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Architecture 544Wood StructuresWood Structures
Design LoadsDesign Loads
Peter von BuelowUniversity of Michigan
ASCE 7-02• Referenced standards• Referencing Codes
3.0 Dead Load4 0 Live Load4.0 Live Load
• Occupancy• Roof
6.0 Wind7.0 Snow
• Flat• Sloped
Peter von Buelow University of Michigan, TCAUP Slide 1 of 32
General 1.0
Building Classification• Used for Importance factor
Peter von Buelow University of Michigan, TCAUP Slide 2 of 32
Combinations of Loads 2.0
ASD• Each combination seperately
• Combine to give worst affect
Peter von Buelow University of Michigan, TCAUP Slide 3 of 32
Dead Loads 3.0Weight of material
• Table C3-1 in commentary
Peter von Buelow University of Michigan, TCAUP Slide 4 of 32
Dead Loads 3.0Weight of material
• Table C3-2 in commentary
• Raw materials
• Also Steel manual
Peter von Buelow University of Michigan, TCAUP Slide 5 of 32
Dead Load of LumberUsing specific gravity GUsing specific gravity, G
• Neglecting Moisture Content
428.62D G
• Including Moisture Content
100
..1
..009.01428.62D
cm
cmG
G
G = specific gravity (unitless)
D = density (e.g. PCF)
m.c. = the % moisture content (e.g. 18)m.c. the % moisture content (e.g. 18)
Section Dead Load:
144DPLF
Area
Member Dead Load:
144
LengthArea
DLBS
Floor Dead Load
Length144
DLBS
12AreaDPSF
Peter von Buelow University of Michigan, TCAUP Slide 6 of 32
..144DPSF
co
Live Loads 4.0
Concentrated• Non-residential = 2000 LBS
• Handrail 200 LBS
Floor• By occupancy
• Reduction with area > 400 SF
• Table 4-1
Roof• Minimum Lr between 12 PSF and 20 PSF
• Lr = 20 R1 R2
• See 4.9.1
Peter von Buelow University of Michigan, TCAUP Slide 7 of 32
Live Loads 4.0Roof
• Minimum Lr between 12 PSF and 20 PSF
• Lr = 20 R1 R2
• See 4.9.1
1 for At ≤ 200 ft2(18.58 m2)
R = 1 2 − 0 001A for 200 ft2 < A < 600 ft2R1 = 1.2 − 0.001At for 200 ft2 < At < 600 ft2
0.6 for At ≥ 600 ft2(55.74 m2)
where At = tributary area in ft2 (m2) supported by t y ( ) pp yany structural member and
1 for F ≤ 4
R 1 2 0 05 F f 4 < F < 12R2 = 1.2 − 0.05 F for 4 < F < 12
0.6 for F ≥ 12
where, for a pitched roof, F = number of inches ofwhere, for a pitched roof, F number of inches of rise per ft and, for an arch or dome, F = rise-to span ratio multiplied by 32.
Peter von Buelow University of Michigan, TCAUP Slide 8 of 32
Wind Loads 6.0
Method 1• 6.4 Simplified
Method 2• 6.5 Analytical
Wind Tunnel• Get Cp and base V & M
• NOT CFD
Peter von Buelow University of Michigan, TCAUP Slide 9 of 32
Wind – Method 2
Minimum force• 10 psf (6.1.4.1)
Basic pressure equation
2
21 vq
ASCE equation• Sec. 6.5.10 eq.6-15
V l it V i i MPH
IKKK00256.0 2dztz vqz
• Velocity V is in MPH
• 0.00256 accounts for air density/2 and conversions
Peter von Buelow University of Michigan, TCAUP Slide 10 of 32
Wind – Method 2
V - Basic Wind Speed• 3 sec. gust speed in MPH (m/s)
• 33 FT height
Peter von Buelow University of Michigan, TCAUP Slide 11 of 32
Wind – Method 2
Kz - Exposure Coefficient• Table 6-3
• Exposure Categories B, C & D (6.5.6.3)with photos in commentary.
