Steel Plate Shear Walls
An innovative structural steel lateral load building system is finally gaining recognition and being used in low and high-rise developments across North America. Known as steel plate shear walls (SPSW...
An innovative structural steel lateral load building system is finally gaining recognition and being used in low and high-rise developments across North America. Known as steel plate shear walls (SPSWs), these systems can be fabricated at low cost, are quickly erected and are expected to have a superior performance, in particular in response to earthquakes. While their concept is not entirely new, their recent application in high-rise structures and incorporation within seismic rehabilitation projects mark the start of an era when they could well grow to be the dominating lateral load resisting system available to structural engineers.
A steel plate shear wall system element consists of a column and beam frame augmented by a steel in-fill panel between the boundary wide flange members. When these in-fill plates occupy each level within a frame bay of a structure, they constitute a vertical SPSW stack.
The variability of the system’s ductile response can be controlled by what type of connection is specified between the integral beam and the column, i.e. simple shear or moment resistant. For zones with high risk seismic activity, designers typically specify moment connections. Therefore, the system’s lateral resistance relies on the post-buckling strength of the thin steel in-fill panels in conjunction with the co-planar integral moment resisting frame. The behaviour emulates the tension field development within web panels of bridge plate girders that are anchored by the surrounding stiff boundary elements (Figure 1).
Early steel plate shear wall system structures neglected the post-buckling benefit of thin in-fill panels and, in fact, incorporated measures to prevent out-of-plane local instabilities of the plate. Hence their designs incorporated heavy stiffening measures — at a fabrication premium. In the early 1980s a suite of analytical and experimental investigations originating in Canada explored the potential of near pure post-buckling strength reliability. Thorburn and Kulak initiated this work at the University of Alberta, and it was immediately followed by physical testing by the author (Timler) and Kulak. Our research had an excellent correlation with the analytical predictions and showed that the frame attained a very good response when subjected purely to cyclic lateral load.
Since then, researchers in Canada have continued to develop codified rules for applying the design, notably at the Universities of Alberta and British Columbia. For example, Driver and Kulak tested a scaled version of a four-storey shear panel under gravity and the Applied Technology Council’s prescribed lateral cyclic loading protocol. The results clearly demonstrated a beneficial response to repeating cycles of in-plane loading and subsequent reversals (“hysteretic behaviour”). Additionally, at the University of British Columbia Lubell and Rezai conducted smaller scale multi-storey tests with Prion and Ventura which focused on the first dynamically tested multi-storey steel plate shear wall system in the world. Results again were favourable. The series of tests in the 1990s highlighted the most notable characteristics of the system as being robust and having a stable hysteretic performance, near perfectly elastic/perfectly plastic response, and high redundancy. It was clear that the SPSWs tested were a superior energy-absorbing lateral framing system.
Coming to code
Preliminary code clauses were first developed as guidelines in Canada’s CAN/CSA-S16.1-94 (S16). Design consultants put the guidelines to test by developing a full design rational and applied the concept on suitable hypothetical buildings of modest height (eight stories) in several cities in Canada. For each location the structural system’s ability to absorb energy was set at levels commensurate with the seismicity of the region. Similarly, parallel alternate designs in reinforced concrete and hybrid forms were done to compare construction costs.
The results generally confirmed that the draft code clauses were appropriate, with the recommendation of some practical improvements. It was here also that the benefit of a dual structural system, i.e. the primary steel plate shear wall system and the moment frame back-up system, were first recognized and implemented.
Equally important, however, was the design and cost evaluation exercise that demonstrated the significant economic benefits of selecting a steel plate shear wall structural system over reinforced concrete core structures (20% to 30%). The benefits of SPSWs included a major shortening of the schedule due to the ease in fabrication, erection and the fact that they involve fewer interfaces between different trades.
Canada will soon be adopting full code rules for the design of steel plate shear wall systems in its next edition of S16 anticipated to be published by early next year.
Recently there has been interest in using steel plate shear wall systems for rehabilitating seismically deficient buildings. It has been surmised that because these systems have demonstrated reliable ductility, economy and have a minimal impact on the building form, there may be merit in integrating them into sub-standard capacity moment framed structures, both of reinforced concrete and structural steel form.
Some owners of nominally reinforced concrete moment framed buildings have expressed interest in the system as a rehabilitative technique. They are driven by the need to bring their structures up to code, and find a reliable and repetitive strengthening method that minimizes the need to displace the building occupants. The concept being considered is the semi-development of a tension field within thin plate in-fill panels framed in reinforced concrete moment frame bays; these are aligned vertically to form a composite shear wall. The SPSW system strategy is much less disruptive than conventional techniques. It minimizes penetrations, lessens the need for foundation improvements, and can be integrated in segments while the building remains occupied. Researchers have been encouraged to further study the viability of the approach.
