From 4e870dbc4ba32db72881b6c15450605ff9b56d80 Mon Sep 17 00:00:00 2001
From: XianjueDeng <Xianjue.deng@gmail.com>
Date: Wed, 5 Mar 2025 11:50:23 -0800
Subject: [PATCH 1/7] Create zeroLengthInterface2DUpdate.rst

---
 .../elements/zeroLengthInterface2DUpdate.rst  | 154 ++++++++++++++++++
 1 file changed, 154 insertions(+)
 create mode 100644 source/user/manual/model/elements/zeroLengthInterface2DUpdate.rst

diff --git a/source/user/manual/model/elements/zeroLengthInterface2DUpdate.rst b/source/user/manual/model/elements/zeroLengthInterface2DUpdate.rst
new file mode 100644
index 00000000..85d31db6
--- /dev/null
+++ b/source/user/manual/model/elements/zeroLengthInterface2DUpdate.rst
@@ -0,0 +1,154 @@
+zeroLengthInterface2DUpdate
+===========================
+
+**zeroLengthInterface2DUpdate** fixes bugs in **zeroLengthInterface2D** and adds a new feature that allows the user to change the friction coefficient during the analysis. 
+ 
+(**zeroLengthInterface2D** was written by Roozbeh Geraili Mikola (roozbehg@berkeley.edu) and Prof. Nicholas Sitar (nsitar@ce.berkeley.edu) on July 02 2010.)
+
+Bugs fixed in zeroLengthInterface2D
+-------------------------------------
+1. Incorrect friction behavior when the interface is under cyclic sliding motion.
+2. Incorrect normal pressure behavior due to the repetitive search algorithm. The original zeroLengthInterface2D overestimates normal pressure at the interface.
+
+User Instruction
+----------------
+
+1. **Element Command Syntax**
+
+   The tcl input command for the zeroLengthInterface2DUpdate element is the same as for zeroLengthInterface2D::
+
+      element zeroLengthInterface2DUpdate eleTag? -sNdNum sNdNum? -pNdNum pNdNum? –dof sdof? mdof? -Nodes Nodes? Kn? Kt? phi?
+
+2. **Tcl Input Example**
+
+   .. image:: https://github.com/user-attachments/assets/7833184d-cea4-40ee-9198-c795f353c3d1
+      :alt: Tcl input example
+
+   **element zeroLengthInterface2DUpdate 1 -sNdNum 6 -mNdNum 2 -dof 3 2 -Nodes 10 9 8 7 6 5 4 1 $Kn $Kt $phi**
+
+   *(The above defines a zeroLengthInterface2DUpdate element between a quad element (in blue) and beam elements (in red).)*
+
+3. **Changing the Friction Coefficient**
+
+   To update the friction coefficient during analysis, modify the parameter named "phi". For example:
+
+   .. code-block:: tcl
+
+      set Kn 1e6
+      set Kt 1e6
+      set phi_ini 0.0
+      element zeroLengthInterface2DUpdate 1 -sNdNum 6 -mNdNum 2 -dof 3 2 -Nodes 10 9 8 7 6 5 4 1 $Kn $Kt $phi_ini
+
+   *(This creates a zeroLengthInterface2DUpdate element with zero friction, and both normal and tangential stiffness equal to 1e6.)*
+
+   Then, update the friction coefficient as follows:
+
+   .. code-block:: tcl
+
+      parameter 1 element 1 phi
+      updateParameter 1 [expr 16.7]
+
+   *(The above changes the friction coefficient in element 1 from 0 to 0.3.)*
+
+  Full tcl example code
+----------------
+.. code-block:: tcl
+  wipe 
