Friction reliant / free-to-move base - Most sliding resistant design?

[nucstructural012]

Anyone worked on a base plate type design where you can't fix the base and must rely on friction to maintain some stability?
I'm working on a design to modify an existing ladder mounted at the top of a concrete deck, extending down about 25 ft to the floor at the base of the ladder. The problem with the existing configuration is that when analyzed for seismic forces vs the requirements for our plant, the ladder deflects too much and fails the anchors at its top side; this is due to a 5 inch gap from the ladder base plate and the stainless steel floor. The hurdle Im facing is that any new attachment to the ladder base can NOT be bolted or welded to the floor below, and therefore must rely on friction/sliding resistance to stabilize the ladder in a seismic event.

The conceptual modification idea is the bridge the gap by welding a few new members to the ladder base (likely a little metal working or shims to bridge the gap perfectly). I thought of adding short circular HSS sections to the ladder bottom for this concept.
However, I've thought about it and wondered if the best solution isn't some type of adjustable column(s) sitting on the floor with a top plate welded to the ladder base underside. Theoretically, I'd think the most stable attachment would be one exerting the most Normal Force back on the floor, giving the most sliding resistance. Since the gap is so small, wouldn't an adjustable column(s) (basically a floor jack) provide the most resistance? Wondering if anyone has dealt with: a base design not allowing fixity? Design using a wedge or relying on friction? Permanent jacking type design?

I've never ran into this type of design problem before. Hoping there are some thoughts out there!

Thanks!

 

[1503-44]

Embed it in some poxy grout?

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[r13]

The best way is to provide lateral support midway on the ladder. Also, you may consider strengthen the top connection to resist the forces. Otherwise, provide wheels/rollers at the bottom may help, better than relying on shear friction anyway.

 

[nucstructural012]

Grout and other similar substances are out due to materials reqs and the fact that the ladder must be left removable.

 

[nucstructural012]

RE:retired13....There are a couple lateral supports coming off the ladder now that provide friction resistance (St steel on St steel) along the wall. The anchor/top connection part of your solution is also something i'm considering. Space is tight where the original anchors are, but may be possible.

Very interested in the rollers idea! Makes complete sense- i know roller/bearing type solutions are used in other seismic structure applications already. I'm definitely going to research some seismic quality type rollers. Thinking they'd be best configured coming out at an angle from the ladder base almost like spider legs, providing that lateral resistanc?

 

[KootK]

The hurdle Im facing is that any new attachment to the ladder base can NOT be bolted or welded to the floor below, and therefore must rely on friction/sliding resistance to stabilize the ladder in a seismic event.

Can you tell us why the ladder base cannot be welded or bolted to the floor? That rules out a lot of stuff.

Permanent jacking type design?

Sure, in concept you could install some rubber soled feet on a screw-able threaded rod and basically pre-stress that down onto the deck. It's clever, non-invasive, and very removable. My only qualm with this would be if you need a definite, reliable capacity from the connection. That would be hard to assess.

 

[r13]

Thinking they'd be best configured coming out at an angle from the ladder base almost like spider legs, providing that lateral resistanc?

Sounds a good idea.

 

[nucstructural012]

Can you tell us why the ladder base cannot be welded or bolted to the floor? That rules out a lot of stuff.

No connection can be made to the floor because this stainless steel liner/floor can only be modified/connected to at certain locations where embedded plates exist, which are not here.

Sure, in concept you could install some rubber soled feet on a screw-able threaded rod and basically pre-stress that down onto the deck. It's clever, non-invasive, and very removable. My only qualm with this would be if you need a definite, reliable capacity from the connection. That would be hard to assess.

The rubber soled foot may be a good idea bc the coeff of friction would improve vs steel on steel. You're saying you'd thread those through the ladder base plate, fasten them w nut, then turn with a torque wrench to get the prestressed load at the rubber foot? As for the definite, reliable capacity, it would moreso be about lessening the deflection of the ladder by adding the best sliding resistance in order to put less force on the anchors up top. The connection to the base plate (fastener & threaded rod in shear + bending) would need to be reliable and checkable vs seismic forces, but with a handful, i'd think it would be reasonable to believe?

 

[IRstuff]

Is there nothing that can be done at the top? It seems to me that you're asking for performance that cannot be designed in, nor overtly maintained. If the floor got wet or oily, you'd be back in the same situation.

Can you not change or add to the ladder mount at the deck?

TTFN (ta ta for now)
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[KootK]

You're saying you'd thread those through the ladder base plate, fasten them w nut, then turn with a torque wrench to get the prestressed load at the rubber foot?

Yeah but, then, I thought this was your idea?

The connection to the base plate (fastener & threaded rod in shear + bending) would need to be reliable and checkable vs seismic forces, but with a handful, i'd think it would be reasonable to believe?

At this point you're more of an expert on this than me. If you'd be dealing with a seismic lateral force at this connection, keep in mind that you'd also be dealing with a concurrent seismic vertical force tending to reduce the friction in the connection. Some parts of ASCE7 actually restrict you from using friction in connections for this reason. Perhaps that could reasonably be waived where the friction is a product of excess prestress.

 

[nucstructural012]

Yeah but, then, I thought this was your idea?

Yes, mostly, but the rubber foot part is something to consider.
That's a good point about the vertical seismic force decreasing friction reliance. Will explore the code more on this.

Thanks!

 

[nucstructural012]

Is there nothing that can be done at the top? It seems to me that you're asking for performance that cannot be designed in, nor overtly maintained. If the floor got wet or oily, you'd be back in the same situation.

My answer there is that I don't yet have a reason that modification can't be done at the top. Spacing near the existing anchors is somewhat tight, and rebar layout in the slab is closely spaced. But I'm starting to think you may be right and that the best answer is to work there rather than @ the base of the ladder where there are more variables and unknowns. Esp considering that adding two anchors to the exst two would half the forces theoretically.

 

[r13]

Can you provide a sketch that show existing connections, and the structures the ladder attached to? Is that possible to provide kickers from the ladder to the upper slab or any structure element that is stable?

 

[nucstructural012]

Photos of sketch attached showing connection assembly at top side. Basically 2 HSS “arms” extend out from the ladder structure and are welded with a vertical plate connection to a base plate anchored to the deck.

 

[nucstructural012]

*photo 2 iso attached here

 

[KootK]

@nucstructural012: what is the actual load case that's causing you grief here? Is it the situation shown below? If so, I've proposed a possible remedy in the sketch.

 

[r13]

Is that possible to extend the fastening plate beyond HSS, use thru bolts, and add kickers from the ladder to the bottom of the deck? By doing all of these, the ladder shall be quite sturdy and stable.

 

[hoshang]

Some parts of ASCE7 actually restrict you from using friction in connections for this reason

Do you mean setion 13.4?

 

[Ron]

Extend the ladder with adjustable all-thread and fashion a metal foot plate with a rubber sole. Install and put the ladder stringers in compression with the adjustment.