Patent Publication Number: US-2012023845-A1

Title: Base Mat Assembly And Method For Constructing The Same

Description:
TECHNICAL FIELD 
     The present invention relates generally to LNG (liquefied natural gas) tanks and methods for constructing LNG tanks, and more particularly, to base mat foundations for containment tanks which are used to support LNG tanks. 
     BACKGROUND 
     LNG full containment tanks typically comprise an inner primary container (hereinafter “inner tank”) for storing LNG and an outer container (hereinafter “outer tank”) which is designed to contain leaks from the inner tank. Large storage tanks, such as those used with LNG full containment tanks, are typically sensitive to differential settlement in the foundations supporting the inner tank which stores LNG. Consequently, standard practice is to build a rigid foundation with a flat top surface supporting the bottom of the inner tank. For example, the foundation can be a grade supported reinforced concrete mat, a deep foundation with a mat resting on the ground, or an elevated slab foundation. In cases where very stiff soil or soft rock is present at a construction site, a grade supported mat is the preferred type of foundation. The conventional design for the foundation mats utilizes cast in place reinforced concrete. 
     Construction using an assembly of precast concrete elements is a known approach when construction time at a site needs to be minimized and when site conditions and locations are problematic for casting in place structures. Assemblies made of precast elements are often used for temporary and, in some cases, for permanent pavement. 
     However, application of precast elements in LNG tank foundations is not a conventional approach. Such foundations would be subject to high forces at connections which makes design and construction of such connections very difficult. Only small allowances can be made for vertical differential displacements across a base mat when an inner primary container or inner LNG tank is to be supported upon an assembly of precast elements forming the base mat. Grade supported mats provide special support to a large tank bottom so as to minimize differential settlements. If a base mat is to be constructed from precast concrete elements, the forces that will occur due to differential settlement or voids in the underlying soil will tend to separate and displace the base mat elements from one another. 
     It is challenging to provide sufficient vertical stiffness to precast base mat elements forming a base mat while keeping the thickness of the base mats within an economically acceptable range. 
     SUMMARY 
     A precast reinforced concrete base mat element, a base mat assembly or foundation and method for constructing a base mat assembly from a plurality of the base mat elements are described. The base mat element comprises a liner plate and concrete body having first and second tensile conduits extending there through. Preferably the tensile conduits extend in first and second generally perpendicular directions. The liner plate is anchored or cast into the concrete body. Ideally the base mat elements have interlocking surfaces formed thereon. 
     A plurality of the base mat elements may be juxtaposed together. The tensile members extend through the first and second tensile conduits and are anchored and clamped about the base mat elements utilizing anchors with a planar membrane surface being formed by the liner plates. The liner plates may be welded together to form a membrane which is generally flat and which utilizes the liner plates as a vapor barrier. Ideally, if interlocking surfaces are provided on the base mat elements, the interlocking surfaces can interlock with one another to limit displacement of the base mat elements relative to one another. Preferably, the base mat elements interlock in both horizontal and vertical directions. In one embodiment, the first and second tensile conduits are located above and below the horizontal center plane of the concrete body so that when tensile members are passed through and post-tensioned and anchored about the base mat elements using anchors, the clamped base mat elements resist relative displacement between one another due to the clamping force provided by the post-tensioned tensile members and the additional stiffness provided by the membrane. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objects, features and advantages of the present invention will become better understood with regard to the following description, appended claims and accompanying drawings where: 
         FIG. 1A  is a perspective view of an inner tank being constructed within an outer containment tank as the roof of the outer tank is being constructed; 
         FIG. 1B  is a perspective view of a completed outer tank, in partial cutaway, showing a base mat assembly, a ring beam, a shell or tank wall and a roof mounted atop the shell as well as the inner tank; 
         FIG. 2  is a plan view of the base mat assembly or foundation and a ring beam or ring beam foundation which are to be joined by a cast in place concrete connection (not shown); 
         FIG. 3A  is a plan view of a pair of base mat elements juxtaposed and interconnecting with one another; 
         FIG. 3B  is a perspective view of a liner plate and attached J-hooks which are to be cast within a concrete body when making a reinforced concrete base mat element; 
         FIG. 3C  is a perspective view of a base mat element, including concrete body, showing a pair of tensile members passing through tensile conduits disposed within the concrete body; 
         FIG. 3D  shows a plurality of base mat elements juxtaposed in lateral and longitudinal directions with tensile members and cooperating anchors clamping the base mat elements together in the lateral and longitudinal directions; 
         FIG. 4  shows an exemplary pile with L-shaped brackets mounted thereon which can be used to support base mat elements or ring beam elements at predetermined heights so that the base mat elements or the ring beam elements can be readily welded together; 
         FIG. 5  shows a matrix of piles which can be used to support base mat elements which can then be welded together to form the base mat assembly; 
         FIG. 6A  is a plan view of a pair of cooperating juxtaposed ring beam elements; 
         FIG. 6B  is a perspective view of a liner plate welded to a thicker base plate and attached J-hooks welded beneath the plates which are used in casting a reinforced concrete ring beam element; 
