Patent Application
20110250023
This invention relates primarily to the mining, tunneling, and construction industries and, more specifically, to an engineered steel crib for the support of hanging wall and foot wall, roof and floor, or upper and lower surfaces in underground mining, or providing temporary support for heavy structures such as ships, cars, houses or buildings being relocated or receiving foundation work.
Wooden posts and wooden cribs, or chocks, are probably the oldest support systems used in the mining and construction industries. A wooden post, typically 4 inches to 10 inches in diameter or square cross-section, loaded axially provides support between two points. A wooden crib or chock provides support over a larger area, typically varying from a 30 to 72 inches square or rectangle. Wooden posts and wooden cribs are extensively used in the mining industry even today.
A wood crib consists of layers of two or more parallel timbers with adjoining layers placed at right angles to each other. Thus, the number of parallel timbers in each direction determines the number of contact areas through which load is transferred or resisted. For example, a 2-by-2 crib means two layers of timber in each direction, resulting in 4 contact areas. A 2-by-2 crib configuration is most common, although 3-by-3, 3-by-2, 3-by-3, and 4-by-4 configurations have been considered and have found limited application.
Underground mines use large numbers of wooden cribs to provide support over an area between two opposing surfaces rather than at a point as with a wooden post. These opposing surfaces are referred to differently in different mining industries. For example, the lower surfaces in mines may be referred to as the floor or footwall, and the upper surfaces as the roof or hanging wall. Typically, cribs are more extensively used in longwall and high extraction room-and-pillar coal mining. Cribs are also extensively used in non-coal underground mining.
A crib is typically constructed of wooden elements of square or prismatic cross-section, 5 to 6 inches across, although other shapes have also been used. The length of elements used typically varies from 30 inches to 60 inches, depending upon the height of the area to be supported. The term ‘aspect ratio’, when used in conjunction with a crib, denotes the ratio of the height of the crib to the distance between centers of contact areas along a timber. Reducing aspect ratio increases the stability of the crib structure, and ratios larger than 2.5 and less than 4.3 are recommended. A crib structure should be designed to have appropriate rigidity, or stiffness, and load carrying capacity to provide early, controlled resistance to rock mass movement to maintain excavation stability.
A typical crib uses solid, prismatic wooden crib elements of 5″-by-5″-by-30″ or 6″-by-6″-by-36″, although other sizes may be used. The load is transferred between upper and lower surfaces through typically four contact areas in a horizontal plane of the size 5″-by-5″ or 6″-by-6″ depending on the size of the crib element. Except at and around the contact areas, there is very little stress or force within the prismatic element. The areas adjacent to the contact areas are in tension while zones away from contact areas have almost no stresses vertically or horizontally. At and below the contact areas are high compressive stresses due to load transfer.
Wood is a transversely isotropic material with much higher strength and stiffness when loaded axially, or parallel to the grain, as compared to loading transversely, or perpendicular to the grain. More specifically, a typical oak timber loaded axially has a compressive strength of 2000-2500 psi and an elastic modulus of 150,000-250,000 psi. Similar data for the two lateral loading directions are about equal to each other, and a typical oak timber has a compressive strength of 500-700 psi and an elastic modulus of 25,000-35,000 psi. Furthermore, the Poisson's ratios for loading in the axial and lateral directions are also significantly different: 0.10-0.20 for loading axially and 0.30-0.40 for loading in the two lateral directions. The type of wood and the engineering data included here are provided as an example and these may vary over a wide range.
A typical solid wood cribbing for support has several disadvantages, including: low rigidity, a limited load carrying capacity, a high resistance to airflow, heavy weight per crib element, limited pre-load capacity, insufficient post-failure characteristics, flammability, and shrinkage.
A typical wood crib's rigidity is low since wood is loaded at right angles to grain. So; the support column allows a significant amount of deformation, as much as 20% of the total height of the column may be reduced through deformation.
