Everything you need to build this bridge by hand: a full-size plan, the cut list, the joints and the glue order.

Paper

Build pack · ASCE Gulf Coast Student Symposium 2023 (Balsa Bridge Over Mobile Bay)

sandwich pratt-6 for ASCE Gulf Coast Student Symposium 2023 (Balsa Bridge Over Mobile Bay)

Mass: 7.9 g; limit 150 g
Length: 19.4 in; 18 in gap
Sticks: 12; 36 in stock
Rule checks: All pass; 23 checked

Loads (simulated, not yet checked on real breaks)

First failure: 0.0 kg
Collapse: 0.1 kg
Efficiency (load ÷ mass): 14

Expected to break first: member crushing/buckling, at FB3 (floor beam).

The full-size plan

One truss seen from the side, 19.4 in long. Both trusses are built on this same plan. The plan prints full size across the pages outlined below: trim each page along its dashed line, lay it over the page to its left or above, line up the ⊕ marks and tape it. Print at 100% (“Actual size”) and check the scale bar on each page with a ruler.

Cut list

Every piece, labelled as on the plan. Lengths are the finished long edge, to the nearest 1/16 in. Angles are between the cut and the stick's edge (90° is square), left end first as drawn (bottom end first for verticals); where an end has two cuts, both are given. For the truss pieces, lay the stick on the full-size plan and mark the cuts from it.

Piece	What	Cut	Stick	Length, in	Ends
B1	bottom chord	4	1/16 × 1/16 in	3 7/16(3.41)	both square
B2	bottom chord	4	1/16 × 1/16 in	2 7/8(2.85)	both square
B3	bottom chord	4	1/16 × 1/16 in	2 1/4(2.24)	both square
B4	bottom chord	4	1/16 × 1/16 in	3 3/8(3.36)

The joints

Every kind of joint on the bridge, once, enlarged. Blue runs through; amber is cut to fit against it. Truss joints are seen from the side, like the plan; joints between the trusses are seen from above.

Side view · 3 like this on each trussOn each layer, two bottom chord pieces meet end to end here. The diagonals and verticals are cut against them.The chord is two sticks with the webs between them: each web runs to the chord's far face and is glued on both faces.Where webs meet each other at the joint, one runs through and the others are cut against it.Glue a gusset plate over the joint on both faces.

Glue order

Pieces that run through a joint go down first, because the others are cut to fit against them: the chords (and the end posts, where they run through the corner). Then the webs between them. Build the second truss the same way on the same plan. The pieces that join the two trusses go in last: the deck members first, then the bracing above them.

1
First truss: chords
B1, B2, B3, B4, B5, B6, P1, P2, T1, T2, T3, T4
Pin them down on the plan over wax paper or plastic wrap.
2
First truss: webs
E1, E2, V1, V2, V3, V4, V5, D1, D2, D3, D4, L1, L2
Cut each end to the plan, then glue it against the chords.
3
Second truss: chords
B1, B2, B3, B4, B5, B6, P1, P2, T1, T2, T3, T4
On the same plan, once the first truss is dry and lifted off.
4
Second truss: webs
E1, E2, V1, V2, V3, V4, V5, D1, D2, D3, D4, L1, L2
The same pieces again.
5
Join the trusses: deck

The rules, in short

Our summary of the ASCE Gulf Coast Student Symposium 2023 (Balsa Bridge Over Mobile Bay) rules. If anything here disagrees with the rulebook or the organizer, they win.

Size and span

The bridge stands on a flat surface; nothing may touch down inside the 18 in gap.
Overall length: 18 to 20 in.
Overall width: up to 6 in.
No member thicker than 3/8 in.

Deck and clear space

It needs a level deck (roadway).
Leave room for the 3 x 3 in roadway: clear path for traffic at least 3 in wide and 3 in high.
Leave room for the ship channel: 3 in vertical clearance for a 3 in wide channel under any portion of the bridge.

The load test

Standard load-testing vehicle: 5 in long, 2.5 in wide, 1 in wheels on a 3 in wheelbase.
The judges choose where it goes.
It must hold at least 35 lb.
Each load step is held for 5 s.
Sagging more than 1/2 in counts as failure.

Materials

Allowed balsa sizes: 1/16 × 1/16 in, 3/32 × 3/32 in, 1/8 × 1/8 in, 3/16 × 3/16 in, 1/4 × 1/4 in, 5/16 × 5/16 in, 3/8 × 3/8 in, 1/16 × 1/8 in, 1/16 × 1/4 in, 1/8 × 1/4 in, 1/8 × 3/8 in, 1/4 × 3/8 in, each up to 36 in long.
Sheet balsa up to 1/16 in thick.
Members glued up from several sticks are allowed, up to 3/8 in.
Any of the usual glues.

Joints

The rules allow butt joints (a stick's end glued against another), lap joints (sticks overlapping side by side), sandwich joints (a stick glued between two others), gusset plates on one side, gusset plates on both sides, notches.

Plan pages (Letter)

These print one per page at full size. On screen they show at your display's scale.

Page A1 of 3 · sandwich pratt-6 for ASCE Gulf Coast Student Symposium 2023 (Balsa Bridge Over Mobile Bay)

Full size. Row A, column 1.

This bar is 2 in (50.8 mm) long.

sandwich pratt-6 for ASCE Gulf Coast Student Symposium 2023 (Balsa Bridge Over Mobile Bay)

The full-size plan

Cut list

The joints

Glue order

The rules, in short

Size and span

Deck and clear space

The load test

Materials

Joints

Plan pages (Letter)

sandwich pratt-6 for ASCE Gulf Coast Student Symposium 2023 (Balsa Bridge Over Mobile Bay)

The full-size plan

Cut list

The joints

Glue order

The rules, in short

Size and span

Deck and clear space

The load test

Materials

Joints

Plan pages (Letter)

From each stock stick

Mass and score