The Design of Subways (1918)
Public Service Record · Vol. V, No. 10/11, October/November, 1918.
The Design of Subways.
By Julius Glaser, Designing Engineer, Division of Designs
Why do we build subways? They're expensive. They cost several times as much, mile for mile, as elevated railroads, and their construction entails more inconvenience to the public and to business, and for a longer time. They interfere with and endanger the sewers, gas pipes, water mains, electric conduits, and other subsurface structures, for an extended period, and then, when finally completed, many people dislike to ride in them.
Yet we build subways, because, when finished, unlike elevated railroads, so far as street conditions are concerned, they are noiseless, invisible and do not obstruct light, air or traffic. Train operation is never interfered with by weather conditions, and real estate along the route is enhanced in value. The permanent advantages of underground railroads far outweigh the temporary inconveniences during the construction period.
A rapid transit system, however, would be too expensive if it consisted entirely of subways. A well-balanced system should consist of subways for the congested parts of a city, with elevated extensions and feeders for the outlying and more open portions. Development of the open portions and a corresponding rise in realty values soon follow the construction of the elevated extensions.
Profile of Line. Once the route of the subway has been decided upon, the first consideration in its design is the determination of its profile. The greatest single determining factor in the cost of subways is excavation, with the exception, perhaps, of underpinning in narrow streets flanked by tall buildings. Both items should be minimized by keeping the structure as close to the street surface as possible. In fact, it should follow the contour of the surface, unless excessive grades result thereby. Only enough room should be left between the street surface and the roof of the structure to allow proper restoration of the usual subsurface structures. Special attention must be paid to trunk sewers, and other existing or proposed subways passing under or over the subway under consideration.
In narrow streets the width of the structure can be reduced by double-decked construction. This leaves sufficient room on the sides for restoration of pipes which would otherwise have to be placed on top of the structure. In this way the amount of excavation is reduced, because the structure does not need to be depressed on account of the largest pipes. In the case of double-deck subways, the express tracks are usually placed on the lower level and can be tunneled so as to avoid heavy grades, while the local tracks should preferably follow the street surface contour, on account of the stations.
Near Surface at Stations. At stations, the structure should be very close to the street surface, so as to reduce to a minimum the number of steps at the entrances. The use of elevators and escalators should be avoided as much as possible, because of the enormous increase in the cost of operation which they involve. At the stations where island platforms are called for, the structure must be depressed to allow for mezzanines. The necessity for future mezzanines should be carefully studied, and proper provision made in the determination of the profile that they may be constructed later if desired. In this connection, special attention should be called to terminal stations where the island platforms are connected at one end by a passage at the same level as the platforms. If no provision were made for a mezzanine, an extension of the line would necessitate a resort to sub-passages. These should be avoided, because they involve extra stair climbing by the traveling public.
Typical express station, four track structure. (Click image to enlarge.)
As to Grades. Where the subway passes under private property, it should be depressed sufficiently to allow for a basement to the building, unless the property value does not warrant it.
In order to obtain proper drainage, the grades of the profile should not be made less than 0.3 per cent, preferably not less than 0.75 per cent, but the latter is not always desirable for other reasons.
The profile might be affected also in some localities where there is rock or good soil under soft material, in that the structure might be depressed sufficiently to gain the advantage of a good bottom.
The grades between stations should generally be not greater than about 3 per cent. More than that seriously affects the cost of operation. But at times it is necessary to make the grades at least 4 and sometimes 5 per cent. At stations and station approaches, grades should not be more than 0.75 per cent so that trains will not creep when the brakes are off. Grades in excess might be used when they are balanced about a high point. At all breaks in grade, vertical curves should be used with a maximum rate of change of 4 per cent. This maximum is generally employed at the ends of stations where it is desired to change the grade as quickly as possible so as to avoid excavation, and, for the same reason, also at points where the structure passes under other subways and especially below water. Vertical curves should be kept off stations entirely, if possible, so that the platforms can be built straight. Crossings at grade should be avoided.
Alignment of Subways. In working out the alignment for subways the object of prime importance is to get a good operating track. This is obviously obtained by keeping the line as straight as possible and by using large radii.
