Efficient Design and Spectacular Effects in AISC's Modern Steel Construction Magazine. Written by George F. Wendt
In an article published in AISC's Modern Steel Construction magazine, high quality tube bending helped create spectacular effects while achieving significant cost savings. The combination of Chicago Metal's tube bending and an innovative translucent covering created a hub for a revitalized downtown Kansas City.
In Modern Steel Construction Magazine, October 2009
A lightweight and semi-transparent covering over an efficiently designed steel frame creates an outdoor courtyard inside Kansas City’s new mixed-use development known as the Power and Light District. The 9 block area is anchored by the ‘Live!’ block and is covered by a single-layer ethylene tetrafluorethylene (ETFE) membrane.
Structurflex, LLC, Kansas City, MO, & Structurflex Ltd., Auckland, New Zealand, built the membrane system, the first single layer ETFE system to be executed in North America. It is the same system as was used on the Beijing Olympic Stadium—also known as the “bird’s nest.” Structurflex has a partnership with German membrane company Seele Cover (formerly known as Covertex), the manufacturer for the ETFE membrane who also designed/manufactured, and installed the membrane on the stadium. With Structurflex’s 25 years of experience in designing, fabricating and building tensioned fabric systems, this new material is the manufactured by the world leading specialist. ETFE systems differ from fabrics in that they are much like a film or foil, rather than a woven fabric that is comprised of a coated scrim. The manufacturing process is quite different.
Pneumatic ETFE membrane systems have been used extensively in Europe for the past decade. The Allianz Arena in Munich is a recent major application completed by Structurflex’s manufacturing partner, Seele Cover. The Germanfirm also pioneered the application of ETFE films for true tensile membranes where it was first used in large scale on the AWD Arena in Hanover, then most recently at the Bird’s Nest in Beijing. In all applications, the accuracy and sophistication of the structural steel support system is of paramount importance for the successful performance of the membrane system. The team assembled by Structurflex for the Kansas City project was able to bring it all together for the major installation.
The canopy was designed and developed to provide protection from rain and snow, to offer some shading and to define the space generally. The original concept of a glass roof greatly exceeded the project’s budget. A conventional steel canopy was then considered, but the large and heavy structure coupled with a galvanized metal deck would zap away the energy that the space was meant to generate. The client had obvious requirements for lightness and transparency.
Structurflex proposed a turnkey system for the design, engineering, membrane, structure and construction of the canopy. Being able to offer both fabric and ETFE was a definite advantage: both options required less structure and therefore less cost than a glass or metal clad canopy. The steel weightis 30%-40% lighter than that needed for a glass canopy or that needed for a steel deck due to the fact that no purlins were needed. Using cables instead reduced not only weight but also the fabrication and installation costs, and additionally enhances the filigree appearance of the structure. The total cost was substantially less than a metal deck canopy and less than half the price of a glass canopy.
After extensive design collaboration with the owner and architect, ETFE was settled upon. The notion that this would be the first single layer ETFE system in the Americas was actually an incentive for the owner to be a part of a groundbreaking system.
The canopy is 191’ x 138’ in plan; the top chord length is 142’; the truss depth is 7’; the large bay has a 45’ span while other bays have a 36’ span; and the end overhangs have an 18’ cantilever at each end. With a 65’ elevation from street level to apex, the membrane spans some 27,000 square feet.
With 336 cable assemblies (1x19 construction) ranging from 5/16” to 7/8” diameterintegrated into the membrane and with stainless steel end fittings, the system can easily handle the live loading conditions that will be imposed by dramatic Midwest seasons. The cable fabricator and supplier was Ronstan International which is typically known for sail rigging but which also does architectural cable assemblies.
Chicago Metal Rolled Products curved all the roof members (about 50 tons) utilizing both its Kansas City and Chicago plants. A number of different sections were considered for the structure including 12” standard pipe and 10” square tube with 0.3125 wall). To simplify the connections and the interface from the membrane system to the structural steel, the design settled upon used square tube. The top chord required angles to be integrated in order to accept the cables and facilitate a seamless method of clamping the membrane.
The Chicago plant provided 28 sections of 10” square tube with a 0.3135 wall rolled to a 160’ 8.5” outside radius and 20 sections of the same size tube rolled to a 153’ 8.5” outside radius. Material arrived on a Friday and on the following Tuesday two truckloads brought the 32’ to 48’ long curved sections to Collins and Hermann in St. Louis for further fabrication.
Due to some modifications in the design, the new Kansas City plant of Chicago Metal rolled 41 pieces of 5 x 3 x ¼ angle and 8 pieces of 3 x 3 x ¼ angle. The angles, 40-43’ long, were all rolled to a 160’ 11.5” outside radius leg in and rushed directly to the jobsite Using two plants minimized transportation costs and saved time. Offering numerous alternatives for the steel framing, Chicago Metal became involved early in the design process first with Structurflex and later with Collins & Hermann. “The structure turned out great,” said Bart Dreiling, President of Structurflex’s American Operations, about the work of Chicago Metal and Collins and Hermann. “Collins & Hermann did a great job on the steel fabrication.Thanks to project manager Craig Pardue, this project went quite smooth from fabrication all the way into steel erection.”
The membrane has a silver fritted reverse dot print in order to provide a certain level of shading. The fritting was also important to achieve nighttime lighting effects through an LED lighting system that covered the entire color spectrum. This design allows for the canopy to constantly change its color throughout an evening to provide a truly lively and dynamic feel. It was also important to maintain enough transparency to see the surrounding downtown high rises as well, so that the feeling of being in the urban core was not lost. The reverse dot print provides approximately 75% translucency and achieves all of the aforementioned design criteria.
Erecting the structure was the most challenging construction element of the $275M development: a crane reach of 170’ from the surrounding streets was required, always lifting over the top of two story buildings. The advantage to the membrane installation was that it was sectionalized into 12 panels therefore allowing other construction trades to work beneath in other areas of the structure. The system installation was so efficient that it did not hinder any other trades on the project. .
The single layer translucent ETFE membrane system over a steel frame has become a Kansas City landmark that defines the city’s most vibrant gathering point. KC Live borders the new Sprint Arena with its transparent glass curtain wall framed by 16” pipe (also curved by Chicago Metal Rolled Products). The views of activities both inside and outside the Power and Light District contribute to the excitement of an area adjacent to the city’s tallest sky scrapers. This oasis attracts locals and tourists at the end of a work day, before and after major sporting events and concerts, or simply for a night on the town.
The architectural effects of the project have resonated well throughout the Kansas City architectural community and beyond. Architects, designers, owners and contractors now see this as an efficient and smart alternative to other systems. An inherent virtue of the system is that itincorporates “green” and sustainable attributes, key issues for the owner. This project won the highest award of the Industrial Fabrics Association International for 2008 in the category of commercial canopies.
Project: KC Live!
Engineer: Wayne Rendely, Wayne Rendely, PE
Architect: Bill Johnson, 360 Architects
General Contractor: Structurflex LLC, Kansas City, Mo., & Structurflex Ltd., Auckland, New Zealand
Structural Steel Fabricator: Collins & Hermann
Roller/Bender: Chicago Metal Rolled Products
Fabrication: Gunda Noatzsch, KFM GmbH
Fabric: Single Layer ETFE by Nowofol
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