Lesson 59 of 69 in this sequence

86%

Not completed yet

Saved on this device only — not synced to an account yet.

Learning path

Step 59 of 69

Continue the sequence

ResumeReturn to this step ReviewRevisit the core idea RemediateGet repair practice Next stepChoose the next lesson manually

Completion

Review objectives and instruction
Complete the activity with manual learner confirmation
Answer every understanding-check item
Show completion feedback and reward only after the learner finishes the check

Progress

Step 59/69
15 content types
AI help source context available
Auto-advance blocked

Review objectives and instruction

AI help

Available only with source grounding, privacy limits, age controls, and measurement.

AI source context

Tracing Tension and Compression Through a Truss · Mechanics of Materials · truss · tension

Evidence: local lesson AI-help metadata only; source-grounding, privacy, age-safety, and measurement hooks do not prove real model quality, production telemetry, child AI approval, or AI tutor production readiness

Production AI readiness not claimed

AI source context

Assessment shape

Local lesson assessment shape is complete.

Local shape only; production answer-attempt telemetry, live ownership/RLS evidence, mastery interpretation, release, and production readiness are not claimed here.

Objectives: 5
Activity: Prompt ready
Quiz items: 3 items, 3 answer keys, 3 explanations
Reward: Completion reward ready

Assessment progress stays manual: do not auto-advance through uncertain answers, failed writes, consent, media, payment, learner choice, or AI-assisted checks.

Learning-loop evidence: Content-map contract only; persistence and production learner-progress writes require route/API evidence.

Manual policy: assessment, media, consent, payment, learner choice, failed writes, and AI-uncertain help stay under learner control.

Prerequisite: Complete or review Torque: Why Distance From the Pivot Changes Everything

AI guardrails

Privacy

Do not send raw child free text without consent
Use source excerpts and non-identifying progress state only
Asset is not registry-gated

Age safety

Apply learn audience controls before AI-help claims
Keep child learner AI paths behind guardian/product review evidence before readiness claims

Measurement

Log AI help usage by surface and asset slug
Evaluate answer quality against source-grounded rubric before production-readiness claims
Tie remediation prompts to completion, retry, or review outcomes

Local AI-help guardrails only; they do not prove guardian approval, production telemetry, model evaluation, release, deployment, or AI tutor production readiness.

Source and rights

Source path: koydo-app/scripts/curriculum-authoring
License: ai-generated-koydo
Attribution: Attribution decision needs owner review
AI provenance: claude-opus-4-8

Source review gates: attribution_missing

Local source evidence only; this does not prove publication rights, attribution approval, AI provenance clearance, release, deployment, or production readiness.

Route review

Current route: /library/learn/engineering_making/en/lesson/truss-load-paths
Route source: Fallback route needs owner canonical review
Candidate route: /library/learn/engineering_making/en/lesson/truss-load-paths

Local route evidence only; fallback candidates need owner-approved canonical route policy before route writes, production discoverability, or readiness claims.

Content gates: canonical_target_web_missing_uses_library_fallback

lessonslessonslesson objectivesmedia scenesdrillspractice setsquizzesfeedbackcharacterscitationsconcept mapsprogressresumereviewremediationnext steps

Tracing Tension and Compression Through a Truss

with Atlas

Atlas stands on a steel bridge over a river, pointing up at the triangular framework of a Pratt truss, chalk lines glowing on each member to show which ones are being pulled apart and which ones are being squeezed together as a truck rolls slowly across.

Identify which members of a simple truss are in tension and which are in compression.
Explain how a load applied to a truss travels through connected members down to the supports.
Compare tension and compression using observable physical evidence such as stretching versus shortening.
Predict whether a given diagonal member in a common truss will experience tension or compression based on its orientation.
Describe why triangles are the fundamental shape in a truss and how they create a stable load path.

Key terms

Truss: A structure of straight members joined at nodes
Tension: A pulling force that stretches a member apart
Compression: A pushing force that squeezes a member shorter
Load path: The chain of members carrying force to the supports

Following the Load Path

When a truck pushes down on a truss, that force cannot vanish; it must travel to the ground. The force flows member by member from where the load lands, through the connected straight pieces, down to the abutments at each end. This chain is the load path. Understanding it means recognizing that no member sits idle: each one passes force along, either pulling or pushing, until the entire load reaches solid support.

Tension, Compression, and Diagonals

Every member carries force in one of two ways. A member in tension is stretched, like a rubber band, with end forces pointing apart, and is often the bottom chord. A member in compression is squeezed shorter, like a sponge, and is often the top chord or a vertical post. Diagonals are the interesting case: their slope decides their role, and in a Pratt truss the diagonals sloping toward center carry tension while the verticals carry compression.

Worked examples

A truck parks at the center of a Pratt truss. Is the bottom chord below it in tension or compression?

The truck's weight pushes down on the top chord, bending the truss.
As the truss bends, its bottom chord is pulled outward at both ends.
Forces pointing apart at the ends mean the member is stretched.

Answer: The bottom chord is in tension.

Hey — welcome to one of my favorite engineering puzzles. See this bridge? Every single piece of it is either being stretched or squeezed right now. Let me show you how to read it. A truss is a structure made entirely of straight members connected at joints called nodes. When a load — like a truck — pushes down, that force has to travel somewhere. It cannot just disappear. It flows through the members, one by one, until it reaches the supports — the solid structures at each end of the bridge called abutments. Engineers call this the load path. Here is the key rule: every member in a truss carries its force in one of exactly two ways. TENSION means the member is being pulled apart at both ends. Think of pulling a rubber band — it stretches, and you can feel it pulling back. A member in tension is like that rubber band. The forces at its ends point away from each other. If you could cut it, the two pieces would spring apart. Tension members are often the bottom chord — the horizontal piece along the bottom of the truss. COMPRESSION means the member is being squeezed, pushed from both ends toward its middle. Think of pressing your palms on the ends of a sponge — it squishes. A member in compression is being shortened. If you could cut it, the two pieces would push toward each other — they were being compressed and want to close that gap. Compression members are often the top chord and the vertical posts. The diagonal members — the angled pieces between top and bottom — are where it gets interesting. Their direction (which way they slope) determines whether they carry tension or compression. In a common Pratt truss, the diagonals slope toward the center of the span and are in tension; the verticals are in compression. Here is the big picture: every load you apply creates a complete load path — a chain of members, some in tension and some in compression, that routes the force from where it lands all the way down to the ground at the supports. No member is just sitting there doing nothing. Each one has a job: pull or push.

Activity

Drag each label — TENSION or COMPRESSION — onto the correct highlighted member of the truss diagram to complete the load path.

Practice

Label the top chord and a vertical post of a Pratt truss as tension or compression.

Explain how cutting a tension member differs from cutting a compression member.

Common mistakes to avoid

Diagonals only hold the shapeDiagonals are active load-path members carrying real tension or compression, so removing one can collapse the truss.
The bottom chord is compressed by the loadThe downward load pushes the top chord, but the load path stretches the bottom chord, placing it in tension.

Check your understanding

A truck parks at the center of a Pratt truss bridge. Which statement best describes what happens to the bottom chord member directly below the truck?

You cut a truss member and the two cut pieces spring away from each other (outward, away from the cut). What does this tell you about that member?

A student says: 'The diagonal members in a truss do not carry any real force — they just hold the shape.' Is this correct?

Recap

In a truss, a downward load follows a load path member by member to the supports, with each piece in either tension when stretched or compression when squeezed, and diagonals carry real force whose type depends on their slope rather than merely holding the shape.

Reflect

Why does a triangle of members create a more stable load path than a square does?