Because of large deformations, the column has limited load carrying capacity; a typical crib column fails due to buckling before achieving its full load carrying capacity.
Air flow in mines is important. Since each crib column reduces the available air flow space when installed, resistance to air flow can be significant.
Installing typical solid wood crib element is difficult in locations where the surfaces are not parallel to each other or irregular.
Each wooden crib element typically weighs about 35 pounds, making carrying them by hand and assembling a crib column an arduous process, especially when one must lift an element above one's head.
Since low-rigidity wedges, cut parallel to the wood grain, are typically used to preload the crib, the amount of preload force that can be introduced to a column is limited and it tends to decay with time. The wedges typically deform under low loads, which means that the column does not support significant loads until the upper and lower surfaces have deformed toward each other, through compression of the column. Preloading is currently applied through wooden wedges, typically 3 to 4 inches wide that are cut at inclination angles of 10 to 20 degrees. These wedges are loaded transversally to the wood grain and yield at the low pressure of 500 to 700 psi. For wedges cut at high inclination angles, the contact areas with prismatic crib elements are small. Therefore, stress concentrations at contact points are high and the wedges yield even at low crib loads. The wedges then become loose providing little or no preload on the installed crib. Industry professionals suggest that there is a need to develop a relatively simple mechanism to apply a sustained preload of 5 to 8 tons when a crib is installed. Moreover, wood shrinks as it loses moisture. Upon shrinking it loses the preinstalled load and industry is seeking ways to minimize this problem.
In addition to the above, the post-failure characteristics of wood do not provide a relatively flat load-deformation curve and wood is flammable.
The use of steel or other similar material for mine support and tunnel cribbing would solve most of the problems inherent in the wood cribbing. Steel is more rigid than wood, and, when used in a support column, has a greater load carrying capacity without the risk of buckling failures present in wood cribbing. A steel crib support column can support a sustained preload when it is installed and does not lose this preinstalled load through shrinking, as is common in wood cribbing. Additionally, steel provides a relatively flat post-failure load-deformation curve and steel is not flammable. However, despite the advantages of using steel, it has only found limited use as a material in cribbing elements because solid steel is heavy and expensive. Understandably, steel would not be a suitable material to construct prior art cribbing elements; a conventional solid cribbing element made of steel would be very heavy. Steel has been extensively used in steel supports such as arches, and bars (U.S. Pat. Nos. 3,991,580; 3,952,525; 7,909,542; 5,484,130), and either as reinforcement for concrete (U.S. Pat. No. 4,497,597) or as only a part of the load carrying element in wood (WO 00/53892) and cement-concrete (U.S. Pat. No. 4,565,469) crib elements.
The present invention utilizes an innovative design to construct a light-weight steel crib element with all of the desired characteristics for a crib element as well as low resistance to airflow. Furthermore, the innovative design should find applications beyond mining and tunneling industries such as in construction, railroad and ship building industries.
This invention provides an improved cribbing support in mines and tunnels through a lightweight, engineered steel crib element to provide support between two surfaces. It is easy to carry manually and offers very low resistance to airflow depending upon the selected embodiment. The elements can be used in multiple ways to construct cribs. The element provides high stiffness support to develop a crib with literally any desired load carrying capacity. However, it is anticipated that initially cribbing with load carrying capacity of 100-tons to 300-tons will be commonly used to minimize rock mass movement in mines and tunnels.
