Cantilever Rigid Diaphragm

[GalileoG]

Hi fellow Eng-Tippers,

Please refer to the attachment. I have a structure where the concrete shear walls are not symmetrically located in one direction, creating a 'cantilever' diaphragm (Gridline 1 to Gridline 2).

Would you break the slab into two areas (when determining center of rigidity, center of mass) and design accordingly or would you design as one slab (despite having only a 'sliver' of slab between the two main cores?) What special considerations are there? Your thoughts and concerns?

Thank you.

 

[msquared48]

If the cantilever is the portion between grids 1 and 2, then the maximum shear would be in the thin section of slab between the two openings - not the best scenario. Depending on the dimensions here and the lateral loading, I would push for at least a RC or steel frame at the end of the soon not to be cantilever slab.

Mike McCann
MMC Engineering

 

[hokie66]

I don't understand your reasoning, Mike. The reaction of the propped cantilever diaphragm is taken by the core walls, so the horizontal shear in the narrow section of slab would be much less than at the left face of the core.

I would definitely treat it as one slab.

 

[msquared48]

You're right Jolie. My bad.

Mike McCann
MMC Engineering

 

[msquared48]

Darn cell has a mind of its own. Will not recognize your name no matter what I do. Sorry.

Mike McCann
MMC Engineering

 

[BAretired]

Jolie has a nice ring to it.


BA

 

[GalileoG]

Hokie66,

I can't imagine how the entire slab (from Grid 1 to Grid 3) can act as one rigid body given the thin sliver of slab between the main cores. If I were to treat slab between Grid 1 and Grid 2 as a separate diaphragm cantilever, I would get much higher forces on the walls due to eccentricity. Your thoughts?

 

[hokie66]

If the entire section were cantilevered off the cores, the cores would have to resist a large amount of torsion. Not the best situation. But with the small core on the right to take the backspan reaction, I would think that it may work, but not knowing more about the loading and dimensions, I don't really know.

Just analyze the diaphragm as a propped cantilever, with the left side of the big cores being a roller and the small core at the right side being a pin. The simplified model is thus determinate, and you get a big reaction at the roller, which makes the shear through the narrow slab section small.

 

[BAretired]

If the plan is to scale, the location of shear walls cannot be recommended even if it can be justified theoretically. If the architect is unwilling to alter the location of shear resisting elements, then I agree with Mike that a frame at Gridline 1 and possibly at Gridline 3 would be worth considering.

BA

 

[asixth]

I dont think the thin slither of slab will experience much diaphram shear between those two cores which is where the force will be attracted too. A central core is a very common system.

 

[BAretired]

It may be a common system in most buildings, but the proportions of this building are not common. It is a very poor system here.

BA

 

[hokie66]

That relatively narrow section of slab won't see much in plane shear, but it does have to take the bending. A lobby between a bank of elevators would normally be 8' or so wide, so you have an 8' deep beam. I understand the misgivings, but still think it may work.

Perhaps GalileoG can enlighten us as to what type structure this represents. If it is a mezzannine slab in the atrium of a high rise, perhaps this level can be bypassed in the lateral load model.