Continuation of the discussion on Stiffness Modifications for Surfaces:
Hello everyone,
I'm comparing a reinforced concrete building in RFEM 6 (version 6.16.0002) with ETABS and have encountered a mass discrepancy in the modal analysis that I can't explain.
Model
- 5 stories, 4 m each, doubly symmetrical floor plan, central reinforced concrete core and columns
- Building Model add-on, story types: “Without | Original Stiffness,” “Flexible,” “Rigid”
- Modal analysis masses taken from a load combination containing only self-weight
- Manual calculation per story: slab 330.8 t, walls 86.4 t, columns 10.8 t, for a total of 428.0 t per story
Observation (sum of masses ΣM in X/Y from the Effective Modal Masses table)
- “Without | Orig.”: approx. 2,128 t, consistent with the manual calculation and ETABS (2,120.7 t)
- “Rigid” and “Flexible”: 2,678.4 t, or 550 t more (approx. 110 t per story)
The additional mass occurs only with “Rigid” and “Flexible.” Numerically, it corresponds to about one third of the slab mass per story. I don't know if that's a coincidence.
Results from the previous discussion:
- Static check with a 10,000 kN horizontal load at the top: displacement in RFEM “Rigid” and ETABS (Rigid Diaphragm) is 15.6 mm in both cases. So the stiffness is the same.
- Natural periods: RFEM “Rigid” T1/T2 = 0.25 s and T3 (torsion) = 0.205 s; ETABS 0.219 s and 0.194 s
- The difference in periods can be explained almost entirely by the additional mass: 0.219 s · √(2,678.4/2,120.7) ≈ 0.246 s.
My questions
- Is ΣM expected to change for “Rigid”/“Flexible” compared to “Without”? What makes up the additional mass?
- How should I define the modal analysis masses in the Building Model so that each mass is counted only once (e.g., self-weight directly in the modal analysis load case instead of importing it from the combination)?
- Do I need to handle “Flexible” differently from “Rigid”?
The example model is attached in the previous discussion.
Thanks in advance!