Drive the truck off-centre and watch the hull heel, then change the wave period and find the heave resonance. Drag across the scene to move the load sideways.
The science
Archimedes. A floating hull displaces its own weight: rho g A d = W, so the draft of a box pontoon is d = W / (rho g b l). The page solves the exact tilted waterline, not a small-angle formula.
Stability. Metacentric height GM = KB + BM - KG with KB = d/2 and BM = b^2 / (12 d). A load that sits to one side shifts the centre of gravity sideways by yg, and for small angles the hull heels by about phi = yg / GM. A tall, narrow load raises KG and shrinks GM.
Spare capacity. The extra load a bay can carry at minimum freeboard f is rho g l b (H - f) minus the bay's own weight.
Heave. m z'' + B z' + c z = F with c = rho g b l, m = structure + added mass (Ca = 0.5, half-cylinder of water), natural period Tn = 2 pi sqrt(m / c). The wave force is Froude-Krylov, reduced by depth exp(-k d) and the beam factor sinc(k b / 2). Near T = Tn the hull resonates.
Simplifications. Two dimensions (per bay), linear waves, a rectangular hull, a rigid cargo with its centre at one point, no current, no mooring, no neighbouring bays. The bridge-wide effects (deck bending, current, anchors) are in the 3D lab.