Investigating the Storm: Assessing Structural Damage
by Tim Marshall, P.E. Meteorologist 1
Presented at the Tornado Summit Oklahoma City, OK February 27, 2018 REFERENCES - FEMA
http://www.fema.gov/library
2 ASCE- AMERICAN SOCIETY OF CIVIL ENGINEERS
3 HURRICANES
http://pubs.asce.org/books/ NIST – National Institute of Standards and Technology
https://www.nist.gov/publications AMAZON.COM
6 REFERENCES AMS HTTP://WWW.AMETSOC. ORG/ PUBS/INDEX.HTML THE NATURE OF WIND
8 TROPICAL STORM WINDS
3 second gust
9 Mean plus or minus turbulence TORNADO – MESOCYCLONE WIND
Each curved vertical line is 5 min
10 MEASUREMENT OF WIND
• Standard anemometer is utilized. • Standard wind measurement is 10 meters (33 feet) above ground in open, unobstructed terrain (Exposure C). • Three second gust is utilized by building codes and standards. • Correction factors are needed to compare winds to other heights or exposures.
11 ASCE – 2005 BASIC WIND DESIGN DIFFERENT SITE CONDITIONS Effects of ground level obstructions and increase of wind with height
Exposure A: Urban Exposure B: Suburban Exposure C: Open Exposure D: Shoreline
14 A B C D WIND INTERACTION WITH BUILDINGS
15 WIND PRESSURE EQUATION
P= .00256 Cp V2 where: P = pressure (psf) Cp = pressure coefficient (< 1 >) V = wind velocity (mph)
16 WIND FORCE CALCULATIONS ON A 4 X 8 FT. SURFACE Wind Velocity Pressure Force 20 mph 1 psf 32 lbs. 50 mph 6.4 psf 205 lbs. 100 mph 25.6 psf 820 lbs. 200 mph 102.4 psf 3277 lbs.
17 Low Wind Wind Direction Velocity
Roof
Box-shaped structure Roll Vortex
18
Smoke plumes in wind tunnel test High Wind Velocity
Laminar Flow
WINDWARD LEEWARD
Roll Vortex
19 Turbulence Wind effects on a gable roof
Ridge Corner 20 Wind effects on wall and eave
Wall corner Roof eave 21 22 Roof Corner Uplift Roof Eave Uplift Wind on roof produces lift; shingles are removed
Open porch catches Wind stagnates at Outward
wind like a box window force rips23 away siding Internal pressure increases when wind breaches a building
Wind Additional uplift on roof and back wall
24 It’s bad news if the wind gets in WIND FORCE ON A BUILDING
Wind tries to topple building or push it over. 25 WIND FORCES ON A BUILDING
Percent increase/decrease (pressure coefficients) of 26 wind around a building when wind comes from the bottom-up. WIND FORCE ON GARAGE DOOR POST
27 WIND UPLIFT ON ROOF
28 THE EFFECTS OF INTERNAL PRESSURE
IN OUT
29 IF WIND GETS IN, IT WANTS TO GET OUT
IN OUT
30 BLOCKING AND CHANNELING OF WIND
• Homes in the middle of the block tend to be shielded from wind especially when wind is parallel to street. • Homes at the end of a block, cul de sac, or open field receive the full brunt of the wind and tend to experience greater damage.
31 WIND FORCES - EXPOSURE
Aerial view of Joplin, MO tornado damage illustrates W W shielding, i i blocking, n n and d d orientation of wind/buildings
Homes at the end of a block, cul de sac, or open field receive32 the full brunt of the wind and tend to experience greater damage. Boarded windows and vents can help save the house from internal pressure effects.
33 SECURING ROOF WITH AIRCRAFT CABLE TIED TO PIERS
34 BASIC WIND SPEED DESIGN
35 ABOUT BUILDING CODES
• Building codes are a “minimum” design.
• Building codes do not include design for tornadoes.
