Abstract:
A structural assembly comprising profile members imparting axial strength to the assembly. The profile members extend along a length. Ribbon holds the profile members together along their length. The ribbon imparts lateral strength to the assembly by being braced to the profile members. A method for manufacturing the structural assembly comprises winding the ribbon about a support structure to form a bracing lattice, positioning the profile members in a predetermined position on the support structure; and bonding the profile members to the bracing lattice to form a structural assembly.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     The present patent application claims priority on U.S. provisional patent application No. 60/738,572, filed on Nov. 22, 2005, by the present applicant. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to structural assemblies and method for manufacturing structural assemblies and, more specifically, to the manufacture of structural assemblies made of composite material. These structural assemblies are suited for assembling strong lightweight towers for the tower industry. 2. Background Art  
         [0004]     Towers serve many purposes for different industries. Amongst other things, towers serve to support broadcasting and telecommunication antennas, cables such as power transmission lines, as well as anemometers and windmills. Towers come in various shapes and sizes, and can be classified in groups that include guyed towers, self-supporting towers, or monopoles.  
         [0005]     Locations and types of towers are chosen strategically to optimize multiple objectives that often include minimizing cost. Other objectives include: optimal radio coverage, optimal transmission corridor, or optimal strength of wind. Towers are erected in urban, rural and remote areas, on top of buildings and directly on the ground. Each tower site has its own constraints, such as climate (temperature range, wind, ice formation, salt), soil or building structure, impact on humans and the environment.  
         [0006]     Depending on its use and its environment, a tower is designed to resist a specific loading (wind, ice, dead load). For example, a telecommunication tower of 200 feet (approx. 65 meters) may be designed to hold sets of cellular antennas, microwave dishes at different heights, withstand ice on the structure, all the while moving minimally while being subjected to strong winds of 150 km/h.  
         [0007]     The required rigidity and strength is typically achieved with steel structures. Steel structures, however, are heavy and are susceptible to corrosion. Because of weight issues, such towers require costly foundation work or building reinforcement. The weight factor also affects transportation costs including the need to build access roads in remote areas. Furthermore, maintenance and protection from corrosion is an issue in certain environments, such as a maritime environment.  
         [0008]     U.S. Pat. No. 6,264,781, issued to Bott on Jul. 24, 2001, teaches an apparatus and a process for the continuous production of structural beams. The process includes: forming a tubular element from fibers of composite material; followed by a separation of the tubular elements into longitudinally extensive circumferentially separated corner caps, then securing sandwich panels between the adjacent corner caps to form a tubular beam. Subsequently, the tubular beam is shaped and cured to produce the structural beam. The apparatus disclosed is such to achieve the process steps previously mentioned. Thus, the apparatus: winds fibers to form them into a tubular member; cuts the member; spreads the member into various sections; adds an inner skin; applies a core material and an outer skin followed by a consolidation of the various layers, and ends with curing of the beam formed.  
       SUMMARY OF INVENTION  
       [0009]     It is therefore an aim of the present invention to provide a structural assembly that addresses issues associated with the prior art.  
         [0010]     It is a further aim of the present invention to provide a novel method of manufacturing a structural assembly.  
         [0011]     Therefore, in accordance with the present invention, there is provided a structural assembly comprising at least two profile members imparting axial strength to the assembly, each of the profile members extending along a length, and at least one ribbon, the at least one ribbon holding the at least two profile members together along their length, the at least one ribbon imparting lateral strength to the assembly by being braced to the at least two profile members.  
         [0012]     Further in accordance with the present invention, there is provided a method of manufacturing a structural assembly comprising providing at least two profile members and at least one ribbon, winding the at least one ribbon about a support structure to form a bracing lattice, positioning the at least two profile members in a predetermined position on the support structure, and bonding the profile members to the bracing lattice to form a structural assembly.  
