Peter L. Datin, Ph.D. Risk Management Solutions David O. Prevatt, Ph.D., P.E. (MA) University of Florida Forest Products Services Convention, Portland, OR June 20111
Hurricane (and Tornado) Wind Damage to Wood Structures Hurricane Main Finding/Recommendation Reference (Year) Alicia Most structural damage due to loss of roof sheathing (Kareem 1985) (1983) “Total collapse of timber-framed houses was a common scene.” Gilbert Most of the damage due to anchorage deficiencies (Adams 1989; (1988) Continuous load path is needed Allen 1989) Hugo Roof loss with subsequent collapse of walls (Sparks 1990) (1989) Most damage was roof and wall cladding failures - extensive rain damage Andrew Excessive negative pressure and/or induced internal pressure (FEMA 1992) (1992) Correct methods for load transfer are needed Iniki Overload on roof systems due to uplift forces (FEMA 1993) (1992) Load path must be continuous from the roof to the foundation Charley High internal pressure from window failure was major cause of roof loss (FEMA 2005a; (2004) Load path needs to be continuous FEMA 2005c) Ivan Structural damage was due to sheathing loss (FEMA 2005b; (2004) Ensure a complete load path for uplift loads FEMA 2005c) Katrina Structural failures roof sheathing loss and roof-to-wall connection failure (FEMA 2006) (2005) A continuous load path must be present3
Motivation and Background• The majority of U.S. homes are light-framed wood structural systems • Structures are typically not engineered and lack rational framing • Problems of brittle failure of connections is well known for decades• Hurricane winds cause extensive damage to wood residences • 2004/2005: $73 billion damage in the US hurricane season • Roof structures (sheathing and roof-to-wall connections) are vulnerable• Current design and analysis methods do not predict failure mechanisms • Is the simplified wind design load methodology appropriate? • Does our limited knowledge of wind load paths hinder better design? Wind Fl
• Most residential LFWS are not engineered • Prescriptive design methods based on anecdotal data (Crandell et al. 2006) • Limited testing of components and assemblies of components • Few studies on field installation on real homes • Few engineering studies/results available to predict load transfer through a complex 3D LFWS (Li et al. 1998; Gupta et al. 2004; Gupta and Limkatanyoo 2008) • van de Lindt and Dao (2008) introduced performance- based wind engineering design • Requires fundamental understanding of wind uplift behavior of LFWS5
• ASCE 7-05 (Minimum Design Loads for Buildings and Other Structures) • Main wind force resisting system (MWFRS) • Low-rise method (≤ 60 ft or 18m) • All heights method • Components and cladding (C&C) • Wood Frame Construction Manual (WFCM) • One- and two-family dwellings • Optional high wind area guide21
• Gable end wall carries up to 70% of load applied to gable end truss (if intermediate connections) • In most residential construction, gable end wall sheathing continuous with gable end truss • No design guides exist to design these connections • End wall carries higher percentage of roof uplift forces than side wall regardless of roof-to-wall connection arrangement • Not anticipated by current design guidelines24
• ASCE 7-05 • DAD showed that MWFRS underestimates peak loads at gable end truss when intermediate connections are not present by 10-33% (non-conservative) • Component & cladding approach is better at capturing DAD-estimated peak loads better (still some underestimation at interior trusses by as much as 19%) • Results strongly suggest the components and cladding wind design uplift load is the appropriate predictor of wind load on wood-framed buildings25
• Previous load stands ~1982 based on pseudo pressure coefficients(GCp) developed by Stathopoulos (1979) • Influence lines developed for simple, steel portal-framed building • New validation shows these not appropriate for light- framed wood construction 24.4m • Structural systems respond differently 7.3m 24’ 9.1m 4.3 14’ 7 .6 m26
• ASCE 7-05 • MWFRS loads underestimate DAD-derived peak loads • C&C better estimate of peak forces • WFCM • Underestimate DAD-derived peak loads at roof-to-wall connections • Conservative values for wall-to-foundation connections • Inconsistencies derive from improper influence coefficients used in ASCE 7-05 for external pressure coefficients leading to vulnerable buildings in high winds27
The authors would like to acknowledge the generous support provided by the National Science Foundation, under Grant #0800023 Dr. Peter L. Datin (Dissertation Work) Co-Principal Investigators: Dr. Rakesh Gupta, Oregon State University Dr. John W. van deLindt, University of Alabama
David O. Prevatt firstname.lastname@example.org University of Florida29
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