Abstract:
A tensioning arrangement uses a tendon to pull together (pre-stress) a hollow steel arm and a bracket in a localized area around the joint between the arm and the bracket in order to induce localized compressive stresses that will reduce the wind-induced, intermittent tensile stresses and hence minimize fatigue at the joint between the arm and the bracket.

Description:
This application claims priority from U.S. Provisional Application Ser. No. 61/977,131 filed Apr. 9, 2014. 
    
    
     BACKGROUND 
     Steel power transmission poles have steel arms to support electric conductors. The steel arms are often tubular in cross section with varying lengths that may reach 20 feet or greater. The steel arms project horizontally from the steel poles. The steel poles also have shorter steel arms, typically located at the very top of the pole, to support the ground wires, also referred to as the shield or static wire arms. Each of the steel arms is made of a steel shell (tube) and has a hollow interior. The steel arms are welded to a bracket, which is then bolted to steel plates that are welded to the steel pole. The tubular steel arms can be tapered or non-tapered. 
     Wind loads on these arms can cause the arms to vibrate. The repeated flexing at the welded joint creates fatigue stresses, which may cause fatigue cracking at the welded joint. In order to address this problem, the practice has been to secure the arms to the pole with tie down cables immediately upon installation. Also, the specifications for welding the arms to the brackets require a substantial weld which requires a substantial expenditure of time and material for the welding process. 
     SUMMARY OF THE INVENTION 
     The purpose of the present invention is to greatly reduce the flexural tensile stresses that occur at the joint of the arm and the bracket in order to reduce or eliminate the problem of fatigue cracking or failure at the joint. This is accomplished by providing a localized prestressing arrangement that includes a short tendon that puts the joint in compression, so that localized compression stresses are created in the critical area of the joint to counteract the high tension stresses caused by the wind induced vibration causing flexing of the steel arm. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a broken-away, perspective view of a steel electric power transmission pole with a horizontal steel transmission arm welded to a bracket which is in turn bolted to steel plates welded to the steel pole; 
         FIG. 2  is a section view through the transmission arm and bracket, taken from the back side of  FIG. 1 , with the arm broken away, and showing the internal tensioning mechanism; 
         FIG. 3  is a view taken along the section  3 - 3  of  FIG. 2 ; 
         FIG. 4  is a view taken along the section  4 - 4  of  FIG. 2 ; 
         FIG. 5  shows an alternative embodiment in which the tensioning arrangement of  FIGS. 1-4  is used to provide tension between a vertical steel pole and a horizontal base plate; 
         FIG. 6  is a view, similar to  FIG. 3 , showing an alternative embodiment that uses several tensioning rods; 
         FIG. 7  is a view similar to  FIG. 2  but showing another alternative embodiment, with eccentric tensioning mechanisms; 
         FIG. 8  is a view taken along the section  8 - 8  of  FIG. 7 ; 
         FIG. 9  is a view taken along the section  9 - 9  of  FIG. 7 ; 
         FIG. 10  is a view similar to  FIG. 2  but showing another alternative embodiment; 
         FIG. 11  is a view similar to  FIG. 2  but showing another alternative embodiment; 
         FIG. 12  is a view taken along the section  12 - 12  of  FIG. 11 ; and 
         FIG. 13  is a view taken along the section  13 - 13  of  FIG. 11 . 
     
    
    
     DESCRIPTION 
       FIG. 1  shows a steel pole  10  with a horizontal arm  12  projecting from the pole  10 . (The arm has a rise but is generally referred to as being horizontal.) The arm  12  also is made of steel. The arm  12  is welded to a bracket  14  along a joint  18  which extends around the perimeter of the proximal end of the arm  12 . The bracket  14  is bolted to the pole  10  by means of bolts  16  which extend through holes  16 A in the bracket  14  (See  FIG. 2 ). As shown in  FIG. 1 , there may be a second arm mounted on the opposite side of the pole  10  in the same manner. There may be additional horizontal steel arms mounted at other positions on the pole  10  at various elevations as well. 
