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Presents Residential Structural Design, the Easy WayBy Paul Malko, Foard Panel Inc.
© 2010
Paul Malko
●In-house engineer @ Foard Panel West Chesterfield, NH
●B.S. Mechanical Engineering●Structural Insulated Panel Association
Technical CommitteeCode Report CommitteeEducation Committee
●Timber Frame Engineering Council●Timber Frame Business Council
Board of Directors
© 2010
DisclaimerI learn new things every day. This is the best of my knowledge with nothing held back. However, I reserve the right to change my mind when I learn something new.
© 2010
Structural Design
The specification of the system resist the forces of nature and occupants.
• Safety• Building Quality• Value Engineering• Code Compliance
© 2010
Structural Design Methods in Building Codes
• 1 & 2 Family Residential•IRC Designed•IRC Prescriptive
• 3+ Family Residential•IBC
• Public Buildings•IBC
© 2010
1-2 Family ResidentialIRC Prescriptive Method
• Based on tradition• Historical performance• No Math• Limits
•“Typical” construction materials•“Typical” types of buildings•100 mph max wind speed
© 2010
1-2 Family ResidentialIRC Prescriptive Method
• “Typical” construction materials
© 2010
1-2 Family ResidentialIRC Prescriptive Method
• “Typical” types of buildings
© 2010
1-2 Family ResidentialIRC Prescriptive Method
• “Typical” types of buildings
1950 1973 1995 20050
500
1000
1500
2000
2500
3000House Size
Livi
ng S
pace
(SQ
FT)
© 2010
IRC Prescriptive Method• 100 mph max wind speed
© 2010
IRC Engineered
• Applied loads from ASCE 7
• Resist loads with conventional engineering techniques
•Foundations•Beams•Columns•Diaphragms
© 2010
ASCE 7• Estimates from comparing:
•Building damage (insurance investigations)•Weather/Conditions that caused damage•Strength of structure
© 2010
Design Loads
• Dead Load•Attached Building Loads•Soil Loads & Hydrostatic Pressure
• Live Load•Distributed•Flood•Wind•Snow•Seismic
© 2010
Design Loads
• Dead Load•Attached Building Loads•Soil Loads & Hydrostatic Pressure
• Live Load•Distributed•Flood•Wind•Snow•Seismic
© 2010
Dead LoadWeight of Building Soil Pressure
© 2010
Dead Load - Soil Pressure
• Pound per square foot of pressure per foot of depth
Soil Type Up to 8' deep
Over 8' deep
Gravel & Sand/Gravel mix
35 psf/f 60 psf/f
Clay Gravel mix 45 psf/f 60 psf/f
Clay/Sand mix & Clay/Silt mix
85 psf/f 100 psf/f
Inorganic Clay 100 psf/f Not Suitable
© 2010
Foundation Design• Keyed
Footings• Horizontal &
Vertical Rebar• Exterior
Grading
© 2010
Design Loads
• Dead Load•Attached Building Loads•Soil Loads & Hydrostatic Pressure
• Live Load•Distributed•Flood•Wind•Snow•Seismic
© 2010
Live LoadPeople &
Furnishings
© 2010
Live LoadPeople & Furnishings
Space Uniform LoadUninhabitable Attic, no storage 10 psfUninhabitable Attic, w/ storage 20 psfHabitable Attic & Sleeping Space 30 psfStairs 40 psfAll other areas 40 psfBalcony <= 100 sqft 60 psfBalcony > 100 sqft 100 psf
© 2010
Floor Stiffness
L/?Organization RecommendationCeramic Tile Institute of America
L/360 w/ decoupling layer
Marble Institute of America
L/720
APA – The Engineered Wood Association
L/480
Floors are usually deflection limited, not strength limited.
© 2010
Beam Deflection, D
W = (distributed + dead load) * span * joist spacingL = SpanE = StiffnessI = 1/12 * width * depth3 (rectangular beams only)
D= 5×W×L3
384×E× I
© 2010
Beam Deflection
© 2010
Design Loads
• Dead Load•Attached Building Loads•Soil Loads & Hydrostatic Pressure
• Live Load•Distributed•Flood•Wind•Snow•Seismic
© 2010
Live LoadFlood Load
Nature Always Wins!
