Lecture 03: Design Loads

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Lecture 03 Design Loads

• By Prof Dr. Akhtar Naeem Khan
• chairciv_at_nwfpuet.edu.pk

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Topics to be Addressed

• Types of loads
• Wind Load
• Earthquake Load
• Load Combinations

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Feeling Responsibility
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Types of Loads

• Determination of loads for which a given
  structure may be designed for is a difficult
  problem.

• Questions to be Answered
• What loads may structure be called upon during
  its lifetime?
• In what combinations these loads occur?
• The probability that a specific live load be
  exceeded at some time during lifetime of
  structure?

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Types of Loads
Design load should be rational such that
considering 150mph wind load for a tower is
reasonable but not the load of a tank on top of
the tower.
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Types of Loads

• Three broad categories
• Dead load
• Live load
• Environmental load

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Types of Loads

• Dead load
• Dead Loads consist of the weight of all materials
  and fixed equipment incorporated into the
  building or other structure. (UBC Section 1602)
• Weight of structure
• Weight of permanent machinery etc.
• Dead loads can be reasonably estimated if the
  member dimensions and material densities are
  known.

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Types of Loads

• Live load
• Live loads are those loads produced by the use
  and occupancy of the building or other structure
  and do not include dead load, construction load,
  or environmental loads.
• Weight of people, furniture, machinery, goods
  in building.
• Weight of traffic on bridge

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Types of Loads

1. Live load

• Buildings serve such diverse purposes that it is
  extremely difficult to estimate suitable design
  loads.
• Different building codes specify live load
  requirements.
• Uniform Building Code (UBC)
• Southern Standard Building Code
• BOCA National Building Code

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Types of Loads

1. Live load (UBC Table 16-A)

Live loads for various occupancies Live loads for various occupancies
Occupancy Live load,psf
Residential 40
Libraries(reading room) 60
Mercantile 75-125
Heavy manufacturing 125-150
Light storage 120-125
Heavy storage 250 minimum
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Types of Loads

1. Live load

The 40psf L.L specified by code for Residential
Buildings is too Conservative to account for the
uncertainties in structural actions Such as
impact, fatigue, temp. effects etc.
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Types of loads

• Environmental Loads
• Environmental loads include wind load, snow
  load, rain load, earthquake load, and flood load.

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Live load reduction

• The Uniform building code and BOCA National
  building code permit reduction in basic design
  live load on any member supporting more than
  150ft2

R r(A-150) Or R
23.1(1D/L) Where R reduction,
percent r rate of reduction
0.08 for floors A area
supported by floor or member D
dead load, psf L basic
live load,psf
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Wind load

• Bernoullis equation for stream flow is used to
  determine local pressure at stagnation point,
  considering air to be non-viscous
  incompressible.

q pressure ? mass density of air v velocity
q (?v2/2)

• This pressure is called velocity pressure,
  dynamic pressure, stagnation pressure.
• This equation is based on steady flow.
• It does not account for dynamic effects of gusts
  or dynamic response of body.

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Wind load

• Resultant wind pressure on body depends upon
  pattern of flow around it.
• Pressure vary from point to point on surface,
  which depends on shape size of body.
• Resultant wind pressure is expressed as

PD CDA(?v2/2) PL CLA(?v2/2)
CD Drag coefficient CL Lift
coefficient
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Wind load

• For buildings bridges and the like pressure is
  expressed in terms of Shape Factor CS (pressure
  coefficient)

P CSq CS(?v2/2)

• Air at 15C weighs 0.0765pcf

V mph
P0.00256CSV2
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Wind load

• Measured wind velocities are averages of
  fluctuating velocities encountered during a
  finite time.
• In US average of velocities recorded during the
  time it takes a horizontal column of air 1 mile
  long to pass a fixed point.
• Fastest mile is highest velocity in 1 day.
• Annual extreme mile is the largest of the daily
  maximums.

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Wind load

• Wind pressure to be used in design should be
  based on a wind velocity having a specific mean
  recurrence interval.
• The flow of air close to ground is slowed by
  surface roughness, which depends on density, size
  and height of buildings, trees, vegetation etc.
• Velocity at 33ft (UBC Sec 1616) above ground is
  used as the basic values for design purpose.

