While structural engineers, contractors, and structural steel manufacturers are consistently coming up with better, more efficient ways of designing and building nonresidential structural systems, a new approach to hybrid core construction seems to be garnering even more attention and promise than usual.

Image courtesy of MKA

As compared to conventional concrete core construction, which takes three to five days to erect a floor, a new high-rise core innovation, the coupled steel-plate composite wall system, can be erected at the rate of one floor per day.

In fact, in his 30-some years in the industry, expert Michel Bruneau, Ph.D., P.Eng., professor, Department of Civil, Structural and Environmental Engineering, University of Buffalo, New York, states, “I have never seen the industry so excited about a new type of structural system.”

In place of a traditional concrete core, Ron Klemencic, P.E., S.E., Hon. AIA, chairman and CEO, Magnusson Klemencic Associates, Seattle, has adapted the steel plate composite system currently utilized by the nuclear power industry. In Klemencic’s version, a high-strength concrete shear wall is sandwiched by two structural steel plates, which serve as both the formwork and reinforcing bars. In addition to lending a high level of seismic support, the coupled steel-plate composite wall system is much faster to erect. As opposed to a concrete core, which takes three to five days per floor to construct, the steel can be erected at the rate of one floor per day.

Consequently, the higher the structure, the more construction efficiencies to be gained, which is why the high-rise market is looking very carefully at the first coupled steel-plate composite wall system currently under construction at the MKA structurally designed Rainier Square Tower in Seattle. It is anticipated that this coupled steel plate shear wall system will actually shave several months off the construction schedule and 2 percent off the construction costs to build the 58-story high-rise.

Beginnings

To appreciate the evolution of this newer composite system, a little history is in order.

Prior to 9/11, much high-rise steel construction, particularly in New York City, used a braced steel core. However, the World Trade Center collapse—where the steel core yielded to the impact of the terrorist-flown aircraft, cutting off exit routes—led the mid-rise and high-rise building market to exclusively start using reinforced concrete construction.

While the impact would have pulverized concrete cores as well, this addressed what was envisioned as public concern. Consequently, for the past two and half decades, with some notable exceptions such as Renzo Piano’s New York Times Building, most tower construction has consisted of reinforced concrete core walls surrounded by structural steel composite floor framing.

The construction of these cores was accomplished using formwork and an internal densely packed reinforcing bar cage. But while reinforced concrete provides the required strength and stiffness, there is a long cycle time involved in setting the formwork, installing the reinforcing steel, placing the embedded plates, installing the sleeves and block-outs, and placing the cured concrete before the next level of the core can be constructed.

Another drawback to the concrete core systems are that they require extensive internal reinforcing. If any penetrations through the wall are required for items such as piping, they may interfere with this reinforcing, and the location of the reinforcing may interfere with the placement of the embedded steel plates required for the attachment of steel floor framing beams.

As a result, the need to reduce construction time and resolve interferences have been major incentives to develop alternative structural core solutions.

Hybrid Core Alternatives

Generally speaking, hybrid core alternatives use the same concrete shear walls but with different types of coupling beams. Coupling beams are structural components designed to connect individual shear walls where openings are required in the shear wall. By connecting the shear walls, the coupling beams stiffen the building and may serve as a fuse to dissipate seismic movement. This could be concrete walls and steel coupling beams, concrete walls and coupling beams made from engineered cementitious composites, or concrete walls and precast post-tensioned coupling beams, explains Amit H. Varma, professor, Lyles School of Civil Engineering, and director, Bowen Laboratory of Large-Scale CE Research at Purdue University, West Lafayette, Indiana.

“In general, all these systems attempt to improve the seismic performance of the core wall system by improving the performance of the coupling beams,” explains Varma. “The hybrid cores replace these conventional beams with other alternatives to alleviate rebar congestion and constructability issues while providing good energy dissipation capacity.”

While these alternate systems are being used in practice, their use is not widespread. Varma conjectures that because their economic advantages are limited to the coupling beams, the cost benefits are therefore not very significant.

Another composite innovation is called Bi-Steel. The lightweight tied-plate steel panels are assembled on-site, erected, and then filled with concrete. This system can be constructed six times faster than conventional concrete, is highly flexible, and offers efficiencies with build sequences and reduced site congestion. In addition, no formwork is required, and the system offers better accuracy in interfacing with the adjoining steelwork.

The original Bi-Steel system included a patented welding procedure to affix interconnecting tie rods between two steel plates. While it has been used for a few apartment buildings in London, the system does have some fabrication limitations, and is not designed for seismic locations so its application has been limited.

Another alternative, called the modular core, is a prefabricated precast concrete modular unit. The columns, beams, and walls are precast concrete; prestressed hollow core flooring and steel units provide the structural hybrid frame solution; and in-situ concrete is used to facilitate structural connections. The system has been used in the United Kingdom.

In yet another configuration, steel plate shear walls, a thin steel web plate is welded or attached to horizontal and vertical boundary elements. As a thinner component, the walls can be thinner as well, thereby offering more usable square footage. Similarly, the building weight and required structural support, particularly when compared to concrete walls, is significantly less.

The relatively thin steel plate also offers excellent post-buckling capacity, is faster to construct, and is designed to handle wind and seismic loads. Most notably, the structural system was designed for the U.S. Federal Courthouse in Seattle, the Hyatt Regency Hotel at Reunion in Dallas, the Century in San Francisco, and a few projects in Japan.

Demonstrating the cost savings offered by the steel plate shear wall (SPSW) system, a study performed for The Century project recorded an average wall thickness, with furring, of 18 inches, as compared to an average concrete shear wall thickness of 28 inches, resulting in a savings of 2 percent in gross square footage.

The Century project study also found that the building weighed approximately 18 percent less than a building designed using a concrete shear wall core system. This led to a reduction of foundation loads due to gravity and overall building seismic loads. In terms of the project schedule, a one-month reduction in time was attributed to the SPSW system.

Because SPSW systems are usually more flexible than concrete shear walls, primarily due to their flexural properties, when designed for tall buildings, additional flexural stiffness must be provided. For example, for The Century and U.S. Federal Courthouse projects, large composite concrete infill steel pipe columns were used at all corners of the core wall to improve both the system’s flexural stiffness and overturning capacity.

There is also the consideration that excessive initial compressive force in the steel plate panel may delay the development of the tension-field action, so it’s important that the construction sequence be designed to avoid excessive compression in the panel. For the Courthouse, this was addressed by delaying the welding of the plate splice connections until most of the dead load deformation occurred in order to relieve the precompression within the SPSW panel.