Best Practices in Metal Construction
February 2012By Jerry Hatch
Engineers are ever-searching for the latest innovations and research in building and design, and will travel the globe in pursuit of insights and solutions that work. In the world of metal construction, Texas is a frequent destination because of the annual engineering conference hosted by NCI Building Systems. The event, now in its second decade, is centered on issues near and dear to the hearts of engineers involved with metal buildings and is the largest seminar of its kind. It also provides engineers with the opportunity for extended time to interface with researchers.
SEMINAR SHARED INSIGHT
This past October marked NCI’s 2011 seminar, held in Houston, Texas, which unveiled the latest research to influence how metal construction engineers will design structures in the near future. It featured presentations from leading researchers and experts in a range of engineering disciplines, and provided attendees with an overview of best practices in topics ranging from foundation design to ethics.
Attendees reported that some of the most eye-opening content was focused on the behavior of steel building system frames for earthquake loading, as well as the cautionary tale of the Las Vegas Hilton Sign, the largest sign ever constructed—the day it collapsed and lessons learned as a result.
SEISMIC PERFORMANCE
Matthew Smith, a Ph.D. candidate at the University of California San Diego, presented on the Metal Building Manufacturers Association (MBMA)-sponsored Moment Frame FEMA P695 Analysis at UCSD. This project focuses on the seismic performance of metal building system frames, and is anticipated to change the seismic performance factors for tapered member frames, to be subsequently incorporated into the International Building Code (IBC) and the American Society of Civil Engineers ASCE7 codes.
Quantifying the behavior of moment frames for earthquake loading is one of the most pressing issues that the metal building industry currently faces. The industry has been very efficient in designing buildings to stay elastic, but the code requirements introduced by IBC now require an understanding of inelastic behavior: what happens when elements start bending and buckling, and what is the progression of the collapse mechanisms? To answer these questions, the team at UCSD tested three different buildings on the largest shake table in the U.S., giving them the first glimpse at how tapered moment frames behave in the inelastic range.
TESTING THE DESIGN LOAD
The first of the buildings, with a metal wall and roof panels, survived to 300 percent of its design load. The second building had concrete tilt-up panels on the walls and a metal roof. The third had one concrete tilt-up wall and one metal wall, as well as metal roof panels and a mezzanine. Both of these buildings performed to 100 percent of their design loads, surviving an earthquake-level shaking without collapse. Over concerns of damage to the shake table equipment, none of the buildings were taken to the point of failure, so the full potential of each building design is not yet known.
Smith is currently incorporating the behaviors observed from the testing into finite element models, with the goal to identify three frame types: one based on elastic analysis, such that engineers would not need to change their current processes; a special moment frame incorporating multiple locations of inelastic behavior; and an intermediate frame with one designated location of inelastic behavior. The results of the analysis are expected to provide seismic performance factors for design of tapered member frames suitable for adoption by the building codes.
LAS VEGAS HILTON SIGN
One of the most spell-binding presentations of the conference was given by Finley A. Charney, Ph.D., P.E., professor of structural engineering at Virginia Tech. Charney recounted the tale of the Las Vegas Hilton Sign, the world’s tallest free-standing sign which, on July 18, 1994, collapsed during a summer thunderstorm in Las Vegas, sending the top half of the sign on a 180-foot free fall to crash into the ground below. His detailed exploration into the original design and construction of the sign, as well as the reasons that it failed, comprised as much an ethics presentation as it provided a comprehensive checklist of every shortcoming a team should be sure to avoid when planning and building a tall structure.
According to Charney, the reasons for the massive failure of the 361-foot-tall sign could be found in every aspect of the design-build process, including concept, management, design, review, fabrication, erection, and inspection. As he found—and explained in his published paper “On The Collapse Of The Las Vegas Hilton’s Spectacular Sign,”—part of the explanation also lay in subjective building code language and unconservative building code regulations. He posited that everyone involved in the design-build process knew that things were going wrong, but the pervasive attitude that “this was just a sign” kept things going when it would have been prudent to stop work, fix the problems, then continue.
More than 3 years after the failure, the Hilton filed suit against the lighting and sign company, the company’s subcontractors, a peer reviewer, and an independent inspection agency, and won an $8 million out-of-court settlement. It was the end of a riveting tale about havoc wrought when time and monetary pressure win out over caution, and how the greatest spectacle in Vegas was lost over a breakdown in professional ethics, because nobody took responsibility for, or had any real pride in, their work.
Over the past several years, metal building manufacturers have supplied greater than 40 percent of the low-rise market in the U.S., according to McGraw Hill Construction Research. With the latest findings in seismic performance factors and other areas, metal building manufacturers will be able to provide building types that they have been precluded from in the past, bringing about a natural growth in the market. Continued research of this type allows manufacturers and designers to refine their technology, ensuring life safety and general health for the industry. ■
About the Author:
Jerry Hatch is manager of Engineering Development for NCI Building Systems and chairman of the Metal Building Manufacturers Association (MBMA) Technical Committee. He has worked in the Metal Building Industry for 17 years as production engineer, engineering manager, director of Research and Development, and most recently as manager of Design Development at NCI. Previous to his work at NCI, he was active in mid-rise and low-rise building design, and also spent several years at a nuclear power generating facility, planning and scheduling construction projects during refueling outages. For more information, visit www.ncibuildings.com.
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