Clash Detection and System Routing Priority: Hard Clashes, Soft Clashes, Gravity-First Routing, and Resolution

How architects identify and resolve conflicts between building systems using BIM-based clash detection, distinguish between hard and soft clashes, apply gravity-first routing hierarchy, and coordinate resolution across disciplines during project development and documentation.

ARE · Project Development & Documentation

Overview

Every building contains dozens of systems fighting for the same space. Structural beams, ductwork, plumbing risers, electrical conduit, fire suppression mains, and specialty systems like medical gas lines all need to fit within the same ceiling plenum, the same wall cavity, the same vertical chase. When two of these systems try to occupy identical space, that's a clash. And if nobody catches it before construction, the field fix costs real money and real schedule time.

Clash detection is the BIM-based process of identifying these spatial conflicts before a single bolt gets tightened. The VA BIM Manual describes it as one component of "coordination management," a broader process that ensures adequate space for equipment layout, accessibility, and maintenance while resolving spatial interferences before fabrication and installation.

But not all clashes are created equal. A duct punching through a steel beam (hard clash) demands a completely different response than a fire sprinkler head sitting too close to a supply diffuser (soft clash). And when you're deciding which system moves, gravity-driven systems like sanitary waste always get priority over pressurized systems that can route around obstacles.

For the PDD exam, you need to understand how these conflicts get identified, classified, and resolved across disciplines. This isn't about running the software. It's about knowing which system wins when space runs out.

Essential

Coordination management is the umbrella process that keeps building systems from colliding. The VA BIM Manual puts it plainly: the commonly used term "clash detection" is just one part of coordination management. The full process includes ensuring adequate space for equipment and systems, verifying accessibility for maintenance, and resolving spatial interferences before anything gets fabricated or installed. Two parties share this responsibility across a project's life. The architect/engineer (AE) team handles Design Coordination using consultant models. The general contractor (GC) takes over Construction Coordination using sub-trade models during their phase. Both need model elements at sufficient development levels to make coordination meaningful. ## Hard Clashes vs. Soft Clashes A hard clash occurs when two modeled elements physically occupy the same three-dimensional space. Think of a 24-inch supply duct running directly through a W12x26 steel beam. Both objects claim the same coordinates in the model.

Professional

Consider a healthcare facility project where the architect leads coordination of a 12-foot floor-to-floor height with an interstitial plenum serving patient rooms. The structural engineer has placed W18 beams at the column lines. The mechanical engineer routes a 36-inch by 18-inch supply duct trunk along the corridor, and the plumbing engineer runs a 6-inch sanitary waste line with a required 1/8-inch-per-foot slope from the patient bathroom group back to the main riser. During the first coordination review at the DD milestone, the BIM coordination run reveals 47 hard clashes and 23 soft clashes in a single corridor section. The sanitary waste line passes directly through the bottom flange of two W18 beams. The supply duct trunk conflicts with a fire sprinkler main at three crossing points. And twelve sprinkler heads fall within 10 inches of supply diffusers, violating the manufacturer's 18-inch minimum clearance requirement.

TLDR

Hard clash: two model elements occupy the same 3D space (duct through beam). Binary conflict that must be resolved.
Soft clash: elements don't intersect but violate required clearance zones (sprinkler head too close to diffuser, insufficient panel working space).
Workflow/4D clash: sequencing conflict where construction timing creates the interference, not geometry.
Gravity-first rule: slope-dependent systems (sanitary waste, storm drains) route before pressurized systems (supply water, ductwork, conduit) because rerouting gravity lines requires slope recalculation across the entire run.
Routing priority (high to low): Structure > gravity drainage > large ductwork > fire protection > plumbing supply > hydronic piping > electrical conduit > low-voltage.
LOD 300 minimum for design coordination; LOD 350 minimum for construction coordination. Lower LOD produces unreliable results.
AE owns Design Coordination (consultant models). GC owns Construction Coordination (sub-trade models). Don't mix these up.
BIM Execution Plan defines tolerance settings, responsible parties, software versions, and resolution workflow for the entire project.
Resolution cycle: Run > Filter > Classify > Resolve by priority > Re-run to verify > Document.
Resolving one clash often creates another. Expect three to four resolution cycles per coordination zone.

ELI5

Imagine you're building a LEGO house. You've got one friend building the walls, another building the plumbing pipes, and a third running electrical wires. Each friend builds their part separately, and then you try to put everything together. Sometimes a pipe goes right through a wall where there's no hole. That's a hard clash. Two pieces trying to be in the exact same spot. Something has to move. Other times, the pipes don't actually touch the wires, but they're so close together that you couldn't fit your hand in to fix either one if something broke. That's a soft clash. Nothing is crashing into anything, but there's not enough room to work safely. Now, here's the tricky part. When something has to move, who moves? Think about water slides at a pool. Water slides have to go downhill because gravity pulls the water.

AREProject Development & Documentation

Clash Detection and System Routing Priority: Hard Clashes, Soft Clashes, Gravity-First Routing, and Resolution

2 min read217 words

Why Clash Detection Keeps Buildings from Failing Before They're Built

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