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Wind load
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Wind load

• Shape factor varies considerably with proportion
  of structure horizontal angle of incidence of
  the wind.

• CS for windward face of flat roofed rectangular
  building is 0.9
• CS for negative pressure on rear face varies from
  -0.3 to -0.6
• For such building resultant pressure be
  determined by shape factor 1.2 to 1.5
• Commonly used is 1.3
• CS for Side walls -0.4 to 0.8
• CS for roof 0.5 to 0.8

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Wind load

• Wind forces on trussed structures e.g. bridges,
  transmission towers, beam bridges, girder bridges
  etc. difficult to assess because of leeward parts
  of structure.
• Recommended coefficients for walls of buildings,
  gabled roofs, arched roofs, roofs over unenclosed
  structures(stadium), chimneys, tanks, signs,
  transmission towers etc. are given in ASCE 7-02
• Wind pressures specified by building codes
  include allowance for gust and shape factors.

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Wind load

• Pressure acts on the windward face of the
  building
• Suction acts on the leeward face of the building
• Suction acts on the sides of the building so a
  person
• standing in The window may be thrown outside
• Suction acts on the floor so that GI sheet
  floors are
• blown away During strong wind storms

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Wind load
The revolving restaurant supported by a concrete
column will Experience suction which will cause
tension in the column and as Concrete is weak in
tension so it may crack. As a result the
lateral Wind load may collapse the restaurant.
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Wind load
AASHTO specification for Bridge Truss The
pressure face is taken as a solid without
openings and suction on the leeward face is
neglected (its still quiet Conservative)
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Wind Pressure UBC 97

• Design Wind Pressure

UBC (20-1)
Ce combined height, exposure and gust factor
(Table 16-G) Cq (or Cs) Pressure coefficient
for the structure or portion of
structure under consideration (Table 16-H) qs
wind stagnation pressure at the standard height
of 33ft (Table 16-F) Iw importance
factor (Table 16-k)
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Wind Load Example

• Example Calculate the wind pressure exerted by a
  wind blowing at 100mph on the civil engineering
  department old building.
• Sol According the formula given above
• For windward face Cs .8 inward (UBC97 Table
  16-H)
• For Leeward face Cs .5 outward (UBC97 Table
  16-H)

V mph
P0.00256CSV2
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Wind Load Example
P0.00256CSV2

• Pwindward 20.48 psf
• Pleeward 12.80 psf
• Ptotal 33.28 psf

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Wind Load Example

• Alternate Method
• Ce 0.76 ( For 30ft height Exposure B, Table
  16-G)
• Cq 0.8 ( For windward wall, Table 16-H)
• 0.5 ( For leeward wall, Table 16-H)
• qs 25.6 psf (For 100mph velocity, Table 16-F)
• Iw 1.0 (According to occupancy category,
  Table16-K)

UBC (20-1)
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Wind Load Example

• Pwindward 15.56 psf
• Pleeward 9.73 psf
• Ptotal 25.29 psf

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Wind Load Example
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Earthquake Load
Earthquake Waves

• Earthquake loads are necessary to consider in
  earthquake prone regions.
• Earthquake waves are of two types
• Body waves
• Surface waves

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Earthquake Load
Earthquake Waves

• Body waves consists of P-waves S-waves
• These waves cause the ground beneath the
  structure to move back and forth and impart
  accelerations into the base of structure.
• Period and intensity of these acceleration pulses
  change rapidly their magnitude vary from small
  values to more than that of gravity.

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Earthquake Load
Earthquake Waves
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Earthquake Load
Earthquake Waves
Body waves reach the buildings first, followed
by the more Dangerous surface waves
A linear increase in magnitude of EQ causes
approximately cubic increase in the
corresponding amount of energy released
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Earthquake Load
Earthquake Waves
Shallow EQ of depth, say, 15-20km are far more
dangerous than deep EQ of depth, say, 150-200km.
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Earthquake Load
Factors effecting earthquake response of
structures

• Structure response to an earthquake primarily
  depends upon
• Mass
• stiffness
• natural period of vibration
• damping characteristics of structure
• location from epicenter
• topography geological formation.

