Engineering Loads - Wind/seismic

7/29/2019 Engineering Loads - Wind/seismic

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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


2/11

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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3/11

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


10/11

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.

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Engineering Loads - Wind/seismic