Modeling with plates

Question

Hello Everyone, 
 
 I have modeled a two-storey building in staadpro using three methods and I have gotten varying results. The three scenarios I tried are as followed: 
 1. I modeled the structure with only the structural frame (ie. beams and columns only), using self weight to account for the frame weight and floor loads to represent my floor loading (inclusive of the slab weight). 
 2. I modeled the structure with the structural fame along with plates to simulate my slab using self weight to account for the frame and slab weight and floor load to represent my floor loading. 
 3. I modeled the structure with the structural fame along with plates to simulate my slab similar to method two, however I used the plate stress command so as to ignore the effect of in-plane bending. 
 I have noticed that the moments generated (based on dead loads only) in my beams has significantly decreased between method one (higher moments/forces) and methods two and three (lower moments/forces). I have also noticed that my storey drift in methods 1 and 3 are fairly similar, while my storey drift using method two is lower. What is the cause of these difference and which method would give my the best representation of my structure (the structural frame of the structure is a rectangle in plane with no horizontal nor vertical irregularities). 
 
 thanks

Sye · Answer

The higher moments in beams for structure 1 is due to the fact that the beams are carrying the whole moment from the applied load whereas when a slab is modeled using plates, it is sharing moments and shears too causing the beam to take less. The story drift for model 1 would be more because the in plane stiffness due to the slab is missing in that model when compared to others. 
 Regarding which one to use, here is what I think you should do 
 If the diaphragm is a rigid one, you should use the FLOOR DIAPHRAGM command. That would NOT require modeling the slab with plates but would still give you in-plane rigidity offered by a diaphragm. You can take care of the loads using FLOOR LOAD. 
 If the diaphragm is flexible, model it using plates. If you DO NOT want the diaphragm to carry any out of plane bending or shear, then only you should specify it as PLANE STRESS but please note in that case you would NOT be able to apply any out of plane loading on it, not even selfweight. Else if the diaphragm can carry out of plane loads, simply model it with plates and it will automatically pick up appropriate moments and shears based on it&rsquo;s actual stiffness which I think would be more realistic.

Sye · Answer

Couple of points. If you are using floor diaphragm you do not need to use plates. In general I would expect the drifts to be different when using a floor diaphragm vs not using it. If you can upload the models in which you observed no difference in drifts with or without the diaphragm, I can look into it. Semi rigid diaphragms would also have to be modeled using plates in STAAD.Pro

Kaushal Aggarwal · Answer

Gilbert, 
 
You may additionally want to go through the following discussion I have on the same topic few months ago and found the answer. 
 communities.bentley.com/.../118398

Sye · Answer

I have not been able to take a detailed look at the models so far but based on a preliminary investigation of the models, here are my findings. The drift values are not exactly same but are pretty close. You should look at results for cases 15 onwards where lateral loads are acting on the structure. If you look at nodal displacements at the top level of the structure ( 59, 60, 61 ), the displacements at these nodes for load case 15 are also different between two models but again the difference is not that much. So the floor diaphragms do not seem to make a whole lot of difference in this case. It still is the more accurate way to model the in-plane stiffness due to the slab.

Sye · Answer

The inherent torsion and accidental torsion are only to be considered for rigid diaphragms. For semi rigid or flexible diaphragms, just model the diaphragm using plate mesh with appropriate material and thickness and the analysis would distribute the lateral forces to the lateral force resisting systems based on their relative stiffness. You should not use the FLOOR DIAPHRAGM option in that case.