US3488972A

US3488972A - Cryogenic storage structure

Info

Publication number: US3488972A
Application number: US666552A
Authority: US
Inventors: John J Closner
Original assignee: Preload Co Inc
Current assignee: Preload Co Inc; Preload Co
Priority date: 1967-09-06
Filing date: 1967-09-06
Publication date: 1970-01-13
Anticipated expiration: 1987-01-13

Description

4 Jan. 13, 1970 J. J. CLOSNER 3,488,972

CRYOGENIC STORAGE STRUCTURE Filed Sept. 6, 1967 5 Sheets-Sheet l ATTORNZEY;

Jan. 13, 1970 J. J. CLOSNER 3,488,972

CRYOGENIC STORAGE STRUCTURE Filed Sept. 6. 1967 5 Sheets-Sheet 2 BNVENTOR JUA A/ f ('1 Jf/I/FE ATTORNEY Jan. 13, 1970 J. J. CLOSNER 3,488,972

CRYOGENIC STORAGE STRUCTURE Filed Sept. 6, 1967 5 Sheets-Sheet 3 INVENTOR Qf/A J. CAOj/VE/Z BY Qwzafiywvz w ATTORNEYf Jan. 13, 1970 J. .1. CLOSNER 3, ,97

CRYOGENIC STORAGE STRUCTURE Filed Sept. 6. 1967 5 Sheets-Sheet 4 INVENTOR JE/M/ J? 64 din/EB m wj Jan. 13, 1970 J. J. CLOSNER CRYOGENIC STORAGE STRUCTURE 5 Sheets-Sheet 5 Filed Sept. 6, 1967 INVENTOR ATTORNEYS United States Patent 3,488,972 CRYOGENIC STORAGE STRUCTURE John J. Closner, Douglaston, N.Y., assignor to The Preload Company, Garden City, NY. Filed Sept. 6, 1967, Ser. No. 666,552 Int. Cl. F25d 23/12; E01b 3/04 U.S. C]. 6245 12 Claims ABSTRACT OF THE DISCLOSURE A cryogenic storage tank made of prestressed concrete having a dual roof system wherein pressure within the tank and pressure on the outer surface of the inner roof are maintained in substantial equilibrium to reduce the size and reinforcement of the roof, and carbon steel barriers are maintained in compression to avoid brittle fracture.

The present invention relates to the storing of liquified gases and more particularly to structures for containing the gases.

Many gases, such as natural gas, oxygen, nitrogen and the like are economically stored at temperatures far below the usual ambient temperatures so that they may be kept in liquid condition. When these gases are liquified they are in a cryogenic state and are known commonly as cryogenic liquids. Cryogenic liquids are stored at essentially atmospheric pressure but at extremely low temperatures, such as about 260 F. for liquified natural gas. The proper storage temperatures for different gases will vary, but they are well known.

In recent years there has been considerable activity to devise means and structures for storing liquified gases. This has been particularly so with the storing of natural gas. In many places there is a seasonal demand for natural gas for use as a heating fuel. During the period of great demand the supply and distribution system from the supply wells is obviously under a very heavy load. Also, during the non-heavy demand season the supply and distribution system is normally only partially used. In order to reduce the size of the gas transmission system it has been found feasible to stock pile liquified gases for use during the peak demand time, thus making greater use of the supply system.

Accordingly, there have been recent improvements in storage facilities for such gases. Storage structures which are useful for this purpose are shown and described in U.S. Patent No. 3,092,933, issued June 11, 1963 and in U.S. Patent No. 2,777,295, issued Jan. 15, 1957.

As shown in U.S. Patent No. 3,092,933 prestressed concrete structures may be used for the storage of cryogenic liquids. With prestressed concrete tanks, a roof system of the concrete dome type may advantageously be used. However, it has been found that vapor or internal pressure builds up within a tank and causes tensile stresses within the dome roof systems. As the tank pressure increases, it must be relieved or resisted to prevent possible damage to the roof system. A solution suggested heretofore has been to counteract the internal pressures within the tank by applying more concrete onto the dome roof and thus increase the dead load of the roof system to balance or counteract the internal pressure. However, the increased dead load of the roof system also creates severe stresses at the peripheral edge of the roof and bending in the shell of the dome. The increased dead load increases the horizontal thrust at the edge of the dome and also requires an increase in load capacity of the tank foundation as well as the dome ring support.

