How does STAAD determine beam deflections?

Question

Hello, 
 I am modeling a floor slab (1 ft thick) with concrete beams (all 24" wide x 36" deep). These beams will be cast monolithically with the floor slab, therefore the beams are modeled as 24" deep. 
 My question is how does STAAD determine the following beam deflections? What are the formulas? What criteria is taken? 
 (The particular beam in question is highlighted in blue. The red beam selected is the one segment determined with the max deflection. The circled deflections on the right of the image is what I'm questioning how STAAD obtained these values.) 
 
 Any help will be greatly appreciated. Basically, I want to check these values, perhaps by hand, and conform my STAAD output is reasonable. 
 Best, 
 Pascual

Sye · Answer

The displacement you circled is the Global displacement in Y at the start of the beam. So basically this is the displacement at the node at the start of the member. For the beam and the slab to be considered as monolithic, please make sure that the beam is broken up into segments in line with the plate mesh. 
 Details on how the stiffness analysis is carried out by STAAD.Pro is available at 
 https://docs.bentley.com/LiveContent/web/STAAD.Pro%20Help-v11/en/GUID-3ED02290-F1A5-4EA7-8313-8A23C75A9F19.html

Sye · Answer

Yes -0.23 in is the deflection at node 1660. The literature I referred you to describes how stiffness analysis is carried out by STAAD.Pro. I am not sure how you can hand calculate and match the value of -0.23 inches because it is not just a simply supported beam in isolation but rather one that is connected to a slab which is a two way type element ( transferring load in both directions) and directly influences the displacement of the beam. Moreover the element matrices/global matrices also involve terms related to shear deformations which are not accounted for in hand calculations. If you want to do some verification on the software, there are a number of verification examples that you can find at 
 https://docs.bentley.com/LiveContent/web/STAAD.Pro%20Help-v11/en/GUID-8C62E6A3-4470-4E9E-92DB-4878EB27DF9D.html