APA - The Engineered Wood Association


Longer Spans Open Design Possibilities

The superior strength of glulam allows longer clear spans than solid-sawn lumber. This opens up the design possibilities in both commercial and residential construction. In commercial design, custom glulam beams can span more than 100 feet. In reticulated glulam framed dome structures, glulam arches span more than 500 feet.

Glulam trusses also take many shapes, including simple pitched trusses, complicated scissors configurations and long span bowstring trusses with curved upper chords. When designed as space frames, glulam truss systems can create great clear spans for auditoriums, gymnasiums, churches, and other applications requiring large, open-floor areas.

Panelized Roofs Top Large, Open Spaces

In large, commercial buildings, such as warehouses and large retail facilities, glulam beams are an integral component of panelized wood roof systems.

In a typical panelized system, pre-framed units are constructed with glulam purlins or trusses spaced 8 ft. on center. Lumber stiffeners spaced 16 inches or 24 inches on center span between the glulam members. Structural wood panels are then attached to the stiffeners and purlins and the entire assembly, 8 x 72 ft. or greater, is lifted to the roof elevation with forklifts. This greatly speeds the erection process and minimizes construction costs.

In some panelized roof designs, a cantilevered glulam girder system allows for long-span roofs with minimal need for intermediate columns, thus providing more open column grid spacings. This feature, combined with extra roof height, maximizes both space efficiency and storage versatility.

Designing for Deflection

In longer span applications, deflection is often a controlling design factor. While any wood bending member can be designed to minimize deflection, glulam is the only engineered wood product that can be easily cambered to reduce the aesthetic effect of in-service deflections. Camber is curvature built into a fabricated member which is opposite in direction and magnitude to the calculated deflection which will occur under gravity loads.

The glulam industry recommends that roof beams be cambered for 1-1/2 times the calculated dead load deflection. This will generally be sufficient to assure that the beam will not exhibit a sag over a period of many years of loading, as may occur with non-cambered wood products. To achieve a level profile it is recommended that floor beams only be cambered for 1.0 times the calculated dead load deflection.

Camber for glulam beams is specified as either “inches of camber” or as a radius of curvature that is to be used in the manufacturing process. Commonly used curvature radii for commercial applications are 1600 and 2000 feet, although any camber may be specified.

Because most residential applications require very little or no camber stock beams are often the ideal choice. Stock glulam beams are typically supplied with a relatively flat camber radius of 3,500 feet or zero camber. If more camber is required, custom beams are available through the manufacturers.