Stair with integral landing 6

[JStructsteel]

when doing a concrete stair with a integral landing, do you guys just deisgn as a continuous beam, or do you have other methods for deisgning?

 

[hokie66]

Stairs with cross landings are usually designed with the landings spanning across, and the flights spanning between the landings. But you can also model them with the entire stair including both landings and the flight as one slab span.

 

[jike]

I usually design slab stairs as one span from support to support (the landing being part of that).

 

[youngstructural]

As long as you can model the behaviour and see the constructability, there is no reason you can't approach this with almost any system.

Is this a precast stair? If it works well for your situation you can span this as one long span beam, but need to be very careful about the compression forces in the angle changes. There have been very spectacular low load failures of precast stairs where the entire compressive block blew out. You should always provide a spiral in the change of angle between stair and landing where compression forces occur against "empty" space.

Regards,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[jike]

A "spiral"?

 

[JStructsteel]

a "spiral" Do you mean a cage, or something to take torsion? I cant see the mode of failure any different than a beam at the compression top

 

[youngstructural]

I'll post a copy of a very good article on the issue Monday,however the basic problem occurs in long staircases when the landing is near centre-span.

If you think about walking up the stairs and arriving at the landing, that last stair is poking "out" from the landing. Think about what happens to your top and bottom bars here. They cross over, and good detailing demand you add a bent bar to reinforce the outter face. The area becomes similar to a beam column joint and in particularly long spans the forces can be so severe that they exceed the shear stress capacity of this joint. The problem relates to the change of direction of the tension and compression forces, causing a localised area of high shear. A spiral stirrup accross the length of this top stair (R10, 100 dia, 75 pitch | #3, 4" dia, 3" pitch) confines this zone and prevents this failure mode.

Hope that's a bit more clear as to what I was talking about. If anyone's interested I'll post the article on Monday.

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[jike]

I do detail the bars here so that I have top and bottom bars. The bars that terminate here get extended to the other face. I assume that you are talking about a stair going up and reaching a level landing where the componenets of the compressive forces combine to want to push upward.

In 38 years, I have never seen a failure of this type but stairs are rarely loaded to design load except perhaps when people assemble to have a photo taken.

 

[youngstructural]

I have never seen this failure either... Not that I have seen 99 percent of the failures I routinely design against. However, this failure is possible, and at quite LOW load levels. I'll post the article; It's an excellent read.

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[youngstructural]

Good Morning All;

Please see the attached paper. It's quite an interesting read.

Would be keen to hear other people's thoughts on the matter...

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[ali07]

This is how we detailed with mid landing. But be careful about development lenght at every point.

 

[youngstructural]

Ali:

Is that staircase meant to span from left to right, as a long beam, with the landing in the middle as an unsupported flexural section? Or is this side to side supported?

IF that is a long span staircase I think you should have a careful read of the article I posted. I am not sure you are adequately guarding against the compression block blow out failure which is possible.

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[ali07]

This staircase is spanning from padestal foundation to slab shown first floor above. I don't think compression block failure would initiate unless you are treating stair waist slab as minimum to behave section in a way your comprssive stress at top exceed the allowable limit. Make sure your allowabel reinforcement ratio does not exceed the maximum. I believe the collapse which is attached is due to that reason that waist slab was kept minimum alongwith poor rebar detailing.

 

[youngstructural]

Ali:

Agreed; However if you continued your down-turned bar at the nose of the mid-way landing (currently terminating in a simple hook) into the body of the stair, you would further increase the strength of the staircase at negligible cost. Something to think about...

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[BAretired]

If you consider a strut and tie model for the change in angle in a stair with a landing at midspan, the tension in the bottom reinforcement, acting in two different directions, produces a resultant force tending to cause the bottom reinforcement to burst downward out of the concrete.

In the top, the two compression components do just the opposite, creating a resultant tending to force the concrete up and away from the bottom reinforcement.

The two resultant forces are equal and opposite. If the bottom reinforcement is not tied to the upper concrete, the only thing resisting these opposing forces is tension in the concrete. Concrete is not reliable in tension, so steel ties are required at each bend which is concave on the bottom.

When the bend is concave on the top, there is no problem. The resultant force from the two tensile forces oppose the resultant force from the compressive forces, producing a compressive force which concrete can easily resist without additional reinforcement.

 

[ali07]

show in sketch your wording guys

 

[engn555]

with respect to YS's article, not sure I understand what the author means when he says "the compression block stress needs a tension normal to the neutral plane to get around the corner." Can someone please explain?

 

[youngstructural]

BAretired: Good summary of the mechanism involved.

Ali & Engn555: This is just like a beam column joint in concrete. Or like any force interaction at a bend. I'll post a sketch when I get to the office.

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[youngstructural]

Please see attached.

These forces become non-trivial with long span staircases. They also become quite high with dead load alone, which is what occured in the failed staircase. Until end restraint is in place (by pouring the precast stairs into the floor slabs) the mid-span moment can cause high enough break out loads to fail the staircase under very minor live loads.

It would be rare for this failure mode to govern once a staircase is fixed into the structure, however not unthinkable.

I look forward to your thoughts.

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[msquared48]

Thanks the article - never seen this type of failure, but I will keep the detailing change in mind in the future.

Mike McCann
MMC Engineering