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100 FT. PULTRUDED GLASS REINFORCED PLASTIC TOWER MANUFACTURED AND ERECTED BY BALAJI COMPOSITES LTD.

Introduction
     
  The technology of pultrusion is not new to India . Pultruded rods used in electrical industry are there for the last 20 years. The use of pultrusion in structural members such as gratings and profiles has been there for over 15 years. Most applications are mainly in the area corrosion resistant structures and the development of pultruded products is mainly for small profiles, typically under 100 mm width. Few products have been developed that are larger in size, typically flat sheets and channels for the cable tray applications. This application of pultrusion, in my opinion was the first attempt in making a large structure with computer aided design, simulation and destructive tests of critical parts. Though structures of this size are fairly common in Europe and USA , the stress was on a clean board approach with locally available parts and locally available technology.

 
 

 

The Requirements of the Client
 
  The specifications of the tower as required by the client can be roughly put as follows ;  
 
  1. The tower should support a circular rail of 25 mtr radius on its underside extending to nearly a quadrant of a circle.
  2. No Supporting pillars beyond the base, requiring a total cantilever structure.
  3. The tower should have absolutely no metallic parts. That meant that use of metallic fasteners; gusset plates or even paints / pigments with heavy metal content were prohibited.
  4. The tower should withstand all the environmental conditions at the site.
  5. The tower should withstand 220 kmph winds from all sides.
  6. The tower should support a point load of 200 kg at the extreme cantilever position with maximum allowable deflection of 20 mm.
 
   
Design
 
  The design of the tower was addressed by putting the requirements of the client in decreasing order of severity  
 
  • Wind load of 220 kmph winds acting on any side of the tower.
  • Cantilever structure, 200 kg load at the extreme position, deflection under 20 mm.
  • No metallic parts, all joinery had to be composite material.
  • Environmental exposure.
  The wind load factor was the most crucial part of the design. At 220 km per hour, the winds formed a considerable pressure on the structure. This combined with the cantilever shape of the tower created large twisting moments.  
     
 

The first approach tried by the designers was to have an aerodynamic structure was the shape of a curved cylinder as shown in fig.1

The design was suited for the winds as it had a very low drag coefficient. However, the cylinder was heavy and the joinery involved in the segments of the cylinder would be difficult. The moulding process also involved very large moulds. Above all, the process of hand moulding or spray-up was not exactly a tightly controlled one. Overall, this design did not get a second look from all agencies concerned with the project.

The design efforts were now focused on a regular lattice like structure. The structure was tapered at the top of the tower, as the stresses were lower there. The design was essentially based on the earlier curved cylinder model. Fig. 2 gives an approximate idea of the design.

This design allowed straight segments joined to form a curved structure. This design facilitated using pultruded profiles. Two additional props were also provided for additional support.

The design was suited for pultruded profiles but posed problems for the joinery. The change in profiles at the joint was in all three dimensions, and hence there were excessive stress concentrations on the joints leading to cancellation of this design.

The design now evolved into a tower of a uniformly tapering profile in the vertical plane. (Fig. 3) This allowed the use of gusset plates in the joinery and the stress concentrations at the joints were lowered to an acceptable value. The base of the tower was also widened so that the critically stressed parts had better factor of safety in the worst-case wind loads.

The design of tower was done using the latest structural design software's and over 150 simulations were done with various wind loads, structural designs and pultruded profiles. The software provided detailed numbers regarding the forces and twisting moments on each member and joint under all loading conditions. As mentioned earlier, wind loads were the worst stressing factors and very little modification was needed to provide for the live loads on the structure.

As no metallic parts were to be used in the tower, the design of the joints was based on GRP gusset plates, GRP pultruded studs and nuts, and adhesives. The design of joints was done in such a way that the shear forces in the joints would be taken entirely by the adhesives. The gusset plates and GRP studs and nuts would just assist in holding the adhesive at its place. The added benefit in this design was that an additional factor of safety was built into the joint by way of gusset plate and GRP pultruded studs and nuts. Wherever design necessitated, GRP molded channels were used in place of gusset plates.

The environmental factors were not as critical as the wind due to the inherent corrosion resistance of pultruded GRP materials. The factors to be considered here were that the tower location was right next to the sea. A visit to the site showed all the damage of saline atmosphere on the exposed metallic parts. Though any of the commercial unsaturated polyester resins would have done the job, we selected Vinyl Ester resins for their superior corrosion resistance. Using suitable additives UV resistance was incorporated.

 
   
Pultrusion
   
  The construction of the tower is cantilever type and hence there were large twisting moments in most of the main and cross members. This meant that the profiles had to have a good torsional resistance. The best profile for this job was round tube as it has good torsion resistance and the least wind resistance. However the round tube is not well suited for joinery. The second best option was a square tube. The profiles selected for the tower were square tubes of varying sizes and thickness based on the actual stress conditions on the Tower. The main members of the Tower were
  • Main member Square tube 150 x 150 mm. Wall thickness 8 mm, 10 mm
  • Cross member Square tube 110 x 110 mm. Wall thickness 6 mm
  • Diagonal member Square tube 75 x 75 mm. Wall thickness 6 mm, 8 mm
  the various wall thicknesses were rationalized to only one thickness per profile for faster production. All profiles were provided with large corner radius to reduce the wind resistance of the profile. The torsional resistance of the profile was further improved by incorporating a +/- 45º stitched mat on the inner and the outer surfaces of the tube. The balance material was the standard combination of roving and mats.

As the tubes were specifically manufactured for this tower, the dies and mandrels were manufactured and chrome plated in-house using our electroforming unit's facilities. During pultrusion, the job lengths were also set dynamically when the tubes were under going pultrusion to prevent re-cutting and wastage. Each section of tube coming out of the machine was marked and numbered as per the fabrication drawing. Additional lengths were provided on some members at regular intervals to take test coupons for tensile testing.
 
 
 
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