Barrier design & dimensions for railway 3

[Jean_Wong]

Good day!

In our railway project, there are concrete barriers at both guide-way edges.

The formula I used is the one on AASHTO LRFD 2012 Page 13-19

If the barrier is sloping (150mm at top, 200mm at bottom, 1200mm in height), what is the thickness that should be considered when calculating for Mc and Mw?
Can I just use the average (175mm)?

I've read that slopes are added on highway barriers to re-orient vehicles and reduce collision damage. But on trains, the body extends over the wheels by a significant amount, would the slope matter?

If the barrier is vertical and could resist collision force, would there be any benefits in adding a slope?

Also, in our design criteria, 1000kN is specified as the collision force. Is this too much for LRT collision force?

Thanks!

 

[SlideRuleEra]

Are the barriers used as "crash walls" to protect a bridge over the railway from a derailed train?

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[dhengr]

Jean_Wong:
Since your OP is pretty vague, all we can do is add some general thoughts and considerations which might help guide your design. 8" thick at the base and 4' high seems like a fairly thin wall to me. How are you going to fix the base of this barrier (conc. wall). Although, in a derailment or impact situation, these barriers should stop the train, and guide it down the track bed, acting almost like an energy absorbing catchment net. They can be all busted up as long as they don’t let the whole car (intact) to leave the guide-way. You really need some drawings of this entire system so people (and you) can better understand what’s going on here.

You are talking about LRT (Light Rial Transit) cars/trains, right? The center of mass/C.G. of those transit cars is right around the floor level. There may be considerable structure above that elevation, but it is all pretty light structure compared to the floor structure, running gear and motors, etc. which are all below the floor level. You should really know the weight of the cars, their C.G. locations, the weight of a train set, the speed at which they travel, etc. to start to judge whether the 1000kN is a reasonable number for an impact force. Your client should be able to provide this info. and know where the numbers in their spec. came from. Then you should make some determination of what length of barrier takes this fairly concentrated load. Remember, that for the most part this large impacting load is a glancing blow, since the car is running parallel and only of few feet away from the barrier.

To add to SRE’s question, one area which is different and difficult is anyplace that there is an opening in the barrier system. The idea being that the train could derail 30-40-50' ahead of the start of a new barrier and have a head-on collision with that new starting structural portion. That is truly a destructive condition for the railcar and the barrier. Otherwise, at a bridge overpass, the idea is to protect the bridge bent columns from a direct impact of the heavy portions of the railcar by causing it to slide down the track bed just inside the columns. At a barrier starting point in this situation, you might flare the barrier out long before the bridge columns to force the train back toward the center of the track bed before it reaches the columns.

 

[chicopee]

The railroad engineering forum may be a better place for your OP but do not double post as it is frown upon.

 

[dhengr]

Another good reason for posting in the railroad engineering forum would be that it is almost a certainty that you wouldn’t have to worry about getting any responses. The last post there was on 15FEB16, way to busy for most of us with that much traffic, we just can’t keep up. Some of those forums should require double posting just to generate some traffic.

 

[Jean_Wong]

SlideRuleEra:
the barriers are used to prevent trains going to the highway on either side of the track.

dhengr:
My first question was about the thickness to consider when designing a single slope barrier? Is the average thickness ok?

With regards to the load, I will just request information from train manufacturer. The 1000kN in our design criteria might be an assumption from a previous project.

Our railway/track engineer has already considered the locations of the barrier but he is on leave so I couldn't ask about specs of the train. I am assigned to structural design of the barrier.

chicopee:
Sorry, I wasn't sure where to post. Since my question was more about loadings and dimensions for moment design, I decided to post in structural engineering.

 

[DLBridge]

Per the commentary CA13.3.1 (near the bottom of the page): "where the width of the concrete railing varies along the height, Mc...should be taken as the average of its value along the height of the railing.

On a side note, 1000kN is a huge force. If you're trying to use a standard single slope barrier from one of the state DOT's good luck.

 

[SlideRuleEra]

Jean Wong - Unless things have changed recently, there is no code for design of light rail (LR) barriers. The loads are somewhat higher than AASHTO requirements and much lower than AREA's for crash walls. Since this project is to protect the highway users, I would scale down AREA requirements rather than scale up AASHTO specs.

I consider 1000kN a reasonable collision force for LR.

IMHO, the proposed 150/200mm thick x 1.2 m high barrier is of little or no value in a collision:

1. Ask the train manufacturer for the height of the center of gravity, measured from top of rail (TOR). It will probably be 1.6 m + 0.3 m. A barrier needs to be a good bit higher to keep a derailed train from rolling over the top of the barrier.

2. AREA requires crash walls to be 30" (0.76m) thick, minimum. For LR, I would assume (constant thickness) 300mm as a basis to begin calcs (no code... can assume whatever you want to start). Also, 300mm is about the minimum thickness that can effectively use two rebar mats.

3. AREA does not offer much other guidance on crash wall design, but would consider (but possibly scale down) what is available. Bottom of wall 4' underground. Minimum height above TOR, 6' or 12', depending on distance of wall from track.

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[Jean_Wong]

DLBridge:
Thanks for pointing to the commentary!

SlideRuleEra:
It's hard to find code for light rail transits so we used AREMA and AASHTO LRFD for the designs.

Anyways,
The height of the collision load is 1.2m high, I might increase the height of the barrier to 1.6m. For the thickness, at least 250mm-300mm is required to resist 1000kN collision load. But there would be issues regarding clearance/property lines that needs to be checked. It's hard to acquire information on the train since our track engineer is on leave and the supplier is from another country so I might just use current calculations until better data is provided by the supplier.

Thanks for the advice!

 

[LittleInch]

Not really on topic here, but I can't see anywhere else where there are barriers stopping the train, which is after all on fixed rails, from impacting with vehicles / people by means of barriers. The only ones I've seen are on elevated curves on the Docklands light railway and then it's personnel barriers.

Normally this is the other way around, i.e. stopping vehicles which are not running of fixed rails, from crashing into, swerving, stopping etc in the way of the train causing it to stop / crash/ de-rail.

This seems to be a large expense for very little reduction in risk and a very low probability.

How many trains do you think are going to jump the tracks and fall over?

Modern light rail systems incorporate safety systems to stop or slow down trains before they do anything fast enough.

This seems to be a very poorly thought out exercise, but maybe there is more to it?

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

 

[SlideRuleEra]

LittleInch - In the USA, you are right. Here is an excerpt from an article on Light Rail Accident Statistics:

Statistics indicate that light rail is by far a safer mode of transportation than other forms of motorized travel...
Most collisions between light rail and motor vehicles are caused by motorists making illegal or improper turns or running red lights.

The issue is no code specifically for light rail design. Lacking a code, the default is to strict AREMA requirements. As a bridge contractor, we build a total of six massive crash walls to protect the piers on new RR overpasses. Two of these walls were on a little used spur where RR traffic was limited to about 10 MPH.

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