Abstract:
A monopole hollow strengthening tower is provided comprising stages that each comprise a pair of half-pipe sections that fit around the monopole. Each pair of sections is connected to the stage below and to each other. A first stage is connected to the footing of the monopole, a second stage is connected to the top of the first stage and includes cable ports. Subsequent stages extend above the second stage, finally there is a top stage which incorporates a clamping system to grip the monopole. This stage is the only stage above the footing where the monopole and the strengthening tower are in contact with each other. This results in minimisation of outages and disturbances, shortest timeframe, minimum strengthening and avoids significant enlargement of the monopole footprint

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention concerns the construction of telecommunications towers on which transmitters are mounted, and in particular it concerns strengthening for monopole towers. 
     1. Description of the Related Art 
     Telecommunications towers are usually tall so that the transmitters and receivers can broadcast and receive over the tops of nearby buildings and hills. There are several different types of towers suitable for mounting transmitters. First, guyed masts that are laterally supported by guy wires. Second, lattice towers that have a wide footprint, taper as they go up and are self-supporting. And third, monopole towers that have a small footprint and are self-supporting. 
     Ubiquitous wireless communication networks require communications towers to be located throughout populated areas. The location, elevation and concentration of these towers are determined by geographic factors and population density. Communication companies differentiate by providing superior network coverage and capacity. These conditions have led to a market for vertical real estate capable of accommodating communications equipment. This market need has been fulfilled by existing communication towers and in urban areas rooftops. The demand in this market is balanced with costs to build and operate telecommunications towers and local authority delays and community resistance to building new facilities. 
     As a result, over time more and more equipment tends to be mounted on each existing tower until eventually the maximum structural capacity for a tower may be approached. Due to network operation issues and associated costs tearing down the existing tower and building a new and bigger tower is not an attractive solution. Accordingly, techniques have been developed for strengthening the towers. 
     Lattice towers and guyed masts are able to be incrementally strengthened, for instance by adding more lattice or additional guy wires, however it is much more difficult to strengthen a monopole. 
     SUMMARY OF THE INVENTION 
     The invention is a monopole hollow strengthening tower comprising stages that each comprise a pair of half-pipe sections that fit around the monopole. Each pair of sections are connected to the stage below and to each other. A first stage is connected to the footing of the monopole, a second stage is connected to the top of the first stage and includes cable ports designed for the particular cabling requirement of the monopole. Subsequent stages extend above the second stage, finally there is a ‘top stage’ which incorporates a clamping system to grip the monopole, and this stage is the only stage above the footing where the monopole and the strengthening tower are in contact with each other. Wherein the footing for the monopole is strengthened by casting a concrete foundation around the existing footing and setting the upper level of the foundation at a first predetermined distance below a lowest cable tray, and wherein the half-pipe sections of the first stage have a height equal to the first predetermined distance; and further wherein the half-pipe sections of a third stage have the same diameter as the half-pipe sections of the first stage. 
     A bolt cage may be embedded in the new foundation to connect to the first stage of strengthening. 
     The half-pipe sections for the first stage may be provided in a range of different diameters. 
     The first predetermined distance, which is 2 m. 
     The half-pipe sections for the third stage and above, may be provided in a range of different lengths. This provides adequate versatility for the height of the hollow strengthening tower. 
     The half-pipe sections for the first stage and the third stage may be selected from multiple predetermined types. 
     The strengthening tower surrounds the monopole, and since it is required to be extremely stiff it may be fabricated from steel or carbon fibre half-pipe sections. 
     The half-pipe sections may include flanges along their vertical edges for connection to each other. The sections may also have semi-circular flanges around their top and bottom edges for connection to the stages above and below. Webs may be incorporated between the flanges and the outer wall of each section for further stiffening. 
     The cable ports are located at joints in the half-pipe sections such that the telecommunications cables remain undisturbed, and therefore operational, throughout the strengthening process. 
