Patent Publication Number: US-7716890-B2

Title: Tapered load plate for transferring loads between cast-in-place slabs

Description:
This is a continuation of application Ser. No. 10/489,380, filed Mar. 12, 2004, which claims priority to PCT Application No. PCT/US02/29200, filed Sep. 13, 2002, which in turn claims priority to U.S. Provisional Application Ser. No. 60/318,838, filed Sep. 13, 2001, all of which are incorporated by reference in their entirety herein. 

   TECHNICAL FIELD 
   This invention relates generally to transferring loads between adjacent cast-in-place slabs and more particularly to a system for transferring, across a joint between a first slab and a second slab, a load applied to either slab. 
   BACKGROUND OF THE INVENTION 
   Referring to  FIG. 1 , when a concrete floor slab  100  is first placed and the concrete starts to cure the volume of the concrete decreases causing the slab to shrink (usually on the order of ⅛ of an inch per 20 feet). Concrete has a relatively low strength when in tension. When the internal stresses due to shrinkage  104  reach a point greater then the tensile strength of the concrete, random stress-relief cracks  102  occur. 
   These random cracks  102  are undesirable as they detract from the performance of the floor slab  100  and reduce its life span. Referring to  FIGS. 2A and 2B , a typical method of controlling where these cracks  102  occur is to induce a weakened plane by saw cutting the top surface  200  of the concrete slab  100  into small panels, as depicted by saw cut  202 . 
   Referring to  FIG. 3 , an undesirable side effect of having the floor slab  100  made up of numerous small sections is that when the floor is loaded, such as with the wheels of a moving fork lift  300 , each section of the floor may be deflected  302  relative to its neighbor causing damage  304  to the joint edge, as depicted in  FIG. 3 . 
   Referring to  FIG. 4 , a conventional technique for reducing this type of deflection  302  is to span the joint  400  with steel bars  402  each having a round cross-section. These bars  402  are commonly referred to as dowel bars. 
   Referring to  FIGS. 5A-5C , dowels of this type are typically assembled into a wirework frame  500  that holds the dowels at a desired depth  502  and orientation. This assembly is generally known as a dowel basket. 
   Using circular-cross-section dowel bars is associated with various drawbacks. For instance, if the dowel bars  402  are misaligned  600  such that they are not oriented totally perpendicular to the joint, the dowel bars  402  can lock the joint  400  thereby undesirably restraining the joint from opening, which in turn may cause random cracks  102 . 
   Referring to  FIG. 7 , if a concrete floor slab, such as slabs  100 - 1  or  100 - 2 , tries to move along the line of the joint  400  relative to the next panel (for instance due to shrinkage or thermal contraction), the dowel bars  402  will restrain this type of movement  700 , thereby causing random cracks  102 . 
   Referring to  FIG. 8 , at an intersection of two joints, movement  800 , which is a combination of the two types of movement discussed above in connection with  FIGS. 6 and 7 , can cause a situation known as corner cracking  802 . 
   Referring to  FIGS. 9A and 9B , the round-dowel-bar drawbacks discussed above have been addressed in the past by using dowel bars  900  having a square or rectangular cross-section in conjunction with a plastic or steel clip  902  that places a compressible material  904  on the two vertical faces of the dowel bar  900 . These clips  902  produce a void in the concrete wider than the dowel bar  900  allowing for sideways movement and a slight degree of misalignment. The clips  902 , however, undesirably add to the expense associated with using dowel bars  900  having square and/or rectangular cross-sections. A more cost-effective solution that overcomes the misalignment problem to a greater extent, therefore, would be advantageous. 
   Under certain conditions, such as outdoor applications, concrete slab placement should be able to withstand concrete expansion, which is typically due to thermal changes, such as colder winter temperatures changing to warmer summer temperatures. Referring to  FIG. 10 , conventionally, a piece of compressible material  1000 , such as foam, fiberboard, timber, or the like, is placed in an expansion joint  1002  between concrete slabs  100 - 1  and  100 - 2 . A round-cross-section dowel bar  402  and an end cap  1004  may be used for transferring a load across the expansion joint  1002 . As the slabs  100  expand, they move together, as indicated by arrows  1006 , the joint  1002  closes, and the dowel bar  402  goes farther into the end cap  1004 . This use of round-cross-section dowel bars, however, is associated with the misalignment drawback discussed above in connection with saw-cut control joints. A cost-effective way of dealing with the misalignment situation while transferring loads between concrete slabs across expansion joints  1002  would therefore be desirable. 