+  # Create ModelBuilder with 3 dimensions and 6 DOF/node
+  model basic -ndm 2 -ndf 2
+  
+  # nDMaterial ElasticIsotropic $matTag $E $v <$rho>
+  nDMaterial ElasticIsotropic   1   1e10   0.49  6.75 
+  uniaxialMaterial Elastic 2 [expr 1e10];
+  # ################################ 
+  # build the model 
+  # ################################# 
+  
+  node 1 6 0
+  node 2 6 1 
+  node 3 4 1
+  node 4 4 0
+  fix 3 1 0 0 
+  element quad 1 1 2 3 4 1 PlaneStrain 1 
+  
+  set Kn 1e6; 
+  set Kt 1e6; 
+  set phi 16.7; 
+  
+  # Gravity loads
+  pattern Plain 1 Linear {
+  # sp $nodeTag $dofTag $dofValue
+    sp 2 2 -1.e-2
+    sp 3 2 -1.e-2 
+  }
+  
+  model BasicBuilder -ndm 2 -ndf 3;
+  
+  node 5 0 0 
+  node 6 2 0 
+  node 7 4 0
+  node 8 6 0
+  node 9 8 0
+  node 10 10 0
+  
+  geomTransf Linear 1
+  # wall section
+  section Fiber 1 {
+  # patch rect $matTag $numSubdivY $numSubdivZ $yI $zI $yJ $zJ
+     patch rect 2 100 1 [expr -15.0] -0.5 [expr 15.0] 0.5
+  }
+  
+  element dispBeamColumn 2 5 6 5 1 1
+  element dispBeamColumn 3 6 7 5 1 1
+  element dispBeamColumn 4 7 8 5 1 1
+  element dispBeamColumn 5 8 9 5 1 1
+  element dispBeamColumn 6 9 10 5 1 1
+  
+  #element ZeroLengthContactNTS2D eleTag? -sNdNum sNode? -mNdNum mNode? -Nodes Nodes? Kn? Kt? fs?
+  element zeroLengthInterface2D 7 -sNdNum 6 -mNdNum 2 -dof 3 2 -Nodes 10 9 8 7 6 5 4 1 [expr $Kn] [expr $Kt] [expr $phi]
+  # element ZeroLengthNew 7 -sNdNum 6 -mNdNum 2 -dof 3 2 -Nodes 10 9 8 7 6 5 4 1 [expr $Kn] [expr $Kt] [expr $phi]
+  fix 5 1 1 0
+  fix 10 0 1 0   
+  
+  # ----------------------------
+  # Start of recorder generation
+  # ----------------------------
+  recorder Node  -file  reactionForce.out  -node  5 10 -time -dof 1 2 reaction;
+  recorder Node -file Node2.out -node 2 -dof 1 2 -time disp 
+  recorder Node -file Node3.out -node 3 -dof 1 2 -time disp
+  recorder Element -file Contactele.out -ele 7 -time force
+
+  system SparseGeneral
+  numberer RCM
+  constraints Penalty   1.e+018   1.e+018
+  test NormDispIncr  1.00e-0010 500 2
+  algorithm KrylovNewton
+  integrator  LoadControl 0.1
+  analysis    Static
+  
+  analyze     5
+  puts stdout "\ngravity finished"
+  flush stdout
+  
+  wipeAnalysis
+  
+  model BasicBuilder -ndm 2 -ndf 2;
+  loadConst -time 0.0
+  remove sp 3 1
+  remove sp 3 2
+  
+  pattern Plain 2 "Series -dt 1 -filePath loading.dat -factor 1.0" {
+     sp 3 1 [expr 0.1]
+  }
+  constraints Transformation
+  test        NormDispIncr 1e-7 500 2
+  algorithm   KrylovNewton
+  numberer    RCM
+  system      ProfileSPD
+  integrator  LoadControl 0.01
+  # integrator  DisplacementControl 3 1 0.01
+  analysis    Static
+  
+  analyze     300
+  puts "\ncyclic slide finished\n"
+.. code-block:: tcl

From 24d435e2ca0a92f2cf655db57bfc239ff6858617 Mon Sep 17 00:00:00 2001
From: XianjueDeng <Xianjue.deng@gmail.com>
Date: Wed, 5 Mar 2025 11:51:58 -0800
Subject: [PATCH 2/7] Update zeroLengthInterface2DUpdate.rst