         [0013]     Still further in accordance with the present invention, there is provided a mandrel for producing a structural assembly, the mandrel comprising at least one faceplate having a face oriented outwardly; corner members positioned at opposite ends of the at least one faceplate, the corner members each being adapted to support a profile member; actuator members supporting the at least one faceplate and the corner members between an extended position in which a ribbon is wound about the mandrel so as to form a bracing lattice interrelating the profile members, and a collapsed position in which the mandrel is separated from the structural assembly resulting from a bonding of the bracing lattice and the profile members; and channels defined in the face of the at least one faceplate to accommodate the ribbon wound on the mandrel.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     Having thus generally described the nature of the invention, reference will now be made to the accompanying drawings, showing by way of illustration a preferred embodiment thereof and in which:  
         [0015]      FIG. 1  shows a perspective view of a guyed tower using structural assemblies constructed in accordance with an embodiment of the present invention;  
         [0016]      FIG. 2  shows a cross-section of a structural assembly according to one embodiment of the present invention;  
         [0017]      FIG. 3  shows an enlarged cross-section of a profile member of the structural assembly illustrated in  FIG. 2  in association with ribbons bracing the assembly;  
         [0018]      FIG. 4  shows a perspective view of a mandrel used in manufacturing the structural assembly of  FIG. 2 ;  
         [0019]      FIG. 5  shows a perspective view of a tower segment constructed with the structural assembly of  FIG. 2 ;  
         [0020]      FIG. 6  shows an elevation view of ribbon-bracing lattice used in the structural assembly of  FIG. 2 ;  
         [0021]      FIG. 7  shows an enlarged perspective view of a ribbon-bracing lattice illustrated in  FIG. 6 ;  
         [0022]      FIG. 8   a  shows a perspective view of a joint element of the tower segment of  FIG. 5  in accordance with one embodiment, including a full flange plate;  
         [0023]      FIG. 8   b  shows a perspective view of joint element of the tower segment of  FIG. 5  according to another embodiment, including a flange plate defining a central hole;  
         [0024]      FIG. 8   c  shows a perspective view of joint element of the tower segment of  FIG. 5  according to yet another embodiment, defining a completely opened central hole and exterior flange plate;  
         [0025]      FIG. 9  shows a perspective view of a portion of a mandrel according to another embodiment of the present invention;  
         [0026]      FIG. 10  shows a cross-sectional view of the mandrel according to  FIG. 9 ; and  
         [0027]      FIG. 11  shows a side view of the mandrel according to  FIGS. 9 and 10 . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]     Composite materials have been used for many applications where weight and specific properties related to weight are issues. The process consists of pulling a fiber reinforcing material though a resin-impregnation bath and into a shaping die where the resin is subsequently cured to form a composite profile. Pultrusion is a known technology with a continuous process that has been used to make composite profiles of constant cross-section. These profiles can be assembled just like steel profiles would be, to form a tower structure.  
         [0029]     Ribbon winding is another known technology that has been used to make support structures such as poles. It is a fabrication process that consists of winding a continuous reinforcing fiber impregnated with resin around a rotating and removable form, the mandrel.  
         [0030]     Both pultrusion and ribbon winding allow the skilled practitioner to build structural assemblies with different strengths.  
         [0031]      FIG. 1  illustrates a guyed tower  100  having a tower structure  150  which may include one or more tower segments  250 . These segments  250  include structural assemblies made of composite material according to the present invention.  
         [0032]     The guyed tower  100  includes a central base  106  supporting the tower  150  centrally.  FIG. 1  illustrates three peripheral anchoring bases  104   a, b, c  used to connect guyed wires  102  to the tower  150 .  
         [0033]     The tower  150  is built by assembling or connecting various lengths of tower segments  250 . The lengths (i.e., height when the tower segments  250  are installed) of the tower segments  250  are in an embodiment 40 feet long (approx. 13 meters), due primarily to transportation constraints. Forty-foot lengths are preferred because they tend to be the maximum length that can be easily loaded onto a truck. The length of a tower segment  250  may be adjusted as needed upon manufacturing the structural assemblies that will determine the length of a tower segment  250 . The length of the structural assembly may be limited by the size of machinery used to produce same and by the nature and location of the site where the tower will be mounted.  
