FRAME BUILDER - VOL7 NO3 / 33 and near the center hinge point of the lower and upper leaves. The wind trusses can be located either inside the building (typical) or exterior to the door. The exterior design is sometimes used on very large doors. Several bi-fold door suppliers indicate that roughly 1/8 of the total wind on the door moves vertically and is resisted by the building header system. The remaining 7/8 of the wind load is transferred to the jamb columns via the previously described wind trusses. Since most of the wind load applied to a bi-fold door is transferred to the door jamb columns and the load is imposed at several points over the door height it is straight forward and slightly conservative to simply add half the door width to the tributary area of the jamb column for the height of the door. The wind imposed on the door will be modeled as a uniform line load when designing the jamb column. The same load distribution strategy can be used for both wind suction and pressure. • Wind pressure for half the door width is applied as a line load to the jamb column. • 1/8 of the total wind load on the door is applied as a line load to the door header. Note: The wind pressure distribution prescribed above is slightly conservative in that 12 percent of the wind at the top of the door is applied twice, once to the head and again to the adjacent jambs. Bi-fold doors are latched to the door jambs just below the hinge point. The latches hold the door closed under suction loads. The latches apply point loads to the jamb column. However, it is simpler and conservative to distribute the suction pressure to the surrounding framing in a manner similar to that used for the pressure load case. Since suction forces rarely control jamb column design this simplifying distribution method will have little impact on design results. Bi-fold Door — Operation Loads A bi-fold door is a simple machine. Two half-height, full width panels are hinged together at the centerline and hinged to the building at the top. Tension straps located intermittently across the door width are wrapped around drums. The straps form the third side of a triangle. As they are wound, that side of the triangle shortens, the bottom door panel rolls up the jamb columns and the door opens. When a bi-fold door is stationary, the entire assembly can be assumed to be a rigid body. The door weight can usually be assumed to be 6 psf for unlined un-insulated doors and 8 psf for lined insulated doors. When the door is closed the door hangs from the upper hinges. Much of the door dead load is transferred via diaphragm action of the door slabs to the exterior hinge points and then to the supporting jamb columns. Though diaphragm action of the continued on page: 34 YOUR TOOLKIT FOR BUILDING EXCELLENCE Photo Courtesy of Midland Doors
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