---
 .../user/manual/model/elements/zeroLengthInterface2DUpdate.rst   | 1 -
 1 file changed, 1 deletion(-)

diff --git a/source/user/manual/model/elements/zeroLengthInterface2DUpdate.rst b/source/user/manual/model/elements/zeroLengthInterface2DUpdate.rst
index 85d31db6..a00f281b 100644
--- a/source/user/manual/model/elements/zeroLengthInterface2DUpdate.rst
+++ b/source/user/manual/model/elements/zeroLengthInterface2DUpdate.rst
@@ -151,4 +151,3 @@ User Instruction
   
   analyze     300
   puts "\ncyclic slide finished\n"
-.. code-block:: tcl

From b315384a214012b97d9c50283dc7645f29759ef0 Mon Sep 17 00:00:00 2001
From: XianjueDeng <Xianjue.deng@gmail.com>
Date: Wed, 5 Mar 2025 12:01:24 -0800
Subject: [PATCH 3/7] Update zeroLengthInterface2DUpdate.rst

---
 .../elements/zeroLengthInterface2DUpdate.rst  | 197 +++++++++---------
 1 file changed, 98 insertions(+), 99 deletions(-)

diff --git a/source/user/manual/model/elements/zeroLengthInterface2DUpdate.rst b/source/user/manual/model/elements/zeroLengthInterface2DUpdate.rst
index a00f281b..a11cdaa2 100644
--- a/source/user/manual/model/elements/zeroLengthInterface2DUpdate.rst
+++ b/source/user/manual/model/elements/zeroLengthInterface2DUpdate.rst
@@ -50,104 +50,103 @@ User Instruction
 
