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Tie-Down Anchor Bolt - Length Change

Question
State MO
Description Text

Ivan Schmidt at MoDOT has raised a question regarding the
anchor bolt length that was used to tie-down the steel straps. The built-up
strap material at the hole locations was ½ in. thick. In the original report, a
57-mm (2¼-in.) long bolt was used. Original report link is provided below.



 



Original Report



http://mwrsf.unl.edu/reportresult.php?reportId=219&search-textbox=tie



 



In the letter report to NDOR, the CAT anchorage testing
program utilized a 1¾-in. long bolt with the RedHead Drop-In anchors. Letter
report link is provided below.



 



Letter Report



http://mwrsf.unl.edu/reportresult.php?reportId=266&search-textbox=tie



 



Do you know why the hex head bolts were shortened by ½ in.
and did not use the original bolt length of 2¼ in.? I have quickly looked
through the original and letter reports as well as current RedHead anchor
information. I was unable to find the published threaded distance within the
anchor cavity. Let me know if you recall as to why this both length was
reduced. Thanks!



 

Keywords
  • Temporary Barriers
Other Keywords Tie-Down
Date July 29, 2015


Response
Response The bolt was shortened based on concerns that the longer bolt could bottom out in the drop in and cause it to break the tabs that expand at the base. 
Date July 29, 2015


Response
Response

So, then why was the original report not revised? Also, the 2007 letter reports that the ultimate tensile capacity 17.3 k of the red head anchor is based on “limited testing and review of manufacturer test data of the drop in anchor conducted during the development of the steel strap.” Seems like the tensile capacity would be a pretty big deal since these anchors are pulled out of the holes in barriers directly impacted.

 

And, if the tensile strength is actually based on a 2 1/4” long bolt, then the alternatives would be overdesigned.

 

Where and when was this change in bolt length documented so we can have for our records and should MoDOT immediately change the 2 ¼” long grade 5 bolt to 1 ¾”?  If the 2¼” long bolt was crash tested and worked why change to 1¾”?

Date July 30, 2015


Response
Response

See my comments below!

 

So, then why was the original report not revised?

**There was not a problem with the original configuration. The RedHead anchors were used with 2¼” long bolts, and the barrier system performed well. The original report documents the successful crash test and installation details. No hardware problems were encountered in the crash test. As such, there was no consideration to prepare a revised version of the report. However and based on your inquiry, we are wondering whether a notice should be posted to better indicate this bolt length change.

 

Also, the 2007 letter reports that the ultimate tensile capacity 17.3 k of the red head anchor is based on “limited testing and review of manufacturer test data of the drop in anchor conducted during the development of the steel strap.” Seems like the tensile capacity would be a pretty big deal since these anchors are pulled out of the holes in barriers directly impacted.

**Per Bob, the RedHead anchor socket has an internal threaded length of 1¼”. Thus, the original longer bolt penetrated farther into the void region below where the internal threads ended within the socket. Those extra threads did not engage the socket and did not provide additional tensile capacity. The new 1¾” long bolt with full threads engaged the entire threaded region of the socket when considering the strap thickness and welded washer plate thickness. Thus, the tensile capacity of the anchor was not changed. The anchor socket behavior was controlled by concrete fracture and/or bond failure.

 

**If bolts are excessively long, there could be a potential for really long bolts to contact and rupture the deformed tabs at the bottom socket. We do not recall that scenario occurring in our actual field installation that was used in the crash test.

 

**With additional information from the manufacturer, we chose to use a 1¾” bolt length in the follow-on study that evaluated alternative mechanical anchor hardware. Again, the tensile capacity of RedHead socket was controlled by concrete strength. Both bolt lengths would provide equivalent tensile capacities when considering a 1¼” threaded length within the socket.

 

And, if the tensile strength is actually based on a 2 1/4” long bolt, then the alternatives would be overdesigned.

**See comments above.

 

Where and when was this change in bolt length documented so we can have for our records and should MoDOT immediately change the 2 ¼” long grade 5 bolt to 1 ¾”?  If the 2¼” long bolt was crash tested and worked why change to 1¾”?

**The 2007 letter report documented this change. Drawings were also prepared for AASHTO Task Force 13 Roadside Barrier Hardware Guide. The latest drawings are attached. Note that the anchor socket did not change. The 2¼” long bolt met crash testing requirements. The 1¾” long bolt will also meet crash testing standards as all bolt threads are engaged in 100% of socket threads. The shorter bolt length can reduce any potential concerns if an excessively long bolt would contact deformed tabs and cause damage. Although we did not have that occur, we took advise and reduce the risk of it occurring.

 

**It does appear that the original 2003 FHWA eligibility letter, B-112, still depicts the 2¼” long bolt for use in the socketed anchors.

 

**Alternatively, the online AASHTO TF13 Hardware Guide provides a 2007 SWC10 detail (8/31/2007) that shows the 1¾” bolt length. The link is provided below. However, the latest version of the SWC10 drawing that is located on our internal server is dated, 10-22-2008. Thus, the different dates causes me to raise the question as to why the online hardware does not depict the most current version of the detail that remains on our server as it should. As such, I need to speak to my colleagues to better understand how often revised details are forwarded to TF13 for replacement in the online guide.

http://guides.roadsafellc.com/hardwareGuide/index.php?action=view&hardware=124

 

**Please let me know if you have further questions regarding my responses to the items noted above. Thanks!

Date July 31, 2015


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