p-delta effect

[nazgol]

Hi,
in my model cantilever 2D with these properties:
lateral load V=1000kg
gravity load P=-20000kg
cross section A=23.9 cm^2
modulus of elasticity E=2.1e6 kg/cm^2
moment of inertia I=1317 cm^4
length L=400 cm
under static analysis, lateral displacement with p-delta effect is:
with opensees:12.5562 cm
with sap2000:14.39 cm
with solve differential equation:14.43 cm
why the result with opensees is different with sap2000 or differential equation?
# --------------------------------------------------------------------------------------------------
# Example 1. cantilever 2D
# all units are in kg,cm
# elasticBeamColumn ELEMENT
#
#
# ^Y
# |
# 2 __
# | |
# | |
# | |
# (1) 400
# | |
# | |
# | |
# =1= ---- -------->X
#
wipe;
model basic -ndm 2 -ndf 3;
file mkdir data3;
node 1 0 0;
node 2 0 400
fix 1 1 1 1;
mass 2 0 1.e-9 0.;
geomTransf PDelta 1;
element elasticBeamColumn 1 1 2 23.9 2.1e6 1317 1;
recorder Node -file Data3/DFree.out -time -node 2 -dof 1 disp;
pattern Plain 1 Linear {
load 2 1000 -20000 0.;
}
constraints Plain;
numberer Plain;
system BandGeneral;
test NormDispIncr 1.0e-8 6 ;
algorithm Newton;
integrator LoadControl 0.1;
analysis Static
analyze 10;

puts "Done!"

[silvia]

do you get the same results without Pdelta???

[nazgol]

yes, I get the same result without pdelta:
sap:7.7136cm opensees:7.71354 cm equation:7.71354cm
It's important that:
pdelta plas large displacements in sap2000 = displacements with corotational in opensees
but
displacements with pdelta effect in sap2000 isn't equal with pdelta effect in opensees

in my model:
for v=1000kg:
pdelta plas large displacements in sap2000:12.5418cm = displacements with Corotational in opensees:12.5335cm

displacements with pdelta effect in sap2000:14.39cm but displacements with pdelta effect in opensees:12.5562cm
of course displacements with pdelta effect by solve differential equation:14.43 cm

for v=10000kg:
pdelta plas large displacements in sap2000:117.1719cm = displacements with Corotational in opensees:117.117cm

displacements with pdelta effect in sap2000:143.905cm but displacements with pdelta effect in opensees:125.562cm
of course displacements with pdelta effect by solve differential equation:144.3134 cm

[ArdalanNejat]

dear nazgol,
did you solve this issue? I'm dealing with it now, I need help please...I checked last revision(5634) but I get same results as you did !

[fmk]

as the documentation points out, OpenSees does P-Delta, i.e. P large Delta. if you want to include P-delta, i.e. p small delta, you either break the column up or if 2d use the new element for p-delta (see example below).

have a look at the verification example AISC25.tcl in the following:
http://opensees.berkeley.edu/WebSVN/lis ... ication%2F

AFTER you familiarize yourself with the difference between P_Delta and P-delta.

you have to know the theory before you go playing with the tools otherwise you will have no clue as to what the results represent.

[ArdalanNejat]

Dear fmk,
thank you so much for quick response, actually I knew that OpenSees do the P-large delta but I was seeking a way to do both P-Delta & P-delta by only one element (not by breaking it up into some elements) and you just showed me a way in 2D thanks! by the way would you please give me OpenSees documentation link? thank you so much

[mhscott]

There is little documentation, but information on the forceBeamColumnCBDI can be found in:

http://dx.doi.org/10.1061/(ASCE)0733-9445(1998)124:6(704)

http://dx.doi.org/10.1061/(ASCE)ST.1943-541X.0000757

and references therein.

[fmk]

the command manual at this moment in time still does not have the documentation on this nice element!

[ArdalanNejat]

Dear all,
thank you so much for your help