• Case 1 – Low Rise• Case 1 – Low Rise
• Use mean roof height z=h
Peter von Buelow University of Michigan, TCAUP Slide 12 of 32
Wind – Method 2
Exposure CategoriesB• Commentary 6 B
B
Peter von Buelow University of Michigan, TCAUP Slide 13 of 32
CB C
DC
D
Peter von Buelow University of Michigan, TCAUP Slide 14 of 32
Wind – Method 2
Kzt - Topographic Factor• Sec. 6.5.7.2 fig. 6-4
• Increase in pressure as wind goes over a hill
Peter von Buelow University of Michigan, TCAUP Slide 15 of 32
Wind – Method 2
Kd - Wind Directionality Factor• Table 6-4
Peter von Buelow University of Michigan, TCAUP Slide 16 of 32
Wind – Method 2
I - Importance Factor• Based on building category (Tab. 1-1)
• Critical occupancy
Peter von Buelow University of Michigan, TCAUP Slide 17 of 32
Wind – Method 2
Design Pressure Equations• Take shape of structure into account
• Interior pressure + or –
• Sec. 6.5.12.2 eq. 6-17 or 6-18
• q windward = qz
piip GCqqGCp q windward qz
• q leeward = qh
• Conservatively use qi = qh
Peter von Buelow University of Michigan, TCAUP Slide 18 of 32
Wind – Method 2
G – Gust Factor• Sec. 6.5.8
• Use 0.85
Peter von Buelow University of Michigan, TCAUP Slide 19 of 32
Wind – Method 2
Cp – External Pressure Coefficient• Fig. 6-6 through 6-17
• Or supplied by wind tunnel test
• Walls – windward, leeward, and side
Roofs slope or flat• Roofs – slope or flat
• Special roof shapes see later figures.
Peter von Buelow University of Michigan, TCAUP Slide 20 of 32
Wind – Method 2
Gcpi – Internal Pressure Coefficient• Fig. 6-5
• Combine with GCp to give worst case
Peter von Buelow University of Michigan, TCAUP Slide 21 of 32
Wind – wind tunnel testing
Boundary Layer Wind Tunnel
Peter von Buelow University of Michigan, TCAUP Slide 22 of 32
Wind – CFD
Computational Fluid Dynamics
Peter von Buelow University of Michigan, TCAUP Slide 23 of 32
Wind – CFD
Computational Fluid Dynamics
Peter von Buelow University of Michigan, TCAUP Slide 24 of 32
Snow Load 7.0
pg - Ground snow load• from map fig 7-1
• PSF
• Based on 50 year mean (2% probability of being exceeded)being exceeded)
Peter von Buelow University of Michigan, TCAUP Slide 25 of 32
Snow Load 7.0
pf - flat roof snow load
pf = 0.7 Ce Ct I pg• Eq. 7-1
• Minimum where pg > 20 = I 20 PSF
Low Slope Roofs• Monoslope < 15°
• Gable slope < (70/W)+0.5
• W = horiz. dist. from eave to ridge in FT
• Low slope < 2.38° see 7-10 for +5PSF
• Low slope < 1.19° see 7-11 for ponding
Peter von Buelow University of Michigan, TCAUP Slide 26 of 32
Snow Load 7.0Ce – Exposure Factor
• Table 7-2
Peter von Buelow University of Michigan, TCAUP Slide 27 of 32
Snow Load 7.0
Ct – Thermal Factor• Table 7-3
I – Importance FactorI Importance Factor• Table 7-4
Peter von Buelow University of Michigan, TCAUP Slide 28 of 32
Snow Load 7.0ps – sloped roof snow load Cs – Roof Slope Factorps sloped roof snow load
ps = Cs pf• Eq. 7-2
• Figure 7-2
• C1 = Ct
• Equations given in commentary C7-4
Peter von Buelow University of Michigan, TCAUP Slide 29 of 32
Snow Load 7.0
Balanced• Ps
UnbalancedUnbalancedFor W ≤ 20FT
• 1.5 ps/Ce
For W > 20FT
• 0.3 ps and 1.2 (1+(ß/2))ps/Ce
Unbalanced Gable Roof Loads• Not for Θ > 70°
• Not for Θ < 70/W + 0.5
Peter von Buelow University of Michigan, TCAUP Slide 30 of 32
Snow Load 7.0
Other considerations• Drifts 7-7
• Projections 7-8
• Sliding Snow 7-9
Peter von Buelow University of Michigan, TCAUP Slide 31 of 32
Other Loads
Soil, Hydrostatic and Flood• Sec. 5.0
RainRain• Sec. 8.0
E th kEarthquake• Sec. 9.0
Peter von Buelow University of Michigan, TCAUP Slide 32 of 32
