US2211589A

US2211589A - Submerged bridge or tunnel construction

Info

Publication number: US2211589A
Application number: US35032A
Authority: US
Inventors: Arthur J Widmer
Original assignee: Individual
Current assignee: Individual
Priority date: 1935-08-07
Filing date: 1935-08-07
Publication date: 1940-08-13
Anticipated expiration: 1957-08-13

Description

Aug. 13, 1940. A. J. wlDMER SUBMERGED BRIDGE 0R TUNNEL CONSTRUCTION Filed Aug. '7, 1.955v

3 Sheets-Sheet l.

Illlmmhnmllll Aug. 13, 1940. A.J.w1DMER SUBMERGED BRIDGE 0R TUNNEL CONSTRUCTION l Filed Aug; 7, 1935 3 Sheets-Sheet 2 Aug. 13, 1940. N A. J. wlDMER SUBMERGED BRIDGE 0R TUNNEL CONSTRUCTION Filed Aug. '7, 1935 3 Sheets-Sheet 3 @Jai Z5 ox? Patented Aug. 13, 1940 PATENTv OFFICE SUBMERGED BRIDGE OR TUNNEL CONSTRUCTION Arthur J. Widmer, Webster Groves, Mo.

Application August 7,

4 Claims.

My invention relates to tunnels. With many streams, such as the Mississippi River, the distance from the water to bedrock is so great and the material of the bed so unstable and so liable to scour that it has heretofore been impracticable to build a tunnel through such bed. 'I'he principal object of the present invention is to devise a construction which, with attributes of both a tunnel and a bridge, is suitable for use in the unstable bed below the water and will continue to function even though the 4surrounding solid matter is washed away. Another object is to devise a tunnel construction which will be stable, rigid and capable of sustaining substantial vertical and horizontal stresses or loads in addition to the hydrostatic pressures to which it is exposed. Another object is to locate the bottom of the tunnel at less depth below the surface of the stream than has heretofore been prac'- ticable consistently with the requirements of navigation. Another object is. to make it practicable to build a tunnel in long sections in a safe and convenient place and float such sections to piers previously built to receive them. Another 25 object is to utilize the current as a means of increasing or modifying the net vertical'load on the tunnel. Another object is to devise a tunnel whose drainage will not be impaired by deflection of the tunnel under load. Another object is. to devise a tunnel of such cross section as to cause minimum obstruction to the flow of the stream, especially in flood stage. Another object is to minimize the effect of the downstream pressure of the current. Another object is to provide a 3,5 more direct and easier passage across the main chamber of incoming fresh air and outgoing foul air. Other objects are to devise an economical construction and to achieve advantages. hereinafter appearing.

The invention consists partly in locating the tunnel proper in or below the water and far enough down to meet the requirements of navigation and in mounting the tunnel proper, after the manner of bridges, on piers that rest on bed- 45 rock or that rest beneath the lowest plane at which scouring is possible, on piles or other substantial supporting medium. The invention also consists in so making the tunnel of reinforced concrete that it is adapted not only to withstand the hydrostatic pressure thereon but is adapted to act as a hollow box cantilever or other type of girder in resisting both vertical and horizontal stresses thereon. The invention also consists in making the tunnel with a relatively wide and low main chamber of generally oblong shape and 1935, serial No. 35,032

(ci. s1-42) with upstream and downstream extensions forming chambers that serve for fresh air and foul air flues and other purposes. It also consists in making such upstream and downstream extensions in the general form of triangles which have their apexes outermost and preferably with the outer surfaces of the inclined sides shaped to conform more or less closely to the requirements of stream-lining or so arranged that the pressure of the current will produce a vertically acting re- 10 sultant force that lightens or increases the net vertical load on the tunnel. The invention also consists in the process of building the sections of the tunnel proper at a convenient location, closing the ends thereof for the sake of buoyancy and then floating said sections to their places on the piers. The invention also consists in the method and in the parts and in the combinations and arrangements of parts hereinafter described and claimed.

