Moment in slab

[oranda]

I'm designing continuous one-way slab which has to support a short precast wall in one of the span (as well as normal SDL & LL). The problem is that I have to design the slab to cater for additional moment for the wall (from wind load). Do I just sum of the moment from the wall to the moments caused by normal dead and live loads?

 

[JedClampett]

If the precast is cantilevered from your slab the answer is yes. Make sure you've detailed the attachment for the precast to the slab to accommodate the moment. The conservative way to design it is to add the moment directly to your DL and LL moments. Or you could do a simple beam analysis with a concentrated moment and DL and LL.
Having said that, most precast is designed to span from a slab to top supports. Maximum moment is at midspan and no moments are transferred to the slab. This saves the detailing nuisance of transferring the moment at the base.

 

[mapestru]

10/13/02
To Oranda from mapestru, re "Moment in Slab":

1. I partly agree with JedClampett, but the situation seems to need clarification and elaboration, since several concepts may be involved, as noted, below:

2. If wall orientation is perpendicular to span direction of slab, then the moment, if any, at base of wall must be transferred to slab in some effective way.( ie: bolting; building in with proper rebar arrangement,and/or anchorage, in slab and wall; and possibly even epoxy adhering, etc. ).

3. The wall effect will cause an abrupt change in the bending moment, and shear diagrams, in the slab -- as Jed Clampett said,-- effectively adding,(algebraically,) the wall base moment to the slab DL and LL moments. It will also affect the shear diagrams.

4.Note that such may involve adding wall moments in two possible opposite directions, if they are generated by winds in two possible directions.

5. Also to be considered, will be the new vertical dead load of the wall on the slab. This will be effectively a concentrated vertical load on the slab, at the wall location,which will cause additional bending moment and shear components in the slab. Those will also have to be added to the slab's existing M and S diagrams due to its own DL and LL.

6. Further complication may exist since the slab is
"continuous". That may be the source of several possible "worst case scenarios", due to variations in slab M and S diagrams due to possible consideration of the slab's variable LL placement.

7. Also, if the precast wall is laterally supported at its top, then its base moment may be less than a full cantilever condition, ---and could be as low as zero, --- all dependent on how the base connection is made.

8. If the precast wall is oriented parellel to the slab span, (which though unlikely, may be a possible interpretation of Oranda's original posting,) much of the above is not applicable in the same way.

9. Then, the wall may or may not even develop any significant vertical load on the slab, due to its providing a self supporting span condition.

10. But its base wind-based moment, if any, must still be transfered to the slab. In this case such moments will load the slab in a direction that it may not be adequately reinforced for. Special rebars will be necessary to provide adequate reinforcement in the slab, to resist the resulting bending moment, in this perpendicular direction.

11. I will not address this possibility further, since it is not too probable. But if it is the intended arrangement, several further suggestions should be made. Let us all know.

I hope the above is helpful------mapestru