CPD: Better definition with BIM

The Ordsall Chord provides a new viaduct and section of railway to connect Manchester’s Victoria, Oxford Road and Piccadilly stations

Understanding how much information to provide at different stages of the BIM process is vital if the benefits of the technology are to be realised. Duncan Reed from Trimble explains.

BIM can deliver huge efficiencies in the construction process – but only if members of the project team clearly understand their roles. This is particularly important when providing information for the model.

In UK building information modelling (BIM) terminology, the acronym LOD usually refers to “level of definition”, meaning the amount of model detail or information detail provided at different stages of a project. It is an acronym that can sometimes be misinterpreted (see below), but a clear understanding of its meaning and purpose is vital.

At the start of a project, agreeing the appropriate LOD throughout the lifecycle of a scheme is central to using BIM efficiently and successfully.

One important thing to understand about LOD is that it will change over the duration of a construction project – with less definition required during the early stages, but potentially much more later on. In this respect, the software used becomes an important guiding tool.

When an organisation enters the BIM process – at any of the eight stages defined by the BIM Task Force’s Digital Plan of Work (DPoW) – it needs to know how much information it is expected to provide. To get the best out of  BIM, it is important that a project team member does not overproduce or underdeliver on information for the model. Otherwise the lean principles which underpin BIM are unlikely to be realised.

This point is illustrated in the table (see below), which shows the process of designing and detailing a building structure using BIM. Here, Tekla Structural Designer has been used to develop the model definition. In the early stages of the project, the structural analysis is the most important factor. By stage three, only the sizes of the structural members have become known. As the project progresses to stage four, the locations of the members, along with their physical and performance properties, have been determined.

Design development of a reinforced concrete column using Tekla BIM software

At the end of stage five, the level of model definition needs to be completely defined, at which point the structure is procured from a specialist supplier, in this case a precast concrete frame manufacturer. Importantly, the Tekla Structures model can be used by the manufacturer to produce all the structural frame components.  Accurate member sizes and accurate geometry as well as end plates, welds, bolts and 2D fabrication information are generated directly from the 3D model.

The example shows how overproducing information for a BIM model during the early stages of a project is as unhelpful as underproducing. Detailing the reinforcement required for a concrete column at stage two is pointless, as the framing material has not been decided, and the scheme may ultimately choose a steel frame solution at stage three.

On the other hand, modelling the optimal level of information ensures an efficient process for designers and detailers. This ensures technical queries are virtually eliminated, reducing delays and waste in the construction process.

Comparison between stage four “design” detail (LOD 300) and stage six “as built” detail (LOD 400 –500). Note the extra detail in the more advanced stage of the model (on the right)

It is very easy to get bogged down in information overload. For BIM to be used successfully, the team should start by defining what information is important and when in the lifecycle of the project. The suite of BIM Standards, PAS1192-2:2013 and PAS 1192-3:2014, help project teams to define the information needed.

The correct level of model definition over the life of a project should be defined and recorded at the outset, so that all parties understand what they are being asked to deliver.

It is also important to identify the software capable of delivering a model to these required levels. The Tekla suite of software provides support through the eight stages of the DPoW and offers a high level of transparency for all project team members – ensuring clarity, removing the need for rework and ultimately driving waste out of the construction process.

Duncan Reed is digital construction process manager, structures division, with Trimble

Definition, detail, development – understanding BIM definitions

The arrival of BIM has created a whole new language for construction organisations to try to understand – and there is plenty of scope for misunderstanding.

One acronym which crops up frequently is LOD, which can mean different things to different roles in different organisations.

Its original definition came from the American Institute of Architects (AIA), which published a BIM guidance document called the LOD Forum in 2011. It defined LOD as “level of development” – referring not to design phases, but instead to project milestones or deliverables.

Much early BIM guidance in the UK referenced the AIA document in the absence of anything else. However, the Digital Plan of Work (DPoW), created by the UK BIM Task Group three years ago, has since provided UK construction with an eight-stage design, construction and operation plan that encompasses the whole life of an asset. From this, a standard has emerged – PAS 1192-2:2013 – which not only defines the BIM project stages but also gives a UK meaning for LOD: “level of definition”: 

3.30. “Level of Definition”. Collective term used for and including “level of model detail” and “level of information detail”. Note: The “level of model detail” is the description of graphical content of models at each of the stages defined. The “level of model information” is the description of non-graphical content of models at each of the stages defined.