In the accompanying drawings, wherein like reference numerals refer to like parts wherever they occur:

Fig. 1 is a transverse cross sectional view of a stream crossed Xby a submerged bridge,

Fig. 2 is a transverse cross sectional view of a stream in which the tunnel sections are entirely surrounded by water,

Fig. 3 is an isometric projection of a pier supporting a tunnel section,

Fig. 4 is a side elevation partly in section of the connection of adjacent tunnel sections,

Fig. 5 is a transverse cross section of a preferred design of tunnel section,

Fig. 6 is a transverse cross section of another tunnel section,

Fig. 7 is a transverse cross section of a tunnel section embodying structural steel members in the vertical walls;

Fig. 8 is a side elevation of a tunnel construction mounted on a pier as a balanced cantilever,

Fig. 9 is a side elevation of a tunnel construction in which a metal bolster caps a pier,

Fig. 10 is a diagrammatic transverse cross section of a tunnel section in which the top of the section extends horizontally throughout the full width of the section,

Fig. 11 is a diagrammatic transverse cross sec.- tion of a tunnel section in which the bottom of the tunnel section extends horizontally the full width of the section,

Fig. 12 is a diagrammatic transverse cross section of a tunnel section in which the exterior surfaces of the top side portions are at a smaller angle with the horizontal than the exterior surfaces of the bottom side portions,

Fig. 13 is a diagrammatic transverse cross section of a tunnel section in which the exterior surfaces of the bottom side portions have a greater area than the exterior surfaces of the top side portions,

Figs. 14 and 15 are transverse cross sections of tunnel sections having triangular chambers on the downstream side,

Fig. 16 is a diagrammatic transverse cross seotion of a tunnel section embodying the main tensile reinforcement and some of the diagonal tension reinforcement of a tunnel section designed as a cantilever to resist a downward vertical load,

Fig. 17 is a diagrammatic transverse cross section of a tunnel section embodying the main tensile reinforcement and stirrups of a tunnel section acting as a cantilever' against the horizontal stream thrust moving in the direction indicated by the arrow, and

Fig. 18 is a diagrammatic transverse cross section of a tunnel section embodying the principal reinforcing steel required to resist hydrostatic pressure.

The ordinary tunnel is supported more or less continuously throughout its length by more or less stable ground and is subject to pressure of the surrounding water, soil or solid matter, whereas the typical bridge is supported on widely spaced piers and is designed principally to carry its own weight and superimposed load. The structure of the present invention embodies characteristics of both tunnels and bridges as it is composed of sections that will withstand considerable hydrostatic pressure and are supported wholly or partly on widely spaced piers and are designed to take care of vertical and horizontal loads or stresses of non-hydrostatic origin.

In consequence of the design of my construction it becomes practicable to locate the tunnel proper at any desired depth below the surface of the water provided it does not interfere with navigation. In the case of deep streams, the water may completely surround the tunnel; in the case of shallow streams, the tunnel may be wholly below the stream; and in other cases, the tunnel may be partly in the water and partly in the bed of the stream.

According to the present invention, series of piers i are built up from below the stream and extend high enough for the tunnel proper to rest upon. These piers may rest on bedrock or upon piles or other suitable supports Provided therefor beneath the lowest plane at which scouring of the river bed is anticipated.

These piers l are relatively narrow crosswise of the stream but on the downstream side they may slope to bedrock or other support at a considerable angle and thus afford increased stability against tlie thrust of the current on the tunnel and piers or they may be guyed upstream near their tops or may be vertically anchored at their bottom near the upstream edges or otherwise stabilized. The tops of the piers, at their downstream ends, preferably project vertically above the main top surface of the pier so as to constitute abutments 2 for the downstream side of the tunnel sections 3 to bearagainst. The upper surfaces of these abutments or vertically extending portions of the piers preferably ccnform to the under surfaces of the downstream portions of the tunnel sections so as to support said portions horizontally as well as vertically.

The tunnel proper comprises one or more alined sections of reinforced concrete that arc preferably pre-cast at a convenient location and moved as individual units to the piers. Many factors enter into the design of the tunnel sections. In the first place, they must be designed to withstand the hydrostatic or soil pressure in accordance with well known principles. Hydrostatic pressure, as used herein, refers to the pressure of the weight of the water on the tunnel sections and does not refer to the pressrlre of the stream current. In the second place, the tunnel sections are designed to act as hollow box beams, preferably of the cantilever type in carrying their own weight and vertically applied load. They are also designed to act as hollow box beams or cantilevers in resisting the horizontal pressure of the moving stream. In addition to the foregoing factors, the tunnel units are designed to minimize the effect of stream pressure after the manner of streamlining and by minimizing the vertical height thereof and to make use of the stream pressure by creating vertically acting components to increase or decrease the net vertical load on the structure. Another important factor is the need for .effective ventilation. All of these factors are provided for in the construction herein described.

