Boomilever

Boomilever is a build event. Before the competition you design and build one wooden structure. At the tournament, you hang it on a bolt that sticks out of a vertical testing wall, the loading assembly is hung from its tip, and sand is poured into a bucket until the structure fails or the load reaches the cap. Your score is the load the structure is credited with divided by the structure's own mass. The winning structure is therefore not the strongest one. It is the most efficient one, and the event rewards teams who understand why a truss carries load better than a solid beam of the same mass, can predict where a structure will fail, keep their construction inside the rules, and can explain their design to the Event Supervisor.

Event format

Teams have up to two students, and the event runs about 10 minutes per team. Each team enters one structure. Everything that touches the structure during testing belongs to the Event Supervisor: the testing wall, the mounting bolt, the loading assembly, the sand, the scales and the bucket stabilizing sticks. You bring the structure, your eye protection, your estimated load and your knowledge.

There is no written test. The design-knowledge clause means the Event Supervisor can ask you about construction and operation, so both partners should be able to explain the structure. The 2027 rules say nothing that permits notes, reference sheets or calculators, and they bar teams from bringing apparatus, equipment, tools or instruments, so plan to bring only the structure and your eye protection. Do your calculations, including your load estimate, before you arrive.

What the rules cover

The 2027 rules manual has the same seven parts in both divisions: description, event parameters, construction parameters, competition, scoring, test apparatus and definitions. Most of the text is shared. Division B and Division C differ in four places: which side of the width lines the structure may touch, which depth line limits the base option, which depth line limits the bonus option, and the size of the bonus.

All depth lines are measured down from the center of the bolt hole, not from the top of the wall.

What changed from last season

If you are reusing a 2026 design or a 2026 study sheet, check these differences against the 2027 rules.

Scoring: the number that decides the event

Score is the load a structure supports, plus a bonus when it qualifies, divided by the structure's mass. The score has no units. It is grams of load per gram of structure, and higher is better.

Load Supported is the loading assembly plus the sand in the bucket, after any pieces of the structure that fell into the bucket are taken out. Three limits follow. The floor is the loading assembly's own mass (at most 1,500 g): a structure that cannot hold it is scored for participation only. The ceiling is 15,000 g; the Event Supervisor sets up 15,100 to 15,200 g of assembly plus sand, but no structure is credited with more than 15,000 g. The largest possible Load Scored is therefore 20,000 g in Division B and 22,500 g in Division C.

Because the bonus requires holding 15 kg and the cap is 15,000 g, every bonus structure is scored at the same Load Scored. Among bonus teams in the same tier, only structure mass separates them.

Tiers and tiebreakers

Structures are placed in tiers before scores are compared, and any Tier 1 structure outranks any Tier 2 structure.

Ties are broken first by the lower structure mass, then by the Estimated Load Supported closest to the actual load. The estimate is submitted at check-in, so write it down before you arrive.

Two bonus details are easy to get wrong. A structure entered for the bonus that touches the wall below the bonus line but above the base line is simply scored as a base-option structure; it is not tiered for that. And a structure that meets the bonus contact rule but fails before 15 kg loses only the bonus; failing to reach 15 kg is not a violation.

What one gram is worth

The score is a ratio, so adding wood and adding load do not have the same effect. Take a structure of mass $m$ holding load $L$ with score $S = L/m$. A small change in mass $\Delta m$ with no change in load moves the score by about $-S\,\Delta m / m$, and a small change in load $\Delta L$ moves it by about $\Delta L / m$. Setting these equal gives a rule of thumb: one gram of structure mass costs about $S$ grams of load.

Two lessons follow. First, when your score is high, wasted wood is expensive: a gram you can remove without losing load is worth roughly your score in grams of load. Second, the bonus is worth a lot, and more in Division C. At the full 15,000 g, a Division B bonus structure scores 20,000 / 15,000 = 1.33 times a base structure of the same mass, which matches a base structure that is 25% lighter. In Division C the factor is 22,500 / 15,000 = 1.5, so a base structure would need to be a third lighter to tie. Once a design reliably holds 15,000 g, the load can rise no further, and the only way left to improve the score is to cut mass.

Structural mechanics: how the structure carries load

The cantilever and the load path

A cantilever is a beam supported at one end and free at the other. Your structure is a cantilever with its support at the wall and its free end reaching out to the loading block. Sand in the bucket pulls the loading assembly down, the loading block transmits that force into the structure at its tip, and the structure passes the force back to the mounting bolt. The path that force follows, from the tip through each member to the wall, is the load path. Every member sits somewhere on that path, and the design question is which members carry what.

