Stair design

[Yankee Chronicler]

A couple of recent discussions have mentioned or involved (at least somewhat) stair and ramp designs. Some of the details I see on construction drawings these days leave me wondering if the designer perhaps slept through all discussions about stairs,

When I was preparing working drawings in architecture firms, in addition to always having a copy of Architectural Graphic Standards next to my drawing table I also used a formula (which came from Graphic Standards) to detail tread-riser proportions. For any architects among the membership here -- do any of you (or your drafters) even know about Architectural Graphic Standards? Do offices still use it as a resource?

The stair formula is 2R + T = 24 to 25

In other words, two risers plus one tread should add up to between 24 and 25. This is based empirical observations of what constitutes a comfortable gait for most adults. The standard 7/11 stair ratio fits this perfectly: (2x7) + 11 = 25

Does anyone actually calculate (and detail) the "real" tread and riser dimensions, or does everyone today just assume 7" riser and 11" treads, and leave it to whoever does the stair shop drawings to make it work?

 

[Yikes]

Yes, our office still uses that formula from Architectural Graphic Standards. We also use the truck turning radius tables, and when designing custom residential closets we may use the anthropomorphic data to consider practical reach ranges. Other than that , the book mostly gathers dust.

More recently, federally-funded public housing in Los Angeles has received a lot of scrutiny after settlement of several Section 504 lawsuits. The court-appointed Neutral Accessibility Consultant (NAC) goes back and measures every riser and every tread on over stair, interior and exterior. Many that were designed to be at 7/11 were found to be noncompliant. Some had been covered with carpet or with rubber treads that were sticking out too far. Some wood-framed stairs were just slightly over 7"; and since 7" is a max. allowable height. there is no tolerance for a 7.01" riser. It has to to be fixed.
On some concrete site stairs, they found the contractor had tooled in some grooves on the tread edge. The grooving tool had pushed the adjacent concrete upward by a small fraction, again over 7.00".

As a result, unless the stairs are steel, shop fabricated, we are staying away from 7/11.
We are designing to 6.75 / 11.25, which formulates to 24.75. If the stairs get covered with carpet that bends over the nosing, we design to 6.75 / 11.5 if space permits.

To your point: once those ratios have determined the total quantity of risers and treads, we leave it to the fabricator to make the final riser height adjustment, based on field measurements of height between the main landings. They also take into account the height of any gypcrete or other underlayment, and/or finish flooring materials that occur on the main landing but not on the stairs themselves.

 

[bill1952]

I will trust Jake Pauls and consider such formulas for rise and run bogus.

In know from reading research that once the rise is above 6 or 6.25 that the number of falls starts to increase faster. It's linear to that point, increasing as rise increases in a straight line than the rate increases.

As for treads, it all relates to foot size. I wear a size 12 or 13 shoe and that fits comfortably on a 13" to 13 1/2" tread. Any less and I gave to turn my foot (outward - I seem to recall that people are split between turning out and in) and am naturally less stable. People are simply less likely to fall while moving if their feet supported the full length and if straight rather than turned.

You all do as you please but I'll stick with research and my experience. I know if a stair in a building I work on is 6 X 13 vs 7 x 11, there will be substantially fewer falls and injuries over the buildings life.

 

[walker.t]

even know about Architectural Graphic Standards? Do offices still use it as a resource?

I never saw a copy of Architectural Graphics Standards in any of the three firms where I’ve worked. I have a copy of the seventh edition which is now 44 years old, I don’t reference it very often.

Does anyone actually calculate (and detail) the "real" tread and riser dimensions

I did not know it was acceptable to not do a detailed stair calculation for drawings for plan review, I assumed that was a guaranteed “Resubmit with stair calculations demonstrating code-compliant stair riser and tread dimensions.”

Architect friend did a three-story commercial project and didn’t include any stair data, I asked why, he said that would force the contractor to build them “to code.” And that’s exactly what they did - contractor had lots of residential experience but not a lot of commercial experience and he built them to residential code standards and they had to tear them out after they failed inspection. I never understood why the building department didn’t turn down the plans and require that stair data be shown on the plans - maybe they missed it, no one’s perfect. But it wouldn’t have taken the architect 15 minutes to do the calculation, I have no idea what it cost to replace the stairs but many times more than 15 minutes of the architect’s time.

 

[Yikes]

A stair calc for design and plan check for a single run is literally as quick as this example:

Given: Floor-to-floor height = 10' = 120" total rise of stair

120" / 7" = 17.14, round up to at least 18 risers.
120" / 18 risers = 6.67" actual riser height (approx. 6 11/16")

Tread = 11.25"
There's always one less tread than riser, so 11.25 x 17 = 191.25 = 15' 11 1/4" total run of stair. Confirm stair length (+ landing space) on the plan.

