Practice Periodical on Structural Design and Construction

The 15th turbine unit in the Three Gorges hydropower station was proposed to use a complete bearing spiral case, a first in large hydropower stations in China. This research performs a three-dimensional, nonlinear, finite-element statical analysis of the concrete structure based on a damage-plasticity model to study the reliability of the spiral case and the safety of the turbine unit under various working conditions. Detailed analyses performed include stress distribution in the spiral case and reinforcement bars, range of potential damages in the surrounding concrete, and displacement of the overall structure. The computational results indicate that the embedment method proposed for the 15th turbine unit at the Three Gorges hydropower station is feasible and will satisfy safety and design requirements.

This article is an investigation of buckling of cold-rolled gusset plates subjected to cyclic axial load for various unsupported length-to-thickness ratios ($L/e$). A total of twelve specimens were tested and their respective time-to-failure (in terms of the number of cycles) were measured. Using the data obtained, the dependence of time-to-failure on the ratio $L/e$ was investigated. The dependence of time-to-failure on the $L/e$ ratio is important and can be rationalized via an equation through analyses and test results. Such an equation will especially be helpful in design and can be used, along with other requirements, for the selection of an appropriate size for a gusset plate. The paper draws conclusions based on the limited test results and discusses the importance of $L/e$ and cyclic loads in design of gusset plates in bridges.

This paper introduces a technically feasible and economically affordable PP-band (polypropylene band) retrofitting for low earthquake-resistant masonry structures in developing countries. Results of the basic material tests and shaking table tests on building models show that the PP-band retrofitting technique can enhance safety of both existing and new masonry buildings even in worst-case scenarios of earthquake ground motion as in the Japan Meteorological Agency (JMA) seismic intensity scale 7. Therefore, the proposed method can be one of the optimum solutions for promoting safer building construction in developing countries and can contribute to earthquake disaster mitigation in the future.

The analytical deformation of a transmission line (TL) tower computed using software is less than the test deformation. A TL tower consists of many splice joints in the leg member. A small rotation owing to bolt slip in the joint may cause additional deformation in the tower, which is difficult to predict and cannot be accounted for in the analysis. Experimental studies are conducted on variation of bolt force with the applied torque and on bolt slip in butt-jointed specimens and the load at which it occurs. The actual behavior of the joints is studied on seven towers recently tested at the Tower Testing and Research Station of the Structural Engineering Research Centre, a national laboratory under the Council of Scientific and Industrial Research, Government of India. The bolt slip occurs when the axial force in the leg member exceeds the clamping force at that particular joint. On the basis of the studies conducted, a factor that gives the relationship between experimental and theoretical deformation is suggested to modify the analytical deformation. Rotations of 0.057° at stub level and 0.034° at intermediate levels for TL towers with double-cover butt joints and 0.125° at all levels for towers with single-cover butt joints are suggested for computing the actual deformation. The proposed factor is useful in estimating the exact deformation of a communication tower and can also be used as a predictor for monitoring the tower deformations during testing.

Seawalls played an important role in protecting harbors and multiple beach properties along the country’s East and West Coasts, coast line of the Gulf of Mexico, and the Great Lakes. Engineers successfully used different types of seawalls, ranging from simple gravity walls to flexible sheet pile walls. Until recently industry interchangeably used the term bulkhead or seawall. The majority of bulkheads can be built as simple flexible sheet pile soil retaining structures, whereas seawalls have to be designed for an additional set of loads that includes large hydrodynamic forces. The difference between two structures becomes apparent in toll seawalls designed for large magnitude wave-generated pressures produced by storm or seismic events. The “Great Storm” that pounded the West Coast in January 1988, was one such events. Significant wave height ($H_s$) along the beach during that event was recorded to be in excess of 16 ft. Waves of such magnitude may carry high energies that can be destructive to the seawall itself. A large percentage of seawalls built in Great Britain required replacement only after 25 years of service. Evolution of seawall design and analysis is based on observation and lessons learned from some past and recent catastrophic events and failures. Another lesson relates to the requirement for wall ductility: Severely distressed areas may develop within the wall length during maximum predicted catastrophic events, but failure will be ductile and prevent the loss of life and loss of protected property. Obviously, wall ductility has a major impact on ease of wall restoration. Given that the desired life span of a major seawall is usually in a range of 50 years, events such as the Great Storm or possibility of tsunami wave during the same life span will be strongly considered for seawall design. The following discussion reviews loads and modified bulkhead approach for design of flexible seawalls.