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Input File Format

The structure input file defines the structure to be analyzed. The calculation engine is unit-agnostic, so the units used as input are also the output units. Units need, nevertheless, to be congruent.

The input file should be a plain-text file with the .inkfem extension. Inside the file, the first line should include the header:

inkfem vM.m

where M and m are the major and minor versions of the binary used to compute the structure. The current version is 1.1:

inkfem v1.1

Then go the definition sections:

• nodes: the structure nodes, referred by id
• sections: the element's sections, referred by name
• materials: the element's materials, referred by name
• loads: the loads applied to the nodes and elements
• bars: the structure bars (linear resistant elements), referred by id

The sections can appear in any order.

The Nodes

The nodes are defined under the header:

|nodes|

Each node is defined following the format:

<id> -> <xCoord> <yCoord> {[dx dy rz]}

where:

• id: the node's unique id
• xCoord: the node's position x-coordinate
• yCoord: the node's position y-coordinate
• {dx dy rz}: set of externally constrained degrees of freedom

Examples

Node with id 23, at position (120, 450) and no external constraints:

23 -> 120.0 450.0 { }

Node with id 48, at position (300, 50) and the displacement in the x and y directions externally constrained:

48 -> 300.0 50.0 { dx dy }

The Materials

The materials are defined under the header:

|materials|

Each material is defined following the format:

<name> -> <density> <young> <shear> <poisson> <yield> <ultimate>

where:

• name: the material's unique name
• density: the material's density
• young: the material's Young or elasticity modulus
• shear: the material's shear modulus
• poisson: the material's poisson ratio
• yield: the material's yield strength
• ultimate: the material's ultimate strength

Examples

Standard steel 275:

'steel_275' -> 0.00000785 21000000.0 8100000.0 0.3 27500.0 43000.0

The Sections

The sections are defined under the header:

|sections|

Each section is defined following the format:

<name> -> <area> <iStrong> <iWeak> <sStrong> <sWeak>

where:

• name: the section's unique name
• area: the section's cross section area
• iStrong: the strong axis' moment of inertia
• iWeak: the weak axis' moment of inertia
• sStrong: the strong axis' section modulus
• sWeak: the weak axis' section modulus

Examples

Standard European IPE-100 section:

'ipe_100' -> 10.3 171.0 15.92 34.2 5.79

The Loads

The loads are defined under the header:

|loads|

There are two types of loads:

• Distributed
• Concentrated

Both types are applied to bars. To define a concentrated load on a node, choose a bar which contains the node, and add the concentrated load to that bar at position t = 0 (start node) or t = 1 (end node).

Distributed loads are defined following the format:

<term> <reference><type> <barId> <tStart> <valueStart> <tEnd> <valueEnd>

where:

• term: is either:
  • fx: force in the x-axis direction
  • fy: force in the y-axis direction
  • mz: moment about the z-axis
• reference: the reference frame in which the load is defined. Can be:
  • l: reference frame local to the bar
  • g: global reference frame
• type: must be d to signify this is a distributed load
• barId: The id of the bar where the load is applied
• tStart: the load start position in the bar's directrix (0 <= t <= 1)
• valueStart: the value for the load at tStart
• tEnd: the load end position in the bar's directrix (tStart <= t <= 1)
• valueEnd: the value for the load at tEnd

Distributed loads are always linear: they have a start and end value, and those values are linearly interpolated. The current implementation doesn't allow any other kind of distributed load interpolation.

Examples (Distributed)

A distributed force in the bar's local y-axis direction, applied to a bar with id 4, starting at t = 0 with value -50 and ending at t = 1 with value -75.

fy ld 4 0.0 -50.0 1.0 -75.0

A distributed moment about the global z-axis, applied to a bar with id 12, starting at t = 0.25 with value 100 and ending at t = 0.75 with value 200.

mz gd 12 0.25 100 0.75 200

Concentrated loads are defined following the format:

<term> <reference><type> <elementId> <t> <value>

where:

• term: is either:
  • fx: force in the x-axis direction
  • fy: force in the y-axis direction
  • mz: moment about the z-axis
• reference: the reference frame in which the load is defined. Can be:
  • l: reference frame local to the bar
  • g: global reference frame
• type: must be c to signify this is a concentrated load
• elementId: The if of the bar where the load is applied
• t: the load position in the bar's directrix (0 <= t <= 1)
• value: the load's value

Examples (Concentrated)

A concentrated force in the element with id 11 local y-axis direction, at position t = 0 (applied in the start node), with value -70.

fy lc 11 0.0 -70.0

The Bars

The bars are defined under the header:

|bars|

Each bar is defined following the format:

Input File Example

Here's a complete input file example:

inkfem v1.1

|nodes|
1 -> 0 0 {dx dy}
2 -> 200 300 {}
3 -> 400 0 {}
4 -> 600 300 {}
5 -> 800 0 {dx dy}

|materials|
'mat_A' -> 1.0 1.0 1.0 1.0 1.0 1.0

|sections|
'sec_A' -> 1.0 1.0 1.0 1.0 1.0

|loads|
fy ld 4 0.0 -50.0 1.0 -75.0

|bars|
1 -> 1{dx dy rz} 2{dx dy rz} 'mat_A' 'sec_A'
2 -> 1{dx dy rz} 3{dx dy rz} 'mat_A' 'sec_A'
3 -> 2{dx dy rz} 3{dx dy rz} 'mat_A' 'sec_A'
4 -> 2{dx dy rz} 4{dx dy rz} 'mat_A' 'sec_A'
5 -> 3{dx dy rz} 4{dx dy rz} 'mat_A' 'sec_A'
6 -> 3{dx dy rz} 5{dx dy rz} 'mat_A' 'sec_A'
7 -> 4{dx dy rz} 5{dx dy rz} 'mat_A' 'sec_A'

Preprocess File Format

The preprocessed structure is saved into a .inkfempre file if the -p flag is passed to inkfem. The file's template is defined in preprocess.template.txt.

Solution File Format

The solution structure is saved into a .inkfemsol file. The file's template is defined in solution.template.txt.