    *(The above changes the friction coefficient in element 1 from 0 to 0.3.)*
 
-  Full tcl example code
+Full TCL Example Code
 ----------------
+
 .. code-block:: tcl
-  wipe 
-  # Create ModelBuilder with 3 dimensions and 6 DOF/node
-  model basic -ndm 2 -ndf 2
-  
-  # nDMaterial ElasticIsotropic $matTag $E $v <$rho>
-  nDMaterial ElasticIsotropic   1   1e10   0.49  6.75 
-  uniaxialMaterial Elastic 2 [expr 1e10];
-  # ################################ 
-  # build the model 
-  # ################################# 
-  
-  node 1 6 0
-  node 2 6 1 
-  node 3 4 1
-  node 4 4 0
-  fix 3 1 0 0 
-  element quad 1 1 2 3 4 1 PlaneStrain 1 
-  
-  set Kn 1e6; 
-  set Kt 1e6; 
-  set phi 16.7; 
-  
-  # Gravity loads
-  pattern Plain 1 Linear {
-  # sp $nodeTag $dofTag $dofValue
-    sp 2 2 -1.e-2
-    sp 3 2 -1.e-2 
-  }
-  
-  model BasicBuilder -ndm 2 -ndf 3;
-  
-  node 5 0 0 
-  node 6 2 0 
-  node 7 4 0
-  node 8 6 0
-  node 9 8 0
-  node 10 10 0
-  
-  geomTransf Linear 1
-  # wall section
-  section Fiber 1 {
-  # patch rect $matTag $numSubdivY $numSubdivZ $yI $zI $yJ $zJ
-     patch rect 2 100 1 [expr -15.0] -0.5 [expr 15.0] 0.5
-  }
-  
-  element dispBeamColumn 2 5 6 5 1 1
-  element dispBeamColumn 3 6 7 5 1 1
-  element dispBeamColumn 4 7 8 5 1 1
-  element dispBeamColumn 5 8 9 5 1 1
-  element dispBeamColumn 6 9 10 5 1 1
-  
-  #element ZeroLengthContactNTS2D eleTag? -sNdNum sNode? -mNdNum mNode? -Nodes Nodes? Kn? Kt? fs?
-  element zeroLengthInterface2D 7 -sNdNum 6 -mNdNum 2 -dof 3 2 -Nodes 10 9 8 7 6 5 4 1 [expr $Kn] [expr $Kt] [expr $phi]
-  # element ZeroLengthNew 7 -sNdNum 6 -mNdNum 2 -dof 3 2 -Nodes 10 9 8 7 6 5 4 1 [expr $Kn] [expr $Kt] [expr $phi]
-  fix 5 1 1 0
-  fix 10 0 1 0   
-  
-  # ----------------------------
-  # Start of recorder generation
-  # ----------------------------
-  recorder Node  -file  reactionForce.out  -node  5 10 -time -dof 1 2 reaction;
-  recorder Node -file Node2.out -node 2 -dof 1 2 -time disp 
-  recorder Node -file Node3.out -node 3 -dof 1 2 -time disp
-  recorder Element -file Contactele.out -ele 7 -time force
-
-  system SparseGeneral
-  numberer RCM
-  constraints Penalty   1.e+018   1.e+018
-  test NormDispIncr  1.00e-0010 500 2
-  algorithm KrylovNewton
-  integrator  LoadControl 0.1
-  analysis    Static
-  
-  analyze     5
-  puts stdout "\ngravity finished"
-  flush stdout
-  
-  wipeAnalysis
-  
-  model BasicBuilder -ndm 2 -ndf 2;
-  loadConst -time 0.0
-  remove sp 3 1
-  remove sp 3 2
-  
-  pattern Plain 2 "Series -dt 1 -filePath loading.dat -factor 1.0" {
-     sp 3 1 [expr 0.1]
-  }
-  constraints Transformation
-  test        NormDispIncr 1e-7 500 2
-  algorithm   KrylovNewton
-  numberer    RCM
-  system      ProfileSPD
-  integrator  LoadControl 0.01
-  # integrator  DisplacementControl 3 1 0.01
-  analysis    Static
-  
-  analyze     300
-  puts "\ncyclic slide finished\n"
+
+    # ----------------------------
+    # Start of model generation
+    # ----------------------------
+    wipe 
+    model basic -ndm 2 -ndf 2
+
+    nDMaterial ElasticIsotropic   1   1e10   0.49  6.75 
+    uniaxialMaterial Elastic 2 [expr 1e10];
+
+    # ################################ 
+    # Build the model 
+    # ################################# 
+    node 1 6 0
+    node 2 6 1 
+    node 3 4 1
+    node 4 4 0
+    fix 3 1 0 0 
+    element quad 1 1 2 3 4 1 PlaneStrain 1 
+
+    set Kn 1e6; 
+    set Kt 1e6; 
+    set phi 16.7; 
+
+    pattern Plain 1 Linear {
+      sp 2 2 -1.e-2
+      sp 3 2 -1.e-2 
+    }
+
+    model BasicBuilder -ndm 2 -ndf 3;
+
+    node 5 0 0 
+    node 6 2 0 
+    node 7 4 0
+    node 8 6 0
+    node 9 8 0
+    node 10 10 0
+
+    geomTransf Linear 1
+    section Fiber 1 {
+       patch rect 2 100 1 [expr -15.0] -0.5 [expr 15.0] 0.5
+    }
+
+    element dispBeamColumn 2 5 6 5 1 1
+    element dispBeamColumn 3 6 7 5 1 1
+    element dispBeamColumn 4 7 8 5 1 1
+    element dispBeamColumn 5 8 9 5 1 1
+    element dispBeamColumn 6 9 10 5 1 1
+
+    element zeroLengthInterface2D 7 -sNdNum 6 -mNdNum 2 -dof 3 2 -Nodes 10 9 8 7 6 5 4 1 [expr $Kn] [expr $Kt] [expr $phi]
+    fix 5 1 1 0
+    fix 10 0 1 0   
+
+    # ----------------------------
+    # Start of recorder generation
+    # ----------------------------
+    recorder Node  -file  reactionForce.out  -node  5 10 -time -dof 1 2 reaction;
+    recorder Node -file Node2.out -node 2 -dof 1 2 -time disp 
+    recorder Node -file Node3.out -node 3 -dof 1 2 -time disp
+    recorder Element -file Contactele.out -ele 7 -time force
+
+    # --------------------------------------------------------------------
+    # Start of static analysis (creation of the analysis & analysis itself)
+    # --------------------------------------------------------------------
+    system SparseGeneral
+    numberer RCM
+    constraints Penalty   1.e+018   1.e+018
+    test NormDispIncr  1.00e-0010 500 2
+    algorithm KrylovNewton
+    integrator  LoadControl 0.1
+    analysis    Static
+
+    analyze     10
+    puts "\ninitiate contact finished"
+
+    wipeAnalysis
+
+    model BasicBuilder -ndm 2 -ndf 2;
+    loadConst -time 0.0
+    remove sp 3 1
+    remove sp 3 2
+
+    pattern Plain 2 Linear {
+      sp 3 1 1.e-1 
+    }
+
+    constraints Transformation
+    test        NormDispIncr 1e-7 500 2
+    algorithm   KrylovNewton
+    numberer    RCM
+    system      ProfileSPD
+    integrator  LoadControl 0.01
+    analysis    Static
+
+    analyze     100
+    puts "\nsliding finished\n"

