Abstract:
A self-expanding and self-undercutting guard rail bolt includes a dowel having a guard rail fastening end and a ring. The fastening end is configured to be driven in rotation, and the ring is configured to rigidly connect to a first screwing end of the dowel by an incipient fracture portion. The self-expanding and self-undercutting guard rail bolt further includes a counter-dowel screwed to the dowel by a second screwing end. The counter-dowel includes an expansion cone, and an anti-rotation head having at least one edge which projects beyond a periphery of the dowel to prevent rotation of the counter-dowel about an axis of the guard rail bolt.

Description:
RELATED APPLICATIONS 
     The present application is a National Phase entry of International Application Number PCT/IB2005/000898, filed Apr. 6, 2005, which claims priority from, French Application No. 0403675, filed Apr. 8, 2004, the disclosure of which is hereby incorporated by reference herein in its entirety. 
     BACKGROUND 
     This invention relates to bolts used in the building industry serving to fasten guard rails to concrete slabs, e.g. apartment balcony slabs, and referred to as guard rail bolts. 
     These bolts traverse the shoes of these guard rails via holes provided to this end and are driven into the concrete drilled in advance. Each shoe is held against the slab by virtue of a nut screwed on to the threaded head of the bolt and mounted on the plate in which the holes in the shoe have been provided. 
     In order to fix a bolt, the concrete slab is drilled to a sufficient length, to the diameter of the bolt, opposite each hole in the shoe, the bolt is driven in by striking its head, then the nut is screwed on, thereby ensuring fastening of the shoe. 
     The bolts used are generally of the self-undercutting and self-expanding type, i.e. capable upon fixing, on the one hand, of enlarging in a conical manner the hole drilled in the concrete at the end of the bolt and, on the other hand, of being expanded there, by the forced deformation of expansion lugs by means of a cone, in such a manner that it remains in contact with the walls of the hole having the diameter achieved in this manner. 
     The aim of the undercut is to relieve the stresses in the concrete around the hole in which the bolt is anchored. As the concrete does not remain locally prestressed once the bolt has been fixed, it consequently does not have the tendency to spall, in particular at the edge of the slab. 
     Normally, once the slab has been drilled, the undercut is produced when the bolt is driven into the concrete by striking and/or rotating its threaded head via a sleeve provided with a carbide coating to this end and covering the bolt, then the tightening of the nut results in the expansion of the lugs over a cone rigidly connected to the bolt, on the one hand, as a result of the reduction in the size of the bolt due to the tightening and, on the other hand, as a result of the fact that the lugs are held at this depth by the sleeve. 
     This fixing or fastening method has several disadvantages:
         the presence of the sleeve means that the concrete has to be drilled to a diameter greater than the diameter of the bolt, e.g. in the case of a bolt having a diameter M 12  of the international metric system, it is necessary to drill a hole of 18;   if the holes in the shoe are not drilled to this diameter, but to the smaller diameter of the bolts, the method also means that the concrete has to be drilled not through the shoe put in place in order to serve as a drilling guide, but before the shoe is placed in position, which may lead to drilling centre distances aligned incorrectly with those of the holes in the shoe;   as the sleeve has a predetermined length, the bolts can only be used for one single driving depth;   the undercut formed by the driving-in operation prestresses the concrete around the bolt before the tightening of the nut and therefore before the expansion of the bolt, as a result of which the mounting of the shoe of the guard rail on the slab cannot be controlled properly.       

