Dangers Involving Bowstring Truss Roofs

Variable Loads

• A bowstring truss roof balances the outward pressure at the ends of the curved roof against the holding ability of the horizontal piece at the bottom of the truss. This works partly because the stress is equal at each end stressing the horizontal segment equally. If the load on the curved segment of the truss varies, such as an uneven load of snow on the roof, it creates unequal stress on the base of the truss that can cause failure.

Component Failure

• A failure of either the base or curved upper portion of the bowstring trusses actually causes the failure of the whole truss. The interaction of the two components to balance the load of the roof is broken if one component fails. If the horizontal base fails the curved roof can spread out at the ends while the roof surface can collapse if the curved top of the truss fails.

Fire Danger

• Interior fires can destroy the horizontal base of the bowstring truss. This could lead to a catastrophic failure of the roof with the ends of the trusses forced out and collapsing. This is an issue with bowstring trusses using wood as the principle construction material. Damage by fire can occur to any wood truss. The danger of the bowstring truss is the additional outward pressure created on the walls if the base of the truss fails.

Upgrades

• Older bowstring truss roof systems may not meet modern building and fire codes. Consult a structural engineer for upgrade bracing that can bring the bowstring truss into compliance. These upgrades may include metal braces that are fire resistant.