     Additional overturning resistance may be provided by means of screw piles, rock anchors or bored piers, that are connected to the concrete foundation. 
     Only the second stage is customised to the monopole by having portholes for the existing, as well as any new, cable bundles. Since the portholes accommodate the cabling there is no need to disconnect the cables from the monopole when the hollow tower is being built. 
     The monopole hollow strengthening tower may extend above the monopole. 
     In a further aspect the invention is a method for strengthening a monopole, by installing a hollow strengthening tower around it, where the hollow strengthening tower comprises a number of stages that each comprises a pair of half-pipe sections that are connected to the stage below and to each other.
         The method comprising the following steps:   Casting a concrete foundation around the existing monopole footing;   Setting the upper level of the foundation at a first predetermined distance below the lowest cable tray;   Installing half-pipe sections for the first stage that have a height equal to the first predetermined distance;   Installing a second stage that is connected to the top of the first stage and includes cable ports designed for the particular cabling requirement of the monopole.   Installing half-pipe sections of the third stage, which has the same diameter as the first half-pipe sections.   Finally installing a ‘top stage’ which incorporates a clamping system to grip the monopole, wherein this stage is the only stage where the monopole and strengthening stages are in contact with each other.       

     When stages are to be fitted around a part of the monopole that includes equipment, a crane may be used to move the equipment to another part of the tower. After the stages are fitted, the equipment may be returned to its original location, if desired. This minimises downtime for the transmitters during strengthening. 
     The strengthening technique has been developed to meet the following operational requirements during strengthening:
         Minimisation of outages and disturbances for transmitters on the monopole.   Shortest timeframe.   Minimum strengthening to meet the additional strengthening required.   And, avoids significant enlargement of the monopole footprint.       

     In yet a further aspect the invention is a monopole hollow strengthening tower comprising stages that each comprise a pair of half-pipe sections that fit around the monopole. Each pair of sections are connected to the stage below and to each other. A first stage is connected to the footing of the monopole, a second stage is connected to the top of the first stage and includes cable ports designed for the particular cabling requirement of the monopole. Subsequent stages extend above the second stage. Wherein the footing for the monopole is strengthened by casting a concrete foundation around the existing footing and setting the upper level of the foundation at a first predetermined distance below the lowest cable tray, and wherein the half-pipe sections of the first stage have a height equal to the first predetermined distance; and further wherein the half-pipe sections of the third stage have the same diameter as the first stage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An example of the invention will now be described with reference to the accompanying drawings, in which: 
         FIG. 1(   a ) is a pictorial view of a monopole including its footing and equipment. 
         FIG. 1(   b ) is an elevation of the lower part of the monopole of  FIG. 1(   a ) including its cable bundles. 
         FIG. 2(   a ) is a pictorial view showing an upgraded footing for the monopole of  FIG. 1 . 
         FIG. 2(   b ) is a pictorial view of the monopole of  FIG. 2(   a ) after a first semi-circular strengthening section has been put in place. 
         FIG. 2(   c ) is a pictorial view of the monopole of  FIG. 2(   b ) after a second semi-circular strengthening section has been secured in place; completing a first stage of strengthening. 
         FIG. 2(   d ) is an elevation of one of the semi-circular strengthening sections of  FIG. 2(   b ) and ( c ). 
         FIG. 2(   e ) is a plan view of the semi-circular strengthening of  FIG. 2(   b ) and ( c ). 
         FIG. 3(   a ) is a pictorial view of a further pair of semi-circular strengthening sections incorporating portholes for cables. 
         FIG. 3(   b ) is a pictorial sketch of the semi-circular strengthening sections of  FIG. 3(   a ) incorporating port holes for cables. 
         FIG. 4(   a ) is a pictorial view of the monopole fitted with the strengthening sections of  FIGS. 2 and 3 . 
         FIG. 4(   b ) is a pictorial sketch of the arrangement of  FIG. 4(   a ). 
         FIG. 5(   a ) is a pictorial view of a ‘top stage’ for the strengthening, which is clamped to the monopole. 