     SUMMARY 
     The aim of this invention is to obviate these disadvantages. 
     To this end, the applicant firstly proposes a method of fastening a guard rail to a concrete slab by means of a self-expanding and self-undercutting bolt comprising a dowel having expanding lugs and an expansion core, the method comprising a phase consisting in drilling a hole in the slab, a phase consisting in driving the bolt in to a desired depth independent of the depth of the hole, a dynamic tightening phase resulting in the formation of the undercut and a static tightening phase of the guard rail. 
     The term “dynamic tightening” refers to tightening leading to the displacement of the expansion lugs or of the formation of the undercut with respect to depth. 
     The term “static tightening”, on the other hand, refers to tightening keeping the expansion lugs or the formation of the undercut at the same depth. 
     The fundamental difference between the method of the invention and that of the prior art is that no undercut is formed when the bolt is driven in, thereby preventing prestressing of the concrete remaining before the terminal static tightening, in particular torsional stress when the undercut is formed by turning the bolt. 
     The dynamic tightening is advantageously carried out by relative screwing of the dowel and the expansion core to a given depth. 
     In order to carry out the method of the invention, the applicant additionally proposes a self-expanding and self-undercutting guard rail bolt comprising a dowel and a counter-dowel screwed together by means of their screwing ends, the dowel comprising at its fastening end a guard rail fastening head designed to be driven in rotation and rigidly connected at its screwing end by means of incipient fracture means to a ring provided with expansion lugs, the counter-dowel comprising at its other expansion end an expansion cone and anti-rotation means, the expansion lugs comprising means for forming an undercut. 
     In addition to the fact that the driving depth can be controlled by virtue of the anti-rotation means, as no retaining sleeve is required for the expansion lugs, the concrete can be drilled to the diameter of the bolt and therefore through the fastening holes in the shoe. 
     In addition, once the undercut has been formed, the internal stresses of the concrete around the hole are relieved and the concrete consequently does not have the tendency to spall, particularly at the edge of the slab, as is often the case when a guard rail is to be fastened. 
     The applicant finally proposes a tool for fixing the guard rail bolts according to the invention comprising means for driving them in rotation and complementary means for controlling the depth to which the bolt is driven into an anchoring hole. 
     As the driving of the bolt into the hole provided in the concrete slab does not depend on the depth of the hole, it can be fastened precisely by the fixing tool without the user having to take any special measures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be more readily understood with the aid of the following description of the bolt of the invention, the tool and the method of fixing this bolt, with reference to the accompanying drawings, in which: 
         FIGS. 1 and 2  are respectively a perspective view and an axial section of the bolt according to the invention; 
         FIGS. 3 and 4  are respectively a perspective view and an axial section of the tool for fixing bolts according to the invention; 
         FIG. 5  is a cross section of the fixing tool along the line AA of  FIG. 4 , and 
         FIG. 6  is an axial section of a guard rail shoe fastened to a concrete slab by means of two bolts according to the invention, each during a different phase of the fixing operation. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIGS. 1 and 2 , the guard rail bolt  1  having a nominal diameter D comprises a dowel  10  having an axis  5  and a counter-dowel  20  coaxial with the dowel  10 . 
     The dowel  10  has a generally cylindrical shape and is provided on its first end  11  referred to as the fastening end with a head  13  for fastening the guard rail or any other furniture to be mounted on a concrete slab, in this case a threaded head. 
     The head  13  is topped by a pin  14  having a height t, designed to be driven in rotation, in this case by two flats  140  which are symmetrical relative to the axis  5  of the bolt. 
     At the second end  12 , referred to as the screwing end, the dowel  10  is provided with a ring  16  comprising expansion lugs  17  rigidly connected to the remainder of the dowel by an incipient fracture groove  15 . Fracture occurs when a certain torque C having an axis  5  is applied to the pin  14  of the dowel. 
     The expansion lugs  17  in this case comprise circular teeth  170  for the formation of an undercut on the wall of the hole drilled in the concrete in order to receive the bolt, as will be described hereinafter. 
     The counter-dowel  20  comprises on its first end  21 , referred to as the expansion end, an expansion cone  23  for the lugs  17  and an anti-rotation head  24  having an, in this case, square cross section, the edges  240  of which project slightly beyond the nominal diameter D of the bolt, or of the hole drilled in the concrete, to a sufficient extent to prevent rotation of the counter-dowel about the axis  5  as a result of the torque C by means of friction. 
     The counter-dowel  20  is screwed by means of its second screwing end  22  into a tapped bore  19  having an axis  5  provided in the screwing end  12  of the dowel  10 . 
     When the counter-dowel  20  is screwed into the dowel  10  in such a manner that the lugs  17  are flush with the cone  23 , the bore  19  still allows the end  22  to be screwed over a length H greater than a certain length h which will be determined hereinafter, and the bolt  1  is ready for use. 
     Referring to  FIGS. 3 to 5 , the tool  2  for fixing the bolt  1 , having a generally cylindrical shape and an axis  5 ′, is designed to rotate the dowel  10  about its axis  5  by means of its pin  14  when the bolt  1  is introduced into the hole drilled in the concrete. 
     To this end, it comprises a cylindrical drive sleeve  35  having a length e 2  designed, as will be seen in the cross section of  FIG. 5 , to follow the contours of the flats  140  of the pin  14  at is lower end  36  when the axes  5  and  5 ′ coincide and when it is positioned on the head  13  of the bolt  1 . 
     This drive sleeve  35  plays freely in translation along the axis  5 ′ guided between two end limits by a stop guide  31 . These limits are determined by the upper face  37  of the lower end  34  of the stop guide  31 , having a length e 1  and a cross section  33  reduced to such an extent that it will not allow for the passage of the sleeve  35 , and the upper limit resulting from the limit compression I of a spring  38 . 
     The spring  38  is compressed between the sleeve  35  and the lower face  41  of a plug  50  closing the cylindrical free space containing it. When the maximum axial length of the spring  38  has been reached, the lower end  36  of the sleeve  35  is in contact with the upper face  37  of the lower end  34  of the stop guide  31  and these two faces are then at a distance from the lower face  41  of the plug  50  by a length L. 
     The spring  38  allows the lower end  36  to be applied constantly to the head  13 , thereby holding the pin  14  in the interior of this end. 
     A spindle  39  having a length b rigidly connected to the plug  50  and having the same cross section as the pin  14  extends the plug until it penetrates into the sleeve  35  and allows it to be driven in rotation when the plug  50  is itself driven in rotation. 
     To this end, the plug  50  is rigidly connected to a hexagon head  32  by means of which the tool  2  can be rotated, being held by the external sleeve  30  without affecting its rotation about the axis  5 ′. 
     The use of the tool  2  and the operation of the bolt  1  will now be described with reference to  FIG. 6 . 
     When a shoe  61  of the guard rail  60  having a thickness E 1  and comprising two holes  62 ′ and  62 ″ having a diameter D 1  compatible with the nominal diameter D is to be fastened to the surface  101  of a concrete slab  100 , in a drilling phase, the shoe  61  is placed in position and holes  103 ′ and  103 ″ having a diameter D 2  corresponding to the diameter D are drilled in the slab, through the latter. 
     There is no need for the drilling length to be precise. It simply has to be larger than the driving depth of the bolt. 
     As the counter-dowel  20  has been screwed into the dowel  10  until the lugs  17  lightly touch the cone  23 , in a driving phase, the bolt  1 ′ ready for use is forced into the hole  62 ′ to the desired depth, in this case in such a manner that the threaded part of the head  13  projects from the shoe  61  by a desired height E 2  generally equal to the sum of the thicknesses of a fastening nut and a washer (not shown) increased by the length h. In  FIG. 6 , the bolt  1 ′ is shown in this position and in this state. 
     The anti-rotation head  24  of the counter-dowel  20  holds the bolt  1 ′ at this depth. 
     The axis  5 ′ of the tool  2 ′ and the axis  5  of the bolt are made to coincide so that the pin  14  engages in the lower end  36  of the sleeve  35  returned by the spring  38  to the pin  14  and the tool is pressed in order to bring the lower face  33  of the stop guide  31  to bear against the upper face  63  of the shoe  61 . 
     Then, during a dynamic tightening phase, the undercut is formed by the teeth  170  of the lugs  17  by rotating the head  32  by means of an allen key. 
     In so doing, the plug  50 , the spindle  39  rigidly connected thereto, the sleeve  35  and the pin  14  are rotated by virtue of the shape of the cross section of the contact surfaces between the sleeve, the spindle and the pin, the dowel  10  being screwed on to the counter-dowel  20  and held fixed in rotation and translation by the anti-rotation head  24 . 
     When it is driven in, the dowel  10  drives the lugs  17 , which are expanded by the cone  23  to a predetermined depth h and thus form the undercut in the hole  103 ′. The tightening torque due to the screwing gradually increases to the value C required to obtain fracture of the groove  15 . The undercut is produced. In  FIG. 6 , the bolt  1 ″ is shown in this position and in this state. 
     Upon the fracture of the groove  15 , the torsional stresses are relieved and the ring  16  locked by the undercut locks the dowel  10  with respect to its depth, the dowel then drawing the counter-dowel  20 , and with it the cone  23 , towards it from that point on. 
     After the anchoring operation and during a static tightening phase, the fastening nut is screwed on to the threaded head  13  of the bolt in order to mount the guard rail by firmly fastening the shoe  61 . In the event of overload, abnormal tensile forces may result in additional expansion of the lugs  17  and the dowel  10  can then be drawn very slightly towards the exterior of the hole. 
     During this tightening phase, the bolt  1 ′ thus remains driven in to the desired depth corresponding to the bolt projecting above the surface  101  of the concrete by a height E 1 +E 2 . 
     The tool  2  is dimensioned to facilitate and ensure the observance of this height so that the fastening nut is screwed fully on to the threaded head  13 , without the latter extending beyond it. 
     In particular, irrespective of the thickness E 1  of the shoe  61 , the following must apply:
 