The main chamber fl of the tunnel section 3 has vertical side walls of reinforced concrete and top slabl 5 and bottom slab 'I of reinforced concrete which are monolithic with the side walls. In the preferred design illustrated in Fig. 5, the top and bottom slabs are extended horizontally beyond the side walls and, beyond such horizontal extensions, one or both of said slabs are inclined to meet each other at the outermost sides of the structure, thereby forming pointed tips or low side walls 8 that are heavily reinforced and monolithic with the rest of the section. Preferably the construction of the downstream side of the tunnel section is substantially the same as the construction of the upstream side. In the preferred form, the crosssectional outline of the tunnel section is substantially a hexagon with relatively wide horizontal top and bottom and with two inclined faces 9, i@ at each side. The main vertical walls divide the sections into a main chamber l of the general form of an oblong or rectangle of greater width than height and two side chambers l l of general triangular form.

In consequence of this arrangement, under hydrostatic pressure, the top and bottom slabs act in a transverse direction as beams continuous over the side walls of the main chamber. In consequence of their continuous beam action resulting from this arrangement it is feasible to gradually reduce the thickness of the top and bottom slabs away from the vertical walls after the manner of beams of uniform strength. This gradual thinning of the slabs saves considerable weight and material Without significant deviation from their horizontal disposition. It also permits the upper surface of the bottom of the main chamber to slope downwardly to the middle thereof and form an efficient drain l2.

For the purpose of carrying its own weight and vertical superimposed loads, each tunnel section is designed as a longitudinally extending hollow box cantilever or other type of beam and is reinforced accordingly; that is, when it is infeo Aproposed pier sites.

.the current. against deflection due to horizontal thrust than tended that the middle of the tunnel section shall -rest on a pier, such section constitutes a cantilever in which the top slab acts in longitudinal tension and thebottom slab actsin longitudinal compression and are reinforced accordingly, while the vertical walls act as the web members of such hollow cantilever and areV reinforced to take care of the shearing and diagonal stresses accordingly. It is noted that the triangular side portions of the tunnel sections act in the same manner as the main oblong portion in taking care of vertical load so that the full width of the .tunnel section functions as a cantilever.

The vertical load on a tunnel section acting as a cantilever or other type of beam may be regulated considerably by varying the buoyancy of the tunnel section and also, where advisable, by utilizing, in the manner elsewhere described, the

-vertical component of the force of the current on the inclined sides. Therefore frequently in designing and in determining the proper span of a tunnel section, the amount of the more difiicultly regulable horizontal thrust of the current will be a governing factor.

In bridging a stream with a submerged Way resting on piers many peculiar conditions and difficulties may be encountered under water at Greater economy and better design of the whole structure results if elasticity is available with respect to number, location and spacing of piers and also with respect to the character of beam supported by the piers.

The shape and proportions of the tunnel section which I prefer offers great strength as a beam resisting vertical load and relatively even greater strength as a horizontal beam resisting The section is also more rigid against deection due to Vertical load. Because of the relatively great expense of piers, it is apparent that economy of construction lies in selecting as long spans as possible and that spans less than about 21/2 -times the width of the tunnel sections will not generally prove economical. Furthermore since cantilever beams offer relatively more rigidity against deflection than freely lsupported beams, it will frequently be advantageous in increasing the spans and reducing the number of piers required, to so rest the tunnel sections about midway of their length on the piers that the sectionsact as balanced cantilever beams, extending longitudinally in both directions beyond the piers. To provide such cantilevers through the use of my tunnel sections requires only suitable quantity and arrangement of tensile steel reinforcement in the sections in the usual manner to take care of the negative bending moments producedy by the non-hydrostatic vertical and horizontal loads, and such supplementary reinforcement and design precautions as are common in the art of reinforced concrete, Cantilever design oiers numerous advantages; for example, with tunnel sections of equal length, cantilevers will in a total given distance require one less supporting pier than simple beams. Cantileversl necessairly require broad pier seats to prevent tipping from unequal loading of the balanced ends but the increased pier breadth is re1- atively inexpensive and may be automatically compensated for by increasing the length of the tunnel section.

Floating debris or ice may produce varying horizontal loads on opposed ends of cantilevered sections as described, and the vertical loads of traflic may not be uniform throughout the length of a tunnel section. Full consideration of these unbalanced loads of oppositely extended submerged cantilevers is extremely important because, Whereas in ordinary cantilever bridge construction the live loads are generally small compared to the dead loads, the buoyant tendency of the tunnel section of the submerged bridge may frequently almost completely neutralize or even exceed the dead weight of the structure. Thus the live loads assume relatively great importance and may govern the design. In cantilever design as described I nd it advisable to provide, between the tunnel sections and the piers, anchor bolts I3 or other anchoring means of any suitable nature near the longitudinal extremities of the pier to resist tendency to tip under unbalanced loading of the opposed cantilevers, and in certain cases bearing shoes may be necessary at the pier seats to prevent crushing of the concrete.

