U.S. patent application number 11/818710 was filed with the patent office on 2009-01-29 for severe storm shelter.
Invention is credited to Henry B. Crichlow.
Application Number | 20090025307 11/818710 |
Document ID | / |
Family ID | 40294014 |
Filed Date | 2009-01-29 |
United States Patent
Application |
20090025307 |
Kind Code |
A1 |
Crichlow; Henry B. |
January 29, 2009 |
Severe storm shelter
Abstract
This invention is related to the development of and installation
of a shelter structure suitable for severe storm safety. The
shelter is a composite structure constructed of precast and post
tensioned structural elements or shells, the tendon post-tensioning
is designed to maximize strength and minimize material costs by
implementing a specific geometric shape, the inverted catenary, in
three dimensions. The shelter is easily constructed and then
assembled from readily available structural materials that are both
economic and mechanically efficient.
Inventors: |
Crichlow; Henry B.; (Norman,
OK) |
Correspondence
Address: |
HENRY CRICHLOW
330 W. GRAY ST., SUITE 504
NORMAN
OK
73069
US
|
Family ID: |
40294014 |
Appl. No.: |
11/818710 |
Filed: |
June 15, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60804840 |
Jun 15, 2006 |
|
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|
Current U.S.
Class: |
52/81.1 ;
52/741.1 |
Current CPC
Class: |
Y02A 50/00 20180101;
Y02A 50/14 20180101; E04B 2001/3276 20130101; E04B 1/3211 20130101;
E04H 9/14 20130101 |
Class at
Publication: |
52/81.1 ;
52/741.1 |
International
Class: |
E04H 9/14 20060101
E04H009/14; E04B 1/348 20060101 E04B001/348 |
Claims
1. An above ground severe storm shelter having a
geometrically-shaped structure, comprising: a plurality of
structural shells connected to each other; a plurality of support
anchors; a plurality of post tensioning tendons; an access assembly
for entry and exit; a protective structural skirt surrounding base
of the shelter; a means for anchoring the structural shells; a
means for connecting the structural shells; and a means for post
tensioning the structural shells.
2. The severe storm shelter of claim 1, wherein the shelter is made
in the form of a catenoid or a three dimensional volume equivalent
to a surface of revolution of an inverted catenary.
3. The severe storm shelter of claim 1, wherein the structural
shell is in the shape of an inverted catenary comprising: a surface
with equation of the curve y=a cosh (kx) where "a" and "k" are
constants and, y is the vertical dimension and, x is the horizontal
direction.
4. The severe storm shelter of claim 3, wherein the structural
shell being a catenoid comprises a minimum surface area.
5. The severe storm shelter of claim 1, wherein the one or more
structural shells have openings to allow a person to enter the
severe storm shelter.
6. The severe storm shelter of claim 1, wherein the structural
shells are post tensioned by use of one or more circumferentially
located high strength tendons which are implemented in a bonded
mode or unbonded mode.
7. The severe storm shelter of claim 1, wherein the post tensioned
tendons are strands of high tensile strength steel wire conforming
to the requirements of ASTM standards.
8. The severe storm shelter of claim 1, wherein the post tensioned
tendons are high strength thread bar conforming to the requirements
of ASTM Standards.
9. The severe storm shelter of claim 1, further comprising a
waterproof seal between contiguous shells to waterproof the bond
between the structural shells.
10. The severe storm shelter of claim 1, wherein walls and roof of
the severe storm shelter form a continuously reinforced integral
unit.
11. The severe storm shelter of claim 1, wherein the means for
connecting the consecutive structural shells comprises bolts
passing through flanges on these shells.
12. The severe storm shelter of claim 6, wherein the means for post
tensioning the structural shells comprises applying tension to the
tendon by stretching the tendon to a prescribed limit of
stress.
13. The severe storm shelter of claim 6, wherein the post
tensioning stress on the tendons is at least 30 percent of the
maximum allowable stress of the tendon.
14. The severe storm shelter of claim 1, wherein the means for
anchoring the structural shells define a cylindrical space for
receiving a pipe.
15. The severe storm shelter of claim 14, wherein the means for
anchoring the structural shells comprises a plurality of pipes
firmly anchored by cement in ground to a specified depth.
16. The severe storm shelter of claim 1, wherein the means for
fastening the structural shell to the support anchoring system
comprises at least one bolt.
17. The severe storm shelter of claim 1, wherein compressive and
lateral forces on the structure are balanced in the catenary
structural nature of the structural shells providing maximum
stability.
18. The severe storm shelter of claim 1, wherein the means for
connecting adjacent shells includes a groove and tongue system.
19. The severe storm shelter of claim 1, wherein the means for
connecting adjacent shells includes an interlocking groove and a
recesses system.
20. The severe storm shelter of claim 1, wherein the means for
connecting adjacent structural shells includes a male pin and a
corresponding female socket to allow firm connection of shells.