         FIG. 5(   b ) is a plan view of the ‘top stage’ of  FIG. 5(   a ) showing the clamping arrangement. 
         FIG. 6(   a ) is a pictorial view of the monopole after the strengthening has been completely installed. 
         FIG. 6(   b ) is a pictorial view of the monopole after strengthening and including an additional stage to the strengthening to increase its height. 
         FIG. 7(   a ) is an elevation of a strengthening tower comprising three stages. 
         FIG. 7(   b ) is a plan view of the semi-circular strengthening of the top of the upmost stage of the strengthening tower of  FIG. 7(   a ). 
         FIG. 7(   c ) is a plan view of the semi-circular strengthening of the base of the upmost stage of the strengthening tower of  FIG. 7(   a ). 
         FIG. 7(   d ) is a plan view of semi-circular strengthening of the top of the lowest stage of the strengthening tower of  FIG. 7(   a ). 
         FIG. 7(   e ) is a plan view of semi-circular strengthening of the base of the lowest stage of the strengthening tower of  FIG. 7(   a ). 
         FIG. 7(   f ) is a sectional view of a reinforcing web of  FIGS. 2(   d ) and  7 ( c ) to  7 ( e ). 
         FIG. 7(   g ) is a detailed view of the mating interface between two semi-circular strengthening sections. 
         FIGS. 8(   a ) to  8 ( e ) are tables of design data for five different types of hollow strengthening towers, each with three stages. 
         FIG. 9(   a ) is a plan view of the ‘top stage’ of  FIG. 5(   a ) showing another example of the clamping arrangement. 
         FIG. 9(   b ) is a plan view of one portion of the clamp of  FIG. 9(   a ). 
         FIG. 9(   c ) is a plan view of a clamp guide of  FIG. 9(   a ). 
         FIG. 10(   a ) is an elevation of the second stage of  FIG. 3(   b ) showing an example of the porthole for cabling. 
         FIG. 10(   b ) is a cross-sectional view of the second stage of  FIG. 10(   a ). 
         FIG. 10(   c ) is a detailed view of  FIG. 10(   b ). 
         FIG. 11  is a table of design data for selecting one of the types of hollow strengthening towers of  FIGS. 8(   a ) to  8 ( e ). 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring first to  FIG. 1(   a ) monopole  10  can be seen to have a subterranean footing  12 , and to be equipped with various transmitters  14 . The transmitters are fed by cables  16  that run from a cable but  18 . The feed to the cable but generally runs in underground ducts. It can be seen that monopole  10  tapers to increasingly narrow sections as it goes up. 
     There are typically a bundle of cables for each customer who uses the tower. The first customer will typically have their cables running up the interior of the tower, later customers will have their cables running up the exterior of the tower.  FIG. 1(   b ) shows a first cable port  20  opening in the tower wall to receive a first cable bundle  22 . A second cable bundle  24  runs up the outside of the tower. Underground, the bottom  26  of the monopole  10  is set in a concrete footing  28 . 
     It will be appreciated that the greatest strain on the monopole is near its base. It follows that the first part that might need strengthening is the footing  28 . Referring now to  FIG. 2(   a ) the interior of the subterranean part of the monopole  26  is filled with concrete. Then a second large concrete foundation  30  is set around the base of the monopole. The upper surface of the foundation is carefully set at a first predetermined distance ‘d’ below the lowest cable tray  16  that connects to the monopole. The distance ‘d’ in this example is set to 2 m. All the extra weight in the foundation increases the overturning resistance of the monopole. Rock anchors  32  are bored to the required depth to provide sufficient additional restraint as required. A bolt cage is ( 34  in  FIG. 4  ( b )) is installed at the upper end of foundation  30 . 
     Next, a stiff hollow strengthening tower  120  is constructed around monopole  10 . The hollow tower is placed around the monopole in stages with the first stage being placed on foundation  30 . Each stage has two ‘half-pipe’ sections which are semicircular in horizontal section. They are placed around the monopole using a crane  40 . 
