switch back stair analysisdesign

huangyhg

switch back stair analysis/design
dear all:
i am a structural engineer who has never done stair analysis/design ...for one of the projects i am currently working on the client wants us to design a "switchback" stair.the spans are such that deflections are becoming a big head ache esp when i apply the uniform live load on one flight of stairs only and nothing on the other flight or landing ...upon searching ibc etc i got the design loads to be the worst effect of a uniform load of 100 psf or a conc. load of 300 lbs; but these do not tell me if i can assume a percentage of this 100psf load to help with the deflections esp. when i apply the load only on one flight of stairs in the unbalanced load case....
please let me know if there is a reference/code out there that addresses this issue. any advice would greatly help me in designing this stair.
thanks.
matrixeric
check out our whitepaper library.
are you doing a stair where the intermediate landing is unsupported?
i have never designed stairs, but have checked several sets of calcs that have come in for jobs we are doing.  i have never seen a stair stringer treated as a continuous span (as you are suggesting).  maybe this is a function of the switchback stair (i have never heard of this).  if, however, you are designing continuous spans i have to think you must consider skip loading to provide the most critical conditions similar to any continuous beam.
what   
i will try to answer all of your questions at the same time:
jike: i have one column in the centre of the intermediate landing which is my only support in addition to the ends of the stringers at the foundations and deck connection.
structuraleit: you are correct. the geometry with which i am dealing with meant that i need to have the stringers analyzed as continuous to make it work.
ucfse: it is the stringer that has deflection problems. this being a aess stair we are trying to limit the deflections to a 0.5"(live load deflection) at this point of time. do not know if the constraints given to us will allow that.i am assuming stringers to be assumed fixed at the bottom(fdn.) and pinned at the level where it is connected to the deck.
just so that it is easy to visualize,  can some one let me know a way to post an image/pdf out here.
if you have a centered column at the intermediate landing, you should be able to reasonably control deflections. this column should be a t support with a fixed base.
it is advisable to have the full ll on one side to obtain the worst possible situation. it is not a common situation, but i can envision the possibility of a group of people getting on the stairs to take a group photo.
click on "process tgml" right above the "submit post" button.  scroll down to the image instructions.
and...you'll have to upload that pic to an image hosting site (like
alright, the stair i am envisioning has an intermediate landing and a landing at the actual floor level.  the stringer is running up to the intermediate landing then reverses directions to run up to the actual floor level such that the intermediate landings are over top of each other and the actual floor landings are over top of each other (but the intermediate landings are not over top of the actual floor landings).
-------  actual floor
        -
          -
            -
              -
                ------  intermediate (pin support)
              -
            -
          -
        -
-------   actual floor (fixed support)
with this being the case, and you designing for fixed at base (foundation connection) and pinned at the landing i am failing to see how this would be considered a continuous beam.
the stair would have a pinned base (not fixed) and each stringer would be designed as a simple beam spanning from the floor to the end of the landing (cantilevering past the t shaped column support depending its exact location).
jike-
i agree, but it sounds from the op that these stringers are being designed as continuous for more than just the intermediate support.  continuous over the intermediate support still means it is supporting only one flight of stairs, but the op says that one flight of stairs (as the unbalanced case) is the worst.