Editor’s Note

This May 2010 issue, consisting of one Forum article, seven technical papers, and one technical note, contains a wealth of new information and insights for both building and bridge structures.

Before this content is introduced, some basic changes in the format of our Forum are described. Instead of two separate Forums that in the past were dedicated more narrowly to topics within structural design or construction, the Forum is being integrated into a single combined section dealing with either or both subjects. This decision was made by the periodical editors and ASCE staff to provide a more meaningful and less rigid format for the delivery of various current items of interest to the readers. This revised Forum is expected to offer greater flexibility to cover issues that transcend the formal boundaries between design and construction, since many practical cases or applications may have pertinent value to both fields. We are also attempting to include in this Forum section more news of current events, highlights of major changes in building codes, standards, views on professional practice issues, structural collapses, and reprints (or digests), of important publications from other sources. The intent of these changes is to further enhance the Forum’s role as a conduit for informally transmitting news, questions, and discussions in a timely fashion and without the normal procedural constraints imposed by a technical paper submission. We encourage everyone to take note of these changes while reading the Forum, and also invite you to contribute items of current and general interest in this regard.

This issue begins with a relatively short but startling event item in the Forum. It was provided by Soliman Khudeira, and discusses the collapse on June 27, 2009, of a thirteen-story building under construction in Shanghai, China, by overturning and falling on its side due to foundation/soil issues. The building’s site conditions before, during, and after the collapse that may have affected its stability are briefly summarized, including soil surcharge, rain, excavation support system, and hollow concrete piles.

Richard Mott and Manuel A. Diaz then examine the loading effects of short-haul vehicular loads on a typical prestressed concrete open box beam (U-beam) bridge. The AASHTO LRFD Bridge Design Specifications uses distribution factors to modify the load effects determined from the beamline analysis to account for the effects at other locations in the structure. The published vehicular design loads adopted by the AASHTO specifications are considered to be an “average design truck.” The average design truck loads exclude vehicles that are above the legal weight limits for the United States, but are regularly allowed to operate on the United States highway systems. These heavier loads are represented by short-haul vehicles, such as solid waste trucks, aerial rescue fire trucks, and concrete mixers. The results of an analytical parametric study has revealed a number of conclusions applicable to the design of open-box beam bridges loaded by such exclusion vehicles compared to the regular distribution factors in the AASHTO specifications.

The next paper by A. Pipinato and C. Modena presents a review of the structural analysis and fatigue reliability assessment of the Paderno Bridge in northern Italy. Because this historical arch railway bridge was placed into service in 1889, it recently underwent a thorough engineering review of its structural integrity. The practical approach for such an evaluation of a deteriorated steel bridge is discussed, including the analytical identification of redundancies, critical members and connections, mechanical characterization of the materials, assessments of traffic and fatigue reliability, inspections, and retrofitting alternatives. The application of such a comprehensive procedure to the Paderno Bridge showed that a detailed analysis and assessment succeeded in extending its service life with limited corrective retrofits at a relatively small fraction of the costs needed for replacement of the entire bridge.

The final paper on the bridge theme deals with a state-of-the-art review of the prediction, modeling, monitoring, and countermeasures for scour. Authors Lu Deng and C. S. Cai introduce different types of bridge scour and the scour development process, followed by description of various approaches that have been developed for predicting and monitoring bridge scour. Laboratory work and field tests conducted for bridge scour are collected. Various scour countermeasures developed in practice are also summarized. All existing scour monitoring instruments and countermeasures have their own advantages and disadvantages. The selection of the optimum bridge scour monitoring instruments or countermeasure should be based on consideration of many factors, such as the nature of the problem, the site condition, the relative effectiveness and cost, etc.

Explosion phenomena and their effects on structures are then addressed in the next three-part series of papers authored by Ronald Pape, Kim R. Mniszewski, Anatol Longinow, and Matthew Kenner. Interest in this subject has grown markedly since $9∕11$ and the related threats of terrorist acts, for which these three papers provide an excellent overview. In Part 1, background was provided on the types of explosions, explosion effects (including air blast, thermal effects, and projectiles), and damage to structures from the effects of explosions. In Part 2, analysis methods for predicting explosion effects were discussed for several explosion types, with emphasis on air blast. In Part 3, various techniques for predicting explosion damage to structures are summarized. Since the explosion-effects subject is so broad, additional reference sources for more in-depth information on many of the subtopics presented are provided, including several for structural design applications. Illustrative examples, charts, and tables help to provide quick insights into the nature of this problem and its possible solutions.

Bharath Gowda and Nader Heydari discuss in their paper three new structural glass systems, each of which were designed to accommodate the elastic and inelastic seismic criteria for large displacements in areas with high seismic activity. These systems include a rail-supported seismic glazing system, a point-fixed seismic glazing system, and a point-fixed seismic fin glass system. These systems were specifically developed for use as cladding facades of buildings in areas of high seismicity, in particular for the state of California, Seismic Zone 4. The seismic performance of these systems was demonstrated by mock-up testing. This testing verified that the glass would translate horizontally and distribute seismic drift evenly over the height of the glazing systems without breaking any components or glass, and with the silicone withstanding the imposed shear demand without rupturing the seal. Upon successful testing, use of these glass systems on three building projects is described.

The final part of this issue is a technical note that covers the graphical determination of the shear center in thin-walled, asymmetrical U-profiles, written by Gilbert H. Béguin. Several graphs are presented that allow for a direct determination of the position of the shear center and of the corresponding warping constant for such unique steel sections. The background theory and mathematical genesis of these graphical design aids are succinctly reviewed. This highly specialized information could be useful to some design engineers or researchers who work with these particular types of unsymmetrical, light-gauge steel sections.

We trust that the content of this issue continues to be worthwhile and a good resource. Please remember to submit to ASCE any relevant information that you could share in this manner as a Practice Periodical Forum, technical paper, or technical note.