#BS1192 considerations to understand when developing a Volume Strategy and #clashdetection.

There are a number of questions posed within this topic which cover the following areas; what is the definition of a volume and volume strategy; additionally what is a clash, what clash types are there and what is the difference between clash detection and avoidance. Finally the best strategy to mitigate and avoid these is requested to be answered, these will be responded to throughout each section with suggestions for improvements.

Volume and Volume strategy

The primary source of information on a ‘volume’ is PAS 1192:2 which states that a volume is a “manageable spatial subdivision of a project, defined by the project team as a subdivision of the overall project that allows more than one person to work on the project models simultaneously and consistent with the analysis and design process” (BSI, 2013, p.6). It goes on to clarify that to confirm that the lead designer is required to allocate the volumes within the project at an early stage (BSI, 2013, p.6).  This therefore confirms that a volume will be project specific and therefore could be multiple and varying from project to project depending on the type; e.g. a school will be very different from an oil refinery.  PAS 1192:2 also gives cautionary advice that the strategy needs to be thoroughly thought through (BSI, 2013, p.20) and this should be confirmed by all of the design team who must ensure that agreement of these volumes is clear at the start of a project.  Additionally there is a note that the volume diagram should be published as a shared document (BSI, 2013, p.20), which appears very difficult to do at an early stage as there may be no more than massed objects at this stage with which to complete this.  Further to this the AEC BIM Protocol document states that the “model structure shall take into account, and be agreed by all internal and external disciplines to be involved in the modelling and documented in the BEP” (AECUK, 2015, p.23).

It appears to resolve this then that the volume strategy should be updated throughout the project, alongside the Building Information Modelling Execution Plan (BIMEP), which is a live document. This would mean that at each RIBA work stage, from stage 0-5, the volume strategy will illustrate volumes from a macro to micro level of detail.

The definition of a volume will be project specific as “a volume may be based on important aspects of design, such as structure or cores, on specialised functions such as HVAC systems or on strategic elements such as cladding” (BSI, 2013, p. 20). This means that for a school a volume may be a services route through a ceiling circulation void and for a hotel may be a very specific servicing element from one location to another.  Additionally, it is worth considering that this would not only be relevant for buildings but other infrastructure and asset types that will be different again (e.g. highway or rail infrastructure) not just buildings.

For clarity between the BS 1192 (BSI, 2007), which provides information for 2-Dimensional and the start of 3-Dimensional project processes, and PAS 1192:2 which provides information for 3-dimensional project processes, PAS 1192:2 states very specifically that “volumes may overlap in 2-dimensional space (plan view) but they cannot overlap in their 3-dimensional space such that clash avoidance is achieved” (BSI, 2013, p.20). This note ensures that when developing a volume strategy it is clear that a diagram should be provided for information to the team and this will give confidence that the agreed volume strategy is going to minimise clashes.  Race (2015) proposed that techniques to define spaces, space area and space planning techniques are required to create initial layouts that fulfil programme needs and meet planned budgets, this is similar to the concept of volume strategy.

Richards (2015) also confirmed that a clash avoided building should be present from inception of a project and that no discipline should touch other peoples models or volumes. His view is that clash detection should be abolished in favour of clash avoidance. Richards (2015) confirmed that at an early stage the design intent model should be identified by the design lead and show the volumes required to define the floor mass; for example: floor construction and services void.  In this example, the initial owner of all volumes will be the architect, therefore, the initial owner of all the volumes is the architect.

At Stage 3 now that the design is progressed to suitable level of detail the volumes can be transferred to consultants at this time. This means that concerns regarding transfer of the model to other consultants and concurrent use being made of a model should be mitigated by proper model management and appropriate stage authorisation sharing.

To further support the early adoption of a volume strategy Gleeds have stated that within the Employer’s Information Requirements (EIR) the BIMEP Requirements should be confirmed by the supplier to include their coordination and clash avoidance processes in regard to volume strategy, tolerance strategy, technical query workflows, responsibilities for coordination and clash avoidance, software to support coordination and clash avoidance, outputs from coordination and clash avoidance processes (Gleeds.com, 2015). This approach thereby places the onus on the employer to ask for a complete and full volume strategy and clash avoidance strategy from their initial pre-contract BIMEP which will strengthen the creation of a process that reduces clashes between disciplines.

Clashes and clash categories

A clash is typically described as either a ‘hard’ or a ‘soft’ clash, Hardin states that “A hard clash is the physical intersection of two 3D components, whereas a clearance clash will report if components within a specified dimension of each other.” (Hardin, 2009, p.131).  To further support this Tommelein and Gholami (2015) state a “‘hard clash’ refers to one building component physically yet unintentionally penetrating another building component” and a “’soft clash’ … refers to components (subsystems) that are closer than a certain distance (a minimum clearance) from one another (e.g., distance in-between outer cylindrical surfaces of two pipes)”.  However, there is greater complexity to this as Aubin and McClelland (2014, p. 158) in their book on BIM Collaboration states four types; hard, hard (conservative), clearance and duplicates, when applied in Autodesk Navisworks.  A hard clash is the same as above, but a hard (conservative) clash is two items intersectioning even though the geometry triangles do not.  A clearance clash is when two items come within a specified distance of each other and defined by the user.  Duplicates refers to when two items are the same and in the same position. Mangan (Virtual Build, 2010) also states that there are 4 types of clash; hard clash, soft clash, time dependant clash (something that is happening in a sequence that can’t happen in reality, Tommelein and Gholami (2015) also agree with this and make the point that the constructability should be considered an additional clash type), automated clashes, manual clashes (driven by experience of operator) and incorrect annotations, a situation where the drawings are creating incompatibilities (e.g. schedules contradicting the model). This means that for clash detection or avoidance there are a number of different types that may need consideration and planning for during the project duration.

