- Week 8 Design Figure 1 Mobile Cantilevered Umbrella Source Amazon A Shadowspec Figure 2 Folding Stages Of A Cantil 1 (42.76 KiB) Viewed 19 times
Design Description: The DOTW for Week 9 is the mobile cantilevered picnic umbrella shown in Figures 1-3. These umbrellas are available quite cheaply from many home improvement type stores and their concept is to avoid a pole in the middle of the shelter, thus allowing the umbrella to provide a larger and more functional shaded area. They are comprised of a canvas-type canopy and a (generally) metal support structure. Your task for DOTW this week is to consider this umbrella and to discuss the internal loadings and forces that you expect the structure will experience, where you predict it might fail and how you might improve the design. The following focus questions are meant to help you explore the DOTW, but your group doesn't have to address all questions or limit your discussion to them. You might find it useful to create some diagrams to help discuss the problem.
Focus Questions to support Student Discussion: 1. What external loads do you anticipate the umbrella will be subject to? What reaction forces will support the umbrella in equilibrium? 2. Given these external forces, explore effective ways of anchoring the umbrella to prevent toppling. Usability is often a design criterion, so you could consider what practicalities may arise from various anchoring choices. 3. Consider again the external forces. What internal loadings will result in the umbrella structure? You might like to focus on the main vertical post, the umbrella suspension members, and/or the ribs that support the canopy. Drawing free body diagrams could be useful here. 4. In the member you investigated for Q3, how will the different loadings combine to create the total stresses in that member? Where do you expect the largest total stress will be? 5. Cheap versions of these umbrellas are somewhat notorious for being under- engineered or for having poor design choices. Where or how do you think this umbrella could fail? Can you identify locations prone to failure from axial, bending, or shear loads (or a combination)? How might you improve the design to prevent these failure modes? You could consider, amongst others, choices of connections and cross-sections in the structural members. 6. Given the requirements of a shade + waterproof shelter with no obstructions or supports within the shaded area, can your group propose an alternative design? What other design parameters do you take into consideration? Would your design have the same kinds of internal loads or different ones?