Plug and play skyscrapers: can it be done?

Modular construction, in which complete assemblies – ranging from plant rooms to apartments –are manufactured and tested off-site and then simply slotted into position on-site, is revolutionising construction worldwide.

Benefits include shorter construction, greater safety, reduced waste and higher performance. It comes under various names, including ‘modern methods of construction’ in the UK, ‘permanent modular building’ in the USA, ‘prefabricated prefinished volumetric construction’ in Singapore and ‘modular integrated construction’ in Hong Kong.

Engineering challenges

However, devising modular systems for high-rise buildings is more technically challenging than those currently used for low- to medium-rise structures, particularly where space is at a premium and strong winds and seismic loading are likely.

Lifting and handling of modules is also a major concern, so reducing module weight is a priority. However, reducing the self-weight may result in excessive tension uplift under strong winds and lead to non-linear behaviour during earthquakes.

In addition, the lateral stability of modular buildings is primarily provided by a reinforced concrete stair and lift core. For high-rise structures, stability can be increased by adding steel outrigger arms to the core and steel edge columns, but analysis of the stresses induced in the modules becomes highly complex.

Solutions and recommendations

The University of Hong Kong recently carried out a feasibility study for the Hong Kong government on the use of modular construction for high-rise buildings.

Structural schemes for 31–40-storey buildings using commonly manufactured steel modules were numerically modelled in the Etabs software and analysed against codified critical wind loads.

The results showed that modular high-rise buildings are feasible regarding structural stability – that is, they can satisfy the limitations in top-storey and inter-storey drift due to wind loads.

This however depends on careful design of the core wall layout, core wall thickness and connections between the modules and the core.

Modules with one side directly connected to the core are recommended so that the majority of lateral loads can be transferred to the core by each module independently.

Also, these connections should be designed as pin-ended in the vertical direction. As a lower-bound approach, pin-ended connections were used in the numerical model and were further checked against finite-element analysis using Abaqus in terms of stiffness, load capacity, deformation and stress distribution.

Higher strength materials

In addition, high-strength steel columns of grades S690 or even S960 can be used for manufacturing the modules to ensure sufficient load-carrying capacity.

For concrete modules, adopting fibre-reinforced concrete walls and/or slabs can help to reduce structural weight to an optimum level.

This article is based on the authors’ briefing article in the latest issue (174 CE4) of the ICE Civil Engineering journal.