         FIG. 6C  is a perspective view of a reinforced concrete ring beam element including the liner plate and base plate anchored to a concrete body with tensile members extending through tensile conduits embedded in the concrete body; 
         FIG. 7  is a fragmentary sectional view of a cast concrete joint joining the base mat assembly and the ring beam with a shell element being welded to the ring beam; 
         FIG. 8A  is a perspective view of a shell element; 
         FIG. 8B  is a perspective view of a pair of shell elements which are welded together to form part of the outer shell with the shell elements also being welded to base plates of ring beam elements; 
         FIG. 8C  shows a sectional view of a second embodiment of a shell made with two courses of lower and upper shell elements rather than using a single course as seen in the shell of  FIG. 8B ; 
         FIG. 8D  shows a bottom view of the upper shell element which includes a base plate supporting two downwardly depending alignment pipes; 
         FIG. 8E  shows a plan view of a top end cap plate of a lower shell element having two downwardly depending alignment sockets designed to receive the alignment pipes of the upper shell element; 
         FIG. 9A  is a roof frame formed by radial and circumferentially extending wide flange beams; 
         FIG. 9B  illustrates a typical welded connection between a tubular mounting beam of a roof assembly and a pair of roof elements; 
         FIG. 9C  shows the roof assembly and attached seal being raised by air pressure within the outer shell so that the roof assembly can be welded to temporary gusset plates mounted to the top of the shell; 
         FIG. 9D  is a fragmentary perspective view of the roof assembly and peripheral seal being raised to be temporarily welded to a gusset plate; 
         FIG. 9E  is a fragmentary perspective view of a roof membrane of the roof assembly being temporarily welded to a gusset plate; 
         FIG. 9F  is a fragmentary perspective view of the roof assembly permanently welded to the shell with the temporary gusset plates and seal having been removed and with a roof element attached to the roof assembly and shell; 
         FIG. 10A  is a perspective view of a roof element frame used in casting a typical roof element; and 
         FIG. 10B  is a perspective view of an exemplary roof element which is to be mounted to the roof assembly during construction of the roof. 
     
    
    
     DETAILED DESCRIPTION 
     I. Overview 
     In a first embodiment, a full containment LNG tank  18  is shown in  FIGS. 1 and 2 . The LNG tank  18  comprises an outer secondary container, hereinafter “outer tank  20 ” surrounding an inner primary container, hereinafter “inner tank  22 ”. Inner tank  22  is intended to store LNG. Outer tank  20  is preferably a full containment tank in which both LNG and gas vapor are fully contained in the event of a leak from the inner tank  22 . 
     In this particular embodiment, outer tank  20  is primarily made of precast reinforced concrete elements which reduce fire risk relative to using a primarily steel outer container or tank. The construction of outer tank  20  involves assembling precast reinforced concrete elements for a foundation  24 , including a base mat assembly  100  and an outer ring beam  200 , an outer wall or shell  300  and a roof  400 . As an example of order of magnitude, outer tank  20  in this embodiment has a diameter of approximately 80 meters, an overall height (to the top of the dome of roof  400 ) of about 50 meters, and a height to the top of shell  300  of about 40 meters. Those skilled in the art of LNG facilities construction will appreciate that much larger or smaller tanks can be built and still use the design considerations described herein. However, tanks having diameters of at least 25 meters are particularly well suited to the construction methods described herein which utilize precast reinforced concrete elements. These elements ideally can be welded together to form structural welds thereby minimizing the number of cast in place concrete structural joints which have to be made to form an outer containment tank. 
     Ideally, inner tank  22  can be assembled at the same time as roof  400  is being built to save time and money in the overall construction of inner tank  22  and outer tank  20 . Note in  FIG. 1  roof elements  402  are being added ring by ring to a roof assembly  404  while inner tank  22  is being constructed. A construction access or shell opening  350  in shell  300  allows for materials to be carried into outer tank  20  so that inner tank  22  can be built as roof  400  is being constructed. After inner tank  22  is completed, special half height shell elements  302 ′ can be installed to close opening  350 . An annular gap of approximately 1.5 meters exists between inner tank  20  and shell  300  to permit circumferential access to build inner tank  22  and to accommodate insulating materials (not shown) which are to be installed between outer and inner tanks  20  and  22 . 
     Inner tank  22  can be built in any of a number of ways which may require welding processes to connect individual plates. For example, the plates can be joined using shielded metal arc welding (SMAW) and submerged arc welding (SAW) for 9% nickel tanks of the size described above, i.e. having a diameter greater than 25 meters. In this particular example, inner tank  22  can be friction stir welded (F SW) by plate at a time erection method. A course of plates at a time can be welded with the bottom (thickest) first and then subsequent (thinner) courses welded there above. Scaffolding (not shown) can be used as each course is added. Alternatively, inner tank  22  can be constructed course down either by supporting the tank from the roof to allow insertion of the next course or by supporting on jack stands to allow insertion of the next course. For the last two methods, the scaffolding need be erected only one time. By way of example and not limitation, another alternative method is to use coil material shaped to the curvature required for the tank. Coil tank building occurs from the top course down either using jacking or by roof supported. Expanded pearlite insulation is typically used in the annulus formed between inner tank  22  and outer tank  22 . Although not shown, an under bottom insulating layer, which can be made of foam glass, is applied on the top of base mat assembly  100  with inner tank  22  prior to inner tank  22  being placed there on. 
