The public and business have a key role to play in establishing the balance between initial cost of construction and improved earthquake performance, or between short term and long term economics. There is a great deal of misunderstanding and inappropriate expectations relating to earthquake resistant design, and this brief article is an attempt to clarify the “behind the scenes” business of earthquake engineering.
Designing buildings and other structures to resist the effects of earthquakes is the role of the structural engineer, and is among the most challenging. Designing to resist the effects of earthquakes is nearly as much an art as it is a science, in part because we go about it in such a uniquely different way than we design to resist other loadings on structures. The most important difference arises because of the fact that the effects of large earthquakes are so severe, yet so infrequent, that it has not been economically practical to design most structures to resist the effects of an actual design-level earthquake within the usual parameters of allowable levels of stress and member capacity.
Typically, the structural engineer designs the elements of a building or other structure to resist forces due to the self-weight of the structure, the contents, snow, wind, etc. within parameters specified in the code regulating the maximum levels of stress or force that can be safely sustained by each component. For all but a few defense and nuclear containment structures, this has not been the case for earthquake design. The reason is economics. It’s not that structural engineers can’t, but rather that it would be too costly to design this level of earthquake resistance into most structures.
The solution, used worldwide in earthquake-resistant design, is to employ a concept called “ductility”, which is a property of some materials and systems to preserve their strength and integrity even after they experience loads which permanently deform them. Some materials, such as steel and properly reinforced concrete, can be produced in shapes that will retain a large measure of strength and integrity even after they experience loads which leave them permanently deformed. In this case, we say these materials are “ductile”, and the members and systems they comprise possess the property of “ductility”.
So for the case of earthquake resistant design, we make use of this concept of “ductility” in a subtle but essential manner. Once we determine what is the maximum probable intensity of ground shaking at a particular location, either as specified in the design code or resulting from a special study by a geotechnical engineer, we convert the actual earthquake force levels to much reduced design levels, and proceed to design the structure. We have not changed the size of the real earthquake we expect could occur. We have just determined to design to a percentage of the real earthquake, which has varied from as little as 8% to as much as 40%. Few people realize that this is what building code provisions actually do, and this subtlety can and does lead to oversights in earthquake-resistant design. The structural engineer then proceeds to select and size all of the members and components to resist the earthquake effects that have been reduced to design levels. What does this mean? For small earthquakes, the actual forces that the structures experience are resisted within the customary parameters of allowable stress or force that each component can sustain, as permitted by the code. Also, additional benefit is realized from the inherent shear resistance of many nonstructural components, such as sheet-rock and stucco wall finishes. In the small earthquake, the structure itself should not have sustained any damage, and its motions during the shaking should be so minor that there is little evidence of damage in the finishes and contents of the structure.
But what happens in a large or great earthquake? Since the structure was really designed for forces and effects much reduced from those that are actually expected to occur, perhaps to only one-eighth, the structural system experiences stresses and forces beyond levels allowed in the design by the design code. When this occurs, portions of the structural system will be permanently deformed, and the energy of the earthquake is dissipated and the intensity of the shaking in the structure is generally reduced. When this happens, the building or structure is expected to remain standing and generally “whole”, so that people can safely exit the building with minimal risk of loss of life or injury.
But what happens to the structure? In the strong shaking, we (and the codes) actually expect the structure to be “damaged” as it experiences cycles of forces that are permanently deforming the members. This is much the same as subjecting a steel paper clip to several cycles of bending. After 3 or 4 cycles of bending, the paper clip is still likely to be whole, and somewhat functional, but it is not the same as before it was deformed. The paper clip is “ductile”, and it could sustain this many cycles of permanent bending without breaking. But with perhaps 3 or 4 additional cycles of additional bending, the “damage” to the paper clip is cumulative and it will finally break. In earthquake-resistant design, the underlying objective is that the limits of “ductility” of the structure exceed the duration of the maximum expected large earthquake.
