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Lateral Loads
Lateral Load sources
WindTornado, hurricane, explosion
Seismic Flood or Tsunami Earth pressure
Basement or retaining walls
Probability
Code specified maximum wind velocities have a frequency of once per 50 years.
Code specified seismic loads have a frequency of once per 500 years.
Hurricanes Floyd 1999 Andrew 1992
Tornado Spawned by Katrina
Wind Damage (Hurricane Andrew)
Hurricane Storm Surge
Camille (1969) storm surge damage
Surf City, New Jersey 1944
BEAUFORT SCALE Original scale developed in 1805 by British naval officer Sir Francis Beaufort.
Beaufort
Number
International
Description
Miles per
Hour
Description
0 Calm Less than 1 Calm; smoke rises vertically.
1 Light Air 1-3 Direction of wind shown by smoke but not by wind vanes.2 Light Breeze 4-7 Wind on felt on face; leaves rustle; vanes move.
3 Gentle Breeze 8-12 Leaves and small twigs in constant motion.
4 Moderate Breeze 13-18 Raises dust and loose paper
5 Fresh Breeze 19-24 Small trees in leaf begin to sway.6 Strong Breeze 25-31 Large branches in motion; whistling heard in telegraph
wires; umbrellas used with difficulty.Moderate
(or near) gale
Gale(or fresh gale)
9 Strong Gale 47-54 Slight structural damage occurs.
Storm
(or whole gale)
11 Violent Storm 64-72 Accompanied by widespread damage.
12 Hurricane 73*-136 Devastation occurs.
*The U.S. uses 74 statute mph as the speed criterion for hurricanes.
10 55-63 Trees uprooted; considerable damage occurs.
8 39-46 Breaks twigs off trees; generally impedes progress.
Beaufort Scale
7 32-38 Whole trees in motion; inconvenience in waling.
Wind Scale
Saffir-Simpson Hurricane ScaleSustained Wind Velocities
Category One Hurricane: Winds 74-95 mph , Storm Surge 4 ~ 5 feet
Category Two Hurricane: Winds 96-110 mph, Storm Surge 6 ~ 8 feet
Category Three Hurricane: Winds 111-130 mph, Storm Surge 9 ~ 12 feet
Category Four Hurricane: Ex. Andrew 1992 Winds 131-155 mph, Storm Surge 13 ~ 18 feet
Category Five Hurricane: Ex. Camille 1969 Winds greater than 155 mph, Storm Surge >18
feet
F-0 Gale Tornado 40 - 72 MPH Chimneys damaged; branches broken off trees; shallow-rooted trees uprooted
F-1 Moderate Tornado 73 - 112 MPH Roof surfaces peeled off; mobile homes pushed off foundations or overturned; moving autos pushed off roads.
F-2 Significant Tornado 113 - 157 MPH Roofs torn off frame houses; mobile homes demolished; box cars pushed over; large trees snapped or uprooted; light-object projectiles generated.
F-3 Severe Tornado 158 - 206 MPH Roofs and some walls torn off well-constructed houses; trains overturned; most trees in forest uprooted; heavy cars lifted off the ground and thrown.
F-4 Devastating Tornado 158 – 206 MPHWell-constructed houses leveled; structures with weak foundations relocated; cars thrown and large projectiles generated.
F-5 Incredible Tornado 261 - 318 MPH Strong frame houses lifted off foundations and carried considerable distance to disintegrate; automobile-sized projectiles hurtle through the air in excess of 100 yards; trees debarked; other incredible phenomena expected.
Tornado Classifications
Hurricane Frequency
0
2
4
6
8
10
12
14
1945 1955 1965 1975 1985 1995 2005
Year
Number of Hurricanes
Global WarmingGlobal Average Temperature vs. Number of
Pirates
13
13.5
14
14.5
15
15.5
16
16.5
35000 45000 20000 15000 5000 400 17
Number of Pirates
Global Avg. Temperature (C)
18201860
1880
1920
1940
1980
2000
Wind Flow across a Low Rise Building
Positive pressure area
Negative pressure areas
Wind flow across a low rise building
Wind Effect on Roofs
Windward roof steep enough to feel pressure. Leeward roof subjected to suction.
Leeward Roof
Windward roof shallow enough to feel suction. Breakpoint approx 20°.
