Modular Engineer: World's Tallest Modular Building

Wolverhampton: The Tallest Modular Building in the World

The project in Wolverhampton was student accommodation consisting of 3 blocks from 8 to 25 storeys, built using modular construction with a total of 805 modules used on the site. The project started on site in June 2008 and was handed over to the client in early September 2009 (a total of 63 weeks). The project is located adjacent to the main railway line in Wolverhampton and the use of off-site technologies reduced both site activity and storage requirements below that of traditional construction, which helped significantly with the planning of the project. This project is the tallest modular building in the world.

Modular construction was first used in medium rise buildings in 1968 when Zachry Construction constructed 16 stories of hotel bedrooms using off-site construction in concrete, but never repeated this success. Since then modular has been most popular up to 10 stories with few systems currently constructing taller. The construction of the modular building in Wolverhampton has shown the industry what is possible and generated significant interest worldwide.

The Vision Modular System

The Vision Modular System was developed from a desire to achieve a flexible off-site solution with potential for high load capacity, and the opportunity to build tall. The structure of the building is contained within the modules that are assembled in a factory and shipped to site. The system was developed with a concrete floor, structural steel load bearing studs in the walls, and a vierendeel roof truss. The concrete floor offers a minimum of 120 minutes fire resistance, and excellent acoustic resistance. The Vision Modular System uses integral corridors which are manufactured as part of the modules. The modules are finished internally both to a very high level of quality due to the factory manufacturing system, and also extensively, as almost all the structural and services connections are completed without internal access. This is a significant achievement of the system allowing completed modules to be delivered to site. For student accommodation, such as the Wolverhampton project, the bed, desk, kitchens and bathrooms are fully complete when the module leaves the factory.

The Vision module system was developed as a collaboration between Vision Modular Systems and Barrett Mahony Consulting Engineers (BMCE). The first building constructed with this innovative system of construction was in Stepaside, Dublin, consisting of 366 apartments in 6 blocks.

Structural Analysis

Each individual module is self supporting for vertical loads while relying on a traditional core for horizontal stability. The load bearing studs in the structural walls of the modules are at relatively close spacing in order to provide support to the finishes, and for efficiency, rather than having members supporting finishes only. This results in an almost continuous support condition to the concrete floor slab, somewhat similar to a wall but importantly made up of discrete supporting members.

The building was analysed with ETABS software, a whole building analysis program. This program allows the modelling of the concrete core and each module with their connections back to the core. The structural models are not limited in size or complexity. Each structural stud may be modelled along with individual floor slabs and connections between the slabs and also connections back to the core. The more complex the model the longer the analysis takes which results in large amounts of output that must be reviewed.

Cantilever Effects

The student accommodation building in Wolverhampton brought some further interesting complications to the structural analysis of the system. The building has a number of recessed floors on one side creating cantilevers at various floors. These cantilevers resulted in high loads in the first stud at the base of the cantilever, and exceeded the capacities that BMCE were working with. Due to the height of the building and the resulting magnitude of the loads, the system already required box sections to be specially manufactured to suitable dimensions, maintaining wall thicknesses and achieving the required structural capacity. Also, some cold formed sections were treated to achieve the capacities of a hot formed section. The loads at the cantilever therefore required an alternative structural solution.

The support system used at the cantilevers in the modules was an independent beam and column arrangement with continuous columns from foundation to the underside of the module above the last recess, and deep steel beams under each cantilever at the various levels. The beam was erected tight to the face of the recessed module so was very close to the last continuous load bearing stud in the modules. The deflection characteristics of the beam, and the elastic shortening of the stud are very different, so when load is applied to the two members the column takes most of the load as the beam deflects. The vertical movement of the stud relates to elastic shortening only while the beam deflections under a similar load are orders of magnitude larger. Therefore if the unloaded beam is placed close to the stud and packed up tight, the amount of load sharing will depend on the relative stiffness and very little load will be transferred to the beam. For this reason, the beams needed to be pre-deflected to a load level consistent with load sharing.

A series of flat jacks were introduced on the beams to induce deflections such that they would take loads off the structural studs. Once the modules were erected the flat jack could be inserted on top of the beam and packed up tight to the underside of the module. A predetermined amount of load could then be pressurised into the flat jack which resulted in axial load in the column and beam system, and a reduced load on the modules. The sequence of loading on the flat jacks required very careful consideration. The flat jacks prevented overloading of the modules but required enough modules above the flat jacks to resist the uplift forces generated. The modules needed to support the full loads of the erected system before the jacks were fitted. Since the jacks were fitted as the modules were being erected the final load could not be introduced into the jacks at the first visit since the final condition of loading above was not present.

Creep and Shrinkage

In tall concrete buildings, the cumulative vertical movement due to creep and shrinkage may be sufficiently large to cause distress in non structural components and, also to induce significant forces in structural members due to displacements. Often the analysis of these effects, in conjunction with elastic shortening, will be used to set different construction levels on columns and walls. In this way, the connecting beam may be erected cast on a slope, and end up level as the project is completed and the majority of movement has occurred. Achieving uniformity of stress in vertical components minimises the relative vertical movement due to creep but this is usually not possible when columns and walls exist together.

This issue would not normally be a significant consideration for a 25 storey building as the columns are usually remote from the core walls and with that height the effects are minimal. With modular construction structural steel studs are erected adjacent (as close as practical) to the concrete and immediately after the core is completed. A significant amount of creep and shrinkage of the concrete still remains and must be considered in the design. Rigid connections from the core to the modules allow the movement of the core to be transferred to the structure in the modules, and these potentially significant forces must be considered in the design. As well as the creep shrinkage and elastic shortening of the core transferring into the modules when the core is subject to horizontal loads, the bending component of that analysis results in tension and compression on opposite sides of the core. These forces may also be transferred into the structure of the modules. The modules are all self supporting so while no vertical load needs to be supported by the core, the connection requirements are different to traditional construction.

Core Connections

A connection system between the modules and the core was developed that allows unrestrained vertical movement, while maintaining the requirements to achieve a horizontal diaphragm. The expected movement at each level was determined from a complex set of parameters. The concrete core was completed before the erection of modules commenced. The remaining movement in the core at the time of connecting each erected level was calculated and then the elastic shortening effects of the modules as further load was added on top. While some of these movements are relatively small the induced forces in restrained connections would have been very significant and somewhat unpredictable and most definitely uneconomical to accommodate in the structure of the modules.