Development Versus Anchorage

[RyUIUC]

I am specifying the use of a post-installed adhesive anchor (HILTI HIT-RE 500-SD) and a fundamental question came to me I have a hard time answering. Why are adehesive anchors, which initially resolve the anchor forces through the bond along the bar, checked similarly to headed anchors where it is assumed the forces are transferred through bearing to the concrete at the head alone?

By this I am talking about the ACI 318 Appendix D checks on concrete breakout(D.5.2)and Side-Face Blowout (D.5.4). For example, when the tension steel in a concrete beam extends beyond the critical zone we ensure the extension meets the required development length such that the full tensile force can be transmitted into the concrete section through the bond. We do not check for concrete breakout or side face blowout. However, when we use an adhesive anchor, which also transfers its tensile force through its bond to the surrounding concrete, we check the surrounding concrete strength virtually the same as if it was a headed anchor where allowable projection areas with edge distances and spacing can severely limit the tensile capacity of the concrete. As long as the bond is able to develop the bar strength shouldnt we be viewing this more similarly to the development of a cast-in-place rebar where this failure mode is neglected?

Refer to the commentary in ACI 318 Section 12.6 (Headed deformed bars in tension) for a description of development versus anchorage.

 

[hokie66]

Excellent question. While we are at it, why do some accept that bars anchored in a hole by adhesive will develop in a shorter length than if cast in?

 

[frv]

It has to do with assumptions about confinement. Development length assumes that there are stirrups or ties to confine the concrete (thus precluding side face blowout and concrete breakout). Appendix D assumes no reinforcement (except where explicitly stated and where you are given the option to disregard the failure mode and design reinforcement to explicitly address that failure mode (see RD5.2.9 and RD6.2.9 in the 08 code).

 

[TXStructural]

Testing identified that a primary failure mode of adhesive anchors involves loss of bond to concrete, and when a crack occurs, the normal failure modes apply.

The other problem is that for cast-in bars, we have to not only develop the bar into the concrete, but develop the forces imparted to the concrete into the internal reinforcement. A dowel is not truly developed unless it laps with internal reinforcement or the concrete itself can otherwise resist the loads applied.

I assume you are using 318-11, since adhesive anchors were not covered prior to that edition.

 

[JLNJ]

Drilled-in dowels don't lap with internal reinforcement. IS it not "truly developed"?

 

[RyUIUC]

TXStructural-I do not have a copy of 318-11 yet, I am using the method of combining the ESR Report essentially as an addendum to 318-08. If the latest edition has anything specifically addressing my question please share.

I submitted the original question to the HILTI site (via. ask an engineer) and the response I got was that I could actually evaluate it either as an anchor (ACI 318 Appendix D) or as a Developed Deformed Rebar In Tension(ACI 318 Ch.12)but account for the bond strength. This is up to the discretion of the Engineer. Thus, if I am able to achieve the development length of the rebar per Ch. 12 and also achieve the required Bond strength per ESR-2322 then I could disregard the concrete breakout and side-face blowout.

hokie66-Perhaps there is no difference between the development length and the length required to achieve a bond strength equal to the tensile capacity of the rebar. Thus in this case Ch. 12 would be moot as it describes the bond between the rebar and cast-in-place concrete.

Regarding frv's comment about confinement, I tend to disagree. Traditional tension steel in flexural members such as footings or walls do not have stirrups or tie confinement and yet they develop past the critical section by the same development length determined in Ch.12 without regard to the concrete limit states described in Appendix D. The (cb+Ktr)/db term in equation 12-1 being limited to 2.5 shows that with a normal side cover or respectable spacing you are likely to reach the limit without even relying on a confinement contribution.

To illustrate this I conceptualize a simple spread footing where the bottom bars must develop beyond the edge of base plate or a suspended slab framing into a wall where flexural steel must develop beyond the wall face. In the first case the loads remain in the footing and in the latter case the loads induce equal flexure into the wall to be resolved by its own flexural reinforcement.

