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CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. Ser. No. 09/166,336, filed Oct. 5, 1998, which issued on Dec. 19, 2000 as U.S. Pat. No. 6,161,339. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     FIELD OF THE INVENTION 
     This invention relates to arrangements for securing the roof of a structure against damage caused by high winds, earthquakes, and the like. 
     BACKGROUND INFORMATION 
     It is known to reinforce a building wall to resist wind and earthquake damage by the use of what will hereinafter be called a “top plate tie down” arrangement in which vertically disposed elongate fastening members that can be loaded in tension (e.g., a threaded metal rod) connect a top plate of the protected wall to an anchor beneath the wall, where the anchor is fixedly attached to a slab or is buried in or otherwise attached to the ground. As described in a parent application now issued as U.S. Pat. No. 6,161,339, a satisfactory anchor for such apparatus may be set in position prior to pouring a concrete foundation slab. The anchor can then be used both to retain a sill plate and to connect an elongate top-plate fastener to the foundation. 
     The use of conventional embedded anchors can lead to problems in installing a top plate tie down system if the anchors are not embedded at the proper positions along a sill or if the anchors are not set in a fully upright position. Because it is difficult to ensure that a correctly oriented anchor is located at each position where a top plate elongate fastener is to be installed, many builders would prefer to fasten anchors to an already hardened slab. 
     It is known, for example, to anchor a sill plate to a slab by driving through the sill plate into the slab and then gluing (e.g., with an epoxy cement) an anchor into the hole formed in the slab. If dust from the drilling operation is not carefully removed from the hole before inserting an epoxy-enrobed anchor, this approach results in an anchor with a very low pull-out strength. Although such an anchor may be satisfactory for retaining a sill plate against lateral forces, it can not safely be used as part of a top plate tie down apparatus. Glued, or otherwise bonded, anchors are generally not acceptable for top plate tie down use because of both the high likelihood of there being at least one dust-contaminated and weakened anchor along a wall, and because of the time and expense involved in running a separate pull-out test on each anchor. 
     Expansion-type anchors are widely used when a high pull-out strength connection must be made to a masonry support. Because this sort of anchor induces a high lateral stress in the masonry, it can cause portions of a masonry body to spall off if the anchor is placed too near a free edge of the body. Top plate tie-down arrangements are, of course, installed on exterior walls near the edge of a foundation slab. Hence, expansion anchors can not be used. 
     Self-tapping threaded masonry anchors are of interest to the present invention. Notable among commercially available hardware of this sort products sold under the trade name “Wedge-Bolt” by Powers Fasteners, Inc., of New Rochelle, N.Y. Patent references in this technical area include: 
     U.S. Pat. No. 5,674,035, wherein Hettich et al. teach a thread forming screw having ratios of the sizes of various portions of the screw selected to reduce screw-in torque; 
     U.S. Pat. No. 5,531,553, wherein Bickford describes a masonry anchor having a dust-relief groove disposed between thread lands; and 
     U.S. Pat. No. 4,439,077, wherein Godsted discloses a threaded fastener for use in hard aggregates. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention provides an improved anchor for a top plate tie down arrangement comprising a plurality of elongate vertical fasteners attached between the top plate and respective anchors disposed beneath the wall. In a preferred embodiment the anchor comprises a stud having one end adapted to be threaded into a concrete foundation slab and a second end threaded to receive a coupling nut for attaching the stud to a respective elongate vertical fastener. 
     It is an object of the invention to provide a top plate tie down apparatus connecting a top plate of a wall to a concrete foundation. As is conventional in construction practice, the wall extends upward from a sill plate placed on the concrete foundation and having a plurality of generally vertical throughholes through it, where the throughholes can be formed either before or after placement of the sill plate on the sill. The inventive apparatus preferably comprises a selected number of anchors, where the number of anchors is generally selected to match the number of throughholes in the sill. Each of these anchors has a respective portion threadably engaging the foundation beneath the wall along an embedment length of the anchor, each anchor has a respective upper portion threaded along at least a selected penetration length that is selected to accord with the accessible threaded depth of a connecting nut, and each of the anchors has a length equal to a sum of the penetration length, the sill thickness and the embedment length. In addition, the preferred apparatus comprises the selected number of vertical tensile fasteners, where each of the vertical tensile fasteners is connected to the top plate—e.g., by means such as those shown in the parent application hereto. Each of these vertical tensile fasteners further comprises a respective lower threaded portion at a respective lower end thereof, where each of the lower threaded portions has a selected lower portion thread length that, like the thread length on the anchor, is selected to accord with a connecting nut Each of the connecting nuts has a length at least as large as the sum of the penetration length and the selected lower portion thread length and has a first end threaded onto a respective one of the anchors, and a second end threaded onto the respective lower threaded portion of a respective one of the vertical tensile fasteners. 
