Design-To Loads are the force and moment vectors that are actually fed into the margin of safety calculations for each Zone. Not to be confused with a Design Load Case, found in the previous section, which is a particular loading scenario - akin to a FEM subcase.
Design-To Loads are the result of taking the FEM forces from the Design Load Case through the load processing steps, to generate a force and moment vector per case, per panel or beam.
The table below shows the Design-To Panel Load Components for mechanical and thermal design loads. For each, panels are loaded using six uniform loads from classical lamination theory (CLT) and two out-of-plane shear loads (\(Q_x\), \(Q_y\)). Mechanical loads have an additional applied pressure. Thermal loads have a uniform change in temperature and a through-thickness temperature gradient.
|
Load |
Description |
Mechanical |
Thermal |
|---|---|---|---|
|
\(N_x\), \(N_y\), \(N_{xy}\), \(M_x\), \(M_y\), \(M_{xy}\) |
Primary loads - ABD matrix |
Yes |
Yes |
|
\(Q_x\), \(Q_y\) |
Out-of-Plane Shear Loads |
Yes |
Yes |
|
\(P\) |
Pressure |
Yes |
No |
|
\(T_{ref}\) , \(T\) |
Uniform \(\Delta = T - T_{ref}\) |
No |
Yes |
|
\(tt_{grad}\) |
Through-Thickness Temperature Gradient |
No |
Yes |
Note
The sign conventions for the loads are shown in the diagram on the Load Property form.
Panel Loading Sign Convention
A positive moment is defined as one creating a "sagging" deflection relative to the orientation of the figure.
The membrane and bending loads are restricted to being uniform across both edges of the plate. Equilibrium is automatically satisfied for these primary loads. Out-of-plane shear loads are not included in the force balance.
Reference Plane
For stiffened panels, the plane of action of the membrane loads is defined at the midplane of the top skin - away from the so-called neutral axis of the panel. See the upper right corner of the Point Free Body Diagram for a visualization of the reference plane.
This choice of reference plane is necessary in order to couple with FEA in a consistent manner. Shell elements of the smeared stiffened panels are meshed at the midplane of the top facesheet.
For a stiffened panel, a pure \(N_x\) load (\(M_x\) = 0) applied at this reference plane will cause significant bending. To apply a uniform end-shortening load, simply set the \(M_x\) degree of freedom as "constrained." When the degree of freedom is constrained, reaction (virtual) moments are computed to ensure zero panel curvature.
User-defined and FEA imported out-of-plane shear loads (\(Q\)) are superimposed on the primary loads (\(N\) and \(M\)) and therefore do not satisfy equilibrium.
For unstiffened and sandwich panels, the reference plane can be adjusted on the Zone Settings form under Loads > Reference Plane.
For mechanical and thermal design loads, beams are loaded with unequal end moments (\(M_{1A}\), \(M_{2A}\), \(M_{1B}\), \(M_{2B}\)); shear loads (\(V_1\), \(V_2\)), torque, and axial loads are applied at the beam-neutral axis. Mechanical loads have an applied distributed load (pressure). Thermal loads have a uniform and through-thickness change in temperature.
Note
For user loads, shear loads are not applied directly but react based on the applied end moments.
|
Load |
Description |
Mechanical |
Thermal |
|---|---|---|---|
|
\(M_{1A}\), \(M_{2A}\), \(M_{1B}\), \(M_{2B}\), \(V_1\), \(V_2\), Axial, Torque |
Primary Loads - Beam Theory |
Yes |
Yes |
|
\(P\) |
Pressure (Distributed Load) |
Yes |
No |
|
\(T_{ref}\), \(T\) |
Uniform \(\Delta{T}=T-T_{ref}\) |
No |
Yes |
|
\(tt_{grad}\) |
Uniform Temperature Gradient |
No |
Yes |
Beam Loading Sign Convention