HILTI HIT-RE 500-SD Max Embedment Depth

[iv63]

I am evaluating existing foundation of 80ft tall vertical vessel. My calculations show that steel capacity of anchor bolts and concrete breakout strength of anchors in tension are NOT adequate to resist applied tension loads per the current codes. To make it work I am proposing FIELD DRILLING (16)~1-1/2"?x36" DEEP HOLES AND SETTING 1-1/4"? ALL THREADED RODS (HAS ASTM A193, GR. B7) WITH HILTI HIT-RE 500-SD ADHESIVE. To increase anchor’s effective embedment depth hef I am adding on top of existing fdn mat new concrete 12” thick doweled to the mat (see attached sketch).

However after running Hilti PROFIS Anchor program with hef=25in (max. approved) these anchors are still failing (concrete breakout 11% and pullout 8% - see attached Design Report from PROFIS).

Question: In order to increase anchors’ capacity can I use actual embedment of 30” with prorated capacity (multiply capacity with max. approved hef=25in from PROFIS by ratio 30/25=1.2)? Also I would like to use bigger diameter rod. Is this allowed since Hilti has data only up to 1-1/4”??

BTW I have called HILTI’s tech support with no luck.

Regards,
iv

 

[JedClampett]

Hilti can only vouch for tested configurations, so if you exceed their tested values or as far as ACI allows them to extrapolate, you're out of luck. And I wouldn't increase the capacity linearly, since it's group action that governs. That's the bad news.
But I would do as much pencil sharpening as I could from the load end, see what you get and leave it at that. You're only 11% over and that's with load multipliers and phi factors. Basically, for an existing configuration, you're slightly reducing the factor of safety for an unlikely load. Note this in your calculation and you're done.

 

[TXStructural]

Look at the newly proposed ACI 318-11 adhesive anchor section. You need sufficient embedment to anchor the rod to full steel capacity, but you may want to debond the end closest to the surface to help move the forces deeper into the concrete. Consider anchor rods rather than all-thread and be sure you are providing significantly more capacity than required to avoid fatigue issues.

Do not oversize the holes unless the adhesive says to do so. 1/4" over is quite a bit. Look at the newest Hilti adhesives, which seem to have better tolerance for bad holes and poor prep.

There is no reason you cannot use a deeper embed than the minimium required to reach capacity (Hilti probably does not recommend an anchor rod longer than they produce, nor one which is embedded deeper than required to develop that rod.)

Using a product for a larger diameter rod may not give predictable performance when scaled from mfg data. The circumference area-to-strength ratio will be lower for larger diameters. It is conservative to use an embedment length which provides adequate capacity for a smaller rod. (For your 1-1/2" diameter rod, use embed length required for 1-1/4" diameter rod for your required capacity.)

As for adding reinforcement, bars which are perpendicular to the anchor rods will confine and provide some restraint to breakout, but you need to develop axial force into reinforcement going into a deep foundation. If the foundation is shallow (you say "mat"), this can be difficult if there is not enough depth to develop forces into reinforcement. This may require strut and tie modeling or similar assessment.

 

[anchorengineer]

Hi IV63,
Once you get past a certain depth the concrete breakout calculations are no longer valid. You will need to design your connection like embedded rebar (although I'm not sure of the effects of the discontinuity of the slab). Besically you'll be designing for development length and need to look at your design like column/footing reinforcement. There is testing (I believe in Europe) that Hilti's RE500 performs almost identical to rebar that is cast into concrete (assuming you follow the exact installation procedures). I believe there is something in ETAG regarding this type of design. The engineers in Tulsa are less flexible when it comes to designing outside the "tested" data. Try calling your local field engineer.
Good Luck!

Clarke Engineering Services, PC
Construction Consulting & Anchor Testing

http://www.anchorengineer.com

 

[delagina]

hmmm i have not retrofit an existing vertical vessel foundation since they dont add new stuff in there.
anyway, why dont you just add more hilti bolts?