Would you stamp this? 6

[dreber]

So in ASCE 7-16, there is a new requirement in § 13.5.10 “If sliding or ductile connections are not provided to accommodate seismic relative displacements [for stairs], the stiffness and strength of the stair or ramp structure shall be included in the building structural model of Section 12.7.3, and the stair shall be designed with Ωo corresponding to the seismic force resisting system but not less than 2.5”

This has put the requirement to ensure bi-directional drift connections on the shoulders of building designers, who are now writing these requirements into their contract documents.

As the stair designer, I'm happy to stamp a connection like the one above, since I will only be relying on it to resist vertical (dead and live) loads.

However, I'm afraid that since this connection isn't a firm fixed connection that the movements in the stairway, and any potential squeaking or banging noises that could arise now, will come back to haunt me when building occupants are unhappy with their stairway that appears to be moving every time the wind blows.

I think I prefer a connection like this, since it's a bit more "positively attached" but I'm not sure that it won't make any noise either.

Keep in mind that one of these is obviously cheaper and my clients (AKA, my bosses) will strongly prefer the end stop connection to the bi-directional slotted connection.

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David Reber

http://www.kadent-structures.com

 

[phamENG]

Interesting. I'm not a seismic guy, so I'm jumping in for my own edification.

Seems to me that this is creating a maintenance issue. It's a moving part, and it'll need to be cleaned, greased, etc. periodically for it to perform as expected when needed most, but also to prevent the other serviceability issues you bring up. That wont' change for either of them. What number two will do is limit pedestrian induced side sway of the flight.

Why a bi-directional joint? The stairs will have the largest impact on stiffness in the directional parallel to stair stringers. Can't you do a long slotted hole with some sort of Ultra-High Molecular Weight Polyethylene bearing pad? That will reduce the lateral motion without having an appreciable effect on global stiffness (in most cases).

 

[r13]

Seems an overkill to me.

 

[radiocontrolhead]

Angle clips with slotted holes. One direction is considered for this requirement as I interpret it. I’d want a positive connection in the transverse direction also so it seems to satisfy both

 

[dik]

I don't think I've ever encountered a 7"+ Grade A325 bolt... what is the objective of the connection?

Dik

 

[MIStructE_IRE]

I’d keep it simple with slotted connections. You dont want your stair to sound like an old car suspension after a few years. The maintenance also simply won’t happen.

 

[Ron]

As the stair designer, I'm happy to stamp a connection like the one above, since I will only be relying on it to resist vertical (dead and live) loads

I disagree with this statement. The bottom connection of a stair stringer picks up most of the shear, and has little vertical uplift, likely only happening under seismic conditions. Under shear, the bottom connection shown above puts the bolt in bending. Shear at the base of stair stringers can easily approach 3 kips.

 

[dik]

The bottom connection of a stair stringer picks up most of the shear

Concur... but the shear is generally only a small fraction of the section capacity.

Dik

 

[phamENG]

It seems to me the whole point of this is to make sure the shear goes elsewhere - particularly during a seismic event.

 

[dik]

phamENG... I've no idea... I've never seen a connection like it.

Dik

 

[dik]

Can you not just have a small strap on it and let the strap yield?

Dik

 

[GC_Hopi]

As others have mentioned I would avoid the spring. If you go with the end stop then you should add a neoprene pad between the two steel bearing surfaces to eliminate noise. This would be similar to the neoprene bearing pad that is used in precast concrete design. The pad would only need to be designed for "normal" movements which might be +/- 1 inch and following an extreme event the pad would need to be inspected/replaced if damaged.

 

[dreber]

phamENG - I have had a pair of recent projects in which the client requested bi-directional release

retired13 - yes. Overkill it is, that's a bit why I'm discussing this with you-all.

radiocontrolhead - I agree, I think I'm going to continue pushing for slotted connections releasing the longitudinal (parallel to stringers) direction.

dik - the objective is to release the stairs in both directions at client's request, and if you were to read the code with a strict eye, you may come to the conclusion that this is the requirement.

Ron - Below is from a recent project, P1, P2, P3 and P4 are pinned in every direction, where P5 and P6 are only intended to resist vertical loads.

dik - I've never seen a "small strap" connection and I'm not sure what that would look like

=========================
David Reber

http://www.kadent-structures.com

 

[dreber]

Because I have become rather OCD about this, here are the 5 options I have considered along with my problems with each.

=========================
David Reber

http://www.kadent-structures.com

 

[r13]

Since ASCE7-16 is relatively new, below is all the information on this topic I can find online. Note the failure of the concrete stair, and the recipe for sliding connections.

 

[dreber]

retired13, check out ASCE 7-16 § 13.5.10, and the associated commentary.

It's new, but it's the rules.

=========================
David Reber

http://www.kadent-structures.com

 

[JoshPlumSE]

AISC has a design guide (DG 34) on steel stairway design. The chapter on connections talks about this a little (I believe).

I know engineers who have been concerned that their moment frame structures are not as stiff as the stair systems. And, that the stringers would effectively act as braces and take the seismic load. Therefore, they want the connection to the stairs to seismically isolate the stair system from the rest of the building.

Seems like a reasonable intent to me. It may be more difficult to achieve than it sounds, of course.

 

[kissymoose]

Got a few questions:
Where did the 6.75"Ø and 10" long slot numbers come from? That seems like a lot of movement.
How come the stop plate and embed anchors aren't sized?
What's the purpose of the nut on the all-thread in the stop plate detail?
Finally, are there not requirements for addressing vertical seismic loads?

 

[r13]

The support must be capable of/detailed to accommodate a seismic relative displacement D[sub]pl[/sub], which is defined in Section 13.3.2. I've no access to the code, quite frustrate to look for online with no avail.

 

[r13]

The background source of this provision, and a few reference materials.