The most important quantity is the bending moment at the wall. For a load $P$ hung a horizontal distance $x$ from the wall, the moment is

Two consequences matter. Moment grows with reach, so the same load at a longer reach puts more demand on the members near the wall. And the moment is largest at the wall and falls toward the tip, because the lever arm to the load shrinks toward the tip. Members near the wall therefore work hardest, and members near the tip can often be lighter. Many strong designs taper for exactly this reason.

The reach is a design choice within a range. The rules allow the chain centerline between 40 and 45 cm from the wall. Reaching only as far as the structure needs keeps the moment down, but the structure must still meet the wall-contact requirements at that reach. Choose the reach deliberately and keep it the same in every test, so that your data compare like with like.

Tension and compression in a truss

When a structure bends, one face is pulled apart and the other is squeezed together. A solid beam has to resist both through its own material. A truss separates the two jobs into different members. Members are tension members if they are pulled along their length and compression members if they are pushed along their length. In a boomilever, the top chord (the long member along the top) is in tension and the bottom chord is in compression, because the arm bends like a beam. The diagonals and verticals between the chords alternate between tension and compression depending on their orientation.

The figure below shows a bridge truss with its members colored by stress. Use it to see the idea of axial forces, then flip the picture: a bridge sags between two supports, while a cantilever hangs out from one. Being able to say which members are in tension and which are in compression, for a given loading, is exactly the kind of design knowledge the Event Supervisor may ask about.

Why a truss beats a solid beam of the same mass

Bending stress in a beam is $\sigma = \dfrac{M c}{I}$, where $M$ is the moment (N·m), $c$ is the distance from the neutral axis to the outermost fiber (m), and $I$ is the second moment of area (m⁴). The neutral axis is the line through the middle of the cross-section that carries no bending stress. For a rectangle of width $b$ and depth $h$, $I = \dfrac{b h^3}{12}$.

The cube in that formula is the important part. Stiffness and strength in bending grow much faster with depth than with width. Doubling the depth of a rectangle multiplies $I$ by $2^3 = 8$, while doubling the width only doubles it. A truss exploits this by making its effective depth the distance between the top and bottom chords and by placing most of its material in those chords, far from the neutral axis. The diagonals and verticals then only need to carry axial force, which uses wood efficiently because a stick loaded along its length carries its load across its whole cross-section.

Compression members and buckling

A slender member in compression can fail suddenly by buckling: it bows sideways at a load far below the load that would crush its wood. Buckling is a stability failure, not a strength failure. The wood can be perfectly sound and the member still fails once it is slender enough. Every compression member in your structure, especially the bottom chord and any long diagonals, is a candidate for buckling.

The classic result for an ideal elastic column is Euler's critical load.

The end conditions set K, which sets the effective length $K L$ that the member behaves as if it had. The standard textbook values are:

The figure below is a teaching model built to show how end conditions change the buckling mode and the critical load. It is not your structure, but the physics is the same.

Why halving the length matters

Because P_cr depends on $1/L^2$, halving the unsupported length of a compression member, with the same end conditions and cross-section, multiplies its buckling load by 4. Adding one brace that splits a long diagonal in half can therefore be a large gain for little mass, because it changes the effective length of a member that might otherwise fail first.

Buckling happens about the weak axis

Buckling happens about the axis with the smallest $I$. The member bows in the direction where it is most flexible, and a stick that is deep in one direction and narrow in the other is weak in the narrow direction. The example below shows why bracing the weak axis matters.

Euler's formula describes the buckling load of an ideal elastic column. Real wood is not ideal, and very short members fail by crushing or splitting before they buckle. Use the formula to compare designs and to see which member is most at risk, and use test data to find the actual failure load. Once a member buckles, its load capacity drops sharply, which is why a test that ends with a sudden bow in one member is usually a buckling failure rather than a joint failure.

Wood, adhesive and joints

What counts as wood

The rules define wood as the hard, fibrous material beneath the bark of trees. Plywood, particleboard, bamboo, paper, wood composites, sawdust-and-glue pieces and any commercially laminated wood are not wood for this event. You may glue layers of solid wood together, which is called lamination, but the layers must be solid wood that you laminated yourself. You may not paint, soak or coat wood, chemically modify it, color it, or stick tape, labels or preprinted paper to it. Ink barcodes and markings from the construction process may stay on the wood. Forgetting these rules is a common way for a good design to end up in Tier 2.