 e 1 +h&lt;E 2 &lt;e 1 +L −1.
 
The height h can be determined precisely as a function of the torque C. It depends on the properties of the concrete. The following must therefore apply:
 
 L −1 &gt;h,  
 
thereby ensuring an optimum dynamic tightening phase.
 
     The tool  2  moreover makes it possible to control the depth to which the bolt is driven into the hole  103 . To this end, the following must apply:
 
 e 2+1 −b=t,  
 
and
 
 e 2 +e 1 +L−b=E 2 +t−h.  
 
     Firstly, the length e 2  of the sleeve  35  must be equal to the length b of the spindle increased by the length t of the pin  14 , but reduced by the minimum length l of the compressed spring  38 . 
     Secondly, the length e 1  of the reduced section of the lower end  34  of the stop guide  31  is a function of the desired projection of the bolt above the surface  63  of the shoe  61 , E 2 −h, i.e. the thickness of the fastening nut. 
     If these conditions are satisfied, the bolt  1  can be driven through the shoe into the hole  103 ′ or  103 ″ in the slab  100  by hammering the hexagon head  32  of the tool  2  in place on the bolt until the tool  2  comes to bear via its lower face  33  against the upper face  63  of the shoe. 
     By using the tool  2  in this manner, the desired driving depth of the bolt, or the projection thereof from the shoe, is obtained in a very precise manner. 
     The advantage of this method resides in the fact that it comprises, after the drilling of the slab, a first phase consisting in driving the bolt in to a depth closely dependent on the desired driving depth, during which no undercut is formed, thereby preventing prestressing of the concrete before anchoring, then a second dynamic tightening phase, the dowel always being driven in while the counter-dowel is static, thereby resulting in the formation of the undercut, and finally a third static tightening phase, continuing the expansion of the bolt, with only the counter-dowel moving, drawn by the dowel.