         [0034]     A cross-section of an embodiment of the structural assembly  200  is illustrated in  FIG. 2 . The assembly  200  includes a bracing lattice defining a triangular cross-sectional shape. The structural assembly  200  is generally hollow and is designed, for instance, to meet telecommunication requirements of towers. The assembly  200  in  FIG. 2  includes a profile member  202  in each of the corners of the assembly  200 . The profile members  202  serve to impart axial strength to the structural assembly  200 , and thus to the tower structure  150  of the guyed tower  100 .  
         [0035]     The structural assembly  200  of  FIG. 2  includes two ribbons  220 ,  240  attached to and holding the profile members  202  together. Ribbon  220  is found in the inside of the assembly  200 , while ribbon  240  is found on the outside of the assembly  200  in this particular embodiment. The ribbons  220  and  240  act as a bracing for the structural assembly  200 , thereby interconnecting the profile members  202 .  
         [0036]     The structural assembly  200  in this embodiment has three profile members  202  that are pultruded profiles. These pultruded profile members are typically made of a composite material composed of fiberglass and a vinyl-ester resin. Composite materials are generally strong and lightweight and give the structural assemblies in tower segments  250  a substantially lower weight when compared to comparable towers produced in steel. This reduction in weight in turn gives the structural assembly  200  a particular advantage during construction of the tower  100  in isolated areas where access is difficult.  
         [0037]     The vinyl-ester resin of the pultruded profile members are in an embodiment formulated with anti-UV additives and additives to facilitate the pultrusion process. Surface treatment may also be performed on the pultruded profiles to allow a better co-curing with the ribbons  220  and  240  that are wound bondingly around the profile members  202 , as will be described hereinafter. The skilled practitioner would understand that a composite material is one that includes one or more types of fiber bonded together chemically.  
         [0038]      FIG. 3  shows an enlarged cross-section of one of the profile members  202  in association with the ribbons  220  and  240  to form a corner or concavity  210  of the structural assembly  200 . The profile member  202  is found in the corner  210  of the assembly  200  and has an interior surface  204  and an exterior surface  206 . Although the profile member  202  is illustrated as cooperating with both the inner ribbon  220  and the outer ribbon  240 , it is contemplated to provide a single ribbon  220  or  240  placed on only one side of the protrusion profile member  202 . One ribbon  220  may be placed on the interior surface  204  of the profile member  202 , or a single ribbon  240  may be placed on the exterior surface  206  Equally multiple ribbons may also be used on the interior or exterior surface of the profile member  202 .  
         [0039]     It is pointed out that the profile member  202  has edges on both its interior surface  204  and its exterior surface  206 , respectively due to a concavity and a convexity. Once the ribbons  220  and/or  240  are bonded to the profile member  202 , this configuration will strengthen the structural assembly  200  by forming a nested engagement.  
         [0040]      FIG. 4  illustrates a mandrel that may be used to produce the structural assembly  200  of the present invention.  
         [0041]     The mandrel  300  may be one of many possible types, such as a collapsible mandrel of constant cross-section which facilitates the extraction of the structural assembly  200  after it is hardened. A non-collapsible mandrel can also be used, requiring the use of an extractor to remove the structural assembly  200  therefrom. Alternatively, a set of conical mandrels may be used to minimize the extraction effort.  
         [0042]     The mandrel  300  may also have a cross-section that varies, thus producing, for example, a structural assembly  200  that has a cross-section gradually decreasing in size toward one end.  
