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Our bridge section has been talking with Bob Bielenberg about the anchor strap regarding the drop-in anchors. I did not cc Bob because I did not have his email address. Our concern is with the 17 kips pullout and 13.5 kip shear capacity stated being needed for the drop-in anchors. The Red-Head Drop-in Anchor used in the crash test does not provide the capacity stated by your staff as being needed for the strap anchor to work properly. According to the manufacturer's data as well as a test done by an independent testing company the ¾" x 3 3/16" drop-in anchor only provides 9480 lbs and 7680 lbs for ultimate tension and shear capacity, respectively. The values stated by MwRSF for 17kips and 13.5 kips is not possible even if the f'c=6000psi. Our bridge decks are not f'c= 6000 psi and I doubt if your test facility pavement has f'c=6000 psi. In lieu of the drop-in anchor we have been reviewing the concrete screw anchors. Bob Bielenberg told me about these awhile back. We really like the screw anchors as compared to the drop-in anchors.
I have attached a PDF file that summarizes the review that we have conducted on the various concrete anchors. KDOT wants to pursue the option of using the concrete screw anchors however we need to know the capacity values needed for the anchors. We also want to minimize the embedment length into the bridge deck. As you can see the screw anchors develop more allowable tension and shear capacity as compared to the drop-in anchor and would like to know if it is acceptable to substitute them for the drop-in anchors. In addition, we would also like to know the actual required capacity. The 17 kips and 13.5 kips values seem high based upon the manufacturer data. Therefore, we do not want to specify values that exceed the recommendations provided by the manufacturer for the drop-in anchor that you used successfully in the crash test. In addition we also want to provide the contractor with several options for anchorage including the drop-in anchor if desired. Note that KDOT will only allow the anchor strap option when we have 2' or more from the edge of the bridge deck to the back of the TCSB. See anchor detail PDF file.
In regards to anchorage for the TCSB I would like to propose the use of the concrete screw anchors in the bolt holes for the TCSB in lieu of the straps. If possible, the size and number of anchors could be optimized based upon the distance from the edge of the bridge deck to the back of the TCSB, shown as "A" in the attached PDF file. Note that KDOT will only use the through bolted option or bolts epoxied (3 bolts per barrier on the traffic side) into the deck when A is less than 2'. An additional part of this research could also look at using smaller diameter bolts (bolted through the bridge deck) in the TCSB with less bolts per barrier when A>2'. This option we allow the bolts to yield or break however the deflection of the barrier would not cause the barrier to topple off of the bridge. Is it possible that this can be done with simulation and design calculations without the need for a crash test? The benefit of this study would be to provide the contractor with more options to anchor the TCSB to the bridge deck, reduce the number of holes in a new bridge deck, as well as reduce the depth that the bolts are embedded into the bridge deck.
|Date||April 26, 2004|
|Attachment||Tie-Down Temporary Barrier System.pdf|
I have come up with some answers to you questions regarding you questions about anchoring of your F-shape barrier. You basically posed two main issues.
1. With regards to the F-shape strap tie-down developed at MwRSF, you had a question about the anchor strengths we gave as guidelines when compared with the listed anchor strengths supplied by the anchor manufacturers. The basic issue was that only one anchor design on the market could meet the listed anchor capacities listed in our report on the system.
In order to address the issue, I reviewed the listed anchor strengths as compared to the design strengths we used in the design of the strap tie-down. The design criteria we used fro the strap tie down anchors were 17.3 kips in tension and 13.5 kips in shear. We selected the REDHEAD Ramset II 3/4" drop-in anchor for our design based on its publish ultimate loads at the time of 17.3 kips in tension and 13.9 kips in shear in 4,000 psi concrete. We then conducted dynamic testing of the tension capacity of this anchor and got a value of 17.3 kips. This anchor was then used in the development and full-scale testing of the strap tie-down.
Subsequently, the manufacturer's listed ultimate strengths for this anchor and many other similar anchors have reduced significantly from the values that were originally referenced. I have attached a chart showing all of the applicable concrete anchors along with their manufacturers' current listed ultimate strengths and embedment depths. Based on the current published values only the Simpson Titen HD Anchor will meet the tension and shear criteria for the design and have an embedment depth less than 5 inches. Your group had noticed this issue as well and notified us.
I contacted REDHEAD and talked with their design engineers about the reduction in load between their original data and their current numbers. According to REDHEAD, all of their anchor testing was originally conducted in the 70's and early 80's. These are the numbers that have been published for years. In the mid to late nineties, the testing criteria for concrete anchors changed and currently all anchors are tested to the an ICBO standard. The ICBO testing required testing of the anchors at critical edge distances. This reduced the capacity rating of the anchors in the new tests. The anchors themselves have remained unchanged.
Based on the component testing anchor, the full-scale test of the strap tie-down system and the fact that the anchor design has not changed, we believe that the REDHEAD Ramset is still a viable anchor alternative for the strap anchor. I have attached a letter from REDHEAD stating that their anchor design has not changed since we conducted our testing here. Therefore, we currently recommend two anchors designs for the F-shape strap tie-down; 1) the 3/4" dia. x 5" long Titen HD anchor from Simpson or 2) the 3/4" dia. REDHEAD Ramset II drop-in anchor.
We don't believe we can safely recommend any of the other anchors at this time because we have not tested them and their listed strengths are lower than the values used to design the strap tie-down system.
2. You had also asked about the possibility of using a reduced anchorage with your F-shape barrier. You had stated that you typically allow for 2'-4' offsets from the edge of the bridge deck or drop off and you wanted to know if you could use some form of reduced anchor in the holes used for the bolt through tie-down. I have looked at this, and I cannot see any way to do this without significant effort and some testing. The available concrete anchors would not work well with the bolt holes currently in the barrier. In addition, it would be very difficult to predetermine the performance of the anchors used in that application. As such, we are recommending that you use either the strap tie-down or the bolt through tie-down we have developed previously. In addition, we will be testing a tie-down for use on asphalt surfaces some time this summer that should be available to you as well.
If you have any questions, or if I missed something, please let me know.
|Date||May 13, 2004|
|Date||May 13, 2004|
|Attachment||Ramset-Red Head Letter.pdf|
|Attachment||Strap Anchor details.xls|
130 Whittier Research Center
2200 Vine Street
Lincoln, NE 68583-0853
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