Inspection Pit

[rlhelton]

I am having trouble picturing a long inspection pit for a "Big Boy" locomotive. What keeps the rails from springing inwards. With over a million pounds of pressure being applied downwards onto the rails and no interior bracing, or was there? the force would tend to move the rails in the direction of least resistance.
Thanks for any help.
Richard

[davekelly]

Just a gut resonse here. Given that the speed of the engine over such a pit would be slow, the depth of the pit relatively shallow, I would think that there would be very minimal forces acting on the pit walls other than the normal force (straight down). Thus, there is great force trying to pu***he rails (and it's support) down, but little trying to push it inwards. It is the strength of the material making up the rail support and pit walls that will determine whether the inspection pit collapses or not.

While I am no expert on inspection pits, all the one's I have seen have the rails attached to and supported by some sort of masonary. Think of the weights that are supported by concrete columns. As long as the forces are directed downwards, such structures can support massive objects without "blowing out" to the sides.

Sorry for the terribly worded answer . . . this is one of those "I know what I am thinking, but can't seem to put it in words."

Dave

[selector]

I would guess, to support Dave, that the engineers (not the train guys [:D]) would have used forumulae available for reinforced concrete and vector analysis to establi***he construction parameters for such pits. Those locos were new, big, heavy, expensive (especially), and came mid-century when our appreciation of the forces involved was well established.

Note: I'm not an engineer. If above response is garbage, consider the source..[:-^]

[Anonymous]

The few pits I've seen (for diesels) have the track supported by closely spaced I-beams under the track. There is also a support beam running along the bottom of the track. The pits were deep enough so maintainers could stand up when working on the locos.

Bob Boudreau

[tatans]

Inspection pits I have seen do have cross braces( perpendicular to the rails) at intervals along the pit.

[DavidJ611]

QUOTE: Originally posted by davekelly

Just a gut resonse here. Given that the speed of the engine over such a pit would be slow, the depth of the pit relatively shallow, I would think that there would be very minimal forces acting on the pit walls other than the normal force (straight down). Thus, there is great force trying to pu***he rails (and it's support) down, but little trying to push it inwards. It is the strength of the material making up the rail support and pit walls that will determine whether the inspection pit collapses or not.

While I am no expert on inspection pits, all the one's I have seen have the rails attached to and supported by some sort of masonary. Think of the weights that are supported by concrete columns. As long as the forces are directed downwards, such structures can support massive objects without "blowing out" to the sides.

Sorry for the terribly worded answer . . . this is one of those "I know what I am thinking, but can't seem to put it in words."

Dave

[#ditto] Furthermore, If you observe closely, you'll notice the railhead is gently rounded on top and wheel tread contours are tapered from inside to out. The net effect is that the weight of the loco (or any rolling stock would actually tend to pu***he rails apart, not inward. As Dave touched on above, on an inspection track, little tractive effort or dynamic side loads would be exerted on the rails. So the loads would be primarily downward with a very small outward component.

FWIW, the wheel tread contour actually helps center the wheels on the rail for better tracking and the taper also helps accommodate for rounding curves where the outside wheel has to travel further than the inside one. The exact shape of the railhead and wheel profile is very highly engineered to allow for optimal dynamic behavior, as well as minimize wheelset and rail stresses and wear. Cool, huh? [8D]

Even in small scale the NMRA sets standards (e.g. RP25) for wheelset/flange contour to get the best performance out of our rolling stock. [:)]

-Dave

[davekelly]

Dave,

Cool info on railhead/wheel shapes! By tapering the wheels the solid axle/wheels set will act sort of like an automobile differential in curves. Never thought about it before, but makes perfect sense. Those RR engineers are sure smart guys!

Dave

[Leon Silverman]

A million pound load on a small area would create quite stress. However, the engine's weight is spread across 12 axles. Using the Big Boy weight quoted in the "Classic Trains" Forum Locomotive section of 762,000 pound, this works out to only 63,000 pounds per axel, or 31,500 pounds per wheel. Other railroads, notably the C& O and the PRR designed their biggest engines around an axle loading of 80,000 lbs.
Reinforced concrete structures can support tremendous weights due to the pre-stretching of the rebars prior to pouring the concrete. Stretching the rebar requires exerting tons of force on the ends of the re-bar. The concrete is then poured in place and allowed to completely cure (harden) before the stretching forces are removed. The released re-bars attempt to return to their original length but are restrained by the now solid concrete. This now puts the concrete under a compressive load which enables the structure to withstand tremendous loads since no portion of the structure will experience a tension load (which would cause the conrete to crack).