Choosing and conditioning sticks

Wood strength varies with density, grain and defects. Straight grain with few knots or other defects gives strength you can count on, and knots and defects are most dangerous in compression members. Orient each stick so its grain runs along the direction of the forces it carries. Weighing sticks to sort them by density is useful, because heavier pieces are often stronger, but a heavier stick adds mass. The right comparison is strength per unit mass, not strength alone.

Humidity changes both the mass and the strength of wood. A stick that gains moisture gains mass, and that changes your structure mass to the gram. Condition all your stock and finished builds in the same place for the same time, and weigh them after the same conditioning. Keeping builds in a consistent environment is part of the same discipline as recording each test.

Adhesive and glue lines

Any commercially available adhesive is allowed except adhesive tape. Cyanoacrylate ("super glue") is common because it sets quickly, but glue, cement, epoxy, hot melt and polyurethane are also allowed. The strength of a glued joint depends on three things: clean, fresh surfaces (lightly sanded and free of dust and oil), a thin and controlled glue line, and enough cure time. Rushed or thick glue lines can be brittle, and a joint that looks solid can fail at the glue line rather than in the wood.

Glue is also mass. Heavy joints, where a team adds glue beyond what the joint needs, cost structure mass without adding much strength. Over-gluing is a common mistake for teams that have not yet measured how much their joints add.

Joint types and how they fail

A joint carries force across an area, so its strength depends on the glued area and on how the force acts on that area. Shear stress across a glued face is $\tau = F / A$, where $F$ is the force (N) and $A$ is the glued area (mm²).

Peel is the other failure mode. A butt joint is poor at resisting a force that tries to pull its two members apart, because the glue line has to carry that pull at its edge. Peel is a force that pulls the two members apart at right angles to the glue face. Where you can, arrange joints so their glue faces carry shear instead, and use lap or gusset details where a pull-apart force is likely, especially at high-moment locations.

Designing around the wall

The structure rests on a connector bolt that the Event Supervisor installs through the testing wall. The wall is a rigid, smooth, low-friction surface at least 40.0 cm wide and 30.0 cm high, usually ¾-inch plywood, set plumb with a level. The bolt hole is about 5.0 cm below the top edge and centered side to side, and its horizontal and vertical centerlines are drawn on the wall. The bolt is a ¼-20 connector bolt, 60 mm long, with a wide flat head (about 17 mm across) and a short unthreaded section next to the head. A nut and washer on each side of the wall hold it, and it is set so the gap between the wall face and the inside face of the bolt head is 3.0 cm ± 0.1 cm. Your structure hooks over or slides onto the bolt within that gap. You may not adjust or take apart the bolt assembly.

Three depth lines are drawn at 10, 15 and 20 cm below the bolt's center, and two width lines run vertically 4.0 cm either side of it. The rules then restrict where your structure may touch the wall:

• Width. Division B may touch only inside the width lines. Division C may touch only outside them. Neither division may touch the sides, top or bottom edge of the wall.
• Depth, base option. Contact only above the 20 cm line (B) or the 15 cm line (C).
• Depth, bonus option. Contact only above the 15 cm line (B) or the 10 cm line (C), and the structure must hold 15 kg.
• Block height. Before any sand goes in, the bottom of the loading block must sit above the level of the bolt's centerline. During loading the tip may sag below that level without penalty.
• Reach. The chain's centerline must be 40 to 45 cm from the wall face, measured horizontally, and within 2.5 cm of the line running straight out from the wall's center.

How the geometry shapes the truss

A typical boomilever hangs its tension member on the bolt and pushes a compression member against the wall below it. The vertical distance from the bolt to that lower wall contact is the depth of the truss at the wall, and it is where the moment is largest. The depth line limits that depth. Because the force in each chord is roughly the wall moment divided by the depth, a shallower contact means larger chord forces.

Leave a margin. A contact pad that is designed to sit exactly on the line can end up just below it once the structure settles on the bolt, and a structure entered for the base option that touches below the base line is Tier 2.

The block-height rule is new for 2027 and applies in both divisions. The tip of the structure, 40 to 45 cm out, has to hold the bottom of the block above bolt level before loading, so a top chord that starts at the bolt cannot slope down toward the tip. Most designs run the top chord level or rising slightly toward the tip, so that its unloaded height leaves room for the deflection you expect early in loading. Measure the block height on your practice wall with the full loading assembly hanging, because the assembly's own weight (up to 1,500 g) already bends the structure a little before any sand goes in.