Earthquakes
Niigata 1964
Tsunami 12/2004
http://www.ldeo.columbia.edu/news/2005/images/tsun_eq.mp3
Tsunami Effects
SOURCE: US NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION PACIFIC MARINE ENVIRONMENTAL LABORATORY, US NATIONAL DATA BUOY CENTER; © 2004 KRT
Scale for Measuring earthquakes was developed by seismologist Charles Richter Class Magnitude
Great 8 or more Major 7 - 7.9 Strong 6 - 6.9 Moderate 5 - 5.9 Light 4 - 4.9 Minor 3 -3.9
Types of Seismic Waves
http://www.analog.com/library/analogDialogue/archives/35-01/earthquake/index.html
Probability (statistics of chance)
http://www.barringer1.com/feb05prb.htm
Defining Lateral Loads Codes specify environmental and live loads
Local geography affects intensity of wind loads
Importance factors are used to adjust loads for more important buildings
Seismic loads are affected by building geometry, mass, structural system and local geological conditions
Soil lateral loads affect by soil type and groundwater level
Seimic Loads
SEI/ASCE 7-02
Total lateral force V in each principal direction should be computed as
V = CsW
Cs = seismic response coefficient
W = Total dead load and applicable portions of other loads
Applicable portions of other loads
Storage areas: minimum of 25% floor live load. Not needed for garages and open parking
When partition load is included in floor load design, actual partition weight or minimum of 10 psf whichever is greater
Total operating weight of permanent equipment Where flat roof snow load exceeds 30 psf, design snow
load should be included in W. When jurisdiction authority approves it, snow load contribution may be no less than 20% of design snow load.
For the given occupancy classification, the appropriate Seismic Use Group and corresponding Importance Factor Is is determine.
Building Classification for Lateral Loads: Category I Buildings and other structures that
represent a low hazard to human life in event of failure, such asAgricultural facilitiesCertain temporary structuresMinor storage facilities
Building Classification for Lateral Loads: Category II Every building or structure that is not listed
in Categories I, III, or IV
Building Classification for Lateral Loads: Category III Buildings with substantial
hazard to humans in case of failure, such as Where more than 300
people congregate Day care facilities greater
than 150 Schools greater than 250 Colleges greater than 500 Heath care greater than 50,
but no surgery or emergency
Jails and detention facilities Power generation facilities
no in Cat IV Buildings not in Cat IV that
mfg, process, handle, store, use or dispose of hazardous fuels, chemicals, waste, or explosives containing sufficient quantities to be dangerous if released
Building Classification for Lateral Loads: Category IV Essential facilities such
as Hospitals/health care Fire, rescue, ambulance Emergency shelter Emergency preparedness,
communication, operations Power generating
facilities/public utilities + ancillary facilities (towers, storage tanks, substations, etc.)
Aviation control towers Water storage facilities and
pump stations Critical national defense Hazardous materials
facilities where quantity
exceeds threshold quantity determined by the relevant authority
Seismic Use Group Designations
Seismic Use Group
I II III
Occupation Category I ✪II ✪III ✪IV ✪
Occupancy Importance Factors
Seismic Use Group Is
I 1.0
II 1.25
III 1.5
Site classification (A-F) must be determined, and then the site coefficients Fa and Fv can be found.
These are the maximum considered earthquake (MCE) spectral acceleration.
Fa is for short period and Fv for 1 second.
Ss and S1 values are taken from the maps
Seismic Site ClassificationSite Class vs N or Nch su
A: Hard Rock >5,000 ft/s NA NA
B: Rock 2,500 – 5,000 ft/s NA NA
C: Very dense soil and soft rock
1,200 – 2,500 ft/s >50 >2,000 psf
D: Stiff soil 600 – 1,200 ft/s 15-50 1,000 – 2,000 psf
E: Soil <600 ft/s <15 <1,000 psf
F: Soil requiring site-specific evaluation
1. Soils vulnerable to potential failure or collapse2. Peats and/or highly organic clays3. Very high plasticity clays4. Very thick soft/medium clays
Vs =measured shear wave velocityN = Standard penetration resistance (blows/ft)Nch = corrected N for cohesionless layers (blows/ft)
su = undrained shear strength
Value of Fa for short-period max spectral accelerationSite Class Ss ≤ 0.25 Ss = 0.5 Ss = 0.75 Ss = 1.0 Ss ≥ 1.25
A 0.8 0.8 0.8 0.8 0.8
B 1.0 1.0 1.0 1.0 1.0
C 1.2 1.2 1.1 1.0 1.0
D 1.6 1.4 1.2 1.1 1.0
E 2.5 1.7 1.2 0.9 0.9
F * * * * *
* Site specific response analysis shall be performed except for structures with periods of vibration less than 0.5 sec. Values of Fa for liquefiable soils may be assumed equal to the values for the site class determined without regard to liquefaction in Step 3.