• Basic design criteria for most of U.S. is 90 M.P.H. for a 3-second gust. 36 THE ENHANCED FUJITA (EF) SCALE
37 THE ENHANCED FUJITA (EF)-SCALE
• Original Fujita scale developed by Dr. Ted Fujita in 1971 after the Lubbock, TX tornado. • The Enhanced Fujita (EF) scale adopted Feb. 1, 2007 by the NWS • EF scale developed by the Texas Tech University Wind Science and Engineering center • Rates houses 0 to 5 based on increasing severity of damage. • Assumes houses are “well-built” otherwise corrections are needed. • Simple to use but has shortcomings. 38 REFERENCES NWS
http://www.wdtb.noaa.gov/courses/ef-scale/lesson2/FinalNWSF- scaleAssessmentGuide.pdf THE OLD F-SCALE OVERESTIMATED THE WINDS AS:
• Wind speeds were not calibrated to the damage. • Homes generally fail at wind speeds between 70 and 160 mph. • F-scale wind speeds were empirically derived by dividing the range between Beaufort 12 and Mach 1 into 12 equal increments. Why 12? • Fujita said he would await engineers to calibrate his scale in the future. 40 How Fujita came up with the wind speed ranges in the F-scale.
41 Corrections to the F-scale by Fujita
(Old)
42 The EF-Scale Project
http://www.spc.noaa.gov /faq/tornado/ef-ttu.pdf 28 DAMAGE INDICATORS
Residences
Commercial/retail structures
Schools Professional buildings Metal buildings/canopies
Towers/poles
Vegetation F-Scale Converted to EF-Scale
F Scale Wind Speed EF-Scale Wind Speed F0 45-78 EF0 65-85 F1 79-117 EF1 86-109 F2 118-161 EF2 110-137 F3 162-209 EF3 138-167 F4 210-261 EF4 168-199 F5 262-317 EF5 200-234
EF-scale Wind speeds in mph, 3-second gust Haysville, KS 1999 100 mph
EF0 46 LaPlata, MD 2002 120 mph
EF1 47 LaPlata, MD 2002 140 mph
EF2 48 Ft. Worth, TX 2000 160 mph
EF3 49 Moore, OK Tornado 1999 180 mph
EF4 50 Jarrell, TX 1997 > 200 mph
EF5 51 WIND DAMAGE TO WOOD- FRAMED RESIDENCES DI 2
52 ASSESSING WIND DAMAGE TO WOOD-FRAMED HOUSES DI 2
• Note the type of foundation. • Check wall/floor or floor/foundation attachment. • Check wall-to-wall attachments. • Were there attached garage door problems? • Check roof/wall attachment. • Note roof shape and covering. • Note siding or veneer attachment. 53
TYPES OF WOOD CONNECTIONS
STRAIGHT TOE SHEAR
55 60 lbs. 100 lbs. 1000 lbs. TYPES OF FOUNDATIONS
• Concrete slab (Southwest) • Pier and beam (South, Southeast) – made of CMU, concrete, blocks, bricks, or wood. • Crawl space or basement (North, Midwest) – made of CMU, concrete, or brick masonry. • Stacked block or brick (South, Southeast) • Timber or concrete pilings (coastal)
56 Typical connection of wall to slab
F indicates likely failure 57 areas Types of connections to slab
58 PROPERLY BOLTED 2 X 6 PLATES AND SOLID WALL SHEATHING
59 Straight-nailed studs pulled away from plate
60 Brick veneer fell over as studs pulled from plate.
61 Closer view showing standing nail tips
62 PLATE BROKE AROUND BOLT
63 STRONGER CONNECTION “ABOVE CODE”
Anchor bolt
Metal strap from stud to plate. Pull-out strength is increased about 10x. 64 “Above code” diagram of wall-to-slab foundation connection by FEMA
65 Studs pulled out from strapped plate
66 OOPS. PLATE STRAP IS NOW OUTSIDE THE INSULATION BOARD.
67 SHOT PIN HAS SQUARE WASHER AND IS LITERALLY SHOT INTO THE FOUNDATION
68 SHOT PINNED PLATE AND TOE- NAILED STUD
Will fail here
69 SHOT PINNED PLATE AND STRAPPED STUDS SURVIVED
70 However, the roof was not attached well and blew away. USE OF CUT NAILS ON PERIMETER WALLS IS A BUILDING CODE VIOLATION.
