Last edited 19 Oct 2016

Off-site prefabrication of buildings: A guide to connection choices

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[edit] Introduction

Off-site prefabrication is a potential solution to many of the issues faced by the UK’s construction industry including; safety record, client satisfaction, profitability, delays, availability of skilled workforce and overall contribution to the national economy.

Prefabrication is believed to be preferable to traditional construction methods in a number of ways:

  • Quality – Higher-quality finishes with defects eliminated prior to completion.
  • Safety – Safer working environment under factory conditions.
  • Cost – Repeated use of moulds through standardisation reduces formwork materials, preliminaries, site storage and on-site facilities.
  • Waste – Reduced off-cuts from formwork and the introduction of prefabricated reinforcement bars.
  • Programme – Increased predictability due to reduced external factors such as weather.
  • Local disruption – less environmental impacts such as dust and noise pollution.
  • Accuracy – Increased accuracy with templates produced using Computer Aided Design (CAD) systems.
  • Timescale – Components built off-site leading to reduced on-site construction time.

[edit] History

The two world wars stimulated research into newer methods of construction, with time and cost being major driving forces along with shortages of skilled labour and building materials. This ultimately led to the construction of hundreds of prefabricated concrete tower blocks and thousands of schools in the 1950s and 1960s which were often low cost and poorly designed. Volumetric construction, the construction technique involving the production of buildings as a number of boxes connected on site, was used throughout the 1960s and 1970s.

This poor quality, along with events such as the collapse of the Ronan Point tower block in East London in 1968 led to a decline in prefabrication in the UK that is only recently being turned around. Companies such as Kier, Interserve, NG Bailey, Arup, Capita Symonds and Laing O’Rourke are some of the market leaders, with Laing O’Rourke for example investing £100m in its Design for Manufacture and Assembly facility at its Explore industrial park. This has been driven by an upsurge in interest from clients such as BAA, Premier Inn, the Department of Education, the Ministry of Justice and the Ministry of Defence (Wright, 2010), in part because of a shortage of skilled workers.

[edit] Precast construction methods

As with in-situ reinforced concrete construction, precast construction lends itself to a variety of different construction techniques, layouts and sequences.

[edit] Frame and deck construction

The structure is formed by a precast deck supported by precast beams and columns. This method is often used in the construction of multi storey car parks with spans of up to 16m allowing columns to be positioned between car parking spaces.

The following connections are utilised in frames and deck construction:

[edit] Crosswall construction

Crosswall construction is a modern method where load bearing walls provide the primary vertical support and lateral stability for precast floors. External wall panels, lift cores or staircases are used to provide the required longitudinal stability. Bridging components such as floors, roofs and beams are supported by the load bearing walls or façade wall. The system is ideal for buildings with cellular and orthogonal grids, with rooms of up to 4mx9m. It creates a structurally efficient building with high levels of sound and fire insulation between adjacent rooms.

Crosswall construction utilises the following connections:

  • wall to wall at vertical joints
  • wall to wall at horizontal joints
  • wall to base/foundation

The precast elements are brought to site 'just in time'. Hidden joints and ties, both horizontally and vertically are grouted in place as the work develops, preventing progressive collapse. Other works such as installation of mechanical and electrical services and finished that are required can start prior to the completion of precast structure.

For more information, see Crosswall construction.

[edit] Volumetric construction

Volumetric construction relates to modules constructed in a factory that are installed on site to form a cellular system (or used independently as a self-contained cell). The modules can be cast as a room or as panels which are subsequently joined together in the factory prior to site delivery. For a cellular system, the ground floor cells are laid on pre-prepared ground floor slabs with individual modules lowered into place usually forming the roof of the unit below.

Cellular systems are used for repetitive designs. Common uses include hotels, prisons, student halls and residential buildings. Self-contained cells are used mostly for specialised purposes where services are needed such as wet rooms, bathroom pods and service utility rooms. Once lifted into place, the modules are secured by a number of methods including bolted and doweled connections.

[edit] Hybrid construction

In hybrid construction, precast elements can be used to provide permanent formwork for in-situ concrete. This removes the need for in-situ concrete formwork, so reducing time on site, and can be used to create safe working platforms. Large spans can be achieved with hybrid construction because of the composite behaviour of the formwork and cast structure. Shear stresses at the interface between the components of the composite can be overcome with shear studs or precast reinforcement within the cast structure.

