Stair Framing: Primary Structural Frame or Not?

[Mark K]

This is not a concept used by structural engineers.

My understanding is that the primary structural frame are those structural elements essential to keeping the building from collapse if the secondary structural elements were damaged by fire.

Remember that there are some building systems that do not necessarily have formal columns which would imply that there was no primary structural frame. Since the primary frame is often dependent on secondary members to brace the primary structural frame these secondary members would be considered as part of the primary structural frame.

 

[sergoodo]

This is not a concept used by structural engineers.

My understanding is that the primary structural frame are those structural elements essential to keeping the building from collapse if the secondary structural elements were damaged by fire.

Remember that there are some building systems that do not necessarily have formal columns which would imply that there was no primary structural frame. Since the primary frame is often dependent on secondary members to brace the primary structural frame these secondary members would be considered as part of the primary structural frame.

Agree not primary with the industry standard for engineering- reflected by CSI which has metal stair structures specified as separately as misc metals. Stair structure misc metal is primary for the life safety function of the building like guardrails etc.

Needs to be cleared up because you can hang hat on either.

 

[Mark K]

The fact that metal stairs are considered miscellaneous metal has nothing to do with safety, use, or performance.

AISC in their code of standard practice makes it clear that metal stairs are not structural steel. This is a commercial, not a technical, decision. Steel fabricators focus on fabricating the typical beams and columns. Other fabricators typically fabricate stairs. Since the AISC members typically do not fabricate stairs they have excluded stairs from the definition of structural steel which they use to define their scope of work. In response to this CSI has placed steel stairs in the miscellaneous metals specification section. This is a commercial scope of work issue that is not based on any technical criteria.

Structural engineers use the same technical standards when designing either the building frame or steel stairs.

 

[steveray]

So does the hanger for a sink.

Times like this I miss Brudgers...

I beleive there is a 2024 clarification in on this....

Stairways are not required to be FRR. What would be the point? If there's already fire in the stair shaft no one is using it for egress.

My thoughts too, but the same reason penetrations are allowed in the outside of the stair, but not the inside...Dumb people are writing the code..

Exception: Membrane penetrations shall be permitted on
the outside of the interior exit stairway and ramp. Such
penetrations shall be protected in accordance with Section
714.3.2.

 

[Mr. Inspector]

I would think the framing for the stairway is the primary structural frame for the stairway.

 

[ADAguy]

I would say no...as the stairs do not resist any forces (wind, snow, seismic I guess) or support anything but themselves (and the users of course)....wouldn't be the first time I was wrong.....just the first time today....

"Wrong" must indeed resist seismic forces.

 

[steveray]

That is why I said "seismic I guess" 8 years ago ......If it is not supporting the frame...It's not the frame.....

PRIMARY STRUCTURAL FRAME. The primary structural
frame shall include all of the following structural members:
1. The columns.
2. Structural members having direct connections to the
columns, including girders, beams, trusses and spandrels.
3. Members of the floor construction and roof construction
having direct connections to the columns.
4. Bracing members that are essential to the vertical stability
of the primary structural frame under gravity
loading shall be considered part of the primary structural
frame whether or not the bracing member carries
gravity loads.

 

[Yikes]

I'm struggling with this one. Most of the time, I can see some type of (mostly) intuitive reason as to the life safety rationale, but I don't see it here.
Example: a required exit stair is exterior to the otherwise-enclosed building, and is made of metal (columns, stringers, pans, etc.). It is bolted to the main combustible building. We all know that steel loses its structural performance characteristics when it gets hot.
A fire breaks out on or near the stair (picture a bad actor dumping trash on the stair and setting it on fire).

My intuition says that of all the components of a structure, an exit / exit discharge stair would be one of the most important items to keep operational as long as possible in event of fire. so why would the columns that support an essential means of egress be allowed to have zero fire resistance rating.

 

[Mark K]

Given the definition of a primary structural frame a structural engineer would not normally consider stair stringers to be part of the primary structural frame.

 

[tbz]

Mark K is correct,

Though one would think that the structural steel erectors, AISC members do steel stairs, one would be mislead, that portion of the building is done normally by a separate fabricator, NOMMA members (National Ornamental and Miscellaneous Metals Association) who work closely with structural fabricators, but are 2 different entities on projects.

 

[Mr. Inspector]

If the only structural is a outside stairway and no building why wouldn't the frame for it be the PRIMARY STRUCTURAL FRAME. Why would it change if it was in a building?

 

[redeyedfly]

Is everyone ignoring the ICC commentary that explicitly says the stair framing is not required to be FRR? The commentary is the intent of the writers of the code.

 

[Yikes]

Is everyone ignoring the ICC commentary that explicitly says the stair framing is not required to be FRR? The commentary is the intent of the writers of the code.

The code commentary you cited in your post #22 makes a distinction between "stair components" and "structure supporting the stairway".

Any structure supporting the stairway and the stairway enclosure must be fire-resistance rated consistent with the construction type; however, the stairway components inside the enclosure need only comply with the material limits for the type of construction.

I would say that "stair components" =the items mentioned in IBC 1011.5, which are risers, treads, (incl nosings). Other people might add "handrails" and "guardrails" and "landings", but to me those are separate from the definition of "stairway".
I would also say that posts and stringers are not stair components, they are the structure that supports the stair components.

 

[redeyedfly]

It's the inside the enclosure part that is key to the code theory. You're already inside a protected shaft. There is no reason for additional protection.

 

[Yikes]

It's the inside the enclosure part that is key to the code theory. You're already inside a protected shaft. There is no reason for additional protection.

Sorry, I should have been more specific, I was responding to the second part of Post #1 from RGLA where he was asking about an exterior, unenclosed stair. Here's what he asked:

Interior Stairs: Located within an exit enclosure. Steel framing for stairs (consisting of columns, stringers, etc.) is independent of the building frame and does not support the enclosure walls.

Exterior Stairs: Steel framing for stairs (consisting of columns, stringers, etc.) is independent of the building frame and is exterior of the building envelope (i.e. free-standing with landings connected to building).

Would you consider one or both of the two conditions part of the "Primary Structural Frame" as defined by the IBC? The definition is not clear in this respect, so I'd though I'd get some opinions.

 

[ADAguy]

Makes for a good discussion, it comes down to the details.