Accordingly, it is an object of the present invention to provide a storage system wherein a tank with a dual dome roof also has means for venting or balancing the ice pressure of the tank to relieve internal pressure against the inner roof. It is another object to provide a system wherein the vapor which is relieved from within the tank may be used to blanket insulation materials which may be positioned between the dual dome roof, and, if desired, the insulation about the tank wall.

In the specification and the accompanying drawing, an embodiment of the present invention for the storage of liquified gases and the venting of tanks is shown. This embodiment is not to be construed as limiting the invention but rather it is for the purpose of informing those skilled in the art so that they may practice the invention in many embodiments and within the spirit and scope of the claims which are set forth hereinafter.

In the drawings:

FIGURE 1 is a perspective view of a cryogenic storage tank in accordance with the present invention;

'FIGURE 2 is a sectional side view of the tank of FIG- URE 1;

FIGURE 3 is an enlarged, partially fragmentary, sectional side view of the tank wall, floor and dome of FIGURE 2;

FIGURE 3a is an enlarged, partially fragmentary, sectional side view of the tank wall and floor of FIGURE 3 showing an alternate expansion joint and floor liner arrangement;

FIGURE 4 is a partially fragmentary top view of a dome of the tank of FIGURE 1 showing the tank pressure equalizing system;

FIGURE 5 is a partially fragmentary side schematic view of the tank pressure equalizing system;

FIGURE 6 is a partially fragmentary and schematic sectional side view of the inlet end of the tank pressure equalizing system;

FIGURE 7 is a partially fragmentary and schematic sectional side view of the outlet end of the tank pressure equalizing system;

FIGURE 8 is a schematic view of the shell roof bracing system;

FIGURE 9 is a partially fragmentary side view of the support members for the bracing system; and

FIGURE 10 is a partially fragmentary view of the support of FIGURE 9 taken in the direction of lines 1010.

Referring to the drawings and to FIGURE 1 in particular, a tank 10 in accordance with the present invention is shown. The tank 10 is comprised of a cylindrical side wall 12, a dome roof construction 14 and a pressure equalizing system 16. One of the functions of the pressure equalizing system is the prevention of pressure buildup in the tool storage system of the tank. The operation and advantages of a pressure equalizing system in cryogenic tank storage Will be described in greater detail hereafter. However, it should be noted at this time that the pressure equalizing system 16 controls the pressure build-up beneath the dome roof and helps to equalize it with the pressure which exists in the insulation space which is found on the other side of an inner dome roof member.

As shown in FIGURES 2 and 3 in particular, further details of the tank structure are shown. In FIGURE 2 the illustrated tank 10 is built above grade or the ground line G. In order to support the tank, a series of pilings 18 are driven in accordance with well-known practice. On top of the pilings 18 a wall foundation 20 and a floor 22 are placed. Over the foundation 20 and the floor 22 a moisture barrier 24 is laid, the moisture barrier 24 being of any desirable selected type. A suitable moisture barrier is a laminate of aluminum foil sandwiched between layers of polypropylene.

This laminate may also be placed over a layer of roofing felt of .15 pound weight. To insulate the interior of the tank from the ambient atmosphere outside the foundation and the floor, suitable insulation is preferably provided over the moisture barrier 24. Accordingly, awall supporting insulation element 26 is provided in a recess in the foundation 20. The insulation must support a substantial load from the side wall in addition to having the property of not providing thermal transfer. Foamed glass is suitable for this purpose.

A wall footing 28 made of high strength concrete and reinforced with steel rods in a conventional manner is placed on top of the insulation 26. The wall footing 28 is the main support for the principal portion of the wall system 12, and a weep hole 29 is provided to remove any moisture which may accumulate beneath it.

Adjacent to the wall footing 28 is an insulation layer 30 overlying the floor foundation 22. The insulation layer 30 is also positioned over the moisture barrier 24. Foamed urethane is a suitable material for the layer 30.

As shown in FIGURE 3 a thin protective layer 32 is deposited on top of the floor insulation 30. The layer may be of any suitable material such as plywood or lean concrete-lean concrete being shown in the illustrated embodiment. Over the protective layer 32 is placed an inner floor liner or barrier 34, the liner 34 being preferably made of 9% nickel steel. It is the liner 34 which is in intimate contact with liquified gas contained within the tank and for this reason the nickel steel heretofore has been selected since it is highly resistant to brittle fracture at extremely low temperatures.