     It is an additional object of the invention to provide a method of attaching a top plate of a wall to a slab disposed beneath the wall. The preferred method begins with a step of drilling a selected number of holes into the slab, where each of the holes extends into the slab by more than an embedment length of an anchor bolt, and preferably by about one bolt diameter more than the embedment length. An anchor bolt is then inserted through a throughhole in a sill plate into each of these holes and turned so as to thread the anchor bolt into the hole. In the preferred method a connecting nut threaded onto an upper threaded portion of each anchor bolt provides a set of flat surfaces that can be gripped by a wrench and used to turn the bolt into the hole. Moreover, it is also preferred to place a washer between the connecting nut and the sill plate before turning the bolt into the hole so as to effectively capture the sill plate between the connecting nut and the slab without deforming the sill plate. It will be understood by those skilled in the art that this method can be carried out by the use of a sill plate that has pre-drilled throughholes, or by drilling through the sill plate when drilling the hole into the slab. 
     Although it is believed that the foregoing recital of features and advantages may be of use to one who is skilled in the art and who wishes to learn how to practice the invention, it will be recognized that the foregoing recital is not intended to list all of the features and advantages. Moreover, it may be noted that various embodiments of the invention may provide various combinations of the hereinbefore recited features and advantages of the invention, and that less than all of the recited features and advantages may be provided by some embodiments. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     FIG. 1 is an elevational view taken perpendicular to a framed wall and showing a plurality of roof framing members transverse to the wall anchored to a foundation beneath the wall. 
     FIG. 2 is a partly cut-away elevational detail view of an anchor embedded in the foundation. 
     FIG. 3 is a partly cut-away elevational view of a preferred anchor. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning now to FIG. 1, one finds a wood framed wall  10  standing on a concrete foundation slab  12  and having a tie down apparatus  13  added thereto. The wall  10  may comprise a sill  14  or foot member laid upon the foundation  12  and bolted thereto; a plurality of vertically disposed framing members  16  or studs, and a top plate  18  that is fastened across the top of the studs  16 . A roof  20 , conventionally supported by the wall  10 , comprises a plurality of roof framing members  22  transverse to the wall  10  and above the top plate  18 . Although the preferred embodiment is depicted with reference to a wooden framed wall, other sorts of wall construction may also be employed. For example, a metal framed wall, of the type commonly used in commercial building construction could be employed. So, for that matter, could a concrete block or brick wall having a top plate  18  disposed thereupon. Moreover, although the invention is herein described and depicted with respect to an exterior wall of a building, the same arrangement could clearly be applied to an interior wall crossed by one or more roof members. 
     It is conventional in wall construction for a foundation  12  to be made with a selected number of anchors  30  set into the slab. These anchors are spaced out along a line for the purpose of bolting the sill  14  to the foundation  12 . One approach to doing this is to insert a plurality of anchors  30  into the wet concrete of the foundation  12  before the concrete has set. Another is to suspend a plurality of anchors from a horizontal board positioned at the top of the pouring frame and to then pour concrete over the suspended anchors. The bottom end of each anchor  30  is configured to extend laterally outwards (e.g., by clamping a washer  32  between two nuts  34 , or by providing a elbow-like bent portion  36 ) so that the anchor  30  can not be pulled out of the foundation  12 . The plurality of anchors  30  are spaced out along the centerline of the wall  10 , and a corresponding plurality of throughholes are cut into the sill  14  so that when the sill  14  is placed upon the foundation  12  a threaded upper end  38  of a respective anchor  30  projects through each hole. A washer  32  and nut  34  are then put on each anchor  30  in order to secure the sill  14  to the foundation  12 . In an embodiment of the invention disclosed in parent application 09/166,336, now issued as U.S. Pat. No. 6,161,339, similar arrangements are used, but the anchors  30  are selected to have a threaded upper end  38  projecting somewhat higher above the sill than would be the case for a conventional wall so that a connecting nut  40  can be used to connect each anchor  30  to a respective vertical rod  42  portion of the tie down apparatus  13 . That is, the anchor  30  of the preferred embodiment serves both the conventional purpose of bolting the sill to the foundation, as well as serving as part of a means of tying the top plate  18  to the foundation  12 . 
     In an embodiment described in parent application 09/166,336, now issued as U.S. Pat. No. 6,161,339, the vertical rod has a threaded region  43  on its lower end. The length of the threaded region is selected to be a bit less than half the length of a connecting nut  40 . In one embodiment the connecting nut  40  is one and three quarters inches long and has an internal stop  51  formed by punching a portion of the connecting nut&#39;s wall inward so as to limit the penetration depth of a screw thread to be no more than three quarters of an inch. In this case a threaded region  43  having a length of three quarters of an inch is provided on the rod. A worker assembling this tie down apparatus  13  is instructed to initially fully thread the connecting nut  40  to the rod  42 . The rod  42  is then placed vertically above the anchor  30 , and the connecting nut  40  is threaded onto the upper end  38  of the anchor  30  by turning the rod  42 . This assures that the same number of threads on each of the two threaded regions  38 ,  43  are captured by the nut so as to provide the strongest possible connection. Prior art top plate bolting arrangements employing a rod threaded along its entire length did not provide this means of assuring that the rod and anchor are joined in a maximum strength configuration. Those prior art arrangement allowed a worker to assemble a connection that is acceptable to all outward appearances, but that is seriously weak because only one thread is engaged on either the rod or the anchor. 
