Excerpts from:  Building Performance: Hurricane Andrew in Florida, Observations, Recommendations, and Technical Guidance, by Federal Emergency Management Agency (FEMA), FIA-22 (2/93), December 21, 1992.

Wood-Frame Modular Buildings:

(Page 29): Overall, relatively minimal structural damage was noted in modular housing developments. Thtmodule-to-module combination of the units appears to have provided an inherently rigid system that performed much better than conventional residential framing. This was evident in both the transverse and longitudinal directions of the modular buildings.

Two end-wall (end wall of end modules) failures were observed in a modular home subdivision. Poor connection of the tops of the walls to the roof diaphragms was evident in these instances. Some roof sheathing was observed missing from rafters [after hurricane Andrew], judged to be due either to building envelope breach, (window and/or door failure) or to external wind and debris. Generally, the rafters themselves were left entirely intact, because of the inherent rigidity developed by the relatively short spans and secure connections.

(Page 30): Structural integrity of individual units (necessary to resist lifting and transportation forces [from the factory to the site]) ensures sound components for systems of connected units. Connection of units provides inherent reinforcement to overall construction with doubled members.

One-to Two-Story Light Wood-Frame [Site-Built] Buildings:

(Page 17): The catastrophic failure of one- to two-story wood-frame buildings was observed more frequently that the catastrophic failures of other types of site-built structures [concrete block, , concrete block and stucco, etc.]. Building failure was determined to be primarily a result of negative pressure and/or induced internal pressure overloading the building envelope.

An absence of or improper installation of framing connections, load transfer straps, or bracing from non-loadbearing walls to connecting wall and roof components was noted. This condition contributed significantly to the primary failure of the framing system.

The wood-frame gable ends of roof structures was found to be especially failure-prone. ...A lack of an adequately defined load transfer path for the gable ends was evident. Bracing of the wood-frame gable ends was not performed with consistency and completeness required to effectively resist the transfer the wind loads in the absence of roof sheathing. The reliance on plywood sheathing to act as the sole stiffener of the roof diaphragm left buildings susceptible to structural damage from roof truss collapse when the sheathing separated from the roof trusses.

(Page 20): Individual structural members were observed to have been built [in site-built framed houses] and connected without adequate attention to design and construction details. ... These deficiencies compromised the structural integrity of the entire wall and roof systems.

(Page 22): The roof framing systems observed were composed typically of prefabricated light wood trusses and plywood sheathing. While the trusses were found to have performed well under the wind forces, the connection of the sheathing to the trusses was inadequate. Substandard workmanship in the anchoring of sheathing to trusses was evident.

(Page 23): It is the opinion of the assessment team that reliance on sheathing for truss-roof bracing and the corresponding loss of he sheathing was a major cause of the total damage the building systems. 

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