The side wall 12 is actually a three-part combination including 1) an inner concrete core wall 36, (2) a wall liner 38 and (3) a layer of prestressing tendons 42. The core wall 36 is the main vertical structural member for containing the liquified gas within the tank 10 and on the outer surface of the core wall 36 a liner 38 is provided, and anchored to the outer face of the core wall with suitable embedded anchor studs 36a. The core wall is prestressed vertically by pretensioned or posttensioned prestressing reinforcement depending on whether the core wall is made of precast sections or cast in situ. The anchor studs 36a insure that a portion of the vertical prestressing force provided by tendons 42a pass to the liner 36 thus placing it in vertical compression. .The prestressing tendons 42 are wound about the core Wall 12 and the liner 38, and stressed to provide the desired circumferential compression in the wall 36 and the liner 38, and then covered with a protective coating of suitable material, usually mortar, in the form of a covercoat 44. This type of wall construction is set forth and discussed in detail in US. Patent No. 3,092,933, issued June 11, 1963 to Closner et al.

The liner 38 acts as a liquid and vapor proof barrier and may be made of carbon steel since it is composite with the prestressed core wall 36 and in compression, both vertically and circumferentially; therefore, it is not subject to brittle fracture by the low temperature.

The wall liner 38 is advantageously connected to the floor liner 34 so that a continuous moisture and vapor proof barrier is provided for containing the liquified gas. Between the floor liner 34 and the wall liner 38 an expansion loop 40 of stainless steel is also included to compensate for the expansion and contraction of the floor liner 34.

Passing under the core wall 36 and connected to the expansion loop 48 and the liner 38 is a plate member 39 which is also preferably made of stainless steel or 9% nickel steel since it is at least partially exposed to the low temperature of the liquified gas.

Brittle fracture may occur with steel when the following conditions are present: (1) a temperature that is below the nil ductility temperature of the steel; (2) the existence of a notch connection or construction such as with a welded joint; and (3) a developed tensile strength above approximately 10% of the ultimate strength of carbon steel.

As stated previously, the liner 38 is advantageously kept in compression and therefore not subject to brittle fracture.

In FIGURE 3a a construction is shown wherein carbon steel may be utilized for a substantial portion of the floor liner. In that construction the fioor liner 34a is separated from, but connected to, the expansion loop 40 by a flat ring 41 which is advantageously made of 9% nickel steel or stainless steel. The width of the ring 41 need only be sutficient to insure that any tensile stress developed at the joint between loop 40 and ring 41 is absorbed by the ring. Accordingly, with a tank having a diameter of 250 feet, a ring 41 having a width of about twelve inches is suflicient to absorb any such stresses.

Since the liner 34a is free to move relative to the protective layer 32, and since the maximum contraction of the liner 34a occurs when the storage container is cooled down to near its operating temperature with vapor and no liquid load is present on the liner to restrict movement, the liner working with the expansion loop 40 is free to contract without significant restraint and, thus, is not subjected to a tensile stress about 10% of its ultimate strength.

At the top of the core wall 36 a sliding joint 46 comprised of joint plates 48 provides a seat for the dome 14. The dome 14 is made of reinforced concrete and includes a haunch portion 58 at its peripheral edge and a central roof portion 52. The design and construction of domes having haunch and roof portions are well known to those skilled in the art. The dome must be designed and constructed to have sufficient strength to withstand the internal pressures which build up within the tank due to boil off, i.e. evaporation of the liquified gas stored therein, as well as the strength to take care of external loads.

Boil off occurs at the upper surface of the liquid where it is exposed to the vapor in the tank. It is kept to a minimum by means of insulation provided around the tank. If there is a too rapid boil off of the liquified gas into the gaseous state, perhaps caused by an external fire, the internal pressure against the dome 14 will be very large. Pressure relief devices provide for venting such too rapid boil off to the atmosphere.

Heretofore, the basic means of handling normal internal pressure has been to increase the strength of the inner dome. The problem with increasing the strength of the inner dome is that in order to do this the dead load and reinforcement of it must also be increased and then added strength must be built into it to take care of the dead load. As a result, an unwieldy and very expensive inner dome structure must result if proper precaution is to be taken against excessive internal pressure build-up.

Surrounding the core wall 36, the haunch 5t) and the roof portion 52 is a large mass of a thermal insulation T. A suitable insulation material is of the granular type, such as perlite. Other insulation materials which may be used are vermiculite, chopped or shredded polystyrene foam, polyurethane foam (which may be foamed in place if desired) and glass wool.

