strength is adequate. Concrete strength in tension is very unreliable.

 

[aenger]

Thanks you very much guys. Really helpful information! I learn a lot. Cheers