With respect to strengthening steel moment frames, integrating a steel plate shear wall structure is an obvious choice since a fully developed SPSW system augments, several-fold, a stand-alone moment frame in terms of lateral resistance. When a structure’s original resistance has been compromised due to previous joint fracturing, an extremely flexible option for the structural engineer is to integrate a continuous vertical bay of in-fill panels to reinstate seismic and wind resistance. Since the SPSW system has been tested and rated for varying levels of ductility dependent on the fixity condition between boundary frame members, this range of performance can be tuned to the damage level within the structure’s existing frame joints. Concentrating an SPSW within a nominally damaged section of the structure may preclude the necessity of reinforcing heavily damaged areas since their remaining resistance could be relied upon strictly for gravity loads. Alternatively, SPSWs might be integrated within heavily damaged stacks without re-strengthening the damaged joints. This strategy would infer a less ductile, but still highly reliable, resistance frame, which in conjunction with the undamaged frames would return the building to its original full strength (Figure 2).
Steel plate shear wall systems are being used in new and retrofit construction along the West Coast of the United States. With respect to seismic rehabilitation, the Oregon State Library in Salem, a designated heritage structure, was recently upgraded to current code requirements with thin unstiffened plates in multiple bays along some of the perimeter walls of the building. The driving reason for the selection of this lateral load resisting system was its unobtrusiveness, in particular, its use adjacent to unreinforced masonry walls. The strengthening scheme was tuned to match the allowable deflection limits of the former system, and was capable of 100% lateral load transfer
to the foundations. Within the confined work spaces, the steel plate elements were arranged in easy-to-handle segments suitable for two welders to place. This hidden, clean, and preservative technique, applied at reduced costs compared to traditional methods, was fully capable of meeting the code-specified resistance.
As an application in a new building, the Canadian, thin steel in-fill panel concept was first deployed within an office structure for a major steel fabricator in St. Georges, Quebec. The structural design of this six-storey building was based entirely on the research completed at the University of Alberta. The shear core panels were fabricated in two three-storey lifts and were bolted at the base of the fourth storey in-fill panel (see photo p. 47). By using this unobtrusive system the usable floor space of the facility was maximized.
Further innovation is going on in significant seismic zones along the western seaboard of the United States. The Century project, a 50-storey residential high-rise located in San Francisco, currently under construction, uses the steel plate shear wall concept as the primary lateral system within the core. Based fundamentally on the earlier Canadian research as well as S16 guidelines, the application makes advances in the technique that also complement fast-tracked construction. Originally SPSW systems were developed as a vertical stack component method, but they are being applied also as horizontally oriented erection components. Fundamental to the use of the SPSW within the Century structure is the concept that the internal boundary frame and large diameter composite column outrigger framing serve as acceptable co-planar back-up systems. A somewhat conservative approach was used in the structure’s anticipated energy absorption capacity (ductility) since the 1997 Uniform Building Code has yet to recognize this system officially.
From the same engineers another significant project is underway which integrates steel plate shear walls in one of the main orientations of the primary structural framing. The Seattle Federal Court House is a 23-storey institutional facility of comparable structural configuration to the Century, albeit with generous inter-storey height. A half scale, three-storey model of the system is undergoing cyclic testing at the University of California at Berkeley.
Finally, as a testimony to the performance of the steel plate shear wall system in an actual major earthquake, one only needs to be aware of the example of the Kobe City Halls. The older, low-level, reinforced concrete building suffered a total collapse at its fifth level following the Great Hanshin Earthquake in 1995. The newer high-rise building of SPSW core construction, which is situated immediately adjacent to the former hall, admirably survived the same ground shaking intensity. An examination of the steel in-fill panels at selected storey levels revealed controlled out-of-plane buckling and yield development within the panels, just as they were designed for. The structure was occupied immediately after the earthquake.
In summary, the relevant research on steel plate shear wall systems has been developed, tested and matched with appropriate analytical methods. A practical design basis has evolved to confirm anticipated code rules. Before the codes are full implemented however, engineers have already moved ahead with applications. Furthermore, as a “heads up” to future designers contemplating using this system, the currently suggested ductility ratings, which are already comparable to other highly rated structural systems, are expected to increase thereby promoting the system’s further use. Truly, there is a great future for steel plate shear wall systems in high-risk seismic zones in North America.CCE
Peter A. Timler, P.Eng., is the regional marketing director (B.C.) for the Canadian Institute of Steel Construction. He is also vice president and operations manager (B.C.) for UMA Engineering in Burnaby. Kurt A. Nordquist recently retired from his post as vice-president of Skilling Ward Magnusson Berkshire in Seattle, Washington. This article is an adaptation of a presentation at the 5th Annual Convention of the Steel Plus Network in Las Vegas in January 2000.