huangyhg

opening in 1-way slab
i have a slab with a 2:1 dimension ratio overall. this is complicated by an approximately square opening adjacent to one of the beams, between which the rc slab spans. the slab span is about 30', while the opening is approximately 10' wide. my intuition is that this shouldn't force me to reconsider the slab as 2-way. on the other hand, i probably have to frame out the opening with headers for reinforcement. what's the standard way of dealing with skylights and elevator shafts in 1way slab?

in your case, i would probably frame out the opening with beams. quite often, you can get away without doing this, but a 10' opening is pretty large.
agree with 271828.
for small opening, it could be handled by providing additional reinf, or push the bars fall within the opening, to its sides. however, for large opening, this method would likelt to cause congestions, and lead to other problems.

271828 and kslee1000:
thanks for your thoughts.
what if i used ribbed slab instead of beam-and-slab system? would you still recommend framing the opening with beams? i'd prefer to avoid these if possible, so as to simplify formwork.
with a ribbed slab (i.e. pan joist and beam system) you'd have to frame the opening with beams i would think.
i agree with using rc beams, as the 10 ft square opening is quite large. as far as keeping forming simple, you could use the same forms and drop the slab form lower (to depth of beams). then you could pack (build up) to the slab underside elevation, ie. leaving cavities for beams. this is not that complicated.
you can analyse this as a two way grid using your favourite structural analysis program (or by hand). divide slab into strips of even width and determine section properties but set torsional stiffness to zero, or a very small value. this is necessary to prevent torsional moments in the beam strips.
just spread the load from the opening out to the slab on either side and design these strips for the additional loading.
i agree with csd72's idea. done that before. however, watch out for strain compatibility between the slabs at the sides, and the ends of the opening, and provide necessary reinforcement to alleviate stress concentration.
agree with csd72.

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