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Can I have 1/4" height change on a stair landing?

Y

Yikes

SAWHORSE
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Nov 2, 2009
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Location
Southern California
The engineer is proposing to seismically isolate a new stairway form an existing motel exit balcony. He needs a 2 1/2" seismic gap, which means I need to put a 1/4" tall plate cover over it. So that creates a 1/4" high change in level.
I know that vertical changes in level are not permitted on ramp landings (CBC 11B-405.7.1), but I don't see any similar prohibition at stair landings. Is this OK?

1732584281931.png
 
This isn't at the nosing. As the sketch indicates, this is a flat metal expansion joint cover at the back edge of the tread, where it transitions to the floor. It's no different than the threshold at virtually every entrance door. If a 1/4" change in elevation is allowed to be square-edged, I would have no problem with a cover that has beveled edges. That's better than what the codes allow.
 
Yankee Chronicler said:
This isn't at the nosing. As the sketch indicates, this is a flat metal expansion joint cover at the back edge of the tread, where it transitions to the floor. It's no different than the threshold at virtually every entrance door. If a 1/4" change in elevation is allowed to be square-edged, I would have no problem with a cover that has beveled edges. That's better than what the codes allow.
Click to expand...
Yeah....the threshold has a landing on either side....I hate the thought of stepping up to the last tread, hitting your toes on a threshold, having a "1/4" heart attack" and falling back down the stairs.....I'd rather see the plate extend to the nosing...Not saying I wouldn't approve it...I just wouldn't like it...
 
I dislike the last tread even with the floor, the last tread should be 1 step down removing tread X then the landing can be isolated from the stairway
 
Not sure why without a deeper look at the actual components the top treads walking surface could not have been designed to overlap the upper floor and cover the joint fully as noted by steveray.

I guess my question is where is this in the process, plan submittal or installed and in place?

If in plan submittal, I would highly question why not look at tread integration.

On the flip side I would agree with cc on the bevel, but what size is this plate?

If the open distance is 2.5", what size is the proposed so called plate? and are there limits on threshold type plates?
 
It is an existing 1980s era motel, being converted into homeless housing. The existing stairs, which were metal and bolted directly to the exterior exit balconies, were flimsy and in our opinion dangerous. They are being demolished and replaced with new stairs and footings. Drawings were approved in plan check and we are now at the shop drawing phase of review.
Engineer did the classic “see architectural” without informing the architect (as required by contract).
 
Why is the Engineer proposing this? Is he trying this as a "nonbuilding structure" or a "seismically isolated structure"? I've never seen what you're describing.
 
Yikes said:
It is an existing 1980s era motel, being converted into homeless housing. The existing stairs, which were metal and bolted directly to the exterior exit balconies, were flimsy and in our opinion dangerous. They are being demolished and replaced with new stairs and footings. Drawings were approved in plan check and we are now at the shop drawing phase of review.
Engineer did the classic “see architectural” without informing the architect (as required by contract).
Click to expand...
If nothing is built yet I would have a zoom meeting with the fabricator, engineer & Architect of record.

If this is going to be a norm moving forward a better design should be hashed out IMO, I would look more into either integrating the plate all the way into the surface of the top tread/landing allowing the full surface to move with the stairs, or as the TheCommish noted drop the tread down, maybe do 12" treads rather than 11" treads and extend the handrail extensions further onto the landing and allow for the possible offset.

IMO the vast 99.9% majority of the time the gap is a separation for the 0.1% of the time needed, and as thus, why create an oddity to be dealt with by 99.9% of the time.

Just because its allowed, doesn't mean it's a good design.
 
Yikes said:
He needs a 2 1/2" seismic gap
Click to expand...
So does the design need to accommodate only a 2-1/2" movement horizontally, or does it also have to accommodate a vertical movement? If the stair can move up relative to the existing building, the plate would be in the way, but I guess it could be secured only loosely, or just hinged on one side?

You obviously can't recess the plate into both sides as a rectangular solid, such a shape would have to crumple if the stairs and building moved closer together. It might be possible to recess it as a very thin trapezoidal shape, so that if the stairs and building move towards each other, you'd have two angled sliding planes that push the plate upwards as necessary. Not sure how well that would work in practice, with dirt fouling the sliding surfaces, etc.