From 1ffc86ceda692c6671d914e01bdf3c0e10b718ba Mon Sep 17 00:00:00 2001
From: XianjueDeng <Xianjue.deng@gmail.com>
Date: Wed, 5 Mar 2025 12:11:30 -0800
Subject: [PATCH 4/7] Update zeroLengthInterface2DUpdate.rst

---
 .../elements/zeroLengthInterface2DUpdate.rst  | 25 +++++++++++--------
 1 file changed, 15 insertions(+), 10 deletions(-)

diff --git a/source/user/manual/model/elements/zeroLengthInterface2DUpdate.rst b/source/user/manual/model/elements/zeroLengthInterface2DUpdate.rst
index a11cdaa2..732d6570 100644
--- a/source/user/manual/model/elements/zeroLengthInterface2DUpdate.rst
+++ b/source/user/manual/model/elements/zeroLengthInterface2DUpdate.rst
@@ -1,30 +1,35 @@
 zeroLengthInterface2DUpdate
 ===========================
 
-**zeroLengthInterface2DUpdate** fixes bugs in **zeroLengthInterface2D** and adds a new feature that allows the user to change the friction coefficient during the analysis. 
- 
-(**zeroLengthInterface2D** was written by Roozbeh Geraili Mikola (roozbehg@berkeley.edu) and Prof. Nicholas Sitar (nsitar@ce.berkeley.edu) on July 02 2010.)
+**zeroLengthInterface2DUpdate** is an updated version of **zeroLengthInterface2D**. 
+
+**zeroLengthInterface2DUpdate** has the following improvements:
 
-Bugs fixed in zeroLengthInterface2D
--------------------------------------
-1. Incorrect friction behavior when the interface is under cyclic sliding motion.
-2. Incorrect normal pressure behavior due to the repetitive search algorithm. The original zeroLengthInterface2D overestimates normal pressure at the interface.
+1. Fixed the incorrect friction behavior in **zeroLengthInterface2D** when the interface is under cyclic sliding motion.
+2. Fixed the incorrect normal pressure behavior in **zeroLengthInterface2D** due to the repetitive search algorithm.
+3. Allows the user to change the friction coefficient during the analysis.
+ 
+\* **zeroLengthInterface2D** was written by Roozbeh Geraili Mikola (roozbehg@berkeley.edu) and Prof. Nicholas Sitar (nsitar@ce.berkeley.edu) on July 02 2010.
 
 User Instruction
 ----------------
 
 1. **Element Command Syntax**
 
-   The tcl input command for the zeroLengthInterface2DUpdate element is the same as for zeroLengthInterface2D::
+   The tcl input command for the **zeroLengthInterface2DUpdate** element is given below:
+
+   .. code-block:: tcl
 
       element zeroLengthInterface2DUpdate eleTag? -sNdNum sNdNum? -pNdNum pNdNum? –dof sdof? mdof? -Nodes Nodes? Kn? Kt? phi?
 
