Abstract:
A lifting hook for lifting open-frame steel joists comprises a casing having a shaft passing through it. The shaft has a lifting eye at one end and a hook member at the other end. The hook member comprises a U-shaped body having a transverse width selected to be less than the gap between the angles that form the upper flange of the open-frame joist. The hook member has a throat that is larger than the width of the flange. The shaft is spring-loaded so that the hook member is pulled toward the lower bearing surface of the casing so that once the hook member has been inserted into the joist, it is keyed to the flange and will not come loose.

Description:
This application claims priority of U.S. Provisional Patent Application 61/551,075, filed on Oct. 25, 2011. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to construction equipment and, in particular, to methods and apparatus for lifting open-web steel trusses. 
     It is well known in the commercial construction industry to use standard prefabricated steel Warren trusses to support floors and roofs of structures such as office buildings and hotels where loads are moderate and the spans between supports are relatively long. These trusses, often referred to as open-web steel joists are generally made of light structural members such as angles, bars and channels. 
     Open-web steel joists are commonly manufactured in three standard categories. The standard K-series joists are fabricated using a double-angle top and bottom chord and a round bar web having a depth of from 8 inches to 30 inches. K-Series joists are recommended for spans from 8 feet to 60 feet in length. Other standard open-web steel joists include the LH-series which have a depth of from 18 to 48 inches and may be used for spans of 25 feet to 96 feet and the DLH-series joists which have a depth of from 52 inches to 72 inches and are recommended for spans of 89 feet 244 feet. 
     Open-web steel joists are very economical structural members since they are fabricated from standard lightweight structural steel shapes such as angles and bars. Because the webs are open, they are able to span long distances without the dead weigh load of a solid I-beam. Moreover, because of the open-web design, is possible to run plumbing, electrical lines and ventilation ducts directly through the web itself, which results in considerable savings in floor-to-floor height and weight. 
     Although open-web steel joists have very favorable strength to weight ratio for vertical loads (i.e. loads applied parallel to the depth, which is the axis having the maximum area moment of inertia), open web steel joists have considerably less strength when resisting side loads (i.e. loads applied to the axis having the minimum area moment of inertia). Consequently, open-web steel joists must be handled carefully especially during loading, transport, unloading and positioning prior to final placement. 
     The Steel Joist Institute recommends that when lifting an open-web steel joist using a crane (either during loading, unloading or during final placement), the crane operator should use two chokers configured in a basket hitch with two-way spreaders. The chokers should be rigged passing through the inside of the inverted V-shaped opening in the web. The Institute cautions that when using chokers (with or without spreaders), care must be taken to avoid damaging (e.g. bending) the rod members that form the web. Carefully rigging and unrigging chokers and spreaders, however, is cumbersome and time-consuming. Accordingly, a need exists for a method and apparatus to quickly rig and unrig the open-web steel joists from the lifting crane. 
     One prior art apparatus, marketed commercially as the E-Z Joist Release™ by Freedom Tools LLC of Mesa Ariz., comprises a horizontal flange welded to a vertical web adapted to receive a lifting hook or shackle. The horizontal flange has a hole at each end through which a vertical rotating shaft is mounted. Each of the rotating shafts has a shank that is sized to pass through the gap between the structural angles that make up the top flange of the truss. The rotating shafts terminate at their lower ends with an inverted triangular tip, which when rotated 90° is unable to pass through the gap in the top flange. The horizontal flange also has two vertical tongues adjacent the rotating shafts. The vertical tongues are also sized to pass through the gap in the top flange and serve as guides to key the device to the top flange of the truss. In operation, the device is placed on top of the truss so that the tongues and rotating shafts pass through the gap in the top flange. A lever attached to the rotating shafts is pulled which causes the shafts to rotate 90° to lock the device in position. Once the truss has been moved to its desired location, the lever is returned to its original position so the device can be released from the truss. The E-Z Joist Release has several disadvantages. It is expensive. It may not be easily adaptable to joists having different-size upper flanges because the rotating tip is at a fixed depth. Additionally, the E-Z Joist Release may damage the web members if the tongues and/or rotating shafts come in contact with the web. 
     Accordingly, what is needed is a joist lifting tool that is inexpensive, easy to use, and safe. 
     SUMMARY OF THE INVENTION 
     The present invention comprises a lifting hook for lifting open-frame steel joists. According to an illustrative embodiment of the invention, the lifting hook comprises a casing having a shaft passing through it. The shaft has a lifting eye at one end and a hook member at the other end. The hook member comprises a U-shaped body having a transverse width selected to be less than the gap between the angles that form the upper flange of the joist. The hook member has a throat that is larger than the width of the flange. The shaft is spring-loaded so that the hook member is pulled toward the lower bearing surface of the casing. 
