Clash detection – Design Stage – All projects

Clash detection is one item which many industry people lump together with BIM deliverables. But is it something which can be part of all typical projects at design stage irrelevant of being a BIM deliverable?

The below article is based on the current workflow configuration of the authoring software Autodesk® Revit® and clash analysis software Autodesk® Navisworks® (2013 products). The article is however written in a vendor neutral format. In directly applying any below mythologies to clash detection software, please contact your preferred software vendor to decide if their software solution is capable of the below approaches.

I have only come across a handful of standard projects (non-BIM  deliverable projects) which included digital virtual clash detection.  Generally it is only employed if it is a client or contractor deliverable. Within design contracts, it is a requirement of building design consultants to “coordinate and integrate” their designs with other consultants (refer to below contractual clauses **). Many believe clash detection software tools are beyond the coordination requirements and resource intensive. This is however a misconception. When the philosophy and approach is well planned, the outcomes can be tailored to meet the desired needs.

During Design Development, Contract Documentation and Contract Admin; the Architectural team spend considerable time coordinating the design consultants and answering construction queries. Large projects often contain a dedicated Architectural role to manage and coordinate the consultant’s work. Building coordination comes in many forms, and can depend on the building type and aspects precious to the design team. Architects are well aware, poor coordination leads to poor building outcomes. Architects are typically the “Lead Consultant”, thus fingers inevitably are pointed their way when things go wrong.

Traditional means of coordination is far from scientific. Visual checks and overlays are carried out at periodic times. Checks focus on the ceiling voids and end up being hit and miss. Risers, under structure services, internal storm water pipe reticulation and component height clearances, are often missed. I once worked with a senior coordinator whom I had great respect for. He would describe coordination as the process of, working through the services/structure from below the foundations, up the building to the plant room; then working the coordinated back down the building to the foundations. For manual coordination this was very thorough, but still leaves a large scope for error.

Coordination communications traditionally consist of the manual generation of screen captured images, or marked-up prints. These are then issued to consultants as items are found. On receipt of new drawings/models the previous conflict resolution are navigated to, and checked. The new drawings/models also need a complete re-checked to identify new issues. You can see, this quickly becomes very time consuming and monotonous. Unfortunately design teams just don’t have time to do the desired level of coordination. A 3D model authoring software environment does help; but again, I have yet to come across a team whom comprehensively overlays and visually checks each consultants model on receipt of new models. You still have the issues of is being a “Visual” (hit and miss) and there is no tracking. So surely, if the design consultants are generating 3D models; we can use them to digitally manage the process.

If we are to use a managed virtual digital coordination and allocate the same resourcing time, we should be able to get a better outcome!

Clash detection software is known for returning to mush information/data, when the designer only wants to focus on key items. A strategic planned approach should be adopted.

Key areas to consider:
  • Prioritise building components/assemblies by their function, movability and risk
  • Use reliable meta data to identify specific systems, assemblies and parts
  • Use Object-oriented rules
  • Predefined strategic clash tests and processes
  • Quickly identify, consolidate, mark-up and report/communicate issues
  • Track issues and resolution outcomes

Prioritise building components/assemblies by their function, movability and risk:
We will create a scale from 1 to 4 (1 being the highest priority and 4 the lowest). List all building components, and identify priority to each of them. The philosophy is generalised in the below table using an element importance order;

Element Importance Order
Priority Definition:
Potential examples:
Elements are virtually immovable.
Primary structure, large duct runs & gravity fed services
Elements can be moved but will require a high degree of difficulty and, or coordination.
Services primary equipment
Non-Gravity fed services > 100mm Ø, secondary structure & penetrations
Elements can be moved with little difficulty.
Pipes < 100mm Ø, fixtures and fittings
Elements has the lowest priority and will be moved without question if found clashing.
Suspended ceiling hangers, pipes < 50mm Ø, linings, dressings, finishes

Element priority is defined by the lead consultant. They can decide what is precious for the preferred outcome. Take into account, some traditional coordination methods will continue along anyway despite formalised clash detection (e.g. fittings in a reflected ceiling plan).

