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#1 2020-06-22 13:39:15

chopboy
Member
Registered: 2020-06-22
Posts: 3

Beginner with Code_Aster - Incorrect Deflections

Hi,
I've begun learning code_aster for some basic FEA simulations, and firstly would like to thank the developers of the software for the excellent and polished packages they have produced.
I'm trying to simulate a floor grillage and have firstly broken the simulation down to a single bearer and joist (bearer intersecting joists)
The bearer stress should be ~44MPa and total deflection ~5.5mm per the below python calculation:

import numpy as np
import matplotlib.pyplot as plt
from math import exp
import math as math




# Room Geometry
l=5.6 # Room Length
b=3.6 # Room Breadth



# Bearer Properties
# Bearer Properties
Bno = 1 # Number of Bearers over the floor breadth
Eb = 210 # Bearer Youngs Modulus in GPa
SIGb = 200 # Bearer allowable in MPa
Ib = 4.028*10**-5 #Bearer Second Moment of Area - Calculate elsewhere
yb = 250 # Bearer depth


# Loads - From AS1170.1 or otherwise
UDL = 2 #kN/m2

UDLe = UDL*(b/(Bno+1)) #kN/m


"""Simple calc method ignoring grillage"""
"""Note - Youngs Modulii are lower than expected by 10**3 as loads are input in kN"""

# Deflections

y2 = (-5*UDLe*l**4)/(384*(Eb*10**6*Ib)) # l/b Factor applied for Joist Span. Deflection subject to UDL - Assumes pinned end conditions


print(y2*-1000,'mm')


Stressb2 = (UDLe*l**2/8)*(yb/2)/Ib

print(Stressb2/10**6,'MPa')

Output:
5.449851042701091 mm
43.79344587884806 MPa

Code_Aster is giving me ~35Mpa, which isn't unreasonable given I have added a single joist, but 0.00015mm deflection. The deflection is not realistic, but I can't see why within the model.

I've attached my model files for review and any assistance or guidance would be greatly appreciated.

Regards,


Attachments:
singleJoist.zip, Size: 51.94 KiB, Downloads: 39

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#2 2020-06-23 22:45:33

mf
Member
Registered: 2019-06-18
Posts: 97

Re: Beginner with Code_Aster - Incorrect Deflections

Hi,

this is most certainly a problem with units.

Looking at your comm-file and the material parameters (E and rho), I'd assume you want to use the N-mm-system (all SI units but mm instead of m). In this case, your material 'Wood' has a very low elastic modulus of only 17MPa. Elastic moduli of woods are about 300-1000 times higher in MPa.

Also, it means your forces are in N. Is this correct? Your force in -z would be 0.932N, seems low if you are talking about kN in your Python example. That is, if both can be compared somehow.

So, I'd recommend to check your units,

Mario.

EDIT: oh sorry, it is a force arete, therefore 0.932N/mm. So this part might be correct?

Last edited by mf (2020-06-24 11:03:33)

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#3 2020-06-25 11:04:19

chopboy
Member
Registered: 2020-06-22
Posts: 3

Re: Beginner with Code_Aster - Incorrect Deflections

Hi Mario,
Thanks for taking the time to review my model. I thought like you that it was likely to be a unit error, but it is interesting that the stress nearly matches the hand calc while the deflection is so far out. I just can't find what it is that I've setup incorrectly.
Thank you for picking up the joist Young's modulus error, I have corrected this and re-solved, but the strange deflection result remains.

Regards,

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#4 2020-06-25 13:44:22

mf
Member
Registered: 2019-06-18
Posts: 97

Re: Beginner with Code_Aster - Incorrect Deflections

Hi again,

if I switch materials and change 'Wood' to E=15000MPa I get ~9mm displacement in the center of the structure. Are your materials assigned as intended?

Regarding your .comm from above: CALC_CHAMP and MECA_STATIQUE (or STAT_NON_LINE etc.) should have the same name (or use 'reuse the input object' in Salome-Meca), otherwise DEPLs are not in the results.. I don't know why.

Mario

Last edited by mf (2020-06-25 13:44:43)


Attachments:
DEPL+comm.zip, Size: 596.5 KiB, Downloads: 36

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#5 2020-06-25 20:56:56

sb1966
Member
Registered: 2015-03-16
Posts: 172

Re: Beginner with Code_Aster - Incorrect Deflections

Hi,
Mario wrote:

.... otherwise DEPLs are not in the results.. I don't know why.

AFAIK, it is normal behaviour of code-aster/salome-meca. Without reuse (or when output with a new name), CALC_CHAMP does not preserve the input to it, i.e., DEPL in this case.

The explanation seems reasonable to me, but I am not confirmed. It would be nice to learn from the Gurus though it might be trivial from users' perspective.

Best regards.

Sukumar

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#6 2020-07-06 00:00:19

chopboy
Member
Registered: 2020-06-22
Posts: 3

Re: Beginner with Code_Aster - Incorrect Deflections

Thanks Mario! That has solved the deflection issue I was having.

Regards,

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#7 2020-07-14 21:04:52

mf
Member
Registered: 2019-06-18
Posts: 97

Re: Beginner with Code_Aster - Incorrect Deflections

Good, could you please mark the thread as solved? Edit your first post and write [SOLVED] in front of the title?

Thank you.

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