Flat Slider Bearing Element

[taneryilmaz]

Hello,

For my bridge models, I have to model PTFE/Elastomeric type of bearings at abutment locations. For that purpose I intended to use Flat Slider Bearing Elements connected in series with linear springs. Flat slider elements are expected to produce frictional resistance at the low-friction surface, and the linear springs are expected to act as the shear resistance attributed to the elastomer.

My problem is that I haven't got any force resistance from my flat slider elements. The element deforms but nearly force response is obtained. It feels that using that element is no different than putting nothing there.

I am using Coulomb friction. I have tried to change Kinit as assigning very high or very low values, but nothing changed.

I'd be happy if you could guide me where I am doing a mistake or explain more about this element (especially Kinit).

Thanks in advance,

Taner Yilmaz
PhD candidate in Penn State Univ.

[vesna]

Two things may potentially cause the problem for you. The first is the definition of the local element axis and the second is the presence of the gravity load. For FSB to work properly you need to make sure that the local x axis goes from node i to node j of the element. For friction to get activated you need gravity load acting on the element.

Here is an example of a flat slider bearing:

# ------------------------------
# Start of model generation
# ------------------------------
# Create ModelBuilder (with two-dimensions and 3 DOF/node)
model BasicBuilder -ndm 2 -ndf 3

# Define geometry for model
# -------------------------
set P 18.0;
set mass [expr $P/386.1];
# tag xCrd yCrd mass
node 1 0.0 0.0
node 2 0.0 0.0 -mass $mass $mass 0.0

# Set the boundary conditions
# tag DX DY RZ
fix 1 1 1 1
fix 2 0 0 1

# Define material models
# ----------------------
uniaxialMaterial Elastic 1 137520
uniaxialMaterial Elastic 2 10

# Define friction model for FP elements
# -------------------------------------
# frictionModel Coulomb tag mu
frictionModel Coulomb 1 0.163
# frictionModel VDependent tag muSlow muFast transRate
#frictionModel VDependent 1 0.085 0.163 0.77
# frictionModel VPDependent tag muSlow muFast0 A deltaMu alpha transRate
#frictionModel VPDependent 1 0.085 0.163 7.0686 0.05 0.08 0.77

# Define elements
# ---------------
# element flatSliderBearing eleTag NodeI NodeJ frnMdlTag uy -P matTag -Mz matTag <-orient x1 x2 x3 y1 y2 y3> <-mass m> <-iter maxIter tol>
element flatSliderBearing 1 1 2 1 0.01 -P 1 -Mz 2 -orient 0 1 0 -1 0 0
# element singleFPBearing eleTag NodeI NodeJ frnMdlTag R h uy -P matTag -Mz matTag <-orient x1 x2 x3 y1 y2 y3> <-mass m> <-iter maxIter tol>
#element singleFPBearing 1 1 2 1 37.28 2.60 0.01 -P 1 -Mz 2 -orient 0 1 0 -1 0 0

# Define gravity loads
# --------------------
# Create a Plain load pattern with a Linear TimeSeries
pattern Plain 1 "Linear" {
# Create nodal loads at nodes 2
# nd FX FY MZ
load 2 0.0 [expr -$P] 0.0
}
# ------------------------------
# End of model generation
# ------------------------------

# ------------------------------
# Start of analysis generation
# ------------------------------
# Create the system of equation
system ProfileSPD
# Create the DOF numberer
numberer Plain
# Create the constraint handler
constraints Plain
# Create the convergence test
test NormDispIncr 1.0e-12 10
# Create the integration scheme
integrator LoadControl 0.1
# Create the solution algorithm
algorithm Newton
# Create the analysis object
analysis Static
# ------------------------------
# End of analysis generation
# ------------------------------

# ------------------------------
# Start of recorder generation
# ------------------------------
# create a Recorder object for the nodal displacements at node 2
recorder Node -file Gravity_Dsp.out -time -node 2 -dof 1 2 3 disp
recorder Element -file Gravity_Frc.out -time -ele 1 force
# --------------------------------
# End of recorder generation
# --------------------------------

# ------------------------------
# Perform the gravity analysis
# ------------------------------
# perform the gravity load analysis, requires 10 steps to reach the load level
analyze 10
puts "Gravity load analysis completed";

[taneryilmaz]

Thank you for the reply.

I'm pretty sure that my bearing is taking axial force on it, and I'm also sure about my local axes as the deformations I get match up with the node displacements. I can also send you my source file. But I couldn't understand the fact that in the code you sent, the element definition is:

# Define elements
# ---------------
# element flatSliderBearing eleTag NodeI NodeJ frnMdlTag uy -P matTag -Mz matTag <-orient x1 x2 x3 y1 y2 y3> <-mass m> <-iter maxIter tol>

However, I followed up the definition in the command manual (http://opensees.berkeley.edu/wiki/index ... ng_Element), where there is no "uy" term, instead there is "Kinit" term. Which one is correct? And can you explain what these terms exactly mean?

[vesna]

Follow what is given in the manual. As it says in the manual kInit is initial elastic stiffness in local shear direction.

[vesna]

The developer of the element has posted several examples on the wiki page: http://opensees.berkeley.edu/wiki/index ... ng_Element

He explained me that in the latest version of OpenSees he replaced uy (yielding displacement) with the Kinit (initial elastic stiffness) as it was computationally more stable.

[taneryilmaz]

Thank you Vesna.

I've just seen your reply and I'll examine those examples.