The Stiffness Matrix from First Principles

Every finite element analysis — ETABS, SAP2000, ANSYS, a Python solver of your own — resolves to the same object: the element stiffness matrix.

It comes out of one principle and one assumption, and all of it fits inside a single integral. No black boxes.

The core idea

A structural element resists deformation. The stiffness matrix $[K]$ captures how much — it maps nodal displacements $\{u\}$ to nodal forces $\{F\}$:

$$[K] \{u\} = \{F\}$$

(1)

For a single element with $n$ degrees of freedom, $[K]$ is an $n \times n$ matrix. For a full structure, we assemble all element matrices into a global $[K]$.

Deriving the bar element

The 2-node axial bar is the smallest element that shows every step, and every step generalises. Four moves get from a principle to a matrix.

Virtual work

Principle of virtual work

For a body in equilibrium, the virtual work done by external forces through any virtual displacement equals the virtual strain energy stored internally.

$$\delta W_{ext} = \delta W_{int}$$

For a bar element under virtual nodal displacement $\{\delta u\}$:

$$\{\delta u\}^T \{F\} = \int_V \{\delta \varepsilon\}^T \{\sigma\} \, dV$$

This is the foundation. Now we need to connect strains to displacements.

Shape functions

A 2-node bar element has 2 DOF: $u_1$ and $u_2$ (axial displacements). We assume the displacement varies linearly along the element:

$$u(x) = \underbrace{\left(1 - \frac{x}{L}\right)}_{N_1(x)} u_1 + \underbrace{\frac{x}{L}}_{N_2(x)} u_2$$

Shape functionsN(x)

N₁ and N₂ define how the interior displacement field is interpolated from the nodal values.

In matrix form:

$$u(x) = [N(x)] \{u\} = [N_1 \quad N_2] \begin{Bmatrix} u_1 \\ u_2 \end{Bmatrix}$$

Strain from displacement

For a bar in axial loading, strain is:

$$\varepsilon = \frac{du}{dx} = [B] \{u\}$$

where $[B]$ is the strain-displacement matrix, found by differentiating $[N]$:

$$[B] = \frac{d[N]}{dx} = \left[-\frac{1}{L} \quad \frac{1}{L}\right]$$

The integral

With linear elastic material, stress follows strain:

$$\{\sigma\} = E \{\varepsilon\} = E [B] \{u\}$$

Substituting into the virtual work equation:

$$\{\delta u\}^T \{F\} = \int_0^L \{\delta u\}^T [B]^T E [B] \{u\} \, A \, dx$$

Since $\{\delta u\}^T$ is arbitrary, we can factor it out:

$$\{F\} = \underbrace{\left( \int_0^L [B]^T E [B] A \, dx \right)}_{[K]} \{u\}$$

For a bar with constant $E$, $A$:

$$[K] = \frac{EA}{L} \begin{bmatrix} 1 & -1 \\ -1 & 1 \end{bmatrix}$$

(2)

That’s the 2×2 stiffness matrix for an axial bar element — derived entirely from first principles.

The beam element

A 2D Euler–Bernoulli beam element has 4 DOF: two transverse displacements $v_1$, $v_2$ and two rotations $\theta_1$, $\theta_2$.

The shape functions are cubic (Hermite polynomials):

$$v(x) = N_1 v_1 + N_2 \theta_1 + N_3 v_2 + N_4 \theta_2$$

where:

$$N_1 = 1 - 3\xi^2 + 2\xi^3, \quad N_2 = L(\xi - 2\xi^2 + \xi^3)$$ $$N_3 = 3\xi^2 - 2\xi^3, \quad N_4 = L(-\xi^2 + \xi^3), \quad \xi = x/L$$

Curvature is the bending analogue of strain, and it plays exactly the same role in the integral:

$$\kappa = \frac{d^2v}{dx^2} = [B] \{u\}$$

So the stiffness matrix follows the same form:

$$[K] = \int_0^L [B]^T EI [B] \, dx = \frac{EI}{L^3} \begin{bmatrix} 12 & 6L & -12 & 6L \\ 6L & 4L^2 & -6L & 2L^2 \\ -12 & -6L & 12 & -6L \\ 6L & 2L^2 & -6L & 4L^2 \end{bmatrix}$$

(3)

This is the Euler–Bernoulli beam stiffness matrix — you’ll find this exact matrix in every structural analysis textbook.

