Patent Publication Number: US-7717642-B2

Title: Buoyancy stabilized pier

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims the benefit of U.S. Provisional Application No. 60/732,268, filed Nov. 1, 2005, the disclosure of which is hereby expressly incorporated by reference in its entirety, and priority from the filing date of which is hereby claimed under 35 U.S.C. §119. 

   BACKGROUND 
   The present invention is in the field of bridge construction and, more particularly, to support structures for over-water bridges. The engineering challenges involved in designing and constructing large bridge structures, such as suspension bridges and cable stayed bridges are legion. The bridge structure must have sufficient strength to support itself, the design live loads such as traffic, while also withstanding environmental loads including, for example, wind and other dynamic fluid loads, potential seismic loads, and the like. Typically the bridge structure will be designed to provide both the requisite rigidity to react certain design loads, and a certain amount of flexibility to endure other design loads without catastrophic failure. Moreover, the bridge structure is generally intended to be a permanent structure, and therefore must be designed to maintain its strength and stability over time. 
   Of course, bridges are often built over bodies of water, and rely on support structures that extend into the body of water, and to and into the bed beneath the body of water. Such supports, which may comprise caissons and piers, for example, that extend generally from the bed, out of the water to the bridge deck. 
   Designing suitable support structures for use in estuarial bodies of water having relatively poorly defined sedimentary layers that include significant quantities of fine particles can be very difficult. The support structure generally must provide a very stable support that will transmit very large reaction forces to the ground, while also being flexible enough to withstand loads relating to large episodic events such seismic events, but must do so in a sedimentary environment that is not conducive to reacting such loads. 
   SUMMARY 
   This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
   In an embodiment of the present invention a buoyancy-stabilized pier for supporting a bridge structure over a waterway is disclosed. The pier includes one or more buoyant chambers, such that the pier produces a large, upwardly-directed force. The pier is sized such that the buoyancy force is at least eighty percent of the bridge dead weight that is intended to be supported by the pier. The pier includes a footing portion that is embedded into the waterway bed. However, due to the buoyancy force, the waterway bed does not need to react all of the loads associated with the dead weight of the bridge. 
   A bumper assembly may be provided surrounding the pier, the bumper assembly being attached to the pier with a plurality of extendible, shock absorbing members. 

   
     DESCRIPTION OF THE DRAWINGS 
     The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
       FIG. 1  is a perspective view of a cable stayed bridge in accordance with the present invention, wherein caisson piers are shown in cross-section; 
       FIG. 2  is a side view of a portion of a cable stayed bridge according to the present invention; 
       FIG. 3  is a front view of the cable stayed bridge shown in  FIG. 1 ; 
       FIG. 4  is a second embodiment of a cable stayed bridge in accordance with the present invention; and 
       FIG. 5  is a cross sectional view of the pier for the cable stayed bridge shown in  FIG. 4 . 
   