   Applicants&#39; U.S. Pat. No. 6,354,760 discloses a load plate that overcomes the drawbacks discussed above, namely misalignment and allowing relative movement of slabs parallel to the joint. Referring to  FIG. 11 , the &#39;760 patent discloses using a load plate  1100  rotated such that the load plate has a widest portion (i.e., opposite corners) of the load plate positioned in the joint between slabs  100 - 1  and  100 - 2 . Using such a load plate  1100  at a construction joint works well because the load plate can be reliably centered at the construction joint between the slabs  100 . 
   A load plate  1100  is not, however, ideally suited for use at saw-cut control joints. As described above, this type of joint results from cracking induced by a saw cut in the upper surface of a concrete slab. The saw cut may be off center with respect to any load plate embedded within the cement, as shown by the dashed line  1200  in  FIG. 12 . If the saw cut and joint are off-center, the load plate will not function as intended because more than half of the load plate will be fixed within one of the slabs and less than half of the load plate will be available for transferring loads to and from the other slab. Another situation for which a load plate  1100  is not ideally suited is when a construction joint, formed by an edge form, for instance, is expected to be relatively wide open. Under such circumstances, an undesirably large area of load plates  1100  may undesirably be removed from slabs on either or both sides of the joint thereby reducing the ability of the load plate  1100  to transfer loads between the slabs. For these reasons, a load transfer device that provides the advantages of the load plate of the &#39;760 patent and that is well suited to use in saw-cut control joints and construction joints, which may become relatively wide open, would be desirable. 
   SUMMARY OF THE INVENTION 
   In accordance with an illustrative embodiment of the invention, a tapered load plate may be used to transfer loads across a joint between adjacent concrete floor slabs. The top and bottom surfaces may taper from approximately 4 inches wide to a narrow substantially pointed end  1308  over a length of approximately 12 inches. As will be apparent, other suitable tapered shapes and/or other suitable dimensions may also be used. 
   A tapered load plate, in accordance with an illustrative embodiment of the invention, advantageously accommodates misalignment of a saw cut for creating a control joint. Misalignment up to an angle substantially equal to the angle of the load plate&#39;s taper may be accommodated. 
   The tapered shape of the tapered load plate advantageously accommodates differential shrinkage of cast-in-place concrete slabs. When adjacent slabs move away from each other, the narrow end of the tapered load plate moves out of the void that it created in the slab. As the tapered load plate retracts, it will occupy less space within the void in the slab thus allowing the slabs to move relative to one another in a direction parallel to the joint. 
   Tapered load plates may be assembled into a load-plate basket with the direction of the taper alternating from one tapered load plate to the next. If a saw cut, used for creating a control joint, is positioned off-center relative to the tapered load plates, the alternating pattern of tapered load plates in the load-plate basket will ensure that the cross section of tapered load plate material, such as steel, spanning the joint remains substantially constant across any number of pairs of tapered load plates. For use in connection with a construction joint, an edge form may be used to position tapered load plates before the slabs are cast in place. 
   In accordance with an illustrative embodiment of the invention, a tapered load plate and an end cap, may be used to provide load transfer across an expansion joint. The tapered shape of the load plate will allow for misalignment. As either or both slabs expand and thereby cause the joint to close, the wide end of the tapered load plate moves farther into the end cap. This results in the allowance of an increasing amount of lateral movement between the slabs parallel to the joint  400  to the central and relatively wider portions of the tapered load plate occupying less space in the tapered void. 
   In accordance with an illustrative embodiment of the invention, a tapered-load-plate basket may be used to position the tapered load plates and compressible material before the concrete slabs are cast in place. 
   Additional features and advantages of the invention will be apparent upon reviewing the following detailed description. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a plan view of a concrete floor slab with random cracks caused by concrete shrinkage. 
       FIGS. 2A and 2B  are cross-section and plan views of saw-cut control joints. 
       FIG. 3  depicts vertical deflection of a floor slab under a load and damage to an adjacent floor slab. 
       FIGS. 4A and 4B  are cross section and plan view of dowel bars positioned for transferring loads across joints between adjacent slabs. 
       FIGS. 5A-5C  are plan and sectional views of a dowel basket for positioning dowel bars before a floor slab is cast in place. 
       FIG. 6  is a plan view of misaligned dowel bars locking a joint and thereby causing a slab to crack. 
       FIG. 7  is a plan view of cracks caused by dowel bars restricting relative movement of slabs parallel to the joint between the slabs. 
       FIG. 8  is a plan view showing corner cracking due to misaligned dowel bars and restricted relative movement of slabs parallel to the joints. 
       FIGS. 9A and 9B  are isometric and sectional views of a square dowel and square-dowel clip. 
       FIG. 10  is a side view of a typical expansion joint with compressible material in the joint. 
       FIG. 11  is a plan view of a diamond-shaped load plate between two slabs. 
