Design of Reinforced Suspended Stairs (ACI-318M-14 , ASCE7-10)

[Envi_Eng101]

I am tasked with the design of this structural system of a reinforced concrete suspended stair. The stair is for public access, the first landing and last landing is a proposed footpath on grade 1.8m wide. The two footpaths are connected via two 3m flights of stair with an interim landing which is a (1.8m x 1.8m slab ).
Scenario 1. The interim slab is supported mid span by a square 300mm x 300mm column on an isolated pad foundation.
Scenario 2. The interim slab is supported on a reinforced masonry wall at mid span on a strip foundation.

What i am essentially struggling with is essentially designing the two-way interim slab. In terms of understanding the behavior of the slab in bending. I assume the stairs and slab will be cast monolithically. The slab essentially has two end conditions two ends being fixed-continuous (My) and the other two sides are free (Mx) i.e ( essentially an overhang on either side of the interior column ).

My Questions
1. Is my understanding or interpretation of the behavior of the slab accurate? If not what is the correct analysis and design of this interim slab.

All i have done so far is design the waist slab for the 3m flight of suspended stair.
Determined the design factored load for the slab and check it for two way and one-way shear.

I don't have much experience doing R.C designs. please guide me accordingly as to what design steps i need to take.
Regards.

 

[Agent666]

I'd just put a beam on top of the column perpendicular to the stair span. Is this a possibility? If not I'd simply put a band of reinforcement in the slab to act as a beam element perpendicular to the span and just design the stair as a continuous 'beam' with the two spans spanning to this 'beam' element. Making sure the punching shear works for any axial and moment going into the column if you go to a beam within the slab thickness. Provide some appropriate detailing at the landings/flights transition to deal with the opening/closing nature of this joint.


If the flights are precast then you might have a similar arrangement to this to break it into two simply supported spans. Top flight simply slides over the bottom flight. I'd suspect most contractors would want to see a precast flight in this type of thing, rather than pouring an insitu stair, with all the formwork, propping etc it adds a lot of extra time/complexity to the build

 

[Envi_Eng101]

@Agent666

I have complete control of the design however in terms of construction it will most likely be in-situ as its a settlement relocation project. From what i gather is its that i should replace the interim landing which was a 175mm two-way spanning slab to a beam and design the entire system as a continuous beam. This continuous beam has two spans of which are simply supported. Yea i agree with the complexity issues of the in-situ construction i will bring it up to my manager.

The analysis for the continuous beam can be considered simply supported for the in-situ scenario as well? . Also for the interest of time how would u recommend i proceed with the design that i initially mentioned if it is indeed a practical approach id like to finish it. The new design which you suggested can be implemented on another project i dont want to abandon this design


Check for adequacy of punching shear at column.
Check adequacy of moment/flexture at column.

 

[Agent666]

If its built/detailed as continuous, then it should be designed as continuous. Effectively its a one way slab spanning to the central support, whether that's a beam/wall the flight design is the same.

I kind of like the wall option as it makes it more simple than a column, providing support over the full width of the stair flight.

 

[TLHS]

Even for cast in situ, I'd want to try and basically cast it on the ground in forms and then erect. Cast in place really feels like the wrong system for this. Pure cast in place only seems to make sense if you're casting the bottom of the stairs on grade, or have reasonably short stair spans low to the ground.

If you're worried about fitup tolerance, perhaps use a cast in place landing at the base that you can slide up and down by an inch or two in the field to make up the tolerance from the precast stair.