System allows for space in Miami University facility in Oxford, Ohio

Although steel plate shear walls (SPSWs) have been used on the West Coast as a system that resists lateral load and horizontal-story shear, it is still a rare solution in the Midwest. For Columbus, Ohio, structural engineering firm Shelley Metz Baumann Hawk, Inc. (SMBH), however, SPSWs proved to be the ideal answer for the new Miami University Psychology Building and Animal Care Facility in Oxford, Ohio.

Previously housed within a facility that was somewhat traditional in terms of planning, the university hoped to create an experience that encouraged interaction between students and faculty, as well as provided orientation within the facility and a view of the campus from major public spaces. Although the accessible conference rooms and laboratories were semi-private, they were naturally lit and still visible to the entire department.

The 105,000-square-foot, three-story plus lower-level facility boasts a brick and glass exterior with sloped roofs. Designed to complement the existing structures on the campus, the owner sought an open feel and natural light within the building, which resulted in a design solution that included an atrium and stepped, floating floors.

Structural steel was the favored framing system because of the engineering considerations, specifically the extensive spans necessary to hang floors in the atrium. The suspended floors not only provide additional space—such as a graduate research room that is hung from the third floor—but they also add architectural interest.

The architect, NBBJ, along with the owner, wanted to create an open lower level to invite students into a series of shared public spaces to encourage exploration and interaction.

"In visiting other research and academic institutions, we found that most contemporary learning environments do not encourage serendipitous spaces outside of the classroom," said Ryan Mullenix, AIA, principal at NBBJ. "Based on these observations and numerous conversations with Miami University officials regarding the needs of the psychology department, we determined collaborative, interconnected environments should be a primary focus for this facility."

SPSW solution

Examples of SPSWs as a seismic retrofit solution for low- and medium-rise structures can be found in the United States beginning in the 1970s. Today, the system is being applied to a number of mid-rise and high-rise steel structures primarily on the West Coast. In this area of the country, many structures are undergoing seismic retrofitting to meet current codes, and SPSWs are extremely effective in resisting seismic and wind loads. However, though applicable, the system is not widely used in other parts of the country.

Typical SPSW systems consist of a steel plate wall, boundary columns, and floor beams. The steel plate wall and boundary columns jointly act similar to a vertical plate girder. The steel plate wall itself acts as the web and the horizontal floor beams act as transverse stiffeners in a plate girder. When loaded, the plate will experience large inelastic deformations, while the vertical boundary elements (VBEs) and horizontal boundary elements (HBEs) must remain elastic. This also needs to be the case under forces generated by fully yielded webs. Thus, the actual-versus-theoretical plate-yield strength becomes extremely important in the design of the system. Recent studies have shown that the ratio of expected yield stress to specified minimum yield stress, Ry, for ASTM A36 plate material is 1.3 rather than 1.1, as specified in previous codes. These new findings significantly increase design loads on the system’s VBEs, anchor bolts, and foundations.

Although the structure was designed with a conventional-braced frame, the SPSW was necessary because the diagonal bracing would have interfered with the layout of the laboratory on the basement level. SMBH opted to use the SPSW to allow for accessible space on each side of the frame. Diagonal bracing would have been required to extend from column-to-column, which made the access impossible because of interference with the desired access points.

Once it was determined that diagonal bracing was not a workable option, alternatives for resisting the lateral loads were evaluated, including reconfiguring the diagonal bracing. This solution was eliminated because of the space constraints in the lower level. It was then decided that the design team incorporate an SPSW. The use of an SPSW that did not extend from column-to-column would allow for the desired access.

SMBH began discussing this option with the American Institute of Steel Construction (AISC), since the organization was in the process of developing a design guide.
SPSWs are often the best solution for a project based on the needs of the space.

Although SPSWs have not been incorporated into many projects in the Midwest, SMBH was confident that it was the appropriate choice for this project. SPSWs have been used in several projects throughout the country, and there was substantial evidence that it would solve the challenge presented in this project. For the Miami University Psychology Building and Animal Care Facility, the steel plate was narrower than the column spacing, which allowed the necessary access while still effectively meeting the lateral-load requirements.

According to Bill Lantz, principal at SMBH, SPSW was clearly the solution after researching its attributes. "Based on the available research and empirical data from other engineers, we were extremely confident that this was the best approach for this project."