Value of Fv for 1 second max spectral accelerationSite Class S1 ≤ 0.1 S1 = 0.2 S1 = 0.3 S1 = 0.4 S1 ≥ 0.5
A 0.8 0.8 0.8 0.8 0.8
B 1.0 1.0 1.0 1.0 1.0
C 1.7 1.6 1.5 1.4 1.3
D 2.4 2.0 1.8 1.6 1.5
E 3.5 3.2 2.8 2.4 2.4
F * * * * *
* Site specific response analysis shall be performed except for structures with periods of vibration less than 0.5 sec. Values of Fv for liquefiable soils may be assumed equal to the values for the site class determined without regard to liquefaction in Step 3.
Seismic design category (A-F) and response factor R for the basic seismic force-resisting structural system must then be identified (SEI/ASCE 7-02 or relevant building code)
R-factor value is proportional to the amount of ductility, overstrength, and energy dissipation the seismic force resisting structural system possess.
For more ductile systems with larger R, the lateral seismic design force will be lower than a more vulnerable system with a lower R.
R = 1.0 is the conservative lower-bound. This is pure linear response
Seismic Base Shear Coefficient
€
Cs =SDSRIs
SDS = design spectral response in short period range (g)R = response modification factor for structureIs = occupancy importance factor for seismic use group
However, Cs has a max value of
Use the smaller value
SD1 = design spectral at 1 secondT = fundamental period of the structure (s)
€
Cs =SDI
T RIs ⎛ ⎝ ⎜ ⎞
⎠ ⎟
Design spectral accelerations
SDS = (2/3) SMS
SD1 = (2/3) SM1
SMS = Fa Ss
SM1 = Fv S1
Fundamental periodActual determination is quite complex. Code allows the
following approximation
Ta = Ct hnx
Ta = approximate fundamental period
Ct = period parameter (table)
x = period parameter (table)
hn = height from base to highest level of bldg (ft)
Values of period parameters
Structure Type Ct x
Moment resisting frame systems of steel where frame resists 100% of the seismic force and not enclosed or adjoined by more rigid components preventing frame deflection
0.028 0.8
Moment resisting frames of reinforced concrete where frames resist 100% of seismic force and not enclosed or adjoined by more rigid components preventing frame deflection
0.016 0.9
Eccentrically braced steel frames 0.03 0.75
All other structural systems 0.02 0.75
Vertical distribution of seismic forcesLateral base shear V should be distributed over the height of the structure as
concentrated loads on each floor level.
At a given floor level
Fx = Cvx Vwhere
wi is the portion of total gravity load of W at level I
hi = height from base to level I
k = 1 for building with T ≤ 0.5 s; 2 when T≥2.5 s (interp between)
€
Cvx =wxhx
k
wihik
i=1
n
∑
Example
Industrial building 180’ x 90’, clear height approx 30’
Supported on spread footings on moderately deep alluvial deposits (medium dense sand)
Astoria, Oregon
Site ClassSite Class vs N or Nch su
A: Hard Rock >5,000 ft/s NA NA
B: Rock 2,500 – 5,000 ft/s NA NA
C: Very dense soil and soft rock
1,200 – 2,500 ft/s >50 >2,000 psf
D: Stiff soil 600 – 1,200 ft/s 15-50 1,000 – 2,000 psf
E: Soil <600 ft/s <15 <1,000 psf
F: Soil requiring site-specific evaluation
1. Soils vulnerable to potential failure or collapse2. Peats and/or highly organic clays3. Very high plasticity clays4. Very thick soft/medium clays
Seismic Use Group
First need occupancy categoryLow occupancy, industrial building
Category I
Seismic Use Group
I II III
Occupation Category I ✪II ✪III ✪IV ✪
Spectral Response Acceleration Ss and S1
Read from maps for short period and 1 second intervals for Oregon.