Completely efcient cantilever bridge design generally indicates substantial increase in the depth of the cantilever tunnel sections at the support. However, such increase would tend to cause objectionable obstruction to the stream flow and to create excessive tendency to overturning of the pier, as stream-lining such a consequently large obstacle would be difcult. Furthermore there might be difficulties incident to launching and floating such tunnel sections if built at the shore. Under conditions necessitating extreme length of cantilever, it may be desirable to use a metal bolster I4, which rests on and, in effect, constitutes part of the Dier. Such bolster is designed as a cantilever extending longitudinally beyond the pier in both directions and formed of suitable shapes to reduce resistance to the stream, said bolster being more or less open in construction in the direction of the stream. This bolster, if desired, may be made:

completely permanent with respect to corrosion or similar deterioration through the use of selected alloys of metal. To assist in resisting overturning due to unbalanced loads in the oppositely extended tunnel section cantilevers, which may extend considerable distance beyond the extremities of the bolster, the metal bolster is firmly anchored near the longitudinally opposite sides of the pier by means of suitably arranged bolts I5 or any other suitable and adequate means; and the tunnel section is similarly anchored to the bolster or the pier or to both. Seats I6 are provided of suitable character on the bolster for the tunnel section. It is obvious that under certain conditions I may use a bolster of reinforced concrete construction in lieu of the metal bolster just described.

As indicated above, the tunnel sectionsl are designed to act as hollow cantilever girders in resisting the horizontal pressure of the stream. In such action, the upstream side of a tunnel section, including the upstream vertical wall, is in the region of tension and reinforced accordingly; and the downstream side, including the downstream Vertical wall, is in the region of compression and reinforced accordingly where necessary, while the top and bottom slabs act as webs of the cantilever and are reinforced accordingly for shearing and diagonal tension. The horizontal pressure against the tunnel sections is transmitted to the abutment provided therefor on the piers.

In practice, it is desirable to pre-cast each tunnel section at some convenient place, float it through the water to a point above th-e pier and then lower it into vits position on the pier and against the abutment provided thereon for it. For the purpose of moving such a section, its ends are rst closed by means of removable bulkheads and other temporary bulkheads may be used to divide the main chamber into smaller compartments and, if the section itself is not sufficiently buoyant to oat or floats too low in the water, pontoons or other suitable means are used to provde additional buoyancy. Then the section is towed to a point above the pier and its buoyancy is decreased gradually, as by properly slackening the cables of the pontoons or by admitting water into the tunnel compartments, so as to lower the section onto the pier. If the top of the pier is lower than the bottom of the stream, the bottom is dredged out or removed by any suitable means far enough to seat the tunnel section on the pier; and after the tunnel section is seated on the pier and suitably secured thereto, the surrounding cavity is allowed to silt up under the action of the stream. The temporary bulkheads are removed after adjacent sections have been connected and made watertight by any suitable means and the water and any silt which may have entered the section are removed. In case the tops of the piers are lower than the existing loose bed of the stream, way for the tunnel sections may be provided as previously described or the sections may be rst lowered on the loose bed after which the loose material is removed by any suitable means to permit lowering the sections on the piers.

In the case of cantilever sections, the ends of the sections are preferably provided with suitable connecting devices for joining them together in alinement either directly or through a short connecting section especially provided for the purpose. In either case, it is desirable to have the ends of the cantilever sections provided with radially extending rings or arms l1 firmly anchored in the reinforced concrete and provided with tie bolts I8 adapted to pass through similar rings or arms provided to receive them on the adjacent section.

In designing the tunnel sections to resist hydrostatic pressure, the top slab 6 and bottom slab 'i of the main chamber are treated as beams supported by the vertical walls 5. In my invention, I prefer to have one main chamber 4 and two side chambers l I as shown in Fig. 5, because continuous beam action is thereby assured, especially for the relatively long central slabs which form the top and bottorrr of the main chamber. The top and bottom slab extensions 9, i forming the side chambers, at their outer extremities merge and support each other and at their inner extremities rest on the vertical walls 5. It is desirable that the slabs 9, ID have horizontal portions i9 substantially in the plane of the top and bottom slabs 6, 'I because they improve the condition of continuity over and adjacent to the vertical supports. By providing this continuous beam action with its consequently reduced bending moments, I am able to effect economy in structural materials. Also I may use thinner slabs to reduce weight or I may accomplish this purpose by designing the slab within practical limits as a beam of uniform strength thus obtaining a slab of varying thickness and thus improve the buoyant condition of the entire section.