71 Such nails will pull out with the plate HOW DEEP DOES A CUT NAIL EXTEND INTO THE SLAB?
72 About a half inch if it doesn’t spall the concrete! SCRAPE IN SLAB WHERE CUT NAIL MOVED LATERALLY
Wind likes code violations. This house was73 destroyed in about 100 mph winds. BOTTOM PLATE IN BACK YARD CONTAINS CUT NAIL.
74 ALL THAT WAS LEFT OF THIS HOUSE WAS THE CUT NAILS LEFT IN FOUNDATION.
At least the nail held as the plate pulled through.75 HOUSE ON SLAB SWEPT CLEAN WALL PLATE NAILED TO SLAB PIER AND BEAM FOUNDATIONS
• Have a floor “platform” that is difficult to attach to the foundation. • Have more connections than slab as floor interrupts wall/foundation connection. • House can be swept clean off foundation in winds of only 100 mph. • I hate these things.
78 CROSS SECTION OF PIER AND BEAM FOUNDATION. F indicates likely failure areas
F if CMU 79 HOMES ON PIER AND BEAM FOUNDATIONS GENERALLY HAVE LITTLE TO NO ANCHORAGE.
Note metal pads on timber piers where beams80 were simply supported. - Jarrell, TX 1997 WHAT ARE “SLIDERS” ?
• A slang term for eating oysters. • Fastballs that break slightly in the same direction as the curve. • Cardboard sleds on snowy hills. • Unanchored homes. • All the above!
81 UNANCHORED HOME SWEPT CLEAN FROM FOUNDATION
82 UNANCHORED HOME ON CONCRETE MASONRY PIERS TRAVELED 295 FT TOWARD TORNADO.
House
83 CMU “slider” at 100 mph in Moore, OK 1999 HOUSE SLIDES OFF FOUNDATION DOD 5 ~110 MPH HOUSE SLIDES OFF FOUNDATION – J-BOLT IN GROUTED CELL UNANCHORED HOME ON CMU FOUNDATION FLOOR PLATFORM SLID OFF CMU FOUNDATION FLOOR JOISTS WERE NOT ATTACHED TO SILLS ROOF UNANCHORED HOME ON BRICK MASONRY PIERS.
89 Concrete porch stayed. Birmingham, AL 1998 UNANCHORED 2-STORY HOME ON CMU PIERS.
Note anchored porch stayed and minimum roof90 damage! Failure was at about 100 mph. WHEN SLIDERS ARE ENCOUNTERED:
• Rate them according to the amount of roof damage and/or • Rate them according to surrounding building damage or • Default at EF1.
91 CONCRETE AND CMU BASEMENTS
• Have a floor “platform” that is difficult to attach to the foundation. • Have more connections than slab as floor interrupts wall/foundation connection. • House can be swept clean off foundation in winds of only 100 mph. • Your house is only as strong as your neighbor’s.
92 CROSS SECTION OF CMU BASEMENT FOUNDATION F indicates likely failure areas
93 Grand Island, NE 1980 80 mph
94 Unanchored home slid from perimeter foundation. Birmingham, AL 1998 100 mph
95 Another CMU slider. Note minimal roof damage. Anchor bolts were set into grouted cells of CMU.
96 Bolt either dragged block or broke out of it. Birmingham, AL 1998 110 mph
Masonry foundation fell as home moved. 97 Rural Georgia 1994 120 mph
98 Poorly anchored two story house moved long ways. SILL WAS STRAPPED TO FOUNDATION. FAILURE OCCURRED WHERE WALL AND RIM JOIST WERE SECURED TO FLOOR.
