US3860140A

US3860140A - Balsa wood footing for lng tanks

Info

Publication number: US3860140A
Application number: US342591A
Authority: US
Inventors: Sven Hjalmar Wichman; Luke Francois Debock
Original assignee: Preload Technology Inc
Current assignee: Preload Technology Inc
Priority date: 1973-03-19
Filing date: 1973-03-19
Publication date: 1975-01-14
Anticipated expiration: 1992-01-14

Abstract

A foundation for cryogenic storage tanks is shown. It is comprised of a balsa wood, softwood or hardwood footing positioned between a floor and subfloor of the tank. The footing is comprised of select blocks of wood glued to one another, their longitudinal axes vertically positioned. In a preferred embodiment the footing may be comprised of a three dimensional assembly of blocks glued to one another, their longitudinal axes positioned alternately in the vertical, radial and tangential directions. Longitudinal and transverse restraining bars hold the wood in place. Also shown is a balsa wood or softwood layer between the wall and the floor which acts as a load equalizer and sliding bearing for the wall.

Description

United States atent [191 Wichman et al.

[111 emo [45] Jan. 14,1975

[ 1 BALSA WOOD FOOTING FOR LNG TANKS [73] Assignee: Preload Technology Inc Garden City, N.Y.

[22] Filed: Mar. 19, 1973 [21] Appl. No.: 342,591

[52] US. Cl 220/18, 52/249, 52/573, 220/9 LG, 220/69 [51] Int. Cl 1365!! 87/24, E04b 1/68 [58] Field of Search 220/9 A, 9 LG, l5, 18, 220/69, 11; 52/245-249, 292, 293, 573, 274

[56] References Cited UNITED STATES PATENTS 2,495,798 1/1950 Wissmiller 220/15 2,520,883 8/1950 Kornemann et al 220/9 LG 2,563,118 8/1951 Jackson 220/15 2,911,125 11/1959 Dasker.... 220/11 2,954,892 10/1960 Dosker 220/11 3,067,901 12/1962 Van Geem et al.... 217/65 3,092,933 6/1963 Closner et al 52/249 Smith et a1. 220/9 LG 3,325,037 6/1967 Kohn et a1.

3,514,913 6/1970 Nelson 3,606,067 9/1971 Jones 220/15 Primary ExaminerWilliam 1. Price Assistant Examiner-Stephen Marcus Attorney, Agent, or FirmCurtis, Morris & Safford [57] ABSTRACT A foundation for cryogenic storage tanks is shown. It is comprised of a balsa wood, softwood or hardwood footing positioned between a floor and subfloor of the tank. The footing is comprised of select blocks of wood glued to one another, their longitudinal axes vertically positioned. In a preferred embodiment the footing may be comprised of a three dimensional assembly of blocks glued to one another, their longitudinal axes positioned alternately in the vertical, radial and tangential directions. Longitudinal and transverse restraining bars hold the wood in place. Alsoshown is a balsa wood or softwood layer between the wall and the floor which acts as a load equalizer and sliding bearing for the wall.

7 Claims, 6 Drawing Figures SHEET 10F 3 Pmmmmmm FIG. 5

PATENTEDJANWQYS SHEET 20E 3 BALSA WOOD FOOTING FOR LNG TANKS This invention relates broadly to cryogenic tanks for storage of liquid methane and other liquified gases. More specifically, this invention relates to the novel construction of foundations with unique structural and thermal properties for such cryogenic tanks.

In recent years the demand for liquid natural gas has substantially increased and the art is faced with the need for providing larger and larger tanks for containment of this liquid. The ever-present and increasing need for safety in the design of such tanks has also created numerous problems in their construction and has presented the art with a number of related problems in material selection and in tank design. Special problems arise in the design of prestressed concrete cryogenic tanks. In these tanks, thermal gradients which cause differential movements of the walls and floor create critical stress conditions both before and after service conditions are obtained, giving rise to a number of unique design problems.