   DETAILED DESCRIPTION 
   A perspective sketch of a cable stayed bridge  100  according to the present invention is shown in  FIG. 1 , extending over a waterway  92 , in this particular sketch, and for exemplary purposes only, the estuarial waters of Elliott Bay, in the Puget Sound. The cable stayed bridge  100  includes a bridge deck  110  that is disposed over the waterway  92 , for at least a portion of its length, some distance above the waterline  93 . For example, the bridge deck  110  may be positioned high enough over the waterline  93  to permit nautical traffic to pass therebelow. The bridge deck  110  is supported by one or more tower structures or pylons  102 , each tower structure  102  extending upwardly from one of the piers  104 . The pier  104  extends into the water, and includes a footing portion  106  that is substantially embedded in the sediment of the waterway bed  91 . 
   Two piers  104  are shown in  FIG. 1 , although more or fewer piers may be utilized for a particular bridge. The piers  104  in this embodiment have an open, generally caisson-type construction, that is open at the bottom. The bridge deck  110  is suspended by a plurality of cable stays  108  that suspend the bridge deck  110  from the tower structures  102 . In the embodiment shown in  FIG. 1 , a bumper assembly  130  is provided around each of the piers  104 . The bumper assembly  130  includes a generally annular, buoyant platform  132  that is attached to the pier  104  with a plurality of extendible connecting members  134 . The connecting members preferably operate as shock absorbers to protect the pier from any impact loads, for example resulting from inadvertent collisions by watercraft into the bumper assembly  130 . 
   It will be appreciated that the bumper assembly  130  will not only protect the piers  104  from potential damage from watercraft, floating debris and the like passing under the bridge  100 , but may also provide a convenient platform for various activities, for example performing routine bridge inspections, docking, or for conducting other activities not directly related to the bridge  100 , such as estuary monitoring programs, recreational activities, or the like. The platform  132  may conveniently be generally polygonal in plan form, for example octagonal, to accommodate such other uses, including, for example, to facilitate watercraft docking. 
   In this embodiment the piers  104  are essentially caissons that extend from the waterway bed  91  up approximately to the water line  93 . A substantial portion of the structure of each pier  104  comprises one or more hollow chambers  103  that are filled with air, or alternatively are at least partially filled with a lightweight polymeric foam or the like, thereby providing an upward buoyancy force, such that the waterway bed  91  does not react all of the forces related to the bridge  100  structure. A watertight lining may be provided covering the walls of the chamber  103 , and/or a polymeric foam or a bladder (not shown) may be provided in the chamber  103 , to prevent undesired water incursion into the pier  104 . 
   An optional anchor or piling  107  extends from each pier  104 , preferably to engage bedrock  94  or other materials more stable and compacted than the material comprising the waterway bed  91 . 
   A side view of the bridge  100  is shown in  FIG. 2 , and a fragmentary front view of the bridge  100  is shown in  FIG. 3 , with the bumper structure  130  removed, and the bridge deck  110  simplified, for clarity in illustrating novel aspects of the piers  104 . The piers  104  in this embodiment are generally hollow cylinders, that may be formed for example of reinforced concrete, each with a footing portion  106  embedded in the waterway bed  91 , and the upper portion extending generally to, or beyond, the waterline  93 . The footing portion  106  is preferably filled at least in part with local sedimentary materials. The net buoyancy force generated by the pier  104 , of course, is approximately equal to the weight of the water displaced by the pier  104 , less the weight of the pier  104  itself, and is directed vertically upwards. 
   As an example, if we assume that the open chamber  103  is a right circular cylinder having a diameter D equal to about 200 feet and an air column height H equal to about 60 feet then the relevant buoyancy air volume of almost two million cubic feet. Therefore, by Archimedes principle, and assuming a sea water density of about 64 lbm/ft^3, we can easily generate a gross buoyancy force of about 60,000 tons. 
   The foot portion  103  of the pier  104  that is embedded in the waterway bed  91  is preferably filled with displaced sediment, and therefore does not contribute substantially to the buoyancy force. 
   The weight of the superstructure of the bridge  100 , therefore, may be substantially offset by the buoyancy force on the piers  104 , such that the waterway bed  91  does not react all of these very large forces. In a preferred embodiment of the invention, the resulting buoyancy force is selected (by appropriate choice of dimensions of the pier  104 ) to be approximately equal to the portion of the bridge  100  weight that the pier  104  is designed to support. For example, the net buoyancy force may be designed to be at least eighty or ninety percent of the design supported bridge weight. It is contemplated that water may be pumped into, and/or out of, the chamber  103 , to achieve the desired buoyancy force, and optionally that the buoyancy may be actively controlled by such pumping. 
   As discussed above, the pier  104 , is anchored in place by being partially embedded in the waterway bed  91 , and may be further anchored through an optional piling  107  extending down into firmer strata, such as bedrock. 
   The bridge  100  utilizes a novel pier  104  structure that provides significant advantages over conventional bridge pier structures. In the embodiment shown in  FIGS. 1-3 , the piers  104  are generally cylindrical, shown illustratively and not by way of limitation, as a right circular cylinder. In particular, the piers  104  are substantially larger in volume than conventional pier structures. The larger overall size of the pier  104  permits the pier  104  to accommodate the watertight chamber(s)  103 . 