Thus a key concept in earthquake resistant design, is this property of “ductility”. Some structures, such as unreinforced masonry, possess almost no useful ductility, and they consistently perform poorly in earthquakes. Most nonstructural components, such as sheet-rock and stucco wall finishes, that afford some initial shear resistance, have a very limited strength and ability to sustain racking displacements in the plane of the finish. When the force or displacement applied exceeds the resistance of the nonstructural finish, the finish fails and the residual benefit is often negligible. This phenomena commonly leads to a misplaced confidence derived from the performance of structures in minor earthquakes, which produced such low shaking intensities that the non-ductile structural components and nonstructural components retained their beneficial contribution to structural integrity.
The objective of modern design codes is to produce structures that have high levels of “ductility”, so that when large earthquakes occur, even those earthquakes that may be different or somewhat larger than anticipated, the structure remains standing and whole, so that the risk of loss of life and injury is minimal. Thus the “ductility” or toughness or the structure is providing a safeguard, offsetting our limited understanding of the exact characteristics of the large earthquake that may occur.
It is interesting to note that in recent history, the codes governing earthquake resistant design in other countries have used the concept of “ductility” to meet different national objectives. In New Zealand, a small country with relatively limited natural and economic resources, codes have specified relatively lower earthquake design forces, but with higher levels of ductility or toughness. With appropriate care and quality control, this results in “tough” structures that utilize fewer natural resources in construction. In Japan, in contrast, following the aftermath of the Great Kanto (Tokyo) earthquake of 1923, the codes for earthquake resistant design were developed to generally make less use of “ductility”, and to design structures for a higher percentage of the actual expected earthquake forces. Thus in general, modern engineered structures in Japan have tended to be designed to be shorter, stockier, and with more strength. Both approaches intend to protect the public from loss of life and injury. But in general, the economic consequences of these philosophies are reflected in relatively lower initial cost of construction in New Zealand, and higher risk of earthquake damage in large earthquakes, with attendant risk of complete loss of use of the facility, and potential loss of contents in the case of buildings. In Japan, in general, the initial cost of modern earthquake code compliant structures is expected to be relatively higher, with an offsetting long-term recovery due to the reduced economic consequences of damage due to large earthquakes.
This illustration leads to an emerging approach in the United States, of recognizing the economic consequences of large earthquakes, and more deliberately establishing higher objectives for earthquake performance than the protection of life only. Certainly many businesses and governmental functions cannot withstand the economic consequences of losing a building and the right of access to the contents, in the event that the building is heavily damaged by an earthquake. Hospitals, emergency response centers, and critical bridges and highway interchange structures are examples of other structures where the functions commonly require a higher earthquake performance objective, such as maintaining full immediate use following, or even during, a large earthquake.
While these higher earthquake performance goals can be envisioned and implemented in different ways, in essence they generally result in designing structures for a higher percentage of the actual forces and motions expected in the design level earthquake, with greater attention to detail and quality control through design and construction. Thus we are not necessarily designing to a bigger earthquake, but rather designing for a level of strength closer to 100 percent of the forces and motions anticipated in the maximum earthquake that could occur.
Other relatively more sophisticated technologies for earthquake resistant design are being developed and used. These include approaches that serve to isolate the structure from the earthquake shaking of the ground, and those that use specialized devices in the structural system to dissipate the earthquake shaking energy within the structure, without damaging the structural members themselves. While not yet commonly used in Alaska, these technologies have been used when a more straightforward “brute strength” approach is not practical, as is often the case when seismic retrofits are performed on large historical structures.
With this in mind, the facility owner or manager who realizes the need for higher earthquake performance than the protection of life provided by the earthquake provisions of the code, has a key role to play with the design or seismic strengthening team. It is critical that the owner/manager be able to determine and communicate what the business or operational objectives are for the facility in the event of the occurrence of a large earthquake. This interaction is essential at the beginning of the process of negotiations for design. While the structural engineer has the primary role in earthquake resistant design, achieving earthquake performance objectives above standard code levels will affect and require the cooperative vision and participation of the entire design and construction team. The initial cost of a higher earthquake performance goal will necessarily vary depending upon the objectives, the type of structure, location, and means selected of achieving improved performance. In any case, the cost will be minimized by considering the earthquake performance objectives at the outset of the project.
Mark D. Anderson, PE
November 21, 2001