ACI 318-99 and 318-02 Anchorage to Concrete / Appendix D Discussion

[kevindav]

Strength Design of Anchorage to concrete by ACI code standards is getting a little out of hand, but that's the way it is. So, in order to help myself and possibly others clear up some questions and/or ambiguities I have started this thread.

Question 1:
In Appendix D of ACI 318-02 equation D-9 is used to calculate the psi 1 mod factor. However it is stated that D-9 is valid only for an eccentricity of the normal force less than or equal to the center to center spacing of anchors divided by 2. For a group of anchors (for example: say nine anchors in a 3 rows of three configuration) this limits the eccentricity in shear to half the distance from the middle anchor to the outside anchors in both directions. This same situation occurs in ACI 318-99 with eq A-19. How can this be? Is it a mistake?

Question 2:
I would like to see an example of the proper method to calculate the strength design of a group of anchors (say 5 or 9)by ACI standards embedded in concrete in a circular pattern.

Question 3:
I would also like to see interesting questions others may have in regard to these standards. (please use this thread)

Thanks,
Kevin

 

[MikeDB]

I can add to your list of questions:

4. Are there a lot of anchor bolt failures out there and is that why we need to complicate this issue?

5. I find it strange that threaded rebar used as anchor bolts was not address. They have been used in the electric utility industry for at least 30 years and I have not found any guidance for that outside of ASCE Manual 72. Any other sources?

 

[BryanStein]

When eccentricities are larger than allowed by the new provisions, the engineer must break the load into components (moment and force) about the centroid of the anchor pattern. THis may greatly increase the loads on a few of the anchors, and thus leads to very conservative anchor patterns. It also assumes a rigid embed plate.

To my knowledge there are not a lot of anchor bolt failures, but their are / were enumerable methods in which designers calculated anchor capacities. I presume that ACI and other organizations just wanted to unify the approach .... right or wrong.

 

[sdz]

For a background paper see ACI Structural Journal, N0 92-S9, "Concrete Capacity Design (CCD)Approach for Fastening to Concrete. Jan-Feb 1995.

 

[RobertoMiguel]

You might also want to check out ACI 349.2R-97, "Embedment Design Examples" (26 pgs). It's part of the nuclear safety-related structures ACI code. But, hey, I have seen no other industry w/ more complete documentation on this subject than the nuclear power folks. Probably because they've had the NRC in their hair for so long now.

Roberto Sanabria

 

[kevindav]

Thank you for your continued comments. The ACI-349 reports have been mentioned by a couple of other engineers I have spoken to. I am wondering, how many people actually implement that code in standard commercial and residential situations?

 

[NateB]

THis method does not discribe how to adjust strength for transverse reinforcement. Surely rebar ties would help strengthen the edge faces. Does anybody have a good reference for this concept?

 

[VirtualEngineer]

I have recently been delving into the Appendix D "Anchoring to Concrete" ACI 318-02 and ACI 318-05 Requirements.

Both years have similar requirements however some of the nomenclature of the 05 code is changed.

I also downloaded the CCD Approach for Fastening to Concrete from the ACI Structural Journal / January-February 1995. This article describes the research used to develop the CCD Approach, compares it to ACI 349, and to actual test results.

Apparently the ACI 349 method is somewhat unconservative for deep anchors, thereby necessitating the change over to the CCD method of analysis.

Surely though there must be a better way present the CCD Method than the way it is presented in ACI 318.

It is like a maze to work your way through all the different modification factors. I think the 05 nomenclature only makes the situation worse.

Some issues are totally neglected. For example, I am trying to calculate the blowout strength of anchors in tension close to an edge.

Equation D-17 gives the blowout strength for a single anchor. That value is modified by the value described in the paragraph below it, for a corner location.

Equation D-18 modifies the blowout strength given in D-17 for group action but leaves out the corner modification.

When you look at the value output from equation D-18 it becomes apparent that the group strength is only for two anchors. What happens if you have a line of anchors along an edge? What happens if you have a rectangular pattern near a corner? What happens in all the so-called “non-usual” situations that seem to occur with more regularity than the usual situations?

I would like to be able to calculate the strength of any anchor, with appropriate boundary conditions. Then I can look at any group with any applied load and moment and using statics, determine the load on each anchor and evaluate each anchor individually.

That is what I use to do with ACI 349, and eventually after digging into more of the underlying research, that is what I will use to design anchors using the CCD approach. Hopefully ACI will make their design methods a little less complex.

Regards,

JPJ