     In operation, the hook member is fed through the gap until the lower bearing surface of the casing comes into contact with the upper flange of the joist. The hook member is then extended downward against the force of spring until it is below the lower surface of the flange of the joist. The hook member is then rotated approximately 90° and released, which allows the spring to move the hook member upward until the flange of the joist is pressed between the hook member and the lower bearing surface of the casing. The flanges of the U-shaped hook member grip the top flange to prevent the hook member from rotating back and disengaging. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The present invention will be better understood from a reading of the following detailed description, taken in conjunction with the accompanying drawing figures in which like references designate like elements and, in which: 
         FIG. 1  is a front perspective view of a lifting hook incorporating features of the present invention being attached to an open-web steel truss; 
         FIG. 2  is a front section view of a lifting hook incorporating features of the present invention in the open position; 
         FIG. 3  is a side section view of the lifting hook of  FIG. 2 ; 
         FIG. 4  is a front section view of the lifting hook tool of  FIG. 2  in the closed position; and 
         FIG. 5  is a partial side view of the lifting hook of  FIG. 2  showing details of the engagement between the lifting hook and the top flange of an open-web steel truss. 
     
    
    
     DETAILED DESCRIPTION 
     The drawing figures are intended to illustrate the general manner of construction and are not necessarily to scale. In the detailed description and in the drawing figures, specific illustrative examples are shown and herein described in detail. It should be understood, however, that the drawing figures and detailed description are not intended to limit the invention to the particular form disclosed, but are merely illustrative and intended to teach one of ordinary skill how to make and/or use the invention claimed herein and for setting forth the best mode for carrying out the invention. 
     With reference to  FIG. 1 , a conventional open-web steel truss  10  comprises a top flange  12  and a bottom flange  14  each of which is formed from two symmetrical structural angles  16 ,  18 ,  20  and  22 . Each of structural angles  16 - 22  have vertical legs  62 ,  63 ,  64  and  65  as well as horizontal legs  66 ,  67 ,  68 ,  69 . Each of horizontal legs  66 - 69  have a length “l 1 ” and each of the vertical legs  62 - 65  have a length “l 2 ” the dimensions of which are selected to withstand the particular loads required. Top flange  12  and bottom flange  14  are connected together in a spaced-apart configuration having a web-depth “D” by means of an open web  24 . Web  24  is made from a structural steel round bar or rod  26  which forms a series of Vee&#39;s typical of a Warren truss. Structural rod  26  has a diameter “d 1 .” Consequently there is a gap “g” equal to or slightly greater than the diameter of structural rod  26  between the vertical legs  62  and  64  of structural angles  16  and  18  forming top flange  12  and a similar gap equal to or slightly greater than the diameter of structural rod  26  between the vertical legs  63  and  65  of structural angles  20  and  22 . In a conventional K-series truss, the legs of the angles leg “l 1 ” and “l 2 ” are typically from 1 inches to 4 inches while the diameter of the structural rod  26  is typically from ⅜ inches to 1-½ inches. 
     With reference to  FIGS. 2-4 , a lifting hook  30  incorporating features of the present invention comprises a longitudinal shaft  32 , made of steel or similar high-strength material, comprising a shank  34  and a hook member  36  at the lower end. In the illustrative embodiment, longitudinal shaft  32  terminates at the upper end in a lifting eye  38 , which is adapted to receive a lifting hook, shackle or other conventional means for connection to a lifting cable from a crane, derrick or similar lifting device. Although the illustrative embodiment comprises a lifting eye  38 , any conventional means for attaching a lifting cable, such as a lug, chain plate or threaded fastener may be substituted within the contemplation of the present invention. Hook member  36  comprises a generally U-shaped steel body having a transverse thickness “T 2 ” and throat “t 2 .” The throat dimension “t 2 .” is selected to be slightly larger than the dimension “t 1 ,” which is the distance between the outer surfaces of the vertical legs  62  and  64  of the angles  16  and  18  that form top flange  12 . The upright flanges  53  and  54  of hook member  36  are chamfered inward as shown in  FIG. 2  to guide hook member  36  into position as described hereinafter. 
     The diameter “d 2 ” of shank  34  is selected to be less than the gap “g” between the structural angles  16  and  18  forming top flange  12 . Similarly the transverse width “w” of hook member  36  ( FIG. 3 ) is selected to be less than the gap “g” between the structural angles  16  and  18  forming top flange  12 . For reasons discussed more fully hereinafter, the selection of the dimensions “d 2 ” and “w” ensure that hook member  36  and shank  34  can pass through the gap “g” in top flange  12 . In the illustrative embodiment, “d 2 ” and “w” are between ⅜ inch and 1½ inch, preferably between ⅜ and ⅝ inch and the overall length of lifting hook  30  is approximately 18 inches. 