In order to maintain the insulation T in place, an outer shell is provided around the side wall 12 and the dome 14 but spaced therefrom. Around the periphery of the tank a compression ring 54 to withstand the pressure on the interior of the roof 58 is located and supported on members 56 which are mounted on the haunch 58. Attached to the compression ring 54 is an outer roof 58. Supporting the outer roof 58 and spacing it from the inner roof portion 52 are a series of spacers 62. concentrically located outside of the core wall 36 is a shell wall 64 which is made of steel. The shell wall 64 is also connected to the compression ring 54 and with the outer roof 58 combines to form the outer shell of the tank 10. If desired, the compression ring 54 may be placed at the top of the shell wall 64. j

The insulation material T is contained in the space formed by the core wall 36, the outer wall 64, the roof portion 52 and the outer steel roof 58.

An anchor 66 is provided in the foundation 20 to hold the outer shell wall 64 in place.

The outer shell wall 64 is connected to the tank floor system 34 by means of expansion loops 40 and 40a.

The outer roof 58 and the outer shell wall 64 may be made of any suitable steel sheet material. No exceptional thermal precautions must necessarily be taken with the outer wall and roof since they are not in contact with the cold liquified gas. The compression ring 54 is preferably made of carbon steel for economy reasons since it is in a warm environment and not subject to brittle fracture. The supports 56 are preferably made of a brittle fracture resistant material, such as 9% nickel steel since they are in contact with the haunch 50 and may be subjected to extremely low temperatures at the point of contact with the haunch. The members 56 are advantageously arranged in alternate bracing configurations and angles are satisfactory for this purpose.

The roof supports 60 which are also in contact with a concrete surface which may also be subjected to low temperatures. Rather than use expensive 9% nickel steel for this purpose, reinforced fiber pipe such as fiber glass reinforced polyester is suitable. In addition to the polyester pipe, cement asbestos or plywood supports may also be used. The roof supports while having good support strength also advantageously have sufiicient flexibility to permit movement of the steel roof. To permit the roof shell 58 to expand and contract as necessary, the support members 60 are preferably not connected to the roof so that the support members merely act as unconnected supports for the roof system.

In order to blanket the insulation material T with a gas free of moisture, it has been found that the boil off of gas from the liquified gas contained within the tank may be used for this purpose. To accomplish this, a series of openings 68 may be provided in the concrete roof portion 52. These openings 68 are fitted with goosenecks 70 which permit the boil off gas to penetrate into the insulation T without the problem of the insulation filling the goosenecks and eventually the openings 68. As the internal pressure builds up within the tank T the pressure on each side of the roof portion 52 will be balanced since the opening 68 and the gooseneck 70 permit the gas within the tank to flow into the insulation space and through the insulation to the inner side of the outer roof 58.

As shown in FIGURES 4, 5, 6 and 7 in particular, a more detailed arrangement for equalizing the pressure within the tank is illustrated.

The pressure equalizing system 16 provides a long gas travel of the boil off gas and resulting positive heating of the gas in the ambient temperature without the necessity of elaborate pumping or gas moving equipment.

The pressure equalizing system 16 includes a tank gas outlet pipe 74 which extends from the interior of the tank through an opening 76 in the concrete roof portion 52. A plate 78 is provided about the outlet pipe 74 and connected to the roof portion 52 and the pipe 74 so that a positive seal is made at the outer extent of the opening 76. The pipe 74 extends upwardly through the insulation T and through an opening 79 in the shell roof portion 58. A seal member 80 is provided about the Opening 79 and connected to the roof portion 58 and the outlet pipe 74. Also surrounding the pipe 74 is a fiber glass sleeve 82. Contained Within the fiber glass sleeve 82 is a stainless steel bellows 84 which is connected to the outlet pipe 74 and the steel outer shell 58. The purpose of the stainless steel bellows 84 is to provide for any expansion or deflection of the outlet pipe 74 and still maintain the seal in the space filled with the insulation T.

After the outlet pipe 74 passes into the ambient atmosphere beyond the outer shell 58 it is provided with an outlet valve 86 which is preferably made of stainless steel.

