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- Subject: [SEAOC] ASCE CE magazine Forum article
- From: Julie Mark Cohen <jmcohen(--nospam--at)emf.net>
- Date: Wed, 12 Jun 1996 23:58:31 -0700 (PDT)
- Cc: Julie Mark Cohen <jmcohen(--nospam--at)emf.net>
I received almost one dozen requests that I post my Dec. 1994 ASCE Civil Engineering 'Forum' article. Thank you! Since many of you have long since discarded or recycled this magazine, I have tried to revise my pre-edited disk file to reflect the published article. It is best to refer to the published article for a copy without typos. The article is enclosed for your general interest. Please note that I have to get back to both hunting for paid work and preparation of a research proposal (a longer-term project to be done in my spare time). I am sorry, but I will be unable for any discussions that ensue. (I have already received over 50 sets of comments from the original publication, so I have a good idea what the reactions will be. The original comments were quite positive, especially from our senior colleagues!) Julie --------------------------------------------------- THE NORTHRIDGE WARNING: HAS 3-D DESIGN BEEN LOST? Underlying the many lessons that have been drawn from the Northridge earthquake that struck Southern California a year ago next month in one critical and fundamental issue: Structural engineering has drifted away from the 3-D, comprehensive, systems-oriented thought processes involved in its parent profession, architecture. Many structures damaged in the earthquake pulled apart in the same manner in which they were designed - that is, as an unrelated collection of 2-D vertical and horizontal planes of framing. The lack of breadth and depth leads to the design of structural framing schemes - not 3-D systems - with inadequate reliability for safety, both globally and locally. Therefore, it is not possible to fully consider soil-structure-nonstructure interaction in order to develop adequate building- specific performance criteria for expected levels of ground shaking. There are several reasons for this loss of 3-D design awareness. First, current approaches to seismic-resistant design are being driven by post- WWII and particularly post-1960s, structural engineering education. As analytical computations became easier, structural engineering students became more enamored and even mesmerized by computer output, especially when post-processor graphics were available. In the post-1960s are, students have never been instructed in the design 3-D structural framing systems, comparison of relative merits, and examination of local issues in conjunction with global issues. As a consequence, each succeeding generation of teachers gives their students given less and less guidance in design. Nowadays, a design course typically includes the sizing of beams and columns, and dimensioning of details, with the most attention on members, often without recognizing the complexities of fabrication and construction of connections. This carries over into research and practice, and is reflected in the codes and licensing exams. To counter this trend, one on short-term possibility would be for professors who teach structures courses to architects to co-teach a course with a structural engineering professor to structural engineering undergraduates, perhaps with joint enrollment. This may not be easy, because even architecture students, who are supposed to automatically think in 3-D, are also being adversely affected by the computer and computer- aided architectural design software. The fascination with computers might be overridden by initially excluding them from the educational process and concentrating on developing strong 3-D visual connections among 2-D structural plans and elevations; 3-D structural framing with architectural elements and systems; examples of gravity-, wind-, and seismic-induced failures; and technical discussions of the implications of the global and local failures on structural performance. This kind of dual approach, which is visual plus technical, was exactly the education I received as an architecture student in the early 1970s, which I brought with me to structural engineering and have maintained ever since in spite of the pressures literally to think otherwise. Research is another contributing factor. For example, research on seismic retrofit techniques is being performed by structural engineers without considering the integration of supplementary seismic-resistant structure into existing architecture. Tight budgets for experimental work have led to the use of small specimens, typically two-dimensional, one bay wide and on story tall, with tributary masses and associated inertia forces that are not representative of their placement within real buildings. However, analytical studies based on the lab tests do not address where and how the seismic-retrofit techniques can be used in classes of real buildings. Also, the load- and strain-rate dependencies of materials and connections are often neglected experimentally, because it is expensive to perform time-dependent tests. Again, analytical studies are following directly from experimental work. Finally, engineering practice must share the blame. Current approaches to seismic-resistant design are also being driven by low fees resulting from job competition. Structural framing schemes are designed and quickly as possible. A code- compliant structural framing scheme can be and often is designed as a collection of 2-D vertical and horizontal planes of framing. Codes can be and are being interpreted as maximum standards, rather than minimum standards as originally intended. Peer-review panels for seismic-retrofit schemes are often required by building owners. I have seen quite a few cases in which the panel supported a scheme that had never been substantiated analytically or experimentally in regard to its appropriate and efficient use. The decisions were usually based on cost. A 3-D systems point-of-view - that is, a comprehensive approach that integrates global and local issues - must be taken to ensure safe and reliable seismic performance. This will provide a basis for code improvements to ensure that structural framing is conceived of and designed in 3-D, not a collection of 2-D vertical and horizontal planes of framing. Perhaps code changes will provide incentives for structural engineers to be involved with architects from the earliest stages of schematic design, and for engineering educators to incorporate 3-D structural systems design into undergraduate and graduate curricula. Julie Mark Cohen, Ph.D., P.E. CLADDING RESEARCH INSTITUTE Emeryville, CA ...
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