Combined Trafic Barrier and Retaining Wall

[dessenza]

Hi Guys

I need help to resolve some problem I have regarding the design of retaining wall combined with traffic barrier. Because the barrier should resist impact load from vehicle, the sliding is a little bit of a problem. I don't really quite sure whether we need to design this combined barrier retaining wall for sliding. The other thing, does anyone knows the typical detail connection between barrier? I came across internet that some provided shear key but it seems unlogical if we only provide shear key without dowel bar. Thanks all for your attention.

 

[noodlehead]

The typical permanent barrier (Cast in place or precast barrier) has approximately 9" footing, and this footing resists barrier from sliding. That's why permanent barrier doesn't have any kind of deflection. Since the barrier is either cast in place or precast barrier with 9" footing, the barrier doesn't need any kind of connection key between barriers except for half section concrete barrier. (When you are using barrier on top of bridge deck, then you use #6 dowels 2' O.C. and patch the dowel holes with concrete.) Depending on your state, you can choose either single slope or Jersey shaped barrier.

Typically, you use half section concrete barrier with 9" footing placed against your retaining wall, install continuity connection plate for half section barrier and seal the 1/2" expansion joint gap with sealant to prevent any water leaks, and you have to be sure that the barrier foundation doesn't conflict with retaining wall footing. To answer your question, I personally don't think you need to design the combined barrier retaining wall for sliding.

Temporary full section precast construction barrier doesn't have this 9" footing, therefore it requires to have dowel bar and connection key to secure the barrier from any kind of lateral movement. However, in real world, the contractors often disregard the dowel bar installation procedure to cut the corner. Make sure your contract drawing specifies installation of both barrier key and dowel bar so if something happens, your ass will be covered.

I've added one of the standard details for your use.

 

[MA_PE]

The typical permanent barrier (Cast in place or precast barrier) has approximately 9" footing, and this footing resists barrier from sliding. That's why permanent barrier doesn't have any kind of deflection. Since the barrier is either cast in place or precast barrier with 9" footing, the barrier doesn't need any kind of connection key between barriers except for half section concrete barrier. (When you are using barrier on top of bridge deck, then you use #6 dowels 2' O.C. and patch the dowel holes with concrete.) Depending on your state, you can choose either single slope or Jersey shaped barrier.
Typically, you use half section concrete barrier with 9" footing placed against your retaining wall, install continuity connection plate for half section barrier and seal the 1/2" expansion joint gap with sealant to prevent any water leaks, and you have to be sure that the barrier foundation doesn't conflict with retaining wall footing. To answer your question, I personally don't think you need to design the combined barrier retaining wall for sliding.

Temporary full section precast construction barrier doesn't have this 9" footing, therefore it requires to have dowel bar and connection key to secure the barrier from any kind of lateral movement. However, in real world, the contractors often disregard the dowel bar installation procedure to cut the corner. Make sure your contract drawing specifies installation of both barrier key and dowel bar so if something happens, your ass will be covered.

I've added one of the standard details for your use.

I understand that the drawing you provide is an approved detail for a traffic barrier. Do you have the caculations supporting the design's conformance to the AASHTO code, specifcally the ability to resist the lateral loads as shown in the AASHTO Standard Code 17th Edition Figure 2.7.4B requiring a highway design load of 10kips? I'd be interested in seeing them.

Thanks

 

[noodlehead]

Hello MA PE.

The detail I've provided is from NJ State traffic standard details, and this detail's been approved for general use in state of NJ. You being a structural engineer, I understand you want to review the calculation. Unfortunately, I couldn't find any backup calculation regarding this barrier. If I can get any information regarding this calculation, I will let you know.

Just for the information purpose, per AASHTO's Roadside Design Guide manual (2002), this barrier is approved by National Cooperative Highway Research Program (NCHRP) 350 Report for test level TL-4. (8,000-kg single-unit truck at 15 degrees and 80 km/h)

 

[MA_PE]

Hello MA PE.
The detail I've provided is from NJ State traffic standard details, and this detail's been approved for general use in state of NJ. You being a structural engineer, I understand you want to review the calculation. Unfortunately, I couldn't find any backup calculation regarding this barrier. If I can get any information regarding this calculation, I will let you know.

Just for the information purpose, per AASHTO's Roadside Design Guide manual (2002), this barrier is approved by National Cooperative Highway Research Program (NCHRP) 350 Report for test level TL-4. (8,000-kg single-unit truck at 15 degrees and 80 km/h)

That's where I'm going with this. As someone who has had to generate calculations for small span bridge headwalls, I'm not sure that a lot of these typical details would meet the current AASHTO requirements. I believe that the details were created long aog and have worked fine and now the original design calculations are no longer around, so the supporting documentation doesn't necessarily exist. I think we'll agree that a 8,000 kg (17.6 kip) truck is not a standard HS-20 (20 ton truck) and 80 km/h (50 mph) is less than the posted highway speeds. We encourage clients to use the standard approved details because if we start running the numbers the connection to the roadway slab becomes rather unsightly.

I'd appreciate any onformation you add.

Thanks

 

[dessenza]

That's where I'm going with this. As someone who has had to generate calculations for small span bridge headwalls, I'm not sure that a lot of these typical details would meet the current AASHTO requirements. I believe that the details were created long aog and have worked fine and now the original design calculations are no longer around, so the supporting documentation doesn't necessarily exist. I think we'll agree that a 8,000 kg (17.6 kip) truck is not a standard HS-20 (20 ton truck) and 80 km/h (50 mph) is less than the posted highway speeds. We encourage clients to use the standard approved details because if we start running the numbers the connection to the roadway slab becomes rather unsightly.
I'd appreciate any onformation you add.

Thanks

Hi all

Thanks for your reply. It's been fascinating seeing your arguments. I do agree with "MA_PE" that many trafic barrier is standarize based on crash test not from design calculation. I tried to design some based on Australian Standard which 250 kN impact (how much is that in kips?) and all the proposed standard barrier will not satisfied sliding/overturning requirements as if it's a retaining wall. But i read from some US patent website that the argument for this kind of problem is really to resist kinetic energy of impact force through retaining/barrier weight so as to make them not fallling/or overturn. So the weight contribution from adjacent barrier must be taken into account where it can be done only if the connection between the interface of the barrier can resist the impact force. And here of course as explained by "noddlehead" that the dowel bar should be provided. But the problem i have here is that it seems, the standard crash barrier in Australia only use shear key without dowel bar. I'm not sure whether we should provide one or not because in their state standard drawing, there is no information whether the dowel is required and if I 'provide it', they might think me as a moron who knows nothing. FYI I've designed for shear key strength with 4d8-100, it doesn't work. The shear plane is only 70 mm thick.

Regards,

Donald Essen

PS: I used to work in SI units not US unit.

 

[MA_PE]

Donald:

FYI 250kN = 56kips (1 kip = 1000 lbf).

Interesting approach using the kinetic energy. I be interested in seeing how the approach is applied.