Clash Detection and Clash Avoidance

It has been argued that clash detection is a reactive approach, completed after a design stage has been occurred. Bond Bryan Architects (2015) confirm that their definition of clash detection is “detecting possible collisions between elements in a building information model which would not otherwise be desired or buildable on site” (Bond Bryan 2015, p.5). In contrast, clash avoidance is a proactive approach that refines the design until the final issue for construction reduces clashes prior to issue.  Tommelein and Gholami (2015, p. 2) describe the verb to clash as “the practice of identifying clashes in an integrated BIM”. The concept of an integrated BIM here appears to confirm that the approach of working collaboratively at an early stage is required to ensure that avoidance is achieved by working together throughout the project.  Tommelein and Gholami, go on to state that they found that the construction industry has a tendency for clashes to be discussed somewhat arbitrarily as errors and omissions which are included and they may not be true clashes (2015, p.3).  A verifiable clash should be a clash that involves a “system component designed/modelled/built by different parties … (that) require extra, interdisciplinary coordination and conversation” (2015, p. 3).  They go on to explain that their view is that clashes are an illustration of waste in the production system, which could be a reflection of a poorly managed and planned project at inception but also throughout the project timeline.  Therefore it appears that design rules for a project, or volume strategy which is continually updated, and adoption of lean processes continually could reduce the need for clash detection thought-out the project. An issue to this ideal position is that today’s projects are complex and reduced timescales are an issue, this leads to designers working to the shorter timescales without the previous planning that should be in place to limit clashes.

All of the clash types above illustrate that a clash is something that is unintentional or not designed and therefore leads to unexpected results. A mitigation strategy, although somewhat ideal, however could be achievable, depending on the project, would be for some level of colocation on a project, this is suggested by Tommelein and Gholami (2015) and would allow for clearer lines of communication, forward planning together, and a strengthened team ethos on a project.  The potential for a reduction in clashes could be seen from this colocation in practice.  At Hampshire County Council as the Property Services Department is multidisciplinary, this approach has been taken for architect, interior designers, landscape architects and Mechanical, Electrical and Plumbing Engineers collocating during intense periods of the project.  The success of this has been demonstrated with a well-connected team and positive clash reduced results.

Therefore, clear planning, continual updating of the BIMEP and volume strategy, open lines of communication and the potential for collocating would appear to assist with a reduction in clash detection required and lead to an approach of clash avoidance over the lifecycle of a project.


AECUK, (2015). AEC (UK) BIM Protocol. [online] Available at: https://aecuk.files.wordpress.com/2012/09/aecukbimprotocol-v2-0.pdf  [Accessed 12 Nov. 2015].

Aubin, P. F. and McClelland, D. (2014) BIM Collaboration with Autodesk Navisworks: Part of the Aubin academy master series covers version 2015. United States: Createspace

Bond Bryan Architects, (2015). BIM Dictionary. [online] bimblog.bondbryan.com. Available at: http://bimblog.bondbryan.com/wp-content/uploads/2015/05/30004-BIM-dictionary.pdf [Accessed 12 Nov. 2015].

British Standards Institute, (2015). BS 1192:2007+A1:2015 Collaborative production of architectural, engineering and construction information – Code of practice. London: BSI

British Standards Institute, (2013). PAS 1192:2 Specification for information management for the capital/delivery phase of construction projects using building information modelling. London: BSI

Eastman, C. (2011) BIM Handbook: Guide to Building Information Modeling for Owners, Managers, Designers, Engineers and Contractors, Wiley-Blackwell, 2nd Edition 2011, Hoboken.

Gleeds.com, (2015). Thinking about BIM: Employers Information Requirements. [online] Available at: https://www.gleeds.com/assets/Global/DownloadThinking_about_BIMpaper_15.pdf?1427120813 [Accessed 12 Nov. 2015].

Hardin, B. (2009) BIM and Construction Management – Proven tools, methods, and workflows. 1st edn. Indianapolis. IN Wiley Technology Pub.

Tommelein and Gholami, (2015). ROOT CAUSES OF CLASHES INBUILDING INFORMATION MODELLING. [online] IGLC20. Available at: http://www.iglc20.sdsu.edu/papers/wp-content/uploads/2012/07/13%20P%20008.pdf [Accessed 10 Nov. 2015].

Virtual Build, (2010). BIM Clash Detection. Available at: https://www.youtube.com/watch?v=gTGquTcmScU [Accessed 13 Nov. 2015].


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