Lateral Stability and 4" Settlement Difference

[connectegr]

We are designing a 3 story external stair tower adjacent to a structure. The specifications require that the two structures allow for 4" settlement difference. The aluminum stair tower requires an enormous amount of bracing to be free standing.

How can I attached to the adjacent structure for lateral stability and still have the vertical movement required?

http://www.FerrellEngineering.com

 

[JedClampett]

Are you trying to avoid the bracing by attaching the tower to the building?

 

[nutte]

Create a 2 3/4" A325 bolt and use long-slotted holes. You'd have a slot length of 6 7/8", giving 2 1/16" movement each way.

All kidding aside, that's a tough one, but you wouldn't be here otherwise.

Could you "clamp" a member to the structure, and provide an elastomeric bearing pad between the surfaces that have to move? You wouldn't develop friction forces this way, allowing the pieces to move vertically relative to each other. The clamping pieces would keep your stairs from pulling away from the building.

 

[connectegr]

"avoid" is probably strong. Reducing the vertical bracing in at least one direction would be nice. The existing structure is adequate for the lateral forces applied perpendicularly. I have two landings adjacent to the structure.

I have considered a bolted connection to the adjacent structure, with 8" vertical slot. But, there is not a structural application for bolts in slots longer than "long-slots". So can this transfer and load orthogonal to the slot? Obviously it can, but is it allowed

http://www.FerrellEngineering.com

 

[connectegr]

nutte
I have considered vertical elastomeric pads. But this would allow for free movement vertical and horizontal. Thus, no lateral restraint.

And again, a very long vertical slot could provide horizontal control. But, there is no design reference for the shear capacity of the bolt. There could even be bending of the slot edge, due to the long size.

http://www.FerrellEngineering.com

 

[nutte]

You'd get the lateral restraint by a clamping mechanism. Imagine two wide flange columns, butted up flange to flange. Put the elastomeric pads between them. Then make a C-shape that wraps around the tips of the column flanges snug. Weld this C to one column. The columns can't pull away from each other, but they can slide vertically relative to each other.

 

[nutte]

You'd also need elastomeric pads between the C-shape and the unwelded column flange.

 

[Ron]

A 4" differential settlement between a structure and its stair tower? Only 3 stories?

Something's wrong with the criteria. First of all the structure itself is not likely capable of handling a 4" settlement. All the plumbing connections would go, and most other utilities would be affected.

4" of differential settlement is generally an unacceptable site condition and would require remediation of the soil conditions. If your building is on deep foundations and the stair tower is supported by poor soil conditions at the ground level, then consider ground remediation for the stair tower.

4" differential would also mean that the stairs won't match the landings. That's a life safety issue and a code compliance issue.

A slip connection could be done, but could you guarantee it would still slip in a year, two years...

This just doesn't sound right...are you sure it isn't a typo or missing a decimal point??!!

 

[JAE]

You could develop a vertically long-enough clamp system where the lateral force perpendicular to the building is resisted at least....maybe even in the other direction if you added stiffners or struts to the slide assembly (see attached sketch).

 

[connectegr]

I appreciate your concern. I made the same argument. But, this is a specified requirement. We are designing only the stair towers 3-4 stories, to be placed on foundations provided by the EOR. The foundations are not connected to the adjacent structure. The location is in Lousiana swamp, if that helps with understanding the large consideration.

We have designed two as self-supporting structures and no attachment to the adjacent structures. But, as usual we are discussing the options for a more efficient design. Using aluminum we are trying to minimize welding and are using primarily bolted X bracing.

http://www.FerrellEngineering.com

 

[JedClampett]

I'd stay away from using the building as bracing. With that much settlement, it might be uneven and develop all kinds of lateral loads on the stair tower. Make sure the post/slab attachments have 4 inches of adjustment (extra long anchor bolts?) and design it as a completely independent structure.

 

[connectegr]

JAE
A little elaborate but I had not considered it. Your detail allows vertical movement. But is there enough horizontal resistance to provide lateral stability?

http://www.FerrellEngineering.com

 

[Lion06]

What about a Fabreeka pad oriented vertically between the existing structure and your stair structure? I know they can accomodate large relative movements and the lateral shears shouldn't be too hard to get the connection to work for given that it's a stair tower and probably has a relatively small wind profile. Try contacting them, they're pretty helpful.

That being said, something seems off with the criteria. 4"
seems like way too much.

 

[connectegr]

Thanks for the quick feedback.

I know the 4" is ridiculous, it may have been their indirect way of keeping me from connecting to their building.

http://www.FerrellEngineering.com

 

[MikeHalloran]

Last year, I was watching USACE contractors drive piles to support a new 'T-wall' along the Harvey Canal.

The piles looked like WF sections, maybe 18" deep, and up to 135 feet long.

Funny thing is, when they set the pile in the guide frame and attached a vibrating hammer, many of the piles slid down and drove themselves, upwards of 50 feet, just from the weight of the pile and the hammer.

"Ground" is just a concept in South Louisiana. I believe the 4" spec is realistic.



Mike Halloran
Pembroke Pines, FL, USA

 

[hokie66]

Guided slide bearings similar to these should work. Just installed vertically rather than horizontally.

http://www.granor.com.au/NewPDFS/GRANOR BGSU datasheet.pdf

 

[msquared48]

Put the stair tower on hydraulic self-leveling jacks so it can be raised as the settlement occurs.

You can figure out if I'm serious or not. Even I don't know.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask