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Can I have 1/4" height change on a stair landing?
- Thread starter Yikes
- Start date
wwhitney
REGISTERED
Well, ASCE 7 is not freely available, so that hampers my side of the discussion. All I have to go on are the excerpts you've highlighted in those two posts.
Those excerpts taken together do not provide a complete logical argument that you can't build an exterior stairs for a building as a separate structure with its own seismic design, independent of the main structure. Part of that independence would be that during an earthquake, the two structures don't pound each other; hence the need for a seismic isolation gap. Which is part of the design in the OP.
Cheers, Wayne
This is not allowed by code.
I am making a couple of assumptions based on the O.P., but they are safe assumption.
Assuming the stairs are being built under some version of code that requires compliance with Chapter 16 of some code that is based off of IBC, then Chapter 16 says that every structure and parts thereof shall comply with Chapters 11, 12, 13, 15,17, and 18 of ASCE 7.
I really don't think I can lay it out any more plainly than this. I'm sorry if you don't understand.
2022 CBC
1604.1 General. Building, structures and parts thereof shall
be designed and constructed in accordance with strength
design, load and resistance factor design, allowable stress
design, empirical design or conventional construction methods,
as permitted by the applicable material chapters and
referenced standards.
1604.2 Strength. Buildings and other structures, and parts
thereof, shall be designed and constructed to support safely
the factored loads in load combinations defined in this code
without exceeding the appropriate strength limit states for the
materials of construction. Alternatively, buildings and other
structures, and parts thereof, shall be designed and
constructed to support safely the nominal loads in load
combinations defined in this code without exceeding the
appropriate specified allowable stresses for the materials of
construction.
1613.1 Scope. Every structure, and portion thereof, including
nonstructural components that are permanently attached to
structures and their supports and attachments, shall be
designed and constructed to resist the effects of earthquake
motions in accordance with Chapters 11, 12, 13, 15, 17 and
18 of ASCE 7, as applicable. The seismic design category for
a structure is permitted to be determined in accordance with
Section 1613 or ASCE 7.
ASCE 7
12.1.2 Member Design, Connection Design, and Deformation Limit Individual members, including those not part of the seismic force-resisting system, shall be provided with adequate strength to resist the shears, axial forces, and moments determined in accordance with this standard, and connections shall develop the strength of the connected members or forces indicated in Section 12.1.1. The deformation of the structure shall not exceed the prescribed limits where the structure is subjected to the design seismic forces.
12.1.3 Continuous Load Path and Interconnection A continuous load path, or paths, with adequate strength and stiffness shall be provided to transfer all forces from the point of application to the final point of resistance. All parts of the structure between separation joints shall be interconnected to form a continuous path to the seismic force-resisting system, and the connections shall be capable of transmitting the seismic force, F, induced by the parts being connected. Any smaller portion of the structure shall be tied to the remainder of the structure with elements that have a design strength capable of transmitting a seismic force of 0.133 times the short-period design spectral response acceleration parameter, Sds, times the weight of the smaller portion or 5% of the portion's weight, whichever is greater. This connection force does not apply to the overall design of the seismic force-resisting system. Connection design forces need not exceed the maximum forces that the structural system can deliver to the connection.
13.1.1 Scope This chapter establishes minimum design criteria for nonstructural components, including their supports and attachments.
Nonstructural components include
1. Components that are in or supported by a structure,
2. Components that are outside of a structure (except for nonbuilding structures within the scope of Chapter 15) and are permanently connected to the mechanical or electrical systems, or
3. Components that are part of the egress system of a Structure.
Where the weight of a nonstructural component is greater than of equal to 20% of the combined effective seismic weight, W, of the nonstructural component and the supporting structure as defined in Section 12.7.2, the component shall be designed in accordance with Section 13.2.9.
13.4 NONSTRUCTURAL COMPONENT ANCHORAGE AND ATTACHMENT
Nonstructural components and their supports shall be attached (or anchored) to the structure in accordance with the requirements of this section, and the attachment shall satisfy the requirements for the parent material as set forth elsewhere in this standard.
Except where permitted in Section 13.6.12, component attachments shall be bolted, welded, or otherwise positively fastened without consideration of frictional resistance produced by the effects of gravity. A continuous load path of sufficient strength and stiffness between the component and the supporting structure shall be provided. Local elements of the structure, including connections, shall be designed and constructed for the component forces where they control the design of the elements or their connections. The component forces shall be those determined in Section 13.3.1. The design documents shall include sufficient information relating to the attachments to verify compliance with the requirements of this section.
13.4.1 Design Force in the Attachment The force in the attachment shall be determined based on the prescribed forces and displacements for the component as determined in Sections 13.3.1 and 13.3.2.
wwhitney
REGISTERED
You've quoted various parts of ASCE 7, but not any section that says plainly "you can't do that".
