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 April 24, 1997 |
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April 24, 1997 |
Structures are held up by both skill and luck
By KEN CARPER
The Pacific Northwest has experienced an extraordinary number of failures this past year due to natural hazards. Heavy snows, high winds and floods remind us of the dynamic character of the natural environment and the vulnerability of the objects we build. Designers and builders who fully appreciate the forces of nature in the region, including the ever-present potential for damaging earthquakes, understand that all constructed projects will eventually fail. All structures will someday succumb to the effects of aging and give way to the forces constantly acting to destroy them. Nothing we build will last forever. The best we can hope for is that the design and construction quality will provide satisfactory performance for the projected useful lifespan of the facility. In the book, "Structures, or Why Things Don't Fall Down," James Gordon writes: "All structures will be broken or destroyed in the end, just as all people will die in the end. It is the purpose of medicine and engineering to postpone these occurrences for a decent interval." Still, most structures survive and perform admirably for many decades, some even for centuries. Why is it that most structures do survive so long? One reason is the contribution of the many dedicated professionals in the construction industry, both designers and builders, who worry a lot about failure and constantly work to prevent it.
Another less comforting (but probably more likely) reason is that a little luck is involved; a large number of structures may remain standing simply because they have not yet been exposed to the load for which they were supposed to have been designed. While the Northwest has experienced some unusual weather conditions this year, the effects of these weather conditions were not all unpredictable. Many tragic failures in the Pacific Northwest (and in other parts of the country) can be traced to poor land-use planning decisions. Despite the availability of hazard mitigation information and qualified technical consultants, the information is often ignored and the consultants bypassed as development continues in the flood plains and on unstable hillsides. Often, unwise site selection and ill-conceived site development results in unnecessary exposure to severe natural hazards. The need for competent technical advice is evident in such failures after the fact. In many cases, millions of dollars of direct losses and litigation costs could have been avoided altogether had a qualified geotechnical engineer been retained at the outset of the project. Roof collapse incidents in several counties have led to calls from some parties to increase the design snow loading in the building codes. On the surface, this seems to be a logical step. However, before adopting such an approach, some important questions should be addressed:
A thorough examination of the problem leads to the conclusion that simply requiring the spanning system to be designed for a higher uniform load is not necessarily a satisfactory solution. Failures result from a variety of causes involving both technical (physical) problems and human error (procedural) factors. Usually, the root causes of a failure or accident are difficult to isolate and quantify. The causes may be a combination of interrelated deficiencies. This is one reason for the complexity of construction disputes and the confusion that often accompanies failure litigation.
A particular civil/structural forensic investigation may find a complex combination of several of the listed factors, all contributing to the failure. For example, in the 1970s, a number of failures caused by corrosion occurred in reinforced concrete parking garages in the eastern United States. Parking structures are among the most vulnerable constructed facilities. They are exposed to a wide range of fluctuating thermal conditions. Vehicles deposit corrosive de-icing salts used on roadways onto the parking deck surfaces. These surfaces are not rinsed periodically by rain, as is the case for most highway structures. Parking facilities are utilitarian structures, low on the typical owner's maintenance priority list. Investigation of a parking structure failure usually uncovers contributing factors from every category in the list of failure causes. Design details may be inadequate, especially provision for drainage, detailing around openings, crack control, joint definition and concrete cover. Specifications for sealant materials and application procedures may have been insufficient. Geotechnical errors or foundation design problems may be present, leading to settlement cracking. Construction errors involving poorly executed connections or improper sealant application may have contributed to acceleration of corrosion. Sealant materials may have been defective. The owner or operator of the facility may bear partial responsibility because of inadequate maintenance. Some owners have even used corrosive de-icing salts in their own snow removal operations. Sorting out an equitable distribution of responsibility for a costly failure in such a case is a formidable task. The use of multiple forensic experts, representing various parties to a dispute, can be helpful in ensuring that all potential contributing factors are considered. Forensic engineering, the study of engineering failures, can bring about an improvement in the way builders and designers go about their business. As the results of forensic investigations are disseminated throughout the construction community, they can contribute to better standards, in the same way the investigations by medical pathologists have contributed to improvements in the practice of medicine. The open discussion of failure case studies is important, for it reminds us all of the possibility of failure, despite modern technology and current building codes. Thoughtful design and construction professionals have always understood that building codes are intended to be minimum standards of performance. Unfortunately, these minimum standards usually become the norm due to the pressures of economics.
Minimum standards do not always provide protection against unusual conditions. This is often the underlying lesson to be learned from the premature failure of a facility. If minimum standards are not sufficient, the owners and operators of facilities and their design and construction professionals must look beyond the codes. One of the most innovative structural engineers of the past generation, the late Lev Zetlin made the following comments when discussing the tragic collapse in 1983 of the Connecticut Turnpike bridge over the Mianus River: "Even a fatal accident like the Mianus bridge collapse can have a positive side. ... I hope and believe that the things we find on the Mianus River bridge will keep other bridges up. What is needed is preventive engineering. I look at everything and try to imagine disaster. I am always scared. Imagination and fear are among the best engineering tools for preventing tragedy."
For more information on why structures may fail, click here. Ken Carper is a professor in the School of Architecture at Washington State University, and editor-in-chief of the Journal of Performance of Constructed Facilities, published by the American Society of Civil Engineers.
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