21. The severe storm shelter of claim 1, wherein the access
assembly structure comprises layers of reinforced material and
steel or other highly impact resistant protective material.
22. The severe storm shelter of claim 1, wherein said access
assembly is configured to allow the assembly to slide laterally
across the face of the shell wall.
23. The severe storm shelter of claim 1, wherein said access
assembly has a multi-point locking mechanism securing the door in a
closed position.
24. The severe storm shelter of claim 1, wherein the shelter is
constructed outdoors.
25. The severe storm shelter of claim 1, wherein the shelter is
constructed indoors in an enclosed space.
26. The severe storm shelter of claim 1, wherein the primary
support anchor is a high-strength circular or polygonal shaped
structural element at the top of the shells viably disposed and
connected to provide integral strength for the connected shell
structure.
27. The severe storm shelter of claim 26, wherein the structural
shells are securely bolted to the high-strength circular or
polygonal shaped element at the top by fasteners.
28. The severe storm shelter of claim 1, wherein the structural
shell is a composite structural member comprising: a concrete
matrix with reinforcing rods called rebar, the shell being formed
by pouring said concrete in an uncured state into a hollow form
containing the reinforcing metal disposed to form a reinforcing
grid and allowing the concrete to harden.
29. The severe storm shelter of claim 1, wherein the structural
shell is a composite structural member comprising a concrete matrix
with reinforcing rods called rebar wherein the shell element
concrete matrix is at least 3 inches thick.
30. The severe storm shelter of claim 28, wherein the structural
shells are constructed with the rebar rods disposed in a grid
network with a maximum distance between rod elements in the
horizontal and vertical directions is less than 12 inches.
31. The severe storm shelter of claim 28, wherein the structural
shells are manufactured at remote locations and transported to the
assembly site.
32. The severe storm shelter of claim 1, wherein the structural
shells are constructed of a reinforced polymer laminate cured to a
prescribed hardness.
33. The severe storm shelter of claim 1, wherein the structural
shells are constructed of a reinforced frame covered with an impact
resistant material like Kevlar.TM..
34. The severe storm shelter of claim 1, wherein the shelter is
provided with a seating arrangement for the occupants comprises a
plurality of seats integrally juxtaposed to the periphery of the
base of the shelter.
35. A method of constructing the severe storm structure of claim 1
comprising the steps of: installing the primary and secondary
anchors in holes disposed at the correct location and depth in the
substratum by filling the holes with concrete and allowing the
concrete to harden thus fixing the anchors in place; mounting and
attaching each previously manufactured structural shell to its
respective secondary anchor with the required fasteners; aligning
each shell in sequence with its respective shell partner using the
tongue and groove system and connecting the shells with fasteners
and emplacing a chemical waterproof bond between respective shells;
attaching the tops of the shells to either the primary central
anchor in one embodiment or in the other embodiment or the central
circular structural element with fasteners; inserting and aligning
the post-tensioning tendons through each of the shell elements;
tightening all fasteners on the structural shells and the anchoring
systems; post tensioning the tendons circumferentially by using the
post-tensioning mechanism which imparts a stress on the cable to
the required design limit; locking the tendons in place; installing
a floor and seating arrangements for the occupants of the
structure.
36. The severe storm shelter of claim 3, wherein the shape of the
structural shell and shelter cross-section is modified by changing
the values of the constants "a" and "k" in the catenary
equation.
37. The severe storm shelter of claim 1, wherein the number of
structural shells is at least 2.
38. The severe storm shelter of claim 1, wherein the means for
connecting and reinforcing adjacent shells includes a keystone
system in which the outer shell perimeter is larger than the inside
shell perimeter.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority from provisional
application 60/804,840 dated Jun. 15, 2006 by Dr. Henry Crichlow,
titled "Severe Storm Shelter".
BACKGROUND OF THE INVENTION
[0002] 1. Field of The Invention
[0003] The present invention relates in general to the field of
shelters, more specifically, to above ground shelters for
hurricanes, tornadoes, and other high wind events that occur in
severe storms.
[0004] 2. Prior Art
[0005] U.S. Pat. No. 4,126,972 approaches the problem by showing an
in-house structure built on a reinforced concrete base. It is a
massive building like structure which looks like a miniature house
complete with toilet facilities. The majority of the sidewalls are
covered with sheet metal of sufficient strength and rigidity to
withstand the tornado force impacts.
[0006] U.S. Pat. No. 4,787,1 81 is a complex of two shell units
which are connected together to provide the support for the
structure.
[0007] Patent D466,220 describes a design patent of a storm shelter
in which a hemispherical "flying-saucer-like" structure with an
opening is utilized.
[0008] U.S. Pat. No. 4,615,158 is specially designed for mobile
homes. It is a buried structure connected to the mobile home with a
slanted slide to allow the occupants to enter via a stairway
directly from the mobile home into the shelter.