© 2010
Design Loads
• Dead Load•Attached Building Loads•Soil Loads & Hydrostatic Pressure
• Live Load•Distributed•Flood•Wind•Snow•Seismic
© 2010
Live LoadWind
© 2010
Wind Load● Wind force depends on:
●Building Importance●Wall & Roof Size●Average Roof Height●Roof Pitch●Exposure●Topographic Factor (hill, escarpment, etc.)●Wind Speed
© 2010
Wind Load
● Wind Force on Structure = Wind Pressure X Area
● Wind Pressure from table●Wind speed●Roof pitch
● Multiplied by adjustments●Height & Exposure ●Topographic●Importance
© 2010
Wind Load
© 2010
Wind Pressure
ps = Pressure (psf)
λ = Height & Exposure AdjustmentK
zt = Topographic Adjustment
I = Importance Factorp
s30 = Base pressure from table
ps=×K zt× I× ps30
© 2010
ps30
, Basic Wind Pressure
● Roof pitch matters● + = pressure toward the wall/roof● - = lift away from wall/roof
Wind Speed
Roof Pitch
Wall Horiz. Roof Vertical Roof
Edge Field Edge Field
Windward Leeward Eave Over-hang
GableOver-hangEdge Field Edge Field
mph psf psf psf psf psf psf psf psf psf psf90 Flat 13 9 -7 -4 -15 -11 -9 -7 -22 -1790 3:12 16 11 -5 -3 -15 -11 -10 -8 -22 -1790 6:12 16 12 3 3 -7 -5 -10 -8 -13 -11
90 8:12- 12:12 14 12 10 8 6 5 -9 -8 -5 -6
© 2010
ps30
, Basic Wind Pressure
Wind Speed Matters
Wind Speed
Roof Pitch
Wall Horiz. Roof Vertical Roof
Edge Field Edge Field
Windward LeewardEave Over-hang
GableOver-hangEdge Field Edge Field
mph psf psf psf psf psf psf psf psf psf psf
90 10:12 14 12 10 8 6 5 -9 -8 -5 -6
100 10:12 18 14 12 10 7 6 -12 -9 -6 -7
110 10:12 12 17 15 12 8 7 -13 -11 -8 -9
120 10:12 26 20 18 14 10 9 -16 -13 -9 -10
130 10:12 30 24 21 17 12 10 -18 -16 -11 -12
© 2010
Wind Exposure● B = Wooded or suburban● C = Exposed● D = Exposed on water
© 2010
λ, Height & Exposure AdjustmentAverage
Roof Height
Exposure B
Exposure C
Exposure D
15 1.0 1.21 1.4720 1.0 1.29 1.5525 1.0 1.35 1.6130 1.0 1.40 1.6635 1.05 1.45 1.7040 1.09 1.49 1.7445 1.12 1.53 1.7850 1.16 1.56 1.81
© 2010
I, Importance Factor
● How important is the building?● If building fails, what is the human loss?● Will people gather there during storms?
© 2010
I, Importance Factor
● I = Minor storage & Agricultural● II = Everything else● III = > 300 people, >150 children, public works● IV = Hospitals, emergency response
Category Wind <= 100 mph
Wind > 100 mph
I 0.87 0.77II 1.00 1.00III 1.15 1.15IV 1.15 1.15
© 2010
Kzt, Topographic Factor
© 2010
Wind Pressure Example● House● 10:12 Roof, 25' tall● Located in middle of a field● No extreme terrain
ps= ps30××K zt× Icorner of wall=14×1.35×1.0×1.0=19 psfedge of roof =−9×1.35×1.0×1.0=−12 psf
© 2010
Wind Resistant Structure
● Diaphragms● Shear Connections● Overturning Resistance
© 2010
Wind Resistant Structure● Exterior walls w/ modest
RO sizes● Interior shear walls● Plan lateral system during
design
© 2010
Wind Resistant Structure
© 2010
Wind Resistant Structure
● Diaphragms● Shear Connections● Overturning Resistance
© 2010
Design Loads
• Dead Load•Attached Building Loads•Soil Loads & Hydrostatic Pressure
• Live Load•Distributed•Flood•Wind•Snow•Seismic
© 2010
Live LoadSnow
© 2010
Live LoadSnow
© 2010
Live LoadSnow
© 2010
Live LoadSnow
© 2010
Snow Load
● Snow weight depends on:
●Roof Size●Building Importance●Exposure●Thermal Factor (Heat Loss)●Ground Snow Load●Roof Shape●Roof Pitch●Roofing Friction
© 2010
ps,
Sloped Roof Snow Load
ps = Sloped roof snow load (psf)
Ce = Exposure Factor
Ct = Thermal Factor
Cs = Sloped Roof Factor
I = Importance Factorp
g = Ground snow load (psf)
ps=0.7×C e×C t×C s× I× pg
© 2010
pg, Ground Snow Load
● Grey = Case Study,Too variable
● Elevation limits
© 2010
Ce,
Roof Exposure Factor
Site Exposure
Fully Exposed
Partially Exposed Sheltered
B 0.9 1.0 1.2C 0.9 1.0 1.1D 0.8 0.9 1.0
Above Tree Line 0.7 0.8 N.A.
© 2010
Ct, Thermal Factor
Category Ct
Heated w/ whole surface R < 2 0.85Default 1.00
Heated w/ whole surface R 16-29 1.10Unheated or
Heated w/ whole surface R > 30 1.20
© 2010
Cs,
Sloped Roof Factor
● Adjusts for Ct
● Adjusts for roof ventilation
Roof Pitch Med.- High Grip Low Grip3:12 1.0 0.96:12 1.0 0.78:12 1.0 0.6
10:12 0.8 0.412:12 0.6 0.3
● Assume Ct = 1.0
© 2010
I, Importance Factor
● I = Minor storage & Agricultural● II = Everything else● III = > 300 people, >150 children, public works● IV = Hospitals, emergency response
Category II 0.8II 1.0III 1.1IV 1.2
© 2010
Roof Plan● Size and shape matters greatly
● Complex roofs must be calculated as individual simple roofs
● Consider several conditions●Balanced snow●Unbalanced snow●Drifts (aerodynamic shade)●Sliding snow●Ponding
© 2010
Drifting and Sliding Snow
pg = 65 psf
112 psf
0 psf
ps = 53 psf
65 psf
© 2010
Ponding● Usually only applies to very shallow pitch roofs● High risk for complete failure
© 2010
Design Loads
• Dead Load•Attached Building Loads•Soil Loads & Hydrostatic Pressure
• Live Load•Distributed•Flood•Wind•Snow•Seismic
© 2010
SeismicNature always wins!
© 2010
A Punching Engineer May Help
Yes, engineers can be annoying... but our job is to be professional worriers and keep people safe.