A new publication entitled Design of Composite Beams Using Precast Concrete Slabs draws together new and existing information to provide guidance on a method of construction that is not properly covered by the current codes of practice, says its publisher the UK-based Steel Construction Institute (SCI)

Precast concrete floors are widely used in building construction, but there is little detailed design guidance on their application in steel-framed buildings, says the SCI. It is estimated that close to 50 per cent of floors used in steel-framed buildings in the UK use hollowcore or solid plank slabs. Most of these applications are of regular steel construction in which the precast slabs sit on the top flange of the beams, but there is an increasing number of composite frames and Slimflor constructions where the precast slabs are designed to interact structurally with the steel frame.
The use of hollowcore or solid plank precast units offers benefits in terms of cost (the long spanning capabilities of the precast slabs lead to fewer secondary beams) as well as the advantages offered by composite construction, SCI adds.
"This publication provides guidance on the design of steel beams acting compositely with precast concrete slabs in multi-storey buildings. The design basis is generally in accordance with BS 5950-3, supplemented by recommendations from Eurocode 4 and data from tests; it applies to hollowcore units of 150 to 260 mm depth, and to solid precast planks. Small scale push out tests, and earlier composite beam tests, have established reduction factors for the design resistance of welded shear connectors as a function of the gap between the ends of the precast concrete units and the amount of transverse reinforcement provided," it says.
The publication also emphasises the importance of the design of the steel beam in the non-composite construction stage, where out-of-balance loads can occur during installation of the precast concrete units.
A step-by-step design procedure is given for composite beams using various forms of precast concrete units, with or without a concrete topping. This is supplemented by a fully-worked design example for a composite beam in a 15.8 m by 7.2 m grid, and a series of design tables for concept design.