Editor’s Note

The November 2010 issue of the Practice Periodical on Structural Design and Construction contains four technical papers focusing on infrastructure (two papers), wood construction (one paper), and foundation engineering (one paper). The issue opens with the forum, a contribution from Dr. Soliman Khudeira, editor, about strengthening deteriorated concrete bridge members using external posttensioning. The forum describes the advantages and limitations of external posttensioning and includes a brief discussion of the design and construction procedures thereof.

The first technical paper in this issue is titled “Methods for Enlarging Transportation Tunnels while Keeping Tunnels Fully Operational” and authored by Dr. Fulvio Tonon of the University of Texas at Austin. Dr. Tonon provides an insightful glance into the innovative construction practices of tunnel widening and is commended on the assemblage of various techniques drawn from worldwide experience. When traffic demand exceeds tunnel capacities, engineers, contractors, and owners are left with few alternatives to alleviate the burden. Most commonly the solution is found in the construction of a new tunnel, which requires environmental and alignment considerations that can be prohibitive. The paper reviews approaches to the widening of tunnels, in both hard rock and soft soils, while the tunnel remains in operation. The approaches and their case studies demonstrate the ingenuity of tunneling contractors and engineers in solving a particularly challenging problem.

Robert E. Kimble (Great West Engineering) and Dr. Donald Bender (Washington State University) contributed the second paper in this issue. Their contribution is titled “Stability of Built-Up Timber Beams and Columns: Accounting for Modulus of Elasticity Variability” and describes design procedures that will better characterize the modulus of elasticity $\left(E\right)$ to be used in the evaluation of built-up members subject to compressive loadings. When dimension lumber elements are fastened together to form a built-up member, the statistical probability of $E_{min}$ for each member occurring at the same critical location is minimized. The unified bending of a built-up member inherently forces an “averaging” of the individual $E$ values. This behavior is taken into account as the authors provide revised design checks to existing National Design Specifications for Wood Construction (NDS). Case studies are presented to illustrate the practical approach of the revised procedures.

The next technical paper in this compendium provides a look at how a new method for the determination of soil-structure interaction can be used with commercially available structural analysis software. The goal of the new method, termed modified Kerr/Reissner hybrid subgrade model, is to use commercial software modeling elements (i.e., axial spring elements and tension membrane elements) to better predict soil-structure behavior. By using a tension membrane sandwiched by axial spring elements arranged in two layers, the Kerr/Reissner hybrid subgrade model better predicts the coupling effects between spring elements that are not considered in existing models and simplified analysis techniques. This noteworthy contribution to practice was authored by Regis J. Colasanti (URS Washington Division) and Dr. John S. Horvath (Manhattan College) and is titled “Practical Subgrade Model for Improved Soil-Structure Interaction Analysis: Software Implementation.”

The final technical paper in this issue was contributed by Dr. John W. van de Lindt and Alex Stone of Colorado State University. Their paper, titled “Rapid Girder Selection and Cost Estimation for Simple-Made-Continuous Steel Bridge Design,” presents a method that makes the construction of steel bridges with longer spans possible. In this method, steel girders are erected as simple spans and then made continuous over the piers through the use of specially designed diaphragms or connection plates. The method is less disruptive to traffic below the bridge because the girders can be erected quickly as single-span units without the need for inflection point splicing, which is common for long-span continuous bridges. The paper focuses on the cost effectiveness of the construction method and presents design provisions from the AASHTO LRFD bridge design specification. Like the paper before it, this paper demonstrates the ingenuity of contractors and engineers in solving a challenging problem.

In addition to the technical papers, this issue also contains a technical note by Dr. James F. Wilson, professor emeritus, titled “Passive Restraint System for Floating Docks with Large Water- Level Excursions.” The note presents a dock design methodology that adequately accommodates large water level fluctuations, on the order of $33\mathrm{ft}$ or more. The design incorporates an angled pile at the land end of the gangway, which allows for change in elevation of this end when water levels change. The dock lateral movement is confined by two mooring lines arranged at $45\text{degrees}$ . The success of the system lies in its ability to track water change elevations while simultaneously maintaining taut mooring lines.

The issue closes with discussion of and an author’s closure to a past Periodical paper titled “Chicago’s Vaulted Sidewalks: History and Structural Rehabilitation.”

While the papers presented in this issue cover a range of topics and structures, they all contribute useful and timely information that can be readily put into practice. This makes them ideally suited for the Practice Periodical on Structural Design and Construction. The Editorial Board thanks the authors of this issue’s papers and encourages readers to submit discussion and comment on the subject matter presented. Discussions and comments should be submitted as described in the front pages of the Periodical.