D. Structures and Mechanisms · Form and Function

Keeping Structures Safe: Engineers, Inspectors and Technology

D2.7 A3.1 A2.1 A2.2About 50 minutes

Learning goals

I can describe methods engineers and other professionals use to assess, improve and maintain the safety of structures.
I can explain what a safety factor and a building code are.
I can plan a simple program for a sensor that monitors a structure, and describe how new technologies help keep structures safe.

Words to know

safety factor
How many times stronger a structure is designed to be than the biggest load it is expected to carry.

building code
A set of rules for how structures must be designed and built so they are safe. In Ontario, buildings must follow the Ontario Building Code.

inspection
A careful check of a structure to find damage or problems.

maintenance
Regular care and repair that keeps a structure safe and working.

retrofit
To add new parts or features to an existing structure to make it stronger or safer.

sensor
A device that measures something, such as tilt, vibration or how much a part stretches, and sends the measurement to a computer.

algorithm
A clear set of steps for solving a problem or doing a task, which can be turned into a computer program.

Hook: the iron ring

Many engineers in Canada wear an iron ring on the little finger of their working hand. It reminds them every day of their duty to keep the public safe.

Who makes sure bridges, towers, arenas and schools stay safe for years and years? A whole team of professionals, using careful methods and new technology.

1. Assess: is it safe?

Calculations and computer models: engineers calculate the loads (dead, live, wind, snow, earthquakes) and use computer simulations to see how the structure will react.
Testing models and materials: scale models are tested on shake tables and in wind tunnels. Material samples are squeezed, stretched and bent in labs until they fail, so engineers know their real strength.
Inspections: building inspectors check work during construction. Bridges and other public structures are inspected regularly for cracks, rust, rot and shifting.
Non-destructive testing: tools like ultrasound can find hidden cracks inside steel or concrete without damaging it.

A bucket truck raises an inspector under a concrete highway overpass while two workers in hard hats and safety vests look up at the bridge. — Inspectors check bridges up close on a regular schedule, looking for cracks, rust and wear before they become dangerous. AI-generated image

2. Improve: make it safer

Safety factor: engineers design structures to be stronger than the biggest load they expect. If a shelf is expected to hold at most 50 kg and the designer uses a safety factor of 3, they design it to hold 150 kg.
Building codes: rules (like the Ontario Building Code and the National Building Code of Canada) set minimum standards, such as how much snow load a roof must carry in your region. Codes are updated as engineers learn from failures and research.
Retrofitting: adding braces, steel plates, stronger connections or new supports to older structures.
Load limits: posting signs that limit the weight of trucks on an older bridge, or closing it until it is repaired.

With a safety factor of 3, a shelf expected to hold at most 50 kg is designed to hold 150 kg. The extra strength covers surprises, like hidden flaws or a heavier load.

3. Maintain: keep it safe

painting or coating steel to stop rust
sealing concrete so water and road salt don't get in
replacing rotted wood, worn bolts and damaged parts
clearing heavy snow off flat roofs
keeping drains clear so water doesn't wash away soil around foundations

Who does this work?

Structural engineers design and assess structures. Architects design buildings with engineers. Engineering technologists and technicians run tests and inspections. Building inspectors check that the code is followed. Skilled trades workers, such as ironworkers, welders, carpenters, concrete finishers and crane operators, build and repair structures safely and correctly.

New technologies

Sensors attached to bridges and towers measure tilt, vibration and how much parts stretch, day and night, and send the data to computers.
Drones with cameras inspect high or hard-to-reach places, like the underside of a bridge, so workers don't have to climb.
Artificial intelligence (AI) programs can be trained to spot cracks or rust in thousands of photos. A person still checks what the program finds, because AI can miss things or make mistakes.

These technologies change the work of skilled trades and engineers: workers learn to use drones and read sensor data, while still doing the hands-on repairs.

REPEAT forever

tilt ← read tilt sensor (degrees)

IF tilt > 2 THEN

turn warning light ON

send ALERT message to the bridge engineer

ELSE IF tilt > 1 THEN

save "WATCH" and the tilt in the log

ELSE

turn warning light OFF

END IF

wait 10 seconds

END REPEAT

An algorithm (written in pseudocode) for a tilt sensor on a bridge tower. Normal tilt is less than 1°.

The same tilt-sensor algorithm as a flowchart. Each reading goes down the decisions until one is true, then the program waits 10 seconds and checks again, forever.

Worked example

Let's apply a safety factor

A footbridge is expected to carry at most 40 people at once. The designers assume each person has a mass of up to 100 kg and use a safety factor of 2. What load must the bridge be designed to hold (not counting its own weight)?

Biggest expected live load: 40 × 100 kg = 4 000 kg.
In newtons: 4 000 × 9.8 = 39 200 N.
Safety factor 2: 39 200 × 2 = 78 400 N.

