Understanding clash detection and making it more efficient

In the world of BIM, clash detection is the terminology used for automating collision finding prior to construction issues on field during design and preconstruction. This activity fits in a constructability analysis used to find issues that leads to potential difficulty, cost overrun and productivity loss in the actual construction. Clash detection is available in a number of software such as Solibri Model CheckerNavisworksTekla BIMsight, Vico Office Constructability Manager, ArchiCADRevit and more. Some software have advanced features (Like Solibri) and offer the ability to create rules and some only offer basic features (Like Revit).

What is a Clash Test?
A test is a predefined set of elements which have to be checked for collisions. There are usually 2 types of validations, intersecting and clearance check (sometimes called “soft check”). An intersecting clash is a collision of geometries of 2 objects. A clearance check is a collision between predefined buffer around these geometries. The buffer can be set in some clash detection software or modelled in the BIM object itself as shown below for mechanical equipment maintenance. It can also be used for doors accessibility clearance, stair run clearances, precast concrete panel installation clearance, steel truss deflection and more.
Clearance box

Figure 1 Clearance box from blog.bradleycorp.com

A best practice is to plan your clash tests and to create what is commonly called a “clash matrix” to define combinations of disciplines and priority based on the construction schedule and deliverable. Chronology of clash tests is crucial in reducing iterations between model updates.

clash matrix

Figure 2 Clash Matrix from bimstar.co

What is a Clash Rule?
A clash rule is some sort of formula used to select specific objects that needs to be included in a clash test. It may be important to check the consistency of model tree if the clash rule is using it to select objects. A good way to make naming convention for object and parameters is to use the Industry Foundation Classes (IFC) by buildingSMART for model sharing.

A clash rule can also include Boolean expressions such as and/or or even specific exclusion criteria.


Figure 3 Clash rule from Navisworks

What is a Clash Report?
A clash report is a document shared with stakeholders on a given project using PDF, XML, HTML or other type of format. It is the “traditional” way of sharing issues found with the clash detection software and requires some extra time to set up and track problem solving over time.

clash report

Figure 4 Solibri clash report from solibri.wordpress.com

How to transfer the issues more efficiently?
Using a “traditional” report is not the most efficient way of transferring data to the users in context in the modeling software where the issues need to be solved. BIM Collaboration Format (BCF) an open format developed by buildingSMART is very good at this because it saves the users the time required to identify the location of the clashes in the building or the time required to find an appropriate view when switching from one software to another. BCF is the key to optimal clash detection and should be used by anyone doing clash detection. There are also platforms specializing in issue tracking such as BIM Track who can help transfer issues across multiple platforms.
open bim workflow

How to fix a clash?
A way to reduce mundane modeling tasks which results in tons of clash found by the clash detection software is to use script based modeling automation solutions such as Dynamo. Let’s say you have a HVAC system going through a fire rated partition and you need to add a special accessory at this location, you may use Dynamo to place the accessories automatically at these locations and create an opening in the wall automatically in Revit.
dynamo clash
Figure 5 Dynamo script posted by Modelical on LinkedIn.

An other growing trend is to use clash-free auto-routing with computational design such as GenMEP. The core feature of GenMEP is the ability to use algorithms to route MEP systems without clashes automatically during the design process.
GenMEP clash
Figure 6 MEP routing from GenMEP website.

Clash detection have been available for a long time but what makes it valuable is the process you define around it. Using software out-of-the-box is not an optimal solution to avoid issues on time and without considerable efforts. Ask yourself what are the common issues you encounter on site, set you clash tests accordingly and then start checking you models for these specific cases at the right moment according to the schedule and prior to the job site. Please do not run clash detection with an entire model, review 10 000 “false-positive” clashes and say that clash detection is not valuable.

Better coordination = Better buildings