     The first section  50  of the hollow strengthening tower  120  is shown in place in  FIG. 2(   b ). This section  50  has a height equal to ‘d’, that is 2 m, so that it fits below cable tray  16  when it rests on top of foundation  30 . Its mating section  52 , which is also 2 m high, is shown in place in  FIG. 2(   c ); which completes the first stage  50  of the tower.  FIGS. 2(   d ) and ( e ) show that each section of the first stage has a semi-circular wall  54  with a top flange  56  and a lower flange  58 . These sections may have a limited range of different diameters, for instance 1600 mm for wide towers and 1200 mm for narrow towers. Reinforcing webs  60 , such as gussets, are positioned around the bottom of the sections between the lower flange  58  and the wall  54  to increase stiffness. 
     Vertical flanges  62  and  64  run up the edges of each section so they can be fastened together. The sections are bolted to each other and to the bolt cage ( 34  in  FIG. 4  ( b ))in foundation  30 . 
     It will be seen from  FIGS. 2  that the first sections  50  and  52  are a predetermined length and the upper surface of the foundation set to a height such that the first section of hollow tower stops below the height where cables  16  arrive at the monopole  10 . 
     Referring now to  FIGS. 3 , the second stage sections  70  and  72  of strengthening tower  120  have custom portholes to allow the cable bundles to enter under the hollow tower  120 . Section  70  has a porthole  74  opening at the bottom, and both sections  70  and  72  have half of a mating porthole  76  that is formed when they are connected together. Sections  70  and  72  are also bolted to sections  50  and  52  of the first stage. Cable tray  24  belonging to the first customer enters through port hole  74  and goes into the interior of monopole  10 . The second customer&#39;s cable tray  22  enters porthole  76  slightly higher up the monopole  10  and runs up the outside of the monopole. The location of the portholes is such that the existing cable trays can remain in place. This further reduces the cost for installing the strengthening. 
     Referring now to  FIGS. 4 , above the cable portholes a third stage  50 ′/ 52 ′ are selected from a catalogue of standard hollow strengthening tower sections as will be described with reference to  FIGS. 7(   a ) and  8 ( e ). These will have the same diameter as the first stage, but may vary in length, for instance being 3 in or 6 m high, depending on the height to which the strengthening tower  120  is required. This stage has no portholes. It can be clearly seen in  FIG. 4(   b ) that the hollow tower does not contact the surface of the monopole  10 . In fact while the monopole  10  tapers as it rises, the hollow strengthening tower  120  remains the same diameter from bottom to top. 
       FIG. 4(   b ) also shows a fourth stage  50 ″/ 52 ″ and a fifth stage  100 / 102  of the strengthening tower  120  fitted around the monopole  10 . This stage is also selected from a catalogue of standard hollow strengthening tower sections, and in this case are identical to the third stage  50 ′/ 52 ′. 
       FIGS. 5(   a ) and ( b ) show a ‘top stage’  100 / 102  which is fitted around the monopole at the top of strengthening tower  120 . The top stage has a top flange  101  that carries four clamps  110 ,  112 ,  114  and  116  that grip the outside of monopole  10 . This is a ‘pinned’ connection that provides ‘propped’ support and changes the existing cantilever structure of the monopole  10  into a ‘propped’ cantilever structure. The majority of any force caused by bending in the monopole  10  is transferred through this ‘pinned’ connection to the hollow strengthening tower  120 . 
     The height at which the ‘top stage’  100 / 102  is clamped to the monopole  10 , is determined by the degree of strengthening required. For instance, when a monopole is loaded to its design capacity and new equipment is to be installed, then the top stage  100 / 102  will be installed to the height required for that degree of strengthening. When further equipment is required then the clamps will be taken off, one or more new stages of the hollow tower are added, and then the clamps are reinstalled at a greater height; giving greater strength. This can happen several times during the life of a monopole. The clamps  110 ,  112 ,  114  and  116  are adjustable so that they can accommodate to clamp different diameters of the monopole. This allows the same clamps to be used and re-used at a different height. In one example, the hollow strengthening extends further than the monopole and therefore results in a higher tower than the monopole itself. 
