Holding It All Together
There are many decision points along a project’s lifespan. One often brushed aside decision is Means of Joining. As anyone who’s delved into the Titanic’s sinking and similar structural disasters might understand, means of joining can often lead to the success or catastrophic failure of a design.
Another such well-known structural disaster occurred in July 1981, when, just a year after the hotel’s opening, the suspended second and fourth floor walkways in the Kansas City Hyatt Regency Hotel collapsed during a teatime music and dance event. The event had drawn roughly 1600 people. After exhaustive investigation, causes for the collapse were attributed to a breakdown in communication, lack of professional ownership and oversight, and flawed design.
At the center of this flawed design was the method of suspension for the walkways. The walkways consisted of opposing c-channels welded together to form three spanning box channels across the width of each walkway. The original design had the second and fourth floor walkways suspended in series from the ceiling; the altered design had the second and fourth floor walkways suspended in parallel from the ceiling with a double hanger rod arrangement to link the second floor to the fourth floor. As such, the original design allowed the load path of the system to flow directly through the single hanger rods at each joining location, loading each floor’s corresponding nuts with just the weight of its floor. However, the altered design resulted in twice as much load on the fourth floor walkway’s hanger rod nut, as it was now supporting the load of the second floor in addition to its own weight. To further strain the system, the hanger rods were run directly through the welded joints. These changes in the hanger rod arrangement were made by the construction company under the convictions that the full-length thread specified in the original design of the hanger rods would have been damaged and rendered unusable when the fourth floor was raised into place. These wrongly approved changes resulted in the fourth floor hanger rods ripping through the fourth floor’s box beams. The fourth floor subsequently crashed into the second, which then collapsed into the main floor of the lobby. This structural collapse would hold the title as the most deadly structural collapse in US history for two decades.
As shown in the example above, when deciding which method of joining is best for you project, several areas should be considered, including but not limited to: loading, materials and compatibility, geometry, accessibility or clearance, environment, and last, but not least, redundancies. In most everyday hobby projects, just general loading conditions and materials of members to be joined are normally considered. When transitioning into projects outside the “maker” or “hobby” spheres, however, further steps are need. One ought to remember to consider:
- Whether environmental conditions could cause malfunction (e.g., corrosive environments, thermal cycling, average temperature);
- Detailed loading conditions (e.g., load path, loading cycles, vibrations) coupled with the geometry of the members (e.g., inherent “weak spots” in the members, stress distribution across the member);
- Outside restrictions on accessibility and clearances (e.g., ease of assembly, interferences with other portions of the system); and, finally,
- A “backup plan” (e.g. safety factors, redundancies) to avoid a catastrophic system failure should the original design fail.
For more in-depth examination, Shigley’s Mechanical Engineering Design is a great resource for all things mechanical and has several great sections on the different options and considerations for joining methods.
If you’d like to read more about the Regency Hyatt Hotel walkway collapse, the Kansas City Public Library has a great summary with links to more resources at the following: http://www.kclibrary.org/blog/week-kansas-city-history/hotel-horror
Engineering.com also has a great write-up of the event and subsequent investigations at http://www.engineering.com/Library/ArticlesPage/tabid/85/articleType/ArticleView/ArticleID/175/PageID/197/Default.aspx
Both sites were used to reference this article.
-Casandra Ceri, Jr. Mechanical Engineer