On this project, the 1/4-inch SPSW is on the lower level of a four-story frame with diagonal bracing used for the upper stories; see Figure 1. The bay is 29 feet, 8 inches wide. The SPSW is 12 feet, 8 inches wide and is centered in the bay. The vertical boundary elements are W10 x 68 columns. The plate is connected to these with a 5/16- by 3-inch bar and welded to the bar with 4 inches of 1/4-inch fillet weld spaced at 8 inches. The bar is welded to the columns with 6 inches of 3/16-inch fillet weld spaced at 12 inches (see Figure 2).

The top horizontal boundary element is a W24 x 68 beam at the first floor, and the connection is the same as the vertical boundary elements. The bottom horizontal boundary element is a 24-inch-wide concrete wall that is reinforced with #5 reinforcing bars spaced at 12 inches horizontal and vertical in each face. The base of the plate is welded to a continuous, 1-1/4-inch by 11-inch plate that is bolted to the wall with 7/8-inch-diameter epoxy anchors spaced at 5-1/2 inches staggered; see Figure 3. This bottom connection transfers the shear from the plate into the foundation. The total shear resisted by this frame is 174 kips. The total base shear for the building is 370 kips.

Overturning in the frame is resisted by the outside columns and the vertical boundary elements. The columns are W10 x 112 with 2-1/2-inch by 18-inch by 21-inch base plates with four, 7/8-inch-diameter anchor bolts. The frame does not experience any net uplift. The overturning results in a reaction of 693 kips that is resisted by spread footings under each column. The overall lateral-load resisting system is ordinary steel concentrically braced frames. Two frames were used in each orthogonal direction. The floor diaphragm is a 5-1/2-inch concrete slab with a 2-inch deck. The roof diaphragm is 1-1/2-inch metal deck.

One of the challenges was the lack of design procedures for this construction method. Design occurred in 2002 and 2003, with construction beginning in 2003. It wasn’t until 2005 that AISC Design Guide 20: Steel Plate Shear Walls appeared in draft format. As such, SMBH had to rely on published research papers provided by AISC that formed the basis of the design guide. The AISC Design Guide 20: Steel Plate Shear Walls is currently available.

Despite the challenges, the project was complete for the fall semester in August 2006. According to Mullenix, the use of SPSWs impacted the design in a very positive way. "The steel plate shear walls allowed us to achieve the openness so highly desired throughout the facility," said Mullenix. "The lateral-load resisting system made it much easier to integrate openings and interior glazing within the system, which enabled a much more transparent feel throughout the building."

Lessons learned

Based on the success of this project, SMBH was able to develop in-house techniques for the design of SPSWs. "Through our work on the Miami University project, we were able to incorporate steel plate shear walls into our toolbox of design solutions," said Lantz. "We recognize that this is a creative way to resist lateral loads when there are large constraints for the use of the space. The system is not as bulky as conventional bracing."

SMBH was so pleased with the outcome of the Miami project that they used SPSWs for a project at The Ohio State University Main Library. The 11-story stack tower, which originally opened in 1952, was in need of a seismic retrofit to meet current codes. There were extreme space limitations, which made it difficult to use diagonal-braced frames.

"The use of diagonal-braced frames would have reduced the number of volumes that could be stored on each floor, because the braces would interfere with the closely spaced shelving," said Lantz. By using the SPSWs, the same number of volumes could be stored on each floor as before the retrofit, which was a tremendous asset to the university. "In the end, the SPSWs proved to be a cost-effective solution, because the university would have had to build additional space to make up for the lost shelving," said Lantz. "Using the steel plate shear walls on both of these projects has allowed us to help the owner and architect achieve their goals."

Stephen J. Metz, P.E., is a principal at Shelley Metz Baumann Hawk, a full-service structural engineering firm that provides structural engineering services for architects, contractors, and building owners. He has designed new buildings as well as additions and renovations to existing structures. He can be reached at 614-481-9800 or via e-mail at smetz@smbhinc.com.

Design & Construction Team
Owner: Miami University, Oxford, Ohio
Architect: NBBJ, Columbus, Ohio
Structural engineer: Shelley Metz Baumann Hawk, Inc., Columbus, Ohio
General contractor: Danis Building Construction Company, Columbus, Ohio