[0009] U.S. Pat. No. 5,794,389 is an elaborate system which
completely encapsulates the home and allows it to be raised and
lowered by a specialized lifting mechanism. This mechanism is a
scissors like system connected to a moveable platform on which the
house rests.
[0010] U.S. Pat. No. 5,408,793 describes a process for constructing
a prestressed composite structure which utilizes a prestressing of
circumferentially wrapped material around a frame. The patent
teaches a dome structure with a membrane sandwiched between layers
of rigidifying material such as "shotcrete" or reinforced composite
which also serve to embed radial wires and circumferential
tensioned prestressing. Various types of circumferential tensioned
prestressing can be applied to minimize bursting stresses and can
consist of steel wire as well as fiber or steel-reinforced tape.
Further layers of rigidifying material can then be applied over the
circumferential prestressing as a final protection and cover. The
radial wires can contain spacers or hooks to preclude the
circumferential prestressing from riding up on the structure.
[0011] U.S. Pat. Nos. 5,953,866 and 6,393,776 show reinforced
rectangular modular systems with composite walls.
[0012] U.S. Pat. No. 6,1 61,345 describes a rectangular
parallelepiped with a door set up so that it can be slid
horizontally to enter and exit the structure.
[0013] Commercial shelters that are available are generally
provided in two categories. Underground i.e buried or aboveground.
Underground structures are simple geometric shapes either
rectangular or cylindrical usually made of steel, reinforced
concrete or plastic. These structures are placed in a large
excavation made in the ground and then covered by the excavated
earth. The structures are then fitted with a doorway and a ladder
or stairway to enter and egress the structure. Though efficient as
physical shelters public acceptance has been limited because of
aesthetic concerns and primitive fear in some individuals when one
has to physically leave your home and run through the wind, rain,
hail and lightning of the oncoming tornado to an outside shelter
and enter a hole in the ground.
[0014] Above ground shelters like the "Oz", Ref. 1, is massive
monolithic cement structure poured around forms placed on location.
A 5.times.5 foot structure weighs 21 tons or 42,000 lbs, and costs
about $8,000, an 8.times.5 structure weighs 60,000 pounds and cost
close to $10,000.
[0015] After careful consideration of the above noted problems and
prior art solutions, the inventor has herein a novel and improved
method and system that allows the design, manufacture and
installation of a more effective and severe storm shelter with
minimal costs.
SUMMARY OF THE INVENTION
[0016] An objective of this invention is to provide a safe severe
storm protective system that is easily constructed and that meets
the requirements for protection from impact loads, wind loads and
weather phenomena found in a large tornado. The US Federal
Emergency Management Agency, FEMA, Ref. 2 details the minimum
requirements for such a structure.
[0017] Another important objective is to provide the type of
structure that is user friendly and inviting to the household
occupants who can be in a state of near hysteria when the tornado
approaches. In such a structure, being close to the house, the
occupants of the house will feel comfortable and confident and
welcome in using the structure in and under all conditions as
opposed to buried outdoor structures which can be uninviting to the
individual in times of stress after the tornado alarm has
sounded.
[0018] In one embodiment the structure has a structurally secure
primary anchor which is augmented by multiple secondary anchors and
supported by multiple interlocking post tensioned structural
shells. In another embodiment, there is no primary anchor and the
shells are held together at the top by a massive circular steel
member in addition to the secondary anchors. The structure is
completed with an easily opened lockable door which is preferably
balanced such that minimum effort is required to open it. This door
is solidly constructed of steel or built from laminated impact
resistant material like Kevlar.
DESCRIPTION OF THE INVENTION
[0019] These embodiments of the invention provide a safe room-like
structure which is easily reached by all occupants of the house. A
preferred location is in or near an outside patio which has a
concrete base. In other embodiments it can be installed inside a
garage or a large room.
[0020] The invention consists of a specialized dome shaped
structure comprising of connected structural shells made of
post-tensioned structural shells which are viably and suitably
anchored to the substratum. The surface shape of the structure is
that of an inverted catenary which is rotated in three
dimensions.
[0021] In one embodiment the shells are made of pre-stressed
concrete panels connected to the each other and suitably reinforced
by steel rebar and further strengthened with a plurality of
circumferential tendons made of steel cables which are post
tensioned or stressed to enhance the structural integrity of the
system. The shell elements form both the sides and the roof in a
form that resembles a dome or Quonset with no clear demarcation of
side and roof. The floor of the structure can be a cement pad or
any suitable substratum since it has no load bearing function. In
one embodiment, the bottom edge of the structure in contact with
the surface or ground, is buried below grade a few inches and a
small skirt is constructed to prevent winds from getting under and
creating an overturning load on the structure. In another
embodiment, a concrete skirt can be poured around the periphery to
cover the bottom of the structure and form a skirt a few inched
high. This also prevents winds from entering the structure from the
ground/shell contact.
