Practice Periodical on Structural Design and Construction

This paper reviews worldwide state-of-the-art methods for enlarging transportation tunnels while maintaining current traffic capacity during construction. Both hard rock and soft ground applications are presented and accompanied by relevant case histories.

Built-up beams and columns comprised of dimension lumber fastened with nails, bolts or lag screws, are common in construction. Current design practice for stability checks is to use the lower-tail modulus of elasticity $\left(E_{\text{min}}\right)$ value tabulated for dimension lumber; however, this ignores the averaging effect on $E_{\text{min}}$ that occurs when the mechanically fastened laminations are constrained to deflect in unison. In this paper, we propose a method for checking stability of these assemblies that is based on established statistical theory. A factor $\left(C_s\right)$ is proposed to adjust $E_{\text{min}}$ value used in column and beam stability calculations to account for the reduced variability of $E$ when the laminations are constrained to deflect together. Using $C_s$ to account for the reduction of $E_{\text{min}}$ variability produces more efficient designs with respect to stability. Two examples are provided and each results in larger design capacities compared to current design methods for built-up beams and columns.

Subgrade models are relatively simple and easy-to-use approximations of the actual load-displacement behavior of the ground and have always played a significant role in foundation engineering research and practice. Winkler’s Hypothesis has been used almost exclusively for this purpose even though it has long been recognized as providing a poor approximation of actual subgrade behavior. The primary flaw of Winkler’s Hypothesis is the lack of coupling or connection between adjacent subgrade “springs.” Although numerous subgrade models that inherently incorporate “spring coupling” into their mathematical formulation have been developed, none has seen widespread adoption and use to date. This is due primarily to various issues that hinder their use with the commercially available structural analysis software used in routine practice. This paper is the second in a series of three that collectively illustrate the development and implementation of a new hybrid subgrade model with inherent spring coupling called the Modified Kerr/Reissner (MK/R) model. The MK/R model has been developed specifically to be both relatively easy to implement using commercially available structural analysis software as well as to evaluate using conventional geotechnical site-characterization methodologies. This particular paper is devoted to practical guidance as to how this is accomplished.

Prestressed precast bridges currently dominate the short to medium bridge market in the state of Colorado. This can be attributed to a lack of competitive steel construction products combined with a strong presence of precast fabricators in the region. For steel girders there is a lack of readily available economical and innovative procedures to design and construct steel bridges, which has hindered the industry in certain areas of the country such as Colorado. A significant percent of the bridges throughout the United States are in need of repair and many in need of replacement. In many part of the country, steel bridges can provide an economic and rapid solution for new and replacement bridges. This paper summarizes the results of a study sponsored by the Colorado DOT to develop a rapid preliminary design and cost estimation tool for simple-made-continuous design and construction. In that method, steel girders are designed as simply supported for the noncomposite dead loads and continuous for composite dead loads and live loads, which can provide economy for spans up to 35 m (120 ft). The methodology summarized herein follows the logic of the AASHTO LRFD specifications. Design charts and tables were created to outline structural steel weight to span length and number of girders, which could be used to prepare initial bids during the material section process.

Anchorage of a recreational two unit floating dock, a gangway-deck configuration, is achieved using a combination of a fixed pile and fixed-length mooring lines. Attached to the floating gangway is a guide that rides along the pile during fluctuations in water level. For a 15.2-m (50-ft) gangway and a pile inclined toward shore at a critical angle of 10.55° with the vertical direction, two symmetrically placed shore-anchored mooring lines attached to the deck corners and diverging at 45° remain at nearly constant length during extreme water-level excursions. Experiments employing a scale model of such a design, together with the observed behavior of a recently installed prototype system in a flood control reservoir, serve to validate the practicality of this passive restraint system for inland water applications.