     Foundation  24 , i.e., base mat assembly  100  and ring beam  200 , is designed to support inner tank  22  and outer shell  300 . Outer shell  300  rests upon and transfers loads directly to outer ring beam  200  preferably through welded connections. A particularly significant load is the torsional load applied to ring beam  200  from outer shell  300 . Base mat assembly  100  is secured to and transfers loads to outer ring beam  200  as well through a cast in place joint  270 , which is best seen in  FIG. 7 . Depending on soil conditions, base mat assembly  100  and/or outer ring beam  200  can be supported upon compacted soil or else on piles, as will be described below. Roof  400  of tank  20  includes a roof assembly  404  comprising a skeletal steel frame  406  ( FIG. 9A ), a membrane  412  ( FIG. 9B ) of liner plates  410  and radially and circumferentially extending tubular mounting beams  414 . Roof  400  is supported atop outer shell  300 . Roof assembly  404  supports and is strengthened by a number of concentric rings or courses of precast reinforced concrete roof elements  402  and  402 ′.  FIG. 2  shows a completed outer tank  22  with shell opening  350  being sealed and closed by special half shell elements  302 ′ which are permanently welded to the remainder of shell  300 . 
     II. Base Mat Assembly  100   
       FIG. 2  shows base mat assembly  100  disposed within outer ring beam  200 . A cast in place concrete connection  270  (not shown) is to be formed there between, as best illustrated in  FIG. 7 . In this particular exemplary embodiment, a large number of generally rectangular base mat elements  102  are connected together to form base mat assembly  100  having a contiguous top membrane  140  made from welded together steel liner plates  104 . Membrane  140  ideally cooperates with similar membranes formed from liner plates on connection  270 , outer ring beam  200 , shell  300  and roof  400  to form an overall air tight vapor barrier within outer tank  20 . As will be described later in greater detail, base mat assembly  100  and outer ring beam  200  preferably have discrete reinforced concrete elements that are clamped together by post-tensioned tensile members  150 ,  250 , i.e., steel cables, and anchors  152 ,  252 . Precast elements  102 ,  202  are then welded to together to form, respectively, base mat assembly  100  and outer ring beam  200 . 
     A. Base Mat Elements  102   
       FIG. 3A  shows a pair of juxtaposed base mat elements  102 .  FIG. 3B  shows a generally rectangular liner plate  104  having a series of projections  106  and recesses  110 . Liner plate  104  is preferably made of carbon steel although it can instead be made of other appropriate metals. The thickness is of liner plate  104  in this exemplary embodiment is about 10 mm. As an example, in this embodiment the size of liner plate  104  and base mat elements  102  are approximately 2 meters by 3 meters. A plurality of J-hooks  112  is welded to the lower surface of liner plate  104 . 
       FIG. 3C  depicts a base mat element  102  having a concrete body  114  cast about J-hooks  112  to anchor liner plate  104  to concrete body  144 . Although not shown to simplify the drawing, formed in the concrete body  114  are two or more layers of reinforcing bars or welded wire fabric running in two generally perpendicular directions. For example, the reinforcing bars can be ASTM A 615 deformed carbon-steel bars for concrete reinforcement. Also formed within concrete body  114  is a plurality of laterally extending lower and upper tensile conduits  120 ,  122  and longitudinally extending lower and upper tensile conduits  124 ,  126  which extend in generally perpendicular directions to one another. These tensile conduits  120 ,  122 ,  124  and  126  can be HDPE (high density polyethylene) tubing or steel piping or other appropriate structural conduits. 
     Concrete ribs  130  and grooves  132  are also formed on the longitudinally and laterally extending edge surfaces of concrete body  114 . Locating projections  106  and ribs  130  and receiving recesses  110  and grooves  132  allow a plurality of base mat elements  102  to be placed in interlocking juxtaposition as suggested in  FIG. 2 ,  FIG. 3A  and  FIG. 3D  when base mat assembly  100  is being constructed. Note that concrete body  114  has downwardly and upwardly opening steps  134  and  136  so mating base mat elements  102  can interlock in the vertical direction as well as in the lateral and longitudinal directions. 
     The rows of tensile conduits are disposed below and above the central horizontal plane  144  of concrete body  114 . These laterally and longitudinally extending tensile conduits are designed to receive tensile members  150  there through which allow multiple base mat elements  102  to be clamped together utilizing anchors  152  ( FIG. 3D ) upon the post-tensioning of the tensile members  150 . Only some of the tensile members  150  are shown in  FIG. 3D . 
     B. Construction of Base Mat Assembly  100   
     Base mat assembly  100  is constructed by juxtaposing base mat elements  102  in both first and second generally perpendicular directions as seen in  FIG. 2 . Locating projections  106  and concrete ribs  130  cooperate with receiving recesses  110  and grooves  132  on adjacent elements  102  to align base mat elements  102  in first and second generally perpendicular directions. Steps  134 ,  136  allow base mat elements  102  to interlock in the vertical direction as well. 
     Tensile members  150  are fed through tensile conduits  120 ,  122 ,  124  and  126  as base mat elements  102  are being juxtaposed to one another. After the mating surfaces on the base mat elements  102  are properly located relatively to one another, tensile members  150 , i.e. cables, are post-tensioned and anchored by anchors  152  to clamp base mat elements  102  together. In this particular example, tensioning can be accomplished such as by using a Williams Strand Anchor System available from Williams Form Engineering of Belmont, Mich., USA. Those skilled in the art will appreciate that other tensioning systems can also be used to post-tension and anchor tensile members  150 . After the tensioning is complete, anchors  152  are locked in place on tensile members  150  to maintain the tension in tensile members  150  with anchors  152  bearing upon base mat elements  102 . 