Putting numbers in it

The derivation stops at symbols, which is the right place for a derivation to stop and the wrong place to leave it. Two plots of what it already contains, then the same thing with a section in it.

Shape functionsStiffness vs span

Enter a section and a length below and both matrices are evaluated as you type.

Element properties

E Young's modulus

GPa

A Cross-sectional area

mm²

I Second moment of area

cm⁴

L Element length

m

Axial (bar) element

EA/L 188,300 kN/m DOF order [u₁, u₂]

Axial (bar) element stiffness matrix in local coordinates, DOF order [u₁, u₂]. Row i is the force or moment at degree of freedom i; column j is a unit displacement or rotation at degree of freedom j.

	u₁ per m	u₂ per m
u₁	188,300 kN/m	-188,300 kN/m
u₂	-188,300 kN/m	188,300 kN/m

Euler–Bernoulli beam element

EI/L³ 81.28 kN/m DOF order [v₁, θ₁, v₂, θ₂]

Euler–Bernoulli beam element stiffness matrix in local coordinates, DOF order [v₁, θ₁, v₂, θ₂]. Row i is the force or moment at degree of freedom i; column j is a unit displacement or rotation at degree of freedom j.

	v₁ per m	θ₁ per rad	v₂ per m	θ₂ per rad
v₁	975.3 kN/m	2,926 kN/rad	-975.3 kN/m	2,926 kN/rad
θ₁	2,926 kN·m/m	11,700 kN·m/rad	-2,926 kN·m/m	5,852 kN·m/rad
v₂	-975.3 kN/m	-2,926 kN/rad	975.3 kN/m	-2,926 kN/rad
θ₂	2,926 kN·m/m	5,852 kN·m/rad	-2,926 kN·m/m	11,700 kN·m/rad

Values are shown to 4 significant figures. Rotations are in radians, which are dimensionless — so kN/rad is kN, and kN·m/rad is kN·m. Local element coordinates; no transformation to global axes.

The 3D frame element

Combining axial, bending (two planes), torsion, and shear DOF gives the full 12×12 matrix used in ETABS, SAP2000, and every general-purpose FEM solver.

The assembly process is always the same:

1. Define shape functions for each DOF
2. Compute $[B]$ by differentiation
3. Integrate $[K] = \int [B]^T [D] [B] \, dV$
4. Transform from local to global coordinates via $[K]_{global} = [T]^T [K]_{local} [T]$

In code

import numpy as np

def bar_stiffness(E: float, A: float, L: float) -> np.ndarray:
    """2x2 stiffness matrix for an axial bar element."""
    k = (E * A / L)
    return k * np.array([[1, -1], [-1, 1]])

def beam_stiffness(E: float, I: float, L: float) -> np.ndarray:
    """4x4 Euler-Bernoulli beam element stiffness matrix.
    DOF order: [v1, θ1, v2, θ2]
    """
    k = E * I / L**3
    return k * np.array([
        [ 12,   6*L,  -12,   6*L],
        [6*L, 4*L**2, -6*L, 2*L**2],
        [-12,  -6*L,   12,  -6*L],
        [6*L, 2*L**2, -6*L, 4*L**2],
    ])

Both functions are the closed forms above, transcribed. Nothing in them is fitted or looked up — an error in either one is a typo, not a modelling decision.

What this tells you

Every result ETABS gives you flows from this integral. The “stiffness matrix” isn’t a magic number lookup — it’s a compact representation of how a physical element resists deformation, derived from strain energy and virtual work.

Understanding the derivation doesn’t change your day-to-day workflow. But when a model gives you a surprising result, it tells you where to look.