People build buildings. However, being human means that nothing we do is perfect. With this thought in mind – why do designers of buildings require handrails to be installed without negative tolerances?
“Buildability” is a valid design objective that includes, among other factors, achievable tolerances. Life Safety Code supports buildability of handrails, but it is ignored or misapplied in most contracts and designs. It becomes low hanging fruit for the inspector. Life Safety Code, as published by The National Fire Protection Association, states in section 18.104.22.168.4.1 “New Handrails on stairs shall be not less than 34 inches, and not more than 38 inches, above the surface of the tread, measured vertically to the top of the rail from the leading edge of the tread.” Most contract documents specify for the top of the handrail to be 34 inches. When we draw the handrails to anything other than 34 inches, the approver typically marks up this dimension, specifying the minimum 34 inches (no negative tolerance).
In the real world of construction, nothing is perfect. You add the bevel of a stair into the equation and problems become magnified. Life Safety Code also allows variance in the placement of treads (22.214.171.124.6 * dimensional uniformity). This variance alone can cause the Fire Marshal to fail the height of the handrail above the treads when the handrail is designed at the minimum height. When you add in variances in the surface flatness of poured-in-place treads, you can experience more variations in the dimensional uniformity. Finally, many treads are covered with vinyl or terrazzo. This allows another possibility of the leading edge of the tread being slightly different than the designer’s intent.
In conclusion, being human, we need both negative and positive tolerances when constructing buildings, and designers need to specify the tops of handrails to be at 36 inches. This will provide tolerance both ways and will allow the best chance for success in passing the Fire Marshal’s inspection.
Macuch Steel – inherently imperfect humans.
William L. Macuch “Bill”
Macuch Steel Products, Inc.