In 2009 Barrett Mahony Consulting Engineers designed a structural solution for the world's tallest modular building. At 25 stories there were a number of difficult structural challenges which pushed the boundaries of this type of construction. Five years on and we could construct this building more efficiently and probably faster that the original 9 month construction programme. The Vision Modular system developed from its first building of six stories to 11 stories and then the 25 at Wolverhampton. Each step introduced new challenges and brought a greater understanding of the system and what was possible.
Since the completion of this ground breaking project we have completed 19 stories of modular construction along with buildings of lower heights. From about 16 stories upwards the structural issues that arise in modular construction require a greater understanding of materials and methods. There are various types of modular construction and the issues are different for each type, but the basic principles remain similar. While it may not be rocket science, and just another form of construction, any lack of understanding of the principles involved could result in very serious consequences.
These taller modular buildings use structural steel in the vertical load bearing members for high strength in small sections allowing maximum flexibility and the lightest lifting on site. Buildings such as Zachary's San Antonio Hotel and Habitat 67, both constructed with concrete, result in heavy modules for lifting, large loads accumulating down through the building, and difficulties reducing structural elements in lighter loaded areas of a building. With a structural steel module each structural member is designed specifically to suit the position, floor and load level in a given building.
When the stabilising cores are constructed in concrete then the vertical structure of the modules will be erected adjacent to the concrete structure. These two material behave differently especially in relation to shrinkage where concrete shortens over its lifetime. This effect increases the taller the construction with the biggest difference in height of concrete and steel members occurring at the top of the building. Early buildings constructed with the two materials discovered that round objects rolled off desks on the upper floors. This type of level change occurs in buildings where the steel columns are remote from the core resulting in differential heights over large distances (at least a few meters). With modular construction the steel columns are immediatelly adjacent to the core so the differential height occurs over a very small distance. In this case, with rigid connections from the column to the core, the shortening of the core will pull down on the steel column. This effect may be very significant.
Cores may also be constructed using structural steel offering some advantages and some disadvantages similar to their counter parts in traditional construction. One significant advantage is the materials in the core and the modules now have the same movement characteristics. The shrinkage issues may be ignored and vertical movement is now just elastic shortening of the steel members. However, another effect that needs to be considered occurs when a core leans over due to horizontal loads such as wind loadings. The horizontal movement of a stabilising element such as a core results in shortening of one side and lengthening of the opposite side. Where the modules are originally connected the shortening effect may transfer loads into the vertical module structural elements. It is unlikely that this will result in efficient structural design. The opposing effect however does offer some advantages, where the other side of the core tends to lift upwards, and the weight of the modules resist this uplift. The important factor is the recognition of the effects and the understanding of the consequences. Once understood they may be considered in the design whether offering benefits or difficulties.
Lining, levelling and tolerances in modular construction are very different to traditional structural steel. Where modular buildings are low rise the effects of inaccuracies within the system may not be significant. As the height of the construction increases these inaccuracies become more of an issue, particularly if there is no facility in the system to correct an error / problem. If the manufacturing system introduces an out of plumb into the vertical columns and this is repeated in module after module, then the problem accumulates, and the modules may separate or come together up through the building. In traditional steelwork individual columns are lined and levelled allowing tight construction tolerances to be achieved. With modular construction the structural members are welded together in a factory very remote from the final position in the building. Once erected the rigidity of the system is unlikely to allow any aligning and the forces needed to do so would most likely cause some structural distress. It is therefore essential that the limitations of the structural system be fully understood and how lining and levelling will be achieved.
As buildings get taller the cost of access for cladding escalates. With traditional high rise, fitting the cladding from inside the building offers significant savings. This is not an option with modular as the external walls are closed in and finished. Therefore the cladding must be either prefitted to the module or fitted with external access after module erection. The difficulties with prefitted cladding relate to the earlier mentioned lining, levelling and tolerances. The joints or mates lines in the cladding will vary in width between modules depending on the tolerance and accuracy achieved. In the worst scenario the prefitted cladding elements may be in contact or overlap. Unless high quality detailing is undertaken a building with prefitted cladding may look modular upon completion. At the upper end of quality modular construction there will be no evidence internally or externally, upon completion, that the building was constructed with modular units.
The above non exhaustive list identifies some of the structural issues that need resolution before constructing modules of a medium to high rise level. Once these issues are all resolved throughout the manufacturing process, a clear set of instructions need to be advised to the module erection crew allowing for all situations that may arise on site. A detailed survey should be undertaken at each floor and reviewed by the engineering team to ensure no surprises, especially at a stage when it might be too late.
Michael Hough
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