[0022] Another innovation of this invention is use of existing
technology for structural concrete as a basic element in the design
and construction of the system. This embodiment provides for an
economical and ease of design and rapid deployment in the field. A
further embodiment which uses pre-stressed and post tensioned
concrete elements allows for a variety of design while still
maintaining simplicity and the same impact and structural integrity
of the shelter. The manufacturing process enables the structure to
be fabricated offsite and then to be assembled economically at the
requisite location.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 shows the overall view of one embodiment of the
invention. The structural shells attached by fasteners and are
interconnected and form a symmetric dome-like structure. In one
embodiment there is implemented a major primary anchor at the
center of the dome and several secondary anchors at the periphery.
These anchors are suitably buried in the substratum and connected
by steel connectors to the reinforced structural shell elements. In
addition circumferential steel tendons connect shell elements and
apply a post tensioning stress to the structure as discussed later
in the application.
[0024] FIG. 2a shows a cross-sectional view of a structural shell
which is connected both to the central primary anchor structure and
a peripheral secondary anchor. Both anchors are suitably embedded
in a cement substructure for added strength.
[0025] FIG. 2b shows a cross-sectional view of one embodiment with
no central support. The system is designed with a robust circular
structural support element designed to withstand the lateral loads.
The shells are secured to this central structural element.
[0026] FIG. 3 shows a top view of the structure showing 8 shell
elements.
[0027] FIG. 4 shows a side view of one embodiment wherein the
shells are expanded to show the post-tensioning tendons and shell
connectors to the secondary anchors.
[0028] FIG. 5a, 5b, 5c and FIG. 5d show embodiments of the
interlocking of the shells where they are joined to provide
strength and a solid locking mechanism.
[0029] FIG. 6 shows one embodiment of the shell construction where
the rebar system is shown and the brackets, male pins and female
sockets necessary for alignment of the shells.
[0030] FIG. 7a shows a vertical cross-section of one embodiment
with a circular contoured bench seating arrangement for several
persons inside the shelter.
[0031] FIG. 7b shows a vertical cross-section of one embodiment of
the post-tensioning tendons attached and held in place on the
structural shell.
[0032] FIG. 7c shows a vertical cross-section of one embodiment of
the shell with a circular tendon implemented inside the shell
concrete structure. In this embodiment the tendon is pulled through
each shell during assembly on location before being tensioned.
[0033] FIGS. 8a, 8b show a top view schematic of the flow of
tornadic winds and debris around the invention as compared to flow
around a typical block shaped shelter showing the beneficial effect
of reduced drag flow around the new invention.
[0034] FIGS. 9a, 9b show a side view schematic of the flow of
tornadic winds and debris around the invention as compared to flow
around a typical block shaped shelter showing the beneficial effect
of reduced drag flow around the new invention.
[0035] FIG. 10 shows an isometric rendition of one embodiment of a
completed severe storm structure with 2 circumferential tendons
implemented for post tensioning.
BACKGROUND OF THE INVENTION
[0036] The invention consists of a specialized dome shaped
structure comprising of connected structural shells made of
post-tensioned structural elements which are viably and suitably
anchored to the substratum. The surface shape of the structure is
that of an inverted catenary which is rotated in three dimensions.
The catenary is mathematically described in two dimensions by the
following equation:
Y=a cosh (k x) (1) [0037] where: [0038] Y is the vertical distance
[0039] x is the horizontal distance, [0040] k, a are constants.
[0041] The catenary has several attributes that are effectively
incorporated into the design and construction of the severe storm
shelter. These attributes are discussed in the specifications.
[0042] The wind research laboratory at Texas Tech University, Ref.
3, states that a safety shelter must be able to resist the forces
that extreme winds or interacting structural components place upon
it. The safety shelter should be designed to withstand wind speeds
of 250 mph, which accounts for virtually all tornadoes, which have
occurred in the US
[0043] Wind load calculations are based on some variation of the
following computation model:
F=Q.sub.z.times.G.sub.h.times.C.sub.f.times.A.sub.f (2) [0044]
Where: [0045] F is Force generated by the wind [0046] Q.sub.z is
wind velocity pressure [0047] G.sub.h is Gust response factor
[0048] C.sub.f is Force Coefficient [0049] A.sub.f is area exposed
to wind force and the wind velocity pressure is calculated as
follows:
[0049] Q.sub.z=0.00256.times.K.sub.z.times.[I.times.V].sup.2 (3)
[0050] where: [0051] Kz is the exposure coefficient [0052] I is the
importance factor [0053] V is the wind velocity.
