Hello everyone!

I’m trying to replicate with Sofistik the displacement time history of a SDOF system subjected to a step load.

The SDOF system should be a single mass oscillator. I created a massless, totally clamped column (with very high shear modulus G to have just Euler-Bernoulli bending) with a single mass on the top node. This node is subjected to a suddenly horizontal force acting for just one second. The structural damping of the system is 5% of the critical.

The report of the following code shows the correct eigenfrequency of the SDOF system and an eigenvector with a displacement of 1 meter of the top node. The problem comes in the DYNA module: the load case that I defined in SOFILOAD (100 kN acting for 1 second) takes the force needed to bend the top node up to 1 meter (=eigenmode), i.e. the stiffness of the system multiplied by 1 meter. I really don’t find how to keep the original load defined in SOFILOAD. The displacement time history corresponds to the analytical solution using the Duhamel’s integral, but only scaling it by the value “load DYNA / load SOFILOAD”.

I searched examples in the Sofistik database but I don’t find my error.

Here is my code, thank you very much for helping me!

PROG AQUA urs:1

KOPF Material and cross-section definiton

NORM EN 199X-200X

MAT NR 1 E 12600E3 GAM 0 G 1E11 ! [kN/m2]

QB NR 1 H 800 B 400 MNR 1 ! [mm]

ENDE

+PROG SOFIMSHC urs:5

KOPF Geometric modelling

SYST 2D GDIR NEGY

STEU OPT MESH 1 $ Connecting beams - Vernetzung von Stabwerken

STEU OPT DELN 0 $ Delete useless structure point and line

SPT NR 1 X 0 Y 0

SPT NR 2 X 0 Y 3

SLN NR 1 SDIV 3 NPA 1 NPE 2 QNR 1

ENDE

+PROG SOFIMSHA urs:2

KOPF Full restrain

SYST REST $ Keep info from SOFIMSHC

KNOT NR 1 FIX F

MASS NR 2 MX 1000.[kg]

ENDE

+PROG SOFILOAD urs:3

KOPF Step load

LF NR 1 TYP NONE FAKT 1.0 BEZ ‘Step load’

KNOT NR 2 TYP PXX P1 100[kN]

FUNK T 0 F 1 $ Time Value

T 1 F 1

T 1 F 0

ENDE

+PROG ASE urs:6

KOPF Load case 1

LF NR 1

ENDE

+PROG DYNA urs:4

KOPF Eigenmode and damping

STEU OPT ELF WERT +1

OPT STAB WERT +8

EIGE NEIG 1 ETYP SIMU NITE 1

MODD 1 D 0.05

STEP N 200 DT 0.01

LF NR 1

HIST TYP U-X VON 2 ERG DRUC LFSP 1

ECHO OPT EIGE WERT EXTR

ENDE

+PROG DYNR urs:16

KOPF Time history results

HIST 1 U-X 2

ENDE

+PROG RESULTS urs:18

KOPF Diagram

LF NR ALLE

KNOT TYP UX DARS DLST

KNOT TYP BX DARS DLST

ENDE