How to model a concrete drop beam

[vsator]

Hi
I am using SAP2000 to model mass concrete subway stations (thickness of slabs are about 1500mm). I appreciate if you answer my questions about modeling drop beams. In attachment you can find my model, hand calculations and results.

1)I made 5 different simple models:
•Modeling beam with frame elements without considering drop
•Modeling drop beam with frame elements using “body constraints”
•Modeling drop beam with frame elements using “Area joint offset”
•Modeling beam with shell elements without considering drop
•Modeling drop beam as a shell element using “Area joint offset”
Could you please tell me which one is more accurate for modeling drop beams.

2)How can I use the results to design the drop beam? For drop beams modeled with shell elements should I just design the beam for forces of their own shell element or I should also include forces of slab’s shell elements. What about using frame elements? How can I found design forces for drop beams modeled with frame elements?

3)I made some hand calculations for the simple slab under its own weight using tributary area but the forces of none of the above models are equal to that. Can you help me to find the problem.

 

[vsator]

the SAP file is in attachment
Thanks in advance for your help

 

[vsator]

S2k file is in following:

 

[sebivaq]

Dear vsator,
I would model the slab as a thick shell element, and the beam as a frame element (you can use the concrete design tool). In order to obtain results close enough to the hand calculations, you should:

1) Set the torsional stiffness of the beams to zero, to model a pinned supoport for the slab.
2) Erase shell elements near the supports, because of the local effect.

Best regards,

Sebastian