[0054] By calculating the wind forces "F" above, acting on the
walls of the shelter the unit is scaled to keep the stress loads
within the limits required by statute and justified by good
engineering practice. In addition the embodiments indicated herein
have utilized the available high strength materials available to
the industry to optimize design and efficiency of the system. The
invention discussed herein in addition to strength of materials
uses specific engineering design to minimize wind load and drag
forces on the structure to lower the forces acting on the
structure. The selected shape, which is an inverted catenary in
cross-section, provides for minimal loading of the structure by
allowing the high velocity flow to careen off the structure because
of the streamlined form of the new invention and the lack of sharp
edges open to the wind. In addition, a catenary structure is
self-supporting, for example in a structure built of blocks,
because every element of the catenary is held in place by the
neighboring blocks, the blocks don't slide off each other, even at
the top, because the forces between the blocks are along the curve
of the catenary itself. The blocks at the bottom are more vertical
because they have more weight to support from the blocks above. The
structure in this invention has its own intrinsic stability.
[0055] The combination of forces provided by inches of 5,000-psi
reinforced concrete shells and the post tensioning of the structure
prevents any penetration by flying missiles in excess of a hundred
miles per hour. In addition the catenary structure provides the
smallest cross section surface area for a given volume of cement
material so that the subject structure provides a maximum strength
for a minimal amount of construction material. Further more
compared to other existing block shaped severe storm shelter
structures which provide a rectangular cross-section to wind flow
and missile contact, the subject invention provides a streamlined
cross-section and a continuously curved cross-section to wind flow
and only a fractional component of the wind force acts on the
structure proposed in this invention. Missile and wind impact is
minimized in the case of the new invention and in the existing
block structures of the prior art the missile and wind impact is
maximized.
[0056] Windborne debris and falling objects are two major risks to
people in severe wind driven storms. It is generally accepted that
tornado-generated missiles as flying debris, create the greatest
threat to occupants of homes that are struck by severe winds. These
missiles, very often, perforate conventional walls and roofs. A
2.times.4 beam flying at 200 mph can produce deadly consequences on
contact with an unprotected human being. In order to provide a high
degree of occupant protection, the shelter must be designed to
prevent perforation by missiles on all surfaces, walls, roof, and
door. Windborne debris can be adequately described by their mass,
shape, impact velocity, angle of impact and motion at impact, i.e
linear or tumbling. Impact momentum calculated from Equation 4 and
impact energy compute from Equation 5 provide reasonable estimated
of the momentum and energy effects of windborne debris that is
striking perpendicular to the surface of the structure.
I.sub.m=(W/g) (4) [0057] where [0058] I.sub.m is impact momentum
[0059] W is the weight of debris [0060] g is the acceleration of
gravity [0061] V is the impact velocity
And
[0062] I.sub.e=1/2(W/g)(V.sup.2) (5) [0063] where [0064] I.sub.e is
impact energy [0065] W is the weight of debris [0066] g is the
acceleration of gravity [0067] V is the impact velocity
[0068] They also provide reasonable information estimates when
there is no rotation of the flying body. For off-angle impacts of
windborne debris, the normal component of the impact momentum and
energy component must be replaced by an effective velocity which
includes the cosine of the angle of impact as a product. Since the
cosine is maximum at zero degrees with a value of 1.00 and minimum
at 90 degrees with a value of 0.00, the larger the angle the
smaller the effective velocity and consequently the smaller the
forces on impact. Additionally, for slender rigid-body missiles
such as wood structural elements, pipes or rods with length to
diameter ratio greater than 4.0, research by Pietras in Ref. 4, has
shown that the missile begins to rotate on impact and that the
impact force drops off much more rapidly than the cosine formula
would predict.
[0069] The impact of windborne debris can produce extremely high
forces on a structure in a very short time. Dynamics teaches us
that the magnitude of the force depends on the type of impact,
whether it is elastic or inelastic and the duration of the
impact.
[0070] The subject invention is designed such that a perpendicular
hit is minimized because of the shape of the structure and that
there is maximum rotation of the object that is hitting the side of
the structure since flow around the structure tends to provide a
rotation of the trailing edge of the projectile at the moment of
impact of the leading edge of the debris.
[0071] Adequate strength, impact resistance and penetration
resistance are the key to satisfactory structural performance of
the shelter. The roof must be securely anchored to the walls, the
walls to each other, and the walls to the foundation. These
connections are necessary to insure structural integrity and to
prevent the shelter from overturning.
The current invention indicated herein meets all the requirements
of a safe severe storm shelter especially the primary areas are the
structural integrity and impact protection are maximized because of
the combination of intrinsic protective properties of reinforced
concrete, by the enhancements afforded by the pre-casting,
pre-stressing and post tensioning in this invention and innovative
design using a catenary geometry which guarantees minimum surface
area for wind driven missile impact. To fully appreciate the
benefits of pre and post-tensioning in enhancing a structure, it is
helpful to know a little bit about concrete. As discussed in Ref 5,
concrete is very strong in compression but weak in tension, (i.e.
it will crack when forces act to pull it apart). In conventional
concrete construction, if a load is applied to a slab or beam, the
slab or beam will tend to deflect or sag. This deflection will
cause the bottom of the beam to elongate slightly; even a slight
elongation is enough to cause tensile forces and cracking.