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Earthquake Load
Factors effecting earthquake response of
structures
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Earthquake Load
Response Modification Factor
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Earthquake Load
Response Modification Factor
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Earthquake Load
Natural Time period of structures
EQ generally have short periods which may match
the natural period of the low rise buildings,
say 10 to 20 stories which causes resonance
results in serious damages. The possibility of
resonance for high rise buildings is low due to
longer time periods.
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Earthquake Load UBC 97
Static Lateral force procedure Limitations

• 1629.8.3 The static lateral force procedure of
  Section1630 may be used for the following
  structures
• All structures, regular or irregular, in Seismic
  Zone 1 and in Occupancy Categories 4 and 5 in
  Seismic Zone 2.
• Regular structures under 240 feet in height with
  lateral force resistance provided by systems
  listed in Table 16-N, except where Section
  1629.8.4, Item 4, applies.
• Irregular structures not more than five stories
  or 65 feet
• in height

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Earthquake Load UBC 97
Static Lateral force procedure Limitations

• 1629.8.3 The static lateral force procedure of
  Section1630 may be used for the following
  structures
• Structures having a flexible upper portion
  supported on a rigid lower portion where both
  portions of the structure considered separately
  can be classified as being regular, the average
  story stiffness of the lower portion is at least
  10 times the average story stiffness of the upper
  portion and the period of the entire structure is
  not greater than 1.1 times the period of the
  upper portion considered as a separate structure
  fixed at the base.

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Earthquake Load UBC 97
1630.2.1 Design base shear. The total design base
shear in a given direction shall be determined
from the following formula V (Cv I/R T) W
(30-4) The total design base shear need not
exceed the following V (2.5 Ca I/R) W
(30-5) The total design base shear shall not be
less than the following V (0.11 Ca I) W
(30-6) In addition, for Seismic Zone 4, the
total base shear shall also not be less than the
following V (0.8 ZNv I/R) W (30-7)
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Earthquake Load UBC 97
1630.2.1 Design base shear. For Seismic Zones
1, 2A, 2B, and 3 V (0.11 Ca I) W V
(Cv I/R T) W V (2.5 Ca I/R) W For
Seismic Zone 4 V (0.11 Ca I) W V (Cv
I/R T) W V (2.5 Ca I/R) W
V (0.8 ZNv I/R) W
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Earthquake Load UBC 97
1630.2.1 Design base shear. V total base
shear Ca CV seismic dynamic response
spectrum values. (table 16-Q table 16-R) Z
seismic zone factor. (Table 16.I) Nv Na near
source factors that are applicable in only
seismic zone 4. (Table 16-T Table 16-S)
Depends on Seismic zone and soil profile
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Earthquake Load UBC 97
Soil profiles
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Earthquake Load UBC 97
Seismic Zone BCP 07
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Earthquake Load UBC 97
1630.2.1 Design base shear. I Importance
factor (Table 16-K) W Total seismic dead
load R Response factor depends on type of
structural system (Table 16-N) T Elastic
fundamental period of vibration.
T Ct hn¾
Ct 0.035 for steel moment resisting frame
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Earthquake Load UBC 97
1630.2.1 Design base shear. (0.11 Ca I) this
coefficient is also independent of the period
of vibration. It is a lower bound value,
keeping V at some minimum value.
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Earthquake Load UBC 97
1630.2.1 Design base shear.
(Cv I / R T) acceleration factor (also known
as a seismic base shear coefficient). This
coefficient will govern V for buildings with
medium to long fundamental period of vibrations.
The forces in these buildings are induced by the
velocity component of the bedrock motion. Hence
the "v" subscript.
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Earthquake Load UBC 97
1630.2.1 Design base shear.
(2.5 Ca I/R) this coefficient is independent of
the period of vibration. It will govern V for
buildings with short fundamental periods of
vibrations, like the buildings being studied in
this class. The forces in these stiff buildings
are generated by the acceleration component of
the bedrock motion. Hence the "a subscript.
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Earthquake Load UBC 97
1630.2.1 Design base shear.
(0.82 N v I / R) this lower bound coefficient is
only applicable to structures located in seismic
zone 4 and within 9.3 miles (15 km) of a known
seismic fault.
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Earthquake Load UBC 97
Typical Base shear coefficient for Masonry

• Typical base shear coefficient values for a
  regular, single-story masonry building not
  located near a fault. In addition, we
  conservatively assumed that a geotechnical site
  investigation was not completed. Because this
  type of building is so stiff, the (2.5 Ca I / R)
  coefficient governs V.
• Zone Coefficient
• 1 V .067W
• 2a V .122W
• 2b V .156W
• 3 V .200W
• 4 V .244W

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Earthquake Load UBC 97

• Vertical distribution Total force shall be
  distributed over height in the following manner

VFt ? Fx

• Concentrated force Ft at top shall determined by

.07TV .25V Ft 0 if T
.7 sec.

