ABSTRACT
We chose reinforced polyethylene film as a canopy material as it is low-cost, light-weight, tear-resistant and fire-retardant. Polyethylene can be easily heatwelded using metal bars or a hot air jet. We have successfully bonded this plastic material using highpressure steam. Polyethylene film is an isotropic material that provides equal strength in all directions. In order to create a saddle surface, it had to be cut and assembled from individual parts, which is a potentially time-consuming process. However, we discovered that a single flat sheet could be manipulated into a three-dimensional saddle surface by what we called the pinching technique. Original experiments in pinching used simple, unframed membranes with individual tension points. Later trials have shown that an integrated frame or a continuous edge is more advantageous in transmitting the tensile forces from the tent’s surface. Although the frame is not structurally necessary, it serves to effectively stabilize, deploy and keep the canopy in tension. We had chosen Tonkin bamboo as the frame material for its properties of natural elasticity, light weight and sustainability. The frame consisted of four poles attached together into a closed loop that spread apart at the peak and base. The canopy was mounted within the straight-edge frame which resulted in a highly stable yet collapsible
Pinching
The tent frame is pre-assembled from four straight bamboo pieces coupled with four flexible (hinged) elbows. A single piece of polyethylene film is cut from a continuous roll and heat sealed onto a hinged frame. The frame and film are then folded and pinched, i.e. sewn or welded together. In one step, two overlapping layers of film are welded and cut along a curved line, thus producing a watertight seam. The remaining crescent is then removed. Next, the frame and film are opened and refolded perpendicular to the first seam. A secondary pinch is applied and the crescents are removed. The frame is once again opened and kept in tension at the peak to produce a double-curved or saddle-surface canopy (Figure 10.1).