     Liner plates  104  of each of base mat elements  102  are then welded together to form a part of a generally contiguous membrane  140  on the top of base mat assembly  100 . As tensile conduits  120 ,  122 ,  124  and  126  are located above and below the central horizontal plane  144  of concrete body  114 , the top and bottom surfaces of base mat elements  102  are held together and do not separate due to the post tensioning of tensile members  150  with anchors  152  clamping about base mat elements  102 , preferably even under construction and operating loads applied to base mat assembly  100 . 
     C. Mounting of Base Mat Elements on Piles 
     Base mat elements  102  can be assembled on a graded, level surface if the underlying surface or soil is sufficiently stiff. However, if the soil does not provide adequate support, base mat elements  102  can be mounted on piles  160 . The upper portion of a typical pile  160  is seen in  FIG. 4 . Pile  160  includes a post  162  which is driven into the ground such as in soft soil  163 . Four L-shaped brackets  164  can be bolted or welded to post  162  at an appropriate location with horizontally extending flanges  166  being at a predetermined height. Ideally, a laser positioning system is used to maintain all of the flanges  166  within a predetermined tolerance of height. Mounting base mat elements  102  on flanges  166  will then provide for easy welding of liner plates  104  together as the liner plates  104  should all be at about the same height. Reinforcing bars  170  extend upwardly from post  162  to facilitate later cast in place connections between piles  160  and base mat elements  102 . 
       FIG. 5  depicts a couple of base mat elements  102  being mounted upon flanges  166  of a matrix of piles  160 . Note in this case of utilizing piles  160  for support, base mat elements  102  are notched in their corners and along their long sides to mate with posts  162 , as seen in  FIG. 5 . 
     The corners and long sides of base mat elements  102  will be supported by six cooperating supporting flanges  166 . Voids  174  are created by the intersection of the corners and along the sides of cooperating base mat elements  102 . In the case where piles are used, base mat elements  102  can include reinforcing bar loops (not shown) which are embedded in concrete body  114  and extend laterally and longitudinally from concrete body  114 . The loops will loop over vertically extending reinforcing bars  170 . Concrete is cast in these voids  174  about the loops and reinforcing bars  170 . Fitted liner plate  176  is anchored by J-hooks in the cast concrete so that liner plates  104  and  176  can be welded together to form generally continuous top membrane  140  on base mat assembly  100 . 
     III. Outer Ring Beam  200   
       FIG. 2  shows a plan view of the foundation  24  including base mat assembly  100  and outer ring beam  200 . The numbers of base mat and ring beam elements  102 ,  202  needed to construct these components will vary depending on the size of the desired outer tank  20  and the selected size of the various ring beam elements  202  and base mat elements  102 . Various methods can be used to join base mat assembly  100  to outer ring beam  200 . Preferably, ring beam elements  202  are clampingly held in a circumferential abutment by post-tensioning tensile members  250  and clamping anchors  252  about the ring beam elements  202 . Then base plates  206  anchored within each of ring beam elements  202  are welded together to form an annular ring of such base plates  206 . Similarly, inner liner plates  204  are also welded to one another to increase strength and stiffness and to form a membrane  244  of liner plates  204 . Post-tensioned tensile members  250  and anchors  252  assist outer ring beam  200  in resisting bending, shear and torsional forces applied to ring beam  200  by outer shell  300  and by base mat assembly  100  to maintain ring beam elements  202  in the proper abutting relationship with respect to one another. 
     A. Ring Beam Element  202   
       FIGS. 6A-C  illustrate an exemplary embodiment of a ring beam element  202 . In plan view,  FIG. 6A  shows a pair of ring beam elements  202  interlocking with one another when placed in side by side position.  FIG. 6B  shows an inner liner plate  204  secured to a thick base plate  206 . Liner plate  204  and base plate  206  are preferably made from carbon steel. Liner plate  204  is approximately 10 mm thick and base plate  206  is about 50 mm thick in this example. J-hooks  212  are welded to the bottom sides of liner plate  204  and base plate  206 . Projection  214  and recess  216  are formed on the edges of liner plate  204 .  FIG. 6C  illustrates that ring beam element  202  includes a concrete body  220  in which liner plate  204 , base plate  206  and J-hooks  212  are embedded. An outer radial surface  222  and an inner radial surface  224  define what will be part of the radially outer and inner surfaces of ring beam  200 . A plurality of locating concrete ribs  226  and receiving recesses  230  allow juxtaposed ring beam elements  202  to be cooperatively interlocked with one another, as seen in  FIG. 2  and  FIG. 6A , to form outer ring beam  200 . 
