H-04 — validation

trust the model only after you've checked it.

validation checker

break the model on purpose. see what bad output looks like.

Drag the "software" reactions away from the correct values to simulate a modeling error — watch the equilibrium check and moment comparison respond.

beam parameters
span20 ft
w (klf)1.5
"software" output
R_left15.0 k
R_right15.0 k
Drag reactions away from correct values to simulate wrong output.
hand calculation
M hand75.0
—
equilibrium error (%)
W = — k   ΣR = — k
Mhand = — k-ft
equilibrium check: ΣFy = 0 ?
moment check: software vs hand calc
live equation
set values above to see the live calculation
ΣF = Sum of all forces must equal zero
ΣM = Sum of all moments must equal zero
R = Support force to compare against hand calculation
explained
Validation catches modeling errors before they reach construction. The simplest check: do the reactions sum to the total applied load? If not, something is wrong with the supports or loads. Compare key values (max moment, max shear) against a quick hand calculation — they should be close.
key concepts
equilibriumIf reactions don't sum to applied load, something is wrong

The first validation check: ΣFy = 0. Sum all applied loads and compare to the sum of all reactions. If they don't match within rounding, the model has a problem — missing load, wrong support type, or convergence failure. This catches the biggest errors fastest.

hand calcA quick hand check catches order-of-magnitude errors

You don't need to replicate the full analysis by hand. For a uniform beam: M = wL²/8, V = wL/2. Compare these to the software output. If they're within 5–10%, the model is likely correct. If they're off by 2x or 10x, something is fundamentally wrong.

common errorsWrong units, wrong supports, and missing loads cause most failures

The three most common modeling errors: (1) unit mismatch — entering kips where the program expects pounds, or feet where it expects inches; (2) wrong support conditions — fixed instead of pinned, or missing a release; (3) missing loads — forgetting self-weight, or applying load to the wrong member.

deflection checkUnreasonable deflection is a red flag for modeling errors

If service-load deflection exceeds L/100 or so, the model almost certainly has an error. Typical limits are L/240 to L/360. A beam deflecting L/40 under normal loads suggests wrong E, wrong I, missing support, or unit error. Always sanity-check the displaced shape.