Practice Periodical on Structural Design and Construction

Few contractors in the southwestern United States ever consider using a tower crane on their projects. This reluctance can be attributed to a mobile crane construction culture. A contractor in New Mexico, Bradbury Stamm, considered the option of using a tower crane over that of mobile cranes for the Jonathon W. Rogers Surface Water Treatment Plant expansion in El Paso, Texas. How the decision to use a tower crane was made and a comparison of the crane’s use over that of mobile cranes are the subjects of this case study.

The Arizona Department of Transportation SR 101 Pima Freeway project included construction of the bridge discussed in this case study. This particular bridge is a mainline span over Tatum Boulevard. The aspects of construction discussed here encompass a description of AASHTO Type 6 Modified prestressed girders, the planning process for setting the 19 girders, and transport and placement of the girders. Thorough planning and control are required every time a crane lifts a load. In the case of a critical lift, planning must be done with great care, and lift control must be extremely tight.

A 3D indeterminate strut-and-tie model is developed for the analysis of a footing system to explain the cause of cracking and to develop a rehabilitation solution. The performance predicted with the model, simulating the sequence of construction, is well correlated with the field observations. The prediction thus explains the cause of cracking and concludes that phased construction is its main source. From numerical prediction, posttensioning forces are recommended to close the existing cracks and to prevent new ones. In developing an approach to rehabilitating a cracked structure, the stiffness of the analytical model needs to be properly selected. For this reason, five different levels of stiffness are used to cover the lower and upper bounds for both cracked and uncracked situations. A 3D finite-element solid modeling is also conducted.

Access hatches (holes) in curved-box-girder bridges are usually provided in the bottom flange immediately before or after an expansion joint. If additional access hatches are required after the bridge is built, they must be placed in such a way that (1) they satisfy such important practical constraints as feasibility, accessibility, water leakage, traffic impact, and unauthorized access, and (2) they do not adversely affect the structural behavior of the bridge—i.e., their installation should not impair serviceability or decrease ultimate strength or fatigue life. This paper discusses both of these issues and proposes approaches that are suitable for identifying appropriate locations for access hole placement. The proposed approaches are used to investigate seven curved-box-girder bridges located in the state of Florida. One of the bridges is chosen for further study using a detailed finite-element model. The numerical model is used to confirm the proposed methods and to further investigate the effects of adding access holes.

This paper presents the finite-element results of a study investigating the effect of multipresence of design vehicles on three- and four-lane steel girder bridges. A total of 220 bridges were analyzed using the computer program SAP2000. Typical three- and four-lane, straight, composite steel girder bridges were selected for this study. Bridge parameters such as span length, girder spacing, one span, two equal spans, design loads in all lanes, and design loads in two out of three, three out of four, and two out of four lanes were investigated. The finite-element analysis (FEA) results were used in calculating the maximum wheel-load distribution factors (DF), which were later compared with the simple AASHTO DF $(S/5.5)$ formula, and the formula developed as part of NCHRP project 12-26. The 10% reduction suggested by AASHTO bridge specifications for three-lane bridges appears to be developed by loading two lanes of the three-lane bridges. It was also observed that the calculated wheel-load distribution factors from the FEA-DF due to live loads in three out of four lanes were similar to the 10% reduction of the calculated FEA-DF results obtained by placing live loads in all four lanes. Also, the FEA-DF due to live loads in two of the four lanes were similar to the 25% reduction of the FEA-DF results due to fully loaded four-lane bridges. This paper demonstrates that the multiple lane reduction is built into the DF formula presented in the NCHRP 12-26 final report. The finite-element results suggest a 10 to 20% reduction factor in three-lane bridges and 20 to 25% reduction factor in four-lane bridges when using AASHTO DF $(S/5.5)\text{,}$ depending upon the girder spacing and span length. Finally, this paper can assist engineers in quantifying the adjustment factors used in analyzing and designing multilane steel girder bridges.