A very basic and important part of the cryogenic tank is the foundation structure. The foundation must be able to withstand very considerable compressive loads that are imposed on it by the wall structure. Furthermore, the foundation structure, particularly when the walls are prestressed, must be able to absorb multidirectional stresses due to relative movement of the walls and floors of the tank. Of equal importance, the foundation materials must have good thermal insulating properties so that the foundation does not comprise an excessive heat source for the tank. Heretofore the foundation structures which have been used in cryogenic tank design have been relatively expensive and have not provided the multiple functions that are required under the varied conditions of stress and temperature.

It is thus the primary object of this invention to provide an economical and easily constructed foundation for cryogenic tanks.

It is a further and related object of this invention to provide a foundation structure which is fully able to absorb the very considerable compressive stresses which are encountered where a high concrete or steel wall is erected over the foundation.

It is still a further and related object of this invention to provide a foundation structure which is able to absorb and redistribute uneven stresses which may develop due'to inaccuracies in the construction of the tank or due to uneven cool-down or warm-up of the tank during its service.

It is a related object of this invention to provide a foundation which can withstand the varying forces imposed upon it in vertical, radial and horizontal directions.

It is still a further and important object of this invention to provide a foundation structure which can absorb vertical bending moments which may be imposed upon it by hydrostatic forces, thermal forces or prestressing of the tank wall.

A related object of this invention is to provide a foundation which has a substantial degree of structural integrity under the influence of earthquakes or other ground shock forces.

It is yet a further object of this invention to provide a foundation which has a high degree of thermal insulating property.

It is yet a further and also important object of this invention to provide a foundation which permits some relative movement of the wall and foundation, as occurs during the prestressing, shrinkage, creep, filling and emptying motions of a prestressed concrete tank.

These and other objects of this invention are obtained in an improved foundation for cryogenic tanks which is comprised of balsa, softwood or hardwood, oriented with the grain parallel to the load on the foundation. Since wood is difficult to obtain in very large uniform blocks, it hasbeen found that good results are obtained if the foundation blocks are themselves comprised ofa plurality of select blocks which are attached to one another with a cryogenically compatible adhesive material. In a particularly satisfactory foundation design, the foundation comprises a subfloor, which may be of steel, and a tank floor located thereabove which is of cryogenic steel, and positioned therebetween a foundation of wood with its grain oriented vertically, i.e. parallel to the tank wall. In a desirable variant, the blocks may be arranged in a three dimensional assembly comprising blocks having their grain oriented in the vertical, radial and tangential directions.

The tank wall is positioned above the tank floor and desirably is separated from the tank floor by a thin layer of balsa wood or softwood having its grain oriented horizontally. The thin layer of balsa wood or softwood has a cryogenic lubricant coating on its lower surface, and at its upper surface is attached by means of cryogenic adhesive to the lower surface of the wall. This balsa or softwood layer helps to smoothen out irregularities inherent in large structures, and acts as a sliding bearing between the wall and the floor permitting inward radial movement of the wall due to prestressing or other forces which may be imposed upon it.

In the preferred embodiment of the invention, which is disclosed in connection with the drawings, the wood foundation is retained in position by circumferentially running members, such as angle irons, which are welded to the subfloor of the foundation. Desirably, restraining shear key bars are provided which are positioned at intervals transversely to the course of the foundation. These are attached to the tank floor and to the tank subfloor extending downwardly and upwardly, respectively. These shear bars prevent relative tangential movement of the foundation and the wall. They do permit some radial movement, however, as aforedescribed.

To protect the wood in the foundation from physical damage as well as water absorption, it is desirable to coat all surfaces thereof with fiberglass. In constructions where the upper thin horizontal-grain layer is not used, it is preferable to cover the upper surface of the blocks comprising the footing with a teflon coated glass fiber sheet. In another preferred embodiment, the wood' in the foundation is sandwiched between panels of plywood.

The invention is further described in connection with the following drawings.