   It is contemplated that the piers  104  may be readily installed as follows: First the pier  104  structures may be prefabricated. The piers  104 , with sufficient buoyancy to float, may then be towed into position at the desired emplacement. Pumps, valves, or other means for transferring water into the pier  104  may then be used until the pier  104  has a net negative buoyancy. The pier  104  is then guided to the desired position in the waterway bed  91 . Hydraulic excavation methods, which are well known in the art, may be utilized to embed the piers  104  in the waterway bed  91 , with local sediment occupying a portion of the interior volume of the piers  104 . It is contemplated, for example, that a pump may be incorporated into the footing portion  106 , that is adapted for hydraulically moving sediment located directly below the footing portion  106 . Ports (not shown) may be provided near the upper end of the footing portion  106 , to allow some of the sediment to be expelled from the footing portion  106 . Once the piers are in place, and after and/or during construction of the bridge superstructure, and in particular those portions of the bridge superstructure that are supported by each pier  104 , water is pumped out of the pier  104 , for example by pumping air, other gas, or a lightweight material such as a polymeric foam, into the chamber(s)  103 , such that the pier  104  produces a net upward buoyancy force that substantially offsets the weight of the bridge superstructure and the weight of the pier  104 , but not enough buoyancy to offset the weight of the ballast of sediment that is in the footing portion  106  of the pier  104 . 
   It is noted that many estuaries wherein the present invention may be most suitable have waterway beds  91  comprising relatively fine sediment and similar small-particle matter, such that the piers  104  may be readily installed using conventional hydraulic excavation. It is clearly contemplated that a preliminary bed preparation step may be utilized to clear larger objects away from the emplacement cite, if necessary. When a piling  107  is desired, the piling  107 , for example a cylindrical metal shaft, may be first driven into the bedrock  94  using conventional piling installation methods, and the pier  104  provided with an aperture for receiving the piling is lowered to slidably engage the piling  107 . The pier  104  may be fixedly attached to the piling  107  after installation, if desired. 
   It will be appreciated that the buoyancy-stabilized piers  104  desirably provide a constant righting force on the piers  104 . As suggested above, bridge  100  structure, including the piers  104  may be designed such that there is virtually no dead load on the soil due to the weight of the bridge. 
   A second embodiment of a cable stayed bridge  150  including a buoyant pier  154  according to the present invention is shown in  FIG. 4 . The bridge  150  includes a tower structure  152 , cable stays  158  and deck  160  that are functionally similar to the bridge  100  described above. In this embodiment the pier  154  includes a relatively large-diameter footing portion  156 , a relatively small-diameter middle section  164 , and a relatively large-diameter upper portion  166 . The footing portion  156 , middle section  164  and upper portion  166  forming fixedly connected to form the pier  154 . 
   The footing portion  156  is fully or substantially embedded in the waterway bed  91 , and may include an optional piling  167  (not visible in  FIG. 4 ) that extends further into the waterway bed  91 , and generally anchors the pier  154  in place. The middle section  164  may be a solid section, or as shown in the cross-sectional view of  FIG. 5 , may be of tubular construction, preferably including apertures or ports  162 , such that the middle section  164  will be substantially filled with water during use. The upper portion  166  comprises a hollow platform, that may conveniently be, for example, octagonal in plan form, as shown in  FIG. 4 , or may have any suitable alternative shape. It is believed advantageous to have one or more sides that are relatively straight, to facilitate docking watercraft thereto. The upper portion  166  may further include a peripheral bumper structure (not shown) such as the bumper structure  130  described above. 
   As seen most clearly in the pier  154  cross-sectional view shown in  FIG. 5 , the upper structure includes one or more chambers  168 ,  170 . The chambers  168 ,  170  are substantially enclosed, although ports may be provided, for example to permit inspection or to permit metering in a desired amount of water to achieve a desired buoyancy. The upper portion  166  is sized to provide an upward buoyancy force that substantially reacts the weight loads applied to the pier  154  by the bridge  150  structure. Similar to the pier  104  of the first embodiment, the second embodiment of the pier  154  according to the present invention, therefore, allows the bridge  150  to be designed such that only a small portion of the dead weight of the bridge is transferred to the waterway bed  91 . In this second embodiment, however, it should be appreciated, that the righting moment on the pier  154  is more positively active because the buoyancy forces are developed at the top of the pier  154 . 
   The enlarged footing portion  156  shown in  FIGS. 4 and 5  will help to distribute the loads (static and dynamic) that are transferred to the waterway bed  91 , and may be most suitable when no piling  167  is to be used. Alternatively, the footing may be formed as generally the same diameter section as the middle section  164 , similar to the first embodiment of the pier  104 . The enlarged footing portion  156  embedded in the waterway will resist any overturning forces because to move or overturn the footing would require the movement of a large amount of soil under the footing and/or the creation of a vacuum under the footing, which is impossible, because there are no other forces available to do the job. 
   It is contemplated that the pier  154  primary structures may be formed of any suitable material, and in the current embodiments are preferably formed primarily from reinforced concrete. The chambers  168 ,  170  may be lined with a suitably watertight material, for example with a tar, polymeric liner or the like. Alternatively, the chambers ( 103  in pier  104 , and  168 ,  170  in pier  154 ) may be filled with a stable polymeric foam or similar material suitable for the proposed environment, such that the buoyancy of the pier  104 ,  154  is assured even if cracks occur or the piers  104 ,  154  are otherwise damaged. 
   It will now be appreciated that by employing the teachings of the present invention, the weight of a relatively massive structure such as a cable stayed bridge, may be very substantially balanced by buoyancy forces resulting from the pier structure. Therefore, the vertical soil loading at the footing portion may be minimized or substantially eliminated. Moreover, the pier  104 , 154  will naturally tend towards the upright position, with the substantial portion of the weight of the pier in the footing portion and the net buoyancy force coming from the upper volume. Moreover, it is also believed that the embedded footing portion  106 , 156  will be substantially homogeneous with the surrounding soil because it is filled with the local sediment, so the tendency for external forces such as tidal currents and earthquakes to move the piers relative to the surrounding soils is minimal. 
   While illustrative embodiments have been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.