Slab system vs Diaphragm 2

[ElyasCivil]

Hello guys hope that you are doing well.

I have a question that might seem silly.

In concrete structures as I'm reading ACI we have two methods of designing for two way slabs:
Direct Design Method and Equivalent Frame Method. Now for example for the DDM it says that this is for gravity load design only. In what instance does this apply?

I am also approaching a chapter called Diaphragms.

What is the difference between a two way slab and a diaphragm? The entire chapter on two way slabs never mentions diaphragms.

Thank you very much in advance

 

[jayrod12]

Diaphragm is regarding the distribution of lateral forces to the resisting system (i.e. shear walls, moment frames, braced frames). The two way slab system will act as your diaphragm and you need to ensure it has the capacity to do so. Mostly that means ensuring the connections are capable of transferring the loads.

The reason the DDM example says it's for gravity load design only is because it's true. That method is only intended on the design of the slab for the gravity loading, it is not intended to be used to try and design the diaphragm actions of teh slab. Generally speaking, with two way slab designs, if it works for gravity, it will work for a diaphragm provided the appropriate attention to detailing is taken.

 

[ElyasCivil]

Thank you very much Mr. Jayrod I really appreciate it. So just to clarify in the DDM or the equivalent frame method we come up with reinforcement in both North South and East and West directions. Based on what I understand this reinforcement is sufficient only for gravity loads. On the same slab that was designed based on the DDM method we must now add ASSIDTIONAL reinforcement so that it acts as a diaphragm and transfers that lateral forces.

Is my understanding from your comments correct?

Thank you soo much again.

 

[jayrod12]

Potentially you don't need to add additional reinforcement. Many times, the reinforcement required for gravity design can pull double duty for any requirements from the lateral design once you take into account the different load cases.

Sometimes you will need additional reinforcement to act as chord reinforcing or collectors. But often the standard bars that will be in your slab edge, and the typical connections from slab to lateral resisting system are enough.

 

[ElyasCivil]

Thank you very much sir you are appreciated.