Traffic barrier on MSE Wall 2

[apriley]

I have seen previous posts about MSE walls with barriers, but I still need clarification.

Currently, Caltrans requires the use of BDS LFD for MSE wall design in lieu of AASHTO LRFD. Caltrans requires designing the moment slab and top row of MSE reinforcement for traffic impact. However, it is unclear what this impact force should be, or how it should be distributed. The 54 kip load seems way too high, but I can't find anything about the 10 kip load from AASHTO in Caltrans BDS. All I can find is the 1.9 k/ft load for the top reinforcement (BDS 5.9.3.8.2), which still seems too high. I checked the standard XS sheet barrier slab overturning with the 1.9 k/ft load, and it doesn't work.

If anyone has an idea about what loads to use for 1) the moment slab, 2) pull-out, or 3) reinforcement tension, and where these are specified, I would appreciate it.

Thanks!

 

[miecz]

Interesting that you've gotten no replies to this. I was involved in a similar problem with a prefab retaining wall. We asked for it to be designed for TL-4, 54 kips. I'm surprised that California would allow anything less for a highway application. We could not get the fabricator's engineer to design for this force to our satisfaction. After a number of rejected submissions, we ended up designing it ourselves.

 

[apriley]

I was surprised as well, actually.

I've read some literature about this, particulary about how the 54 kip load doesn't make sense for designing using pseudo-static method. It ends up requiring a very long and heavy moment slab to resist overturning. Obviously, when you consider stability, the barrier isn't going to fall off the wall if a truck hits it.

Anyway, how did you distribute the 54 kip load? And did you assume some transfer to the wall through friction?

 

[miecz]

For beginners, Table A13.2-1 allows 3.5ft. As our parapets are heavily reinforced longitudially, it was easy to distribute the length of a parapet section. That still wasn't nearly enough. We counted on the torsional and flexural strength of the footing to pick up the necessary dead load.

 

[apriley]

The standard sheet for Caltrans shows a 6' barrier slab 1.75' thick, but it doesn't allow for breaks in the slab so I don't have a definite length. I think I have to go with the standard barrier and moment slab, but I'll have to figure something out for transfer to the top row of MSE wall reinforcement.

Thanks for your help!

 

[miecz]

In our case, the moment slab was cast right on top of, and keyed into hollow precast blocks (doublewal). Even then, I was concerned about the lateral force shearing off the face of the block. I've always wondered how this shear is handled with an MSE wall. That's why I was dissappointed that you got no reply from the experts.

 

[apriley]

There's a 2" gap between the slab and the wall in Caltrans' barrier slab detail, so it seems to me that the shear doesn't directly transfer to the wall. Instead, it must be transferred through the soil and resisted by the soil reinforcement.

I've checked the reinforcement tables Caltrans released for MSE wall design, and they don't account for traffic barrier impact load. The first level of MSE wall reinforcement in their tables isn't strong enough, and the 1.9 k/ft load the code suggests for top reinforcement due to barrier impact is pretty high. So I'm in limbo now as to how to interpret the loads for a reasonble design. And this project has about 7 MSE walls.

I actually put this question into two forums as well, with no bites except yours!

 

[MSEMan]

Actually the barrier isn't such a big deal if it's designed correctly. There are a few things that matter a lot. If the barrier is placed on top of the wall facing before the anchor slab is cast then it will rotate about the support point at the top of the facing. On the other hand if the barrier is placed on top of an anchor slab and stitched to make it integral, then it will want to rotate about the leading edge of the slab. The latter arrangement provides a greater lever arm and thus more overtunrning moment to resist. The second thing that matters a lot is the length of the slab. A 30' slab is prefferable to a slab cast in 20' bays.

 

[miecz]

MSEman

Actually the barrier isn't such a big deal if it's designed correctly.

What constitutes a "correct design" isn't readily apparent to me. Even basic things challenge me like "path of the forces" and "distribution width". Is a proper design published anywhere?

 

[MSEMan]

You can infer one from the AASHTO MSE wall sections on traffic barriers and FHWA-00-044 publishes a detail.

 

[MIBridgeEng]

Check out AASHTO LRFD Article 11.10.10.2. It specifically tells you to design for 2 kips/ft, not the 54 kips bridge barriers are designed for.

Also, I do recall seeing a recent paper on the internet regarding this specific issue. I don't have it right now, otherwise I'd attach it.

 

[miecz]

Does that mean that the barriers are designed for 54 kips and the supporting structure is designed for 10 kips?

 

[apriley]

MIBridgeEng - thanks for the reference, but Caltrans has not adopted AASHTO for earth retaining structures at this point. Furthermore, the barrier slab that Caltrans shows in the standard doesn't seem to work for a 2.0 kip/ft load.

meicz - Yes. I think the idea is that some of the 54 kip load will be transferred to the barrier slab along a length.

In my research, I found the article "Design of roadside barrier systems for MSE retaining walls," which discusses this issue. However, that article assumed transfer over a 6m length (distance between joints in the slab), and Caltrans does not allow joints in the slab.

In the end, I used the barrier slab shown on the standard sheet, which *sort of* works for the load indicated in the article (10 kips over 20 feet). However, I can find no documentation of this in the code, although I believe it was an old AASHTO standard.

 

[Deedy]

Try this link:

http://www.reinforcedearth.com/Tech...e Barrier Systems for MSE Retaining Walls.pdf

Or type this into your search engine: Design of roadside barrier systems for MSE retaining walls. Also get a hold of an old FHWA publiction for MSE Walls. I can't remember the pub No. I believe Reinforced Earth also has some guidelines. There is a guideline book for segmental walls that all proprietary wall systems have to follow. Try that.