Hard Clash vs Soft Clash: What’s the Difference?

If you are reading this article, it means you can imagine or know how much impact, in terms of time and costs, addressing an interference in the construction process only during the execution phase can have.

In BIM design, clash detection allows you to identify and resolve hard and soft clashes even before the construction site is opened.

What is a clash in BIM?

In the context of BIM (Building Information Modeling), a clash is an interference or a conflict between elements of the digital model of a building or infrastructure.

Imagine a plumbing pipe passing through a structural beam or an electrical conduit occupying the same space as a ventilation duct.

These situations are called clashes because two objects cannot simultaneously occupy the same physical space.

There are 3 types of clashes:

• Hard Clash (physical clash) – Two elements geometrically overlap.
  Example: a pipe intersects a column.
• Soft Clash (distance or clearance clash) – The elements do not touch, but do not respect the minimum distances required for maintenance, safety, or installation.
  Example: a valve is too close to a wall to be inspected.
• Workflow or Time Clash – Conflict related to construction phases or scheduling.
  Example: two teams need to work in the same space at the same time.

Why do interferences arise in federated models?

Interferences primarily arise in federated models because each discipline involved in the project develops its own model independently. Architects, structural engineers, MEP designers, and specialists indeed work on distinct models that, once federated, are overlapped within a single coordination environment.

This overlap can highlight conflicts that were not visible in the individual disciplinary models. The most frequent causes include design changes not shared in a timely manner, modeling errors, lack of coordination between disciplines, or absence of common rules for clearances and tolerances.

The complexity increases with the size of the work and the number of parties involved. For this reason, interference verification represents one of the fundamental activities of BIM coordination.

Clash detection, clash avoidance, and clash management: differences

Although often used as synonyms, clash detection, clash avoidance, and clash management refer to different activities.

Clash detection involves the automatic identification of interferences present within the federated model using dedicated software.

Clash avoidance represents the set of strategies adopted to prevent the appearance of interferences during the design phase, through shared standards, continuous coordination, and periodic checks.

Clash management encompasses the entire process of managing interferences: identification, classification, assignment of responsibilities, monitoring of corrections, and final verification of resolution.

What is hard clash?

Hard clash identifies the physical intersection between two or more objects in the model and requires a solution at the geometric positioning level.

For example, a hard clash can be an external fixture that intersects a column. The architect created the architectural model with their BIM software and positioned the fixtures based on a specific concept design, while in a second phase, the structural engineer took care of their relevant modeling.

When the federated model is generated and clash detection is performed, the physical interference between fixture X and column Y is detected. We are still in the design phase and can resolve the interference simply by modifying the positions of the objects.

Imagine the benefits of detecting this type of interference even before the construction site is opened!

Example of hard clash

Examples of hard clash in BIM

Among the most common examples of hard clash, we can cite:

• a pipe that crosses a structural beam;
• a HVAC duct that interferes with a column;
• a service walkway that crosses a wall without provisions;
• a ceiling that conflicts with above mechanical systems.

In all these cases, the interference prevents the correct realization of the work.

How to resolve a hard clash

Resolving a hard clash requires an analysis of the involved disciplines and the identification of the most efficient solution from a technical and economic standpoint.
Possible actions include relocating systems, modifying elevations, revising structural crossings, or introducing dedicated openings. Once changes are made, the model must be updated and undergo a new coordination check. It is advisable not to reach the resolution of the clash during the operational phase but to detect it beforehand during the design phase.
From the federated BIM model, in fact, using a clash detection software, the BIM coordinator performs a series of checks aimed at preventing issues during the construction phase of the work. In this case, to resolve a clash, it will only be appropriate to review the project and update the model in accordance with the other figures involved in the BIM process.

What is a soft clash?

The soft clash, also known as Clearance Clash, identifies elements with incompatible parameters that could interfere with each other in physical or functional terms. The interferences identified by this type of clash highlight model objects whose excessive proximity could generate difficulties during execution/assembly and subsequent maintenance or even hinder these same phases.
Returning to the previous example: a window very close to a column is not detected as a hard clash, because the two objects are close but do not physically intersect. At the same time, however, installing the window may prove difficult and require additional intervention such as modifying the frame. The installation of the fixture can be completed but requires an additional step, namely modifying the frame.
Identifying these types of interferences beforehand is therefore extremely advantageous in terms of costs, time, and aesthetics.

Example of soft clash

Examples of soft clash

Some typical examples of soft clash are:

• insufficient space for the maintenance of a valve;
• a reduced distance between an electrical panel and a wall that does not allow accessibility by an operator;
• the inability to fully open a technical door;
• the absence of safety zones required around certain equipment.

