Concrete Tension capacity utilization 1

[Canada580]

This may be silly questions but may not be at the same time.

I want to use concrete tension capacity. Say, you have 1m diameter 30 Mpa concrete. Steel anchors at middle that load the pile into tension. The pile then resist load through skin friction with soil. Say the pile have minimum rebar and not sufficient to carry the anchor pull load at all.

Now, my question is; even if I consider the tensile strength of concrete - only 1 MPa and do not consider the steel bar strength, it has almost 800 kN tensile capacity. Can we say, the pile have this much uplift capacity ? no problem on soil side. it is say plain concrete with anchor rods on it for uplift.

Thanks,

 

[klaus.]

the basic issue with concrete and tension is the cracks
if there are cracks in concrete no tension can be taken
i don't know your system,,, but maybe you can use the dead load of concrete for uplift !?!?

 

[StrucPEng]

I want to say that the steel anchors to the concrete pile is where you would have your issue. Are you able to develop the full uplift force without any exceeding any of the anchor bolt limit states (Break-out, pull out, etc)?

I believe the uplift would normally be handled by the skin friction of the pile but you would more likely have a local failure at the anchor bolt connection rather than a global failure of the pile.

 

[Ron247]

What does the code you are using allow for concrete in tension when it is not a prestressed member? I am not familiar with all concrete codes. I am old-school, I tend to not trust concrete in tension. Failure may give very little warning.

 

[BridgeSmith]

What minimum reinforcement percentage are you using? By my calculations, you'd have to be below 0.25% of the concrete shaft area to have more capacity for the concrete in tension at 1Mpa than the tension capacity of the reinforcement. Minimum drilled shaft reinforcement per the AASHTO code we use is 0.7%.

 

[rapt]

The basic answer is no. The anchor loads have to be transferred to the bottom of the pile with reinforcement. Concrete tension capacity cannot be assumed as once it cracks, the tension capacity is zero.

 

[MIStructE_IRE]

Think of it the other way round - if I hung a floor slab from an unreinforced tension ‘column’ and asked you to stand under it while I load up the slab on the basis that the anticipated stress in the hanger is less than 1MPa...what would you say to me...?

One crack and the whole lot’s gone!!

 

[KootK]

I sympathize but agree with others that you probably ought to have reinforcing over any portion of your pile relied upon for tensile capacity. Whenever I'm tempted to rely upon concrete's tensile capacity, I ask myself this question: can I imagine a scenario where cracking of the member may occur prior to the development of the load to be resisted? Usually, I can. In this, consider this scenario:

1) The pile is cast.
2) The pile develops shrinkage strains and stresses as all concrete does.
3) Soil is highly variable and resists the shrinkage more at some sections of the pile than others.
4) Because of #3, there's the potential for cross sectional tension cracking most anywhere, between locations of local soil resistance.

How much stock do you really want to place in something like this not happening?

 

[rapt]

Ron247

Even with prestressed concrete, tensile strength of concrete is 0.

Codes allow a level of tension at service, but this is actually just a way of indicating that a full cracking check is not required as the steel stresses even assuming no concrete tension will be very low. Some codes do word this badly (ACI especially), but that is what it actually means.

 

[Ron247]

Thanks for the info rapt. I have never done any prestressed design. I was referring to the following 2005 ACI statement and section 18.4 is about serviceability of prestressing as you stated.

 

[BridgeSmith]

The difference is between allowing tension to exist in the concrete (which is allowed), and counting on the concrete to provide tensile resistance (which is not allowed by any codes I'm familiar with).