[0072] Steel reinforcing bars "rebar" are typically embedded in the
concrete as tensile reinforcement. Rebar is what is called
"passive" reinforcement however; it does not carry any force until
the concrete has already deflected enough to crack. Post-tensioning
on the other hand is considered "active" reinforcing. Because it is
pre-stressed, the steel is effective as reinforcement even though
the concrete may not be cracked. Post-tensioned structures can be
designed to have minimal deflection and cracking, even under full
load. In the present invention, a post-tensioning "tendon" is
defined as a complete assembly consisting of the anchorages, the
prestressing strand or bar, the sheathing or duct and any grout or
corrosion-inhibiting coating (grease) surrounding the prestressing
steel. There are two main types of post-tensioning: bonded
(grouted) and unbonded. An unbonded tendon is one in which the
prestressing steel is not actually bonded to the concrete, which
surrounds it and its compressive force is transferred to the
concrete by its anchorages only.
The most common unbonded system (single strand) tendons which are
used in slabs and beams for buildings, parking systems and
slabs-on-ground. A monostrand tendon consists of a seven-wire
strand that is coated with corrosion-inhibiting grease and encased
in an extruded plastic protective sheathing. The anchorage consists
of an iron casting in which the strand is gripped by a conical,
two-piece wedge. Bonded systems are more commonly used in bridges,
both in the superstructure (the roadway) and in cable-stayed
bridges, the cable-stays. In buildings they are typically only used
in heavily loaded beams such as transfer girders and landscaped
plaza decks where the large number of strands required makes them
more economical. In bonded strand systems, two or more strands are
inserted into a metal or plastic duct that is embedded in the
concrete. The strands are stressed with a large, multi-strand jack
and anchored in a common anchorage device. In one method of
stressing a turnbuckle type device can be used to stress the cables
or tendons. The duct is then filled with a cementitious grout,
which provides corrosion protection to the strand and bonds the
tendon to the concrete surrounding the duct. In the anticipated use
of this invention, bonded or unbonded systems can be used in the
construction and installation.
[0073] The inherent stiffness of the concrete shell means that pre
and post tensioning could be used efficiently. This reduced the
long-term effect of deflection due to creep and other effects on
the structural integrity of the current embodiment.
[0074] Another embodiment of the invention comprises the use of
high impact laminate material instead of reinforced concrete as the
impact resistant material of the structural shells. In this
embodiment a carbon fiber type material with suitable epoxy matrix
material can be used instead for the strengthening of the shell.
Modern material like Kevlar.TM. can be used for penetration
prevention and impact absorption but since these are extremely
expensive their routine use is not expected until prices have been
reduced considerably.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0075] As shown in FIG. 1, in the preferred embodiment, the severe
storm shelter is a geometric shaped structure with a primary
support element 1 to which are connected structural concrete shells
2 and these shells 2 are connected by horizontal tendons 7 with
upper and lower tendon anchor elements 3 and 4 located
circumferentially around the structure on the shells 2. The shells
2 are also connected by fasteners 5a to each other and the shell
bases are connected by tie down brackets 5 to the shell secondary
support structures 6 which are anchors that are set in a concrete
base 19 implemented in the substratum 18. The top of the structure
has a cover 8 which allows for ventilation of the structure. FIG.
2a shows how the top of the shell 2 is connected by tie down
brackets 5 which are securely bolted to the primary support 1 by
tie down bolts 22. This figure shows the detail of the bottom
anchoring of the shell 2 via the shell lip 10 through the tie-down
brackets 5 which are connected to the secondary support 6. A
protective skirt 9 covers the base of the shells 2 and prevents
wind from blowing under the structure. The floor 11 of the
structure is shown above the substratum 18. In this figure the
access system is implemented by a sliding door 15 attached to a
doorframe 29 by a hinge system 16. The door 15 is opened by door
opener 17.
[0076] FIG. 2b shows another embodiment in which the central
primary support 1 is replaced by a high strength circular or
polygonal shaped central support collar or element 30. This
embodiment allows more internal free space to the structure and
thus greater human occupancy of the structure. The individual
shells 2 are securely fastened to the central support element 30.
This central support 30 provides rigidity to all the concrete
shells 2.
[0077] Further, FIGS. 2a, FIG. 2b show a side view and FIG. 3 is a
top view which shows the secondary anchors 6 which are constructed
at the base of each concrete structural shell 2 in a symmetric
manner around the severe storm shelter. The shell 2 is connected to
the anchor 6 by a steel tie-down fastener 5. As shown in FIGS. 2a,
2b, the lower end of the shell 2 is buried about 6 inches below
grade of the substratum 18 and a circumferential skirt 9 of cement
is poured forming a berm and this prevents wind from blowing under
the structure and creating an overturning force on the structure
and its occupants. One or more shells 2 are modified to allow for
an easily opened structurally competent door 15 and a door opening
mechanism 17 which can be opened from the inside and outside.
