Epoxy anchoring new concrete pier to existing mat.

[chrisrosebud2001]

I have a situation where I am designing a new concrete pier to be installed to an existing mat foundation. The pier is to be fixed at the base and I am having trouble getting the anchorage to the mat to pass code. Bars are to be anchored to the mat using an epoxy system and I am having no trouble developing the strength of the bars, but breakout calculations per ACI 318 appendix D is controlling the design. I have even tried an embedment up to the recommended maximum of 25". The bars are spaced close enough together that the Anco/Anc factor will never be large enough to make this work. Does anyone have a suggestion of what to do? The mat is approximately 3' thick, so I can still increase the embedment, but this goes beyond the scope of appendix D. Could possibly the old 45 degree cone method be used for a deeper embedment?

Thanks for any help.

 

[SlideRuleEra]

Since your project does not have an easy solution this sounds like a good time to contact the technical services of Hilti

http://www.hilti.com

, or another reputable expoy achoring system manufacturer. They should be able to suggest ways to make maximum use of their products.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[delagina]

if i remember right the tables for hilti (or equivalent) epoxy to develop rebar to full tension is not deep.

 

[chrisrosebud2001]

This issue is not the strength of the epoxy bond. The problem is when you consider the bars to act as post installed anchors per ACI 318 appendix D. The concrete breakout strength is controlling. Should I neglect this consideration since I am using reinforcing bars instead of anchors?

 

[asixth]

I've never been thru the ACI318-D equation but how does the code capacities compare to the breakout capacities of plain concrete the the epoxy suppliers provide accredited test data for? My own ignorance more than any because I haven't been thru the calculations in Appendix D but I thought Hilti has test data for group effects.

 

[chrisrosebud2001]

There is an ICC report for the epoxy anchors. You can download it for free. The report is ESR-3013. The breakout strength should be calculated in accordance with Appendix D. The main difference between ACI and the ICC report is determination of the pullout strength of the anchors. After looking into this further, I realize that I am also having trouble meeting the pullout requirements as well as the breakout requirements. It seems that the only way I may be able to come to a solution is to make the pier larger in order to reduce the tension in the bars. It's just so frustrating to have no trouble designing the pier with plenty of reinforcement, but not being able to develop the force at the base. Any other ideas or more than welcome.

 

[dcarr82775]

If concrete breakout is the problem there isn't much you can do except make the breakout cone bigger (deeper anchors or larger spacing). Williams Form Engineering makes some rock bolt type expansion bolts so you could drill 30-32" in a 36" mat. They don't have up to date ESS reports so you would have to use your judgement there.