Cantilever Slab Deflection
Cantilever Slab Deflection
(OP)
Dear All,
I have a 6'-6" cantilever slab that is supporting a 12 foot tall masonry wall at the end of the cantilever. Strength is not a problem but I'm concerned about the deflections particularly long term. The backspan of the cantilever slab is a 17'x17' two way slab supported on beams and walls. How do I account for the impact of the backspan on the deflections. It seems to me that analyzing the slab as a beam with a 17 foot backspan is too conservative.
Thanks
I have a 6'-6" cantilever slab that is supporting a 12 foot tall masonry wall at the end of the cantilever. Strength is not a problem but I'm concerned about the deflections particularly long term. The backspan of the cantilever slab is a 17'x17' two way slab supported on beams and walls. How do I account for the impact of the backspan on the deflections. It seems to me that analyzing the slab as a beam with a 17 foot backspan is too conservative.
Thanks






RE: Cantilever Slab Deflection
Mike McCann
MMC Engineering
RE: Cantilever Slab Deflection
Since the backspan on the slab is a two way slab, I thought it would help with the rotation around the support and reduce the overall deflection. Analyzing the 17' backspan as a one way spanning member seemed conservative.
RE: Cantilever Slab Deflection
RE: Cantilever Slab Deflection
Is there a design procedure to account for this if these calculations are being done by hand?
RE: Cantilever Slab Deflection
1. I would assume all 4 supporting walls/beams around the back span stay level, i.e. at the same elevation.
2. Reinforce the supported wall so that no deflection occurs parallel to the wall. That way, the cantilever deflection has to be constant.
3. Now just calculate/estimate deflection of the cantilever assuming it is an encastre cantilever. Apply whatever long term estimates you would normally use. Some here will tell you this calculation can be done with great accuracy, but there are so many things during construction which can affect deflection, I take it all with a grain of salt.
4. Be conservative and add maybe 20% for the contribution due to the upward deflection of the two way internal span.
RE: Cantilever Slab Deflection
If the back span is treated as a one way slab, then all of its loading is considered to deflect the back span downwards, which creates a rotation at the cantilever support thus reducing the cantilever deflection. The extra cracking induced by the load in the one way assumption will increase this effect.
If the back span is actually a two way slab then only part of the loading will be acting in the direction of the cantilever and will reduce the back span deflection and its effect on the cantilever deflection. If the back span is square with the same boundary conditions in both direction, only half of the loading will be acting in that direction. This will reduce the elastic deflection but also the amount of cracking in that direction and will significantly increase the cantilever deflection compared to the one way back span slab assumption.
RE: Cantilever Slab Deflection
RE: Cantilever Slab Deflection
In a one way slab, all of the load would be carried by the slab in this direction.
So the back span in the two way case deflects far less than it would in the one way case. Forgetting cracking and long term effects, the deflection in the one way case would be double.
The more deflection you get in the back span, the larger the rotation caused at the cantilever support by the back span deflection. This rotation reduces the cantilever deflection, so the larger deflection in the one way back span case causes more rotation at the support giving a smaller cantilever deflection.
RE: Cantilever Slab Deflection
RE: Cantilever Slab Deflection
RE: Cantilever Slab Deflection
For the OP case with 2-way back span of 17' x17' (1:1 aspect ratio) the actual upwards deflection of the backspan would be considerably less (about 3 times) when you compare plate bending with span aspect ratio of 1:1 vs infinity (infinity being the special plate bending case of 5/384 for a strip).
Cracking and LT deflections need to be encountered for.
RE: Cantilever Slab Deflection
Yes, the case of back span loaded only with SW should be considered. That is why RAPT now automatically does alternate span deflection cases, much to the dismay of some designers who do not like to see the bigger deflection estimates and would like me to remove the option! This will increase the cantilever deflection.
The SW still distributes the same way. Without the point load, with a span/cantilever of about 3, the span would be dominating normally if one way, but probably not if 2 way. Adding the point load will change this.
But it does not matter if the cantilever deflection dominates or not, the effect of the back span on the overall cantilever deflection will still be underestimated if the back span is considered as one way if the actual slab is two way.
TTJT
Same result. Less back span deflection so less rotation at the cantilever support so more cantilever deflection.
RE: Cantilever Slab Deflection
RE: Cantilever Slab Deflection
RE: Cantilever Slab Deflection
RE: Cantilever Slab Deflection
-Deflection in concrete depends on stiffness of the section on consider
-Back span stiffness defiently affects deflection od the cantilever.
Assumptions shall be based on knowing if the slab is assumed 1 way what will be the actual bending moment compared to Mcrack and the same for two ways actions , if the slab will crack we talk about reduced back span stiffness
RE: Cantilever Slab Deflection
RE: Cantilever Slab Deflection
I am not an advocate of patterned loadings. I feel that the probability of live loads is remote enough as it is without the consideration of having the most unfavourable live load pattern.