STEEL PLATE SHEAR WALL SYSTEMS IN HEAVILY SEISMIC REGIONS

Libby Digman Brad Soltys

High seismic activity demolishes buildings and devastates cities. Densely populated areas, such as places in the Northwestern United States, that encounter high magnitude earthquakes may suffer greatly economically, socially, and structurally. In order to avoid mass destruction of infrastructure which can lead to injuries or even fatalities, engineers and contractors need to implement certain design techniques in order to construct sustainable buildings that will resist seismic forces and prevent structural collapse. In order to ensure public safety in highly seismic regions such as the Northwestern United States, the Steel Plate Shear Wall system needs to be implemented in order to sustain structures both during and after an earthquake.  

THE SEISMIC DILEMMA

STEEL PLATE SHEAR WALL SYSTEMS (SPSW)

The Steel Plate Shear Wall System is a system that is integrated into the frame of a structure that serves to accept and dissipate shearing forces that may be associated with seismic activity. In general, the system consists of boundary elements of horizontal and vertical steel beams and an array of thin steel plates, known as web plates. The horizontal and vertical boundary elements form part of the building frame, with the space between each horizontal boundary element being equal to one story of the building. The steel web plates are bolted to the boundary elements and receive energy associated with lateral forces from the building frame. Through development of what is known as a tension field, the steel web plates consume much of the energy from the shear force and prevent it from destroying the building.

A TWO STORY SPSW TENSION FIELD

TYPES OF SPSW    The Ring-Shaped Steel Plate Shear Wall (RS-SPSW) has a steel web plate with ring patterns cut into it. The rings are connected by steel diagonal links. The ring pattern, along with the diagonal pattern, resists common buckling tendencies under pressure. When force is applied to the plate, the ring’s diagonal will compress and shorten while the diagonal pattern connecting the rings will expand and elongate. Slack is removed in the ring, and tension adds in the diagonal portion. This movement then balances the forces in the steel plate. The reduced buckling effect in this technology yields to improved stiffness and performance.     The Steel Plate Shear Walls with Coupling (SPSW-WC) is made of a special type of steel plate shear wall with two wall piers. The two wall piers are joined at the base level by coupling beams. This system is similar to conventional SPSWs, but it provides greater structural efficiency and flexibility allowing for a greater structural sustainability. The addition of the second wall pier provides a certain amount of support while the coupling beams at the base allow for greater top flexibility. Although the ultimate purpose of this technology is to increase flexibility as much as possible while maintaining a sound structure, recent models have not held expected, necessary results.

ADVANTAGES OF SPSW

TESTING DIMENSIONS OF THE STEEL PLATE: POST-BUCKLING STRENGTH

         Compared to the structural wall thickness of regular concrete shear walls, the steel walls of SPSWs are considerably thin. In fact, on average, the walls of SPSW structures save about 2% in wall thickness in gross square footage compared to buildings without SPSW systems. Reduction in building weight results in a decrease of building foundation load. The reduced weight means that there are fewer gravitational forces on the structure, thus an overall reduction in the effects of seismic loads and forces. The steel flexibility allows the structure to absorb seismic forces, reducing structural damage. Another key advantage of the Steel Plate Shear Wall technology is the fast construction time and cheaper cost compared to concrete based structures. In concrete wall structures, there is a period needed for curation and hardening of concrete, but because of the large use of steel in these structures, there is no need for the curing period. Generally, steel is less expensive than concrete. The reduced amount of construction time and a more inexpensive material results in a reduced cost.

RS-SPSW

WORLDWIDE APPLICATIONS OF SPSW

AN experiment conducted by the Harbin Institute of Technology School of Civil Engineering in Harbin, China, demonstrated that relatively thin web plates can perform just as well as stiffened web plates that require more material. This result can be attributed to post-buckling strength, as the thin, ductile web plates are able to perform better with increasing tension field action. The experiment was done entirely by computer modeling, using finite element analysis software, and virtually modeling SPSW systems with different width, thickness, and height ratios in a twenty-story building. Through a method called “Pushover Analysis”, each experimental SPSW system was applied to a twenty-story building model, and subjected to a varied shear force. The maximum displacement of the SPSW system was measured for each group, and the shear force was plotted as a function of the displacement. From these results, each of the six groups were assigned a rigidity ratio, which is simply the ratio of the rigidity of each SPSW system to the rigidity of the overall building frame. In this case, rigidity is defined as the ratio of the shear stress applied to the displacement it causes. Next, the same procedure was used to determine the rigidity ratio values for five experimental groups with varying span-to-width ratios.

