1) The document summarizes findings from past hurricanes regarding structural damage to wood structures, noting that roof sheathing loss and deficiencies in load transfer were common causes of failure.
2) It then discusses how current prescriptive design methods may not adequately predict failure mechanisms in light-framed wood structures subjected to hurricane winds, and that improved understanding of wind load paths is needed.
3) The results of a wind tunnel test on a 1:3 scale model wood house are then presented, showing how connection influences and peak loads vary by location, and how prescriptive design standards could be improved based on such testing data.
Workshop - Best of Both Worlds_ Combine KG and Vector search for enhanced R...
Session 12 ic2011 prevatt
1. 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 2011
1
2.
3. 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 present
3
4. 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
5. • 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
LFWS
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9. 30 Pressure sensors
12 roof-to-wall
load cells
9 foundation
load cells
Floor plan: 3 m wide by 4.1 m long (full-scale 9 m X 12.2 m)
10. Wind generator: 2,800 hp engines driving eight 1.5 m diameter axial fans
Test wind speed: 22 m/s
Three wind directions: 000o, 045o, 090o
3 repeats, each at 10 minute periods
Pressure data collected at 200 Hz
Wind speed measured using Cobra Probe (5000 Hz); (5% TI)
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21. • 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 guide
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24. • 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 guidelines
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25. • 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 buildings
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26. • 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 m
26
27. • 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 winds
27
28. 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