engineering loads - wind/seismic
TRANSCRIPT
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Loads
Structural Loads 2
Structural Loads Hibbeler, Sections 1.3, 1.4 (7th Ed.) Dead
Weight of structure itself & permanent fixtures
Fixed magnitudes & position
Generally known with high level of confidence
Live Type, magnitude and position generally vary over time
Less confidence about magnitudes, position and frequency
of recurrence
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Structural Loads 3
Dead Loads
Can be computed once estimates for sizes ofstructural elements are developed
Mechanical loads also known (plumbing, A/C units,furnaces, etc.) Must also include other non-structural elements
(interior partitions, ceiling, lighting, etc)
Rough estimates for structural system (used to startanalysis/design cycle)
Steel framed buildings: 60-75 lbf/f2
Reinforced concrete buildings: 110-130 lbf/f2
Structural Loads 4
Material Densities & Common
Design Dead Loads
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Structural Loads 5
Example: Dead Loads
Structural Loads 6
Live Loads
Occupancy loads for buildings Wind loads Snow & ice Loads Seismic loads Soil & water pressure
Traffic loads on highway & railroad bridges
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Structural Loads 7
Occupancy Loads SEI/ASCE 7-02 Standard (American Society of Civil
Engineers) Based on surveys of buildings & long history of successful
designs Includes some protection for emergency overloads,
construction loads, insures serviceability (deflections-vibrations)
Structural Loads 8
Reduced Live Loads
Reduced lived loads are permitted for ordinaryfloor loads acting on large floor areas (notroofs, hallways, garages, etc.)
Tributary area must be > 400 ft2
See Example 1-2 (ASCE 7-02)
150.25o
LL TL L K A
= +
Reduced liveload: lbf / f2
Unreduced liveload: lbf / f2
Live load elementfactor: = 4 forinterior column
Tributary area, f2
TA
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Structural Loads 9
Wind Loads (1)
Kinetic energy of wind converted to potential energy of windpressure forces that act on buildings
Factors:
Density of air (varies with altitude, temperature) Velocity
Angle of incidence between wind & building
Shape & stiffness of structure
Simplified Design approach Treated as static pressures applied normal to surfaces of structure
Exceptions are tall buildings and long span bridges: require windtunnel testing and aero-elastic analyses
Fundamental equation
q= ForceArea
= 12
= massdensityof air) V = velocity)2
miles per hour2lbf fq= 0.00256V 2
Structural Loads 10
Wind Loads (2) SEI/ASCE 7-02 guidelines for wind loading Starts with key energy balance equation and builds
in complications for real structures:
Height of building surfaces above ground
Gusts, turbulence
Nearby terrain
Effects of nearby buildings
20.00256z zq K IV
miles per hour
lbf / f2
Varies withheight z aboveground
Dimensionless velocity pressurecoefficient: varies with heightabove ground and nearby terrainfeatures (0.85 1.09)
Dimensionless structureimportance factor: 0.87-1.15
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Structural Loads 11
Wind Loads (3)
Velocities to use: 3 second wind gust measured 33 ft. above the ground
50-year recurrence interval
ANSI/ASCE 7-02 has U.S. wind maps showing this data
See map in text (Fig. 1-12, pg. 17)
Chicago: V = 90 mph, qz = 20.7 lbf/f2(for Kz=1, I =1)
Velocity, V (mph) qz (Kz=I=1.0)
70 12.5 psf
80 16.4 psf
90 20.7 psf
100 25.6 psf= 2560 lbf on a 10 x 10
wall panel (Ford Explr.weighs 4000 lbf.)
Structural Loads 12
Wind Loads (4)
wind velocities
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Structural Loads 13
Wind Loads (5)
Final design pressures Includes a dimensionless gust factor (G): = 0.85 for a rigid
structure
Includes a dimensionless Cp pressure coefficient (positive on
windward side and negative on leeward side). Also reflects theaspect ratios of the building.
( )p h pip qGC q GCDesign pressure
= qz for windward side and varieswith height Z above ground= qh on leeward sides, sloped roofsand ends (just = qz computed at meanheight of surface)
L, B, h, see next page
= 0.18 for fully enclosed bldgs
2lbf f
Structural Loads 14
Example Wind Loads (1)
Windward Side:Load varies w/height. SeeTable 1-5, pg. 18
Leeward Sideand Roof: Loadintensitycomputed atmean height
Building Outside Chicago On Flat Terrain
I = 0.87 (agricultural use)V = 90 mphqz = 18.04 Kz
20.00256IV
General Set-Up
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Structural Loads 15
Example Wind Loads (2)
( )p h pip qGC q GC+ ( )p h pip qGC q GC
Loads on roof and leewardside are constant over areas
Must analyze & design for worst case:positive and negative suction pressure
Structural Loads 16
Snow & Ice Loads (1)
Very significant in northern climates and/or at higher elevations SEI/ASCE 7-02 guidelines for snow & ice loading
Snow/ice treated as static roof loads
Based on a ground snow cover to roof conversion
Use 50 year recurrence interval
General geographic maps provided to set ground snow cover but
local conditions may vary widely Key load equation
0.7f e t gp C C Ip
Site groundload (lbf/f2)
Load on anunobstructedflat roof (lbf/f2)
Ce: dimensionless exposure factor (0.8-1.2)Ct: dimensionless thermal factor (1.0-1.2)I: dimensionless importance factor for structure (0.8-1.2)
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Structural Loads 17
Snow & Ice Loads (2)
For sloped roofs, the loading is further modified
s s fp C p
Cs: dimensionless slope factor (0.0-1.0). Depends onslope and thermal factors. Typical value is 1.0.
Must also consider loaded vs. unloaded portions ofroof, drifting and sliding snow, ice damformation
Structural Loads 18
Loading Combinations: Buildings
Not all loads act on the structure simultaneously Some loads are known with more confidence than
others (e.g. dead vs. wind)
Building design codes (ACI, AISC, etc.) usuallyspecify which combinations of loads must beconsidered and the uncertainty factors to use.
Examples: 1.4 dead + 1.7 (occupancy+wind)
1.2 dead + 1.6 (occupancy+wind) + 0.5 snow
1.2 dead + 1.5 seismic + 0.5 (occupancy+wind)
Structure must be analyzed for all requiredcombinations and each element designed tosafely resist the maximum imposed forces
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Structural Loads 19
Traffic Loads on Bridges
Dead load + traffic (cars & trucks) Traffic load is very complex (number,
type of trucks, wheel spacing,weights, )
AASHTO simplified procedures (Am.Assoc. State Highway & Transp.Officials)
Specifies sets of standard loads forwhich different bridges must bedesigned
Loads vary with importance of bridge:an interstate bridge is not subjectto the same traffic as a rural bridgecrossing a creek
http://www.transportation.org/aashto/home.nsf/FrontPage
Structural Loads 20
Traffic Loads on Bridges Highway load sets composed of:
A uniform lane loading + a concentrated load placed to
cause maximum moment and maximum shear
Multiple truck loads placed to produce maximum moments
and shears at each location on bridge
Bridges are designed to specific loadings, e.g., HS20-44
http://www.transportation.org/aashto/home.nsf/FrontPage
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Structural Loads 21
Traffic Loads on Bridges (2)
These static loads are modified by an impact factor toapproximate effects of dynamic loads (vehicles bouncing).Varies with span length and increases loads 30% max.