What's the Difference Between Clash Detection and Variance Reporting?
A short guide to what these construction technologies do.
Both clash detection and variance reporting are great software tools that find problems ahead of time, that would be costly to discover later in the process. Neither technology existed just 20 years ago, and both provide far more accuracy and speed than manually looking for the problems through human observation. They both improve efficiency and thus profit for the contractor.
Aside from these commonalities, they’re quite distinct. Clash detection and variance reporting are used in different stages of the AEC process. And neither replaces the other.
Clash detection is done in the building design phase. It finds issues before construction; in particular, places where components from two trades inadvertently intersect. The software compares the 3D volumes of the trade components to find these intersections in the BIM model.
- For example, an electrical conduit in the BIM model might pass through a return air duct. The components occupy the same space in the design, which would be impossible to actually build.
By finding intersections in the design phase, the trades can coordinate how to resolve the intersection in the design before construction begins.
This prevents delays (RFIs, change orders, rework costs) that would happen if a clash wasn’t identified until after one of the trades installed their components and then another trade discovered that it was installed in the same location that they need to use.
What is variance reporting?
Variance reporting is done in the building construction phase. It finds installation issues; in particular, it finds differences between what was designed and what was built—either something was incorrectly installed in the wrong place, or it was missed and not installed at all.
The software compares the BIM or CAD files (the design intent) with the reality scan point cloud (the actual construction) and identifies variances between the two.
- For example, the location of a gas pipeline installed at the job site might be ten inches away from its specified location in the BIM model.
By finding construction discrepancies right away, the problems can be assessed and fixed while the crews are still on the job site when it’s far easier to do the rework than if the problem wasn’t identified until after the building was closed up. This saves substantial time and the cost of wasted materials and labor, schedule delays, and liabilities.
When are clash detection and variance reporting used?
The best way to understand the difference between the two is to look at which building phases they apply to.
Clash detection software is used during design coordination to find conflicts within the BIM model or CAD drawings.
Variance reports are typically used during the construction phase, to find differences between what’s specified in the design model and what’s built in the field.
Variance reports are also used after construction, to identify differences between the building today and the most recent construction documentation. For example, prior to making design changes for a new tenant, a building owner may want the current construction documentation to be updated to accurately reflect any changes in the building that weren’t previously documented over the life of the building.
What types of clashes and variances can be detected?
Clash detection and variance reports both excel at finding all the location differences of components at a point in time, and variance reporting can find some other differences as well.
➤ Clash detection identifies design intersections between MEPF trades.
➤ Variance reports identify discrepancies between design intent and construction for MEPF trades, as well as architectural, structural, and process piping. Contractors are also using variance reports to identify missing or mis-aligned concrete in pre-pour components. GC’s are also using variance reports to find omitted or mis-aligned concrete floor components like hangers, embeds or inserts before a concrete pour is scheduled.
➤ In addition, variance reports can be used to report new variances such as an increase in beam/column deflection under load, or an increase in concrete crack length or width in a wall or floor.
➤ Variance reports can also take the form of heat maps based on floor elevation data from the construction to report on floor flatness and floor levelness differences.
Before clash detection and variance reporting software were available, the construction industry was resigned to paying the high costs of rework as “a cost of doing business.” Today’s tools make it possible to cut those costs while delivering a higher quality building to the owner.
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