Practice Periodical on Structural Design and Construction

Developing trends in important aspects of blast and construction vibration monitoring and control are identified through consideration of research and consulting experiences during the last $35\text{years}$ . This retrospective view is written to identify strong currents in a large river of progress rather than to present a comprehensive state of the art survey. Specific case studies set the stage for the following discussion of trends, which in turn set the stage for the prediction of future developments.

Circular base plates are commonly used for pipe columns, such as pylons in cable-stayed bridges, lighting poles, and electric power line posts. Although the explicit solutions for rectangular base plates can be found in many textbooks and AISC design procedures for base plates, a complete design procedure for circular base plates under different loading conditions has not been documented except for a large eccentric load. In this paper, a complete allowable stress design procedure is presented for circular base plates with axial loads, small eccentric loads, and moderate eccentric loads. Several design examples are shown for different loading conditions.

When designing a structure for safety against applied loads, engineers will determine stresses and strains experienced by individual materials and building systems and will compare those magnitudes with known allowable values or other limit states criteria. When investigating the failure of a structure, however, the engineer will be required to review stress and strain under conditions of overload and use that research to evaluate visual signs of distress or collapse. An investigation typically takes many avenues, some of which may even turn out to be unproductive or misleading. The purpose of this paper is to provide an investigating engineer with valuable information regarding material and building systems behavior and to suggest a procedure that will minimize the occurrence of distracting paths of investigation.

Stepped footings are often required for earth retaining structures to suit grade changes in the site. The behavior of walls supported on stepped footings is not well understood due to the presence of reentrant corners and double curvature. In this study, a large number of reinforced concrete retaining walls with different geometry are analyzed using the finite element method. The bending moments throughout the wall and shear forces at the support are determined and presented in a tabular format that is easy to use by practicing structural engineers. The results of the analysis showed that positive and negative bending moments can possibly exist in a retaining wall supported on stepped footing; hence both faces of the wall should be reinforced for flexure. Further, very high stresses due to shear and bending moment occur at the reentrant corner of the stepped footing. These forces, however, cannot realistically exist in thin reinforced concrete structures due to postcracking deformations. Therefore, structural design at such a location should be based on values of shear force and bending moment just off the reentrant corner, provided that proper detailing with diagonal steel reinforcement is utilized at the tip of the step.

Considering the diversity of excavation work, municipalities are always trying to find new methods of shoring. This is how the idea grew up about using a high density polyethylene (HDPE) sewer pipe or a corrugated galvanized steel (CGS) pipe as shoring on the water supply and sewer network for small-dimension excavations of short duration where proven shoring and shielding systems cannot be used and/or a backhoe cannot be used for excavating. HDPE and CGS pipes have been designed according to the requirements for water and sewer pipes. However, when they are used for vertical shoring and subjected to earth pressure, their behavior has never been the subject of quantifications or tests. This study describes the validation by calculations and tests of the use of HPDE and CGS pipes as shoring for small-dimension excavations of short duration on the water supply and sewer network.

Cylindrical tanks are commonly used in water and sewage treatment plants. These tanks are usually built of cast-in-place concrete with flat or conical bases to facilitate the treatment process. The design codes specify high standards of serviceability and rigid requirements for water tightness and crack control to prevent leakage and corrosion of reinforcing steel. In view of these stringent requirements, special attention is focused on accurately determining the governing design forces to satisfy economic, as well as serviceability, requirements. In transferring liquid or earth pressures into forces acting on the tank wall, the designer can utilize the design coefficients provided by the Portland Cement Association (PCA). The PCA tables consider different conditions for the wall-base joints but do not, however, consider the effects of varying subbase soil conditions or the geometry of the base slab. The purpose of this study is to investigate how different soil conditions and base geometry affect the design forces of cylindrical tanks. A finite element analysis, using shell elements, was employed utilizing the SAP2000 program. The subbase soil is represented by a number of springs with constants reflecting the soil stiffness. Finite element results indicated significant discrepancies in the design forces when compared with those obtained from PCA tables.

Although fly ash as a partial replacement for cement has been utilized for many years, it has been almost exclusively used in low-volume percentages, such as 10 or 20% cement replacement. This paper looks at high-volume percentage replacements from 40 to 70%. A minimix study revealed that 50 and 60% cement replacement percentages were the best candidates for full-scale testing. The environmental benefits included a 25% reduction in smog, human health effects, and fossil fuel use compared to the same element built with a 100% Portland cement mix. The economic benefits included a 15% capital cost reduction and a 20% life-cycle cost reduction when compared with a 100% Portland cement mix. Full-scale testing included a complete mix design in addition to the construction of four concrete infrastructure products. The products built included an alley panel and curb, and gutter sections in the City and County of Denver, a precast manhole and lid, and a twin-tee prestressed girder.