Why would the development in cast-in-place reinforcement not induce the pullout failures described in Appendix D but that an adhesive bond development might? Perhaps the rate or distance at which the bond transfers the load is critical. For example, the Ch.12 development rate between the rebar and concrete is low enough to not cause splitting or side-face blowout with minimal cover and spacing. If the adhesive bond which achieves the same strength much faster, this may cause higher stresses along the concrete interface and cause the kind of failures Appendix D attempts to address.

Intuitively I can see how a shallow(<=12")embeded adhesive anchor may pullout similar to a cast-in-place headed anchor but I can also see how a longer (>=18")embeded adhesive anchor might develop the anchor in such a way that the concrete breakout limit states are unlikely no matter what Appendix D says.

 

[TXStructural]

I cannot address whether your approach covers everything in ACI 318-11 App D, as I am not familiar with what is covered in your ESR.

Ry said: "Intuitively I can see how a shallow(<=12")embeded adhesive anchor may pullout similar to a cast-in-place headed anchor but I can also see how a longer (>=18")embeded adhesive anchor might develop the anchor in such a way that the concrete breakout limit states are unlikely no matter what Appendix D says."

The Ch 12 development equations generally apply to fully embedded reinforcement across no more than a cold joint, rather than partially embedded bar which is then connected under tension. [Also, for reference, the Chapter 12 ld is generally limited by splitting forces (see R12.1.) and is conservative in many cases.]

Embedded sufficiently, it is quite likely that an adhesive-embedded rod will develop similarly to CIP embedded deformed bar in uncracked concrete. As for deeply embedded adhesive-anchored bars mimicking CIP rebar, the "tooth" of a deformed bar cast in place is primarily due to the deformations interacting with the concrete. Adhesive placed into a substantially smooth hole will fail more readily upon splitting of the concrete. It takes a narrower opening of a crack to allow loss of anchorage of an adhesive anchor than for an embedded deformed bar. (Testing shows that when installed correctly, the amount of adhesive anchor strength lost after cracking is about 50%.)

Going farther into the analysis, 12.10 discusses at what point reinforcement may be terminated. If adhesive-anchored reinforcement is required to transmit flexural loads (and this would frequently apply to CIP dowels in tension as well) then it must move tension into other reinforcement through splices (mechanical or lap). 12.14.2.3 provides spacing limits on non-contact lap splices, since that would be the most likely situation. Without confining and/or lap spliced reinforcement, the load is effectively going into unreinforced concrete and would need to be designed as such.

It is my understanding that simply providing App D anchorage depth is not sufficient when 12.10 or 12.14 apply. An example of how App D would be beneficial is where hooked or headed bars terminate in the area of the anchors, since you would not need embedment to ld (possibly as modified by 12.14), but rather the larger of ldh or values obtained using App D.

 

[ishvaaag]

I agree with hokie66 in seeing rare the extreme difference in total length required to anchor a tensile force when one uses, say, one adhesive anchor and deformed rebar; the length is, say, one in the order of 5 times the other... This points quite likely to the far more thorough stablishment (and higher built-in, average, safety factor) of the standard for development lengths than of connectors. The worrying aspect is that the necessity of using the short (anchor) connections may have led to override the usual understanding of the reliability of the connections needing be bigger than that of the members to just attain some standard of safety, average, in the whole of the structure.

Quite likely, when subject to the random solicitations expected to be able to cause ruin in structures, anchor connections should be expected to fail first -contrary to the usual intent for connections (and a shadow of this can be seen in the worries about the correct behaviour of stud-rail shear outfits). So the anchor industry has managed to make a living from the necessity of the devices, and efficient lobbying of the results of their tests focused to particular (yet well practical) intents.

 

[hokie66]

Right, ishvaaag. Lobbying, politics, call it what you like. The current situation is illogical. How did we do without all these drilled in anchors in the past? Maybe the builders didn't make as many errors?