• Force Fx at each level including level n

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Earthquake Load UBC 97
Vertical distribution

• The single story building is a special case. In
  most cases, T .7 and Ft then is taken as zero.
• From equation 30-15

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Base Shear Example

• Calculate and distribute the base shear for a
  five story residential steel building 50 ft high,
  located at Peshawar. Assuming SD soil profile.
• V (0.11 Ca I) W V (Cv I/R T) W V
  (2.5 Ca I/R) W

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Base Shear Example
Z 0.2 Table 16.I Ca 0.28 Table
16-Q CV 0.40 Table 16-R I 1
Table 16-K R 4.5 Table 16-N
T Ct hn¾ (.035)(50)¾ 0.66 sec
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Base Shear Example

• V (0.11 Ca I) W V (Cv I/R T) W V
  (2.5 Ca I/R) W
• (0.11 Ca I) 0.0308
• (Cv I/R T) 0.1347
• (2.5 Ca I/R) 0.1556
• Therefore base shear is equal to
• V 0.1347W

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Earthquake Load UBC 97

• Total force shall be distributed over height in
  the following manner

W5
W4
W3
W2
W1
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Base Shear Example

• As T lt 0.7 therefore Ft 0
• W2 W3 W5 14 kips
• W1 W4 20 kips
• W 14 x 3 20 x 2 82 kips
• V 0.1347 x 82 11.04 kips

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Base Shear Example

• Story level 1 F1
• Wxhx 20 x 10 200 k-ft
• ? Wihi 20 x 10 14 x 20 14 x 30 20 x 40
  14 x 50 2400 k-ft
• F1 0.92 kips

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Base Shear Example

• Story level 2 F2
• Wxhx 14 x 20 280 k-ft
• ? Wihi 20 x 10 14 x 20 14 x 30 20 x 40
  14 x 50 2400 k-ft
• F2 1.29 kips

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Base Shear Example

• Story level 3 F3
• Wxhx 14 x 30 420 k-ft
• ? Wihi 20 x 10 14 x 20 14 x 30 20 x 40
  14 x 50 2400 k-ft
• F3 1.93 kips

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Base Shear Example

• Story level 4 F4
• Wxhx 20 x 40 800 k-ft
• ? Wihi 20 x 10 14 x 20 14 x 30 20 x 40
  14 x 50 2400 k-ft
• F4 3.68 kips

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Base Shear Example

• Story level 5 F5
• Wxhx 14 x 50 700 k-ft
• ? Wihi 20 x 10 14 x 20 14 x 30 20 x 40
  14 x 50 2400 k-ft
• F5 3.22 kips

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Mean Return Period

• The average Time Period (in years) based on
  geological and historical records in which there
  is a good statistical probability that an
  earthquake of a certain magnitude or a hurricane
  will recur is called Mean Return Period or
  Recurrence Interval R.

Probability of Exceedence of the event in any one
year is the inverse of the Mean Return Period
1/R
Probability that an event will be exceeded at
least once in the n years is Pn 1-( 1-1/R)n
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Mean Return Period
Considering 150mph with a return period of, say,
100years is Reasonable as compared to 500mph
with a return period of, say, 1000 years.
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Mean Return Period
Example- A structure expected to have a life of
50 years built in locality where mean recurrence
interval of an windstorm of 150mph is 95 yrs. The
probability that structure will encounter an
windstorm exceeding 150mph during its life is?

• P501-( 1-1/95)50
• 1- 0.589
• 0.41 or 41

There is 41 percent chances that the structure
will be exposed to a windstorm exceeding 150mph.
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Mean Return Period
Example- A structure expected to have a life of
50 years built in locality where mean recurrence
interval of an earthquake of 0.4g is 95 yrs. The
probability that structure will encounter an
earthquake exceeding 0.4g during its life is?

