Distributed-mass model

[openc]

Hello:

I was wondering if there is any option in Opensees to model systems with distributed mass? I am looking at modelling a beam with some x value of mass per unit length. Thanks for your time.

[fmk]

use the -mass option (it is mass per unit length and it just does lumped mass)

[url]
http://opensees.berkeley.edu/wiki/index ... mn_Element
[/url]

[openc]

Hi fmk,

I am working on a 1 storey high problem where I am modelling columns as springs and the diaphragm as a beam. I have assumed mass and storey stiffness values to get a fundamental period of 0.5s. I checked this in SAP too and I am getting 0.5s.

I am trying the same example in Opensees and not quite getting the same 0.5s. I may be doing something wrong. Could you please correct me? The code is as below -

*****************************
wipe;

model basic -ndm 2 -ndf 3 # Basic 2D model having 3 degrees-of-freedom per node.

#Node details: #Node no., X-coord., Y-coord.
node 1 0 0
node 2 0 0
node 3 0 0
node 4 4 0
node 5 8 0
node 6 12 0
node 7 16 0
node 8 20 0
node 9 24 0
node 10 28 0
node 11 32 0
node 12 36 0
node 13 40 0
node 14 40 0
node 15 40 0

puts "Node defined"

#Boundary Conditions: #Node no., DX, DY, RZ
fix 1 1 1 1
fix 2 1 0 0
fix 3 1 0 0
fix 4 0 0 0
fix 5 0 0 0
fix 6 0 0 0
fix 7 0 0 0
fix 8 0 0 0
fix 9 0 0 0
fix 10 0 0 0
fix 11 0 0 0
fix 12 0 0 0
fix 13 1 0 0
fix 14 1 0 0
fix 15 1 1 1

#Slaving node:
equalDOF 2 3 1 2 3
equalDOF 14 13 1 2 3

puts "Boundary Conditions assigned"

#Specifying material: #Material Tag, Stiffness value
uniaxialMaterial Elastic 1 8.0486
puts "Spring stiffnesses defined"

#Section details for columns: #Section Tag, Material Tag, Axial force deformation (use "P")
section Uniaxial 1 1 P

puts "Section forcolumn defined"

#Section details for beam:
section Elastic 2 2.1e+11 37853629000000 0.00063878
puts "Beam sections defined"

#Coordinate Tranformation:
geomTransf Linear 1

#Element details: #Element Tag, Nodei, Nodej, no. of iteration points, Section tag, Coordinate transformation tag, -mass (mass/unit length)
#Element details: #Element Tag, Nodei, Nodej (both nodes at same coordinate), Section Tag
element zeroLengthSection 1 1 2 1
element forceBeamColumn 2 3 4 8 2 1 -mass 0.00254841997961264
element forceBeamColumn 3 4 5 8 2 1 -mass 0.00254841997961264
element forceBeamColumn 4 5 6 8 2 1 -mass 0.00254841997961264
element forceBeamColumn 5 6 7 8 2 1 -mass 0.00254841997961264
element forceBeamColumn 6 7 8 8 2 1 -mass 0.00254841997961264
element forceBeamColumn 7 8 9 8 2 1 -mass 0.00254841997961264
element forceBeamColumn 8 9 10 8 2 1 -mass 0.00254841997961264
element forceBeamColumn 9 10 11 8 2 1 -mass 0.00254841997961264
element forceBeamColumn 10 11 12 8 2 1 -mass 0.00254841997961264
element forceBeamColumn 11 12 13 8 2 1 -mass 0.00254841997961264
element zeroLengthSection 12 14 15 1

puts "Element details provided"

#Eigen Values: #Displays 4 fundamental periods
set Lambda [eigen -fullGenLapack 4]
puts "Lambda Omega Period"
foreach Lambda $Lambda {
if {$Lambda > 0.0} {
set Omega [expr pow($Lambda,0.5)]
set Period [expr (2*3.14)/$Omega]
puts "$Lambda $Omega $Period"}}

***************************************

Please note that:
(1) The results are senstive to the value of $numIntrgrPts (used for beam element). This may be one reason? What should be the right value please?
(2) Following the manual example, I have given a very high value for the beam c/s area. Is this OK?

Thanks for your time.