Suppose the existing building is designed to ASCE 7 Chapter 12 "Seismic Design Requirements for Building Structures" (or maybe as it's existing that doesn't require verification), and the new stairs is designed as an independent building also to Chapter 12. Your excerpt from Chapter 12 clearly discusses "separation joints" and that no continuity is required across separation joints.
So where's the violation?
Your excerpt from Chapter 13 just says that the parts of the new stairs building that are non-structural need to be rigidly attached to the structural parts of the new stairs building, which is pretty obvious. It doesn't say they have to be attached across the separation joint to the existing building's structure.
Cheers, Wayne
wwhitney
REGISTERED
P.S. Here's a mostly imagined narrative about why you might want to do something like this: there's an existing wood framed building with wood exterior stairs, and the wood stairs have deteriorated and require replacing. The building owner says "no more wood, give me metal stairs, they will be more durable."
So the engineer designs some metal stairs. But the result is a structure that is quite a bit more laterally stiff than the existing wood building. Maybe in a design level earthquake, the existing building would want to move up to 2.1" laterally, while the metal stairs structure would only move up to 0.3" laterally. If the metal stairs were rigidly attached to the wood building, the greater displacement demand of the wood building would result in very large forces in the metal stairs. I.e. the metal stairs would try to restrain the wood building from moving, until some part of the metal structure or its foundation fails because the demand is too much.
So the engineer decides to design the stairs to withstand an earthquake as an independent structure. That requires the 2.5" seismic isolation gap, so that the two structures don't smash into each other during a design level event.
Cheers, Wayne
So the engineer designs some metal stairs. But the result is a structure that is quite a bit more laterally stiff than the existing wood building. Maybe in a design level earthquake, the existing building would want to move up to 2.1" laterally, while the metal stairs structure would only move up to 0.3" laterally. If the metal stairs were rigidly attached to the wood building, the greater displacement demand of the wood building would result in very large forces in the metal stairs. I.e. the metal stairs would try to restrain the wood building from moving, until some part of the metal structure or its foundation fails because the demand is too much.
So the engineer decides to design the stairs to withstand an earthquake as an independent structure. That requires the 2.5" seismic isolation gap, so that the two structures don't smash into each other during a design level event.
Cheers, Wayne
Regardless of the structural issues there is the original question of the 1/4" hump in the floor. 1/4" doesn't seem to be a big deal. That's because it is a small deal..right up until someone catches their foot and careens head first down the stairs. I am surprised that there is a question about this. Don't do it. Tell the engineer to engineer it without a hump in the floor.
I've already said that I can't help you understand this, nor do I need to or want to. That's not my problem.
If this were new construction (as the issue I dealt with was) then the path I laid out makes it clear that the stairs need to be connected to the building and cannot move independently. Since this is an existing building, they should go to whatever version if the existing building code that is adopted in their jurisdiction. If they wanted to repair the existing stairs (or even replace with like-for-like) then I'd say, "no problem". The scenario is not that so if they want to build a compliant design the CA Existing Building Code says it has to be designed to the Building Code, and the path I provided shows you can't design the stairs as an independent structure.
If the owner or authorized agent wants to present an AMM proposal to the BO of that jurisdiction, go for it. If the BO of that jurisdiction doesn't understand this the way I do, they might just approve it, go nuts. Before I had the experience, I had I wouldn't have known better.
@Yikes seems to me like he wants to know and understand the code and he has provided some really valuable information to this community, so I felt like returning the favor and share some insight I learned from a very specific situation.
wwhitney
REGISTERED
So far, everything you've posted is consistent with the following interpretation:
ASCE 7 Chapter 12 allows for separation joints within the structure as per 12.1.3. The structure only needs to have a continuous load path between separation joints. Nonstructural components need to be connected to the structure, but again, not across separation joints. A building can be designed with a separation joint at the top of a stairway, and the structure on each side of that separation joint will have its own seismic design. The stairway will be attached to the structure on the stairway side of the separation joint.
I didn't read all of ASCE 7 Chapter 12, and don't have access to the latest version, but I found this section in ASCE 7-03:
"12.12.3 Building Separation. All portions of the structure shall be designed and constructed to act as an integral unit in resisting seismic forces unless separated structurally by a distance sufficient to avoid damaging contact under total deflection (δx) as determined in Section 12.8.6"
That separation is a separation joint as referred to in 12.1.3.
Cheers, Wayne
I didn't have the current ASCE at the time the issue came up for me, and seeing as how this is my job, I went out and got it. A lot has changed in 20 years, including the standards. If you want to have a meaningful discussion about seismic design standards then get access. Otherwise, I'm done trying to help you understand something when all you seem to want to do is tell me I'm wrong without the references available to back up your argument.