So, although the UK has defined LOD as “level of definition”, it is also possible that teams will use it to mean “level of detail” too, either as model detail or information detail. With so much potential for misinterpretation, it is critical that project team are clear what they understand the acronym to mean at the start of the BIM process.

Northern Hub proves BIM value
BIM 3D modelling software is helping with the structure’s design on a key project in the Northern Hub rail enhancement programme in the north west.
The Ordsall Chord provides a new viaduct and section of railway to connect Manchester’s Victoria, Oxford Road and Piccadilly stations

The Northern Hub capacity rail enhancement works is a complicated and multifaceted project. Together with the North West Electrification Programme (NWEP), it totals over £1bn of rail investment.

A key part of the programme is the Ordsall Chord, where a Skanska/Bam joint venture is the main contractor. The work includes building a new viaduct and section of railway to connect Manchester’s Victoria, Oxford Road and Piccadilly stations, and widening existing brickwork viaducts using new concrete arches.

Construction is due to get under way in the summer. In the meantime, the design is being developed by the joint venture engineering team of Aecom and Mott MacDonald. The structures are complex, comprising a mix of insitu concrete, precast concrete and steelwork. But the design is being helped enormously by BIM 3D modelling.

“Building virtual, constructable models for the requirements of stage five in the Digital Plan of Work [DPoW], before starting work on site has revolutionised the design engineering for the project,” says Brian Duguid, engineering manager for the scheme with the Aecom/Mott MacDonald JV.

“It has meant substantial benefits in terms of reducing risk of errors, increasing design accuracy, and providing better information for the construction team – and this is all before building work actually commences.”

The Ordsall Chord provides a new viaduct and section of railway to connect Manchester’s Victoria, Oxford Road and Piccadilly stations

The project delivery team set out from the start to build a common BIM model, for use both during construction and for future asset maintenance. The civil engineering design team selected Tekla Structures to create the 3D model.

One area where the model has helped particularly is in the reinforcement design. “Reinforced concrete drawings are traditionally schematic in nature, typically providing information that would now be deemed suitable at stage three or four of the DPoW,” says Duguid.

“Although easy to understand, they don’t always help contractors and reinforcement fixers to plan the sequence of their work accurately, especially on complex structures. They also provide limited information on how reinforcement should be arranged in tight locations with limited tolerances. So being able to visualise the full reinforcement cage in 3D, using information from stage five of the DPoW, will potentially make it much easier to plan the construction.”

The modelling capabilities are crucial on the Ordsall Chord, as the widening of the existing brick-arched viaduct involves 25 varying concrete arches in total. Every new arch will have a different geometry as a result; the taper of the concrete widening and the demanding architectural parameters means that no two arches are the same.

“A key feature of Tekla 3D modelling software is that any changes made to the model during stages three, four and five of the DPoW can be fed through into updated 2D drawings and reinforcement bar schedules with a minimum of manual intervention,” explains Duguid. “With a large number of parties involved in the project, being able to quickly respond to changes and revisualise the reinforced concrete works will be very helpful.”

Another complication on the project is the amount of structural steelwork hidden within the reinforced concrete.

“The Tekla software allowed us to detail this steelwork and reinforcement, which was invaluable,” says Duguid. “It provided the opportunity to identify any potential glitches and clashes, and see how everything will fit together before the steelwork is fabricated off site. This will greatly reduce problems out on site and prevent substantial delays and cost.”

He continues: “Fully developed stage five BIM data provides enhanced reassurance and confidence to all parties involved, especially on a project like this with multiple subcontract suppliers, all of whose work must fit together perfectly. For example, there are precast elements supported on insitu concrete, which in turn support further insitu concrete panels, as well as the instances where structural steelwork is embedded in the concrete. Those three key suppliers have a number of geometrically complex interfaces.

“The use of Tekla software for coordination throughout the design, planning and delivery phases has improved accuracy and greatly reduced the likelihood of unexpected clashes.”

Tekla software for the construction and structural engineering industries is produced by Trimble, a technology company with the vision of transforming the way the world works. Tekla solutions are used for realising projects around the world, from housing and bridges to factories and skyscrapers.

This article has been created by Construction Manager in partnership with Tekla

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