The width rule shapes the wall end. In Division B, the whole wall-side footprint must fit in an 8 cm strip around the bolt, so the compression member usually comes straight down to a single narrow pad. In Division C, the contacts must lie more than 4 cm either side of the bolt, so the compression side splits into two legs that spread outward to pads beyond the lines. Spread legs also resist sideways twisting, but they add length and joints, and every leg is a compression member that can buckle.

The Event Supervisor checks the bolt seating, the loading block, the reach, the width zone, the block height and the depth line before loading begins, and checks the bonus line to decide bonus eligibility. The bucket must hang with clearance above the floor (the 2027 rules give about 2 inches as an example) so it can swing or the structure can deflect. Nothing may be adjusted once sand starts flowing. You pour the sand yourselves, and you may steady the bucket only with the tips of the two bucket stabilizing sticks, never by lifting or pushing it. Loading stops the moment the structure fails, the load rests on anything but the structure, or the structure gets vertical support from anything other than the bolt. A structure that is almost right in the workshop can still fail a check on test day, so build a practice wall with the same lines, bolt and block and check every build against it.

Design principles

The following principles follow from the mechanics above. Each one names the mechanism that makes it work, so you can explain it to the Event Supervisor and apply it to designs the principles do not cover.

1. Triangulate the load path. Triangles are stable, and a truss turns bending into axial forces in its members, so each member carries force along its length rather than bending. Keep long compression members short by bracing them, because $P_{cr} \propto 1/L^2$.
2. Use all the depth the rules allow. Chord force falls as the depth at the wall grows, so place the wall contact as low as your option permits, with a safety margin above the line.
3. Brace the weak axis. Buckling uses the smallest I. A stick that is deep in one direction and narrow in the other needs bracing or orientation that matches its weak axis to the lateral load.
4. Work the wall-side members hardest. Moment is largest at the wall, so wall-side members should be the strongest, and tip-side members can often be lighter. A design that is uniform along its length may be carrying mass near the tip that it does not need.
5. Make joints efficient, not heavy. Use lap or gusset details where the force is high, avoid end-to-end butt joints at high-moment locations, and stop adding glue once a joint has enough area. Each extra gram of glue costs about your score in grams of load.
6. Design the tip for the block-height rule. Build in enough rise at the tip that the block is still above bolt level with the empty loading assembly hanging.
7. Keep the test repeatable. The same reach, the same conditioning and the same glue cure time make each test a fair comparison with the last. Changing three things at once tells you nothing about which change helped.

Building and testing logs

The design-knowledge requirement asks you to know your structure, and a testing log is the fastest way to learn it. Keep one log per team and write an entry for every build and every test. The best logs record what you expected, what happened, and what you changed because of it. Without an expectation written down before the test, you cannot tell whether your understanding was right.

Record at least these items for each test:

• Date, room humidity and temperature, and any change in conditioning.
• Glue type, joint types used, and cure time before weighing.
• The mass of each member group and of the whole structure, to 0.01 g.
• The reach, the wall-contact points measured against the depth and width lines, and the block height with the empty loading assembly hanging.
• The load supported, to the nearest gram.
• The first failure location and the failure type: joint peel, glue-line shear, member buckling, crushing, splitting or slipping on the bolt.
• Your predicted load before the test, and the difference from the actual load.

The last item matters for the second tiebreaker, because the team whose estimate is closest to the actual load ranks higher. Each test that records a prediction teaches you how good your estimates are, and an estimate that is systematically high or low is a correctable error. A bonus structure that holds the cap should estimate 15,000 g.

Read the log for patterns. If your first failure is always at the same joint, that joint is the weakest point of the design, and changing it will move the failure to the next weakest point. If buckling appears every time, bracing the compression members is often a cheaper fix than adding wood. Score improvement usually comes from removing mass where the structure does not fail, and adding mass where it does.

Common pitfalls

Most lost points come from a small set of mistakes, and several of them are rule violations rather than design errors.

How tournaments test this

Boomilever has no written test, and the scio.ly test archive holds no Boomilever questions, so there are no question counts to report. The judging is the test: the structure is scored on test day, and the design-knowledge requirement means the Event Supervisor can ask how your structure was built and how it works.