[edit] Precast connections

[edit] Classification of connections

Precast connections range in their level of rigidity, from fully rigid to a completely pinned connection:

  • Rigid connection – This connection can sustain vertical and horizontal actions as well as bending moment. The relative angle between connected members is maintained due to the stiffness of the connection.
  • Pinned connection – This connection can sustain vertical and horizontal actions but not bending moment. The connected members are free to rotate in one direction with the connection having no stiffness.
  • Semi-rigid connection – This connection is between the rigid and pinned classes as it is able to sustain vertical and horizontal actions and some amount of moment.

A true pinned connection containing zero moment capacity is rare. In fact, many connections have some degree of rigidity but are generally assumed to be pinned. This is a conservative measure as beams spanning pinned connections are subject to the full action moment. Due to the connection having some degree of stiffness and therefore moment capacity, the negative bending moment acting upon the beam will be overestimated.

Within the steel industry, research has shown cost reductions of between 10 to 20% for semi-rigid frames over rigid frames (Kurobane et al., 2005). Therefore the level of rigidity is an important consideration when choosing a method of connecting precast concrete elements.

[edit] Continuous column with Corbel connections

Corbel connections are most often used to support long span beams or heavy loads. Due to the visual and physical intrusion caused by the corbel or haunch widening the column, this connection is not widely used in the construction of multi-storey concrete frames. The basic corbel connection is designed as simply supported, dowel bars and/or fixing cleats. This type of connection can be used to prevent lateral movement and provide some joint fixity, although research has proven that the basic dowelled connection is best modelled as pinned.

In-situ, structural screed can be used to increase continuity of the connection, thus allowing the tension reinforcement to resist the forces arising from beam movements. This can either be at the end, or across the whole length of the beam or floor slab. It has been shown that corbel / haunch connections with small amounts of cast in place reinforced concrete, although designed as a simply supported pinned connection, can improve strength and stiffness resulting in a semi or often fully rigid connection.

[edit] Continuous beam connection

This type of connection is mainly used in portal frames or in skeletal frames when beams need to be continuous over supports, as is required for a cantilever. The beams are seated on dry pack mortar on top of the vertical members and reinforcing starter bars are projected through sleeves in the beam from the lower column up into the upper column. These sleeves are subsequently grouted to provide vertical continuity. Once the beam is lowered into place, the connection requires no additional formwork providing the grout is poured through vents in the upper column. This means that provided the remaining beam end is secured, loads for construction access can be placed upon the beam. This enhances the simplicity of installation and therefore safety on site.

[edit] Wall and column shoes

The hidden corbel connection is now a popular type of precast connection in the UK. It produces fireproof connections which minimise visual intrusion whilst maximising floor to soffit height.

The connection area is minimal, protecting the reinforcement steel used in the connection. The connection also benefits from superior adjustability, utilising a small adjustable plate, allowing fine tuning of the column corbel prior to installation of the beam.

[edit] Ground foundation column connections

There are three main methods of connecting columns to foundations:

  • Projecting starter bars – The in-situ foundation houses cast-in starter bars which the precast column is later lowered onto and grouted to provide continuity.
  • Pocket connection – This is the most rigid connection and is utilised when the moment resisting capacity of the connection is required for the lateral stability of the structure. A pocket is provided within the foundation into which the precast column is lowered. The surrounding area is grouted or filled with in-situ concrete.
  • Baseplate connection – The base of the precast column contains steel base plates which cast-in bolts are fed through and bolted into place. The surrounding area to the holding down bolts is then filled with non-shrink grout to complete the connection.

The three types above are conservatively modelled as pinned connections resulting in an underestimate of the moments transferred to the columns and beams above. The foundation column connection is subjected to a certain degree of variability such as possible rotations due to ground conditions.

[edit] Evaluation of connection options

[edit] Continuous beam connection

The bolted steel shoe is considered to be the most favourable of the continuous beam connection type as it is simple to produce and quick to assemble on site. This connection requires no structural in-situ works compared to other sub-types. The connection can have a number of different bolt arrangements depending on the size and shape of the column. When used correctly the anchor bolts can be utilised to transfer both tensile and compressive load through to the column below, thus minimising stress on the beam / slab in between. Alternatively the beam / slab can be suitably designed to transfer the load to the column below.

