Residential Vehicle Guardrail and Retaining Wall

[STR-DB]

Hi,

I am designing a guardrail to be atop a 6 ft retaining wall along a residential driveway. The architect is showing 3x8 rail members spaced between posts @ 8' o.c. I have not found any loading in the residential code to use so I have been using the IBC/ASCE 6000 lb load applied at 1'3-2'3 vertical height to produce worst case loads. This produces a very high moment at this rail that a 3x8 is not even close, but arch says this has been used in town previously. Am I right in using the 6000 lb load or is there a way I can reduce this to get a more reasonably sized rail member?

Any insight would be greatly appreciated.

Thanks,
EIT

 

[phamENG]

Can you provide a plan view? Is there a reasonable possibility that a vehicle can strike this at a 90^o angle? In other words, if this retaining wall is in a straight section of driveway with trees tight on the other side, the only possible impact angle may be very oblique, causing the vehicle to (hopefully) glance off. If that's the case, you may be able to justify reducing the design a little bit. Now, if this is in a bend in the driveway, and somebody could plow straight through it, stick to the 6000#.

Don't let the "we've always done it this way" argument scare you. Do you need to be aware of common practices? Yes. Should you follow them? Yes - if it makes sense to do so. Just because it was used somewhere else in town doesn't mean it's right for this situation. And who knows, maybe it wasn't right for the other one either.

You are considering an impact duration factor in the wood design, correct?

 

[bridgebuster]

Your best bet is to check with the town building department. Typically, a barrier should be crash tested but there are different rules for different roads. For example, on low volume rural roads the geometric standards tend to be less stringent; the logic being that the users are aware of conditions. Is this a private or public driveway? If it's a private residence, it may not matter.

3x8 plank railing is something you find in a park or along; not intended to stop anything. It's just a warning device. You could reinforce it with a steel plate, if necessary.

 

[phamENG]

I guess another thing to consider is that the value in ASCE 7 is intended to be used in parking garages. Depending on the design and how empty it is, you could get a car going pretty quick, but generally speaking they only need to stop a car going a few miles an hour. If you need to stop something going 10+, you may need to pull out AASHTO.

bridgebuster is right, though. Contact the town/city/county building department and see what is required. If you don't need anything there, you can pose it to the owner. Do they want a barrier? If so, what do they want it to do? (By residential driveway I'm guessing you mean a single family residence, not a multifamily complex with a public driveway.)

 

[STR-DB]

Thanks for the reply guys. Yes, house will be a single family and there will be a 6 foot drop around a bend so I believe it is necessary there. 2.0 impact factor along with flat use factor, etc. are being used. Calling the town may not be a bad idea as if it is not required maybe we can design this more of a guide rail and not a barrier. Steel plate could also work as it could be hidden.

 

[STR-DB]

Also, the average car width is 6-6.5, so I am thinking of placing the posts at maybe 5' so they will act almost as a bollard and stop vehicle instead of rail

 

[JoelTXCive]

Here's an article on some design loads from Structure Magazine in 2008..

I'm guessing you are limited on space, but is there a way to do anything with landscape rocks or any other natural barriers?

 

[ldeem]

I rarely do any residential work and I am not familiar with AASHTO but I have done several impact designs for heavy loads (ships, haul trucks, etc.). Reading the thread I would suggest paying attention to managing the expectations of the owner (maybe the Architect) as to what the guard rail will be designed to do. If the owner wants something that will survive a 5 or 6 mph impact will be very different design than something that is just a visual indicator.

The liability cautious side of me is worried if the owner thinks it will survive impact and that is documented somewhere, then someone runs through the railing and totals an expensive car and/or is injured they may come back on you saying the guard rail was under designed.

 

[LittleInch]

There's a question like this every month and that article looks pretty good to me.

Maybe substitute some steel rope type barrier to absorb and deflect any collision. Might look nicer as well.

Maybe look into these guys design info?

https://www.armcodirect.co.uk/armco-barriers/armco-barrier-dimension-guide/

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[bridgebuster]

STR-DB - something just came to mind. Is this a cast-in-place wall or a block wall, i.e., Keystone, Unilock, Redi-Rock, etc? If it's block, check with them about mounting the railing on to their wall. I suspect they'll say you can't do it and any railing should be anchored in a sonotube behind the wall. When we do these types of walls along roadways we alsways provide a moment slab.

 

[LittleInch]

It's probably one of those things that if you design for industrial or public space it's one design, domestic is another.

but I would get some sort of indemnity from the architect or walk away.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[JLNJ]

Putting a guardrail at the end of a straight run and having a car run into it will induce a force of much MUCH higher than 6000#.

Be careful of the “it can’t be directly run into so the load will be less than the full 6000#” line of thought.

 

[STR-DB]

So we were able to get an 8x8 post spaced at roughly 5.5 ft with treated glulam rails to work for the 6000 lb load. And this will be embedded into our 6 foot retaining wall with a 14" stem. Thanks for all the input!

 

[phamENG]

You should still manage expectations. That will not stop a vehicle moving more than a few miles per hour.

 

[XR250]

I would think a short 8x8 would be pretty effective at stopping a vehicle - however it would be hard to quantify. Even small trees do a number on many vehicle collisions.

 

[bridgebuster]

Absolutely right. Nothing beats crash testing but in situations like this crash testing isn't practical. Something like LS-DYNA can be used to simulate things but then again, it's high-end software. I did a "guillotine beam" or as some call them a "headache beam" for a low-clearance bridge. No fancy modelling was done - way too $$$$ for the client - you assume a load and a velocity and take it from there. This thing is massive - most likely over designed - 75' long; weighs about 80 K. It works - doesn't stop the truck per se. The driver hits the brakes because the beam demolishes the trailer .

I also did some seat of my pants analysis for bollard installations. The was one project where someone one in the office was doing a streetscape design and the client wanted their non-crash tested bollard detail used. To top it off, the PM asked me if I would sign & seal the detail "why me?" "You're a structural guy; I'm not". I did a very conservative analysis to determine a impact load that the bollard could sustain; wasn't a lot of loading. The client was OK with it.

 

[phamENG]

XR250 - no arguments about the tree, especially if it's 8" in diameter. But a post next to a 6' retaining wall doesn't have the advantage of a strong root system radiating out 10' with a 15' tap root. It's probably set in 3' or concrete, 18" around. It's probably not the post that will fail - it'll be pulled out of the ground.