FIG. 1 is a side-elevation view of a foundation structure according to the present invention;

FIG. 2 is an expanded side elevation view showing the essential detail of the foundation structure and of a balsa wood layer which acts as a sliding bearing and bearing pressure equalizer;

FIG. 3 is a plan view in section of the foundation of FIG. 2, taken along lines 3-3 of FIG. 2;

FIG. 4 is a plan view in section of the foundation of FIG. 2, taken along lines 4-4 of FIG. 2;

FIG. 5 is a side elevation view of a portion of a foundation representing a preferred embodiment of this invention; and

FIG. 6 is a side elevation view of a portion of a foundation representing another preferred embodiment of this invention.

In FIG. 1 reference numeral 10 refers generally to a cryogenic tank structure and more specifically to the foundation area thereof. Reference numeral 12 refers to a steel subfloor positioned above the concrete foundation 14. Reference numeral 16 refers to a primary steel floor of the cyrogenic tank. Steel subfloor l2 and steel floor 16 are separated by a layer of insulation 20 which may be plastic foam or other suitable insulating material. Between subfloor 12 and floor 16, at the edge of tank 10, is a foundation structure, referred to generally by reference numeral 22, which includes the wood supporting members and various restraining members, all as further described in connection with FIG. 2. Above floor 16 is a layer of balsa 24 which acts as a load distributor and as a sliding bearing between floor I6 and concrete wall 26 which is located immediately thereabove. A steel liner 28 is provided at the exterior of wall 26.

With reference to FIGS. 2, 3 and 4, reference numeral 30 refers to a wood footing or foundation wherein the grain of the wood is positioned vertically, i.e. parallel with the load imposed upon it by the weight of of concrete wall 26. The wood footing may be a single block of wood, however, preferably and typically it will be an assembly of wood blocks attached to one another by means of a cryogenic adhesive. Each of the component blocks should be sterilized and kiln dried to a controlled moisture content of between 5 percent and percent. The blocks should be relatively free of holes and knots or decay which would affect their structural properties. The blocks are then typically laminated under pressure to one another with an adhesive such as phenol resorcinol, which is stable under the cryogenic temperatures of liquid natural gas storage, i.e. stable at temperatures of 25 to 300F.

The wood footing which is obtained has a number of unique properties when used for the service for which it is herein intended. This is especially true when the wood used is balsa. When very high insulating value is needed, loads are moderate, and a light weight wood is preferred, balsa is most suitable. Its weight ranges between 6 and 16 lbs. per cubic foot. The balsa wood footing is orthotropic having three principal axes: a longitudinal, a radial and a tangential axis. The compressive strength is greatest in the longitudinal or end grain direction and its compressive strength in that direction is nearly 10 times that in either the radial or tangential flat grain direction. For this reason, the balsa wood is used with its grain parallel to the height of the wall which is erected above it. The footing is thus extremely difficult to deform due to the relatively small percentage of solid matter in the wood structure, and the wood has significant elasto-plasticor rebound properties under the different loads which may be imposed on it. Composite blocks, which are obtained by gluing smaller select blocks to one another, have strength properties which are superior to those of the individual component blocks themselves. The footings are able to redistribute excessive local loads which may occur and are able to rebound after high intensity loading is removed. Furthermore, the blocks have high resistance to splitting. The thermal qualities of the footings are also beneficial in that the porous nature of the balsa wood provides a relatively high and consistent insulating value to avoid heat leak into the cryogenic tanks. Furthermore, the balsa wood has good strength characteristics at cryogenic temperatures, its strength being 50 percent greater at 260 than at F.

To protect the balsa blocks from moisture and from abuse in handling and construction steps, the blocks are desirably fiberglassed on all exterior surfaces. This material is applied togetherwith a layer of 2 oz. per square foot of fiberglass mat. A coating thickness of 60 mils has been found to suffice to protect the balsa wood. If desired, an ultra-violet stabilizer can be added to the resin.

Before insulating, the top faces of the footings are desirably covered with a thin teflon-coated glass fiber sheet. The fiberglass sheets should lap over the inner and outer sides and over the end sides and the edges can be taped, glued or stapled. The top covering of the footing is designated in FIG. 2 by reference numeral 32.