The steel crib element consists of one or more center elongate structural elements, connected at the distal ends of the elongate structural element to at least two outer hollow or solid steel load carrying members. The hollow or solid steel load carrying members may be perforated with holes, and loaded axially or parallel to its longitudinal axis, or laterally or perpendicular to its longitudinal axis depending upon a selected embodiment among the many in each configuration. The hollow load carrying members may be internally reinforced or filled with appropriate materials to achieve desired load-deformation characteristics. The elongate structural elements may be prismatic or round or any other shape in cross-section and may use steel or any other suitable material (metal, non-metal, polymer composites, etc) that have appropriate load carrying capacity, load-deformation characteristics, and material cost. The connection between elongate structural elements at the distal ends and load carrying members may be through nuts and bolts, welds, screws, or other suitable fastening means or polymeric materials. The elongate structural elements, connecting the load carrying members, may also be interconnected among themselves at suitable intervals to provide required flexural rigidity and load carrying capacity in different spatial planes. The crib structure is constructed similar to wooden cribbing by superimposing only these crib support elements in 2×2 layers, 2×3 layers, 3×3 layers, or other suitable layered system. To ensure full contact area between load carrying members of different elements and to minimize likelihood of slippage or limit lateral movement between mating surfaces of different elements, the crib element design includes indexing as well as interlocking stops. The upper and lower plates installed on the hollow load carrying steel tubes may be flat with skid resistant material on the top, “diamond plate” design with curved raised surfaces to allow interlocking and minimize lateral movement or slippage, embossed with suitable design, or may have short pins that fit into holes in the mating plates depending upon the embodiment selected.
The novel crib element offers the following advantages: it has much higher stiffness (10-20 times or more) than the wooden cribs (about 40,000 psi) commonly used today; it can be designed to have variable stiffness; it can be designed to support any loads but typically varying from 100 tons to 300 tons for a 6-ft high cribs; it has the ability to sustain almost peak load even after the steel starts to yield; it is lightweight; it may offer very low resistance to airflow in underground mine roadways; and, it does not shrink like the wooden cribs that are almost exclusively used today in mining and tunneling industries.
FIG. 1—(a) A side view of a steel crib element with a solid load carrying member and a flat plate or bar elongate structural element; (b) A top view of the steel crib element.
FIG. 1A—(a) A sectional view of a steel crib element with hollow load carrying members and a flat plate or bar elongate structural element; (b) Another sectional view of the steel crib element.
FIG. 1B—(a) A top view of a steel crib element with hollow load carrying members with recessed grooves for mating adjoining steel crib elements and a flat plate or bar elongate structural element; (b) A sectional view of the steel crib element.
FIG. 1C—(a) A top view of a steel crib element with hollow octagonal load carrying members and a flat plate or bar elongate structural element; (b) A sectional view of the steel crib element. This figure may need revision.
FIG. 1D—(a) A side view of a steel crib element with hollow perforated load carrying members and a flat plate or bar elongate structural element; (b) A top view of the steel crib element.
FIG. 1E—(a) A sectional view of a steel crib element with hollow load carrying members with indexing and mating top and bottom caps and a flat plate or bar elongate structural element; (b) A top view of the steel crib element.
FIG. 1F—(a) A top view of a steel crib element with hollow cylindrical load carrying members and two interconnected elongate structural elements; (b) A sectional view of the steel crib element.
FIG. 1G—(a) A top view of a steel crib element with three hollow load carrying members and interconnected elongate structural elements; (b) A side view of the steel crib element.
FIG. 1H—(a) A top view of a steel crib element with hollow load carrying members and two interconnected elongate structural elements; (b) A side view of the steel crib element.
FIG. 2—(a) A sectional view of a steel crib element with hollow load carrying members reinforced internally with steel plate and two interconnected elongate structural elements; (b) Another sectional view of the steel crib element.
FIG. 3—(a) A top view of a steel crib element with hollow load carrying members with cylindrical mating and indexing elements and two interconnected elongate structural elements; (b) A sectional view of the steel crib element.
FIG. 4—(a) A top view of a steel crib element with hollow load carrying members with diamond pattern cap plates and two interconnected elongate structural elements; (b) A side view of the steel crib element.
FIG. 5—(a) A sectional view of a steel crib element with hollow steel load carrying members with wood reinforcements and two interconnected structural elements; (b) Another sectional view of the steel crib element.