We get Ss = 1.5 and S1 = 0.6
MCE Spectral Acceleration
Use the tables!Site Class Ss ≤ 0.25 Ss = 0.5 Ss = 0.75 Ss = 1.0 Ss ≥ 1.25
A 0.8 0.8 0.8 0.8 0.8
B 1.0 1.0 1.0 1.0 1.0
C 1.2 1.2 1.1 1.0 1.0
D 1.6 1.4 1.2 1.1 1.0
E 2.5 1.7 1.2 0.9 0.9
F * * * * *
For Fa, we have Ss = 1.5 so Fa = 1.0
Site Class S1 ≤ 0.1 S1 = 0.2 S1 = 0.3 S1 = 0.4 S1 ≥ 0.5
A 0.8 0.8 0.8 0.8 0.8
B 1.0 1.0 1.0 1.0 1.0
C 1.7 1.6 1.5 1.4 1.3
D 2.4 2.0 1.8 1.6 1.5
E 3.5 3.2 2.8 2.4 2.4
F * * * * *
For Fv, we have S1 = 0.6 so Fv =1.5
SMS, SM1, SDS, SD1
SMS = Fa Ss = 1.0 * 1.5 = 1.5
SDS = (2/3) SMS = 1.0
SM1 = Fv S1 = 1.5 * 0.6 = 0.9
SD1 = (2/3) SDS = 0.6
Parameters
Site Class D
SS 1.5
S1 0.6
Fa 1.0
Fv 1.5
SMS 1.5
SM1 0.9
SDS 1.0
SD1 0.6
Seismic Use Group I
Period Parameters
N-S we have moment resisting frame system of steelFrom table, Ct = 0.028 and x = 0.8 and R = 5
E-W we have a braced frame systemThus Ct = 0.02 and x = 0.75 and R = 4.5
Seismic Base Shear Coefficient
Step 1 - Determine SS and S1 Step 2 - Determine site Soil Classification Step 3 - Calculate Response Accelerations Step 4 - Calculate the 5% Damped Design Spectral
Response Accelerations Step 5 - Determine the Seismic Design Category Step 6 - Determine the Fundamental Period Step 7 - Calculate Seismic Base Shear Coefficient
Seismic base shear coefficient
N-S: Cs = 1.0/(5/1.0) = 0.20
E-W: Cs = 1.0/(4.5/1.0) = 0.22€
Cs =SDSRIs
Seismic base shear coefficient
But, there are limits
Max value:
€
Cs =SDI
T RIs ⎛ ⎝ ⎜ ⎞
⎠ ⎟
We need the Period, T
Approx: Ta = Ct hnx
hn = 30.5
N-S: Ta = 0.028 (30.5).8 = 0.43 s
E-W: Ta = 0.02 (30.5).75 = 0.26 s
Maximum Cs values
N-S: Cs = SD1 I / (TR) = 0.6 * 1 /(0.43 * 5)
= 0.278
E-W: Cs = 0.514
Seismic Base Shear Coefficient
N-S: Cs = 0.20
E-W: Cs = 0.22
Vertical Distribution
We have just one story – mezzanine does not count because it is less than 1/3 of the footprint
F = Cv V = 1 V for single story building
V = Cs W
Loads
Snow load from map 25 psf Dead load on roof = 15 psf Mezzanine live load, storage = 125 psf Mezzanine slab/deck dead load = 69 psf Wall panels = 75 psf
Loads
On mezzanine, need 25% of storage load69 + 25%(125) = 100.25 = 100 psf
On roof, snow load is less than 30 psf, so not needed.
Roof
Projected roof area: 90 x 182 = 16,380 ft2
Inclined roof area: 90.32 x 182 = 16,438 ft2
Roof load: 15 * 16,438 = 246.5 kips
Mezzanine
Mezzanine area: 40 x 90 = 3,600 ft2
Mezzanine floor: 360 kips Mezzanine frames: 35 kips Main framing: 27 kips
Walls
Long walls: 2 x 32 x 180 x 75 / 2 = 437 Short walls: 2 x 35 x 90 x 75 / 2 = 224
Load by direction
Source E-W N-S
Roof D+L 243 243
Long walls -- 437
Short walls 224 --
Mezzanine slab 360 260
Mezzanine framing 35 35
Main framing 27 27
Seismic Weight 889 kips 1,102 kips
Shear force
V = Cs W N-S: V = 0.2 * 1,102 = 220.4 kips E-W: V = 0.22 * 889 = 195.6 kips
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