Session 12 ic2011 prevatt
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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.
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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
- 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 5
- 6. Derive/predict (analytical) influence functions Estimate reactions 1/50 wind tunnel Compare model Determine wind Create wind pressure distributions Predicted reactions 1/3-Scale Evaluate influence Wood Compare functions House Validation Measure structural Measured reactions reactions
- 7. 00o Wind Direction 45o 90o -0.5 -0.5 -2 -1 .2 -2 -0.8 -3 -1 -3 .5 -3 -2 .5.5 -212 -2 .5 3 -- -0.6 -2 -0 .8 -0.6 -2 -1 -1.5 -2 .52 - -2 -2 Clemson University Wind Tunnel -1 -2 -1.0 - -1 -1 .5 -1 .2 -1 -1 .5 -1.5 -1 .5 5 -1 . -1 -1 -1 .2 -0.8 -1 -1.5 .4 -1 .5 -0 -1.5 -1 -0 5 -1 . -1 -1 -0 .5 -1 -2 -1 .2 -1 .8 -1-1.5 -0.6 -2.0 1 -0.6 -2 -1 .4 .5 -0 -1 .2 -0- -2.5 -2.5 -1 -0 .8 -1 .2 .5 -1 -1 -1 -0.5 -1 .5 -1.4 2 -1. -3.0 -3 -1 -1 -1 -1 -0 .6 -0.5 -0 .8 -0 .6 5 -1 .4 -0 . -0 .5 -1 -3.5 -3.5 -0 .5 .2 -1 -1 .6 -1 -1 -0.8 --11 -1 .2 -4.0 -4 -1 -0 .5 -1 --11 .2 -1 -0 .5 -1..4 05 -1.6 -0.5 -1 .4 -0.6 - -1 -1.2 -0 .6 -0 . 5 -1 -1.4 -1 -1 -0 .8 -4.5 -4.5 Peak Pressure Coefficients Mensah, A.F., Datin, P.L., Prevatt, D.O., Gupta, R., van deLindt, J.W., (2010) “Database-assisted design methodology to predict wind-induced structural behavior of a light-framed wood building”, Engineering Structures, http://dx.doi.org/10.1016/j.engstruct.2010.11.028 7
- 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) 10
- 11. R2 R2 R1 R1 S3 S2 S1 L3 L4 L5 L3 L4 L5 L2 L6 L2 L6 L1 L1 SF4 SF4 SF3 LF5 SF3 LF5 SF2 LF4 SF2 LF4 LF2LF3 LF2LF3 SF1 LF1 SF1 LF1 With intermediate gable end connections Without intermediate gable end connections 11
- 12. • Roof-to-wall connections R2 R1 S3 S2 0.2 S1 L3 L4 L5 L6 0.4 0 0.2 L2 L1 SF4 SF3 LF5 0 LF4 0.4 SF2 LF2LF3 0 SF1 LF1 0.6 0 0. 4 0.6 0 0.2 0 0. 8 12
- 13. • Roof-to-wall connections 0 0 R2 R1 S3 0 S2 L3 L4 L5 0.2 S1 L2 L6 SF4 L1 0.4 SF3 LF5 LF4 0 SF2 LF2LF3 SF1 LF1 0. 6 0.2 0.8 0.4 0 13
- 14. • Wall-to-foundation connections R2 R2 R1 R1 S3 S3 S2 S2 S1 L3 L4 L5 S1 L3 L4 L5 L2 L6 L2 L6 L1 0.1 L1 SF4 SF4 SF3 LF5 SF3 LF5 SF2 LF4 SF2 LF4 LF2LF3 0.2 LF2LF3 SF1 LF1 SF1 LF1 0.1 LF2 SF1 14
- 15. • Gable end connections 0 R2 R2 R1 R1 S3 0.3 S3 S2 S2 S1 L3 L4 L5 S1 L3 L4 L5 L2 L6 0.1 L2 L6 L1 0.1 L1 SF4 SF4 SF3 LF4 LF5 0.3 SF3 LF4 LF5 SF2 SF2 LF2LF3 LF2LF3 SF1 LF1 SF1 LF1 15