       FIG. 6(   a ) shows a monopole  10  that is completely encased by hollow tower  120 .  FIG. 6(   b ) shows a hollow tower  120  that extends above the existing monopole. In one example, the hollow tower  120  extends above the monopole and stands on its own, that is it is not clamped to the monopole  10 . 
       FIG. 7(   a ) illustrates another example of a strengthening tower  700  comprising a lower stage  702 , a feeder entry stage  704  including a first porthole  705  and a second porthole  706 , and a top stage  708 . The stages  702 ,  704  and  708  are taken from a catalogue of standard strengthening towers. In this example, the lower stage  702  extends from 0 m to 2 m, the feeder entry stage  704  from 2 m to 4 m and the top stage from 4 m to 10 m above ground level. The three stages  702 ,  704  and  708  are also referred to as modules or jacket sections. 
       FIG. 7(   b ) illustrates the semi-circular strengthening  710 , such as a flange, at the top of the top stage  708 . The flange  710  has multiple holes  712  for affixing the brackets  110 ,  112 ,  114  and  116  of  FIG. 5(   b ) to the flange  710 . In this example, the flange  710  has ten equally spaced holes  712  but a different number of holes is possible according to the standard design data in  FIGS. 8(   a ) to  8 ( e ) for five different standard types of steel jackets. For example,  FIG. 8(   b ) gives design data of a type B steel jacket. This data contains “24” (row labelled “3 (Top)” and column labelled “FLANGE BOLTS/#BOLTS”) as an example for the number of bolts  821  in  FIG. 8(   b ) for the entire circumference, that is 12 holes in each semi-circular flange. 
     The diameter of the holes is such that the diameter of the holes corresponds to the diameter of bolts used, as given in  FIGS. 8(   a ) to  8 ( e ). For example, a 26 mm hole is used for a 24 mm (M24) bolt and a 33 mm hole is used for a 30 mm (M30) bolt. 
       FIG. 7(   c ) illustrates a flange  720  of the base of the top stage  708  of the strengthening tower  700 . Similar to flange  710  in  FIG. 7(   b ), the flange  720  comprises holes  722 , where the number and diameter of holes is given for different jacket types in the standard design data in  FIGS. 8(   a ) to  8 ( e ) in rows labelled “3 (Base)”. In addition to the features of flange  710  in  FIG. 7(   b ), flange  720  is reinforced by reinforcing webs, such as gusset plates  724 . 
       FIG. 7(   f ) illustrates gusset plate  724  in sectional view. The gusset plate  724  has the shape of a right triangle with the three corners cut off. The height  725  is defined as the length of the cathetus that extends along the steel jacket and is given in  FIGS. 8(   a ) to  8 ( e ). In one example, the other measurements of the gusset plate do not change between different jacket types. That is, the top horizontal edge  726  and the lower vertical edge  727  are both 20 mm long while the gusset plate  724  is set back from the external edge the flange  720  by 10 mm. 
     The gusset plate is welded to the flange  720  and the wall  728  of the steel jacket  708 . The thickness of the wall  728  is denoted as “tw”, while the exterior diameter of the wall  728  is denoted as “do” in the tables in  FIGS. 8(   a ) to  8 ( e ). In this example, the centre of hole  722  is spaced apart from the exterior surface of the wall  728  by 50 mm (“m1”) and the flange extends outwardly from the centre of hole  722  by 50 mm (“n”). The flange  720  extends inwardly by 25 mm (“x”) from the exterior surface of wall  728  into the interior of jacket  708 . 
