Residential structure

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.

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Structural Design

The specification of the system resist the forces of nature and occupants.

• Safety• Building Quality• Value Engineering• Code Compliance

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Structural Design Methods in Building Codes

• 1 & 2 Family Residential•IRC Designed•IRC Prescriptive

• 3+ Family Residential•IBC

• Public Buildings•IBC

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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

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• “Typical” construction materials

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• “Typical” types of buildings

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• “Typical” types of buildings

1950 1973 1995 20050

500

1000

1500

2000

2500

3000House Size

Livi

ng S

pace

(SQ

FT)

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IRC Prescriptive Method• 100 mph max wind speed

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IRC Engineered

• Applied loads from ASCE 7

• Resist loads with conventional engineering techniques

•Foundations•Beams•Columns•Diaphragms

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ASCE 7• Estimates from comparing:

•Building damage (insurance investigations)•Weather/Conditions that caused damage•Strength of structure

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Design Loads

• Dead Load•Attached Building Loads•Soil Loads & Hydrostatic Pressure

• Live Load•Distributed•Flood•Wind•Snow•Seismic

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Dead LoadWeight of Building Soil Pressure

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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

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Foundation Design• Keyed

Footings• Horizontal &

Vertical Rebar• Exterior

Grading

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Design Loads



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Live LoadPeople &

Furnishings

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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

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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.

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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

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Beam Deflection

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Design Loads



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Live LoadFlood Load

Nature Always Wins!

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Design Loads



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Live LoadWind

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Wind Load● Wind force depends on:

●Building Importance●Wall & Roof Size●Average Roof Height●Roof Pitch●Exposure●Topographic Factor (hill, escarpment, etc.)●Wind Speed

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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

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Wind Load

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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

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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

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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

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Wind Exposure● B = Wooded or suburban● C = Exposed● D = Exposed on water

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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

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I, Importance Factor

● How important is the building?● If building fails, what is the human loss?● Will people gather there during storms?

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● 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

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Kzt, Topographic Factor

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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

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Wind Resistant Structure

● Diaphragms● Shear Connections● Overturning Resistance

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Wind Resistant Structure● Exterior walls w/ modest

RO sizes● Interior shear walls● Plan lateral system during

design

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Snow Load

● Snow weight depends on:

●Roof Size●Building Importance●Exposure●Thermal Factor (Heat Loss)●Ground Snow Load●Roof Shape●Roof Pitch●Roofing Friction

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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

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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.

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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

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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

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● 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

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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

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A Punching Engineer May Help

Yes, engineers can be annoying... but our job is to be professional worriers and keep people safe.

Residential structure

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