     The arm  12  and bracket  14  are symmetrical about an imaginary vertical plane. 
       FIGS. 2-4  show the details of the arm  12  and bracket  14  which include a pre-stressing mechanism that puts the joint  18  in compression in order to reduce or eliminate the flexural tensile stresses, induced by wind induced vibrations, occurring in the vicinity of the joint  18 . 
     As shown in  FIG. 2 , this particular bracket  14  is made of plates welded together, with a first plate  14 A welded to the proximal end of the arm  12  around substantially the entire perimeter of the arm  12  at the welded joint  18 , and a second plate  14 B, welded to the back of the first plate  14 A along the welded joints  18 A (See also  FIG. 4 ) at the top and bottom of the second plate  14 B. The portions  14 A′ of the bracket  14  that have the bolt holes  16 A are made up of two plates welded at right angles to the portion of the first plate  14 A that is welded to the proximal end of the arm  12  at the welded joint  18 . The two plates  14 A and  14 B function together as the base plate for securing the arm  12  to the bracket  14  and for tensioning the arm  12 . The bracket  14  alternatively could be made as one piece from cold bent steel by bending a single plate  14 A, and reinforcing plates could be added as desired. 
     The pre-stressing mechanism includes a steel plate  15 , which is welded to the inside of the arm  12  at a desired axial distance from the joint  18 . That axial distance (the length of the tendon  22  between the bracket  14  and the internal steel plate  15 ) preferably is from 1.5 to 3 times the outside diameter of the arm  12  at the joint  18  in order to provide a localized prestress that induces compressive stresses at the joint  18 . 
     In  FIGS. 2-4 , the plate  15  is substantially perpendicular to the longitudinal axis  28  of the arm  12 . In this embodiment, the tendon  22  is a threaded rod. The tendon  22  alternatively could be a cable or other linear tensioning member. The proximal end of the tendon  22  bears against the bracket  14 , and the distal end of the tendon  22  bears against the plate  15 . The tendon  22  extends through a central through opening  24  in the plate  14 B of the bracket  14  and through a central through opening  20  in the plate  15  and is tensioned by means of nuts  26  at both the distal and proximal ends. There is a beveled washer  26 A between the plate  14 B and the nut  26  at the proximal end, which compensates for the angle between the plane of the plate  14 B and the axis  28  of the tendon  22  being slightly off of perpendicular. The beveled washer  26 A provides a surface that is perpendicular to the axis  28  of the tendon  22 , and the proximal nut  26  bears against that surface. The distal nut  26  is welded to and bears against the plate  15 , and the proximal nut  26  bears against the plate  14 B of the bracket  14 . (As an alternative, instead of using the distal nut  26 , the through opening  20  in the plate  15  could be threaded.) The openings  20 ,  24  are aligned along the central axis  28  of the arm  12 , so the tendon  22  provides an axial tensioning force, pulling the arm  12  and bracket  14  together in the axial direction and thus inducing axial compression. 
     This axial compression greatly reduces the flexural tensile stresses at the joint  18  induced by flexing of the arm  12  due to wind induced vibration (depicted schematically by the arrow  30  of  FIGS. 5 and 10 ) thereby greatly reducing or eliminating fatigue cracking/failure at the joint  18 . 