       FIG. 7(   d ) and  FIG. 7(   e ) illustrate flanges  730  and  740  of the top and base of the lowest stage  702  of the strengthening tower of  FIG. 7(   a ), respectively. The number of holes and gusset plates, as well as other standard design data is presented in the tables in  FIGS. 8(   a ) to  8 ( e ) in row labelled “1 (Top+Base)”. 
     In this example, the wall thickness of the feeder entry stage  704  matches the wall thickness of lowest stage  702 . The configuration of the top and base flanges of feeder entry stage  704 , including number of holes and design of gusset plates, is such that the configuration matches that of the bottom flange of top stage  708  and the top flange of the lowest stage  702 , respectively. 
       FIG. 7(   g ) is a detailed view of a mating interface  760  between two semi-circular strengthening sections. In the following description of the mating interface  760 , the reference numerals of the base flange  720  of the top stage  708  are used but the same design may be applied to the other stages  702  and  704 .  FIG. 7(   g ) depicts the flange  720  having hole  722 , gusset plate  724  and wall  728 . A seam plate  761  is welded to the flange  720  and the wall  728 , such that the seam plate extends vertically between the top and the base flange of the top stage  708 . The edge of flange  720  and the exterior surface of seam plate  761  define a smooth surface, such that two identical semi-circular strengthening sections can abut without forming any gaps between them. 
     A pattern of multiple vertically align holes, such as hole  762 , extends along the seam plate  761 , the centre of hole  762  being located outwardly from the exterior surface of wall  728  by 35 mm. The seam plate extends inwardly from the exterior surface of wall  728  by 20 mm. When in use, two semi-circular strengthening sections  70  and  72  in  FIG. 3(   b ) are bolted together through holes  762  in the seam plate  761 . 
       FIG. 9(   a ) shows another example of the ‘top stage’  900 . This example of top stage  900  is similar to the example described with reference to  FIG. 5(   b ) in that the top stage  900  comprises four clamps  910 ,  912 ,  914  and  916  that form a pinned connection with the monopole  10 . One difference to  FIG. 5(   b ) is the design of clamps  910 ,  912 ,  914  and  916  which will now be described with reference to  FIG. 9(   b ). 
       FIG. 9(   b ) shows one of the four clamps  910  in more detail. The clamp  910  comprises a horizontal, arcuate clamp plate  932  that supports an downwardly extending bearing plate  934  for engaging the monopole  10 . The connection between the clamp plate  932  and the bearing plate  934  is reinforced by reinforcement gussets  936  and  938 , which are, in this example, arranged such that they are equally distanced from the centre of the clamp plate  932  and one end of the clamp plate  932 . A third reinforcement gusset  939  is located at the centre of the clamp plate  932 . 
     At the centre of the clamp plate  932 , there is a outwardly extending round bar  940  attached to the top surface of the clamp plate  932 . The round bar  940  has a thread  942  at the outward end. When in use, the round bar  940  is received by clamp guide  920  in  FIGS. 9(   a ) and  9 ( c ). 
     The clamp  910  further comprises two vertical coupling plates  942  and  944  located at opposed ends of the clamp plate  932 . Each of the coupling plates  942  and  944  has a central hole for receiving a bolt that connects two adjacent clamps as shown in  FIG. 9(   a ). 
       FIG. 9(   c ) shows the clamp guide  920  in more detail. The clamp guide  920  comprises a horizontal rectangular base plate  952  with two holes  954  and  956 . The distance between the holes  954  and  956  and their diameter correspond to the distance and diameter of holes  712  in top flange  710  as described with reference to  FIG. 7(   b ). The clamp guide  920  further comprises a vertical interface plate  958  with a central hole  960  for receiving the round bar  940  of the clamp  910 . The connection between the interface plate  920  and the base plate  952  is reinforced by reinforcement gussets  962  and  964 . 
     When in use, the clamp guide  920  is located on the top flange  710  of the top stage  708  such that the holes  954  and  956  align with the holes  712  of the top flange  710  and such that the interface plate  958  is facing outwardly. Two bolts as specified in  FIGS. 8(   a ) to  8 ( e ) are inserted into the holes  954  and  956  and through the holes  712  of the top flange  710 . The bolts are then secured by corresponding nuts. 
