D.5.2.9

[pravinagashe]

Dear All,

I have a query pertaining Clause D.5.2.9, Appendix D of ACI-318M-08.
I am posting below the clauses under discussion for your ready reference.

D.5.2.9 — Where anchor reinforcement is developed
in accordance with Chapter 12 on both sides of the
breakout surface, the design strength of the anchor
reinforcement shall be permitted to be used instead of
the concrete breakout strength in determining φNn. A
strength reduction factor of 0.75 shall be used in the
design of the anchor reinforcement.

RD.5.2.9 — For conditions where the factored tensile
force exceeds the concrete breakout strength of the
anchor(s) or where the breakout strength is not evaluated,
the nominal strength can be that of anchor reinforcement
properly anchored as illustrated in Fig. RD.5.2.9.


If factored tensile force exceeds concrete breakout strength, how anchor reinforcement can help when it is not at all receiving the tension directly from base plate?

If we observe the fig. RD.5.2.9 – Anchor reinforcement for tension
1. What advantage we have by providing the Anchor reinforcements as shown in above mentioned figure?
2. Suppose we provide the reinforcement as per the clause, then

The flow of tensile force remains as under…
From Bolts to Concrete > > > > > From Concrete to Anchor reinforcement > > > > > > From Anchor reinforcement to concrete around it.

Thus in no-way the anchor bolts are relieved from taking the tension. Ultimately they are the weak link if Concrete breakout strength governs.

So even though we provide the necessary development length and design the provided reinforcement for 0.75 x As x Fy, the weak link remains anchor bolt only.

3. Even If we match the Anchor reinforcement strength (0.75 x As x Fy) to Concrete breakout strength (Ncbg) by adjusting the “ As ”, what are we achieving?
4. Even after providing this reiforcemnt, code is not allowing us to reduce the edge distances less than 1.5 hef.
5. No where it states that we can have reduced clear cover than 1.5 hef.
6. I was thinking in a way that, this clause may be for the reason,

Every time it is not possible to provide the edge distances of 1.5 hef,(Pedestal size may go very high) thus we can transmit the tension in anchor bolt through Anchor reinforcement to the pedestal with no more need of failure surface to form.

Please explain me what would be the difference in my pedestal size requirement in case if
1. I do not consider the use of anchor reinforcement as per D.5.2.9
2. I do consider the use of anchor reinforcement as per D.5.2.9

In other words,

1. Why to provide Anchor reinforcement, if the failure surface formed is sufficient to transmit the load?
2. And if anchor reinforcement is taking care of the applied tension, why failure cone is needed ? We should be able to reduce the edge distances to less than 1.5 hef.

Regards,
Pravin.

 

[KootK]

If factored tensile force exceeds concrete breakout strength, how anchor reinforcement can help when it is not at all receiving the tension directly from base plate?

Think of it as effectively a lap splice between the nelson studs and the anchor reinforcement.

1. What advantage we have by providing the Anchor reinforcements as shown in above mentioned figure?

You're obviating the tension breakout failure cone by mobilizing a much larger chunk of concrete in tension.

From Bolts to Concrete > > > > > From Concrete to Anchor reinforcement > > > > > > From Anchor reinforcement to concrete around it.

Exactly right unless you lap the anchor reinforcement with other reinforcement. Conceivably, you could just rip out a larger chunk of concrete. If you run the numbers on a larger failure frustum passing beyond the anchor bars, however, you'll find that the capacity jumps up pretty quickly and other things are likely to govern rather than tension breakout. It's always worth remembering that the appendix D provisions originated from testing that concerned pretty small scale elements. Halfen curtain wall anchors etc. Larger scale applications may well require additional attention and/or alternate methods.

3. Even If we match the Anchor reinforcement strength (0.75 x As x Fy) to Concrete breakout strength (Ncbg) by adjusting the “ As ”, what are we achieving?

You'll have gotten a lot closer to ensuring that the governing mode of failure is a ductile mode of failure. That can be important in seismic areas.

5. No where it states that we can have reduced clear cover than 1.5 hef.

It's still considered an anchorage problem that relies on concrete in tension. As such, the breakout frustum logic still applies. If you want smaller piers and the ability to circumvent the appendix D provisions, you need to witch to true reinforced concrete principles as embodied in the strut and tie method and this paper which addresses an example quite similar to yours I suspect: Widianto

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[msdmoney]

Agree pretty much with what Kootk wrote.

Similar to what you wrote above about the transfer of forces, I've always thought of the reinforcement keeping the chunk of concrete from pulling out because it is crossing the potential concrete failure surface and developing above and below. This detailing came in to play from the 05 to 08 ACI because it was very difficult to meet the original concrete breakout provisions and simultaneously the requirement for a ductile failure mode (failing the steel).

As for the 1.5hef, I've always taken that diagram as showing the distances of the failure surface not necessarily mandating 1.5hef to use that detailing. Your bars can only practically occur so far away from the anchors anyways, otherwise there isn't space to develop above and below the failure cone wedge. Sometimes it takes many more bars than shown in the diagram and as they get farther away from the bolts your ldh distance becomes smaller and smaller.