What is Projected Concrete Failure Area, ANC for this? 1

[pob11646]

Appendix D of ACI 318-08 provides formulas for calculating the projected concrete failure area, ANC on page 421 for varying number of rods. These formulas are to calculate the ANC when ALL of the rods are in tension, i.e. when ALL the rods are experiencing uplift when in the process of being pulled out.

Let's take an example. One base plate, i.e. one of many such base plates on a slab with 4 anchor rods. All rods are located more than 1.5hef from the edge of the slab. ANC per ACI would be (ca1*s1*1.5hef)*(ca2*s2*1.5hef). This is Fig. RD.5.2.1 (b) in ACI 318-08. In this case, since the rods are all more than 1.5hef away from the edge of the slab,

ANC = (s1 + 2*1.5hef) * (s2 + 2*1.5hef) Agreed?

Here is my question, what if I had just one base plate - same size as mentioned above, still with the four rods - supporting an W shape column, for instance. Just one base plate. There is a moment acting about the base place such that at any one time, two of the four rods will be in tension, and the remaining two rods will be in compression. A common occurrence. A load on the top of the column causing a moment about the base plate, and thus a couple on the rods with two rods in tension, and two rods in compression. Now, not all of my four rods are in tension at any one time. What will my ANC be like?

1. Will it still be same as that provided by ACI, i.e. will ANC = (s1 + 2*1.5hef) * (s2 + 2*1.5hef)?

Or 2. Will my ANC be different now, because only two rods are in tension, and two are in compression? Thus, will ANC be (s1 + 2*1.5hef) * (1.5hef + s2/s), for instance? The way I got this formula is to take one full side of the projected concrete failure area, multiplied by half the other side of the projeced concrete failure area because only half is in tension, the other half being in compression, obviously. Is such a projected concrete failure area possible?

Or 3. Some other formula. Please provide and please elaborate.

Thanks, everyone. If I have not stated my problem clearly enough, please let me know. Hope someone can assist and have a happy Thanksgiving. Be safe.

 

[Lion06]

If you can look at the two anchors in tension as being the only anchors on the baseplate (i.e. act like the two anchors that aren't in tension don't even exist), then it makes it much easier to visualize what your Anc should be.

 

[JStephen]

As I understand it, the anchor bolts that are not loaded at the time are disregarded, so Anc is based on the two loaded bolts only.

 

[southard2]

I think the formula should read:

(s1 + 2*1.5hef) * (2*1.5hef)

I don't have the manual here but I think this example is essentially in the manual. In your equation you are stopping the projected area halfway between the rows of bolts. With the other bolts in compression they would only help prevent the failure. I agree with the person above I'd just treat the problem as if there were only two anchor bolts when determining the tensile breakout failure. hef can also be limited by thickness.

Of course this is a simplified way of looking at the problem. See the following way for another way at looking at the problem. In this example was considers the baseplate as being stiff and prying, etc....

http://www.structuremag.org/article.aspx?articleID=511

Ramsteel has a nice baseplate program that can determine the loads on each bolt for a variety of load cases and size your baseplate for the moment. If only two bolts are in tension then I think analyzing the bolts as a two bolt pattern is correct for concrete failure analysis.



John Southard, M.S., P.E.

http://www.pdhlibrary.com