• P501-( 1-1/95)50
• 1- 0.589
• 0.41 or 41

There is 41 percent chances that the structure
will be exposed to an earthquake exceeding 0.4g
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Mean Return Period
Uniform Building Code specifies that the
earthquake for which a building has to be
designed should correspond to an earthquake with
a return period of 475 years. Assuming that a
building has service life of 50 years. The
probability that it will experience and
earthquake of mean return period 475 in its
design life would be

• P501 - ( 1 - 1/475)50
• 1- 0.90
• 0.01 or 10

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Impact Load

• Spring Example
• It is customary to express Impact load as
  percentage of static force.
• Effect of impact load is taken into account in
  calculation of loads.
• If impact is 25 , Live load is multiplied by
  1.25
• According to AISC live load on hangers
  supporting floor and balcony construction should
  be increased by one-third for impact.

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Load Combinations
ASD Load combinations

• 1.0D 1.0L
• 0.75D 0.75L 0.75W
• 0.75D 0.75L 0.75E
• D dead load
• L Live load
• W Wind load
• E Earthquake load

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Load Combinations
ASD Load combinations

• You can use following load combinations with the
  parameter ALSTRINC (Allowable Strength Increase)
  to account for the 1/3 allowable increase for the
  wind and seismic load
• 1.0D 1.0L
• 1.0D 1.0L 1.0W
• 1.0D 1.0L 1.0E

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Load Combinations
LRFD Load Combinations

• 1.4D
• 1.2D 1.6L 0.5(Lr or S or R)
• 1.2D 1.6(Lr or S or R) (0.5L or 0.8W)
• 1.2D 1.3W 0.5L 0.5(Lr or S or R)
• 1.2D 1.0E 0.5L 0.2S
• 0.9D (1.3W or 1.0E
• D Dead load L Live load
• Lr Roof Live Load W Wind load
• S Snow Load E Earthquake
  load
• R Rain Water or Ice

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Load Combinations
LRFD Load Combinations

• Why only Dead load in equation (1) ?
• There may be a significant live load on a
  structure during construction.
• Moreover, the structure may have not reached its
  full 28 days strength as further construction is
  usually carried out .

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Load Combinations
LRFD Load Combinations
Example increase in dead load on the ground
floor due bricks lying on the roof for the
construction of the first floor
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Load Combinations
LRFD Load Combinations

• Why negative sign in equation (6) ?
• It accounts for the stability of structures due
  to lateral loadings.

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Load Combinations
LRFD Load Combinations
The stabilizing effect of gravity is reduced and
the destabilizing effect of lateral load due to
wind or earthquake is increased to have the worse
situation
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Load Combinations
Example Roof beams W16X31, spaced 7ft-0in
center-to-center, support a superimposed dead
load of 40 psf. Code specified roof loads are 30
psf downward (due to roof live load, snow, or
rain) and 20 psf upward or downward (due to
wind). Determine the critical loading for LRFD.
D 31 plf 40 psf X 7.0 ft 311 plf L 0 (Lr
or S or R) 30 psf X 7.0 ft 210 plf W 20 psf
X 7.0 ft 140 plf E 0
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Load Combinations

• 1.4D
• 1.4(311 plf) 435 plf
• 1.2D 1.6L 0.5(Lr or S or R)
• 1.2(311 plf) 0 0.5(210 plf) 478 plf
• 3) 1.2D 1.6(Lr or S or R) (0.5L or 0.8W)
• 1.2(311 plf) 1.6 (210 plf) 0.8(140 plf) 821
  plf
• 4) 1.2D 1.3W 0.5L 0.5(Lr or S or R)
• 1.2(311 plf) 1.3(140 plf) 0 0.5(210 plf)
  660 plf

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Load Combinations
5) 1.2D 1.0E 0.5L 0.2S 1.2(311 plf) 0
0 0.2(210 plf) 415 plf 6) 0.9D (1.3W or
1.0E) a) 0.9 (311 plf) 1.3 (140 plf) 462
plf b) 0.9(311 plf) - 1.3(140 plf) 98 plf The
critical factored load combination for design is
the third, with a total factored load of 821 plf.
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Thank You!