op continuous bar in two-way slab

huangyhg

top continuous bar in two-way slab?
i wonder practically if some percentage of top bars should be continuous through the entire span in two-way slab design. thanks a lot!
this would depend upon whether top bars are needed for negative bending and/or to prevent shrinkage cracks.
sometimes loading in adjacent spans may cause negative bending which would necessitate top bars. also, if the slab thickness is large enough, you may want a top layer to prevent shrinkage cracking.
top steel also reduces deflections.
it's a good idea to make some bars continuous just for structural integrity.  how many is up to you.  if there ever were a failure, those continuous bars help tie everything together, even if it's only long enough to get everybody out of the building.  it's a relatively cheap insurance.  that goes along with making some bottom bars continuous and hooking bars into columns and walls even when a hook is not required by analysis.
aci 318, figure 13.3.8 shows top and bottom bars for two-way slabs...top bars are not required to be continuous for structural integrity.
instead, they use the bottom bars and require a portion of them to be continuous.  if a column failure occurs, or other type of failure, whether the continuous bars are in the top or bottom isn't all that critical when you are simply trying to avoid a collapse progression like what happened in oklahoma city some years ago.
the bottom bars are spliced over the support so there is some concept that the additional concrete above the bars at the support helps to pin the bars in place and allow for a cantenary type behavior after collapse.
one of our biggest problems as structural engineers is that we try to save on items that do not greatly affect cost ... such as rebar.  weigh the constructability issues, not so much the design or material quantity issues.  designing top bars as continuous will negate many possible field mistakes related to placement of top reinforcing.  no longer will there be a worry of bar lengths and thier placement over negative bending regions.
do not be afraid of using more rebar than required ... unless constructability will be affected greatly.  if you do use continuous, be ready for a call from the contractor however - they may have some opinions on the constructability that would govern wheather or not you design them as continuous again.