Retaining walls and vehicle barriers: guidelines ?

[Saxifrax]

I'm involved in a hillside residential construction project where there's a requirement to :

1)Build a roadside turnout for a firetruck
2)Widen the existing driveway to 15ft

Both of these require retaining walls ranging between 2-6ft on the downhill side, which have been designed by our structural guy (8in slab wall with 18-24in pier + pillaster every 6ft). We submitted the design showing the wall with a 6in lip above the driveway grade: the county came back with a request we raise the wall another foot in order to "stop the wheels" of a vehicle (basically to prevent a car from "rolling off" the driveway).

OK with us - the civil will redraw to show this, but now the structural refuses to redesign to this, stating he cannot do so without a code to work against (and also noting that he would also like to have a design which would stop a truck from going over, as we have designed the driveway to take the load of a truck driving on it).

The owners are not happy with the prospect of a regular highway crash barrier along their driveway, but the structural will not budge. We asked the county for specific requirements but they admit there is no code to work to here.

Anyone have any experience with this type of situation ?

T.

 

[kslee1000]

I think your structural guy has over-reacted a little on this. You shall obtain written letter from both the owner and the county official to waive the requirement of a standard highway barrier, and it shall clearily state the 1' requirement. It is really meant to cover your bottom line, should there be any crash over incidence (though it is highly unlikely event for a slopped low speed drive way, but you never know).

 

[Saxifrax]

Thanks for the reply.

I have to 'fess up and say I'm the owner in this case...

We've tried suggesting we provide some kind of "waiver" which would not hold him liable in the event of someone going over. His response is that this would not protect him in the event someone was looking to sue him. We already have in writing that the County is not requiring crash barriers, just that we raise the wall height.

He's still insisting more or less on a "highway-designed" solution.

The other difficult aspect is he's also done the structural for the house, and we're about to submit those plans, so at this point replacing him would be expensive and likely cause us a big delay.

T.

 

[kslee1000]

T:

Thanks for your confession

My impression is your structural has designed a rigid reinforced concrete retaining wall with 6" above pavement. After raising the county requested 1', the barrier would be 1'6" high, which should be able to prevent a vehicle to "run-over" with limited speed. If shaped and reinforced properly (I would consider thicken the wall to act as beam in between pillasters), the crash force (a point load) may cause some damages to the wall but avoiding complete fail.
This is the whole point he needs to consider.

Also, you may spend a little more to detailing the wall face (using liner form) to be more pleasant looking than the cold look of concrete mass. Thus, increase your property value.

 

[Ron]

The barrier should be designed for a 10,000 lb load by general code (IBC) or by anticipated vehicle weight and speed. Impact point should be 27 inches above grade per AASHTO.

 

[kslee1000]

Ron:

The question on T's mind is the explicit need & criteria of crash absorbent barrier for road under county jurisdiction. Though AASHTO has the highest authority over highway issues, however, I doubt it generally applies to local roadways. The arguments include difference on traffic patterns, travel speed, and type of vehicles.

However, for a drive way with steep slope, it wouldn't hurt to be prudent. Also, I forgot to remind T to check with state DOT to see their's stance on this matter.

 

[Ron]

kslee1000...thanks for the clarification; however, most county governments do not have the technical sophistication to establish their own criteria. They will typically yield to their state DOT, which often yields to AASHTO.

 

[kslee1000]

Ron:

I agree to a certain extent. It's not unusual to see lack of crash protection even along interstate highways. I guess the main reason is money, otherwise, every roadway would be liken race way with high barrier walls on edges, and in the medians.

One interesting point here is, if both the owner and the county agree to waive the crash protection requirement, what liability remains sticking with the designer? Sure, he might not escape the fate to be summoned to the court as co-defendent, or witness shall accident occurs. But, what he is guilty about?

 

[csd72]

He should design it for the 10 kips at 1'6" and be done with it.

 

[Ron]

I agree with CSD72....the owner can leave it in or take it out, but the design should be done. The engineer is required by law in most states to protect the health, safety, and welfare of the PUBLIC, not just the owner.

 

[kslee1000]

T:

I have failed to find a code provision that clearily declares the safety barrier requirement for local, and/or residential access road. However, most sites suggest a roadside barrier system shall be considered if the roadway geometric is likely to cause fall over accident, and from which the driver would suffer lesser injury by with than without. I think the suggestion is worth to think about.

The question remains as does your barrier require to be in compliance with the standard, which was tested for a minimum driving speed of 40 mph with the impact force and point of impact as mentioned above by others. If required to comply, the 18" suggested previously is obviously out of the window. I don't think 50/50 is the solution.

At this juncture, I would suggest you to look into the flexible type roadside barrier system - metal guardrails, in liu of a 3' solid wall. Hope this will solve the problem from any angles.

Attached is general consideration/description on barrier systems, by TXDOT. Have a look, and good luck.

 

[RHTPE]

Why not design the wall for a post & rail barrier, with the top of the wall concrete 12" above grade? Design to whatever standard is appropriate. If the "owner" neglects to install the posts & railing, how would the wall designer be responsible?

Just a thought.


Ralph
Structures Consulting
Northeast USA

 

[kslee1000]

Ralph:

Wow, finally there is light from the end of tunnel.

 

[Saxifrax]

Thanks all for your input here: I will check the various referenced material.

At the risk of sounding like a whining client:
I guess what we're trying to avoid is having our driveway look like a road. We bought this property for the "rural" situation, and we're already putting a big hole in that by needing to widen the existing driveway to 15 ft (and having to remove a number of trees in the process) Adding highway guardrails would just be salt in the wound.

What bugs us is because there's "no code" which applies to private driveways, the only recourse seems to be to jump straight from "stop the wheels of a slow-moving passenger vehicle" to some sort of highway-certified system which can stop a car at 70mph. Is it really not possible to design a wall to the County's request ?

Bottom line: we've had to roll over *so many times* for the County in this project that it's kind of galling that the first time they're seeming reasonable in their request we're unable to just meet it due to one or "our" team.

Cheers,
T.

 

[kslee1000]

T:

As suggested, you can make the (concrete) wall pleasant to view by using special forms, like stone face, brick...Also, the concrete barrier above pavement needs not to be continuous, it may consist of blocks with gaps in between. In my opinion, it is better than other opyions, and potentially increase your property value.