This article intends to explain how the P-Delta effects are taken into account in RM Bridge. Initially, the user must understand that this function “P-Delta effects” generates a stiffness matrix that depends on the normal forces in the structure:

1. If only one load case the program calculates this load case many times in an iteration loop until the change of normal forces satisfies a certain limit.
2. Where you want to consider the influence of the normal forces from several load cases, you should use the calculation type Accumulate stiffness.

Accumulate stiffness means the program takes the normal force from the accumulated load cases (SUM-LC given in the calculation pad) and sums up to these normal forces the normal forces from the actual load case (accumulate stiffness).  With that state of normal forces, the software builds the dependent stiffness matrix and calculates the actual load case. In this actual load case the P-Delta effects are now included with the total normal forces times the transverse deflections. This situation will be shown in the second schedule of the FAQ Example: Analysis - Second Order Effects.

In Schedule 2, the load case with the normal forces will be added, calculated, and stored in the load case LC1000. Now the load case with the transverse load will be calculated with the calculation type “accumulate stiffness”. Therefore, in the results of the second load case, the P-Delta effects due to the normal forces of the first load case (LC11) and the transverse deflection of the second load case (LC12) are included in the results.

In Schedule 3, the load case for the normal forces (LC11) will be calculated after the load case with the transverse load (LC12). Verify that. in this case, despite the active “accumulate stiffness” button is active, the software doesn’t take into account the P-Delta effects of both load cases. The reason for that exists in the circumstance that the second load case (LC11) has no transverse deflection.

To overcome that problem the calculation type Accumulate perm. Load can be used instead. With that button activated the program sums up the load input of both load cases and calculates the summation load case together. Therefore, with that method, you take into account the P-Delte effect of both load cases. Note that, however, this Accumulate Permanent Load method is only applicable if the calculation model has no changes between both load cases. In other words, this method is not usable in the case of having any construction stages analysis.