Why soft clashes are harder to detect

Unlike hard clashes, soft clashes do not produce an obvious geometric collision. Their identification requires the prior definition of control rules based on distances, operational spaces, and regulatory requirements.

Without adequate parameters, these interferences may go unnoticed until the construction or maintenance phase.

How to set rules and tolerances to detect soft clashes

To correctly identify soft clashes, it is necessary to define a clash matrix that establishes minimum distances, buffer areas, and tolerances for each category of elements.

The rules can be based on technical standards, client requirements, or company standards. BIM clash detection software allows for the creation of specific checks that automatically and directly signal on the model the non-compliance with the established distances.

What is the difference between hard clash and soft clash?

The substantial difference between the two types of clash detection lies in the severity of the identified interference:

• the hard clash identifies a physical collision between two objects in the model;
• the soft clash alerts on excessive proximity between two objects that could generate problems during execution/installation or maintenance.

The third category of clash: Workflow Clash

There is a third type of clash, less straightforward than the first two: the Workflow Clash. Also known as Clash 4D, this check concerns temporal interferences, manifesting when the installation or construction of two objects is designed with an incorrect sequence of timing.

An example could be the installation of furniture of particular dimensions, whose installation must necessarily be carried out before the assembly of a glass wall.

Examples of workflow clash

Among the most frequent examples, we find:

• two teams that need to work simultaneously in the same area;
• the installation of systems scheduled before the completion of the structures;
• the simultaneous use of the same equipment by multiple companies.

What are the benefits of Clash Detection?

The benefits of BIM Clash Detection are observable, as with all applications of the BIM methodology, in a considerable gain in time and costs.

Working with a BIM model allows for the verification of interferences and conflict management:

• substantially automatically through specific BIM clash detection software
• in the design phase, even before the construction site is opened and materials have been ordered.

For these reasons, the advice is to try the BIM clash detection software for free for a month, activating the free trial month directly from the usBIM store.

How clash detection works in the BIM process

Clash detection follows a structured operational flow that allows for the systematic identification and resolution of interferences.

1. Federation of disciplinary models

Architectural, structural, and plant models, created separately by different disciplinary specialists, are aggregated into a single federated model to facilitate coordination checks. It is always advisable to perform checks on the individual models before federation. Clashes can indeed occur due to design errors.

2. Definition of control rules

Following federation, in a clash detection software, the rules are set that determine which elements need to be compared, with what criteria, and according to which type of clash (hard, soft, etc.).

3. Automatic analysis of interferences

The software scans the federated model and identifies all possible clashes.

4. Classification by severity and priority

The interferences are visualized directly on the 3D model, analyzed, and classified based on their impact on the project and the urgency of resolution.

5. Clash report, issue tracking, and resolution

The clashes are documented through reports and assigned to responsible parties via issue tracking systems until complete resolution.

Who manages hard clashes and soft clashes in the BIM project?

The management of interferences involves various professional figures that operate in a coordinated manner, particularly the BIM Coordinator who is the executor and responsible for coordination checks.

The role of the BIM coordinator

The BIM Coordinator carries out interdisciplinary coordination activities, organizes clash detection checks, and monitors the resolution of interferences.

The role of disciplinary designers

Disciplinary designers analyze the assigned clashes and update their models to eliminate the identified conflicts.

The role of the BIM manager and CDE

The BIM Manager defines procedures, standards, and information requirements. The Common Data Environment (CDE) ensures the controlled sharing of models, reports, and issues among all project participants.

FAQ about Clash Detection in BIM

Below are the answers to the most frequently asked questions regarding interferences in BIM models and clash detection processes, useful for understanding how to prevent errors and optimize design.

What is a clash in BIM?

In the context of BIM (Building Information Modeling), a clash is an interference or conflict between elements of the digital model of a building or infrastructure. It occurs when two objects cannot simultaneously occupy the same physical space.

What are the types of clash in BIM?

There are three main types: hard clash (physical overlap between elements), soft clash (non-compliance distances between elements), and workflow clash (conflicts related to the timing of activities).

Why do interferences arise in federated models?

Interferences arise in federated models because each discipline works on separate models that are then overlaid. This can lead to conflicts due to lack of coordination or unshared changes.

What is the difference between clash detection and clash management?

Clash detection automatically identifies interferences in the model, while clash management encompasses the entire process of managing, assigning, and resolving them.

What is a hard clash?

A hard clash is a physical interference between two elements of the model, such as a pipe passing through a structural beam. It requires a geometric modification to be resolved.

What is a soft clash?

A soft clash indicates insufficient distance between elements that do not physically touch but may create installation, maintenance, or safety issues.

What are the benefits of clash detection?

Clash detection allows for the identification and resolution of interferences in the design phase, reducing costs, time, and problems on site through the use of coordinated BIM models.