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# Fiberglass Pergola Beam Design — The Box-Girder Principle ## Overview The structural behavior of a Baldwin Pergolas beam system is more sophisticated than it first appears. What looks like two parallel beams is, in engineering terms, a partial composite built-up girder — a configuration that dramatically increases bending stiffness without adding material weight. Layered on top of this are two additional manufacturing practices — pre-cambered crown and CNC-notched purlins — that produce a pergola structure that is stiffer, more precisely made, and more durable than anything achievable with conventional methods. ## Key Concepts - Two FRP beams spaced 5" apart behave as a partial composite built-up girder, not two independent beams - The parallel axis theorem governs: stiffness increases with the square of beam separation distance - Effective moment of inertia is approximately 3.9× that of two beams simply touching - All Baldwin beams are pre-cambered with 3/8" to 1/2" of upward crown over 20 feet — built in at pultrusion, not shimmed in the field - CNC-notched rafters at 18–20" spacing act as shear connectors, mobilizing composite action - Curved aluminum beams add torsional rigidity that further stiffens the overall structure --- ## The Standard Baldwin Beam System The standard Baldwin beam configuration uses two FRP rectangular tube members installed in parallel with 5-inch face-to-face spacing. Structural rafters notch over both beams and are fastened with stainless steel screws at 18–20 inch spacing. This creates a partial composite built-up girder whose effective stiffness far exceeds two independent beams. For longer spans, Baldwin upgrades to: - **Double 4×12 FRP tubes** — 1/4" top and bottom flange walls, 1/8" side walls, standing 12" tall - **Double 2×8 aluminum tubes** — same geometry, 4× the modulus of elasticity All three systems use the same composite girder principle and the same pre-cambered crown specification. --- ## Pre-Cambered Crown — Engineered Upward Bow > Baldwin pergola beams are ordered with a built-in upward crown of 3/8" to 1/2" over 20 feet. Dead load brings them to level. They never sag below that. Pre-cambering is standard practice in bridge engineering and steel construction. Members are fabricated with a deliberate upward bow — called crown or camber — sized to offset expected dead load deflection. When dead loads are applied, the beam deflects down to a level position. Live loads then deflect from a level baseline rather than a sagging one. Baldwin specifies this crown directly with its FRP pultrusion suppliers for both the 2×8 and 4×12 profiles. Every beam arrives at the Middletown facility with 3/8" to 1/2" of upward bow over 20 feet — not applied in the field, not shimmed at installation, but built into the member at the time of manufacture. ### Why This Matters The most common long-term concern about any pergola is visible sag. Beams that were once level gradua
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