FIG. 6—A perspective view of the crib elements stacked into a 2×2 crib structure.
FIG. 7—A planar view of stacked crib elements forming a crib support between the mine floor and roof.
FIG. 8—A planar view of stacked crib elements forming a crib support between the mine hanging wall and footwall.
A conceived steel crib may consist of two or more pieces of metal load carrying members of any geometrical cross-section connected to each other through one or more elongate structural elements. The elongate structural elements may be rod of any shape or plate metal or any other material that provides appropriate load-deformation characteristics prior to and after yielding, has appropriate flexural rigidity in different orientations, and is reasonable in cost. The material used may also have any geometry that satisfies the above requirements. Mechanistically, the size of the load carrying member, its wall thickness, its geometry (square, rectangle, circular), and its loading orientation control its stiffness and load-deformation characteristics. The elongate structural elements may be metal rod or bar, non-metal, or polymers. The elongate structural elements may be interconnected at suitable intervals to provide appropriate strength and stiffness in different spatial planes. Similarly, the internal hollow portion of the hollow load carrying members may be reinforced with any material such as: metal, wood, plastic, and cementitous or pozzolonic material in various geometric configurations to achieve desired strength and to further modify the load-deformation characteristics of the crib element. The type and spatial distribution of lateral connections between the tubes (round, prismatic, solid, hollow) and the type of reinforcements (square, round, prismatic) between the lateral connections allow the ability of the crib to carry differential loading in different planes and twisting of the cribbing structure. The crib element is loaded so that it is either loaded transversely or loaded axially with respect the axis of the load carrying members.
In the embodiments where the hollow tube load carrying member is loaded axially, the upper and/or lower surfaces of the load carrying member may be covered with a lateral-movement resistant designs including but not limited to the following: solid or perforated steel plate; steel plate with roughened surfaces, such as “diamond plate” material available commercially; suitably embossed plates of any material, any thickness, and any shape; or protruded surfaces on one end with appropriate mating surfaces on the other end.
Additionally, the design of the hollow load carrying member may be shaped to provide interlocking between stacked crib elements.
To date, all experimental studies have been performed on Grade B or Grade C steel 30-inch long crib elements with load carrying members constructed of ASTM A-500 steel tube of 3/16-inch wall thickness. Grade B steel has a minimum yield stress of 46,000 psi and minimum tensile strength of 58,000 psi, while Grade C has a minimum yield stress of 50,000 psi and minimum tensile strength of 62,000 psi. The elongate structural elements also have similar strength and elastic properties. Steel tubes with up to 100,000 psi yield stress are available in a variety of wall thicknesses. Several of these may provide feasible desired crib element but the weight of the element and cost of the element could be very different.
The design of the structure above allows it to be lightweight, with the ability to withstand large amount of deformations because of the characteristics of steel or other materials used and reinforcements within the hollow load carrying members. The weight of each crib element will vary based on load carrying capacity. For a single element with ability to carry about 120-tons of load for a 2×2 crib, the weight of the designed element is only 20-21 pounds. Since most cribs used in mining and tunneling applications are designed to carry loads varying from 100-tons to 200-tons with four loading surfaces, all embodiments tested are suitable for use in mine and tunnels. The design of the cribs may be varied to meet the load carrying requirements of different structural applications.
The length of the load carrying members and elongate structural elements can be varied to achieve desired aspect ratio (height to width ratio). All studies to date have been performed on 30-inch or 36-inch long steel cribs with two load carrying steel members per crib. Longer cribs such as 42-inch, 48-inch, 54-inch, or 72-inch cribs can easily be developed based on this disclosure by adjusting the dimensions of the load carrying members and elongate structural elements and/or by connecting three or four load carrying members in series. Crib elements with three or four load carrying members may be configured at equi-distances along the length of the crib element (a preferred embodiment) or staggered.
