What is the difference between a trip and a slip?
A trip and a slip are different events: in a slip, the foot travels forward or sideways and the body's mass is left behind it, while in a trip, the foot stops and the body continues over it. The fall direction, the injury pattern, and the way witnesses describe what they saw all differ between a trip and a slip, and the mechanism usually declares itself early.
The distinction between a trip and a slip matters practically. A geometry case, one that turns on the shape of a stair or walkway rather than its traction, does not require the walking surface to be tested in its incident condition. A stair riser measured a year later is still the riser that existed, provided nothing has been rebuilt in between.
Why are stairs so unforgiving of an inconsistent step?
Stairs are unforgiving of an inconsistent step because descending a stair is largely open loop: after the first two or three steps, a walker stops looking at each tread and lets a learned rhythm place the foot. That learned rhythm is efficient, and it works precisely because the steps are the same as one another. A single riser that differs from its neighbors breaks the walker's assumption at the moment the walker is least able to correct for it.
The open-loop way people descend stairs is why stair uniformity provisions exist alongside absolute dimensional limits, and why a stair can be non-compliant on uniformity while every individual step falls inside the permitted range.
What limits apply to stair risers and treads, and which code governs them?
Model codes cap stair riser height and set a minimum tread depth, with the International Building Code applying tighter limits to most occupancies than the International Residential Code applies to dwelling units. Both the International Building Code and the International Residential Code also limit how much the risers and treads within a single flight may vary from one another, and that tolerance is small.
The governing text for a particular stair is the code the jurisdiction adopted when the stair was built or last altered, with local amendments. Existing stairs may be lawfully non-conforming to current code editions, so construction and permit history is part of the analysis. In workplaces, OSHA's walking-working surfaces rules at 29 CFR 1910 subpart D apply alongside the building code rather than instead of it.
Why does the nosing on a stair matter in a fall?
The stair nosing matters because the nosing, at the leading edge of the step, is where the foot lands on descent. Building codes address nosing projection, the nosing's radius or bevel, and whether open risers are permitted at all.
A worn, rounded, loose or damaged stair nosing changes the effective tread depth and the visual edge cue simultaneously. The nosing is also the part of a stair most often repaired quietly after an incident, which is reason enough to photograph stair nosings early.
What role does a handrail play in a stair fall?
A handrail is the recovery mechanism on a stair, not primarily a guide: it is what a person reaches for once balance is already lost. That is why handrail standards address height above the nosing line, continuity along the run, extensions past the top and bottom risers, clearance from the wall, and a graspable cross-section. The ADA Accessibility Guidelines set the handrail height range most jurisdictions have followed into their building codes.
A stair handrail too large in section to be encircled, interrupted at a newel post, or ending at the top riser instead of extending beyond it can be entirely present and still unavailable at the instant it was needed.
Why are small changes in level on a walkway a trip hazard?
Small changes in level are a trip hazard because an elevation change too small to be conspicuous is still large enough to catch a toe at walking pace. Heaved sidewalk joints, settled pavers, thresholds and floor-covering transitions all live in exactly that range.
Accessibility standards treat vertical changes in level in tiers: very small ones are permitted as they are, intermediate ones must be beveled at a limited slope, and anything beyond that requires a ramp. ASTM F1637, the practice for safe walking surfaces, addresses the same problem of small changes in level for walkways generally, including single steps and abrupt transitions.
Why are transitions between floor surfaces a fall hazard?
Transitions between floor surfaces are a fall hazard because, where carpet meets tile or an exterior walkway meets a lobby floor, two things change at once: the traction and the geometry. A proud transition strip, a curled mat edge, or a gap at a threshold introduces a trip hazard in the same place a walker is already adjusting to a different surface.
Building entry areas concentrate both the traction problem and the geometry problem of surface transitions, and entry areas deserve their own survey.
Why does lighting matter in a trip or stair fall case?
Lighting matters in a trip or stair fall case because a stair or walkway geometry defect is only avoidable if it can be seen. Illuminance at stair nosings and at changes in level, the contrast between tread and nosing edge, glare from a low sun or a wet surface, and abrupt transitions from bright to dim all determine whether a hazard is perceivable in time to adjust a step.
Lighting readings taken under the conditions that actually applied — the same time of day, the same lamps in service, blinds and fixtures as found — belong in the same site visit as the dimensional work on the stair or walkway.
How is a stair and walkway geometry survey done?
A stair and walkway geometry survey measures every riser and tread in the flight rather than a representative sample, at consistent points across the width, since stairs are frequently out of level side to side. The survey also takes handrail height at several stations along the run, nosing projection and condition step by step, and slope and cross-slope on the approach walkway.
The output of a stair and walkway geometry survey is a table, and its value is that it is reproducible. Another engineer with a tape, a level and the adopted code can repeat the survey and get the same answer, which is why geometry findings tend to be harder to dislodge than friction findings.
This article is general technical orientation, not a failure analysis, an engineering opinion, or advice on any specific matter. Determining the cause of a particular incident requires hands-on examination by a credentialed expert.