         [0043]     The mandrel  300  illustrated in  FIG. 4  includes an outer shell, with three faces  302  having a length  304 . The triangular cross-section shape is visible on a front face  305  of the mandrel. This cross-sectional shape produces the cross-sectional shape of the structural assembly  200  as illustrated in  FIG. 2 . The corner  306  at the intersection of the faces  302  are shaped so as to receive in coplanar relation the profile members  202 .  
         [0044]     For instance, the mandrel  300  may also have in an embodiment rounded corners  306  at the three angles  307  indicated on the front face  305  of the mandrel  300 , as a function of the cross-section of the profile member  202 . Each of the corners  306  may be produced by chamfering and polishing the shell to the appropriate smoothness along the meeting line of the three faces  302 . The faces  302  or sides of the outer shell may be outwardly rounded along the entire length  304  of the mandrel  300 .  
         [0045]     A ribbon-winding process is used to wrap the ribbons  220  and  240  around the mandrel  300 . This wrapping is achieved by a relative motion of the mandrel  300  with regard to the ribbon applicator (not shown). In a preferred embodiment the mandrel  300  rotates about the longitudinal axis  310  while the ribbon  220 / 240  is applied from a spool moving back and forth along the length  304  of the mandrel  300 , or set at predetermined locations along the length  304  of the mandrel  300 .  
         [0046]      FIG. 6  illustrates a bracing lattice having a first pattern  220   a  obtained by the first wrapping of the ribbon  220  along the mandrel  300  (the mandrel is not illustrated) in a first direction, as well as similar patterns  220   b  and  220   c  axially offset from the first patterns  220   a.    FIG. 6  illustrates that at a regular interval  222 , the ribbon  220  wraps around the mandrel (not shown) at a first axial position (i.e, along the Y axis). Between these intervals  222 , the ribbon  220  is wrapped at a substantially 45 degrees diagonal angle  228  on all three faces  302  of the mandrel  300  by way of the patterns  220   a,    220   b  and  220   c.  Similarly,  FIG. 7  illustrates an enlarged perspective view of ribbon  220  produced where the interval  222  and the angle  228  are shown in greater detail.  
         [0047]     Referring back to  FIGS. 3 and 4 , the profile members  202  are positioned on all three of the rounded corners  306  of the mandrel  300  upon which the ribbon  220  has been applied previously. In an embodiment, the profile members  202  and the mandrel  300  are designed such that the profile members  202  have a stable position in contact with the ribbon  220  or  240  upon the mandrel  300 . Alternatively, the profile members  202  may be positioned directly on the rounded corners  306  of the mandrel  300  without any ribbon  220  preapplied to the mandrel  300  as a function of the desired configuration of the bracing lattice.  
         [0048]     With the profile members  202  now in place (either directly on the mandrel or on the ribbon  220 ) and fixed, the second ribbon  240  is wound upon the mandrel  300  supporting the profile members  202  using the ribbon-winding process. In an embodiment, the wrapping sequence of the ribbons  220  and  240  is such that the ribbons  220  and  240  overlap each other along the entire length  304  of the structural assembly  200 . In such a case, the respective shapes of the mandrel  300  and the profile members  202  are such that the second layer of ribbon  240  is in full contact with the first layer of ribbon  220 . However, the ribbons  220  and  240  need not overlap, and may be wound so that the they no longer a substantially co-linear bracing lattice, but make a continuous bracing wall between the profile members  202 . Alternatively, the ribbon  240  may not be necessary according to the contemplated use of the structural assembly  200 .  
         [0049]     The ribbon-wrapping sequence described previously may be a complete wrapping of the mandrel with ribbon-winding orientation varying from close to perpendicular to diagonal. This variant of the wrapping process produces a reinforced monopole tower. Once the profile members  202  and the ribbons  220  and/or  240  are layered into one another, the assembly is heated or cured (e.g., with UV light) to chemically bond the profile members  202  to the bracing lattice of ribbons  220  and/or  240 .  