In justice to the owners of abutting property, it is ordinarily endeavored to keep the structure centered on the street so that the interference to the building vaults under the sidewalks will be a minimum and will affect both sides of the street alike. This is, however, not always possible, especially where the line runs through a narrow and winding thoroughfare. In such cases, it often becomes necessary to disregard the center line of the street and run out a series of tangents in such a way as to reduce the number of curves and enable the use of large radii.
Where Curves Occur. Curves occur either at changes in the direction of the street or of the line itself. Where the line turns off, curves become necessary and the structure must generally pass under private property. The radii then depend on property values in the particular locality. At all curves in the line, care must be taken that the tracks are spaced far enough apart to allow for end and center excesses, as well as excess due to super-elevation, in addition to the standard clearances and construction. At stations located on curves, center and end excesses make the platform construction such that the gap between the car body and the platform edge is a possible source of danger to passengers. For this reason, curves at stations should be avoided if possible. At ends of island platforms, however, it is generally necessary to use curves, so as to narrow the structure quickly. In such cases, care must be taken that the gap is not large enough to be dangerous.
Typical four track double deck subway. (Click image to enlarge.)
Curves also occur where loops are used. The advantage of loops is that trains are enabled to reverse direction without holding up traffic behind them. When used, they are located either at terminals or at points from where it is desirable to return some but not all of the trains in operation.
Location of Storage Yards. Storage yards are generally built near the terminals of the elevated railroad extensions. It is necessary, however, to provide emergency storage tracks at convenient points along the line, so that disabled trains can be switched off quickly and not hold up traffic. Emergency storage tracks are also useful for the purpose of adding extra trains to the service during rush hours.
In order to obtain good and economical operating conditions, no curve of less than 150 feet radius should be used, and all curves of less than 2,000 feet radius should be transitioned. Reverse curves should be separated by a tangent as long, at least, as the distance between car trucks, so as to give the train a chance to straighten out before reversing.
Placing of Crossovers. In addition to providing for full interchange between all tracks, by means of crossovers at terminals, crossovers should also be located at all points where a change in heavy traffic is expected, so that trains can be reversed from these points. Besides these crossovers, others allowing for full interchange should he placed about a mile apart for emergency purposes, so as to make operation of a part of the line possible in case of accident. Crossovers on running tracks should not have a smaller frog than number eight, while for emergency a number six frog is sufficient. Larger frogs are, of course, desirable, but, in subways, owing to the length of crossovers, large frogs mean a more expensive structure. In this connection, it should also be pointed out that for the sake of economy crossovers should be so located that if possible not more than two tracks need to be spanned by the roof beams. From an operating point of view, special frogs should be avoided and the number of different kinds be a minimum, so that only a small stock is necessary for renewal.
As stated before, all curves of less than 2,000 feet radius should be transitioned on account of super-elevation. A length of 150 feet is generally sufficient to gain this result, although shorter ones, even curves half that length, are giving satisfaction. This, of course, is not possible at crossovers and at points of reverse curves.
Type of Structure. The determination of the standard type of structure to be used between stations, except at fan chambers, duct manholes and pump chambers, is subject to the nature of the ground, the depth of the subway, the width of the street, the relation of the structure to mean high water or to ground-water, the number of subsurface structures to be maintained and the general traffic conditions.
When the subway is near the street surface, in earth and above water, the most economical structure has been found to be one composed of steel bents, 5-foot centers, connected by concrete arches. Each steel bent consists of roof beams, sidewall columns, interior columns and knee braces. Should the subgrade be below water and in earth, steel beams are added to the bents just described in the track floor of the structure.
Where the roof of the proposed structure is to be at least 10 feet below the top of rock, or where the subway is to be in earth and far from the street surface, the best type of structure has proved to be a concrete tunnel, with an arched roof and reinforced flat invert. This type reduces the amount of excavation and does not require the erection of cumbersome steel members which an open cut steel bent structure at this depth would necessitate.
In narrow streets, a steel bent double-deck structure might be used, and, if tunneling be more economical, a combination of steel bent and concrete tunnel is possible.
If the line is to be built in open cut without cover through undeveloped territory, where few subsurface structures have to be maintained, and the room occupied by the finished structure is not a governing factor, reinforced concrete design might be advantageous.