The bridge must be designed to hold 78 400 N (the weight of 8 000 kg) of people, twice the biggest expected load.

Worked example

Let's trace the algorithm

Use the tilt sensor algorithm. What happens for each reading: 0.4°, 1.6°, 2.3°?

0.4°: not more than 2, not more than 1, so the ELSE part runs: warning light OFF.
1.6°: not more than 2, but more than 1: save "WATCH" 1.6 in the log. (The light is not changed.)
2.3°: more than 2: warning light ON and an ALERT is sent to the engineer.

0.4° → light off; 1.6° → logged as WATCH; 2.3° → light on and alert sent.

Just for fun

Why do bridge inspectors love drones?

Checking the underside of a bridge is a lot more fun when you don't have to hang upside down to do it.

Hands-on activity

Be a structure inspector (and a programmer)

Part 1: Inspection. With a grown-up, inspect a structure at home or in your neighbourhood: a deck, a fence, a shed, a playground climber or a bike. Use a checklist:

cracks, splits or breaks?
rust or rot?
loose, missing or bent bolts, nails or screws?
leaning, sagging or wobbling?
worn or damaged surfaces that could be a safety hazard?

Record what you find, rate each problem (OK / watch / fix now), and suggest a repair or maintenance step. Look only; don't climb on anything unsafe, and don't touch rusty sharp edges.

Part 2: Program a sensor. Plan an algorithm for a snow-load sensor on a flat arena roof. The sensor measures the snow load in kilograms per square metre (kg/m²). The roof is designed for 200 kg/m². Write pseudocode that: turns on a yellow light and alerts the maintenance crew at 120 kg/m² ("plan snow removal"), and turns on a red light and alerts the manager to close the arena at 160 kg/m². If you have a block-based coding app such as Scratch, build it using a slider as the sensor and test it with several values.

What to look for: Part 1: specific observations with a rating and a sensible fix (e.g., "rusty swing chain: watch; loose railing bolt: fix now, tighten"). Part 2: the program checks the bigger limit first (160, then 120), repeats, and has a normal case. Example:

REPEAT: load ← read sensor; IF load ≥ 160 THEN red light ON, alert manager "close arena" ELSE IF load ≥ 120 THEN yellow light ON, alert crew "plan snow removal" ELSE lights OFF; wait 1 minute

Your turn

What is a safety factor?

What is a building code?

Name two ways engineers assess whether a structure is safe.

Name two maintenance jobs that keep structures safe.

What is retrofitting?

How do drones help inspect bridges?

Name two skilled trades that build or repair structures.

More practice: warm-up

What metal ring do many Canadian engineers wear as a reminder of their duty?

Which code must buildings in Ontario follow?

What does a sensor do?

What is an algorithm?

Why do workers paint steel bridges?

More practice: on your own

A shelf bracket will hold at most 20 kg of books. The designer uses a safety factor of 4. What mass should the bracket be designed to hold?

Why does a building code require stronger roofs in some parts of Canada than others?

In the tilt algorithm, what happens if the reading is exactly 2°?

Why should a person still check the cracks an AI program finds in bridge photos?

A town posts a 10-tonne limit on an old bridge. Is this assessing, improving or maintaining safety? Explain.

Challenge

An engineer says, "A safety factor makes up for the things we can't know exactly." Give three things that engineers can't know exactly when designing a structure.

Change the tilt algorithm so it also sends an alert if the sensor stops sending readings. Write your new step.

Explain how inspections, maintenance and sensors work together to prevent failures like the ones in the last lesson.

Check your understanding

Quiz

A safety factor of 3 means a structure is designed to hold...

What is the Ontario Building Code?

Which is a maintenance task?

Adding steel braces to an old school to make it stronger is called...

In the tilt algorithm, a reading of 2.5° makes the program...

Why do people still check results from AI crack-finding programs?

For parents and teachers

Big idea (D2.7): Engineers and other professionals assess (calculations, models, tests, inspections), improve (safety factors, codes, retrofits, load limits) and maintain (painting, sealing, repairs) the safety of structures. The lesson connects to careers in engineering and the skilled trades (A3.1), uses an algorithm for a sensor (A2.1) and discusses drones and AI (A2.2).

How to help: Do the inspection walk together. Point out maintenance work you see (bridge painting, road crews, roofers). For the coding part, ask your child to "be the computer" and trace their pseudocode with different numbers.

Common difficulties:

Order of IF checks. The bigger limit must be checked first, or the big alert never runs.
Thinking a safety factor means the structure is overbuilt for no reason. It covers unknowns: material flaws, extreme loads and aging.
Safety: inspections are look-only; no climbing on damaged structures.