Free-Standing Stair 1

[Yazan Atoum]

Hello Engineers,

I have a steel freestanding stairs to design, is there any additional consideration I have to take into account rahter than designing a simply supported stairs? is it right that I need to apply 2-3 live load cases to give max tension in higher flight and max compression in the lower flight.

Thanks for any help or recommondation

 

[Trenno]

Be careful with the connection detailing. What members will be continuous? What will the connections look like? Will the hollow section members be required to transmit torsion?

 

[JAE]

You need to check the connections VERY carefully as Trenno suggested. The four end connections take everything so their strength and stiffness is important.

And the connections at the ends may not be correctly modeled as an infinitely rigid end boundary constraint - for example - the top two may be connected to a floor beam which has flexibility to some extent. That needs to be included in your model.

Also - you need to check the vibration of the assembly - making sure to keep the frequency as high as possible - i.e. definitely > 5 Hz but higher would be preferred.



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[atrizzy]

Ensure that your model includes live load cases that adequately represent the possible eccentric loading and maximize torsion in your stringer members.

 

[KootK]

This would be all about vibration for me. You can get surprising amounts of lateral movement at the mid-landing slab. I've investigated single stringer versions of this setup where the foot fall loading from a single person resulted in about 3" of lateral sway at the landing. If the supporting structures at the top and bottom of the stairs are anything less than convincingly rigid laterally (concrete slab), take care to consider that flexibility in your vibration assessment. Much comes down to the type of tread and riser that you're dealing with. If those elements form an effective diaphragm between the stringers, you'll probably be in good shape.



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[racookpe1978]

"Looks pretty" does not equal "Feels stable/comfortable/re-assuring/strong to a commercial user when they walk up and down a "miraculously supported" stairwell. NOBODY (except teenagers and 3-year-olds and me) will want to "wobble" on a suspended set stairs as they go up and down.

Emergency exit access also will get running and crowding conditions you'd not normally see.

 

[dik]

Make it good and stiff and, as noted, pay particular attention to connections. Modelling dynamics may not be appropriate (I have trouble with dynamics of simple things). I've done a couple of 'big' ones in concrete (dist from top to bottom in order of 18' to 20'), but, not in steel. Good luck...

Dik

 

[Trenno]

I would try to get the frequency up around 9-10 Hz to mitigate any effects of higher mode excitation.

When checking dynamic behavior - any pin supports can be probably be now considered to have some rotational restraint.

Do you have folded plate treads? As Koot eluded to, this will increase system stiffness. Do the stringers have any welded upstand plates attached? This again will increase stiffness.

Can you put some hangers at the free end of the stair? Belts and braces solution if you get stuck.

 

[hokie66]

Making the central stringer a stiff tube, floor to floor, will mitigate most of the vibration problems. Ever seen the way the do ornamental stairs on cruise ships? A big built up tube single stringer, sometimes with the stair treads on one side only.

 

[mbullism]

I've done one set of stairs like this, 10-15 years back, in a middle school. Our largest concern was that the kids discover that a few of them acting in concert could treat it as an amusement ride .

I'll second everything above, and add that part of our solution was making the header beam supporting things at the top stiff vertically, studs into the slab to resist lateral loads at the top, cantilevering the upper run through the header down to the landing, and a decent sized footing at the bottom such that the lower run could be modeled as cantilevering out of the slab on grade up to the landing. The resultant stairs were sound underfoot without looking stocky, and the concrete filled pans stiffened everything laterally.

YMMV...