Double check using Graphic Standards (not a code requirement): (2x 6.67) + 11.25 = 24.59, which is between 24 and 25, therefore good.

Mark on the stair plans: 18 risers @ 6.67" / 17 treads @ 11.25"

DONE

 

[Yankee Chronicler]

A stair calc for design and plan check for a single run is literally as quick as this example:

Given: Floor-to-floor height = 10' = 120" total rise of stair

120" / 7" = 17.14, round up to at least 18 risers.
120" / 18 risers = 6.67" actual riser height (approx. 6 11/16")

Tread = 11.25"
There's always one less tread than riser, so 11.25 x 17 = 191.25 = 15' 11 1/4" total run of stair. Confirm stair length (+ landing space) on the plan.

Double check using Graphic Standards (not a code requirement): (2x 6.67) + 11.25 = 24.59, which is between 24 and 25, therefore good.

Mark on the stair plans: 18 risers @ 6.67" / 17 treads @ 11.25"

DONE

That's how we used to do it when I was doing working drawings. Except that I converted the decimals to fractions, because carpenters and stair shops typically don't have rulers that measure in hundredths of an inch. But yes, the starting point is the floor-to-floor dimension, from which the number of risers is derived, and the rest follows.

As in many other respects, I'm seeing architects today getting lazier and lazier, and trying to show less and less on their "construction documents."

 

[Yankee Chronicler]

It rarely works out to exactly a 7" riser, so the idea of aiming for slightly less in order to avoid going over (after all, 7 inches is a maximum, not a dimension with a plus-or-minus tolerance) is usually self-resolving. The 11-inch tread is a minimum dimension, but the code doesn't even mention the ratio or proportion between treads and risers, A stair with a 6-1/2" riser should have a tread depth between 11 and 12 inches, ideally 11-1/2". A stair with 6-inch risers should have treads between 12 and 13 inches, ideally about 12-1/2. I suspect many, if not most, architects (at least in my area of operations) will just stay with an 11'inch tread and ignore the stair formula -- if they've even heard of it.

 

[tbz]

More recently, federally-funded public housing in Los Angeles has received a lot of scrutiny after settlement of several Section 504 lawsuits. The court-appointed Neutral Accessibility Consultant (NAC) goes back and measures every riser and every tread on over stair, interior and exterior. Many that were designed to be at 7/11 were found to be noncompliant. Some had been covered with carpet or with rubber treads that were sticking out too far. Some wood-framed stairs were just slightly over 7"; and since 7" is a max. allowable height. there is no tolerance for a 7.01" riser. It has to to be fixed.
On some concrete site stairs, they found the contractor had tooled in some grooves on the tread edge. The grooving tool had pushed the adjacent concrete upward by a small fraction, again over 7.00".

Yikes,

I found this interesting, as Cali Has Section "1011.5.1 Dimension reference surfaces. For the purposes of this section, all dimensions are exclusive of carpets, rugs or runners."

in Chapter 11B-504.2 Treads and risers. only references uniform, not the 3/8" max dif. from Sec. 1011.5.4 Dimensional Uniformity. and if they are using the 3/8" variable from chapter 10, then why are they dismissing 1011.5.1.?

A little off topic, but on topic.

As to the main topic, I personally like a 6x12 stair flight.

As to using the formula which dates back centuries in text and actual historical building sites. J. Templer and others have discussed this in their papers and books. I can't remember at this moment which book or paper I was reading and if it was Fernie, Templer, or someone else, but the actual sweet spot was noted to be just under 10", like 9.863. Now most of these studies are 30 plus years old now, and foot sizes are not the same, but though tread size does matter I remember reading someplace that gait to support point of the foot in stride is the key.

after 40-years in the industry I still don't understand why designers believe general contractors will be holding to 1/16", on stair construction. especially masonry flights.

CC, I have 3 different editions of the graphic standard, I am always on the look out for early editions in used books stores around universities when traveling.

 

[Yankee Chronicler]

Yikes,

I found this interesting, as Cali Has Section "1011.5.1 Dimension reference surfaces. For the purposes of this section, all dimensions are exclusive of carpets, rugs or runners."

That's straight from the ICC IBC. 2021 language:

1011.5.1 Dimension reference surfaces. For the purpose of
this section, all dimensions are exclusive of carpets, rugs or
runners.

Unfortunately, that doesn't prevent the Access Board or the DOJ from deciding that they're going to include carpets or tread mats in their measurements.

 

[tbz]

That's straight from the ICC IBC. 2021 language:

Unfortunately, that doesn't prevent the Access Board or the DOJ from deciding that they're going to include carpets or tread mats in their measurements.

CC ,

Yikes noted section 504 law suits, Cali inspectors don't have the authority to enforce what is not adopted by the state or local AHJ.

My question is directed at how do the Cali inspectors flag a measurement that the code they are required to enforce, exempts carpets, rugs and runners?

Cali State inspectors aren't employees of the access-board, nor the DOJ.

 

[Yikes]

That's straight from the ICC IBC. 2021 language:



Unfortunately, that doesn't prevent the Access Board or the DOJ from deciding that they're going to include carpets or tread mats in their measurements.

Right, and the 7” max / 11” min. Is not only in the IBC, it’s also stated in 2010 ADA Standards 504.2, which unlike the I C does not have any allowances or exclusions for carpet, runners or other applied finish materials.
So if you’re trying to not only pass building department inspection but also avoid ADA claims, it’s better not to push the limits.

Also, the 3/8” variability in CBC/IBC occurs within the max 7” R / min 11”T limits.
Example: If I have 17 risers at 6 7/8” and one riser at 7 1/8”, the stair will neither meet code nor ADAS, because that one riser exceeded 7”.

 

[Inspector Gadget]

Does anyone actually calculate (and detail) the "real" tread and riser dimensions, or does everyone today just assume 7" riser and 11" treads, and leave it to whoever does the stair shop drawings to make it work?

As an aside, I require stair detail drawings for most of the buildings I deal with. I do this to verify the rise/runs meet Code, but also to make sure the design can fit the rise-run in the space allocated in the drawings.

You'd be surprised how many people just drop some random template into place that, upon evaluation, doesn't fit. It's not rocket surgery to check (esp in Canada, with metric everything to make division super-de-duper simple) and yet ....

 

[Paul Sweet]

There also used to be a rule-of-thumb R x T between 72 & 77.

Outside stairs should have slightly lower risers and wider treads.

I still have my Time Saver Standards, as well as Architectural Graphic Standards, and calculate stair dimensions like Yikes showed.

 

[Jay Marx]

I have Graphic Standards and have had it since I was a student, when I had the student version. I look at the online version mostly. I calculate my stairs like @Yikes does exactly. I think I might start making my stairs 6" x 12" though now that I hear that everyone is doing this. More comfortable and much easier to calculate! And usually when we design a building, we like to use round numbers for the floor-to-floor heights.

More recently in my career, I've started to make ADA clearances larger than they needed to be unless we were in a really tight spot. I think I had undue confidence in the builder early in my career. But one time we sandwiched the restrooms in the core between two stair enclosures where the GC had shifted the concrete stair walls 3" towards each other and we had a situation where the restrooms didn't quite fit with 6" less to work with. We had to redesign the bathrooms!

 

[tbz]

ADA Clearances are a big issue being over looked by many designers and contractors still today.

When I teach M.O.E. for walkways, ramps and stair flights we preach heavily to draw the minimum clear "No Fly Zones" on the plan view with a 3/4" offset line all around it and then build outward.

We then instruct the designers to push all ADA details for these type items to a separate drawing sheet for informational notes and enlarged details with notes.

One point we explain so the contractor can understand the drawings better is they need to imagine if they had a refrigerator the exact size of the minimum clear turn landing size of 60"x60" that was 80" high. The final construction must have clearance to move this imaginary fridge around without touching or hitting anything on the landing except the floor. Otherwise it will not be accepted at time of construction.

Conveying the what fails acceptance and therefore not being paid seems to work well for those that migrating towards the, what are you talking about direction.

It does generate RFI's from contractors when they read it on the prints.

I am also big on the ADA details to insert the actual complete section number text, with each detail.

A big one is 505.10 through 505.10.3, no one ever seems to grasp that you can't connect the handrail of a ramp to a stair flight, nor the opposite. Both in the 2010 ADA & Model A117.1.

Went a little off course here from stair geometry to ramp handrails.

 

[TheCommish]

When I teach M.O.E. for walkways, ramps and stair flights we preach heavily to draw the minimum clear "No Fly Zones" on the plan view with a 3/4" offset line all around it and then build outward.

We then instruct the designers to push all ADA details for these type items to a separate drawing sheet for informational notes and enlarged details with notes.

One point we explain so the contractor can understand the drawings better is they need to imagine if they had a refrigerator the exact size of the minimum clear turn landing size of 60"x60" that was 80" high. The final construction must have clearance to move this imaginary fridge around without touching or hitting anything on the landing except the floor. Otherwise it will not be accepted at time of construction.


^^^Great advice^^^

I always had trouble putting the 33" refrigerator in the 33" space