Structural Steel versus Concrete. The question of whether structural steel or reinforced concrete should be used in subway construction is an interesting one. The opinions of contractors differ radically as to the ease and facility with which a reinforced concrete job can be carried out, and the careful designer will prepare alternative designs for bids where it seems that reinforced concrete would he more economical than steel construction. The advantages of reinforced concrete are the ease and promptness with which the various materials can be procured; the smaller cost of the reinforcing rods as compared with steel beams and riveted sections; the cheapness with which it can be placed; its adaptability to any form, and the fact that it does not need painting.
Steel is advantageous for subway work, because it permits the cover load to be shifted to it immediately after erection and adapts itself readily to the restoration of subsurface structures. It also reduces to a minimum the amount of excavation.
Typical local station on four track structure. (Click image to enlarge.)
As stated above, reinforced concrete might be employed in open cut construction where decking is not necessary, where few subsurface structures need to be maintained, and where the room occupied by the finished structure is not a governing factor. The unit bid for excavation would probably be low, because, in subway contracts, where construction is by cut-and-cover method, the price for excavation includes decking and the maintenance of many subsurface structures. The contractor would not be delayed on account of the fabrication of steel and would prosecute the work with cheaper labor than that necessary for steel construction.
Where Steel is Imperative. In narrow streets, however, bearing a heavy traffic and congested with many pipes, ducts, and other subsurface structures, steel bent construction is practically imperative. On account of its superior strength, the roof, sidewalls, and invert of the subway can be made much thinner with steel than with reinforced concrete. Where pipes and ducts cross the structure, a very thin construction can be employed, over which longitudinal pipes and street railroad construction can pass with a minimum depression of the subway, thereby reducing the amount of excavation. On the sides where the streets are very narrow, similar depressions afford room for sewer manholes. In steel bent construction the contractor can place his timber bracing between the steel bents without greatly interfering with its erection. After the steel is erected, the cover load Can be shifted to it at convenient points, allowing greater freedom for the restoration of subsurface structures and concreting.
With reinforced concrete this is difficult, because the forms and bracing cannot be removed until the concrete has set. Furthermore, the timber bracing prevents the desirable continuous laying of concrete and makes it difficult to get a close spacing of rods, besides necessitating patchwork in the waterproofing after the removal of the bracing.
These various advantages and disadvantages of the materials have practically standardized the design of subways. In general, steel bent construction is used except where changes in the design have to be made, due to the discovery, in the progress of excavation, of bad soil, surface water, or the absence of expected rock. Here reinforced concrete, for which rods can quickly be obtained, comes to the rescue without delaying the work.
Ventilating Flue Construction. Reinforced concrete is also made use of in the construction of ventilating flues. The flues follow the street surface and are so dependent on the location of the subway, with reference to the curb lines, sewers, and other subsurface structures, that their shapes are irregular and cannot be entirely determined until these are nearly ready to be built. The adaptability of reinforced concrete to any form, as well as the promptness with which rods can be obtained, is thus taken advantage of for the construction of these flues.
It is interesting to note the development of the present standard types of subway structures front that of the first subway built in New York City. The original structure is of the steel bent type, with plate and bulb angle columns. It is entirely enveloped by waterproofing. The ducts carrying the current for operation are of the four-way type and are placed outside of the steel for practically the entire height of the sidewalls.
The present types use standard angles instead of bulb angles. Every steel mill rolls standard angles, and the delay which might arise through the shutting down or other contingencies of mills rolling bulb angle columns is obviated. The ducts in the sidewalls of the old type, forming an air space, as well as the continuous waterproofing envelope, which is a poor conductor of heat, retain the heat generated by train operation and do not allow it to radiate into the surrounding earth. In the present structure, the waterproofing on the sides and bottom is omitted, except below mean high water, and the ducts are banked inside of the sidewalls, forming, with their concrete protection, a walk 2' 1" wide and 4' high, allowing the generated heat to be dissipated through the sidewalls above the ducts and through the bottom. This in turn reduces the amount of grating necessary to ventilate the structure. The type of ducts has also been changed from four-way to one-way. The one-ways are laid with broken joints, both horizontally and vertically, so as to prevent an accident to one cable affecting others.