     Also formed within concrete body  220  are circumferentially extending lower and upper tensile conduits  232 ,  234 . In this particular embodiment, there are four such tensile conduits  232 ,  234 . Two of these tensile conduits  232  are generally arranged below and two conduits  234  are arranged above the horizontal center plane  236  of ring beam element  202 . Also, two of the tension conduits  232 ,  234  are arranged beneath base plate  206  and two are located radially inwardly beneath liner plate  204  closer to the inner radial surface  224 . These cooperating locations of tensile conduits  232 ,  234  allow tensioning members  250 , i.e., cables, to pass through each of ring beam elements  202  and assist in counter balancing the bending, shearing and torsional loads applied to base plate  206  by outer shell  300 . Also, tensile members  250  and anchors  252  cooperate to clamp about ring beam elements  202  to prevent the abutting ring beam elements  202  from displacing with respect to one another. In this particular example, high density polyethylene (HDPE) tubing is used to form the tensile conduits  232 ,  234  in concrete body  220 . Of course, the tensile conduits could be made of other suitable materials such as steel or other structurally strong materials. Downwardly and upwardly opening steps  238  and  240  allow elements  202  to be vertically interlocking as well. Also, located within concrete body  220  is a plurality of reinforcing bars (not shown) which are conventional for adding tensile strength to cast concrete bodies. Extending radially inwardly are reinforcing bars or J-hooks  242  which are later to be included in cast concrete connection  270  which connects base mat assembly  100  with ring beam  200 . 
       FIG. 6C  illustrates a ring beam element  202  having four circumferentially extending tensile conduits  232  and  234  extending there through. This embodiment may be appropriate for cases where outer ring beam  200  is supported by stiff underlying soil or surface. In the event that a softer underlying soil is available at a site, it may be preferably to include an additional pair of tensile conduits located radially outboard of the locations where the tensile conduits  232 ,  234  are positioned in  FIG. 6C . This allows additional clamping force to be applied across the ring beam elements  202 .  FIG. 7  shows a ring beam element  202  with three pairs of vertical spaced apart tensile conduits  232  and  234 . Also, as discussed above with respect to base mat elements  102 , ring beam elements  202  can also be mounted on leveled flanges of piles to insure proper support and vertical alignment between ring beam elements  202  which are to have liner plates  204  and base plates  206  welded together. 
     B. Construction of Outer Ring Beam  200   
     Ring beam elements  202  are juxtaposed with respect to one another with locating concrete ribs  226  of one ring beam element being held within locating recess  230  of the adjacent ring beam element. Similarly, cooperating steps  238  and  240  assist in vertical alignment between ring beam elements  202 . Tensile members  250  are placed through tensile conduits  232  and  234  as the ring beam elements are being positioned adjacent one another. As seen in  FIGS. 1A and 1B , at four locations along the circumference of outer ring beam  200 , there are pairs of special abutting ring beam elements  202 ′ through which the ends of tensile members  250  extend. Tensile members  250  are tensioned utilizing tensioning devices  260  and anchors  252  are anchored about ring beam elements  202 . In this particular example, tensioning can be accomplished such as by using the aforementioned Williams Strand Anchor System. Those skilled in the art will appreciate that other tensioning systems can also be used to post-tension and anchor tensile members  250 . 
     After the individual ring beam elements  202  are aligned and clamped together using tensile members  250  and anchors  252 , base plates  206  are welded together along their radially extending abutting edges to form an annular ring which strengthens outer ring beam  200 . Finally, liner plates  204  on ring beam elements  202  are also welded together along their radially extending edges to form a continuous membrane  244  on the radial inner side of outer ring beam  200 , as best seen in  FIG. 8B . 
     C. Connection Between Base Mat Assembly  100  and Ring Beam  200   
       FIG. 7  illustrates an example of how a base mat assembly  100  can be connected to the outer ring beam  200 . Concrete is cast in place about reinforcing bars  142 ,  242  extending outwardly from adjacent base mat elements  102  and radially inwardly from ring beam elements  202  to form a cast in place joint  270  between the base mat assembly  100  and outer ring beam  200 . Liner plates  272  are cut to the necessary size and shape to mate between liner plates  104  of base mat assembly  100  and inner liner plates  204  of outer ring beam  100 . J-hooks  274  are welded to the bottom side of liner plates  272 . Concrete is cast in place between base mat assembly  100  and base ring  200  to create a concrete body  276  anchoring liner plates  272  and J-hooks  274 ,  142  and  242  in the cast concrete to form joint  270 . Although not shown, appropriate reinforcing bars will also be positioned in the space to be occupied by joint  270  prior to the concrete being cast to enhance tensile strength of joint  270 . 
     IV. Outer Shell  300   
       FIG. 8A  shows a perspective view of a typical shell element  302 .  FIG. 8B  depicts a pair of shell elements  302  which are welded together and form a part of outer tank wall or shell  300 . To reduce construction time for outer tank  20 , shell  300  is built using precast reinforced concrete shell elements  302  that are joined by welding shell elements  302  together to form a generally annular, tapered thickness annular shell  300 . Shell elements  302  are thicker at their bottoms to handle greater loads applied there to near ring beam  100  as compared to the loads imposed by roof  400 . Preferably, there is minimal or no need to cast in place any significant structural connections to join shell elements  302  together or to ring beam  200  or to roof  400 . Shell elements  302  will be precast to the number, widths and heights required for the size of outer tank  22 . Typically, the inner diameter of outer tank  20  will be about 3 meters larger in diameter than inner tank  22  to allow for construction of inner tank  22  and to provide for insulation in the annulus between the tank shells. Precasting of shell elements  302  can be started as soon as the tank diameter and height are known. 
