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by Rania Bedair, Ph.D. P.Eng.
A considerable number of buildings in North America present a risk of poor performance during an earthquake event as they were constructed prior to seismic design codes or the available code used was immature and presents flaws. The 1985 Mexico City and 1994 Northridge earthquakes proved that even large steel frame buildings can be vulnerable to major damage or sometimes collapse.
The practice of improving the seismic performance of existing buildings is known as seismic rehabilitation or retrofitting. In order to design an efficient retrofit scheme, it is imperative to have a thorough understanding of the expected seismic response of the existing building and all of its deficiencies.
According to design codes, buildings are aimed to be strong enough to resist small earthquakes without damage and major earthquakes without collapse. To accomplish this goal, the structural design requires a combination of basic lateral force-resisting strength with proper structural detailing and appropriate ductile connections of the structural elements.
The main difference between the design of a new structure and the evaluation of an existing one, is the design perspective according to the relevant codes. The design of new structures is based on calculated forces and the structures are modelled as elastic systems with stresses proportional to the strains. The design lateral forces are obtained from a base shear formula that includes a response modification factor, which reflects the connection between required strength and ductility. However, existing buildings are usually examined based on displacements, drifts and the ductility of each action element individually. The goal is to determine how the structure will respond to a design earthquake by finding weak links in the system and to identify how they will affect the response of the structural systems. In fact, a building may survive an earthquake by dissipating energy in the yielding of its components even if the actual forces and deflections are larger than the calculated forces.
The seismic vulnerability of existing buildings can be determined by the evaluation of the structural system, materials and detailing. An analysis should quantify the resistance capacity of the structure. An important part of this analysis is the application of a model with elastic and inelastic properties that takes into consideration all structural components, which participate in the resistance of the building. A building’s design and construction characteristics with the condition of materials used affect the seismic performance. In order to find an appropriate cost-effective seismic strengthening solution for an existing building, the structural engineer must understand the existing lateral force-resisting system and any constraints on the desired performance of the structure due to design or construction characteristics or deteriorated materials.
Retrofit of a concrete parking structure using exterior X-braced steel frames (FEMA P-154)
Strength, stiffness, ductility and damping govern the dynamic response of a structure to ground motions. It is usually possible to compensate for a deficiency by enhancing one or more to the other. A deficiency in global strength is commonly found in older buildings due to a lack of seismic design completed in accordance with early code requirements, which are inadequate per current regulations. It can be retrofitted by strengthening the existing members or adding new members that will increase the overall strength of the structure. Moreover, increasing the global stiffness of the structure is an effective strategy in the case of seismic deficiency. The location of the added member and the added stiffness is important due to the transfer of loads among the elements of the structure, depends on the relative stiffness of those elements (FEMA P-547).
Without enough ductility, structures would be vulnerable to brittle failures during an earthquake. Ductility capacity depends on the material type, loading mechanism and loading history. Connection detailing is a key to achieve robustness through ductility. The most common example of a detailing deficiency is poor confinement in concrete gravity columns. Moreover, studies of existing buildings during earthquakes have demonstrated that ductile buildings (ex. flexible moment frames) perform much better in seismic events than rigid buildings (ex. masonry shear wall buildings) due to the inherent ability of flexible systems to dissipate energy of ground motions.
The primary focus for determining a viable retrofit scheme is on vertically oriented components (ex. column, walls, braces, etc.) due to their significance in providing either lateral stability or gravity load resistance. Deficiencies in vertical elements are caused by excessive inter-story deformations that either create unacceptable force or deformation demands. However, depending on the building type, the walls and columns may be adequate for seismic and gravity loads, while the building is inadequately tied together, forming a threat for partial or complete collapse during an earthquake. Generally, there are five classes of measures taken in the retrofit a building:
Adding elements: increasing strength or stiffness
Enhancing performance of existing elements: increasing strength or deformation capacity
Improving connections: assuring a complete load path
Reducing seismic demands: removing upper floors or other mass from the structure, adding damping devices or seismically isolating part of the structure
Removing selected elements: preventing damaging interaction between different systems
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