Variation of Concrete strength - Columns&Walls

[Murali27]

Dear All,

Is applicable in the practice if the variation of column concrete strength changes from 10 ksi to 5 ksi at a level?
Even this variation is safe to resist the internal moments & shears.

Your input will be appreciated.

Murai

 

[JAE]

ACI doesn't directly deal with differences between columns in adjacent storeys. However, section 10.15 does set limits on concrete strength variations between a floor structure and the column supporting it.

With a limit of 1.4 maximum between floor and column, you have to do some different design work to accommodate 10 ksi vs. 5 ksi in the columns....in other words, at least one of your two columns will most likely be more than 1.4 times the floor f'c. See 10.15 and the few sections that follow for the required actions.

 

[Murali27]

JAE


Section ACI 10.15 is transmission of column loads through floor system. When the f'c of column is more than 1.4 times f'c of floor, either of 10.15.1 to 10.15.3 adopted.

Once i take care of the floor system as above, Is it allowed to change my concrete strength from 10 ksi to 5 ksi at a level from the design point of view alone.

Is there any limitation in the view of practice ?

Your input will always be appreciated.

 

[ishvaaag]

There may be some code limitation where the seismic code does not allow for some degree of change in the stiffness of consecutive stories, what the change of concrete may cause to happen.

 

[JAE]

Yes... I think ishvaaag has hit on the primary concern. If you satisfy all of the 10.15 provisions (assuming you exceed the 1.4 limit) then there is really no other issue to consider EXCEPT the fact that you now have a column story that possesses a much lower Ec modulus and thus, is a lot less stiff than the story below.

The seismic provisions for "soft stories", etc. should be carefully looked at to verify whether or not you have created an irregular structure.

But other than that, I do not believe that there is any code/spec provision that would keep you from varying the f'c as you propose.

 

[Murali27]

Thanks ishvaaag & JAE. In this structure the predominant lateral force is due to wind only. I understood that one can use this option once satisfying 10.15 provisions