By Li Ming Tang, Vice President, Structural
For earthquake preparedness, performance-based seismic design (PBSD) has proven to be a useful decision tool in the rehabilitation of existing critical facilities. As an alternative design paradigm to prescriptive code-based design, PBSD enables designers to assess performance levels of structures, through rational analysis, to address seismic design issues that are either not appropriately handled by building codes or are without the scope of the codes. Common uses of PBSD method includes the establishment of seismic performance levels of existing structures that do not meet current code requirements for safety, as well as the development of innovative rehabilitation options that meet the operational and budgetary constraints. This piece describes the seismic rehabilitation study of a pre-code steel hospital building in southern Ontario on a major site redevelopment.
Southern Ontario has low to moderate seismicity. While it has a potential for large and damaging earthquakes, they are expected to be relatively infrequent. Hence, many of the buildings constructed prior to the introduction of modern seismic provisions in the building code likely do not meet current code requirements or were not designed for seismic loads at all. In these cases, the seismic performance for these buildings is unknown and proves to be an issue when modifications and alterations need to be made. When it comes to modernizing critical infrastructure like hospitals, the cost and time necessary to bring these buildings to code can sometimes be a deterrent for modernization.
For this pre-code hospital building project, a 3-story portion of the target building is slated for demolition and change in the redeveloped site, while the building itself needs to continue providing critical healthcare services during and after the redevelopment. For the campus redevelopment project, the proposed demolition constitutes a building alteration that would trigger a full retrofit of the building to bring it up to current seismic standard. However, since the building does not have a qualified lateral force resisting system recognized by the code, the retrofit will require the construction of a new lateral force resisting system, which is cost prohibitive and would require prolonged disruption of the critical functions currently facilitated by the building. Unless the proposed demolition can be done with only minor intervention, alternative options, such as rebuilding the hospital in a new site, may need to be considered. To assist decision-making, the owner of the modernization planning, the project structural team, which consists of EXP and Kinetica Risk, undertook a performance-based assessment with the following goals:
1) Assess the seismic performance of the building before and after the proposed partial demolition to understand the impact.
2) Propose a minimal intervention seismic retrofit that would meet the intended performance level of the current building code.
The work was performed in accordance with the latest performance-based standards, including the latest edition of the ASCE-41 at the time of the project. For the first goal, due to the use of non-conforming connection types by modern code standard, a field investigation was required to develop a good understanding of the lateral load resisting system, in particular the beam column connections. Based on the field investigation, most of the connection along the short building axis consists of welded flange cover plate moment connections with a bolted shear tab, while shear tab connections dominate in the direction of the long building axis. While these are considered partial moment connections, they can still develop lateral resistance that may be enough to withstand the relatively low seismicity at the site. Samples of steel coupon from beams, columns and bolts were removed and tested in the lab to establish the material properties.
The seismic response of different types of beam-column joints were first assessed using solid finite element models. The results were then incorporated into 3-D nonlinear structural models of the building before and after the proposed demolition to be assessed using nonlinear dynamic analyses with a suite of ground motions selected and scaled per the latest building code requirements. The nonlinear dynamic analysis did not find evidence that the seismic performance of the building would be worsened by the demolition. While this result provides a basis to justify the proposed demolition, it does not demonstrate that the building has adequate structural integrity to meet the current intended performance of a post-disaster facility. In fact, the building is obviously deficient seismically due to its reliance on non-conventionally, and potentially non-reliable partial moment connection mechanisms for lateral resistance. To rectify this, the second part of the assessment is to develop a minimally interruptive retrofit scheme that meets the stringent operational requirements of the post-disaster building.
To accommodate the building’s future use after the redevelopment, it needs to have a level of performance that would be equivalent or better than the intended performance of post-disaster buildings designed to the current code, which means it should be operational following a design level earthquake. Since the building does not have a qualified lateral force resisting system per the current building code, a performance-based approach was used to develop a structural retrofit scheme that makes maximum use of the existing capacity. In addition to this consideration, there are several project constraints that were considered by the structural team. Namely, the structural upgrade must minimize the disruption to the existing emergency and in-patient units, it should provide a level of structural performance similar to what is expected of a new hospital building, and it should meet the expected levels of function post-earthquake. Since the building was not designed for earthquake forces, a significant increase in stiffness would likely require substantial strengthening of the existing cast-in-place concrete diaphragm, and the foundations. Furthermore, increased stiffness will increase floor accelerations causing further damage to the mechanical, HVAC and architectural components to the building, which were not designed and anchored in accordance with modern seismic standards.
From a cost and functional disruption perspective, it was desirable to maintain low stiffness and use passive supplemental damping to control damage. A final proposed solution, incorporating selected exterior joint upgrades and supplemental viscous dampers, was developed following repeated analysis and optimization using the performance-based approach. It was demonstrated that the upgraded structure would not only meet the structural safety requirements stipulated by the current building code, but will also have reduced levels of acceleration and shear forces in foundation and diaphragms compared with the original structure. It is expected that the upgrade structure can be safely reoccupied immediately, and has a high probability of maintaining functional continuity even under an extremely rare earthquake prescribed by the current code, which meets the intended code performance level.
In conclusion, our examination of a real-life case involving a 1950s hospital undergoing a significant renovation in a region with infrequent earthquakes has highlighted the challenges and solutions associated with retrofitting older structures not originally designed to meet modern seismic standards. Despite the building’s initial vulnerability to seismic hazard as per current building codes, our application of performance-based seismic design was able to help the project team in establishing compliance, and develop performance insights that guide the current redevelopment project, and intended future use.
Through this project, we were able to identify effective upgrade strategies, including reinforcing specific components and strategically implementing seismic protection technologies, which elevate the hospital’s seismic structural performance to the level of newly constructed healthcare facilities adhering to the latest safety guidelines. Our evaluation of building system performance, which includes non-structural elements, underscored the importance of ensuring the hospital’s operational continuity during significant seismic events, contributing to the long-term viability of the facility.
Ultimately, this study reinforces the importance of employing advanced tools like performance-based seismic assessment to make sound decisions pertaining to investments in aging structures, striking a balance between safety, cost-effectiveness, and resilience for the future.