Shoot structure varies enormously across species, yet how such variations interact with their mechanical stability is poorly known. We present a model to calculate safety factors along stem axes against stresses imposed by gravity and wind. The model integrates side branch architecture, leaf and stem mass distribution, stem tapering and stem mechanical properties, and is applied to 16 coexisting woody species from an Australian arid shrubland. There was substantial variation in leaf and stem mass for a given length of shoot across species (140 and 20 fold respectively). In contrast, safety factors for gravity or wind varied only 3-4 fold across species. Safety factor along stem at 0.4-1.2m length was roughly uniform in tree species, but was more variable in shorter shrub species. Stem modulus of rupture (MOR) was unrelated to safety factors across species. Stems with high MOR had slender structures (higher shoot length/stem diameter), which compensated against the positive effect of high MOR on safety. These results suggest that diverse shoot structures across species are constrained to satisfy similar mechanical safety factors with internal trade-off among traits. Our model strengthens mechanistic understanding about how key shoot traits are coordinated to satisfy mechanical requirements.