99 These CMU cells were grouted and had metal straps, however, they never secured the sill plates.
100 WALL PLATE PULLED OUT OF FLOOR. NOTE STRAIGHT NAILS.
101 BOTTOM PLATE WITH NAILS IN BACK YARD
102 FAILURE OF NAILED WALL/FLOOR CONNECTION ROTTED FLOOR BENEATH PLATE Split-level homes and ravines are deadly combinations.
105 8 ft. tall CMU back wall fell over and homes fell into ravine. BOLTS DID NOT HAVE NUTS OR WASHERS ON THIS HOUSE.
106 BOLT HAD NO NUT OR WASHER
Direction house moved.
107 FLOOR PLATFORM SLID OFF CONCRETE PERIMETER FOUNDATION INTO RAVINE.
108 Omaha, NE 1976 120 mph
109 Unanchored home on concrete foundation. WALL ATTACHMENTS
• Let-in brace at corners, a code minimum. • Solid sheathing and/or strapping is best to resist racking. • Wall corners must be well attached. • Shear walls are needed to resist applied loads. • Brick ties are needed to hold the masonry against the structural wall.
110 LET-IN BRACE HELPS STIFFEN WALL.
111 Gap between let- in brace and wall stud was improper reducing lateral strength of wall.
112 OOPS. WHAT YOU DON’T SEE WILL HURT YOU.
113 STEEL STRAP DIAGONAL BRACE AND SOLID WALL SHEATHING AT CORNER.
114 SOLID SHEATHING HELP STIFFENS WALL CORNER IN LIEU OF LET-IN BRACE.
115 POOR WALL CORNER ATTACHMENT
116 Wall failure locations- Multiple wall failures
117 LACK OF SHEAR WALLS MADE THIS HOUSE FOLD LIKE A STACK OF CARDS.
118 VERTICAL STRAPS HELP RESIST ROTATION.
119 STRAPPED STUD AND TOP PLATE
120 How to stiffen two-story wall by FEMA
121 THIS HOUSE HAS SEVEN SHEAR WALLS WITH SMALL WINDOWS.
122 THIS HOUSE ONLY HAS 4 SHEAR WALLS WITH BIGGER WINDOWS.
123 DOOR AND WINDOW FAILURES
• Failure of doors and windows allows internal pressure to build up rapidly causing additional roof uplift. • Attached garages are detrimental to the house. When doors fail, the roof then fails or a sidewall blows out. • Coastal homes are notorious for having the “weak” side of the building face the ocean.
124 Arlington, TX 2000 80 mph
125 Another example of door/sidewall failure. Channelview, TX 1992 100 mph
126 Doors blew in and sidewall blew out. Arlington, TX 2000 120 mph
127 Similar pattern of garage door failures in homes. Modes of roof EAVE failure initiation in Grand Island, NE Tornado in 1980
128 Deflection of the double door and rotation of the roller from the track
129 Improper attachment of track to door jamb Insufficient Nail Penetration
130 STIFFENING BRACKET AND GLIDER TRACK BY FEMA
131 Bracing of garage door for hurricanes by FEMA
132 FAILURE OF WINDOWS LEAD TO LOSS OF ROOF
133 BOARDING UP WINDOWS AND VENTS IS A GOOD IDEA!
134 Steel shutters are better but expensive
135 Taping windows does nothing to strengthen them and won’t stop debris from impacting.
136 ROOF FRAMING
• Codes require rafters/trusses to be toenailed to the wall top plates. • Such toenailed connections fail easily when the roof is uplifted. • The “above code” approach is to strap and/or clip the rafters/trusses to the top plates. • Bridging is needed between trusses as lateral bracing. 137 LaPlata, MD 2002 80 mph
Oops. Poorly attached roof slid off house due to 138 internal pressure. Note no shingles missing! 40 ft. truss was toenailed into top plate.