[0078] FIG. 4 which shows the structural shells 2 in a "stand-up"
view. Each shell 2 is connected to its adjacent partner by means of
shell fasteners 5a. In the middle of the base of each shell 2, the
shells are connected to the secondary support anchors 6 which are
connected to the concrete base 19 which is further buried in the
substratum 18. The horizontal tendons 7 and the respective anchors
3 and 4 are also shown traversing the shell elements.
[0079] FIG. 5a, FIG. 5b, FIG. 5c and FIG. 5d show embodiments of
detail connections between adjacent shell elements 2. In one
embodiment, the shell 2 has tongue 14 fits into the groove 13 of
the adjacent shell. In another embodiment, the connection can be
made by overlapping the shells 2 as shown in FIG. 5c. In FIG. 5d,
since the outer edge of the shell 2 is longer than the inner edge
of the shell, this embodiment behaves as a keystone and this
keystone arrangement further interlocks the contiguous shells 2.
Each shell 2 behaves like a keystone and thus transfers the load
forces laterally to the other shell elements 2 that make the
structure more rigid.
[0080] As shown in FIG. 6, the concrete elements 2 are constructed
or "formed" from a combination of steel rebar 12 and high strength
cement with approximately 5000 psi strength rating. The forming
process is well known in the industry and the pre-casting can be
efficiently done by many skilled in the field. These shells are
almost an article of industry and given adequate "shop drawings"
can be manufactured to the desired specifications by any competent
pre-casting cement company at a centralized location and then
shipped to the construction location. The shell element 2 uses a
rebar frame 25 which is fabricated and curved to meet the design
requirements of the severe storm shelter catenary surface
structure. In one embodiment, at selected parts of the rebar frame
25 steel pins 23 are added as male inserts, either by extending the
rebar 12 or by welding a steel pin 23. At the same time female
sockets 24 are made in the shell form such that the male pins 23
will later fit when the shells 2 are locked together.
[0081] FIGS. 7a, 7b, 7c show cross-sections of one embodiment with
the central support element 30. In these figures the steel tendons
7 are installed circumferentially around the structure either
externally on the shell 2 as shown in FIG., 7b and held in place by
tendon anchors 3 and 4. The external positions of the tendon are
also maintained by tendon guides 32a and 32b. In the case of
internal tendons 7 these are implemented inside the shell element 2
as shown in FIG. 7c.
[0082] The steel tendons 7 are used to post-tension the structural
elements. These tendons 7 can be bonded or un-bonded. The tendons
are tensioned by well-known industry practices such as a screw
mechanism or a hydraulic jack device, which allows the tendon 7 to
be pulled together and stretched thus imparting a load on the
shells 2 and thus increasing the unit strength of the structure.
Post tensioning is a well-known process in the construction
industry and by itself is not part of the invention but its
implementation in this type of shell element 2 is novel to the
severe storm shelter industry. In this application tendon, cable
and tensioning element are used interchangeably to describe the
element that is post tensioned. FIG. 7a also shows a circular bench
31 for the occupants during their stay in the structure.
[0083] In assembling the structure by referring to FIG. 1, the
primary support 1 is planted in place and anchored with cement. In
a similar manner the secondary supports 6 are emplaced and anchored
in place in one embodiment in a circumferential trough about 6
inches below grade as shown in FIG. 2a and FIG. 2b. The first
structural shell 2 is attached to its secondary anchor 6 using
bracket 5 and tie-down bolt 22. The top of the shell is bolted to
the top of the primary supports 1 or 30 using a similar bracket 5
and bolt 22. A second shell 2 is put in place and attached to the
first shell 2 by fitting the male insert 23 into the female socket
24 and allowing the tongue 14 to fit into the groove 13. A further
embodiment shown in FIG. 5c, of the invention allows an
interlocking groove and recess system in which adjacent shells are
assembled to form the shelter body. A waterproof chemical bond
compound 20 is inserted between the shell elements 2 to provide a
sealant and to maintain an impermeable seal against wind blown
water entering the structure. Each shell is attached at both the
top of the primary supports 1 or 30 and bottom secondary support by
the brackets 5 using the tie-down bolts 22. The shells 2 are also
connected by the fasteners 5a which laterally connect the shell
elements. All tie-down bolts 22 are tightened to the required
torque and the skirt 9 is poured from cement around the structure.
The door 15 is installed and bolted to its frame 29.
[0084] The post tensioning is implemented with methods current in
the industry usually by hydraulic or mechanical means to stress the
tendons. The amount of post tensioning is calculated and designed
to maximize the strength of the structure. This is usually done by
a computer program well known in the industry and available to many
skilled in the art. In one embodiment, the horizontal tendons 7 are
threaded through the tendon anchors 3 and 4. The upper and lower
tendons are emplaced and the tendons are stretched to the requisite
limit. The tendons are anchored off using standard industry
hardware and components as shown in Ref. 6 by SureStress.TM.. A
plurality of structural shells are modified as need to allow the
post tensioning hardware and components to be installed and
implemented. The fact that the tendons are kept in a permanently
elongated state causes a compressive force to act on the cement
shell elements. This pre-compression which results from the post
tensioning counter balances the tensile forces created by the
subsequent loading during a severe storm event and increases the
load carrying capacity of the subject structure.
