There are instances where it may appear that MecaStack is not properly considering external point loads applied to the model. If the stack is self supported (Free standing) then the check to see if loads are included in the reactions is very simple. For example, in this self supported stack we have applied a 1000 lbs shear load at the top of the stack and classified it as dead load (ExtLoad_Self.stk):
Within MecaStack there is a section of the output labeled "Static Summation of Forces". What this does is sum the reactions acting on all restraints in the system in the X, Y and Z directions. Looking at the Summation of Forces in the output, we see the 1,000 lbs that we applied as shear load under the dead load case.
Now if we take this exact same stack and add guy wires (ExtLoad_Guy.stk), things become much less clear.
Now the summation of forces indicates that our 1,000 lbs of shear because 1,690 lbs of total reaction at the base. In a linear system if we apply 1,000 lbs of force we expect to get 1,000 lbs in the reaction; however, a guy wire stack is not a linear system. Not only are the guy wires non-linear, but we are also essentially performing a P-Delta analysis on every guyed stack. In MecaStack we break up the loads into increments, or load steps. In this example we used the default of 10 load steps. This means we take our 1,000 lbs force and divide it by 10, and we apply 100 lbs per step. After applying the first 100 lbs, we then calculate deflection of the stack and then determine the new tension in each guy wire. The stiffness in the guy wire is dependent upon the tension, and so as we change the tension in each guy wire we are also changing the stiffness of that guy wire. As the tension increases, the stiffness increases, and as the tension decreases the stiffness decreases, and this is non-linear behavior. Also, the P-Delta effect is taking place also. This means that after each load step our weight components are acting at the deflected center of each element, considering the deflected shape. The weight is creating a secondary moment, which in turn causes more bending in the stack and can also impact the resulting tension in each cable. All of this makes the situation very complicated.
In the above example, we applied the shear load at the top of the stack. Now lets apply it at 50 ft elevation and see what happens.
Now the 1,000 lbs force translates to only 50 lbs of net reaction. If we take the exact same stack but remove the guy wires, we once again see that the forces total the 1,000 lbs because we once again have a linear system.
