Slab Run

[Junior1234]

Hi All,

Just a question about a one way nominal width slab run

In the run i have imaginary supports which are picked up in the run going perpendicular

My question is how much load (self weight, live load and deadload) should be considered before the run is stiff enough to be considered to transfer to these imaginary supports. it is important to mention the load will also be picked up in the other run so considering to much will be inaccurate and inefficiently doubling the loads. Considering to less and the element is not stiff enough to attract the load and transfer to the intended areas

 

[jayrod12]

The question isn't clear. Why are the supports imaginary?

 

[BAretired]

Good question, jayrod12. Also, I'm not clear on what is meant by a slab "run". That is not common terminology, so far as I know.

BA

 

[jayrod12]

I assumed he's taking a design strip of the slab. But then talks about imaginary supports. If this is a two way slab, and you're trying to run a middle strip, then I guess your imaginary supports would be the perpendicular column strips. But in those cases, I don't understand where the issue about enough stiffness comes from. Two-way slab design is fairly straight forward for most regular layouts. And for irregular layouts there's many analysis programs and simplifying assumptions that can be made. Reinforcing is cheap in the grand scheme of things, so unless this will reduce it signficantly and it's a repeated floor plan so the overall reinforcing savings are substantial, I'd just throw extra reinforcing at it.

If it is a one way slab like the first sentence indicates, I don't understand how the supports can be imaginary.

 

[rapt]

If the "imaginary supports" are not stiff enough, then they are not supports and it is a two way flat slab.

In all designs, all of the load is carried in both directions, unless the slabs span one way only to supports that are walls in the other direction that carry the loads vertically to other supports below or the ground below.

 

[MIStructE_IRE]

There’s no such thing as imaginary supports.

 

[Junior1234]

imaginary supports as locations in a slab strip direction picked up by the other direction?

 

[rapt]

Sorry, your final "question" still does not make sense to me.

 

[Settingsun]

Are you asking how far your column grid can deviate from square before it stops acting two-way? Eg 6m x 6m is clearly conventional two-way, but 6m x 24m will act somewhat like one-way?

 

[MIStructE_IRE]

Can you post a sketch?

 

[Junior1234]

alright ill go into more detail hopefully this will make more sense

theres the uploaded image. The highlighted area is where i want the imaginary support to be. What im aiming for is the cantilever portion loads at the bottom gets pulled in the vertical run, then the horizontal run pulls it to the imaginary supports. Hence when the vertical run is done that imaginary support is pulled to an actual support

what do yall think any suggestions otherwise on how these cantilever portions approx 3m can be supported

 

[Settingsun]

I think you should do a two-way analysis using software rather than trying hand calcs using strips for this.

 

[Junior1234]

any free software that yo know of @steveh49

 

[rapt]

You could approximate it with 2D software by running both directions as 2D frames with cantilevers.

Then apply equal and opposite point loads at the ends of the 2 cantilevers and adjust the loads until the deflections of the 2 cantilevers are equal.