Concrete Tensile Stress Area In Ringwall 1

[PEStructural]

I am designing an AWWA D100 circular steel tank that is supported on a concrete ringwall. I am designing the anchorage to the tank using ACI 318-02 Appendix D "Anchoring To Concrete". When designing for pullout strength ( Section D.5.3 ), there is a variable Y4. It then gives a stipulation "For an anchor located in a region of a concrete member where analysis indicates no cracking ( ft < fr ) at service load levels, the following modification factor shall be permitted, Y4 = 1.4".

I have the hoop tension in the concrete ringwall calculated.

My question is to get the tensile stress, ft, do I divide the hoop tension by the gross area of the ringwall because it is all in tension?

 

[AlohaBob]

Yes.

In my mind, it would seem that the hoop stress of the tank would be resolved within the tank itself. And the foundation merely supports vertical load or wind or earthquake or some unbalanced loading. If that's the case, the concrete ring would just be in compression from applied load.

 

[KBadger]

The concrete does not carry any hoop tension. The reinforcing steel in the ringwall carries all the tension. Use your steel area, not the concrete area.

 

[AlohaBob]

I think mjflorio is sizing anchor bolts to secure a steel tank to a concrete foundation.

My ACI is 318-99. I don't have this Appendix D Anchorage.

However, by the description I see above, it appears the design values for anchor bolts are dependent on their location in a tension or comression zone in the concrete.

In this case, although true that the actual tension of a concrete tension ring would be provided by the steel reinforcement. The average tensile stress would be the tension over the gross area of concrete to determine whether ft < fr. I suppose you can also check the strain in the steel under the load and compare this to the strain corresponding to fr.

But this misses the observation if the steel tank itself resolves the hoop tension stress from the containment of some product, then the concrete should just be in compression from vertical load.

 

[PEStructural]

AlohaBob,

You are correct. I was looking to see if the section where the bolt resides is in cracked concrete ( ft > fr ) or uncracked concrete ( ft < fr ). The hoop stress in the ringwall is actually from the soil next to the ringwall ( not taking into account the passive pressure on the other side ) per Structural Engineering Handbook, 3rd edition, Gaylord & Gaylord, Section 27-4.

Thanks for confirming what I designed for.

 

[JedClampett]

Ringwalls (actually the reinforcing) carry a tension force. This is due to the active pressure component of the vertical soil load. In Gaylord and Gaylord "Structural Engineering Handbook" the amount of reinforcing in square inches is calculated by:
Area of Steel=.00052*H*D*d*g
g=the specific gravity of the fluid
d=height of the ringwall (ft.)
D=Tank Diameter (ft.)
H=Permissible Height of Liquid (ft.)
D=Tank Diameter (ft.)
This assumes an active soil component of about 1/3 and a tensile stress in the reinforcing of 20,000 psi.
I would use cracked sections for the anchor bolt analysis.

 

[KBadger]

The surcharge weight of the water inside the tank plus the lateral soil pressure (usually at-rest as opposed to active due to the stiffness of the ringwall) creates tension in the ringwall. Whether or not the concrete is in tension depends on the modulus of rupture of the concrete, the section area and whether or not there is reinforcing steel in the ringwall.