 2. **Tcl Input Example**
 
-   .. image:: https://github.com/user-attachments/assets/7833184d-cea4-40ee-9198-c795f353c3d1
+   .. image:: https://github.com/user-attachments/assets/ec851ee3-1093-4894-bbaf-b72d0e6b1184
       :alt: Tcl input example
 
-   **element zeroLengthInterface2DUpdate 1 -sNdNum 6 -mNdNum 2 -dof 3 2 -Nodes 10 9 8 7 6 5 4 1 $Kn $Kt $phi**
+   .. code-block:: tcl
+
+      element zeroLengthInterface2DUpdate 1 -sNdNum 6 -mNdNum 2 -dof 3 2 -Nodes 10 9 8 7 6 5 4 1 $Kn $Kt $phi
 
    *(The above defines a zeroLengthInterface2DUpdate element between a quad element (in blue) and beam elements (in red).)*
 

From 9759c0d5cfda1c7b74236946ca31b8faf7429b1c Mon Sep 17 00:00:00 2001
From: XianjueDeng <Xianjue.deng@gmail.com>
Date: Wed, 5 Mar 2025 12:13:48 -0800
Subject: [PATCH 5/7] Update zeroLengthInterface2DUpdate.rst

---
 .../user/manual/model/elements/zeroLengthInterface2DUpdate.rst | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/source/user/manual/model/elements/zeroLengthInterface2DUpdate.rst b/source/user/manual/model/elements/zeroLengthInterface2DUpdate.rst
index 732d6570..6437c4c8 100644
--- a/source/user/manual/model/elements/zeroLengthInterface2DUpdate.rst
+++ b/source/user/manual/model/elements/zeroLengthInterface2DUpdate.rst
@@ -27,11 +27,12 @@ User Instruction
    .. image:: https://github.com/user-attachments/assets/ec851ee3-1093-4894-bbaf-b72d0e6b1184
       :alt: Tcl input example
 
+   To define a zeroLengthInterface2DUpdate element between a quad element (in blue) and beam elements (in red).
+
    .. code-block:: tcl
 
       element zeroLengthInterface2DUpdate 1 -sNdNum 6 -mNdNum 2 -dof 3 2 -Nodes 10 9 8 7 6 5 4 1 $Kn $Kt $phi
 
-   *(The above defines a zeroLengthInterface2DUpdate element between a quad element (in blue) and beam elements (in red).)*
 
 3. **Changing the Friction Coefficient**
 

From c062fe990bda4ebc0f3e3a3d453a245817e63de1 Mon Sep 17 00:00:00 2001
From: XianjueDeng <Xianjue.deng@gmail.com>
Date: Thu, 6 Mar 2025 09:24:49 -0800
Subject: [PATCH 6/7] Update zeroLengthInterface2DUpdate.rst

---
 .../user/manual/model/elements/zeroLengthInterface2DUpdate.rst  | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/source/user/manual/model/elements/zeroLengthInterface2DUpdate.rst b/source/user/manual/model/elements/zeroLengthInterface2DUpdate.rst
index 6437c4c8..4df760ae 100644
--- a/source/user/manual/model/elements/zeroLengthInterface2DUpdate.rst
+++ b/source/user/manual/model/elements/zeroLengthInterface2DUpdate.rst
@@ -109,7 +109,7 @@ Full TCL Example Code
     element dispBeamColumn 5 8 9 5 1 1
     element dispBeamColumn 6 9 10 5 1 1
 
-    element zeroLengthInterface2D 7 -sNdNum 6 -mNdNum 2 -dof 3 2 -Nodes 10 9 8 7 6 5 4 1 [expr $Kn] [expr $Kt] [expr $phi]
+    element zeroLengthInterface2DUpdate 7 -sNdNum 6 -mNdNum 2 -dof 3 2 -Nodes 10 9 8 7 6 5 4 1 [expr $Kn] [expr $Kt] [expr $phi]
     fix 5 1 1 0
     fix 10 0 1 0   
 

