Issue 4

The design of semi-continuous composite beam systems requires that the rotation capacity of the joint regions exceed that required in those regions necessary for the degree of redistribution of moments assumed in the design to be safely accommodated. Concern has previously been expressed that some combinations of connection detail and adjacent composite beam properties might not provide sufficient rotation capacity. One hitherto relatively unexplored possibility to improve the situation is to use the concept of partial shear connection. This paper considers the effects of partial shear connection (employing the concepts of partial interaction) on the required and the available rotations of semi-continuous composite beam systems. Partial shear connection can be considered to occur in two distinctly different regions in a semi-continuous composite beam, these being characterised by sagging and hogging bending zones. Generally, the effects of partial interaction, which is increased by the use of partial shear connection, will result in reduced strength and stiffness and potentially enhanced ductility of the overall system. It is the perceived increased ductility of these systems which may be considered to alter the demands for rotations at the joints, resulting from the reduced curvatures within the span. Furthermore, the use of partial interaction within the hogging moment regions potentially increases the available rotation capacities of the joint regions. This study provides a method to evaluate the effects of partial shear connection on both the strength and ductility of beams in both the sagging and hogging bending zones. The method is validated against existing independent and original experimental results for sagging and hogging bending zones. A parametric study is conducted herein on unpropped composite beam systems with composite slabs spanning between secondary beams and supported by primary beams to elucidate the effects of various influential parameters. It is found that partial shear connection has the effect of increasing both the available and the required rotations. Furthermore, these two competing parameters reach parity in a design sense when the spans exceed 14m. For design cases which may require larger spans it is generally required that more localised issues are considered to try and satisfy this ductility limit state. Some suggestions are given for this in the paper. Recommendations suitable for design and more general conclusions are then provided which provide the basis for safer and more economic designs. B. Uy, BE, PhD, CEng, CPEng, MIEAust, MASCE, MIStructE, MICE University of Wollongong, Wollongong, Australia D. A. Nethercot, BSc (Eng), PhD, DSc, FREng, FIStructE, FICE, FCGI Imperial College London, UK

Steel bridges in Germany have traditionally been equipped with orthotropic decks. However, their number has strongly decreased against composite bridges with concrete decks. The reason for the increase in concrete decks is cost: 1m2 of roadway costs about € 800 for an orthotropic deck but only about € 300 for a concrete deck. In addition, an orthotropic deck is more sensitive to freezing which means its roadway can suddenly become slippery due to icing. A concrete deck is more robust and thus less sensitive to fatigue damage and is also stiffer than an orthotropic deck and thus the asphaltic wearing surface adheres better to it. German codes were changed in the 1980s, omitting limitations on tensile stresses in concrete roadway decks under service conditions. Instead, crack control has to be applied by a sufficient amount of finely distributed reinforcement so that the crack width is limited to 0.2mm. In this way the ineffective longitudinal post-tensioning is no longer necessary – ineffective because much of the compression force runs from the concrete deck into the steel trough due to shrinkage and creep. Today, only transverse post-tensioning is used. An exception to this rule is the latest development for bridge beams with corrugated webs. The cost for simple stiffened web steel plates have gone down in Germany during the last 20 years from about € 3000/t to about € 2000/t today, including erection and corrosion protection. Composite steel bridges are competitive for bridges with spans above about 60m because of lower costs and shorter construction periods. Corrugated webs can be less expensive than those with welded stiffeners if their production is automated. Even if a composite beam itself is more costly than an all-concrete one, the complete project may be more economic with a composite superstructure due to additional savings in shorter approach dams due to reduced beam depth, pier and foundation costs, shorter construction period and traffic interruptions. Taking into account life cycle costs the maintenance requirements of steel bridges in Germany are similar to those of concrete bridges with about 0.8% of construction costs per year, and smaller demolition costs for steel bridges than for concrete bridges. In Germany, the total length of all existing Federal bridges today comes to about 1600km, nearly 10% of which are all-steel or composite bridges. Their share is much higher for recent large bridges, e.g. for the new motorways in mountainous regions in East Germany. While in 1970 only 21% of all steel bridges were composite, their share has recently increased to 60% due to the above mentioned reasons. The German Public Roads Administration currently actively supports the following trends for major bridges: • for spans up to about 50m use incrementally launched concrete bridges, for longer spans use composite bridges, • for casting the concrete roadway slab use formtravellers which are supported from underneath in order to avo