BRE BR463 PDF

The aim of the work was to investigate the validity of the conclusions drawn from the author's work on deflection at the European Concrete Building Project's in-situ concrete frame building at Cardington. That work showed that slab deflections can be increased significantly by cracking induced by construction loading. Measurements of backprop forces at Cardington showed that the peak construction load on a slab occurs when the slab above is cast. Analysis of backprop forces measured at Cardington showed that the upper floor in a supporting assembly carried a greater proportion of the load from casting the slab above than that given by the conventional assumption that the load is shared equally between floors. The generality of this conclusion was investigated at St George Wharf by measuring backprop forces during the construction of floor 6. The work at St George confirms that most of the conclusions drawn from research into construction loading and deflection at Cardington are valid for practical purposes. The most important conclusions are:

- Designers should recognise that slabs are subject to peak construction loads and design slabs accordingly

- Forces in backprops can be significantly underestimated by elastic analysis if increases in load due to temperature and preload are neglected.