Changing the tread size of a staircase in a building

[VTAdam]

I am aware of the fact that changing the rise dimension of a staircase might cause accidents. So I did not change that. However, I changed one of the floors tread by 20mm. The entire building has 300mm steps except for one of the middle floors which has 280 mm steps. I would like to point out that every floor of the building has the same steps; except for one floor which has the 280mm ones. Are there any rules against it or would people notice it? Thanks.

 

[XR250]

Check your building code on minimum tread dimensions

 

[SlideRuleEra]

VTAdam - IMHO, making that change is not a good idea. Use of stairs is human factors issue. Under certain circumstances, users will definitely notice. If the stairs are intended for rapid building evacuation (say, for a fire), users develop a cadence to their step that will be interrupted by the sudden change. As they descend quickly from upper floors this will happen twice, when they first encounter the 280 mm run and when they leave them. Could cause some people to trip, especially in an emergency. The combination of rise and run (the angle of the steps) is just as important as each dimension.

Other details that you have not mentioned, such as the dimension of the rise, number of floors in the building, and which floor is different could make this issue more important. If it is not too late, suggest you keep all rises identical and all the runs identical.

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[thaidavid40]

So long as your tread length and your rise fit within IBC, OSHA, or NFPA requirements (depending on your application and jurisdiction), you are allowed to have the individual stair rises for one flight of steps differ from other flights. You just can't have varying rises within the same flight, as the uneven pattern could cause a trip hazard.
Dave

Thaidavid

 

[4thorns]

Interesting question. I believe the US code says that tread widths within any flight of stairs can vary no more than 3/8".
Key words being "within any flight of stairs" I haven't found anything regarding successive flights of stairs (separated by landings).

Not sure if this is the case here but you can see the Run (Tread width). Can the human mind react quick enough to a
sudden, approximately 3/4" (20mm) difference in tread width to maintain a stride down the next, completely different set of stairs?

Somebody might have an answer but untill you find it I echo XR250,

Check your building code on minimum tread dimensions

PS. Here's a little food for thought.

https://www.youtube.com/watch?v=cji3Ih1GP0k

Looks like a riser height issue but entertaining just the same.

Doug

 

[Ron]

SRE is correct as usual. There is an expectation of consistency in stair tread depth and rise in order to prevent tripping or an uneven gait when descending the stairs in a hurry as SRE noted. Stairs are almost always a part of the life safety system for a building and as such should be designed and constructed with that in mind. Inconsistent tread depth does not accomplish this. I would not do it.

 

[RPMG]

Is this a change in the work line? That is, if you drew a line connecting the tips of the tread noses, would you're slope change? If so, that is not acceptable.

If you are cropping off an inch at the back of the stair tread due to some obstruction and the stair is not used for egress, I would ask an architect.

 

[hokie66]

RPMG,
It is not one "work line", it is two different stair flights.

I would agree with SRE and Ron in principle, but these stairs are not your ordinary egress stairs. 300 and 280 are both wider than "normal", and I think as long as the treads in each flight are correctly proportioned with the risers, the 20 mm difference in tread length would not be noticeable.

 

[3DDave]

I've read of experiments in stair tread changes - very disruptive for a single step to be out by a small amount. Personal experience suggests that going up and hitting a greater rise than expected is the worst case; the foot is brought only high enough to slide into position on the next step, but hits the edge instead, stopping the momentum of the foot, but the rest of the body continues and falls.

I know I read a study where stair tread and rise were altered, but I don't know where to look to find it again. I think the study indicated that the key was variation from one step to the next; keeping it the same on an entire flight is best, but continuous increase or decrease is not insurmountable, as long as the variation is small. Human locomotion is adaptable as long as it's not too much change.

 

[Guastavino]

The IBC codes have a section on Dimensional uniformity. In the 2009 IBC its 3/8", or about 10mm for those in the rest of the world. It's in 1009.4.4 for the 2009 IBC if you want the exact reference.

 

[hokie66]

njlutzwe,
But for those in the rest of the world, none of us who use the IBC, does it address uniformity between separate flights? That section sounds like just a dimensional tolerance thing.

 

[RPMG]

Thanks hokie. I re-read it, and this is acceptable. I once marked this up on a drawing to make them uniform, and received highlighted code sections and was told that it can change between landings.