- 16. • Gable end connection differences With Without intermediate intermediate gable gable end end connections connections R2 R2 R1 R1 S3 S2 S1 L3 L4 L5 L3 L4 L5 L2 L6 L2 L6 L1 L1 SF4 SF4 SF3 LF5 SF3 LF5 SF2 LF4 SF2 LF4 LF2LF3 LF2LF3 SF1 LF1 SF1 LF1 16 -0.2 0 0.2 0.4 0.6 0.8 1.0 1.2
- 17. Time Influence Coefficients Roof Pressures (Wind Tunnel) 17
- 18. • Wind tunnel time histories 18 (Simiu and Stathopoulos 1997; Main and Fritz 2006)
- 19. • 3m x 2m cross-section • Suburban terrain • 1:50 scale • (z0 = 0.22m, TI = 0.27) • 387 roof pressure taps • Open terrain • 300 Hz sampling rate • (z0 = 0.047m, TI = 0.20) 19
- 20. o o o o o 0 45 90 135 180 2000 2000 2 Peak Reactions (lbs) 1750 1750 1 Peak Reactions(lbs) 1500 1500 1 R2 R1 1250 1250 1 S3 S2 1000 1000 1 S1 L3 L4 L5 750 750 L2 L6 SF4 L1 500 500 SF3 LF5 250 250 SF2 LF4 LF2LF3 SF1 LF1 0 Mean (lbs) Mean (lbs) 500 500 250 250 0 0 LF5 LF4 LF3 LF2 LF1 SF1 SF2 SF3 SF4 LF1 SF1 L6 L5 L4 L3 L2 L1 S1 S2 S3 R1 R2 R3 R4 R5 R6 LF5 LF4 LF3 LF2 SF2 SF3 SF4 L6 L5 L4 L3 L2 L1 S1 S2 S3 R1 R2 R3 R4 R5 R6 Roof-to-Wall Connection Wall-to-Foundation Connec Roof-to-Wall Connection Wall-to-Foundation Connectio Gable end wall carries significant load and reduces corner reaction loads 20
- 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 21
- 22. 2500 MWFRS Low-Rise Design Load (lbs) 2000 MWFRS Low-Rise All Heights C&C: EWA > 100 1500 WFCM Low-Rise MWFRS All Heights MWFRS = 10 C&C: EWA > 100 MWFRS Low-Rise C&C: EWA WFCM0o All Heights DAD - o MWFRS 1000 MWFRS o = 10 DAD 0o All C&C:-EWA Heights WFCM0 DAD - 45o DAD - 45o 500 C&C:-EWA > 100 DAD 45 C&C: EWA = 10 0 L1 L2 L3 L4 L5 L6 Connection R2 MWFRS loads underestimate DAD-derived peak loads R2 R1 R1 S3 S2 L3 L4 L5 L6 C&C loads capture gable endL3 L4 L5L6 S1 peak load L2 L2 L1 L1 SF4 SF3 WFCM more conservative thanSF3 LF5 SF4ASCE 7-05 MWFRS loads LF5 SF2 LF4 SF2 LF4 LF2LF3 LF2LF3 22 SF1 LF1 SF1 LF1
- 23. • Wall-to-foundation connections Peak Reaction Load (lbs) 2000 1500 1000 500 0 LF5 LF4 LF3 LF2 LF1 SF1 SF2 SF3 SF4 Connection DAD - 0o DAD - 45o R2 R1 S3 WFCM - Table 2.2A S2 S1 L3 L4 L5 WFCM - High Wind Guide L2 L6 L1 ASCE 7 - All-heights SF4 SF3 LF5 23 ASCE 7 - Low-rise SF2 LF2LF3 LF4 SF1 LF1
- 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 24
- 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 25
- 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
- 29. David O. Prevatt dprev@ce.ufl.edu University of Florida 29