     In a preferred embodiment, as depicted in FIG. 3, a threaded anchor  24  is turned into a hole  26  drilled into a hardened foundation slab  12  so as to capture a sill plate  14  between a connecting nut  40  and the slab  12 . In one particular case the threaded anchor  24  has an overall length of about nine inches. At one end of this anchor there is a embedment length portion  44  about six inches in length that has a nominal half inch self-tapping lead thread formed on it. The lead thread preferably comprises a helical land having a relatively high helix angle and a helical dust relief groove formed in the body of the anchor. At the other, upper, end there is a second threaded portion  38  adapted to engage a connecting nut  40 . This portion generally has a length about one half the length of an associated connecting nut and may, for example, be about three quarters of an inch long with a {fraction (7/16)}×12 thread. In a preferred embodiment an unthreaded intermediate portion  49  of the anchor has a length approximately the same as the thickness of lumber used for forming a sill plate  14 . As depicted, the preferred arrangement accommodates a washer  32  between the connecting nut  40  and the sill plate. In the exemplar case, the intermediate portion  49  has a length of about one and three quarters inch. It will be understood by those skilled in the art that as long as enough of the upper portion of the anchor is threaded, the penetration depth of the anchor into the connecting nut is limited by an internal stop  51  in the middle of the connecting nut, and not by the threaded length of the upper portion, Hence, it is really not important whether the intermediate portion of the anchor is threaded or not. In any event, as long as the hole is deep enough, the overall length of the self threading anchor will be approximately equal to the sum of the penetration depth of the anchor into the connecting nut, the sill thickness and the embedment length. 
     To install the preferred self-threading anchor  24 , a hole is drilled through the sill plate and into the concrete slab, preferably by using a special drill bit designed for drilling pilot holes for fasteners that have the self tapping lead thread on the anchor. The preferred hole extends into the slab to a depth of about one anchor bolt diameter (e.g., one half inch) longer than the embedment length  44  of the anchor, and is preferably cleaned (e.g., by means of one or more blast(s) of compressed air) before the anchor  24  is inserted. A connecting nut  40  having a limited thread extent (e.g., that has a detent or other center stop  51 ), is turned onto the end of the anchor that will be uppermost after installation, a washer  32  is placed around the anchor shaft, and the anchor  24  is turned into the hole so as to tightly capture the sill plate  14  between the washer  32  and the foundation  12 . 
     In the top plate tie down system taught in parent application 09/166,336, now issued as U.S. Pat. 6,161,339, a cable  60  disposed above the top plate  18  is tensed by tightening a respective turnbuckle  48  on each of a plurality of rods  46 . In an arrangement of this sort, if any one of the anchors disposed along a wall pulls out of the slab, the tension in the cable  60  is relaxed. Hence, it is important that each anchor be reliably tied to the slab. 
     It is easy to test the preferred anchoring arrangement disclosed above to ensure that each and every anchor is secure. Inspection of the anchor is a two-step process in which the inspector first checks to see that the washer  32  is not loose and then tries to apply a test torque to the connecting nut with a pre-set torque wrench. If the connecting nut does not turn responsive to the test torque, the inspector can conclude that the embedment portion of the anchor is securely engaging the slab. 
     From the foregoing, it can be seen that the invention provides a preferred method of securing a wall to a foundation so as to resist severe wind loads and other stresses tending to detach the roof from the wall, the method comprising the steps of: 
     a) inserting each of a selected number of anchor bolts into respective holes drilled into a hardened foundation slab so that each anchor bolt presents a vertically oriented threaded upper end extending above a sill. These anchor bolts are spaced out along the center line of the wall and extend through respective throughholes in the sill. 
     b) tightening each anchor bolt, by means of a respective connecting nut threaded onto the threaded upper end, so that the sill is captured between the connecting nut and the foundation slab; 
     c) threadably connecting a rod having a length less than the distance between the sill and a top plate of the wall to the upper end of each anchor bolt by means of the respective connecting nut. 
     d) connecting each rod to tie-down apparatus above the top plate. 
     Although the present invention has been described with respect to several preferred embodiments, many modifications and alterations can be made without departing from the invention. Accordingly, it is intended that all such modifications and alterations be considered as within the spirit and scope of the invention as defined in the attached claims.

Summary:
Top plate tie-down arrangements are used for securing the roof of a structure against damage caused by high winds, earthquakes, and the like by anchoring the top plate of a wall to a foundation slab. An anchor for use in a top plate tie-down arrangement has a self-tapping thread on one end that allows it to be threaded into a hole drilled into the slab. The upper end of the anchor, which protrudes through a sill plate, is threaded to engage a connecting nut that ties the anchor to an elongated vertical fastener attached to the top plate. The lengths of various portions of the anchor and of the hole into which it is threaded are selected so that the sill plate is captured between the connecting nut and the slab.