Why is a concrete railroad tie formed with a sag in the middle? Wouldn’t it be easier, and cheaper to just make it a 6X8 rectangular cross-section, like a wooden tie?
Good question, Murph. Just to foster discussion, I would surmise that it gives more clearance for low-hanging stuff without compromising strength, and possibly better control of derailed cars?
Total and complete shot in the dark here - concrete costs money. If I can reduce the amount of concrete in a tie by 5% without compromising the structural integrity, then I get 21 ties for the same amount of concrete as 20 “full” ties. Over the course of thousands of ties, that starts to add up…
Just a guess on my part, however.
The weight pushes the ends down and forces the tie into compression, which is a good thing with concrete. However this is just a guess.
Good guess by Tree68. The extra cost of each steel form amortized over its lifetime of several tens of thousands of casts is very small, but the cost of extra concrete unnecessary for strength adds up to a very large number.
Technical detail: A tie is a beam with two connections to an applied load, that must take that load and transfer it to the ballast, and do this under millions of load cycles without failure. The tie has to have sufficient negative bending strength. The section of the tie that you see at the center of the tie is enough for the “beam” to gain the desired bending strength. However that section if carried to the ends would be too thin to obtain a sufficient bearing area for the point load under the rail within the limits of the material, and is too thin to get a good attachment for the steel shoulder insert or to transfer the load from the connection with the insert to the “beam” of the tie itself. Thus the tie needs one size section for the beam, and another size section for the connection. You could thus more correctly ask, “Why is the tie thickened at the ends?”
S. Hadid
My guess would be because of weight more then money. Concrete ties are heavy and if they were just straight you’d be able to carry less per flatcar plus the machinery replacing the ties would have to be bigger. The handling of thousands of heavier ties could cost more than the cost of the extra concrete used in itself, especialy if it is not structurely needed.
(1)If a tie is in compression [like it’s supposed to be], the top structural element is sitting there doing nothing. Stupid thing already weighs way too much. The structural footprint under the center of the tie is about zilch, supports nothing but the tie.
(2) Ideally, the ballast section should be at least an inch or two below the rail. Makes it easier to blow out the surplus ballast with a ballast regulator. You want the rail and the fasteners up high and dry, especially in signal territory.
No, it’s not weight or handling expense. It’s the expense of the concrete. The tie inserter doesn’t care if it’s picking up a 660-lb. tie or a 700-lb. tie as the force required to insert the tie is a heck of a lot more than the effort required to lift the tie. Yes, you save a little bit of fuel on the tie inserter and the train that hauls the tie to the worksite, but it’s very small compared to the cost of the concrete. If you’re saving 1/100 of a cubic yard of concrete per tie, and you’re paying $100 cubic yard for high-early concrete, you save $1 per tie. At 2640 ties per mile, that’s $2640. Buy 1000 miles worth and you’ve saved $2,640,000.
S. Hadid
…Opinions all over the place…maybe a bit of truth in most of them to add up to make a reason for doing the low section in the center.
S. Hadid
I know this is available somewhere, but what is the expected life of concrete ties vs wooden ties? Have there been any structural failure trends of concrete ties which have popped up? or any other issues of costs or operational problemes?
thanks,
ed
Wood ties – 30-35 years nominal, but climate and drainage are big variables.
Concrete – 50-60 years, but they’re highly vulnerable to damage from dragging equipment, which is why we now put in a dragging-equipment detector at every intermediate signal.
Suggest you read the pdf I posted earlier to learn about structural issues. Rail seat erosion and shoulder erosion are major issues. Concrete ties are stiffer and more vulnerable to being center-bound, which will snap them in two. That can be solved with the right ballast, good maintenance practices, and proper drainage.
Cost is somewhat higher for concrete at most locations in the U.S. for new construction or out-of-face installation.
S. Hadid
This doesn’t answer the original question about why concrete crossties have a sag in the middle, but here are the reasons given by a manufacturer of concrete crossties about their advantages over wood.
Something I found interesting in their reasons that no one else has mentioned – no creosote and no fire hazard.
With the demise of steam engines, mentioning the fire hazard may seem insignificant, but there was a major blaze near Yuma, Arizona last year that destroyed a wooden trestle on the Union Pacific’s Sunset Route mainline that closed the line down for two or three weeks while it was rebuilt out of concrete.
The “no fire hazard” sounds nice, doesn’t it? But in reality concrete ties are often a poor choice for bridges due to ablation issues and section modulus problems (and I’ve never seen them on an open deck bridge except for some real oddballs). Concrete ties are OK over concrete box culverts or short-span bridges where you can get a good depth of ballast between the bridge structure and the bottom of the tie. In a long span bridge you can’t afford that amount of weight, and thin ballast sections lead to ablation of the bottom of the tie.
Creosote is figured into the life-cycle cost, including disposal and paying the tie treatment company to solve its environmental issues. For light-rail and heavy-rail applications concrete avoids having to argue with do-gooders from the public.
Asking a vendor if their product is better is like asking your landlord if you should pay your rent on time.
S. Hadid
The answer is to increase the load weight the tie can handle before cracking. I’m not going to bore you with the detailed explaination because that dives deep into physics and strengths of materials. Simply put, a wooden tie can be nice and squared-off because if can retain its shape after under going the weight of a train passing over it. The sag in the concrete tie is mean to do the same. If a concrete tie was shaped like a wooden tie, it would crack more easily. It all has to do with tension, compression and a material’s elasticity.
The ties on the left are covered more with ballast than ties on the right. Is one condition better than the other?
The way I read it, they did answer the original question. The center section doesn’t need as much concrete volume to do it’s job as the ends do. Some enterprising fellow figued out, that if the center could be smaller, may as well take it off the top surface to provide additional clearance under the train. (?)
Talk of tension and compression makes me wonder if the concrete ties are built with a camber?
Hope the track on the left is not signalized. Unfinished surfacing/broom job.
I wish I had the figures in front of me, but…The cost difference between concrete and wood, when all costs are figured in (Cost of tie, labor for installing the tie, etc.) is astronomical.