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BR27 Railing

Question
State VA
Description Text A question has come up related to the BR27 railing that VDOT uses.

It has been suggested that the anchor bolts should be fully tensioned using turn of the nut tightening (the same tightening we use for beam splices). VDOT uses a 1/8” elastomeric “leveling” pad, so I don’t think that will work, but no pad was shown on the crash test. I am 99% sure that no special tightening was used in the crash test and that, in fact, tensioning the connection might even degrade performance, but that 1% say I should get confirmation that snug tight is tight enough and fully tensioned is not desirable or required.

Our detail is at this link:

http://www.virginiadot.org/business/resources/bridge/Manuals/Part3/BR27C-12.pdf 
Keywords
  • Bridge Rails
Other Keywords none
Date April 11, 2018


Response
Response

I am not aware of any special tightening on these anchor bolts as well. I do not think that it would significantly influence the performance of the system either when anchors are finger tightened or more snugly tightened. I am enclosing a copy and link to a recent report where alternative anchorages were provided.

https://mwrsf.unl.edu/reportresult.php?reportId=313&search-textbox=iowa

https://mwrsf.unl.edu/researchhub/files/Report313/TRP-03-325-15.pdf

 

Bearing pads may likely be used to deal with uneven concrete surfaces, lower stress concentrations, or better distribute loading.

 

Since Bob worked on this system a few years ago, I will see if he has additional thoughts.

Date April 11, 2018


Response
Response

I would concur with run that the amount of torque and associated preload in those anchors is not critical for the bridge rail to function. You would want them tightened and not loose.

 

One related issue is that you have to be careful with the amount of torque and preload you use if you use an epoxy anchored rail. To much preload in the epoxy anchor can create issues with the epoxy bond capacity and creep. Most manufacturers have recommendations for the torque/preload.

Date April 12, 2018


Response
Response

Yes, that is why the pad is used. To help eliminate unevenness. 

 

The concern I have with going to a turn of the nut condition is that the intent of turn of the nut is to ensure a consistent clamping force for slip critical connections, we do this by yielding the bolt.  In a conditions where we are counting on friction (this is fine), in a condition where a collision would add load, then the bolt which has already yielded would deform until fracture with no additional strength (unless there is a reliance on strain hardening).

 

The addition concerns with turn of the nut are:

yielding will debond concrete locally near the top of the anchorage

poisons ratio will create tension stresses in the 10 or 12” barrier which may lead to premature failure.
Date April 14, 2018


Response
Response

I have looked over our last correspondence on the anchor topic for bridge posts. Although I do not have much more to comment, we can further discuss if need be.

 

I assume that your anchor rods are cast into the parapet. Is this true? I really do not think that finger tight or some moderate torque requirement would negatively impact the post and anchor performance. I am hesitant to think that the poison effect will cause problems in the upper parapet region but have not studied this issue. For anchors epoxied into concrete and subjected to higher long-term dead loading, I believe that there can be concerns for anchor creep within the epoxy resin material. Under those scenarios, anchor failure can occur. If the anchors were installed with epoxy resin, I would suggest that one follows the epoxy manufacturer’s guidance. Again and for now, I do not think that the finger tight or moderately tightened would drastically change post and railing performance.

 

Let us know if we need to further discuss. Thanks!

Date April 25, 2018


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