The use of a central main chamber and two side chambers to `create three continuous spans provides another great advantage with respect to the reduction of weight of the tunnel section. Hool, in vol. 3, page 443, of his book entitled Reinforced Concrete Construction (1st edition, 1916), shows how four walls similar to the top, bottom and sides of the main chamber 4, when the top and bottom are under hydrostatic pressure, may be treated in the manner of a symmetrical arch vwith fixed ends, with the resulting moments in the top and bottom slabs very similar, if not identical, to lthose of the continuous beams previously mentioned. However, in order to create this condition the vertical walls 5, particularly at their extremities, must substantially equal in thickness the top and bottom slabs at their supports. Therefore, if designed according to the above arch theory, as increases in hydrostatic pressures increase the thickness of the top and bottom slabs, so also must the vertical walls increase in thickness and weight. Under the method of design set out in the preceding paragraph (since in designing for hydrostatic pressure the walls are considered merely as supports, and for this purpose require only relatively small areas of concrete), increases in weight of the top and bottom slabs may be compensated for by reducing the Ithickness and weight of the vertical walls. Since these Walls serve also as beam webs in the treatment of the entire section as a beam under vertical load, this reduction in the thickness of the vertical walls may be accompanied by the use of either a thin steel girder 2U to take non-hydrostatic vertical loads as indicated in Fig. 7, or by the use of a thin reinforced concrete wall with suitable structural steel sections introduced, in designing for the non-hydrostatic vertical loads, as a supplementary means of strengthening or assisting said reinforced concrete wall.

In the central slab of three uniformly loaded continuous spans the positive bending moment at the center of the span is only one-half as great as the negative bending moment at the supports. Advantage is taken of the reduced moment to reduce the slab thickness in the center of the bottom slab of the main chamber to create a central longitudinal gutter or drain as indicated at l2. The finished roadway surface of a vehicular tunnel may be a thin cement or mastic coating of uniform thickness applied directly on the reinforced concrete bottom slab to maintain the drain in the center, or the slab itself may form the roadway surface.

In the interior` span of a uniformly loaded beam continuous through three spans, the maximum bending moment is the negative moment over the supports. This is twice the amount of the positive bending moment in the center of the span. The point of contra-exure is about twenty-one one-hundredths of the span from the support; therefore the bending moment decreases very rapidly from the support. For example, in a span of twenty-one feet, under about five thousand pounds per square foot hydrostatic pressure, the moment has decreased suiciently one foot from the support to permit nearly twelve inches reduction in the thickness of the slab from about thirty-nine inches thickness required at the support, while six inches still further away about sixteen inches total reduction in thickness is possible without increase in the tensile and compressive stresses in the steel and concrete. A theoretically ideal condition would be realized if the floor slab forming the bottom of the main chamber were altered in thickness through its entire length to correspond exactly with the changes in the bending moment. However, shearing stresses and other considerations make it impractical to realize the ideal condition. Normally in a structure as described, except adjacent to the supports, it will be desirable merely to approximate the minimum slab thicknesses permitted by consideration of the tensile and compressive stresses only. However, near the supports advantage may be taken of the rapidly decreasing bending moment sharply to reduce the slab thickness. Such designmay be used to provide sharp upward inclines in the top surface of the chamber bottom at its sides as indicated at 2|. These sharp inclines form brackets at the sides of the roadway and thus create effective wheel guards for vehicles. Obviously, if desirable to provide increased effectiveness as wheel guards, the inclined surfaces may be altered in angle and position but should not lie within the surfaces established by the structural design.

The construction hereinbefore described is especially useful in the case of streams whose beds are liable to scour for a considerable depth in time of ood. In such case, the tunnel is located at a level that is determined by considerations of economy and the requirements of navigation rather than by the soil and water conditions and may normally be embedded in the silty or gravelly matter that forms the bed of the stream and remains more or less stationary under normal conditions but is likely to scour to a considerable depth in time of flood and thereby leave the tunnel sections supported wholly by piers after the manner of bridges. My construction is well adapted to meet these conditions and the stream-lining thereof minimizes its effect as an obstruction to the current.