What the rules set up, and what you should prepare for, is this. The score rewards efficiency and is measured to the gram. Mass is taken at check-in, the geometry is checked against the wall lines before sand is poured, and the load is read after loading stops, so the procedure matters as much as the design. Many tier problems are found before any load is measured: a contact pad below the line, a pad on the wrong side of a width line, or a block that sits below bolt level. The tiebreakers reward a light structure first and an accurate estimate second. Questions on construction and operation can cover the physics behind the design: why a truss works, why members buckle, which members are in tension, how the reach and contact depth change member forces, and why you chose the base or bonus option.

When you practice, run the full sequence the event uses: submit an estimate, weigh, hang the structure, hang the loading assembly, check the block height and contacts, and load sand inside a six-minute timer. Ask your Event Supervisor what form the design questions take at your tournament.

Strategy

Split the work by skill, not by job title. One partner can own the calculations and the testing log: the moment at the wall, the chord forces, the buckling checks, the score comparisons and the prediction tracking. The other can own the build and the rules: the wood selection, the glue schedule, the wall geometry and the check-in procedure. Swap roles for a few builds so that both partners can explain the whole structure.

Plan around the testing window. Six minutes sounds generous until you add hanging the structure, reassembling the loading assembly in the same order if you took it apart, the supervisor's pre-load checks and the pour itself. Practice the full sequence with a timer until setup is routine.

Check mass carefully. Weigh the structure after conditioning, on a scale that reads to 0.01 g. Mass errors are priced the same way as design changes: at a score of 30, 0.5 g of structure mass is worth about 15 g of load, and mass is also the first tiebreaker.

Decide early between the base and bonus options. The bonus contact line is 5 cm higher, so the wall members work about a third to a half harder, and the structure must survive the full 15 kg. In return the Load Scored jumps to 20,000 g (B) or 22,500 g (C). The bonus is especially strong in Division C, where a base structure must be a third lighter just to tie. Build both if you can, and let your test data decide which scores better and more reliably.

Decide whether to appeal. A team that appeals must leave its structure with the Event Supervisor, so make sure your score sheet is right during the data review before you acknowledge it.

Keep a spare. Build at least one backup of your best design, because a single bad glue line or a dropped structure can end a team's day. Record the backup's mass and joint details so you can judge whether it is competitive.

Test yourself

These questions are written for this guide; the test archive has no Boomilever questions to quiz you with. Work each one before you read the answer. Units and rounding are part of the answer.

1. A structure weighs 200.00 g and supports 4,000 g with no bonus. What is its score?

2. A 125.00 g structure holds the full 15,000 g and meets every bonus requirement. What is its score in Division B, and in Division C?

3. At a score of 20, how much extra load offsets 10 g of extra structure mass?

4. Where may a Division B base-option structure touch the wall? A Division C one?

5. A team's structure cannot hold the loading assembly by itself. How is it scored?

6. A Division C structure meets the bonus contact rule but breaks at 13,000 g. Is it Tier 2?

7. Before loading, the bottom of the loading block sits 0.5 cm below the level of the bolt's centerline. Which rule does this break, and when is sagging allowed?

8. A load of 15,000 g is applied at a reach of 45 cm. What is the moment at the wall?

9. A bonus structure of 200.00 g holds 15,000 g. How light must a base-option structure holding 15,000 g be to tie it, in each division?

10. Two Division B teams in Tier 1 both score 125.0. Team X (bonus) weighs 160.00 g and estimated 15,000 g exactly right; Team Y (base, holds 15,000 g) weighs 120.00 g and estimated 12,000 g. Who places higher?

11. A 30 cm pinned–pinned strut is changed to a fixed–fixed strut of the same length, section and wood. What happens to its buckling load?

12. Name the tension and compression members in a cantilever truss under a downward load at the tip.

References and further reading

• Science Olympiad, 2027 rules manual, Boomilever (Divisions B and C): https://www.soinc.org
• SciOly Wiki, Boomilever (another summary of the event, linked for comparison; check any rule it states against the 2027 rules): https://scioly.org/wiki/index.php/Boomilever
• Euler's critical load, including the effective-length factors used in this guide: https://en.wikipedia.org/wiki/Euler%27s_critical_load
• Truss, the terms for chords, webs and members: https://en.wikipedia.org/wiki/Truss
• OpenStax, University Physics Volume 1 (statics, moments and the cantilever): https://openstax.org/details/books/university-physics-volume-1
• USDA Forest Service, Forest Products Laboratory, Wood Handbook: Wood as an Engineering Material (grain, density, moisture and strength): https://www.fpl.fs.usda.gov/
• Figure credits and licenses are in each figure's caption and in its linked Wikimedia Commons page.