FEM - Mass group based on self-weight load case

Introduction

This chapter is to provide a closer information about change introduced in SCIA Engineer version 24.0. This change is applicable for modal analysis.

Upon opening of the project where modal analysis is present, this warning is shown:

Example

The content of the warning message is explained on the example provided below. There is a model of concrete frame structure, with two defined mass groups:

MG1 = mass from self-weight (automatically generated).

MG2 = 20 nodal masses of 50 kg each on the nodes of the uppermost floor.

Three combination of mass groups are considered and results between "before v 24.0" and "since 24.0" are compared below:

As seen, no matter whether the MG1 (mass from self-weight) was explicitly assigned by the user to combination of mass group CM1 - CM3, and what was the multiplier (whether 1 or 5 for CM1 and CM2 respectively), it was always considered in the background by multiplier 1. Since SCIA Engineer version 24.0, the user definition is being exactly respected, hence this is more transparent. Even though, there does not seem to be frequent use for turning off the mass which is automatically generated based on self-weight, or introducing the mass from self-weight by different multiplier then 1.0, what was the reason of the behaviour before v 24.0.

Note: the example does not consider the moments of inertia (Rotation around global X,Y,Z axes is turned off within the advanced analysis settings) in order to exclude the cause of other expected difference since v 24.0, which is explained " FEM - Neglecting of the moments of inertia of the beam members".