The outlet valve 86 acts to control the flow of gas from the interior of the tank 10 and through the pressure equalizing system. Connected to the valve 86 is a gas travel pipe 88 which is of an arcuate shape and is positioned so that it is in the ambient atmosphere and on the outer surface of the shell 58. Supporting the travel pipe 88 are a series of supports 90 which are mounted on the shell 58. At various locations along the travel pipe 88 of a series of return valves 92 are provided. The return valves 92 are also preferably of stainless steel and are connected to return pipe conduits 94. The return pipe conduits 94 communicate with an opening 96 in the outer shell 58.

By the time the boil off gas has traveled through the extent of the pressure equalizing system 16 and returned by means of the return pipe 94 to the space inside the shell 58 which is filled with the insulation T, it has been heated up to a temperature approaching that of ambient outside the tank. The heating up to ambient temperature is advantageous since the outer steel shell 58 is made of carbon steel and would be subjected to possible brittle fracture if it had impinged upon it cold boil off gas. However, by means of the arrangement shown in FIG- URES 4, 5, 6 and 7 in particular, the gas deposited through the return pipe 94 is of high enough temperature so that the possibility of brittle fracture is substantially eliminated.

For economy sake the shell roof 58 is of relatively thin steel sheet and it is exposed to the elements of nature such as wind, rain and perhaps snow. Therefore, it is preferably stabilized against external forces. As shown in FIGURES 8, 9 and 10 a bracing system may be incorporated for use in conjunction with the supports 60 to stabilize the shell roof 58 against the external forces. The bracing system is preferably positioned between the upper portion of the supports 60 and the shell roof 58.

To accommodate the bracing system, cut outs 98 are provided in the upper end of the supports 60. A series of angles 102 are concentrically located in the cut outs 98 in concurrent fashion beneath the roof 58. The individual angles 102, which in the illustration of FIGURE 8 are chords, are joined together to make several complete concentric units. The angles 104 comprise a series of radial aligned supports connected at 100 to the angles 102. The angles 104 are joined at their center intersection by a splice plate 106. On top of the

angles

102 and 104 the shell roof 58 is placed.

Since the shell roof 58 rests on and may be connected to the grid system formed by the

angles

102 and 104 or restricted at points of support so that it is free to move radially with respect to the center of the spherical segment but not circumferentially, it is braced against external loads such as wind and the like. By limiting the size of the cut outs 98 there is a restriction to the tangential movement of the roof 58 and thus the entire dome shell is strengthened against the external forces. The amount of restraint exercised by the grid system is determined by the limits of the vertical supports and the size of the cut outs 98.

As shown in FIGURE 3, a gas pressure controller 108 may be used if desired to maintain the desired pressure in the insulation space. If the controller 108 is utilized, the openings and communication between the interior of the tank and the insulation space may be eliminated. In order to substantially equalize the pressure between the tank and insulation space, the valve 110 is controlled by pressure controller 110a actuated by pressure sensing means 11011 which is connected to the tank proper.

With the controller 108 a valve 110 is provided to shut off the flow of gas into the space through the pipe 112 which is connected to an opening 114 in the wall 64.

When the controller 108 is used, an inert gas, such as nitrogen, is pumped into the insulation space to blanket the insulation. As pressure builds within the tank and against the inner dome 52, the controller increases the pressure against the outer surface of the dome 52 to counteract the buildup within the tank. In such a system the pressures on each side of the inner dome 52 are kept substantially equal.

By substantially equal is meant that the pressure differential in favor of the internal pressure is preferably not substantially greater than the dead weight of the inner dome 52. Normally, this differential would be expected to be about 2-3 inches of water.

It will be appreciated that the present invention provides means for maintaining equalized pressure on the inner dOrne roof of a cryogenic storage structure. Such a system is also desirable in respect to the sidewall system. However, such control of sidewall gas or vapor pressure will not normally come into consideration as the liquid level of the liquified gas will generally rise to the upper portion of the sidewall.

It is claimed:

1. A structure for storing liquified gases maintained at low temperature comprising a floor, a side wall and a roof system, said side wall including an interior prestressed concrete core wall connected to the floor and a moisture and vapor proof barrier substantially coextensive therewith, said roof system including a reinforced inner roof connected to and supported by said core wall and a second outer roof spaced from the inner roof to form an insulation space, said outer roof having compression means connected therewith said space containing insulation material of the type through which gas may pass, and gas connecting means between the interior of the structure and the insulation space whereby gas pressure on each side of the inner roof may be substantially equal.

2. A structure as defined in claim 1 and further including an outer shell wall spaced from the concrete core wall, said shell wall joined to the outer roof and insulation placed in the space formed by the inner core wall and the outer shell wall.

