# Additional mass for projectile type “Rigid” or “Compliant” **URL:** \ **Category:** Product Support\ **Tags:** addon-building-model, addon-modal-analysis, bug-report, dlubal-calc\ **Created:** [October 28, 2026, 10:01am UTC](https://community.dlubal.com/t/zusatzliche-masse-bei-geschosstyp-starr-bzw-nachgiebig/5461 "2026-10-28T10:01:38Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![FLlamoca](https://community.dlubal.com/letter_avatar_proxy/v4/letter/f/91b2a8/32.png) [@FLlamoca](https://community.dlubal.com/u/FLlamoca)\ **Post date:** [October 28, 2026, 10:01am UTC](https://community.dlubal.com/t/zusatzliche-masse-bei-geschosstyp-starr-bzw-nachgiebig/5461/1 "2026-10-28T10:01:39Z")
Continuation of the discussion on [Stiffness Modifications for Surfaces](https://community.dlubal.com/t/steifigkeitsmodifizierungen-fur-flachen/5423/4): 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. ![image](https://community.dlubal.com/uploads/default/original/2X/e/ed0eb23445c59009adf0a412da97319c183bb6e7.png) ![image](https://community.dlubal.com/uploads/default/original/2X/f/ff7b9d9fe942986e34108f2f5b938cd6e933b04e.png) 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 1. Is ΣM expected to change for “Rigid”/“Flexible” compared to “Without”? What makes up the additional mass? 2. 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)? 3. Do I need to handle “Flexible” differently from “Rigid”? The example model is attached in the previous discussion. Thanks in advance! ---
**Author:** ![FLlamoca](https://community.dlubal.com/letter_avatar_proxy/v4/letter/f/91b2a8/32.png) [@FLlamoca](https://community.dlubal.com/u/FLlamoca)\ **Post date:** [October 28, 2026, 12:40pm UTC](https://community.dlubal.com/t/zusatzliche-masse-bei-geschosstyp-starr-bzw-nachgiebig/5461/4 "2026-10-28T12:40:51Z")
I was able to solve the problem with AI. I’m including the link in case anyone’s interested. > **[Dlubal CALC – AI-Powered Structural Calculations](https://calc.dlubal.com/share/v/d974e9de0688be7cf8df052b46d56e79db3d3a85eb459e4fccfa19603ad5aaed)** > > Dlubal CALC is an AI-powered web app for structural engineers – conversational calculations, direct RFEM integration, and professional PDF reports in a modern chat interface. ---
**Author:** ![FLlamoca](https://community.dlubal.com/letter_avatar_proxy/v4/letter/f/91b2a8/32.png) [@FLlamoca](https://community.dlubal.com/u/FLlamoca)\ **Post date:** [October 29, 2026, 2:48pm UTC](https://community.dlubal.com/t/zusatzliche-masse-bei-geschosstyp-starr-bzw-nachgiebig/5461/6 "2026-10-29T14:48:48Z")
Hello everyone! 🥲 Unfortunately, the suggestions from MIA didn’t really solve the problem. I decided to try the same approach with a larger model (with area and line loads), and the same problem has come up again. The masses considered in my modal analysis don’t match the actual masses. I’ve attached the model I used for the tests. Situation 1: Line loads (façade) ![image](https://community.dlubal.com/uploads/default/original/2X/7/7ab3366678ed30f1cf5a9e3df29b209f184ce966.png) The masses of the load-bearing elements on the floor are shown in blue. The line load would amount to 126 t (21 m \* 6 \* 1.0 t/m³). The following masses are calculated in the modal analysis: ![image](https://community.dlubal.com/uploads/default/original/2X/3/3df18f7e4999d6f8a5f782f523f11fde88b65560.png) Almost 100 t more per floor! To fix the problem, I defined the line load directly as a mass, and then everything worked perfectly. Unfortunately, this solution doesn’t work for area loads, since it isn’t possible to define them as masses when the floor type is set to “rigid” or “flexible.” This means I’m limited to obtaining realistic results only with the slab’s original stiffness. I’d appreciate a detailed explanation of why this phenomenon occurs, as well as any tips on how I can carry out my seismic design without problems and obtain realistic results. Best regards, Frank [Test Kern 2.0.rf6](https://community.dlubal.com/uploads/short-url/uk8THaHiSCisMlulys3sGzOrfbQ.rf6) (1.9 MB)