Tall Modular Construction

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.

There may be a struggle between accessibility and completion of a modular system. Vision have delivered modules to site with rooms finished, curtains, tiling, bathrooms, furniture and cutlery in the drawers. This will usually mean that one corner of the room is completely inaccessible from a structural point of view. How will this work for connections both vertically and laterally? Sometimes this is solved with cassette corridors but the preference will be to include the corridor in the finished module. The Vision Modular system includes everything outside of the core in finished modules and yet allows for completion of all necessary connections.

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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Wembley Hotel and Apartments

Situated at a prominent junction midway between the tube stop and Wembley stadium this mixed hotel and residential development provides a focal point. The project was the first element of the master plan for Olympic Way to be delivered and this building has set a precedent for the standard of regeneration.

The development is designed around a central courtyard with a 20 storey modular residential tower rising from a 7 storey block of housing on the southern boundary. A 237 bedroom hotel in a 12 storey block and 4 stories of housing also constructed in modular completes the courtyard perimeter. This is one of the first developmements for the Vision Modular System that was designed from the outset with a modular solution.

The building is constructed with traditional reinforced concrete up to a podium transfer slab. the structure below the transfer is set out to suit commercial and retail occupancies and is unrelated but mindful of the construction above. The modular construction commences above the transfer slab with varying heights of buildings on the four sides of the site. The hotel building is constructed with 12 stories of modules including a sloped roof. The hotel bedrooms each offer views of Wembley stadium through the projecting feature windows.

The system employed allows construction of irregular shaped modules and also changes in the profile of the upper levels of the modules as they are constructed. The tower element of the building includes 19 stories of modules with cantilevering perimeter balconies and featuring cladding elements on the outer edge of the balconies. The oval tower extends upwards from the lower rectangular housing block demonstrating what can be achieved with the system. The modules in the tower were erected with the balconies in place allowing the balconies to act as a type of scaffolding during the construction.

The striking elevations are created from the attention to detail by HTA Architects and their understanding of the modular system. The west elevation fronting onto Olympic Way reveals a shifting pattern depending on the time of day and the viewers perspective. Each bedroom projects outwards in a random pattern catching the light in a variety of shades across the elevation.

Commended, British Homes Awards: Mixed Use Development, 2014

Shortlist, Housing Design Awards, Project Category, 2012

 

Michael Hough.

 

 

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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.

 

OSC Gold Award for Overall Offsite Excellence, 2009

OSC Award for Best Use of Steel, 2009

OSC Best Offsite Education Project, 2009

 

Michael Hough

 

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