Below is a sample “possible” priority table for Hydraulics:

Hydraulic Services:
Clash Priority 1-4
Piping systems – Gravity-fed services ≥ 100mm Ø (pipes, fittings, & insulations)
Piping systems – Non Gravity-fed services ≥ 100mm Ø (pipes, fittings, accessories & insulations)
Piping systems – Gravity-fed services < 100mm Ø & ≥ 55mm Ø (pipes, fittings, accessories & insulations)
Piping systems – Non Gravity-fed services < 100mm Ø & ≥ 55mm Ø (pipes, fittings, accessories & insulations)
Piping systems – Gravity-fed services < 55mm Ø (pipes, fittings, accessories & insulations)
Piping systems – Non-Gravity fed services < 55mm Ø (pipes, fittings, accessories & insulations)
Plumbing Fixtures
Plumbing Equipment

During design stage clash detection; the decision may be made to only test items with a priority 1 and 2. The method clearly identifies what needs to be resolved first.

Reliable meta data to identify specific systems, assemblies and parts:
I have come across many professionals whom believe clash detection is not BIM; your just clashing 3D geometry. Clash detection is a prime example of “rubbish in; rubbish out”. To carry out “effective and efficient” clash detection; quality and reliable data is needed from the BIM. When carrying out clash detection, I request the models to have the following element data from the consultants:

Authoring identification
Who created the element? Many consultants model pipes (can be with the file name)
Element Classification
Specific identification of category
Pipe/Fittings Diameter
Pipe and pipe fitting sizes
Insulated pipe identification
Ability to identify pipes with insulation
Duct Size
e.g. identify of Duct sizes e.g. ≥750mm in size
Insulated duct identification
Ability to identify ducts with insulation
Structural material type
Identify; steel, precast, concrete etc…
Service system type
Distinguish between, supply, return, cold, hot, gravity-fed & non gravity-fed pipe systems
Service System name
Identify group of components creating the unique system. i.e. Supply are for level 2.
Element Room Data
Identify the room/space name and number equipment is within. This assists in reporting the location of clashes.

None of the above is onerous, and most competent firms (claiming BIM knowledge) are producing BIM’s with this Meta data all ready. Much of the above is automated by the authoring applications if used correctly.

Object-oriented rules;
There are very few true construction “Object-oriented” clash detection software’s available (the computer gaming industry has used it for several years now).
Object-oriented is where classified objects have specific associated rules. E.g. in many BIM compatible authoring applications,
       Walls: can have penetrations, define room extents and cannot be horizontal,
      Furniture: cannot have penetrations, can be allotted to a room and are always vertical

Within Clash Detection, rules can remove irrelevant clashes and create a pseudo object-oriented environment. Some examples:
  • Ignore clashes of parts, if a previous clash is found within the parent assembly: i.e. A beam is clashing with a door assembly (both leaf and frame geometry), only report one clash
  • Ignore clashes between parts, if they are within the same unique system: i.e. In a service system, if due to poor modelling a duct branch intersects with the parent duct, ignore it
  • Ignore clashes if an element has a specified value: i.e. ignore duct clashes, if the duct is a flexi-duct.
  • Ignore parts which have an external wrapping: i.e. ducts and pipes are often wrapped in insulation. If it is modelled with the insulation, ignore the wrapped component
  • Ignore parts if a specific numerical parameter value is less than a specified number: i.e. Ignore pipes less than 90mm diameter in size

I do look forward to the day when clash detection is truly object-oriented.
E.g. a pipe is clashing with a floor slab:
the algorithm determines the slab is horizontal and the pipe vertical. A role states a slab core drill penetration size up to 150mm is acceptable; thus the clash is not reported.
However if the pipe run is parallel to the slab the clash is reported.

Predefined strategic clash tests and processes;
Comprehensive clash test templates are prepared and documented, allowing all team members to clearly understand outcomes. Sample clash test:

Structural Elements
Hydraulic Elements
Rules – Ignore items in:
Floor Slab >= 350mm Thickness
Framing & trusses
Mechanical Equipment
Gravity-fed piping systems <= 100mm Ø
Same File
Same Assembly object
Previously found pair of assembly objects
have Insulation Defined

Explanation of above test:
      Name – Clash test name. Identifies discipline, priority of component and clash tolerance
      Clash Type – is it a clearance test or physical intersection
      Rules – Rules to remove false positives and irrelevant objects.