The balsa wood footings are retained in position by means of retaining

bars

34 and 36 which are suitable angle irons welded to subfloor 12. These retaining bars restrict the sideways expansion of the balsawood under the compressive load. Located at intervals along the length of the foundation are shear

key bars

38 and 40. Bar 38 is welded to subfloor l2 and shear bar 40 is welded to tank floor 16. Pairs of these shear brs are provided transverse to the course of the foundation at intervals therealong. These bars may be comprised of any suitable barstock material.

The sliding bearing 24 is comprised of a relatively thin layer of balsa wood with its grain oriented horizontally, i.e. perpendicular to the direction of loading, so that the load is across the grain. The layer may be comprised of strips glued to one another. The layer should be precompressed under conditions which prevent lateral spreading, conditions which obtain in service, since each piece of balsa abuts its neighbor and has no room to spread sideways. The layer 24 is covered with a suitable cryogenic lubricant 25 at its bottom and is adhered to the lower surface of the concrete wall 28. The balsa layer slides on floor 16 when the prestressing operation forces wall 26 to move inwardly. Once wall 26 is prestressed, its liner 28 is welded to the steel floor plate 16 and the flat grain balsa wood 24 then acts as a cushioning element.

With reference to FIG. 5, reference numeral 50 refers to a composite footing of balsa wood, each block having its grain oriented vertically. Blocks are glued to one another as described above, and all surfaces of the balsa wood are sealed with fiberglass as described above. A plywood panel 52 is glued with cryogenic adhesive to the top surface of the balsa wood and a similar plywood panel 54 is provided on the lower surface. The purpose of the

plywood panels

52 and 54 is to provide increased resistance against tangential forces applied to the foundation.

With reference to FIG. 6, reference numeral 60 refers to a composite footing of balsa wood, softwood or hardwood, consisting of an assembly of numerous small blocks, their axes oriented in the vertical, radial and tangential directions. The resulting composite is of approximately equal strength in all three principal directions, vertical, radial and tangential. lf superior strength is required in any of these directions, a higher percentage of end grain blocks is used in this particular direction. Thus, a wooden structure can be created to suit given loading and stress conditions.

What is claimed is:

l. in a cryogenic tank construction including a subfloor, a tank floor and a wall supported above said tank floor, the improvement which comprises:

a. a foundation comprised of wood positioned between said floor and said subfloor, said wood having its grain oriented parallel to said wall;

b. restraining means for said wood positioned along the sides of said foundation and fastened to said subfloor, running essentially parallel to the course of said foundation; and

c. restraining shear key bars for said wood positioned, at intervals, transversely to the course of said foundation and attached respectively to said tank floor and said subfloor, for preventing relative tangential movement of said foundation and wall.

2. In a cryogenic tank construction as recited in claim 1, the further improvement which comprises coating said wood with fiberglass and covering the upper surface thereof with teflon-coated glass fiber sheet.

3. In a cryogenic tank construction including a tank floor and a wall supported thereon and wherein said wall is free to move radially inward under the influence of prestressing forces thereon, the improvement which comprises:

a slide bearing situated between the bottom surface of said wall and the top surface of said tank floor, said bearing comprising a thin layer of wood its grain oriented horizontally, said wood being attached by adhesive to said wall and having a cryogenic lubricant coating on its lower surface.

4. In a cryogenic tank including a steel subfloor, a steel tank floor and a prestressed concrete wall supported above said tank floor, the improvement which comprises:

a. a foundation comprised of wood positioned between said floor-and said subfloor, said wood being comprised of a plurality of select blocks adhered to one another with a cryogenic adhesive and having a coating of fiberglass thereon;

b. restraining bars for said wood positioned along the sides of said foundation and welded to said steel subfloor, said bars running essentially parallel to the course of said wall;

c. restraining shear key bars for said wood positioned, at intervals, transversely to the course of said foundation and attached respectively to said tank floor and said subfloor, for preventing relative tangential movement of said foundation and wall; and

d. a slide bearing situated between the bottom surface of said wall and the top surface of said tank floor, said bearing comprising a thin layer of precompressed balsa wood or softwood, its grain oriented horizontally, said balsa or softwood being attached by adhesive to said wall and having a cryogenic lubricant coated on its lower surface.