Another important advantage of the construction hereinbefore described is that its at top and bottom are well adapted for resisting stresses due to cantilever or other type of beam action and permit the vertical height of the tunnel sections to be reduced fairly close to the height needed for the accommodation of traic. This reduction in the height reduces the area of resistance to the pressure of the stream and also enables the bottom of the tunnel sections to be located at a higher level and consequently where the hydrostatic pressure is less than would other- Wise be practicable.

Another important advantage is the streamlining effect arising from the sloping of the upstream and downstream sides of the tunnel. The ideal condition for stream-lining would call for the surfaces of the sides to be reverse'ly curved with the concave surface portions outermost on both .upstream and downstream sides; but for practical purposes it is sufficient to make such sloping surfaces substantially flat provided their angle with the horizontal is not more than about forty-live degrees. For a xed position in a stream, the total horizontal thrust due to the action of the current on a body depends upon the velocity of the current, the size and shape of the body and the roughness of its surface. If the body is pointed at both ends with its axis parallel to the current, as in my preferred design, the stream lines on closing together smoothly at the downstream extremity, exert normal pressures against the downstream surfaces whose components longitudinal to the said axis approximately balance the corresponding components of the normal pressure on'the upstream surfaces; so that the total horizontal thrust consists mainly of that due to skin friction alone distributed along the external surface of the body.

Since the skin friction is comparatively slight f due to the smoothness of the external surfaces of my tunnel sections, I prefer to approximate the ideal condition by shaping the downstream extremity similar to the upstream extremity.

Another important advantage is that the sloping sides of the tunnel proper may be optionally so designed that the pressure of the current will act either in aid of or against the buoyancy of the construction, as the pressure of the current is normal to the surface of the tunnel sections and varies as the sine of the angle between the direction of the current and such surface. Thus, if as in Fig. 10, the top of the tunnel section extended horizontally throughout the full width of the section, while the side portions of the section sloped all the way from bottom to top, the pressure of the stream would have an unbalanced component acting vertically upwardly against the sloping portions and it would thereby lessen the net vertical load required to be carried by said section. If it were desired to decrease the buoyance of the construction, the bottom of the section could be made horizontal for the full width thereof and the side portions of the top sloped, as in Fig. 1l, so that the Vertical component of the current pressure would act downwardly thereon. As the total pressure of the current varies according to the area under such pressure, in some cases, it may be desirable to slope one or both of the top side portions for a different width than the bottom side portions as in Fig. 13. Regulation of the net vertical load may also be obtained by having the topand bottom of the side portions at different angles with the horizontal as in Fig. l2. To sufciently lessen the net vertical load on the section, it may be desirable to take advantage of the hereinbefore mentioned reduction in the slab thickness at the center of the bottom slab of the main chamber to create further inclined surfaces against which the current may act in the manner just described. Such inclined surfaces may be provided by keeping the upper surface of the bottom slab horizontal and inclining the bottom surface upwardly from the sides thereof as shown at 22 in Fig. 6.

Another important advantage of my construction is that it provides a very eflicient system of ventilation. For this purpose one of the side triangular chambers may serve as the fresh air duct and the other side triangular chamber may serve as the foul air duct, suitable openings 23 being formed through the vertical walls at intervals throughout the length of the wall and at various heights. By this arrangement, the air passes more or less horizontally from one side duct to the other, being preferably propelled or drawn by suitable blowers or fans provided for the purpose.

In the foregoing description, I have described each tunnel section as seated at its middle on a single pier. With a long section and a narrow pier, a heavy unbalanced live load would tend to unbalance such a structure. For this reason, I prefer to cap the pier with a ,wide metal bolster firmly secured thereto by -bolts or otherwise and having projecting brackets 24 firmly braced against and secured to the sides or the top adjacent to the sides of the pier. The tunnel section is seated on said bolster and firmly secured by bolts or otherwise to the wide spreading brackets of said bolster. The wide bearing and anchorage thus afforded for the tunnel section reduces the beam stresses therein and protects it against imbalance under live load.

