Here's an interesting issue...

I created a Beam Calculator calculation like this:

Load Type: fixed at one end, uniformly loaded

Deflection calculation

Beams button > Select W16x45, click Done (This makes Moment of inertia 32.8 in^4)

Modulus of Elasticity: 30460000

Length: 144 inches

Load: 6000 lbs

Solve: Deflection = 2.24 in

Next, I tried the same calculation in SimulationXpress...

1) Office Products > Structural Steel

2) Select W16x45 beam (same as above), click create, done

3) Extrude sketch to 144 inches (same as above)

4) Apply material: Alloy Steel (Modulus of Elasticity ~30460000, same as above)

5) Open Simulation Xpress

6) Fix one end of beam, Apply 6000 lb load to top face

7) Mesh and Run the study

8) View results for displacement

9) Note maximum displacement is only 0.132 inches, much less than beam calculator results.

First question: Why are the results so drastically different?

Second Question: Which result is correct?

estimate, but when using weak tools that don't apply correctly to the situation, the results should be considered suspect. I built your model, and both the beam calculator and Xpress results you list are what I got, too, so we are either making identical errors, or showing the same results of these inappropriate assumptions on the part of the Xpress tool.