CompositesWorld
Issue link: https://cw.epubxp.com/i/697353
47 CompositesWorld.com "Te method is identical to that used for decades by the aero- space industry with one exception, we use a smaller sample size," Humphries notes. Representative laminate samples were tested at room temperature for tensile and compressive modulus, Pois- son's ratio, tensile, compressive and fexural strength, interlaminar shear strength, ultimate bolt-bearing strength and other mechan- ical properties. Although the statistical implication of a smaller sample size is a larger "knockdown factor," resulting in lower allowable strengths than might be achieved with more testing, CONSTRUCT compen- sates for this by designing in large margins of safety — in the shroud's case, 300% — for maximum expected loads in all load conditions. Te critical load, in this case, was wind loading. In parallel, multiple trade studies were conducted to assess the benefts/shortcomings of options posed by various combinations of resins, fbers, layups, tooling and fabrication methods. Tis refned the design, minimizing unknowns and risks. Te selected laminate layup featured two types of Texonic (Saint-Jean-sur-Richelieu, QC, Canada) non-crimp structure (NCS) woven E-glass, 22 oz and 54 oz, with at least two-thirds of the fabric laid in the axial direction of the shell. Both fabrics were selected for their good draping characteristics to curvature. After FEA modeling confrmed the design's capability, the fnal drawing specifed a shroud fabricated in fve separate curvilinear panels, each approximately 13.4m long, and varying in width to a maximum of 4m (see drawing, p. 93). Key shroud design features would include integral 90° fanges, which would be molded on the sides of each panel. Te fanges would be about 51 mm wide and about 10.5 mm thick — 50% thicker than the shroud walls. To accommodate a skylight, the top portion of each panel would feature a fange about 305 mm wide (see drawing). To enable fange molding and post-cure part removal, removable fences would have to be installed along the sides and top of the mold. Molding and installation JRL Ventures (Cape Coral, FL, US) was selected as moldmaker and molder. All fve panels were molded in the same tool, but two panels — designed with a 1.5m-wide by 11m-tall opening to permit pedestrian trafc in and out of the stairwell — were marked for post-cure trimming by scribe lines etched into the mold surface that defned the opening outline. Te total infusion time for each panel was about 30 minutes, with a cure temperature of 65.5°C for at least 30 minutes and a post-cure at about 74°C for 2 hours. Nominal panel thickness is 6.35 mm with a glass fber volume of 65-68%. After cure, each panel was de-fashed and painted with Break-Trough water-borne acrylic, supplied by PPG Architectural Coatings (Pittsburgh, PA, US). At installation, the panels were attached After cureat the 90° fanges by 3/8-inch bolts through holes spaced about 254 mm apart. Joints were sealed with Dynatrol urethane construction sealant, supplied by Pecora (Harleysville, PA, US). Te tubular shape of the shroud is maintained by attaching the panel joint fanges to tubular steel rings at each stair landing. Prior to attachment to the steel support plates on the right and left of the stairway, the bottom of the shroud panels were trimmed to ensure they were level with the polycarbonate foor that covers the refecting pond below. Te stairwell shroud mimics the efciency of a domed structure: Tension and compression loads are evenly distributed within the plane of the composite shell. Tis compound curvature not only looks good, but also enabled designers to realize a lighter, thinner structure than would be possible in a fatter, rectilinear shape. Now in use at the LTU Taubman Complex, it's a compos- ites showcase that could dramatically change the perception of composites as mere aesthetic add-ons in the architectural engi- neer's toolbox. Class A + curvilinear = composite choice One of the two opening panels is shown here, after coating with a water-borne acrylic paint. The requirement of a Class-A surface fnish, along with the structure's compound curvature, tilted the selection of the material for the shroud's fve panels in favor of composites, albeit one with structural properties. Source | CONSTRUCT Michael R. LeGault is a project manager and technical writer for Information Development Inc. in Houston, Texas, US, and the former editor of Canadian Plastics magazine (Toronto, ON, Canada). mlegault@compositesworld.com LTU Taubman Stairwell Shroud