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RE: More confusion with concrete
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- Subject: RE: More confusion with concrete
- From: "Elias Hahn" <ehahn(--nospam--at)eepdx.com>
- Date: Wed, 8 Jun 2005 11:13:37 -0700
I was hoping I was doing something wrong, and not at the limits of the equation. I'm trying to use an 18" footing, with 1.5" anchor bolts, and I'm running into the limits of this equation at about 10" edge distance (for a corner condition, c1=c2). Plus, you all have to admit, the commentary is poorly worded when talking about Avo as representing the failure prism of an anchor "unaffected by edge distance" when, in fact, it is a function solely of edge distance. Seems like it should be more a function of diameter an embedment depth... -----Original Message----- From: Arvel L. Williams, P.E. [mailto:awilliams(--nospam--at)gwsquared.com] Sent: Wednesday, June 08, 2005 10:44 AM To: seaint(--nospam--at)seaint.org Subject: RE: More confusion with concrete Elias, The problem is the boundary conditions that this equation will work in. Try looking at some of the theoretical ranges from the original equation development. I expect that somewhere there is a published range of value that this equation is valid. Taking any working empirical equation to near the boundary will result in erroneous results. Arvel -----Original Message----- From: Elias Hahn [mailto:ehahn(--nospam--at)eepdx.com] Sent: Wednesday, June 08, 2005 12:35 PM To: seaint(--nospam--at)seaint.org Subject: RE: More confusion with concrete Agreed, the problem I have with formulas is that if you increase c1 until it is "big" (ie, 1.5c1 is greater than c2 and/or h), Av/Avo gets small. (or conversely, have a thin slab, or a corner condition where c1 is roughly equal to c2.) Ok, now my long hypothetical - feel free to not read: The problem in my mind is if you take a long beam, and put an anchor in it, and look at the calculated strength of that anchor (side-face blow out) in both directions. Now, in one direction, c1 is "big", much bigger than either c2 or h, while in the other direction c1 is "small." Now on the case were c1 is "big" Avo is great, but Av is small, (assume c2 and h are negligible in that direction, Av/Avo tends to 1/(3*c1), which makes the overall equation roughly (a number less than one)*(Square root(c1)). Now if you look at the other direction, were Av/Avo tends to one, you now end up with an overall equation of (a number greater than one)*(c2)^1.5)). It is easy to see that the direction without much edge distance could have a larger concrete side-face blow out strength... -----Original Message----- From: Scott Maxwell [mailto:smaxwell(--nospam--at)engin.umich.edu] Sent: Wednesday, June 08, 2005 10:21 AM To: seaint(--nospam--at)seaint.org Subject: Re: More confusion with concrete Elias, First, I would say that you need to be a little clearer about what kind of condition you are looking at. The edge distance, the effective thickness of the concrete (h), and the number of anchors can affect the "view" of things. Take a simple basic condition...a single anchor in a concrete element that is MUCH thicker than the depth of the anchor and is only at a side, not a corner. Kind of like what is shown in Fig. RD.6.2.1(a) and RD.6.2.1(c) in the 2002 ACI 318. In such a case, c2 and h don't enter the picture. In such a case, then Av is equal to Av0. Both have a value of 4.5*c1^2. Av/Av0 will be 1. Ok, now make it two anchors that are spaced 3*c1 apart. This will be that same as above but Av will be twice as big because the failure surface (Av) will still be bottom of two FULL half pyramids with a base of 3*c1 by 1.5*c1. This is because you now have two anchors with FULL failure surfaces that do not interesect (remember Av is the failure surface for the entire group of anchors). So, Av/Av0 will be 2. OK, now decrease that space of the two anchors. The failure "half pyramids" will now overlap. Av will become 1.5*c1 (depth of failure plane) times 1.5*c1+s+1.5*c1 where s is less than 3*c1. The result is that Av/Av0 will be less than 2 and approaching 1 as s decreases toward zero. I will leave it to you to look at more complitcated, "less perfect" cases such as when concrete thickness is not much greater than anchor depth (i.e. h<1.5*c1) or corners or multiple rows of anchors parallel to the edge. This is as it should be. Av0 is the shear breakout of ONE anchor assuming "perfect" edge conditions (i.e. no other anchors to overlap the failure surface, at just a plain edge not corner, and concrete thickness much greater than the anchor depth). Av is the shear breakout of the entire anchor group, which could in fact be single anchor in the group [if you have an isolated anchor] or an actual group of anchors, under potentially less than ideal conditions (i.e. close spacing, at a corner, and/or concrete thickness less than 1.5*c1). Thus, things like spacing, another edge (i.e. at a corner), or "thin" concrete will affect Av. The point is that under "perfect" conditions (i.e. spacing greater than 3*c1, edges not corners, concrete thickness much greater than anchor depth [i.e. h>1.5*c1], anchors all in one parallel row/line that is parallel to the edge) Av will be a integer number multiple of Av0, where the integer multiple is the number of anchors. This multiple becomes a non-integer and less than the number of anchors as the conditions become less than perfect. Now, I can only hope that I am remembering/explaining this right...I am little scatter-brained today. HTH, Scott Adrian, MI On Wed, 8 Jun 2005, Elias Hahn wrote: > So, I come again to this list with a question stemming primarily from ACI > 318. > > > > I'm "designing" some anchor bolts, and I'm confused about the formula for > concrete breakout strength of anchor bolts. Specifically about Av/Avo. > Because Avo is a function of 1.5*c1, and Av is a function of 1.5*c1, c2, and > h - it seems like the larger c1 gets, the smaller Av/Avo gets, which seems > counter-intuitive. > > > > The reason it gets so small is because as c1 gets large, 1.5*c1 will be much > larger than h and c2, so that Av becomes much smaller than Avo. > > > > My confusion is why is Avo a function of c1 if it supposed to approximate > the "surface area of full breakout prism. unaffected by edge distance"? > > > > Thank you, > > Elias Hahn > > Evergreen Engineering, LLC > > phone 503.502.0698 > > > > ******* ****** ******* ******** ******* ******* ******* *** * Read list FAQ at: http://www.seaint.org/list_FAQ.asp * * This email was sent to you via Structural Engineers * Association of Southern California (SEAOSC) server. To * subscribe (no fee) or UnSubscribe, please go to: * * http://www.seaint.org/sealist1.asp * * Questions to seaint-ad(--nospam--at)seaint.org. 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