Introduction
Until the late 19th century, the tallest buildings in the world were religious structures, whose spires and domes pierced the sky. This changed in 1885, with the erection of the Home Insurance Building – the world’s first tower block – in Chicago. Framed in structural steel, it stood 42m high with 10 storeys of office space.
The new technology demonstrated in Chicago caught on in cities across the world and the Home Insurance Building was quickly dwarfed by subsequent structures as the race to the sky began. However, it brought with it a significant concern: the vertical movement of vertical loadbearing elements in multistorey structures.
This Technical Guidance Note concerns the concept of axial shortening – a phenomenon that occurs in vertical loadbearing elements within all structures, but whose effects are especially pronounced in those over 15–20 storeys high.
Reinforced concrete frames are most prone to axial shortening due to the impact of shrinkage and creep on composite materials.
This note covers the causes of axial shortening, how it is assessed and predicted, and the mitigation measures structural engineers and building contractors employ to counter its effects, particularly in relation to fit-out and facade installation following building construction.
The note also refers to ways in which current codes of practice offer a means to analyse structures for axial shrinkage.