Or you could have the seismic gap take a right turn before it emerges out of the landing, and instead emerge out of the face of the topmost riser. The tread and portion of riser above the gap would be rigidly attached to the existing building. Any required vertical movement allowance would show up as a gap in the face of the riser.

Cheers, Wayne
 
Joe.B said:
Why is the Engineer proposing this? Is he trying this as a "nonbuilding structure" or a "seismically isolated structure"? I've never seen what you're describing.
Click to expand...
Seismically isolated. The main building is wood framed, the stair structure is metal. They have different resonant frequencies. The existing stair runs perpendicular to the building and due to parking requirements this orientation must be preserved.

1732642208735.png
 
I don't think this is allowed. I was involved in a disagreement between two Engineers regarding a metal set of stairs and a building. One Engineer was insisting that he could design this as a seismically isolated structure, the other said that it can't be because it is a part of the main structure. I'm looking back to see if I can find why, but punchline is the stairs are a part of the structure and need to be connected.

Here's a couple of things I did find on my first dive:

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.

Nothing in chapter 16 specifically references stairs, but this does back up that point of view:

1604.8 Anchorage. Buildings and other structures, and
portions thereof. shall be provided with anchorage in accordance
with Sections 1604.8.1 through 1604.8.3, as applicable.

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.

The argument between these two Engineers basically came down to which chapters of ASCE 7 could and could not be used. Chapter 15 clearly cannot be used for what you're describing.

15.1.1 Nonbuilding Structures. The provisions of this chapter shall apply to unoccupied nonbuilding structures and buildings whose primary purpose is to enclose equipment or machinery and whose occupants are engaged in maintenance or monitoring that equipment, machinery, or their processes. ...

So, then it comes down to whether a stair system is allowed to be a seismically isolated structure designed per Chapter 17 of ASCE 7. The answer in the case I delt with was "no", and I'll try to find the exact reason. I suspect that it may simply come down to 1604.1 that I referenced above. I'll dig deeper and get back to you.
 
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Here's a couple more references from ASCE 7 that I just found, they seem to support the position I outlined here.

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.

I think based on this (and the info in the last post) you can confidently say "No, this cannot be designed per Chapter 17 (or 15). This must be designed per Chapter 13, and must be connected to the building."

Let me know where to mail my bill.

P.S. Any bets on how long it will take for [NAME REDACTED] to but in and say I'm out of line?
 
Joe.B said:
Here's a couple more references from ASCE 7 that I just found, they seem to support the position I outlined here.
Click to expand...
Not so familiar with your references, but what about this part, which you didn't highlight:
Joe.B said:
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.
Click to expand...
I'd need check the definition and rules on separation joints, but what says you can't place a separation joint at the location shown, between the stairs and the stair landing?

Cheers, Wayne
 
Joe.B said:
All parts of the structure between separation joints shall be interconnected to form a continuous path to the seismic force-resisting system,
Click to expand...
Let's pretend I know nothing about seismic (OK, don't pretend)....If the stair resists, and the building resists....Why do they have to be connected?
 
steveray said:
Let's pretend I know nothing about seismic (OK, don't pretend)....If the stair resists, and the building resists....Why do they have to be connected?
Click to expand...
The Engineer that prevailed in the situation I dealt with said that during a seismic event you don't want things moving independently. They start banging together and causing far more damage. You especially don't want that if the element you're talking about is a means of egress.
 
Joe.B said:
ASCE 7 13.1.1, item 3 as I referenced above.
Click to expand...
I don't see how that language precludes designing the stairs as its own structure with a separation joint from the building and landing. You'd need some language that says that the egress path can't pass over a separation joint or can't pass from one structure to another.

Cheers, Wayne
 
Joe.B said:
I provided exactly that. See posts 14 and 15.
Click to expand...
Well, I guess you could have a separation joint, it just wouldn't actually doing anything because the stairs would be required to be connected. They wouldn't move independently.
 
Joe.B said:
Well, I guess you could have a separation joint, it just wouldn't actually doing anything because the stairs would be required to be connected. They wouldn't move independently.
Click to expand...
Not following your logic--if you have a separation joint, the text I bolded says you don't have to connect across it. Nonstructural components on each side of the separation joint would still be rigidly connected to the structure on their respective side of the separation joint.

Cheers, Wayne
 
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