     A. Shell Element  302   
     Each of shell elements  302  includes a pair of tapered carbon steel side plates  304  and a carbon steel liner plate  306  forming a generally U-shaped steel cross-section. In this example, the liner plate  306  is about 10 mm thick and side plates  304  are about 25 mm thick. The width of side plates  304  is about 800 mm at their bottom and 400 mm at the top providing the tapered shape to shell element  302 . A base plate  312  connects side plates  304  and liner plate  306  at the bottom of shell element  302 . Numerous reinforcing bars  310  are welded to and extend from side plate  304  to the opposing side plate  304  and from base plate  312  to top end cap  314 . Base plate  312  is about 50 mm in thickness in this example. A steel end cap plate  314 , also about 50 mm thick, is welded to side plates  304  and steel liner plate  306  at the top of shell element  302 . Concrete is cast in place in the U-shaped volume defined by side plates  304  and steel liner plate  306  and about reinforcing bars  310  and J-hooks  316  to form reinforced concrete body  318 . Liner plate  306  acts as a vapor/gas barrier 
     B. Constructing Shell  300   
     Shell  300  is constructed by arranging shell elements  302  vertically upon outer ring beam  200 . Initially, base plates  312  are arranged radially slightly outside of their final position for welding with shell elements  312  being slightly radially spaced apart. Shell elements  302  are then moved radially inwardly until all base plates  312  and side plates  304  are brought into abutment at the proper radial position atop of base plates  206  of ring beam  202 . Erection gear will be used to finally align and pull the side plates  304  closely together to begin welding. 
     As seen in  FIG. 8B  and  FIG. 1A , vertically extending weld joints  320  are made on the inside and outside of abutting side plates  304 . Weld joints  320  ideally are designed to be strong enough to carry the loads on shell  300  without any or little other significant reinforcement, in particular, the need for post tensioning or utilizing any type of significant cast in place joints. End cap plates  314  are welded to one another forming weld joints  322  and creating a continuous upper ring on shell  300 . As best seen in  FIG. 7 , base plates  312  of shell  300  and base plates  206  of ring beam  200  are welded together along the inner and outer radial surfaces on base plate  312  to form base plate welds  324 . Shell elements  302  are splice welded to accommodate load transfer even under earthquake or missile impact. If additional strength is needed, shell elements  302  may be designed to be mechanically interlocking with one another as well as being joined by welds such as weld  320 . Ideally, the welded connections between shell elements  302  will be comparable in strength to conventional cast in place connections formed between shells. 
     All of shell elements  302  are permanently welded together to form shell  300 , with the exception of 2-4 special half shell elements  302 ′. Half shell elements  302 ′ are similar to shell elements  302  except they are only about half the height of regular shell elements  302 . These special shell elements  302 ′ will be temporarily sealed with shell opening  350  to accommodate air raising of roof assembly  404 , as suggested in  FIG. 9C . These special half shell elements  302 ′ are removed after roof assembly  404  is attached to shell  300  to provide access so that inner tank  22  can be concurrently erected as reinforcing roof elements  402  and  402 ′ are welded to roof assembly  404 . When inner tank  22  is completed, along with insulation added in the annulus formed between inner tank  22  and outer tank  20 , the special half shell elements  302 ′ are permanently welded to the remainder of shell  300  to seal shell opening  350 , as shown in  FIG. 1B . Although not shown in  FIGS. 1A and 1B , manhole access openings will be provided in roof  400  to provide access to perform welds on the interior of outer tank  20 . Also, the manhole access openings allow future maintenance to be performed within outer tank  20 . 
     As an alternative to permanently closing construction opening  350 , liner plates can be welded to close shell opening  350 . Then reinforcing bars and steel rib stiffeners can be placed in a form and concrete can be cast in place to form a cast in place shell element similar to that used in the design of the precast shell elements  302 . Shell  300  should have great strength when completed due to the welded connections  320  on the inner and outer edges of side plates  304  which cooperate to form a large number of radially extending stiffeners arranged around the circumferential periphery of outer tank  20 . 
     C. Alternate Shell Design 
       FIGS. 8C ,  8 D and  8 E show an alternative shell design. Rather than using a single course of precast reinforced concrete shell elements  302  to form shell  300 , multiple courses of similar upper and lower shell elements  302   a  and  302   b  are utilized. Those skilled in the art of LNG tank construction will appreciate that any number of courses of shell elements could be used to form a shell. However, shorter shell elements are easier to transport and move but require additional welds as compared to using a single course of shells to construct a shell. 
     Lower shell element  302   a  includes a concrete body  316   a  to which a pair of circumferentially spaced apart side plates  304   a , liner plate  320   a , upper end cap plate  314   a  and lower base plate  312   a  are anchored or embedded (anchors and reinforcement bars not shown). Base plate  312   a  is welded by welds  324   a  to base plate  206  of ring beam  200 . Upper end cap plate  314   a  has two downwardly depending sockets  362  formed therein. 
