How We Won a KIWI Award for Our Cable Stayed Portal in Chicago
Did you know that to visit the largest convention center in America, you enter a portal? Not just any portal, but an award-winning cable-stayed portal.
Novum engineered a cable-stayed portal (17 meters tall and 55 meters long), clad in stainless steel and laminated windows. The predominant cable structure system welcomes visitors who enter from the South Entrance of Chicago’s McCormick Place.
Cable Structures
The two major elements, tension cables, and disc clamps have been employed in several other of Novum projects utilizing cable structures, including Wrigley’s Global Innovation Center, Johns Hopkins University’s Gilman Hall, the World of Coca-Cola in Atlanta, and GM’s Wintergarden.
Unlike a suspension bridge, where the load-bearing forces run across cables from one column to another. A cable-stayed structure uses cables that are fixed onto the columns, so these columns work alone in carrying the load.
Novum Structures’ combined expertise from the departments of structural design, engineering, and installation created a safe, well-designed cable-stayed portal that withstands the Windy City’s wind-bearing loads.
Designing Structures and Calculating Cable Length with ProSteel 3D
ProSteel 3D was utilized throughout the design process, which helped with managing the cables on the twenty-inch diameter, horizontal round tube and its connecting points on the steel frames that encircle it. This program aided the designers and engineers with practical information, including how long the cables should be. ProSteel 3D CAD system’s software developers, Bentley Systems, awarded Novum Structures a top place for the KIWI Awards in 2006, for their savvy use of ProSteel 3D.
The Structure and The Forces That Affect It
The cable-stayed structure comprises a fish-bellied design. Its parabolic form was selected both for its aesthetics and its stress-strain characteristics. The structure’s basic components are two columns with a 30-inch diameter connected to a horizontal round tube with a 20-inch diameter. 21 steel frames consisting of tubes 3” by 3” in size hold the cables that weave across the structure at specific points from both columns using stainless steel cable clamps.
18 steel cables divided into three groups provide various functions to the portal ranging from stiffening the structure against wind loads and uplift forces to increased stability and aesthetics.
After the initial installation, stainless steel sheeting and laminated glass were added at a later date.
The Design Process
ProSteel 3D and other related software’s computational abilities created the spatial course for each cable for maximum efficiency, aesthetics, and dampening. This task, generally speaking, required a complex computer program. Correlating where the cables intersect at which nodes on the steel frames needed a level of acute discernment that could only be achieved by these programs.
Wind Tunnel Testing in the Windy City
In order to receive the green light from Chicago’s Department of Buildings to pass the Chicago Construction Codes, the cable-stayed portal—including the stainless steel cladding and laminate windows—must not have gust-excited vibrations in the horizontal direction transverse to the portal’s entire span.
In order to prove this assertion, the Alan G. Davenport Wind Engineering Group figured out both the portal’s natural frequencies and natural modes of vibration using wind tunnels. At the relevant natural modes of “Horizontal vibrations transverse to the portal span,” frequencies 1,706 Hz, 2,609 Hz, and 4,590 Hz (roughly corresponding with musical notes A, E, and C#) don’t initiate gust-excited vibrations. With the portal’s sophisticated design and engineering, the structural dampening needed amounted to .5 percent.
The Alan G. Davenport Wind Engineering Group analyzed the portal structure with spectral analysis (range .1–5 Hz) to investigate its dynamic behavior with higher excitation frequencies under fluctuating wind effects. Also, a deterministic gust impulse load with variable parameters was used along with Schlaich’s deterministic gust model and Newmark-Wilson integration for the calculation.
When the wind-tunnel test began, the steady-state wind load gradually increased to 40% and held there for the portal structure to adapt to this load. After ten seconds, the load pulse was applied for the full wind load.
The Wind Tunnel Results of Our Cable-Stayed Portal
Using control nodes 217 and 218 to capture rotational effects, static horizontal displacements of 5.197 inches (132 mm) for node 217 and of 5.536 in (141 mm) for node 218 illustrate the only minor influence of the gust impulse load on the portal structure. The dynamic enhancement factor is just about 1.03, and the wind-induced portal vibrations quickly decay.
They increased the number of wind gusts to two to three in short succession with 5-second interval pauses and shorter pulse duration from 2 to 1 second produced a slight increase in the dynamic enhancement factor to 1.10, which showed no change in the system’s behavior. Following this analysis, gust-excited vibrations could not be detected.
Based on these results and data of Chicago’s wind energy spectra, the Alan G. Davenport Wind Engineering Group investigated the sensitivity of the portal structure to higher-frequency gust-excited vibration. Their conclusion: due to the low energy density, there is no risk of gust-excited vibration even at excitation frequencies above 1 Hz.
How We Built Our Cable-Stayed Structure
Carefully, the portal was first preassembled close to the thoroughfare near its installation site. Four major tasks needed to be completed. First, mounting the steel structure on temporary columns. Second, installing and pre-tensioning the cables. Third, installing the concrete column bases. Last, lifting the steel structure onto the column bases.
16 hydraulic cylinders, connected to parallel-attached pressure hoses, were powered by two power team type RH206 hydraulic cylinders to pre-tension first H1 to H4 (inner horizontal), then H5 to H8 (outer horizontal). Pressure gauges were monitored, and the permanent column bases were allowed to move freely longitudinally, so that the cable forces become compressive forces into the horizontal tube. The team checked the tension with a Pian Sweden RTM 200 tension meter at the cable ends and in the middle of the structure.
Once the vertical load cables V1 to V4 were pre-tensioned, the structure lifted from the scaffold towers.
Two Cranes To Completion
In the evening after the Chicago traffic subsided, two cranes were used to move the structure. The assembly joints were secured onto the concrete column bases with screws and welding.
Following the installation, the laminate windows, stainless steel cladding, and the text “McCormick Place” were added. Measuring the structure with the theoretical data generated earlier in the project showed congruence with regards to deformation and deflection of the horizontal round tube.
The combination of engineering, installation, construction, and design came together to create a new portal to the Windy City’s convention center, engineered and designed to last for years to come.
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