Due to the presence of a continuous beam, the large hogging moments generated at the connection will be transferred to the column. The moment capacity of this connection is high due to the high tensile capacity of the steel holding down bolts resisting the rotation of the column due to buckling, which may result from the hogging moments transferred from the beams.

The amount and positioning of the holding down bolts will determine the connections rigidity. The closer the bolts are to the centre point in the plane of rotation the more the connection will represent a pinned connection between the columns and the beam.

[edit] Corbel/Billet connection

The bearing only type connection can be favoured because of its simple straightforward design which facilitates quick assembly time on site. This fully pinned connection type transfers vertical and horizontal loads into the column, but all moment will be contained within the beam. For this reason, the beam must be designed to resist greater moment and so the connection is less efficient than moment sustaining connections.

The bearing with bolted dowel bar connection allows reduced connection width due to the dowel action acting as horizontal restraint. The connection is fixed at the top using a bolt which extends through the column. This is positioned to resist maximum bending moment as the point of pivot will be within the underside of the beam. It is common practice in design to ensure that fixing elements of connections are not the limiting element and therefore the bolt will be able to transfer a considerable amount of moment to the column. The flat landing of the corbel, although unsightly, when combined with the dowel, acts as a torsional restraint. This can be further improved by using a cleat which extends the width of the beam. As the column is continuous, the beam will be required to sustain the majority of bending moment. This connection is semi rigid; therefore it can sustain vertical and horizontal loads with a degree of hogging moment transferred to the column.

[edit] Concealed fixing

The concealed bolted steel billet, which is one of the most modern connections, is favoured within this category. It has the advantages of other connections with an aesthetically pleasing and simple design, which allows minor adjustments to be made to the plate rather than the beam. There are other connections which show greater rigidity but require longer installation times on site, therefore are less favoured. This connection transfers horizontal and vertical loads to the column through the bolted connection. Its moment resisting capacity is small when compared to the bolted doweled corbel. However, as the connection extends to the full height of the beam it is well positioned to sustain some moment. As there are just two bolts per beam to column connection, the level of moment transfer will be limited. For this reason the connection will act as a semi-rigid connection.

[edit] Conclusion

[edit] Vertical load resistance

All three connections are capable of transferring vertical load both from the column above and from beams. As all three of the preferred connections from each group utilise a bolted mechanism to provide fixity, it is feasible that a structure constructed using only these connections would be able to resist vertical loading without using any in-situ casting.

[edit] Horizontal lateral restraint

The connections identified above, unless suitably designed using over-sized members and reinforcement, will struggle to resist lateral loading. The lateral loading will need to be taken by shear walls and/or concrete cores such as lift shafts or steel bracing to create a hybrid structure. But as with the initial problem of over-engineered connections through neglect of their moment capacity, the lateral loading capacity of the preferred connections would need to be assessed and accounted for in order to produce the most efficient design.

Combination of continuous column and continuous beam joints can be used to help transfer moments to stiffer areas of the structure. This also allows for a more efficient structure with only critical members designed to facilitate the load transfer.

[edit] Frame analysis

In-situ frames have fully rigid connections. Should a precast connection be capable of transferring moment to the columns and thus down to the supports, then it can be assed as a complete frame or a series of sub frames. Moments, either hogging or sagging are attracted to stiffer members. Should the connection be capable of transferring these moments, the moments at the columns will then be in hogging and will need to be accounted for. Many published papers (Gorgun, 1997; Aguiar et al., 4 June 2012; Baharuddin et al., 2006) suggest that some precast connections (including the ones mentioned above) can sustain hogging moment, and are therefore over engineered using the current design process. Therefore the structural frame should be modelled similar to a steel frame, where if almost no moment can be sustained then the connections are designed as pinned.

[edit] Disproportionate Collapse

Since 2004, the Building Regulations in England and Wales have been revised to ensure all buildings are designed against disproportionate collapse. The connections above have been analysed with moment capacity as the desired attribute, but they will also provide some tensional resistance which would inherently provide resistance against disproportionate collapse.

[edit] Find out more

[edit] Related articles on Designing Buildings Wiki

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