While I have hereinbefore specifically described the tunnel sections as cantilevers, it is obvious that they may be designed as simple or freely supported beams with their ends resting on different piers or as continuous beams supported on several piers. In such cases, there would be little, if any, difference in the shape of the structure; but the type of beam selected would materially affect the position of the main tensile reinforcement. For example: if a cantilever is selected, the main tensile reinforcement is placed to extend longitudinally in the top member of the tunnel section, as at 25, to resist downwardly acting non-hydrostatic load and near the upstream side extremity, as at 25, to resist the horizontal stream thrust; while in the case of a tunnel section freely supported at its ends, the main tensile reinforcement is placed opposite to that just described to extend longitudinally in the bottom member of the tunnel section to resist downwardly acting non-hydrostatic load and near the downstream side extremity to resist the horizontal stream thrust. In the case of tunnel sections continuous over several supports, the main tensile reinforcement in the middle portion between points of contra-flexure is placed as just described for freely supported tunnel sections, but, at and adjacent to the supports, such reinforcement is positioned after the manner used in cantilevers. Except for the different location just mentioned of the principal reinforcement as governed by the type of beam selected, the hollow beam construction of freely supported tunnel sections and sections continuous over several supports would be similar to and embody the advantages hereinbefore stated. fis in other engineering' structures, every tunnel will present problems as to the amount, location and arrangement of the reinforcement used therein, but, with the guidance afforded by tl 's specification, such problems are well within the skill of engineers familiar with the reinforcement of concrete.

In cases where it may be desirable to increase or decrease the buoyancy of the tunnel sections by making the top or bottom horizontal for the full width of the section as previously described herein, it is obvious that a very deep beam is formedby the wide top 2l or bottom 28 which might be capable in itself, if properly reinforced, of resisting all the stresses induced in the section by the horizontal stream thrust. In such case, this deep beam alone might be designed to carry either all of the horizontal load or all of the shear and diagonal tension resulting from the horizontal load.

Hereinbefore I have described a submerged bridge composed of several tunnel sections and piers. However, since a tunnel section may be several hundred feet long, it is obvious that one such section may sulce to cross a relatively narrow waterway. In such cases the tunnel section may either be cantilevered from a pier near the middle of the stream, or support for the single tunnel section may be furnished by two piers close to or on the stream banks.

In swift current the preferred shape of the tunnel section previously described offers many advantages, of which one is the reduction of the effect of the horizontal thrust of the stream current on the section. However, the unit horizontal stream thrust on the tunnel section varies as the square of the current velocity so that, without adverse effect on the net horizontal tunnel load, as slower currents are encountered, the angle of the sloping sides with the horizontal may be increased so as to approach ninety degrees in still water. It is therefore obvious that under certain circumstances deviation from the preferred tunnel section without materially affecting some of its advantages may be possible. In a relatively short tunnel in a slow current, two side Ventilating chambers may not be needed. In such a case one triangular shaped Ventilating chamber might normally be placed on the upstream side of the main tunnel chamber, but circumstances might dictate its placement on the downstream side, and even in this position, as in Figs. 14 and 15, the resultant stream-line effect would, by elimination of downstream eddies, minimize the horizontal load on the tunnel.

When the tunnel section of preferred shape is under consideration as a beam resisting horizontal stream thrust, the neutral axis normally will lie, as at 29 or 30, in a longitudinally extending vertical plane inward from the vertical wall dividing the main chamber from one of the triangular side chambers. The side of the tunnel on which it will lie depends on the type of beam selected, and, in the case of continuous beams, depends on the longitudinal distance, from the middle of the span, of the transverse plane under consideration. However, if one triangular chamber is omitted, as in Figs. 14, 15, the vertical neutral axis just described may lie in the said vertical dividing wall, or inward from said wall as shown at 3l, if the triangular chamber extends but a short distance from the main chamber, or outward from said wall, as shown at 32, if the triangular chamber extends a relatively long distance from the main chamber. In the latter case the walls or portions of the walls forming the exterior sides of the triangular chamber may require especial consideration to provide and suitably place sufficient cross-sectional area of concrete to satisfy the compressive stresses induced in the tunnel section by the horizontal thrust.

In tunnel sections, as hereinbefore described, whichact as beams of long span, excessive or unprovided-for deflection under load may seriously impair drainage of the tunnel and provide other undesirable features. As the deflection under load is at least approximately calculable,

the pre-cast tunnel sections may desirably be constructed with cambers or bends opposite in direction to and about off-setting in amount the expected deflection under load so that, when in place on the piers and under load, each tunnel section throughout its entire length would lie as closely as practicable in the true lines vertically and horizontally established as desirable for the tunnel.

Hereinbefore I have described the top and bottom of the tunnel as being supported against the hydrostatic pressure by means of side walls of the main chamber. In some cases it may be advisable to use spaced columns in lieu of said walls.

