• State Suggested Answer
  • Replies 6 replies
  • Answers 3 answers
  • Subscribers 111 subscribers
  • Views 5684 views
  • Users 0 members are here

Diaphragm Problems in STAAD

Mohamed El-Deeb
Offline

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 , 

  • 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

    Thanks Sye for your usual quick response,

    but I think you didn't get what I meant exactly .. so let me explain the situation in this simple example ,

    In the attached photo, If I have RC metal deck over the top cord and bottom cord of that truss , and I will model the metal deck as a floor diaphragm in STAAD Pro to get the earthquake analysis results for torsion , story shear and so on as I mentioned before .

    So you are trying to convince me that it's impossible to find Normal (axial) forces in the TOP and BOTTOM cords of the truss due to the diaphragm .. I think that's surely not right and it completely contradict the basics of structural analysis !

    that's the problem I was refering to in my question , so please let me know how can I model this case in staad , to make sure that the analysis is right, seismic analysis is working , seismic output is available !

    thanks in advance for your precious time Sye.  

  • 0 Offline in reply to Mohamed El-Deeb
    As I already explained in my earlier post, if you are idealizing the slab as a rigid diaphragm, there cannot be relative movements in between the members and you cannot have axial forces in these members. If you want to consider in plane deformations in the diaphragm resulting in axial forces in members, model the diaphragm using plates.



  • 0 Offline in reply to Sye
    I think the problem may be that the members are part of both a diaphragm and a truss. Getting the convenient output via the Floor Diaphragm option removes the necessary output for the truss.

    Be careful however you model this as if you model the diaphragm directly as a plate , the plate will suck up much of the axial loads from the truss which may undersize your truss members. It would be worth a hand check to make sure the members have the correct internal loads.
  • 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

Communities
Support and Services
Training and Learning
Social Media