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Temporary Barrier on Bridge 9123

Question
State MN
Description Text

I need your help with this one (See specifics below).



 



We use the Iowa F-Shaped Temporary Concrete Barrier System
(SWC09), and we do have the Tie-Down Strap System for the F-Shape Concrete
Barrier in our standards (SWC10).



 



We essentially will have TL-2 conditions (45
mph Posted Speed). 



 



Is there a configuration/location that would
not require anchorage of the barrier for these TL-2 conditions.  I realize
that a barrier pushing up against the curb would easily tip over as the curb
would act like a hinge, but how close is too close?



 



Is the top of the sidewalk an option at
all?  I can visualize barrier deflection followed by vehicle vaulting on
the curb (assume an 8” curb).



 



If we place a system with the Tie-Down Strap on
the shoulder,  How close to the curb could it be placed?  In the
Tie-Down test FHWA letter, I believe that I saw 12 “ (305mm) of dynamic
deflection for the area of impact, with a TL-3 test.  Could we go 6” with
TL-2 conditions?  We would like to give some room to the vehicles (the
barrier is 22.5” wide).



 



 



Of course the designers need an answer asap.



 



Please give me a call if you need more
information.



 



Thanks






_____________________________________________________________________________



 



I left you a voicemail, but I thought I would follow up with
a clearer view of what was needed. 



 



 



Some information about the site:



-         
TH 21 over UPRR (Bridge 9123)



-         
45 mph on the bridge



-         
Expect that this condition will last for
approximately 5 years until the bridge is replaced



 



 



We are looking for guidance on the following questions:



-         
Can we place temporary barrier on the shoulder
or on the sidewalk?



-         
If so, how far from the edge of the sidewalk or
from the gutter line does it have to be?



-         
What sort of anchoring is needed?



-         
Any other guidance we need?



 

Keywords
Other Keywords none
Date August 4, 2015
Attachment bridge 9123.jpg


Response
Response

I will try to provide my thoughts and be as brief as possible due to MnDOT’s time constraints.

 

First, the duration of the work zone is noted to be five years, which is a long duration . Thus, a conservative approach may be justified using best available information.

 

Second, the placement of a single row of F-shape PCBs on top of the sidewalk may create some concerns. Although the F-shape PCBs will displace laterally, I wonder whether impacting passenger vehicles will reach a higher effective climb height on the barrier face relative to the lower bridge deck surface, thus potentially leading to slightly greater vehicle instabilities. Granted, I believe that this risk would be much reduced at TL-2 conditions as compared to TL-3 conditions. However, I just want to denote this low to moderate potential safety concern.

 

Third, the laterally-deflecting PCB on the sidewalk could create a scenario where a partially-redirected vehicle allows for a wheel to catch on the sidewalk, thus leading to increased roll or yaw behavior and increased vehicle instabilities. Although this behavior and risk is uncertain, I still need to raise this potential concern.

 

Fourth, a free-standing PCB at TL-2 of NCHRP 350 would result in reduced dynamic deflections as compared to TL-3. Unfortunately, we have not previously conducted testing nor simulations at the TL-2 conditions. Instead, MwRSF conducted a study to evaluate conditions at the 85% impact condition – 2000P pickup truck, 36 mph, and 27.1 degrees. Details of this R&D is provided using the link below. Note that the TL-2 impact condition provides approximately 35% more impact severity than the 85% impact condition used for the noted study. Thus, the anticipated free-standing deflection for F-shape PCBs would range between 24 and 43 in for NCHRP 350 TL-2 impact conditions. One might estimate between 32 and 36 in. for TL-2.

 

http://mwrsf.unl.edu/reportresult.php?reportId=243&search-textbox=concrete%20barrier

 

Next, I agree with you that free-standing PCBs on the bridge deck and adjacent to the sidewalk would laterally deflect backward and strike the sidewalk. Once the PCBs had bottomed out against the raised sidewalk edge, the PBCs would be prone to rotation without translation, which could increase vehicle climb and subsequent roll and pitch angles as well as vehicle instabilities. As such, adequate space would be necessary between a free-standing PCB and the raised sidewalk, even for TL-2 conditions.

 

Based on this information, I believe that a reduced-deflection PCB system may be worth considering for your application and located on the bridge deck. There exists the (1) tie-down strap system, (2) vertical through-bolt tie-down system, and (3) WisDOT steel tube and saddle system for use with the F-shape PCB system. All three systems alone, or in combination with one another, may provide a workable solution, especially where you have limited width for PCB placement. If you can accept deeper holes drilled into the bridge deck for either through-anchor bolts or rods epoxied into partial-depth holes within the deck, the through-bolt system offers a low-deflection system, especially at TL-2. If that option is not acceptable, the tie-down strap system could be used with a larger gap between the back of barrier base and face of raised sidewalk, say 6 to 9 in. and at TL-2. Finally, it might seem reasonable to use a combination of the tie-down strap system and the WisDOT tube/saddle system at an even closer offset to front face of sidewalk, say 3 to 6 in., for TL-2 impact conditions of NCHRP 350. Actually, I might consider the latter option more preferred when considering the 5-year work-zone time period. Of course, it should be noted that these options have not been tested under TL-2 conditions and/or when positioned this close to a raised sidewalk edge.

 

Finally, one last consideration would be to further reduce posted speed limits from 45 mph to 35/40 mph to help control potential impact speeds.

 

If you have any further questions regarding the enclosed information, please feel free to contact me at your earliest convenience. Thanks!

Date August 6, 2015


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