     Upper shell element  302   b  includes a concrete body  316   b  to which a pair of circumferentially spaced apart side plates  304   b , liner plate  320   b , upper end cap plate  314   b  and lower base plate  312   b  are anchored (anchors and reinforcement bars not shown). Two downwardly depending and circumferentially spaced apart alignment pipes  360  are welded to lower base plate  312   b . As seen in  FIG. 8C , alignment pipes  360  can be used to align and guide an upper shell element  302   b  as it is positioned upon lower shell element  302   a . Also, alignment pipes  360  held within alignment sockets  362  provide mechanical interlocking between shell elements  302   a  and  302   b . Base plate  312   b  is welded by horizontal welds  364  to end cap plate  314   a  of lower shell element  302   a  along their inner and outer radial edges. A horizontal extending cutout  364  on the outer radial edges of shell elements  302   a  and  302   b  permits the outer radial weld  364  to be easily made. After welding, cutout  364  is filled with grout for fire protection. 
     V. Roof  400   
     Referring in general now to  FIGS. 9A-F , roof  400  has a roof assembly  404  which includes a skeletal roof frame  406  ( FIG. 9A ), liner plates  410  covering roof frame  406  ( FIGS. 9B and 9C ) to form a generally air tight membrane  412  and tubular mounting beams  414  welded atop membrane  412 . In this exemplary embodiment, after a temporary skirt or seal is attached to its circumferential periphery, roof assembly  404  is air lifted to the top of shell  300  from upon base mat assembly  100  by pressurizing the inside of shell  300 , as suggested in  FIG. 9C . Roof membrane  412  of roof assembly  404  is welded to circumferentially spaced apart steel gusset plates  450  ( FIGS. 9D and 9F ) that are temporarily attached atop end cap plate  314 . This prevents the roof seal from blowing out and allows weld joints to be made between the roof membrane  412  and gusset plates  450 . Air pressure is maintained while welds are made between end cap plate  314  and roof membrane  412 . 
     Once enough weld joints have been formed to safely support roof  400 , the pressurization within shell  300  is removed. Personnel can then access the inside of the outer tank  20  to construct inner tank  22 . Additional welding is done to complete the circumferential weld joint between the roof membrane  412  and shell membrane  340 . Special extra strength roof elements  402 ′ are attached to shell  300  and roof frame  404  forming the radial outermost course of roof elements  402 . Then typical roof elements  402  are attached to roof frame  404  with the radial outermost course of roof elements  402  being attached first. Then the successive next radial outermost course of roof elements is added until all the rings of roof elements  402  are in place forming roof  400 . 
     A. Roof Assembly  404   
     Roof assembly  404  is built on the ground in this particular preferred exemplary embodiment. Roof frame  406  comprises a plurality of radially and circumferentially extending wide flange beams  420 , as best seen in  FIG. 9A . In this particular exemplary embodiment, eight concentric rings of openings  422   a - h  are formed between the wide flange beams  420 . Wide flange beams  420  used in the radial direction are W18×76 in the center and gradually increase in size to W18×143 near the outer radial ends. In the circumferential direction each row of wide flange beams  412  has the same section size such as W18×40, W18×50, W18×76 and W18×97 with the larger wide flange beams being used nearer the outer radial edge. Of course, other sizes of wide flange beams can be used, particularly if outer tank  20  is to be of a smaller or larger size than suggested in the present example. 
       FIG. 9B  shows a typical joint construction for roof assembly  404 . Liner plates  410  are appropriately sized and are welded to the top flange  424  of wide flange beams  420  to cover openings  422  and thus provide a generally air tight roof membrane  416 . A tubular mounting beam  414  is shown which is welded atop of adjacent liner plates  410 . Tubular mounting beams  414  generally run in the same direction as the web of the underlying wide flange beams  420 , i.e., radially and circumferentially. As can be seen in  FIG. 9B , roof elements  402  are designed to mount and be welded to tubular mounting beams  414 , as will be described in greater detail below. 
     B. Raising and Mounting Roof Assembly  404  to Shell  300   
       FIG. 9C  shows a roof assembly  404  being raised by air pressure along an outer tank wall or shell element  302 . The three dots shown adjacent shell element  302  indicates that shell  300  is comprised of a large number of such shell elements  302 , which are not shown. A temporary skirt or seal  424  is attached around the circumferential periphery of roof assembly  404 .  FIG. 9D  shows a fragmentary sectional view of roof assembly  404 , seal  424 , and end cap plate  314  and liner plate  306 . Air pressure is applied to seal  424  from within shell  300 . Seal  424  bears against liner plate  306  of shell  300  to minimize the loss of air there between. A temporary gusset plate  450 , one of 240 such gusset plates  450  located around the periphery of the ring of end cap plates  314  in this example, is temporarily welded to end cap plate  314  and extends radially inwardly. 
     Referring now to  FIG. 9E , when roof assembly  404  is raised to the appropriate height, welds  452  are made between liner plates  410  and gusset plates  450 . An outer portion of a circumferential weld  454  is then made between end cap plate  314  and roof liner plates  410  to form an air tight seal there between joining shell membrane  340  and roof membrane  412  of roof assembly  404 . Ideally, weld  454  is a full fusion, full penetration weld requiring welding from both outside and inside outer tank  22 . When sufficient welds have been made to safely secure roof assembly  404  to shell  300 , air pressure can be removed from shell  300 . Then, seal  424  and gusset plate  450  are removed. 