         [0050]     The mandrel of  FIG. 4  is only illustrative and may take up many rodlike structures having any of a wide variety of cross-sectional shapes. The alternative mandrels whose cross-sections include edges would in these cases also have in an embodiment rounded edges for nesting engagement with the profiles members  202 ; thus, any profile member produced preferably has a concavity for greater rigidity.  
         [0051]     The method may make use of a mandrel of an outside shape with the following characteristics: a convex cross-section which helps to maintain a tension within the interval  222 , required by the wrapping technology used; a curvature that matches, after considering the thickness of the inside ribbon if required, the inside curvature of the profile member  202 .  
         [0052]     The present method may advantageously use the wrapping sequence to minimize residual stress and/or stress resulting from large variation of temperature by ensuring that at the contact between the pultrusion profile member  202  and the ribbon  220 ,  240  bracing the fibers are preferably in diagonal rather than perpendicular relative to one another. Moreover, the ribbons may be selected so as to be thinner and wider at a point of contact with the profile members  202  or other layers of ribbon.  
         [0053]     In an alternative embodiment of the process of the present invention, the mandrel  300  may be replaced with a skeletal structure having the appropriate length and cross-sectional shape, adapted to support the profile members  202  so as to be rotated about an axis, thus also allowing the application of a ribbon winding.  
         [0054]     In an embodiment, the ribbons  220  and  240  are made of a composite material composed of fiberglass roving and a vinyl-ester resin. Like for the pultruded profile members  202 , the vinyl-ester resin is formulated with additives for UV protection and for facilitating the ribbon-winding process. Other additive can be added to the formulation depending on the needs, e.g., exposure to fire.  
         [0055]      FIG. 5  illustrates a perspective view of an embodiment of the tower segment  250  which includes the structural assembly  200  illustrated in  FIG. 2  and joint element  400   b  of  FIG. 8   b,  attached at opposing ends of the assembly  200 . The structural assembly  200  has three profile members  202  and ribbons  220  and  240  bracing the profile members  202  together.  
         [0056]     The method of assembling the structural assemblies  200  to build a tower structure  150  comprises the step of rigidly attaching one type of the joint elements  400   a,    400   b  or  400   c,  respectively illustrated in  FIGS. 8   a,    8   b  and  8   c,  at the extremities of the structural assemblies  200 . The tower segments  250  are then assembled by coupling the opposed joint elements at the end of two structural assemblies.  
         [0057]     The rigid assembly of the joint elements with the structural assemblies  200  preferably involves the bonding of the assembly  200  to the outer surface  402   a ,  402   b  or  402   c  of the joint elements. The joint elements  400   a ,  400   b  or  400   c  include the similarly numbered features as the mandrel  300 , and a cross-sectional shape chosen to match that of the structural assembly  200 . The joint elements  400   a ,  400   b  or  400   c  include: inner faces  409   a ,  409   b  or  409   c ; rounded edges  406   a ,  406   b  or  406   c ; and the rounded edges and the faces defining substantially same angle  407   a ,  407   b  or  407   c . The joint elements  400   a ,  400   b  or  400   c  are designed to fit with the structural assembly  200  framework.  
         [0058]     The rigid assemblies of the joint elements pair wise are advantageously bolted assemblies which allow replacement of a structure assembly  200  when needed. The joint elements  400   a ,  400   b  and  400   c  respectively have flange plates  405   a ,  405   b  or  405   c , which may define a plurality of bolt holes  412   a ,  412   b  or  412   c , respectively. Two complementary joint elements  400   a ,  400   b  or  400   c  of two rigid assemblies to be attached can be connected with bolts which in an embodiment are made of stainless steel or of other suitable strong and corrosion-resistant material.  
         [0059]     The joint elements  400   a ,  400   b  or  400   c  are preferably made of composite materials in a sandwich configuration using reinforcing fibers such as fiberglass and a resin such as vinyl-ester for the skins, and a core element that can provide structural stiffness for the part.  
