Practice Periodical on Structural Design and Construction

This paper describes a simple method to estimate tensile stresses due to early age time dependent loadings in concrete bridge decks with full composite action between the deck and girder. These stresses can cause transverse deck cracking if they exceed the modulus of rupture for concrete, and need to be checked. A system of equations for a pin-roller supported composite girder is developed to estimate the deck stresses due to the volume change in concrete caused by shrinkage and/or other effects. These equations are further extended to consider different boundary conditions. The results of this method agree very well with the results of finite-element analyses. It is proposed that this practical method be used as a tool during bridge design to examine concrete bridge deck stresses against the possibility of transverse cracking. A Microsoft Windows application that is easy to use is developed and presented. Furthermore, an overview of factors causing the volume change in bridge deck concrete and their typical magnitude is presented herein to further simplify the use of this practical method.

About two years ago, the Alabama Department of Transportation (ALDOT) increased its minimum bridge deck thickness to 178 mm (7 in.), and thus ALDOT’s deck thicknesses now range from 178 to 197 mm (7–7.75 in.). Even with the upgrade, ALDOT still utilizes the thinnest decks in the country. Thicker bridge decks are stiffer, stronger, and should provide a longer service life; however, they cost more and require a stronger and more costly support girder system. All of the parameters investigated in the parameter sensitivity study, with the exception of two (deck unit weight and initial cost), support increasing Alabama’s minimum bridge deck thickness. An increase in thickness from 178 to 203 mm (7–8 in.) would cause the deck unit weight to increase 287–575 N/m² (6–12 psf), depending on whether the concrete is added to the top or the underside of the deck. This would increase the cost of the bridge deck around $2.15/m² ($1.80/yd²), and would translate into an increase in deck initial cost of around 2–3%. However, increasing the deck thickness from 178 to 203 mm (7–8 in.) would also increase the deck service life, which would reduce the life-cycle cost of the deck/bridge.

The owner of a recently constructed facility described in this paper specifically required that a portion of their facility, referred to as the “concrete structure,” be constructed as a self-supporting structure. Additionally, no portion of this concrete structure could bear loads from upper building walls or floors. Structural retaining walls were required to isolate the subgrade portion of the concrete structure from lateral soil loads, while supporting loads of the exterior cladding system and roof structure. These requirements presented a unique construction challenge. The combination of a cast-in-place structural enclosure, exterior tilt-wall panels, and precast double-tee roof members provided a facility that met the owner's criteria.

Depending on the application, self-supported tubular steel pole (cantilever) structures in the United States are designed for extreme wind loadings based on either ASCE 74 (for electric utility applications), ANSI/TIA/EIA 222 (for antenna applications), AASHTO (for highway sign and traffic signal applications), or ASCE 7 (for general applications, such as stadium billboards). This paper compares the margins of safety of self-supported tubular steel structures designed for extreme wind loadings based on ASCE 74, TIA/EIA, AASHTO and ASCE 7. Based on this comparison, a critical review of the ASCE publications and their respective design requirements and safety factors are presented.

This research studied the seismic performance of equipment support structures with knee-braced moment-resisting frames in petrochemical facility construction. Frame performances were evaluated using the structural stiffness, strength, and ductility. It was found that frame stiffness increases when the magnitude $\left(e_b\right)$ and slope of knee braces $(e_c/e_b)$ increases. For practical purposes, it is suggested that the $e_c/e_b$ ratio be kept at 1 and $e_b$ less than 0.25 times the beam length. It was also observed that the maximum member forces in traditional special moment-resisting frames are shifted from the critical beam-column joints to the beam-knee brace intersections, which greatly reduces the risk of brittle fracture at those regions and significantly enhances the structural ductility.

The competitive and negotiated bidding processes are not new. Competitive or hard bidding has been and is currently more prevalent in the commercial construction industry. However, in the last 20 years, evolutionary changes have occurred in the subcontracting and architectural fields of commercial construction. The result has been a depression in prices for competitively bid jobs. A need has arisen for a change in the bidding process. The negotiated bid process needs to be seriously considered as an alternative process in today’s environment.