Post Installed anchors to an existing concrete column
Post Installed anchors to an existing concrete column
(OP)
Hello all,
I am hoping some of you may shed some light on the concrete breakout strength equations of ACI 318-14. The background is we are trying to connect a new steel beam to either an existing 24" wide x 11" deep concrete column with a min of (4) #11 vertical bars and #4 ties spaced at 11" o.c. or to a 24" x 24" concrete column with a min of (8) #11 vertical bars and #4 ties at 22" o.c. The building was built in the mid '70s. The load on the connection is 121.2 kips factored. I am looking at (2) rows of 1" dia anchors spaced @ 8" apart with the anchors spaced 13" apart within each row, please see attached diagram. The load in parallel to the edges of the column. I have used both Simpson's Anchor design program and Hilti Profis program as well as going through the calcs by hand. For the most part I match both programs, expect for the loads used when comparing the concrete breakout strength in shear. Hilti is using the full load on one row where as Simpson is using only 1/2 the shear. The ACI, for concrete breakout, states to look at 2 conditions, one using the shear split evenly between the rows of bolts and the second to look at just the back row of bolts. The issue is that ACI does not make reference to when you have 2 equal edge distances parallel to the direction of shear. So when looking at trying to design this connection how you go about it?
Thanks for your help in advance,
Phil S
I am hoping some of you may shed some light on the concrete breakout strength equations of ACI 318-14. The background is we are trying to connect a new steel beam to either an existing 24" wide x 11" deep concrete column with a min of (4) #11 vertical bars and #4 ties spaced at 11" o.c. or to a 24" x 24" concrete column with a min of (8) #11 vertical bars and #4 ties at 22" o.c. The building was built in the mid '70s. The load on the connection is 121.2 kips factored. I am looking at (2) rows of 1" dia anchors spaced @ 8" apart with the anchors spaced 13" apart within each row, please see attached diagram. The load in parallel to the edges of the column. I have used both Simpson's Anchor design program and Hilti Profis program as well as going through the calcs by hand. For the most part I match both programs, expect for the loads used when comparing the concrete breakout strength in shear. Hilti is using the full load on one row where as Simpson is using only 1/2 the shear. The ACI, for concrete breakout, states to look at 2 conditions, one using the shear split evenly between the rows of bolts and the second to look at just the back row of bolts. The issue is that ACI does not make reference to when you have 2 equal edge distances parallel to the direction of shear. So when looking at trying to design this connection how you go about it?
Thanks for your help in advance,
Phil S






RE: Post Installed anchors to an existing concrete column
RE: Post Installed anchors to an existing concrete column
RE: Post Installed anchors to an existing concrete column
Again thanks for you thoughts.
Phil
RE: Post Installed anchors to an existing concrete column
RE: Post Installed anchors to an existing concrete column
I suppose this floor is unbonded PT. If the US were to ban it, like a lot of other countries do, it would save a lot of headaches.
RE: Post Installed anchors to an existing concrete column
1. Load in vertical direction divided evenly amongst all anchors.
2. Load in vertical direction only on top two anchors.
3. Load in horizontal direction divided evenly amongst all anchors (Vcbg is multiplied by a factor of 2).
4. Load in horizontal direction only on furthest row of anchors (Vcbg is multiplied by a factor of 2).
You could make an argument that cases 1 and 2 could never cause shear breakout since there is not an edge distance in that direction because the column is continuous to the foundation (ca1 is infinite). Also, shear breakout fails due to tension in the concrete and load in that direction does not cause this, plus there is developed reinforcing on both sides of the breakout plane (which would be the entire area of the column) which D.6.2.9 permits you to use in lieu of concrete breakout strength (must still check the capacity of the reinforcing).
Cases 3 and 4 are required per D.6.2.1c (this is where you get a factor of 2 on Vcbg). Fig RD.6.2.1(b) shows the two assumptions that are required for multiple rows of bolts so you must check half the load in both rows AND all the load on the furthest row. You are checking a column so you're right that there's two edges but only one is relevant in each horizontal load direction so that should not affect your results. For Case 3 you would use half the load with the breakout area from the row of bolts closest to the edge. For Case 4 you would use the entire load with the breakout area from the row of bolts furthest to the edge. Case 4 will likely control the design since it is twice the load with only a slightly larger breakout area.
As Leftwow mentioned, this connection will likely be transferring moment so remember to account for that. If adding a plate and bolting to it in order to reduce the end moment, I would still include the eccentricity from the column to the bolts since a small amount of moment will have a large effect on your anchor capacity.
www.structuralcentral.com
RE: Post Installed anchors to an existing concrete column
Their check appears to be slightly conservative since I think they should be checking the furthest row for all the load which has a ca1 of 16" (which only increases the capacity by 27% since your breakout area is already through the depth of the column).
So I guess my conclusion is just that the result from PROFIS appears to be only slightly conservative. I would definitely NOT be doubling the values from the program like it seems you are suggesting since ACI 318 pretty clearly states that the furthest row must be capable of taking the entire load itself and that only increases the capacity from the PROFIS results by 27%.
Also, if you are attaching another beam to the opposite side of the column, it would cut your breakout capacity in half since the beam on the opposite side is also using up the column's breakout capacity.
www.structuralcentral.com
RE: Post Installed anchors to an existing concrete column
ProgrammingPE - I was not not thinking about doubling the values, I was more curious as to why Hilti and Simpson use different applied shear values when checking the concrete breakout. I agree that Hilti seems to be conservative and that it should be applied to the back rows. What my colleague and I were discussing was since the back row of bolts is also 8" from the back edge shouldn't the applied shear be split in half, as Simpson is doing, when checking against the breakout strength. As for the second beam, yes there will be, but I was only checking for a little less than half the column depth.
Thanks Again,
Phil
RE: Post Installed anchors to an existing concrete column
Without that......you are risking the people installing your Hiltis cutting the column reinforcement. Especially with the stirrups, its never where you think it is.
You've probably already thought of this.....but I wanted to mention it just in case because it's one of those pop up costs that people get mad at you for not anticipating.