anchors set in epoxy adhesive 1

[v2]

This question relates to rebar or threaded anchor dowels set into epoxy adhesive with certain embedment into REINFORCED concrete members.

Is it acceptable to ignore the effect of anchor SPACING and/or EDGE distance on their allowable bond/concrete strength, due to the fact that reinforcing rebars intersect the assumed failure cone surface of concrete?

Thank you.

 

[anandaganesh]

The anchor spacing and edge distance are very much affecting the load capacity of these adhesive anchors. You have to refer to the manufacturer's catelog for calculating the reduction coefficients on load capacity of anchot due to spacing and edge distance. The HILTI catelog gives clearly these reduction coefficients.

 

[redhead]

Hilti also has software for their products, which will calculate the reduced load effectiveness based on the variables you input. The program is available on a CD. You can obtain it fro their website -

http://www.hilti.com.

 

[ishvaaag]

Even when reinforced you need to account for the edge and spacing effects.

 

[v2]

Thank you Ishvaag,

How can you get credit - improving the anchor capacity - for the fact that reinforcing exist, knowing that the load capacity of such anchor is based on plain "unreinforced" concrete test setup?

Thank you.

 

[Ron]

The capacity of the anchor, particularly an epoxy set anchor is independent of the reinforcing. The only time reinforcement will help is when it intersects a tensile/shear failure cone, typical of wedge anchors. Epoxy anchors typically do not fail in the same manner as wedge anchors.

 

[v2]

Thank you Ron,
Your statment is not clear as to why, how and by what amount reinforcing can help when the reinforcing intersect a tensile/shear failure cone, or in other word the concrete failure cone.
epoxy anchors can fail in one of the following three modes of failures:

1. Steel failure
2. bond failure
3. concrete failure

I am intrested in the concrete failure surface that takes the shape of a cone in most cases.
Now, since reinforcing does intersect the concrete failure cone, shouldn't the concrete failure capacity be higher than the one that has no reinforcing intersecting the concrete failure cone?

secondly, what is the difference between the concrete failure cone of a wedge anchor and that of an epoxy anchor?

and finally i am interested to know why/how reinforcing can help the wedge type anchor? by what amount?

Thank you.

 

[ishvaaag]

Since I don't see it in the codes that are reasonably conservative in these things it means to me that your question is not thought must be addressed by the codes at the present state of the art of anchor design.

This is more than a simple statement in that the code resumes usually what recommendable and what not.

Basically and till I see a reasonable and general treaty on anchors to whatever (which surely will be some tomes) I think I have to agree with the code stance, since except quite precise clarification of what kind of anchors are referred to and what relative disposition of the reinforcement there is, designers would misinterpret the new allowance, as in fact I think is the case in general with the design of anchors at the present, for which my common sense and experience about what the materials and structures stand indicates bigger anchors than those allowed should be specified.

The enormous variety of anchor systems, influencing variably the surrounding concrete and for some proportions and closenesses having bursting and slippage modes of failure neither help.

In any case, you can still come to an strut and tie mechanism to guess how the re-bar will influence a simple case of tension, it would be something rigid footing-like...for the shear case a pair may be thought acting and then bencing of both the rod and an assumption of bending or sutrut and tie again be made to take the forces in the pair...then interaction you can use some of the standing formulas...and then better you check what you are doing is certified be safe 95% of the test times, for this may have rational basis but lacks both correlation to any tests and code support.

 

[anandaganesh]

The capacity of the anchors are based on whether the concrete is cracked or uncracked. So, you must be careful while choosing the anchor types for using in the cracked concrete zones. As per the HILTI catelog, I believe that adhesive anchors are not fit for using in the cracked zones.

 

[steve1]

A new concept called the Concrete Capacity Design Method for determining anchor capacity is beginning to be implemented. The IBC2000 and ACI 318 currently have methods for calculating the capacity of cast-in-place headed anchors. The 2002 ACI 318 will contain Appendix D, "Anchoring to Concrete" and require post-installed anchor pre-qualification testing. The 2003 IBC will include Appendix D. Unfortunately bonded anchors are not currently included at this time. The design concepts for these anchors are currently under development and are expected to be implemented in about three years. Note that the ACI 318 Committee provisions will contain a methodolgy for both cast-in-place and post-installed anchors that would apply for use in both uncracked and cracked concrete.

The above information was derived from the paper entitled, "Building Codes and Anchoring to Concrete" by Richard Wollmershauser. The paper can be found on the Hilti web site.