     Once the clamp guide  920  is installed, the round bar  940  of clamp  910  is inserted into hole  960  of clamp guide  920  and secured with a nut from the outside to secure the clamp against moving inwardly. In one example, a second nut is screwed on the round bar  940  before the round bar  940  is inserted into hole  960  to also secure the clamp  910  against moving outwardly. Both nuts are then tightened such that the interface plate  958  is tightly held between the two nuts. 
     In another example, the round bar  940  is inserted into the hole  960  of clamp guide  920  before the clamp guide is affixed to top flange  710 . This may be the case if the dimensions of the monopole and the clamp  910  are such that there is not enough space to mount the clamp  910  after installing the clamp guide  920 . 
     The other clamps  912 ,  914  and  916  are installed in a similar manner such that the monopole is held by a pinned connection between the four claps  910 ,  912 ,  914  and  916 . In different examples, the number of clamps may not be four but any other suitable number, such as three or eight. 
       FIG. 10(   a ) shows another example of the second stage  70  in  FIG. 3(   b ). As described with reference to  FIG. 3(   b ), the second stage  70  comprises porthole  76 . In the example of  FIG. 10(   a ) the second stage  70  further provides longitudinal reinforcement webs  1010 ,  1012 ,  1014  and  1016 . The reinforcement webs  1010 ,  1012 ,  1014  and  1016  compensate for the loss in integral stability caused by inserting the porthole into the second stage  70 . 
       FIG. 10(   b ) shows the reinforcement webs  1010 ,  1012 ,  1014  and  1016  in cross section and  FIG. 10(   c ) shows a more detailed view of the cross section. The reinforcement webs  1010 ,  1012 ,  1014  and  1016  have a triangular cross section and extend downwardly from the top flange  76  of the second stage  70  on the inside of the second stage  70  and on either side of the porthole  76 . 
       FIG. 11  is a table of design data for selecting one of the standard types of strengthening towers of  FIGS. 8(   a ) to  8 ( e ). Typically, a designer of the monopole uses a model of the monopole that incorporates the equipment, such as transmitters  14 , currently installed towards the top of the monopole. Depending on the size and shape of the equipment a moment is created by wind asserting a force on the equipment. 
     The monopole  10  is designed for a maximum moment capacity at high wind speeds and if the installation of additional equipment causes the moment to exceed the maximum moment capacity of the monopole, the proposed strengthening tower is constructed around the monopole. Since a moment depends on the length of a lever, that is monopole  10 , the moment is greatest at the base of the monopole and smallest at the top. 
     The table in  FIG. 11  contains data for the maximum moment capacity of the different types of strengthening towers of  FIGS. 8(   a ) to  8 ( e ). In one example, monopole  10  is designed for a maximum moment capacity at the base (at 0 m) of 800 kNm but due to additional equipment the moment at the base has increased to 1200 kNm. In this example, a tower of type B would be selected, since the design moment at 0 m is 1500 kNm, which is more than the required moment. This selection is made under the assumption that the diameter of the type B tower (1200 mm) is sufficient to accommodate cabling along the outside of the monopole. Otherwise, a type with a larger diameter needs to be selected. 
     In the same example, the moment at 2 m also exceeds the maximum moment capacity of the monopole but at 4 m the moment is less than the maximum moment capacity of the monopole. As a result, only one more stage, that is two stages in total, are required. More stages may be added later, when more equipment is installed that causes the moment at 4 m or above to exceed the maximum moment capacity of the monopole. 
     In cases where installation of large amounts of equipment is expected for the future, a type is selected that has a much higher maximum moment capacity at 0 m than is required. For the example above, although type B is sufficient type C may be selected in order to be able to withstand moments of up to 2400 kNm at 0 m in the future. 
     Although the invention has been described with reference to a particular example it should be appreciated that it may be embodied in many other forms and variations.