Practice Periodical on Structural Design and Construction

This paper reviews the basic differences between allowable strength design (ASD) and LRFD and the suitability of each methodology for the design of structures in the mining industry. The reasons the LRFD approach is of limited use for this industry are discussed. The LRFD method, as formulated in current codes, would need extensive and complex reformulation to be of general use for mining structures. The ASD method offers a rational straightforward way to design structures. Safety and design criteria for the ASD methodology are presented for various types of mining structures.

The Mississippi embayment (ME) of the New Madrid seismic zone (NMSZ) is characterized by deep soil deposits. This places much of the ME outside the realm of code-based spectral shape assumptions. Only the top 30 m of the subsurface profile is used as input for developing current code-based response spectra. Rules for effective damping and response modification for effective damping vary depending on the structure type—conventionally designed plastic hinging or isolation design. The goal of this study was to build on previous research and propose an alternative, rational method for including each of these effects—profile depth, effective damping, and response modification for effective damping—into the development of seismic displacement estimates for bridges in the NMSZ. Displacement estimates for a bridge near Memphis, Tennessee, for both conventional design and isolated design alternatives, were made using both code-based and alternative criteria. The bridge was moved to five other sites of various profile depths to examine the effect on response. It is demonstrated that current code-based displacement demand estimates may be unconservative for both short-period, conventionally designed bridges and isolated structures.

The architectural design of buildings with nontypical floors can create risky situations in which cantilevered slabs and beams are extended while the floor beneath has no cantilevered slabs on which the false work can rest. The design proposed in this paper is based on using steel cantilevered false work to support the formwork system, with no need for extending the false work along the height of the building. A closed-form solution for the temporary structure was formulated and validated using a commercial software program. A parametric study was performed using the closed-form solution to study the variations of the straining actions within the members with the changes in dimensions. The study was extended to design for the lightest member configuration of the false work for each cantilevered span. The design was extended to include the addition of a diagonal bracing member. The variations between the two designs were included in the study.

Pile bent bridges are a frequently chosen solution for stream crossings in the New Madrid Seismic Zone (NMSZ). Ease of construction makes the pile bent bridge a favorable choice in challenging subsurface conditions. When seismic loading requires conventionally constructed pile bents to behave in an inelastic fashion, the wall thickness of unfilled steel pipe piles necessary to preclude local buckling virtually doubles relative to that required if the piles remain elastic. Partial isolation—in which isolators are located at the piers with the abutments constructed integrally—is proposed as a strategy deserving of consideration for such situations. If piling can be designed to remain elastic, then the savings in material costs will, at least partially, offset the cost of isolation bearings. Along with isolator displacement demands, the yield strength necessary to keep the piles in the elastic range is a key parameter to be determined.

Intermediate diaphragms are used in precast concrete girder bridges for three primary reasons: (1) to prevent torsional girder rotations during girder erection and deck placement operations, (2) to increase vertical load distribution between girders, and (3) to transfer and spread horizontal impact loads from an overheight vehicle to adjacent girders. Significant variation currently exists in intermediate diaphragm specifications between states. The importance of bracing girders after erection and during construction is widely accepted as essential. Although it is clear that intermediate diaphragms have some effect on bridge performance, the benefit is considered by some to be negligible. Increasing numbers of states have adopted permanent steel or temporary intermediate diaphragm alternatives in place of traditional cast-in-place concrete. Wide variations are found between states regarding the configuration, spacing, and transverse alignment of diaphragms. This article presents a detailed survey of U.S. design practices by individual state bridge design agencies, including all 50 states, and details the usage of steel intermediate diaphragms and temporary bracing methods across the United States.

The design of bridge structures for seismic loading by displacement-based procedures has become preferred over force-based procedures in modern specifications. One key step in displacement-based design (DBD) is the estimation of inelastic displacement. Current AASHTO specifications rely on a linear response spectrum analysis with an amplification factor, R_d, for conventionally designed bridges and on the substitute structure method (SSM) for analysis of isolated bridges to estimate inelastic displacements. Both methods are used in this study for a selected site and target acceleration response spectrum. The procedures for both are outlined in detail. Displacement spectra from nonlinear response history analyses (NLRHA) are generated for seven ground-motion sets and compared to results from the two simplified methods. The SSM is shown to produce inelastic displacement estimates that more closely match NLRHA results than do results from the AASHTO R_d method over a wide range of periods for the specific conditions of (1) a large modal magnitude earthquake, (2) a class D subsurface profile, (3) accelerations characteristic of the selected site, and (4) reduced initial damping to mimic tangent-stiffness-damping.

This paper provides details and a comparative review of change-order-related clauses in major standard contract agreements. The information from the review is used to highlight the impact of the contractual agreements and their general conditions on change order procedures and pricing. The main categories of analysis include the change order process, direct and overhead cost items, and overhead and profit practices and percentages. Four sets of standard contract documents are reviewed: American Institute of Architects (AIA) A201-2007 and A401-2007, ConsensusDocs 200 and 750, Engineers Joint Contract Document Committee (EJCDC) C-700 and C-523, and Canadian Construction Documents Committee (CCDC) 2-2008/Canadian Construction Association (CCA) 1-2008. The comparative review points to the prevailing confusion and conflicts related to change order procedures and pricing among owners, designers, general contractors, construction managers, and subcontractors.

An analysis-based vehicular-access door design protocol for pre-engineering metal buildings is introduced. Existing access door curtain and jamb design approaches are viable for rigid door jambs. However, it was shown with experiments that wind-lock forces decrease and door out-of-plane deflection increases when the door jamb and framing are more flexible (e.g., in the case of typical cold-formed steel-framing details in which a C-section jamb is discretely braced by wall girts). A beam strip model based on an Euler–Bernoulli elastica solution is implemented in a freely available wind-analysis computer program that considers jamb and framing flexibility. This tool is integrated with an access door design protocol that provides designers both door serviceability and jamb strength limit-state checks that are explained with step-by-step videos and spreadsheets. Maximum and minimum values for the wind-lock spacing, wind-lock gap, and door curtain moment of inertia are provided for various door spans to accommodate situations in which the door jamb and framing design must be completed before the door type is selected.

This paper identifies the challenges and risks in the design, construction, and product conformity of shared (pedestrian and bicycle) path bridges. Also, the paper describes how these challenges were managed and the risks mitigated. The biggest challenge in the design of this project was the erection of the superstructure with minimum disruption to the traffic. The steel arch structure was designed to span 42 m over the width of 6-lane carriageways plus the median on the busy Epping Road at Marsfield. This was further complicated when the fabricator proposed procuring circular hollow sections from other economies that generally use a steel that is manufactured to other national standards without providing enough evidence of compliance to Australian product standards. This could jeopardize the validity of design that was undertaken in accordance with the Australian bridge design standards. The project was designed and managed by a multidisciplinary team from Roads and Maritime and constructed by a contractor who subcontracted the fabrication of the steel arch, deck, and other minor steelwork.

An important consideration in the design of structures for seismic effects is residual displacement. However, few design specifications have residual displacement criteria. As performance-based design gains popularity, the need arises to estimate residual displacements in structures from strong ground shaking. A total of 16,896 nonlinear analyses of single-degree-of-freedom bilinear oscillators were performed to estimate residual displacement ratios. Comparisons were made to previously developed models for estimating residual displacement demand.