Rigidity Ratio 4 9Stories 1-4 9 mm 20.2 mmStories 5-10 5.7 mm 12.9 mmStories 11-13 4.6 mm 10.4 mmStories 14-17 3.2 mm 7.3 mmStories 18-20 2.6 mm 5.8 mm

Rigidity Ratio

Shear Force (1.2 m roof drift)(N)

Span-to Width Ratio (First 4 Stories)

2 3,000 0.94 3,500 1.25 4,000 1.5

Based on the results, the increase in rigidity of the SPSW system causes a dramatic increase in the amount of material that needs to be used. Since more material will correlate to a higher overall cost of fabrication, it is ideal to try to minimize the amount of material so that the cost is practical for widespread, sustained use. The first part of the experiment demonstrates that the SPSW with rigidity of 4 perform similarly to the SPSW with rigidity of 9, and the second part determined that the specimen with rigidity of 4 performed best overall. It can thus be concluded that the additional material used to construct SPSW with rigidity any higher than 4 is unnecessary. As the thinner, more ductile web plates are allowed to stretch past the point of initial buckling, a strong tension field is formed in the plate. The tension field supports the building as shear displacement increases, holding frame elements together and dissipating force at the same time.

CONCLUSION

Not only can the Northwestern United States benefit from the implementation of SPSW technology, but so can countries recently devastated by high-magnitude earthquakes, such as Haiti. On January 12, 2010, Haiti was faced with a 7.0 magnitude earthquake that killed more than 160,000 people. The country was burdened with a massive amount of disease and homelessness as a result of the earthquake. Although the majority of those displaced by the earthquake have moved off of the streets of Haitian cities and towns in recent years, there are many Haitian citizens still living in makeshift shack neighborhoods. These communities lack necessary infrastructure, making it apparent the country is still in need of major rebuilding. According to the ASCE Code of Ethics, it is the duty of Civil Engineers to concern themselves with the welfare of the public and strive for sustainable developments. In order to address the devastation caused by the 2010 Haitian earthquake, engineers need to find the means and resources to build sustainable buildings in this area with the Steel Plate Shear Wall technology. The SPSW structures will ensure safety of the community in a case of future highly seismic activity, as Haiti is located in a highly seismic region and is bound to face additional earthquakes in the future. In addition, the low cost of the SPSW system due to material savings will ensure that the country’s government is able to support construction of the buildings, even without significant financial resources. The application of SPSW structures can sustain communities like this worldwide by not only providing homes for those currently without, but by also ensuring safety in the event of a future earthquake, thus sustaining quality of life, materials, and infrastructure.

The SPSW system is an effective technology that can be used to increase structural resistance to seismic shear forces and sustain a building’s foundation as a whole when it encounters an earthquake. Recent experimentation has demonstrated the possibilities that this system presents to the future of structural engineering. Once thought to be painstaking to construct and much too flexible to carry loads associated with large buildings, research has shown that thin, unstiffened SPSWs perform well after buckling due to formation of the tension field. This allows very thin SPSWs to support large buildings during earthquakes and dissipate appreciable amounts of energy, effectively preventing the building from collapsing in response to seismic waves, sustaining the overall building structure. Increased use of SPSW will immensely benefit heavily seismic regions with large populations. If more earthquake resistant buildings with SPSW structures are implemented, the less likely there is for abrupt, extreme devastation to the population, sustaining not only building structures, but also lives. SPSW systems are a sustainable technology proving to be increasingly effective, and with further development, will protect communities across the globe against the negative effects of the earthquake.