What I claim is:

l. An internally metal reinforced concrete construction comprising a tunnel section of greater width than height and a pier construction holding said section, said section comprising one chamber of oblong section of greater width than height and a chamber of substantially triangular section monolithic with said first chamber on one of the sides thereof, said tunnel section further comprising substantially vertical reinforced concrete walls and substantially horizontal top and bottom members of reinforced concrete, one of said members comprising longitudinally extending metal reinforcement in the region of tensile stresses induced in said tunnel section by nonhydrostatic vertical load, one of said members comprising metal reinforcement in the region of and adapted to take care of the shearing stresses and diagonal tension induced in said tunnel section by horizontal stream thrust, said vertical Walls comprising metal reinforcement in the region of and adapted to take care of the shearing stresses and diagonal tension induced in said tunnel section by non-hydrostatic vertical load, one of said Walls comprising longitudinally extending metal reinforcement in the region of and adapted to take care of the tensile stresses induced in said tunnel section by horizontal stream thrust, and said top and bottom members comprising laterally extending metal reinforcement in the region of tensile stresses induced by hydrostatic pressure.

2. An internally metal reinforced concrete construction comprising a tunnel section of greater Width than height and a pier construction holding said section, said section comprising one chamber of oblong section of greater Width than height and a chamber of substantially triangular section monolithic with said rst chamber on one of the sides thereof, said tunnel section further comprising substantially vertical reinforced concrete Walls and substantially horizontal top and bottom members of reinforced concrete, one of said members comprising longitudinally extending metal reinforcement in the region of tensile stresses induced in said tunnel section by nonhydrostatic Vertical load, one of said members comprising metal reinforcement in the region of and adapted to take care of the shearing stresses and diagonal tension induced in said tunnel section by horizontal stream thrust, said Vertical walls comprising metal reinforcement in the region of and adapted to take care of the shearing stresses and diagonal tension induced in said tunnel section by non-hydrostatic vertical load, said top and bottom members comprising laterally extending metal reinforcement in the region of and adapted to take care of the tensile stresses induced by hydrostatic pressure, and the exterior Walls of said triangular chamber comprising longitudinally extending metal reinforcement in the region of tensile stresses induced in said tunnel section by horizontal stream thrust.

3. An internally metal reinforced concrete construction comprising a tunnel section of greater width than height and a pier construction holding said section, said section comprising one chamber of oblong section of greater Width than height and a chamber of substantially triangular section monolithic with said first chamber on one of the sides thereof, said tunnel section further comprising substantially Vertical reinforced concrete walls and substantially horizontal top and bottom members of reinforced concrete, one of said members comprising longitudinally extending metal reinforcement in the region of tensile stresses induced in said tunnel section by nonhydrostatic vertical load, one of said members comprising metal reinforcement in the region of and adapted to take care of the shearing stresses and diagonal tension induced in said tunnel section by horizontal stream thrust, said vertical Walls comprising metal reinforcement in the region of and adapted to take care of the shearing stresses and diagonal tension induced in said tunnel section by non-hydrostatic vertical load, one of said Walls comprising longitudinally extending metal reinforcement in the region of and adapted to take care of the tensile stresses induced in said tunnel section by horizontal stream thrust, said top and bottom members comprising laterally extending metal reinforcement in the region of tensile stresses induced by hydrostatic pressure, and those portions of the enclosing Walls of said triangular chamber which lie on the compression side of the Vertical neutral axis of the said tunnel section comprising sulicient crosssectional area of concrete to satisfy the principal compressive stresses induced in said tunnel section by horizontal stream thrust.

4. An internally metal reinforced concrete submerged bridge construction comprising a tunnel section of greater Width than height and a pier construction holding said section, said section comprising a main chamber of oblong section of greater width than height and substantially triangular chambers on the upstream and downstream sides of said tunnel section, said tunnel section further comprising substantially vertical reinforced concrete Walls and substantially horizontal top and bottom members of reinforced concrete, one of said members comprising longitudinally extending metal reinforcement in the region of tensile stresses induced in said tunnel section by non-hydrostatic vertical load, one of said members comprising metal reinforcement in the region of and adapted to take care of the shearing stresses and diagonal tension induced in said tunnel section by horizontal stream thrust, said top and bottom members comprising laterally extending metal reinforcement in the region of and adapted to take care of the tensile stresses induced by hydrostatic pressure, the exterior Walls of one of said triangular chambers comprising longitudinally extending metal reinforcement in the region of tensile stresses induced in said tunnel section by horizontal stream thrust, and one of said triangular chambers comprising in those portions of its enclosing Walls, which lie on the compression side of the Vertical neutral axis of said tunnel section sufficient cross-sectional area of concrete to satisfy the principal compressive stresses induced in said tunnel section by horizontal stream thrust.

ARTHUR J. WIDMER.