Quickly identify, consolidate, mark-up and communicate issues:
Following running the test and identification of clashes; they are to be quickly consolidated into specific issues (groups). E.g. a run of two parallel pipes clash a single perpendicular duct. Most software applications will report two separate clashes. The consolidation of this into one reportable item is somewhat manual, but it must be a quick and easy process.

Following clash consolidation; an ideal viewpoint is captured and the clash area marked up.
The report needs to be concise. It should include:

  • Marked up Image of clash, clearly showing context and adjacencies
  • ID number and name of conflicting components
  • Location of clash – Level, Grid coordinates and room number where possible
  • Identify disciplines involved and identify party to lead the issue resolution
  • Identify priority of components involved
  • General Comments

Track issues and resolution:
All BIM authoring software applications generate a unique identification (ID)  number for each component. When components are modified, the ID number should remain unchanged. Clash detection applications use and track this ID number; thus, when models are updated, the application reports the updated clash status.

Another BIM myth is 100% of all clashes will be resolved. This is just not necessary at design stage. The point of the tool is to test the design and ensure proof of concept. Thus; if systems clash and there is ample adjacent free space to easily relocate then (and will not affect design requirement), the clash is approved. If there is no quick/simple fix, then the clash is to be addressed.
Bear in mind we are in the design stage.  The approach at construction stage is slightly different. During the construction, the subcontractors will:

  • convert all the generic equipment to manufactured components
  • tweak solutions to better fit their approach
  • value manage
  • apply construction and OH&S tolerances

Thus, the design intent model and constructed building are two different things. Like everything a balanced approach is always best. Clash detection tools are quite simple to use. Strategically planning the approach is another thing. When implemented at design stage correctly, it will greatly improve all outcomes. 

The next item is; how do you get buy-in from the design team? Again remember, we are just using a tool to more efficiently find and track what is already occurring. Below is a part sample “Request for Tender” (These will vary, and thus please refer to your project specific contract). The sample below also specifies: Finalise design with full coordination with all other design team members”. Monthly reports are commonly now part of projects. The clash detection summary report (including resolution status) can be an attached appendix. Clients love to see these and are well able to act when adequate design resolution is perceived to be not occurring. From a legal point; if a design issue is raised, but not addressed, and this in-turn leads to costs incurred by the client; the client may have the right to pursue the negligent party.

When it comes to BIM deliverables, clash detection is one of the easiest a client can specify. All the above methods are 100% scalable, so when the client does require a “high level” Clash Detection deliverable at design stage; it has minimum impact on resourcing, workflows and could potentially be very profitable for the lead consultant. The above text is just scraping the surface in regard to “strategic clash detection”. But is should provide ample information to start your investigation and seek potentially further help.


References: (all direct quotes are in italics)
Client and Architect Agreement – AN10.01.131 – Australian Institute of Architects - 2009
Clause A. Core Architectural Services: (i.e. required professional services)
-Coordinate and integrate the work of other specialist consultants
- Coordinate the construction services provided by other specialist consultants
The architect must.
d.  coordinate and integrate the work of all specialist consultants engaged for the project;

Below extract from a "Sample Part" design consultants Request for Tender I have come across;
Request for Tender (RFT): Hydraulic: (N.B. please refer to your project specific contract documentation)

General Services
 Allow for attendance and participation at fortnightly consultant coordination meetings during the schematic design, design development and documentation phases of the project. During peak periods of the project these meetings may be weekly.

Design Development
 Expand and develop schematic up to Design Development status. Prepare design development drawings in CAD format. Allow for coordination with the Consultant team during the design development process.
Finalise design with full coordination with all other design team members. Allow for peer review and cross referencing. Provide evidence of this taking place if requested by the Project Manager.

Full Documentation and Tender
Provide full, complete, coordinated set of design documentation relevant to the consultant’s discipline. 

1 comment:

  1. Another great post Brian. Thank you for sharing your knowledge.


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