The toenailed connection was simply139 pulled apart when the roof was uplifted. Skewed nail holes in top plate where trusses had been attached.
140 TYPICAL RAFTER-TOP PLATE CONNECTION THAT PERFORMS POORLY WHEN IT IS UPLIFTED
141 THE “HURRICANE CLIP” CONNECTS RAFTERS TO THE TOP PLATES
142 WELL ATTACHED ROOF BUT WHAT ABOUT POST/BEAM CONNECTION?
143 NICE CLIPS. WHAT ABOUT STUD- TO-PLATE CONNECTION?
144 NEW HOUSE BUILT AFTER IT WAS DESTROYED BY TORNADO. NOTE NAILS THROUGH CORNER OF TOP PLATE.
145 How well will this roof do in the next tornado? Scissor trusses have high center of gravity
146 Such trusses are used for “vaulted” ceilings TRUSSES FELL LIKE A STACK OF DOMINOES DUE TO NO LATERAL SUPPORT WHEN ROOF DECK WAS REMOVED
147 TRUSSES FELL LIKE A STACK OF DOMINOES
148 LATERAL BRACING OF ROOF TRUSSES PER FEMA
149 IMPROPERLY STRAPPED TRUSS ROTATED
150 Proper strapping of roof trusses. However, roof deck and covering failed making house an economic total loss.
151 METAL TRUSS CLIP CAN ALSO HELP PREVENT ROTATION
152 IMPROPERLY BRACED GABLE END FELL OUTWARD
153 IMPROPERLY BRACED GABLE END PUSHED INWARD
154 GABLE WAS PLACED AGAINST A BOARD SHOT INTO THE TOP OF THE CONCRETE WALL.
155 PROPER BRACING NEAR GABLE END
156 COLLAR BEAMS CONNECTED TO RAFTERS
157 STRUCTURES THAT CREATE HIGH UPLIFT PRESSURES.
158 GABLE FLAT STRUCTURES WITH MORE STREAMLINING
HIP MANSARD 159 HIGH UPLIFT PRESSURE AT ROOF CORNER
Wind Direction
160 ROOF DECK REMOVED FROM TRUSSES – LADDER RAKE DETAIL
161 ROOF DECKING WAS NOT INSTALLED CORRECTLY
162 ROOF DECK REMOVED AND CEILING SHEETROCK CAVED IN
163 WHOLE SHEETS OF ROOF DECKING WITH SHINGLES
164 STAPLED ROOF DECK ON NEW HOME AFTER TORNADO
165 DAMAGE TO ROOF COVERINGS
• Roof coverings must meet the same 90 mph 3 sec. gust requirements as the structure (higher in hurricane prone areas). • Asphalt shingles must be nailed correctly. • Tile roofs must be bonded or anchored. • Built-up and single-ply roofs must be mechanically fastened to the roof deck.
166 Undamaged structure but no roof shingles
167 ASPHALT SHINGLES WERE STAPLED INCORRECTLY
Too few staples placed too high on shingles
168 THE NAIL HOLES IN THE TILES WERE LARGER THAN THE FASTENER HEADS.
169 DAMAGE OBSERVATIONS
• About half the homes failed where they were attached to their foundations. • The remaining homes failed where the roofs were attached to the tops of the walls. • Wind damage to the house was greater when the garage door failed. • Most houses fail to meet the minimum wind load requirements.
170 OVERALL FINDINGS
• Homes began getting into trouble when wind gusts neared 100 m.p.h. • Homes were completely destroyed at wind gusts less than 160 m.p.h. • What is NOT damaged provides an upper bound of wind speeds.
171 WIND-RESISTANT DESIGN
• Anchor bolt bottom plates to foundation. • Strap or clip rafters to top plates. • Put solid sheathing on homes. • Reduce variances that allow nailed plates, etc…..
172 THE END
173