From 644f2b30d2b526ae9f389831e4aafa98ac018afe Mon Sep 17 00:00:00 2001
From: XianjueDeng <Xianjue.deng@gmail.com>
Date: Thu, 6 Mar 2025 10:18:33 -0800
Subject: [PATCH 7/7] Update zeroLengthInterface2DUpdate.rst

---
 .../elements/zeroLengthInterface2DUpdate.rst  | 31 ++++++++++++-------
 1 file changed, 20 insertions(+), 11 deletions(-)

diff --git a/source/user/manual/model/elements/zeroLengthInterface2DUpdate.rst b/source/user/manual/model/elements/zeroLengthInterface2DUpdate.rst
index 4df760ae..81731a73 100644
--- a/source/user/manual/model/elements/zeroLengthInterface2DUpdate.rst
+++ b/source/user/manual/model/elements/zeroLengthInterface2DUpdate.rst
@@ -20,7 +20,23 @@ User Instruction
 
    .. code-block:: tcl
 
-      element zeroLengthInterface2DUpdate eleTag? -sNdNum sNdNum? -pNdNum pNdNum? –dof sdof? mdof? -Nodes Nodes? Kn? Kt? phi?
+      element zeroLengthInterface2DUpdate eleTag? -sNdNum sNdNum? -pNdNum pNdNum? –dof sdof? pdof? -Nodes Nodes? Kn? Kt? phi?
+
+   $eleTag:	unique element object tag
+ 
+   $sNdNum:	Number of nodes on secondary surface
+ 
+   $pNdNum:	Number of nodes on primary surface
+ 
+   $sdof $pdof:	Nodal degree of freedom for secondary and primary surface
+ 
+   $Nodes:	Secondary and primary node tags respectively
+ 
+   $Kn:	Penalty in normal direction
+ 
+   $Kt:	Penalty in tangential direction
+ 
+   $phi:	Friction angle in degrees
 
 2. **Tcl Input Example**
 
@@ -31,7 +47,7 @@ User Instruction
 
    .. code-block:: tcl
 
-      element zeroLengthInterface2DUpdate 1 -sNdNum 6 -mNdNum 2 -dof 3 2 -Nodes 10 9 8 7 6 5 4 1 $Kn $Kt $phi
+      element zeroLengthInterface2DUpdate 1 -sNdNum 6 -pNdNum 2 -dof 3 2 -Nodes 10 9 8 7 6 5 4 1 $Kn $Kt $phi
 
 
 3. **Changing the Friction Coefficient**
@@ -43,14 +59,7 @@ User Instruction
       set Kn 1e6
       set Kt 1e6
       set phi_ini 0.0
-      element zeroLengthInterface2DUpdate 1 -sNdNum 6 -mNdNum 2 -dof 3 2 -Nodes 10 9 8 7 6 5 4 1 $Kn $Kt $phi_ini
-
-   *(This creates a zeroLengthInterface2DUpdate element with zero friction, and both normal and tangential stiffness equal to 1e6.)*
-
-   Then, update the friction coefficient as follows:
-
-   .. code-block:: tcl
-
+      element zeroLengthInterface2DUpdate 1 -sNdNum 6 -pNdNum 2 -dof 3 2 -Nodes 10 9 8 7 6 5 4 1 $Kn $Kt $phi_ini
       parameter 1 element 1 phi
       updateParameter 1 [expr 16.7]
 
@@ -109,7 +118,7 @@ Full TCL Example Code
     element dispBeamColumn 5 8 9 5 1 1
     element dispBeamColumn 6 9 10 5 1 1
 
-    element zeroLengthInterface2DUpdate 7 -sNdNum 6 -mNdNum 2 -dof 3 2 -Nodes 10 9 8 7 6 5 4 1 [expr $Kn] [expr $Kt] [expr $phi]
+    element zeroLengthInterface2DUpdate 7 -sNdNum 6 -pNdNum 2 -dof 3 2 -Nodes 10 9 8 7 6 5 4 1 [expr $Kn] [expr $Kt] [expr $phi]
     fix 5 1 1 0
     fix 10 0 1 0