5. In a cryogenic tank construction including a subfloor, a tank floor and a wall supported above said tank floor, the improvement which comprises:

a. a foundation comprised of wood positioned between said floor and said subfloor, said foundation comprising select blocks being oriented in the vertical, radial and tangential directions with respect to said foundation;

6. In a cryogenic tank including a steel sub-floor, a steel tank floor and a prestressed concrete wall sup ported above said tank floor, the improvement which comprises:

7. In a cryogenic tank, an improved construction comprising the elements of: a foundation comprised of wood oriented with its grain in the vertical direction, said wood being coated with fiberglass and having the upper surface thereof covered with teflon-coated glass fiber sheet.

Claims

1. In a cryogenic tank construction including a subfloor, a tank floor and a wall supported above said tank floor, the improvement which comprises: a. a foundation comprised of wood positioned betwEen said floor and said subfloor, said wood having its grain oriented parallel to said wall; b. restraining means for said wood positioned along the sides of said foundation and fastened to said subfloor, running essentially parallel to the course of said foundation; and c. restraining shear key bars for said wood positioned, at intervals, transversely to the course of said foundation and attached respectively to said tank floor and said subfloor, for preventing relative tangential movement of said foundation and wall.

3. In a cryogenic tank construction including a tank floor and a wall supported thereon and wherein said wall is free to move radially inward under the influence of prestressing forces thereon, the improvement which comprises: a slide bearing situated between the bottom surface of said wall and the top surface of said tank floor, said bearing comprising a thin layer of wood its grain oriented horizontally, said wood being attached by adhesive to said wall and having a cryogenic lubricant coating on its lower surface.

4. In a cryogenic tank including a steel subfloor, a steel tank floor and a prestressed concrete wall supported above said tank floor, the improvement which comprises: a. a foundation comprised of wood positioned between said floor and said subfloor, said wood being comprised of a plurality of select blocks adhered to one another with a cryogenic adhesive and having a coating of fiberglass thereon; b. restraining bars for said wood positioned along the sides of said foundation and welded to said steel subfloor, said bars running essentially parallel to the course of said wall; c. restraining shear key bars for said wood positioned, at intervals, transversely to the course of said foundation and attached respectively to said tank floor and said subfloor, for preventing relative tangential movement of said foundation and wall; and d. a slide bearing situated between the bottom surface of said wall and the top surface of said tank floor, said bearing comprising a thin layer of precompressed balsa wood or softwood, its grain oriented horizontally, said balsa or softwood being attached by adhesive to said wall and having a cryogenic lubricant coated on its lower surface.

5. In a cryogenic tank construction including a subfloor, a tank floor and a wall supported above said tank floor, the improvement which comprises: a. a foundation comprised of wood positioned between said floor and said subfloor, said foundation comprising select blocks being oriented in the vertical, radial and tangential directions with respect to said foundation; b. restraining means for said wood positioned along the sides of said foundation and fastened to said subfloor, running essentially parallel to the course of said foundation; and c. restraining shear key bars for said wood positioned, at intervals, transversely to the course of said foundation and attached respectively to said tank floor and said subfloor, for preventing relative tangential movement of said foundation and wall.

6. In a cryogenic tank including a steel sub-floor, a steel tank floor and a prestressed concrete wall supported above said tank floor, the improvement which comprises: a. a foundation comprised of wood positioned between said floor and said subfloor, said foundation comprising select blocks being oriented in the vertical, radial and tangential directions with respect to said foundation; b. restraining bars for said wood positioned along the sides of said foundation and welded to said steel subfloor, said bars running essentially parallel to the course of said wall; c. restraining shear key bars for said wood positioned, at intervals, transversely to the course of said foundation and attached respectively to said tAnk floor and said subfloor, for preventing relative tangential movement of said foundation and wall; and d. a slide bearing situated between the bottom surface of said wall and the top surface of said tank floor, said bearing comprising a thin layer of precompressed balsa wood or softwood, its grain oriented horizontally, said balsa or softwood being attached by adhesive to said wall and having a cryogenic lubricant coated on its lower surface.