       FIG. 12  is a plan view illustrating an off-center saw cut relative to diamond-shaped load plates. 
       FIG. 13  shows a top and two side views of a tapered load plate in accordance with an illustrative embodiment of the invention. 
       FIG. 14  is a plan view showing a misaligned saw cut relative to a tapered load plate. 
       FIG. 15  is a plan view of a tapered load plate, two slabs, a joint, and a void created by the narrow end of the tapered load plate. 
       FIG. 16  shows tapered load plates in a tapered-load-plate basket, wherein the orientation of the tapered load plates alternates from one tapered load plate to the next. 
       FIG. 17  is a plan view showing an off-center saw cut relative to three alternately oriented tapered load plates. 
       FIG. 18  is a plan view of an open expansion joint, a tapered load plate, and an end cap. 
       FIG. 19  is a plan view similar to  FIG. 18  with the joint having closed relative to  FIG. 18 . 
       FIG. 20  is a side view of an expansion-type tapered-load-plate basket, compressible material, a tapered load plate, and an end cap. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 13 , in accordance with an illustrative embodiment of the invention, a tapered load plate, such as tapered load plate  1300 , may be used to transfer loads across a joint between adjacent concrete floor slabs. The tapered load plate  1300  may have top and bottom surfaces that are tapered, substantially planar, and substantially parallel to one another. A triangular-shaped tapered top surface  1302  and two generally rectangular-shaped side surfaces  1304  and  1306  are shown in  FIG. 13 . The top and bottom surfaces may taper from approximately 4 inches wide to a narrow substantially pointed end  1308  over a length of approximately 12 inches. As will be apparent, other suitable tapered shapes and/or other suitable dimensions may also be used. 
   A tapered load plate  1300 , in accordance with an illustrative embodiment of the invention, advantageously accommodates misalignment of a saw cut for creating a control joint. Misalignment up to an angle substantially equal to the angle of the load plate&#39;s taper may be accommodated. Referring to  FIG. 14 , a misaligned saw cut  1400  is misaligned by an angle  1402  from correctly aligned saw cut  1404 , which is oriented perpendicular to the tapered load plate&#39;s longitudinal axis  1406 . The load plate&#39;s angle of taper is depicted in  FIG. 14  by angle  1408 . 
   Referring to  FIG. 15 , differential shrinkage of cast-in-place concrete slabs is advantageously accommodated by the tapered shape of the tapered load plate  1300 . When adjacent slabs, such as slabs  100 - 1  and  100 - 2 , move away from each other, as indicated by arrow  1500 , the joint  400  is said to open. As this occurs, the narrow end of the tapered load plate  1300  moves out of the void  1502  that it created in the slab  100 - 2 . As the tapered load plate  1300  retracts in this manner, it will occupy less space within the void in the slab  100 - 2  thus allowing the slabs  100 - 1  and  100 - 2  to move relative to one another in a direction parallel to the joint  400 . In other words, as the slabs move apart, the narrow end of the tapered load plate occupies less of the width of the tapered void  1502 . 
   Referring to  FIG. 16 , tapered load plates  1300  may be assembled into a load-plate basket  1600  with the direction of the taper alternating from one tapered load plate  1300  to the next. Referring to  FIG. 17 , if a saw cut  1700 , used for creating a control joint, is positioned off-center relative to the tapered load plates  1300 , the alternating pattern of tapered load plates  1300  in the load-plate basket  1600  will ensure that the cross section of tapered load plate material, such as steel, spanning the joint remains substantially constant across any number of pairs of tapered load plates  1300 . For use in connection with a construction joint, an edge form may be used to position tapered load plates before the slabs are cast in place. 
   Referring to  FIG. 18 , in accordance with an illustrative embodiment of the invention, a tapered load plate  1300  and an end cap  1800  may be used to provide load transfer across an expansion joint of the type discussed above in connection with  FIG. 10 . The tapered shape of the load plate  1300  will allow for misalignment, as discussed above in connection with  FIG. 14 . As either or both slabs  100 - 1  and  100 - 2  expand and thereby cause the joint  400  to close, the wide end of the tapered load plate  1300  moves farther into the end cap  1800 . This results in the allowance of an increasing amount of lateral movement between the slabs  100 - 1  and  100 - 2  parallel to the joint  400  due to the central and relatively wider portions of the tapered load plate occupying less space in the tapered void  1900 . 
   Referring to  FIG. 20 , in accordance with an illustrative embodiment of the invention, a tapered-load-plate basket  2000  may be used to position the tapered load plates  1300  and compressible material  1000  before the concrete slabs  100  are cast in place. 
   While the invention has been described with respect to specific examples including presently preferred modes of carrying out the invention, the invention is limited only by the following claims.