         [0060]     The manufacturing process for the joint elements  405   a ,  400   b  or  400   c  may be an infusion process or a composite closed molding process such as resin transfer molding, light resin transfer molding, compression molding, and autoclave molding.  
         [0061]     The shape of the joint element  400   a ,  400   b  or  400   c  is designed to match that of the structural assembly, whereby its cross-section is “L”-shaped. Depending on the application, the “L”-shape of the cross-section may be oriented towards the inside ( 400   a  and  400   b ) or the outside ( 400   c ) of the structural assembly. If the “L”-shape is oriented towards the inside, the joint element  400   a,    400   b  or  400   c  may define a center hole  414   b  (in the case of the joint element  400   b ). For the joint element  400   c  with an exterior bolting flange  405   c , the central hole  414   c  is not obstructed, which is particularly advantageous for telecommunications towers where wiring is often run within the structural assemblies of a tower  100 .  
         [0062]     The structural assembly  200 , now including the joint elements at one extremity, may be cured by heat in a oven, by light such as ultraviolet light or by other means known to the skilled practitioner. The mandrel  300  is then collapsed and removed, or extracted. Curing with UV light provides the advantage of curing as the ribbon winding progresses.  
         [0063]     The thickness and width of the ribbon bracing may be adjusted to minimize the gaps where the ribbon bracing  220  and  240  overlap. These overlaps occur opposite the profile members  202 , and thus the widening contributes to a better contact between the pultrusion profile members and the ribbon bracing.  
         [0064]     The structural assemblies  200  described and produced by the above-described method may be used for telecommunication towers  100 . The said method can also be used to produce structural assemblies  200  for other purposes such as broadcasting antennas, power transmission towers, windmills, anemometric towers, building or stage structures.  
         [0065]     The number of profiles allows different configurations that can serve a variety of purposes. For example, a structural assembly with two profiles could serve as a support column; a structural assembly with four profiles could serve as a power-transmission pylon.  
         [0066]     According to another aspect, there is provided a structural assembly made of composite material to build towers. This structural assembly  200  has structural properties that are not found in the components (profile members  202  and ribbons  220  and  240 ) when considered alone. For example, a pole made by ribbon winding is weaker in flexion than in torsion.  
         [0067]     The structural assembly resulting from the previously described method can be designed to withstand different loading conditions by adjusting, globally or locally, the thickness and properties of the different components.  
         [0068]     For certain loading conditions, a ribbon  220  bracing on the inside or the outside may be sufficient. However, the structural assembly  200  is advantageously resistant when the ribbon  220  and  240  bracing constrain the displacements of the V-shaped profile members  202  by being physically in contact on both the inside and the outside of the profiles. The shear stress on the contact between profile members and ribbons  220 ,  240  is thus reduced as the strains are distributed through physical contact.  
         [0069]     Depending on structural requirements, other kinds of matrices can be used for the composite material, Polymeric resins are the preferred choice: thermosets such as unsaturated polyester, epoxy, urethane and polyurethane and thermoplastic such as polyether-ether-ketone (PEEK), polyethylene and polypropylene.  
         [0070]     Depending on structural requirements, other kinds of fibers can be used for the composite such as carbon, aramid and basalt.  
         [0071]      FIG. 9  illustrates another embodiment of the mandrel  500 . The mandrel  500  includes an outer shell  501  formed by three faces  502  of faceplates, each extending along length  504 . The generally triangular cross-sectional shape  505  of the mandrel  500  is visible at one end.  
         [0072]     The mandrel  500  presented in  FIG. 9  also includes retractable corners  506 , as will be described in greater detail hereinafter, at the edge of each of the faces  502 . The corners  506  each support one of the profile members  202  ( FIG. 2 ) when the assembly  200  is produced on the mandrel  500 . The faces  502  each have two opposite longitudinal edges that each meet at an opposite one of the corners  506  The faces  502  include at least one channel  512  defined therein which produces a pattern with respect to the length  504  of the mandrel  500 . The channel  512  is sized to accommodate ribbon  220  therein ( FIG. 7 ), so as to allow for the secure and precise placement of the ribbon  220  along the length  504  of the mandrel  500 .  