The combination of strength provided by several inches of 5,000-psi
reinforced concrete shells and the post tensioning of the structure
prevents any penetration by flying missiles at more than a hundred
miles per hour. In addition the catenary structure provides the
smallest cross section surface area for a given volume of cement
material so that the subject structure provides a maximum strength
for a minimal amount of construction material. Further more
compared to other existing block shaped severe storm structures
which provide a rectangular cross-section to wind flow and missile
contact, the subject invention provides a streamlined cross-section
and a continuously curved cross-section to wind flow and only a
fractional component of the wind force acts on the structure
proposed in this invention. Missile and wind impact is minimized in
the case of the new invention and in the block structure the
missile and wind impact is maximized.
Procedure:
[0085] The procedure used in installation of this invention is as
follows.
[0086] 1. The selected location for installation is made and the
design template for the anchor piles is used to fix the holes for
the anchor supports.
[0087] 2. Support holes are excavated or drilled for the primary
anchor if needed, and the secondary anchors. The support anchors
are inserted into the holes.
[0088] 3. High strength cement is poured into the holes to anchor
the steel supports.
[0089] 4. The cement is allowed to cure to maximum strength or for
at least 2 days.
[0090] 5. The shelter is then assembled by emplacing each shell in
turn ensuring alignment by fitting the steel pins into the female
sockets.
[0091] 6. The steel tendons are connected using the tendon hardware
attached to each shell or by threading the tendon circumferentially
around the structure.
[0092] 7. The adjacent concrete shells are aligned, connected and
bolted together into the primary and secondary supports anchors. In
those embodiments where a flange is used instead of a primary
vertical anchor the shells are bolted to the top steel flange. All
connections are made to the required torque to ensure rigidity and
strength.
[0093] 8. The structure is post tensioned by stressing the tendons
and anchoring the tendons to the required level of stress. The
stress level is at least 30% of the maximum allowable strength of
the tendon.
[0094] 9. The structure joints are then sealed using a commercially
available sealer or caulk especially at the floor slab and shelter
connection planes.
[0095] 10. The door is installed on its frame.
Further modifications and alternative embodiments of various
aspects of the invention may be apparent to those skilled in the
art in view of this description. Accordingly, this description is
to be construed as illustrative only and is for the purpose of
teaching those skilled in the art the general manner of carrying
out the invention. It is to be understood that the forms of the
invention shown and described herein are to be taken as the
presently preferred embodiments. Elements and materials may be
substituted for those illustrated and described herein, parts and
processes may be reversed, and certain features of the invention
may be utilized independently, all as would be apparent to one
skilled in the art after having the benefit of this description of
the invention. Changes may be made in the elements described herein
without departing from the spirit and scope of the invention as
described in the claims.
List of Elements of Invention.
TABLE-US-00001 [0096] TABLE 1 No Item Description 1 Primary Support
Structure 2 Concrete Precast Post tensioned Shell element 3 Tendon
Anchor upper 4 Tendon anchor lower 5 Shell Tie down Bracket 5a
Shell Fasteners 6 Secondary Support Structure 7 Horizontal Tendon 8
Ventilator dome cover 9 Protective skirt 10 Shell Lip 11 Floor 12
Cement reinforcing rebar 13 Upper and lower portion of shell edge
14 Male insert portion of shell edge 15 Sliding Door System 16
Hinge system 17 Door opener system 18 Substratum 19 Concrete base
for primary and secondary anchors 20 Chemical bonding compound 21
Welded metal bracket 22 Tie down bolt 23 Male insert pin 24 Female
socket 25 Rebar grid frame 26 Wind blown missile 27 Block shaped
tornado structure 28 Tornadic wind flow 29 Sliding Door Frame 30
Central Support Collar 31 Contoured Seats 32a Tendon guide - upper
32b Tendon guide - lower 33a Vortex at front of structure 33b
Vortex at back of structure
REFERENCES
[0097] (1): Oz Storm Shelter, www.ozsaferooms.com
[0098] (2) FEMA Booklet: Design and Construction Guidance for
Community Shelters.http://www.fema.gov/fima/tsfs02.shtm
[0099] (3): Texas Tech Wind Research Lab. http: /
/www.wind.ttu.edu/Research/research.asp
[0100] (4): Pietras, B. K. 1 997. "Analysis of Angular Wind Borne
Debris Impact Loads." Senior Independent Study Report. Department
of Civil Engineering, Clemson University, Clemson, S.C.
[0101] (5): Post Tensioning Institute Publications:
http://www.post-tensioning.org/
[0102] (6): SureStress Company:
http://www.dur-o-wal.com/surestress.html
* * * * *
References