Skip to main content

​
Overview

Member releases allow you to control how forces and moments are transferred at member connections. By releasing specific internal forces (axial force, shear, or moment) at either end of a member, you can model various connection types from fully fixed to pinned or custom partial releases. Releases are essential for accurately representing real structural connections, which often don’t transfer all force components. For example, a simple bolted connection might transfer shear and axial forces but not significant moments (a pinned connection).

​
When to Use Releases

Pinned Connections

Model connections that don’t transfer moment (e.g., simple beam connections)

Truss Members

Create axial-only members in truss structures

Slotted Connections

Model connections that allow movement in specific directions

Simplified Connections

Represent connection behavior when full fixity isn’t realistic

​
Syntax

model.add_releases(
    member_id,
    pi=False,   # Release axial force at start node
    vi=False,   # Release shear at start node
    mi=False,   # Release moment at start node
    pj=False,   # Release axial force at end node
    vj=False,   # Release shear at end node
    mj=False    # Release moment at end node
)

​
Parameters

​
member_id
int
required
ID of the member to apply releases to
​
pi
bool
default:"False"
Release axial force at the start node (node i). When True, the member cannot transfer axial force at this end.
​
vi
bool
default:"False"
Release shear force at the start node (node i). When True, the member cannot transfer shear at this end.
​
mi
bool
default:"False"
Release moment at the start node (node i). When True, the connection acts as a pin at this end.
​
pj
bool
default:"False"
Release axial force at the end node (node j). When True, the member cannot transfer axial force at this end.
​
vj
bool
default:"False"
Release shear force at the end node (node j). When True, the member cannot transfer shear at this end.
​
mj
bool
default:"False"
Release moment at the end node (node j). When True, the connection acts as a pin at this end.

​
Common Connection Types

​
Pinned Connection (Both Ends)

A simply supported beam with pinned connections at both ends: 
from milcapy import SystemModel, BeamTheoriesType

model = SystemModel()

# Define material and section
model.add_material("concrete", modulus_elasticity=2.1e6, poisson_ratio=0.2)
model.add_rectangular_section("beam", "concrete", base=0.3, height=0.5)

# Create beam
model.add_node(1, 0, 0)
model.add_node(2, 6, 0)
model.add_member(1, 1, 2, "beam", BeamTheoriesType.EULER_BERNOULLI)

# Release moments at both ends (pinned-pinned)
model.add_releases(
    member_id=1,
    mi=True,  # Pin at start
    mj=True   # Pin at end
)

# Add supports
model.add_restraint(1, True, True, False)  # Pin support
model.add_restraint(2, True, True, False)  # Roller support

# Add distributed load
model.add_load_pattern("Live Load")
model.add_distributed_load(1, "Live Load", qa=0, qb=-10, typ="fy")

# Solve
model.solve()
model.show()
Pinned connections (moment releases) are the most common type of release in practice. They’re used to model simple shear connections that don’t develop significant moment.

​
Fixed-Pinned Connection

A cantilever with a moment release at the free end: 
# Create cantilever beam
model.add_node(1, 0, 0)
model.add_node(2, 4, 0)
model.add_member(1, 1, 2, "beam", BeamTheoriesType.TIMOSHENKO)

# Fixed at start, pinned at end
model.add_releases(
    member_id=1,
    mi=False,  # Fixed at start (no release)
    mj=True    # Pinned at end
)

# Boundary conditions
model.add_restraint(1, True, True, True)  # Fixed support
model.add_restraint(2, False, False, False)  # Free end

​
Truss Member (Axial Only)

Create a member that only carries axial force: 
# Create truss member
model.add_member(5, 3, 4, "truss_section", BeamTheoriesType.EULER_BERNOULLI)

# Release shear and moment at both ends (axial only)
model.add_releases(
    member_id=5,
    vi=True,  # Release shear at start
    mi=True,  # Release moment at start
    vj=True,  # Release shear at end
    mj=True   # Release moment at end
)
For truss members, release both shear and moment at both ends. This ensures the member only develops axial forces, which is the fundamental assumption of truss analysis.