The overall cribbing structure is constructed similarly to conventional wooden cribbing structure and that is by stacking crib elements. The cribbing structure is tightened between the roof and floor or hanging wall and footwall through steel inserts, wedges, grout bags, wooden wedges or other suitable materials which have high rigidity and will not shrink. Thus, a high preload can be applied to the crib during its construction process. Preferably, the material used for the wedges has a similar stiffness to the load carrying member to apply a maximum preload to the constructed steel crib structure. In preferred embodiments, the wedges are constructed of steel and are 3-inches wide to 5-inches wide to keep their weight to a minimum. Other sizes and materials for tightening the crib are within the scope of this invention. The steel wedges or inserts can also be manufactured from hollow steel tube.
The embodiments of the present invention are best described in reference to the figures. The crib element embodied in
The crib element embodied in
Alternately, to ensure full contact area between load carrying members of different elements and to minimize lateral movement between mating surfaces of different elements, the crib element design may include different designs for indexing or mating elements as well as limiting displacements under load between the mating elements. The upper and lower plates installed on the hollow load carrying steel tubes may also be embossed, may have short pins that fit into holes in the mating plates, or have curved raised surfaces to provide interlocking and indexing. In the embodiment represented in
Indeed, mating of upper and lower surfaces of elements in a crib structure can be accomplished through a variety of methods, such as in the embodiment represented in
In the embodiment represented in
In another embodiment of the present invention, represented in
The crib element embodied in
In another embodiment of the present invention, represented in
Yet another embodiment of the present invention, constructed similarly to the crib element of
In yet another embodiment of the present invention, constructed similarly to the crib element of
In a similar embodiment, the load carrying member 21 is constructed with 6-inch×6-inch steel tube with 3/16-inch wall thickness with a cylindrical reinforcements within each tube. These load carrying members 21 are connected through an elongate structural element 10 comprising two round bar elongate structural elements 27 with interconnections 36 between them. This crib element is loaded axially and the load carrying members 21 are constructed so that the hollow tube load carrying member 21 of one element rests at right angles to the hollow tube load carrying member 21 of the upper or lower element. In this configuration, each steel tube load carrying member 21 is allowed to yield and punch into the lower or upper tube. The two punched tubes interlock and provide load carrying and buckling strength to the crib. This embodiment of the present invention with a 6-inch square hollow steel tube load carrying member 21 and a 6-inch high cylindrical reinforcement 23 carried about 160-tons. Alternatively, other gauges and sizes of steel may be used and other metals may be used for the elongate structural element 10, the interconnections 36, the reinforcements 22, and the load carrying members 21; and other tubing sizes (3-inch to 10-inch or more), with different wall thicknesses (0.125 inch to 0.5-inch) could be used as the load carrying members 21. This design can be modified for different load carrying capacity and stiffness.
In another embodiment of the present invention, represented in
Yet another embodiment of the present invention, represented in
In another embodiment of the present invention, represented in
Yet another embodiment of the present invention, where the crib elements 1 are stacked in a 2×2 crib structure, is represented in
Yet another embodiment of the present invention, where the crib elements 1 are stacked in a 2×2 crib structure between the floor 5 and roof 6 of a mining excavation, is represented in
Another embodiment of the present invention, where the crib elements 1 are stacked in a 2×2 crib structure between the hanging wall 7 and footwall 8 of a mining excavation, is represented in
Although the above discussion relates to steel construction, the principles apply equally to construction of similar designs using other materials. The sizes of the load carrying members, elongate structural elements, and reinforcements indicated throughout the application are only suggestions and not meant to be limiting. The developed concepts can also be utilized in the design of tunnel arches.
This application claims priority to and benefit of an earlier filed co-pending Provisional Patent application Ser. No. 61/323,537 filed Apr. 13, 2010 entitled Engineered Steel Crib for Use as Mine and Tunnel Support.
| Number | Date | Country | |
|---|---|---|---|
| 61323537 | Apr 2010 | US |