3. A structure as defined in claim 1 and further including spacer means for maintaining the outer roof spaced from the inner roof.

4. A structure as defined in claim l and further including spacer and support means for maintaining the outer roof spaced from the inner roof and supporting the outer roof against external forces.

5. A structure as defined in claim 4 wherein the spacer and support means includes a grid system of radially aligned members and concentric members underlying the inner roof, said grid system supported in place by spacer members mounted on the inner roof.

6. A structure as defined in claim 1 wherein said floor comprises an inner sheet member resistant to brittle fracture at low temperatures, an insulation layer supporting said sheet member, a moisture barrier underlying said insulation, and a reinforced support member beneath the moisture barrier.

7. A structure for storing liquified gases maintained at low temperatures comprising a floor, a side wall and a roof system; said side wall including an interior prestressed concrete core wall having a moisture and vapor proof barrier substantially coextensive therewith and an outer shell wall spaced from the concrete wall; said roof system including a reinforced inner roof connected to and supported by said core wall and a second outer roof supported and spaced from the inner roof by a grid system of radially aligned members and concentric members underlying the inner roof with said grid system supported in place by spacer members mounted on the inner roof; said floor comprising an inner sheet member resistant to brittle fracture at low temperatures, an insulation layer supporting said sheet member, a moisture barrier underlying said insulation, and a reinforced support member beneath the moisture barrier; insulation material of the type through which gas may pass in the space formed by the inner and outer walls and the inner and outer roofs; and gas connecting means between the interior of the structure and the insulation space whereby gas pressure on each side of the inner roof may be substantially equal.

8. A structure for storing liquified gases maintained at low temperature comprising a floor, a side wall and a roof system, said side wall including an interior prestressed concrete core wall connected to the floor and a moisture and vapor proof barrier substantially coextensive therewith, said roof system including a reinforced inner roof connected to said core wall and a second outer roof spaced from the inner roof to form an insulation space, said outer roof having compression means connected therewith, said space containing insulation material of the type through which gas may pass, and gas pressure control means for maintaining a gas pressure in the insulation space substantially equal to the gas pressure within the tank and against the inner roof.

9. A structure as defined in claim l, and further including a mositure and vapor floor barrier substantially coextensive with the floor and placed thereon, said floor barrier having an inner portion of carbon steel and a peripheral portion made of a brittle fracture resistant steel, and an expansion loop connecting said peripheral portion to the wall barrier.

10. A structure as defined in claim 8 and-further including a moisture and vapor floor barrier substantially coextensive with the floor and placed thereon, said floor barrier having an inner portion of carbon steel and a peripheral portion made of a brittle fracture resistant steel, and an expansion loop connecting said peripheral portion to the wall barrier.

11. A structure for storing liquified gases maintained at low temperatures comprising a floor, a sidewall and a roof system, said sidewall including an interior prestressed concrete core wall and a steel moisture and vapor proof barrier substantially coextensive therewith and maintained in a state of compression in two directions by prestressing tendons wrapped about the core wall to prestress said wall horizontally and vertically by vertical prestressing tendons in the core wall, a steel moisture and vapor proof floor barrier substantially coextensive with the floor and free to contract without restraint to prevent brittle fracture at low temperatures, and a brittle fracture resistant expansion loop connecting said barriers.

12. A structure for storing liquified gases maintained at low temperatures comprising a floor, a sidewall and a roof system, said sidewall including an interior prestressed concrete core wall and a steel moisture and vapor proof barrier substantially coextensive therewith and maintained in a state of compression in two directions by prestressing tendons wrapped about the core wall to prestress said wall horizontally and vertically by vertical prestressing tendons in the core wall and a steel moisture and vapor proof floor barrier substantially coextensive with the floor, the inner portion of said floor barrier being brittle fracture resistant steel, and a brittle fracture resistant expansion loop connecting said barriers.

References Cited UNITED STATES PATENTS 3,406,526 10/1968 Lusk 6250 1,876,047 9/1932 Edmonds 6250 2,520,883 8/1950 Kornemann et al. 6245 2,777,295 1/1957 Bliss et a1 6245 3,151,416 10/1964 Eakin et al. 6245 3,196,622 7/1965 Smith et al. 6245 3,326,011 6/1967 Sparling 6245 LLOYD L. KING, Primary Examiner US. Cl. X.R. 61-.5; 62260