Designing retaining wall with a crash force at the top

[Martahue]

I have been tasked with modifying a standard median barrier for offset road elevations to increase the height of the offset.

The height of retained roadway is approximately 5' with a 2'-8" crash barrier on top.
The contractor has asked for the retaining wall to have no toe so that the wall can be directly adjacent to drainage structures.

My question is about the crash loading. I have looked at standards from several states and I cannot find any state that seems to have actually designed their barrier to resist overturning at the extreme event II condition for the crash loading. However, for our modification, we need to meet all of the stability requirements for this load case.

Without increasing my footing to an unreasonable length, what are some ways that I can reduce the crash loading transferred to my footing? (I am distributing the moment to the footing over a max of 20' for location of construction joints, but my moment per foot is still high).

I know that MSE walls have barriers mounted on moment slabs which resist the overturning and then the shear is transferred into the top few rows of tie straps. If did something similar, then I could at least lower my crash force by the height of the barrier. Could I also consider some of the shear to be taken by the friction from the moment slab to the soil?

I can provide some sketches if this is not a clear question. I apologize as this is my first post.

Thank you in advance for any suggestions

 

[BridgeSmith]

Yes, it is typically done with a moment slab, since the moment at the bottom of the wall from a vehicle impact loading would be huge, even with a moment arm of about 7'.

Yes, you can use the moment slab to resist shear. I haven't done one myself, so I don't know how much vertical live load can be assumed, though. You may need to incorporate a shear key under the moment slab or some tie-backs, deadman anchors, or soil reinforcements to increase the sliding resistance.

There may be a fully precast solution available, so it might be worthwhile to reach out to some precast concrete fabricators in the area.

I hope your soil bearing capacity is high; that's typically the hardest to meet with no toe.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[GC_Hopi]

FDOT has guidance on the design of retaining walls for vehicle impact on the guardrail/barrier. Try searching their website. I will try posting the reference later. The issue I see is going to be the modified standard median barrier. There barriers are tested for compliance so if you modify then technically you are out of compliance. Generally you have to use so judgment on this but everyone's opinion varies so you can see push back on using a modified barrier.

Edit: The reference I mentioned is the FDOT Structures Manual section 6.7.9.

 

[STrctPono]

Everything you said makes perfect sense. I can say, personally, that I have designed dozens of conventional retaining walls using a brute force method approach when it comes to addressing AASHTO vehicular collision loads. In the end, a larger footing is many times the answer. There are times, however, where you are limited and you do need to come up with a more creative solution.

I have also used moment slabs in conjunction with MSE walls and soil nail walls. The footings still end up being rather large with these and if you don't tie it back with the grids/nails, then you have a deep keyway.

I just designed a median barrier for the Civil Engineer on a recent project and we had to deviate from our State DOT Standard detail due to it being obsolete and us changing some parameters. The footing ended up being quite a bit larger than what the standard plans showed. Reason being, the standard plans barrier was crash tested in Texas and was proven to perform reliably.

EDIT: One trick that we sometimes employ when designing retaining walls if the lateral static and dynamic earth pressures are high is to backfill the retaining wall with CLSM. It can be an expensive proposition but it reduces the forces significantly and may be enough to reduce your other pressures so you can address the collision load without the addition of a massive earth pressure force too.

EDIT EDIT: Are the "drainage structures" discretely located catch basins along the wall length? If so, then the omitting of the toe is only at these select locations and not the entire length of the wall? If the "drainage structure" is a swale running along the length of the wall on the low side, then you could always incorporate the swale as part of your footing toe. I've done that before on another job.

 

[Martahue]

Thank you all for your feedback, this is quite helpful! I am sending my current design up the chain to see if they will accept my footing sizes. If not then I will take a look at the moment slab.

STrctPono, I have two different places where the wall will be used. One has catch basins and for those, I am using a small toe which will be omitted at the drainage structures and continued on. Another location has a trench drain and that is the reason that we are eliminating the toe at that location. I like your suggestion about incorporating it into a footing. I will look into that.

GC Hopi, thank you for sharing the FDOT standards. Those are much more robust than some of the other states that I have looked at.

BridgeSmith, that is a good point. If my proposed solution is not accepted, then I will look into prefab solutions with moment slabs.

 

[BridgeSmith]

You may want to consider integrated systems, where the trench drain or catch basin is reinforced adequately and incorporated into the force resisting system for the wall. It's more design, but could provide significant savings in construction.

Rod Smith, P.E., The artist formerly known as HotRod10