​
Advanced Examples

​
Partial Frame with Mixed Connections

A frame with different connection types: 
model = SystemModel()

# Define materials
model.add_material("steel", modulus_elasticity=2.1e8, poisson_ratio=0.3)
model.add_rectangular_section("column", "steel", base=0.3, height=0.3)
model.add_rectangular_section("beam", "steel", base=0.25, height=0.4)

# Create frame
model.add_node(1, 0, 0)      # Base left
model.add_node(2, 0, 3)      # Top left
model.add_node(3, 5, 3)      # Top right
model.add_node(4, 5, 0)      # Base right

# Members
model.add_member(1, 1, 2, "column", BeamTheoriesType.TIMOSHENKO)  # Left column
model.add_member(2, 2, 3, "beam", BeamTheoriesType.TIMOSHENKO)    # Beam
model.add_member(3, 3, 4, "column", BeamTheoriesType.TIMOSHENKO)  # Right column

# Beam with pinned connection at right end only
model.add_releases(
    member_id=2,
    mi=False,  # Fixed to left column
    mj=True    # Pinned to right column
)

# Supports
model.add_restraint(1, True, True, True)   # Fixed base left
model.add_restraint(4, True, True, False)  # Pinned base right

# Loads
model.add_load_pattern("Wind")
model.add_point_load(2, "Wind", fx=20, fy=0, mz=0)

model.solve()

​
Slotted Connection (Axial Release)

Model a connection that allows axial movement: 
# Member with axial release (can slide axially)
model.add_releases(
    member_id=7,
    pi=True,   # Release axial force at start
    vi=False,  # Transfer shear
    mi=False   # Transfer moment
)
Releasing both axial force and shear at the same end can create an unstable mechanism. Ensure your releases produce a stable structural system.

​
Understanding Release Effects

​
Moment Release Effects

When you release moment at a connection:
  • The member develops zero moment at that end
  • The connection can rotate freely
  • Member stiffness is reduced (lower bending stiffness contribution)
  • Moment diagram shows zero moment at the release point

​
Shear Release Effects

When you release shear at a connection:
  • The member develops zero shear at that end
  • The connection can translate perpendicular to the member axis
  • Member doesn’t contribute to lateral stiffness

​
Axial Release Effects

When you release axial force at a connection:
  • The member develops zero axial force at that end
  • The connection can translate along the member axis
  • Member doesn’t resist axial loads

​
Validation and Debugging

After adding releases, verify your model: 
# Solve the model
model.solve("Load Pattern")

# Get member forces at ends
results = model.get_results("Load Pattern")

# Check that released forces are zero
member_forces = results.get_member_forces(member_id=1)
print(f"Start moment: {member_forces.mi}")  # Should be ~0 if mi=True
print(f"End moment: {member_forces.mj}")    # Should be ~0 if mj=True
Use the visualization tools to inspect deformed shapes and internal force diagrams. Releases should show as discontinuities in moment or force diagrams.

​
Stability Considerations

Critical: Excessive releases can create unstable mechanisms. Always ensure:
  1. The structure has sufficient restraints to prevent rigid body motion
  2. Members with multiple releases don’t create unstable conditions
  3. The global stiffness matrix remains positive definite
If the solver fails or produces unrealistic results, check for over-released members.

​
Common Patterns

​
Simply Supported Beam

model.add_releases(member_id, mi=True, mj=True)

​
Cantilever Beam

model.add_releases(member_id, mi=False, mj=True)

​
Truss Member

model.add_releases(member_id, vi=True, mi=True, vj=True, mj=True)

​
Continuous Beam Interior Support

# Left span
model.add_releases(left_member_id, mi=True, mj=False)
# Right span
model.add_releases(right_member_id, mi=False, mj=True)

​
Best Practices

  1. Match physical behavior: Use releases to represent actual connection details
  2. Document assumptions: Keep notes on why specific releases were applied
  3. Check stability: Verify the structure is stable after adding releases
  4. Validate results: Compare internal forces to hand calculations when possible
  5. Use symmetry: Apply symmetric releases when the structure is symmetric

End Length Offsets

Model rigid end zones at connections

Elastic Supports

Add spring supports to nodes

Build docs developers (and LLMs) love

Get started for freeTalk to us