     Half shell elements  302 ′ are removed to provide access from within shell  300 . Referring now to  FIG. 9F , the inner portion of circumferential weld  454  is then completed between end cap plate  314  and liner plates  410  to form an air tight seal there between joining shell membrane  340  and roof membrane  412 . A series of circumferentially spaced apart permanent flange extension plates  462  are welded between liner plate  306  and beneath the lower flanges of wide flange beams  420 . Also, web extension plates  464  are welded on to the web of wide flange beam  420  and to liner plate  306 . On the outside of roof assembly  404 , extension tubular mounting beams  414   a  and  414   b  are, respectively, welded atop liner plates  410  and end cap plate  314  extending radially from mounting beam  414 . With tubular extensions  414   a  and  414   b  in place, the outermost row of roof elements  402 ′ can then be welded to shell  300  and to roof assembly  404 , as will be described in greater detail below. 
     C. Roof Elements  402  and  402 ′ 
     Roof elements  402  and  402 ′ are mounted to roof assembly  404  after roof assembly  404  has been affixed by weldments to shell  300 .  FIG. 10A  shows a typical roof element frame  428  which includes four L-shaped brackets  432  welded together to form a generally open rectangular frame. Depending inwardly from brackets  432  are a plurality of J-hooks  434 . Reinforcing bars  436  extend laterally and longitudinally between opposing brackets  432 . 
     Roof element frame  428  is used to construct precast reinforced concrete roof elements  402 , as seen in  FIG. 10B . Roof element  402  includes a cast concrete body  440  which surrounds J-hooks  434  and reinforcing bars  436  and supports roof element frame  428 . The L-shaped brackets  432  are located on all four sides of roof element  402  to provide attachment to tubular mounting beams  414  of roof assembly  404  in the case of typical roof elements  402 . The size of each roof element  402  is generally between about 2 meters to about 3 meters in width and between about 5 meters to about 6 meters in length in this exemplary embodiment. Of course, other size roof elements could be selected and will depend on the size of outer tank  22 . 
     For the outermost concentric course of roof elements  402 ′ which secure to both roof assembly  404  and to shell  300 , the design is slightly different from that of roof elements  402  disposed on the inner radial concentric courses. As shown in  FIG. 9F , concrete body  440 ′ in this example tapers from a thinner 150 mm on its inner radial end to a maximum thickness of 300 mm adjacent its radial outmost end. The typical concrete body  440  of a typical roof element  402  has a constant thickness of about 150 mm Second, roof element  402 ′ includes L-shaped bracket  432  on only three edges with the radially outermost fourth side instead having an overhang  470  sized and shaped to fit over end cap plate  314 . Provided in concrete body  440 ′ are four access openings  472  (two seen in  FIG. 9F ) which provide access to four studs  474  which are to be stud welded to end cap plate  314 . 
     D. Mounting of Roof Elements  402  to Roof Assembly  404   
     The outer concentric ring or course of roof elements  402 ′ are located adjacent shell  300  and this outer ring of roof elements  402 ′ is first welded to roof assembly  404  and to shell  300 . This outer ring of roof elements  402 ′ adds significant strength to roof  400 . Subsequently, second through eighth courses of roof elements  402  are sequentially welded to roof assembly  404  starting from the radially outermost course and then each concentric course of roof elements is added until all rows of roof elements  402  are in place to form a complete roof  400  such as seen in  FIG. 1B .  FIG. 1A  shows two courses of roof elements  402 ′,  402  mounted to roof assembly  404 . 
     L-shaped brackets  432  of roof elements  402  are mounted atop and are welded to tubular mounting beams  414  creating welds such as the weld joint  436  seen in  FIG. 9B . In the case of special roof elements  402 ′, roof elements  402 ′ are welded to tubular mounting beams  414 ,  414   a  and  414   b . Studs  474  are placed in access openings  472  and are stud welded to end cap plate  314 . The thickness of tubular mounting beams  414  is less than the thickness of concrete bodies  430 , as seen in  FIG. 9B . Any voids formed between any of concrete bodies  440 ,  440 ′ of the roof elements  402 ,  402 ′ are grouted in place to form concrete joint  442  to provide fire protection to roof  442 . An example of a concrete joint  442  is shown in  FIG. 9B . 
     While in the foregoing specification this invention has been described in relation to certain preferred embodiments thereof, and many details have been set forth for purpose of illustration, it will be apparent to those skilled in the art that the invention is susceptible to alteration and that certain other details described herein can vary considerably without departing from the basic principles of the invention. For example, any one or more of base mat assembly  100 , ring beam  200 , outer shell  300  or roof  400  could be constructed using conventional cast in place construction techniques while the other components are constructed using the precast elements as described herein. As another example of an alternative embodiment, the roof assembly could be directly built atop shell  300  such as by using a crane. In this instance, roof assembly  404  would not have to be airlifted and attached atop shell  300 . However, ideally even in this embodiment, the shell elements  402  could be welded to the roof assembly concurrently with the construction of the inner tank  22  to save construction time on building outer tank  20  and inner tank  22 . 
     While beam ring  200  described in the above particular exemplary embodiment was used in conjunction with a full containment LNG tank, a beam ring could certainly be used in other applications. For example, the ring beam could be used in cases where neither a liner plate or thick structural base plate is necessary. In this case, the ring beam need only comprise ring beam elements having precast reinforced concrete bodies with embedded tensile conduits which receive tensile members there through so that the tensile members may be tensioned and anchored by anchors to form the beam ring. Although not required, preferably the ring beam elements would be interlocking with one another.