Clearances. Proper provision for the installation of signals, for the swaying of car bodies on their springs, and for the safety of workmen in the subway, requires a clearance of 1' 6" from the car body to any wall. This distance may be decreased to a minimum of one foot at isolated points, such as at the first column between the tracks where Crossovers occur. This decrease in clearance may obviate the necessity of spanning three tracks, by using a deep girder, which would add to the cost and interfere with the restoration of subsurface structures. Openings 2' wide by 7' high, spaced on about 10' centers, should also be provided in interior walls for the protection of workmen. The clearance at duct benches, for a height of 4' above the base of rail, should not be less than 5".
Track Trough. The depth of the track trough, as used in the New York subways, for installation of rails, ties, ballast, third rail and for drainage, is 1' 2" below base of rail at center line of track, decreasing to a depth of 12" on both sides of the center line of track and at distances of 5' 2" from it. Above a point 2" above base of rail the clearance from the center line of track to any obstruction must not be less than that required for third rail protection and contact shoe clearances. In addition to these standard clearances, provision must be made for center and end excesses due to horizontal curvature and super-elevation. Excesses due to vertical curves are so small that they can be neglected.
Super-elevation in New York subways is calculated on the basis of a train velocity of 30 miles per hour, the formula used being:
|S = Super-elevation in inches
G = Gauge in inches
V = Velocity in feet per second
R = Radius of curve in feet
At a velocity of 30 miles per hour this formula reduces to:
Stations. Typical local stations are generally provided with side platforms and accommodate trains of less cars than the trains required for express service. Those local stations, however, at which express trains stop, occurring on lines having only a partial express service, must be as long as express stations. The width of the side platforms should not be less than 10 feet. This sometimes necessitates relocating the street sewer underneath the platforms. The height of all platforms is generally about 4 feet above base of rail. The length of platforms depends upon the number of cars to each train required by the probable traffic. In New York City the original subway station platforms were built 200 feet and 350 feet long to accommodate five and eight-car local and express trains, respectively. The increase in traffic required their lengthening to 225 feet and 480 feet for six and ten-car trains. The new platforms for the Dual Subway System in New York are 405 feet and 480 feet long for the I. R. T. lines, and 495 feet and 530 feet long for the N. Y. M. R. R. lines. Typical express stations have island platforms, generally reached by mezzanines over the tracks. Sub-passages, because they increase stair climbing, should be avoided, but in some cases local conditions make their use imperative. Since express stations must be depressed to allow for mezzanine construction, that part of the station not occupied by mezzanines should be built with a high roof. This adds to the appearance, besides being economical, because the roof is required to carry less backfill. Stations in Narrow Streets. In narrow streets and deep structures even local stations are constructed with island platforms and mezzanines. In the case of narrow streets this is done because one island platform accommodating traffic in both directions takes up less room than two side platforms and in the case of deep structures advantage is taken of the available room for a mezzanine. This, besides affording a natural rest in stair climbing, eliminates one control and balances to some extent the extra expense of a deep structure, by the saving in the cost of operation. In very narrow streets it is possible to design a station where the roof over one track is the platform for the other track. The station could of course be constructed on two levels, but the above method saves a considerable amount of excavation and reduces stair climbing. Station entrance stairs are sometimes built on the sidewalks next to the curb. This practice, however, has been largely abandoned and is resorted to where the entrances are built under existing elevated railroad stairs. Generally the practice is followed of endeavoring to convince abutting property owners of the fact that a subway approach through their property will enhance its value. Where this can be accomplished it removes the station entrances entirely from the street, leaving the sidewalk clear for the use of pedestrians. When an owner cannot see his way clear to make the alterations to his building necessitated by the above procedure, he can often be persuaded to exchange window display space, or an entrance to his building from a stair platform, for beam rights. This saves the construction of a supporting wall adjacent to the building line, thereby reducing cost and saving sidewalk area. Escalators Considered. Where the distance from the surface of the street to the platform level exceeds 32 feet, the installation of escalators is considered, and where this distance exceeds 50 feet elevators must be used. Ramps, not exceeding a grade of 10 per cent, are resorted to where the difference in elevation, between levels to be connected would require only a few steps, and also at all other places where possible, the difference between their cost and stair construction not being prohibitive. They should not be used instead of stairs where the large opening they require on account of head room would monopolize valuable areas, as at sidewalk levels and island platforms. [[Category:Subway Article Dual Contracts]] [[Category:Author Public Service Record]]