         [0073]     The channels  512  are positioned on the mandrel  500  as a function of the desired cured configuration of the tower segment  250  ( FIG. 1 ) or like structure produced with the mandrel  500 . In an embodiment, one of the channels  512  runs in a spiral along the length  504  of the mandrel  500  so as to produce the patterns  220   a / 220   b / 220   c  of the ribbon-bracing lattice as illustrated in  FIG. 7 . Other ones of the channel  512  are optionally provided at a fixed position along the length  504  of the mandrel  500 , so as to receive the ribbon that will define the axial bracing at the intervals  222  (i.e., annular members), as visible in  FIGS. 6 and 7 .  
         [0074]     As the ribbon is wound about the rotating mandrel  500 , the channels  512  ensure that the ribbon is positioned on the mandrel  500  in the desired patterns, by accommodating the ribbon. In one embodiment, the rotational speed of the mandrel  500  is synchronized with the velocity of displacement of the ribbon spool  
         [0075]     Because of various factors associated with the production of the ribbon bracing (e.g., non-circular cross-section of the mandrel, thickness of the ribbon-bracing lattice, coordination of speed between spool and mandrel), without the channels  512  axially restraining the ribbon-bracing, small slippage could be experienced with the ribbon-bracing, producing not completely uniform shape which is preferable to ensure a standardization of mechanical properties for the structural assembly  200  ( FIG. 2 ) formed with the ribbon-bracing lattice.  
         [0076]     The channels  512  defined axially in the faces  502  afford the mandrel  500  the advantage of allowing the ribbon to be maintained under greater tension during the winding process, which grater tension would otherwise result in slippage of the ribbon axially along the faces  502  of the mandrel  500 .  
         [0077]     A close-up of one end of the mandrel  500  is presented in  FIG. 10 . The mandrel  500  rotates about an axis illustrated as direction  510 . The faces  502  are part of faceplates supported by a plurality of actuators  550  placed at intervals along the length of the mandrel  500 . Accordingly, the faces  502  are collapsible. Similarly, the corners  506  are supported by actuators  552 , and are thus retractable themselves. The actuators  550  are typically cylinders, such as pneumatic, hydraulic or electrical ones, and project radially from a shaft  514 .  
         [0078]     In  FIG. 10 , the mandrel  500  is illustrated in a configuration suited to receive the profile members  202  ( FIG. 2 ) as well as the ribbon  220  (FIG. C) After the various layers of ribbon  220  have been wound about the mandrel  200  with profile members  202  to form the structural assembly, the curing process takes place, after which the actuators  550  and  552  are actuated to cause a collapse of the mandrel  500 .  
         [0079]     After the structural assembly has cured to sufficient hardness on the mandrel  500 , the actuators  550  and  552  are actuated to retract, and thus release the produced structural assembly.  FIG. 10  further indicates that the faces  502  in an embodiment may have an outwardly rounded surface, as opposed to being flat. The retractable corners  506  have a cross sectional shape of a “v”.  
         [0080]      FIG. 11  represents a side view along the length of the mandrel  500 , where the channels  512  and one retractable corner  506  are visible along the length of the mandrel  500 . The channels  512  are both for the spiral pattern and axially fixed patterns at intervals along the length  504 .  
         [0081]     In a simplistic configuration of the mandrel  500 , only one faceplate is provided between a pair of the retractable corners  506 . In such a case, the channel  512  in the face  502  is sufficient to ensure the desired positioning of the ribbon with respect to the profile members.  
         [0082]     Although the present invention has been described hereinabove by way of preferred embodiments thereof, it can be modified without departing from the spirit and nature of the subject invention, as defined in the appended claims.