Boomilever

Overview

Boomilever structures maximize load supported per unit mass. Performance depends on efficient geometry, material properties, and joint quality.

Structural mechanics

• Load paths: map forces from load point to wall attachment via tension and compression members; minimize bending in tension members.
• Compression stability: Euler buckling risk increases with slenderness λ = L/k; increase area moment of inertia I (e.g., box/triangular sections, bracing) rather than only adding mass.
• Shear and local failures: joint peel and shear at interfaces; bearing stresses around fasteners or hooks; introduce gussets/fillets where stress concentrates.

Materials and joints

• Wood selection: straight grain, low defect density; weigh sticks to sort by density; orient grains along principal stress directions.
• Adhesives: surface prep (fresh, clean, lightly sanded); controlled glue line thickness; cure time and humidity influence.
• Joint types: lap and scarf joints distribute stress better than butt joints; avoid prying configurations.

Geometry and optimization

• Triangulation: convert bending into axial loads; avoid long unsupported compression members.
• Cross‑sections: place material far from neutral axis to raise I; use thin webs with flanges (I‑like behavior) where rules allow.
• Attachment: wall brace geometry influences moment arm; minimize lever arm for compression members to reduce required capacity.

Testing and calibration

• Subcomponent testing: column coupons for buckling; joint shear/peel tests; collect strength vs mass data.
• Full tests: record load vs deflection; identify first failure location; iterate geometry and joints accordingly.
• Environmental control: humidity swings alter wood strength and mass; condition and store builds consistently.

Worked micro‑examples

1. Column buckling estimate (qualitative)

• Halving unsupported length roughly quadruples buckling load (P_cr ∝ 1/L²), holding cross‑section constant.

2. Joint redesign

• A butt joint at a high‑moment location fails early; replacing with overlapping lap joint and gusset increases shear area and shifts failure elsewhere.

3. Mass placement

• Moving 10% of mass from web to flanges increases I disproportionately, improving stiffness without increasing total mass.

Pitfalls

• Over‑gluing (heavy joints) without strength benefit; brittle glue lines from rushed curing.
• Ignoring grain orientation; placing knots/defects in compression members.
• Focusing only on ultimate load instead of efficiency (load/mass) across iterations.

Practice prompts

• Sketch a triangulated boomilever with indicated tension/compression members and justify member sizing.
• Propose a joint detail that reduces peel stress at the wall plate.
• Design an experiment to compare compression member buckling for different bracing spacings.

References

• SciOly Wiki: https://scioly.org/wiki/index.php/Boomilever