ASCE 7-22 is the current standard, 7-16 before that. 7-16 might be acceptable to some jurisdictions, but I wouldn't. Certainly, nobody should be referencing 7-03. For example, the 12.12 section you're referencing now says this:
12.12.2 Structural Separation All portions of the structure shall be designed and constructed to act as an integral unit in resisting seismic forces unless separated structurally by a distance sufficient to avoid damaging contact as set forth in this section.
Edit
If the Engineer does the math there is a formula and maybe 2.5" is that result, but that section is talking about "adjacent structures on the same property" and I don't believe that you can legitimately call the means of egress a separate structure. It is a portion of that building and this section would not apply.
Last edited:
wwhitney
REGISTERED
Agreed it's clear, and it's 100% consistent with the interpretation in my last post. The stairs would obviously have its own portion of the structure, designed to handle the seismic forces on them. The engineer in the OP presumably did the calculations "set forth in this section" and determined that a 2.5" separation would be "a distance sufficient to avoid damaging contact" between the two portions of the structure.
Cheers, Wayne
Alright, it was clear to me because I have the correct book in front of me and I'm reading the entire section instead of relying on google AI to answer code questions for me.
wwhitney
REGISTERED
Hah, that's an unfounded inaccurate accusation.
You've presented various code/ASCE 7 sections, but nothing that says "you can't have a separation joint in such and such a location" that would prohibit the configuration shown in the OP. My involvement in this part of the discussion has primarily been to carefully read the sections that you've posted and conclude that they don't say what you seem to think they say.
Cheers, Wayne
arwat23
SAWHORSE
Ignoring the non-accessibility issues people have brought up, I don't see a code requirement for a level landing for stairs.
Is it ideal? No, probably not. Does it comply with 11B? As far as I see, yes.
Is it ideal? No, probably not. Does it comply with 11B? As far as I see, yes.
Yankee Chronicler
REGISTERED
Those posts don't in any way prohibit an egress path from passing across a seismic joint or an expansion joint.
What section of ASCE 7 would you use to design the stairs?
Clearly not in the scope of Chapter 15 or 17. Chapter 12 and 13 clearly states that they need to be anchored to the building.
Y'all don't have to understand it or agree, I didn't when this issue came up. This is a very fine nuance of structural engineering and most BO's (myself included) just blindly trust the P.E. who stamps the plans.
In both professions (BO's and PE's) there are a wide range of experience and understanding. You and @wwhitney are always very quick to jump in and tell people they're wrong. That doesn't make you right.
@Yikes, have you and the Engineer you work with discussed what I presented?
wwhitney
REGISTERED
They do not state that. Chapter 12 states that the structure should be continuous, except at separation joints. Chapter 13 states that the non-structural portions should be anchored to the structure.
What you seem to be missing is that the stair side of the separation joint may have its own structure under Chapter 12, with its own lateral force resisting system. So the non-structural portions of the stairs are anchored to the structural portions of the stairs, on its side of the separation joint. That structure does not need to be anchored across the separation joint to the rest of the building.
You could think about it this way: take the whole building as partially shown in the picture in post #13. Drawn a line down the middle. The building design could make that line a separation joint, and seismically you'd design it as two separate structures, one on each side of the separation joint, and then compute the required size of the separation joint "to avoid damaging contact" during an earthquake. The stairs shown in the picture would be attached to the righthand structure. Hopefully you agree that is allowed?
Now let that line move across the building towards the right hand side of the building and the landing/stairs junction. For any location of the line, we get another valid way to design the building with a separation joint. And that includes the limiting case where the line just separates the stairs from the rest of the building. We still have a structure on the stairs side of that line, and that structure still has to resist all the seismic forces on it, which would be small, as that structure is just supporting a stairway.
Cheers, Wayne
Yikes
SAWHORSE
To clarify, yes, both the existing motel and the new stair serving the existing motel are each self-supporting for their own vertical and lateral loads. In other words, either structure is capable of standing on its own, independent of the other structure.
There is a 2.5" seismic gap, and like many buildings with seismic gaps, the gap will have a seismic joint cover over it, capable of supporting the live loads of occupants existing the building. The joint cover would be 1/4" tall, and look something like this example:
There is a 2.5" seismic gap, and like many buildings with seismic gaps, the gap will have a seismic joint cover over it, capable of supporting the live loads of occupants existing the building. The joint cover would be 1/4" tall, and look something like this example:
wwhitney
REGISTERED
With the ability to accommodate 2.5" of lateral movement in either direction between the two pieces of the cover? So it can close up 2.5", or expand out 2.5"?
If you have such a joint cover, it could have square edges and be recessed into the landing/step.
Cheers, Wayne