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Diaphragm Problems in STAAD

Mohamed El-Deeb
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Hello all,

I am modeling a multi story steel structure building using STAAD.pro , every story is an RC metal deck floor , which requires modeling a diaphragm 

and I have faced a big problems in the diaphragm definition.

I know that I can define the diaphragm by the following methods

1- Floor Diaphragm 
      it's really amazing in the earthquake analysis , it's also help me to know the story shear, story load , drift ..etc. , also the earthquake tab is amazing !
but I am facing a problem which RUINS my structure, that the floor diaphragm in staad is only RIGID .. which lead to make the NORMAL force in every horizontal member of a truss or beam in the diaphragm floor = ZERO , which not logic and impossible !!    
 

2- Master/Slave Nodes
     by making a master node and assigning rigid ZX  , but it makes me lose the out put of story load, story shear , drift ..etc. 
    also it didnt help in the normal forces issue , it's still = ZERO  , also it let me lose the earthquake tab !!! 

3- Slab definition 
     it's slightly not bad option to model that semi reigid diaphragm , I think it works in analysis results and the axial force output. 
   but it let me lose each story shear, load, drift , CR, CG , and also the earthquake tab .. 


so how could I model a suitable diaphragm for my structure to let the model analysed well, and to extract the data I want !!! 

Thank you in advance for helping , 

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  • 0 Offline
    Since you have RC metal deck floor, these should be modeled as rigid diaphragms. The FLOOR DIAPHRAGM option is the best method to use in your case. It also automatically generates the outputs you are looking for.

    Regarding the forces developed in the beam members which are in the plane of the diaphragm, you will only have non-zero values for out of plane shear and out of plane bending. All in-plane forces like FX, FZ and MY should be zero because there will be no relative in-plane movement between the members due to the rigid diaphragm. The whole floor system will displace laterally or rotate about the vertical axis as a rigid body and transfer the shears to the lateral force resisting systems in your structure. This is an expected behavior.



  • 0 Offline in reply to Sye

    Hi Sye,

    This is with respect to your response above.

    I have a situation where there are Moment frames within a Mezzanine floor in both X-and Z directions which form a part of the Lateral force resisting system, such that the beams of these moment frames are in the plane of the diaphragm and form part of the Rigid diaphragm.

    The in-plane forces are transferred to these Moment frames, the beams of which should experience axial forces too in addition to out of plane shear and bending when considered in isolation as a lateral moment resisting frame. However they show zero in-plane axial forces as the rest of the beams of the rigid diaphgram.

    Is there any way that we can "disconnect" these Moment frame beams from the Diaphgram, or do we just perform hand calculations to account for Axial forces in these beams.

    Also what about those beams which are identified as collector beams in the system.

    Thanks

  • 0 Offline in reply to SATJAY

    What you can try doing is assign the floor diaphragm to a joint list and include only the corner nodes in that list. In other words instead of assigning the floor diaphragm to the entire set of nodes on a particular floor, just assign to the nodes at the corner of the floor. The beams and the collectors should then develop axial forces.



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