       FIG. 5  shows an alternate embodiment in which a similar tensioning mechanism is used to provide axial tension between a vertical steel pole or arm  10  and a bracket that includes a horizontal base plate  14 ′, which secures the pole or arm  10  to a foundation (not shown) by means of bolts  16 ′. The pole  10  is welded to the horizontal base plate  14 ′ at the welded joint  18 ′. As with the first embodiment, this embodiment is symmetrical about an imaginary vertical plane. In this case, the tendon  22 ′ provides tension along the vertical axis  28 ′ of the pole  10 , putting the joint  18 ′ between the pole  10  and base plate  14 ′ in compression. A horizontal steel plate  15 ′ is welded to the inside of the pole  10 , and through openings  20 ′ and  24 ′ in the plate  15 ′ and base plate  14 ′, respectively, are aligned along the vertical axis  28 ′ of the pole  10  and receive a threaded rod  22 ′, which threads into the nuts  26 ′ at both ends to tension the threaded rod  22 ′. The distal nut  26 ′ preferably is welded to the plate  15 ′ to prevent it from spinning as the threaded rod  22 ′ is threaded into the nut  26 ′. The length of the rod  22 ′ (the tendon) between the base plate  14 ′ and the internal plate  15 ′ preferably is between one and three times the outside diameter of the pole  10  at the joint  18 ′ between the pole  10  and the base plate  14 ′. Again, this tensioning arrangement pulls the pole  10  and base plate  14 ′ together in the axial direction, putting the localized area around the joint  18 ′ in compression. This reduces the flexing in the area of the joint  18 ′ and thereby reduces the induced tensile stresses in the area of the joint  18 ′ as the majority of the pole  10  flexes relative to the base plate  14 ′, thereby reducing or eliminating the problem of fatigue at the joint  18 ′. 
     By putting the localized area of the joint between the pole/arm and the bracket in compression in this manner, it may be possible to use a much simpler and less expensive weld at the joint or even to completely eliminate the need for a weld at the joint. If the weld were eliminated, the arm and bracket would be secured together by compression alone. 
     While these embodiments show a single tendon aligned with the axis of the arm, it would be possible to use one or more tensioning rods or tendons  22 ″ (See  FIG. 6 ) which is (or are) offset from the axis of the arm  12 ′ and still apply a localized tensioning force in the axial direction. The tensioning rods (or tendons)  22 ″ in  FIG. 6  are concentrically arranged, but various other eccentric arrangements, or an arrangement that includes offset tendons and an axial tendon could be used as well to induce compressive stressed at the most desired locations, and the number of tendons may be one or more, as desired. 
       FIGS. 7-9  show an example of an eccentric arrangement which is similar to the first embodiment, shown in  FIGS. 2-4 , but in which two tendons  22 * are located above the central axis  28 * of the arm  12 *. In this embodiment, instead of having a nut welded to the plate  15 *, the openings in the plate  15 * are threaded, and the threaded rods  22 * are threaded into those threaded openings. 
       FIG. 10  shows another alternative embodiment, which is similar to the embodiment of  FIG. 2  except that, instead of welding the plate  15 A to the inside of the pole  12 A, a ring  15 C (or a series of blocks or lugs or abutments) is secured (as by welding or bolting, for instance) to the interior of the pole  12 A, and the plate  15 A is secured to the pole  12 A by compression force, with the tendon  22 ** causing the plate  15 A to bear against the ring  15 C (or blocks or lugs or abutments). 
       FIGS. 11-13  show another alternative embodiment, in which the interior plate  15 D projects through rectangular openings  15 E in the wall of the arm  12 B, so the interior plate  15 D is not only welded to the wall of the arm  12 B but bears directly against the edges of the openings  15 E. The edges of the openings  15 E against which the interior plate  15 D bears lie in an imaginary plane that is perpendicular to the axis  28 * of the tendon  22 B. In this arrangement, the weld between the interior plate  15 D and the wall of the arm  12 B may be omitted if desired. In this embodiment, the tendons  12 B lie on opposite sides of the axis  28 * of the arm  12 B and are equally spaced from the axis  28 *. The openings  26 B in the interior plate  15 D are threaded, so the threaded rods  22 B thread into those threaded openings  26 B. 
     The bracket in this embodiment differs from the first embodiment in that, instead of a single plate  14 B, there are two separate plates  14 B′, both of which are welded onto the plate  14 A. 
     It will be obvious to those skilled in the art that modifications may be made to the embodiments described above without departing from the scope of the invention as claimed.