     Lifting hook  30  further comprises a casing  40  which comprises a generally tubular or conical shell  42  having an upper opening  44  and a lower opening  46 . Casing  40  further comprises a rigid floor  48 . A resilient member, such as a compression spring  50  acts between the floor  48  and spring perch  52  formed on or attached (e.g. welded) to shank  34  of longitudinal shaft  32 . Spring  50  urges longitudinal shaft  32  upward towards a closed position as shown in  FIG. 4 . Compression spring  50  may be of any suitable size, but in the illustrative embodiment comprises a conventional cylindrical compression spring having a free length of about 8-¾ inches and a spring rate of preferably between 2 lb/in and 20 lb/inch preferably about 6-¾ lb/in such that in the maximum open position (4-½ inch stroke) the spring is exerting a restoring force of about 35 pounds and in the fully-closed position (installed height of 8 inches) is exerting a force of about 5 pounds. 
     With reference in particular to  FIG. 1 , in operation, lifting hook  30  is moved into position by the crane operator (not shown) with sufficient slack to enable the user to insert the hook member  36  and shank  34  through the gap between angles  16  and  18  forming upper flange  12 . With the hook member  36  oriented longitudinally with respect to the gap “g” between angles  16  and  18  as shown in  FIG. 1A , lifting hook  30  is fed through the gap until the lower bearing surface  58  of the casing  40  comes into contact with the horizontal legs of angles  16  and  18 . Because lower bearing surface  58  is larger than the gap “g” the downward motion of casing  40  is arrested. Pressing against lifting eye  38 , the user extends hook member downward against the force of spring  50  until upright flanges  53  and  54  of hook member  36  are below the vertical legs  62  and  64  of angles  16  and  18 . Still manipulating lifting eye  38 , the user rotates hook member  36  approximately 90° to orient hook member as shown in  FIG. 1B  The user then releases lifting eye  38 , which allows spring  50  to move lifting hook  30  toward the closed position with angles  16  and  18  pressed between hook member  36  and lower bearing surface  58  of casing  40 . The upright flanges  53  and  54  of hook member  36  extend past the vertical flanges  62  and  64  of angles  16  and  18  as shown in  FIG. 1C  which locks lifting hook  30  against rotation thereby preventing hook member  36  from disengaging top flange  12 . The lifting force from the crane, of course, only further locks the engagement between lifting hook  30  and truss  10 . 
     With reference to  FIGS. 4 and 5 , the lower bearing surface  58  of casing  40  is conical in shape, having a conical angle φ selected to match the maximum anticipated angle between the lifting cable and the top surface  60  of top flange  12  of truss  10 . For example, if the lift is to be made with two 30 foot cables without spreaders, spaced apart along the truss by 30 feet, the cables would make a 60° angle with respect to the top surface  60  and, therefore, upper jaw surface  58  would have a conical angle of 90°-60°=30°. Use of spreaders will, of course reduce the cable angle to below 60°. Accordingly, conical angle φ is typically between 15° and 45° and, most preferably between 25° and 35° and most preferably about 30°. 
     Although certain illustrative embodiments and methods have been disclosed herein, it will be apparent from the foregoing disclosure to those skilled in the art that variations and modifications of such embodiments and methods may be made without departing from the invention. For example although in the illustrative embodiment, lower bearing surface  58  is conical, other tapered surfaces such as a spherical lower surface are considered within the scope of the invention. Similarly, although in the illustrative embodiment shank  32  is circular in cross section, a rod with square, hexagonal or other cross-sectional shape is considered within the scope of the invention. Accordingly, as used herein, “diameter” when used in connection with shank  32  means the maximum diagonal of a rod with a non-circular cross section as well as the diameter of a rod with circular cross-section. Additionally, although in the illustrative embodiment hook member  36  is generally U-shaped, other hook members such as a T-shaped or W-shaped hook member that can be rotated to lock the lifting hook to the truss are also considered within the scope of the invention. Accordingly, it is intended that the invention should be limited only to the extent required by the appended claims and the rules and principles of applicable law. Additionally, as used herein, references to direction such as “up” or “down” are intend to be exemplary and are not considered as limiting the invention and, unless otherwise specifically defined, the terms “substantially” or “generally” when used with mathematical concepts or measurements mean within ±10 degrees of angle or within 10 percent of the measurement, whichever is greater.