lecture -3 design loads

N-W.F.P. University of Engineering and Technology

Peshawar

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Lecture 03: Design Loads

By: Prof Dr. Akhtar Naeem [email protected]

CE-409: Lecture 03 Prof. Dr Akhtar Naeem Khan 2

Topics to be Addressed

Types of loads

Wind Load

Earthquake Load

Load Combinations

CE-409: Lecture 03 Prof. Dr. Akhtar Naeem Khan 3

Feeling Responsibility


Types of LoadsDetermination 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?


Design load should be rational such that considering 150mphwind load for a tower is reasonable but not the load of a tank ontop of the tower.

Types of Loads


Types of LoadsThree broad categories:

1. Dead load

2. Live load

3. Environmental load


1. 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 structureWeight of permanent machinery etc.Dead loads can be reasonably estimated if the member dimensions and material densities are known.

Types of Loads


2. 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

Types of Loads


• 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

Types of Loads2. Live load:


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

Types of Loads2. Live load: (UBC Table 16-A)


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

Types of Loads2. Live load:

CE-409: Lecture 03 Prof. Dr Akhtar Naeem Khan

Types of loads3. Environmental Loads

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

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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(1+D/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

Live load reduction


Bernoulli’s equation for stream flow is used to determine local pressure at stagnation point, considering air to be non-viscous & incompressible.

q = (ρv2/2)

• 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.

q: pressureρ: mass density of airv: velocity

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(ρv2/2) PL= CLA(ρv2/2)

CD : Drag coefficient CL : Lift coefficient

Wind load


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

P = CSq = CS(ρv2/2)

P=0.00256CSV2

•Air at 15C weighs 0.0765pcf

V: mph

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.

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.

Wind load


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

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.

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

Wind load


The revolving restaurant supported by a concrete column will Experience suction which will cause tension in the column and asConcrete is weak in tension so it may crack. As a result the lateralWind load may collapse the restaurant.

Wind load


AASHTO specification for Bridge TrussThe pressure face is taken as a solid without openingsand suction on the leeward face is neglected (its still quietConservative)

Wind load


Wind Pressure UBC 97

Design Wind Pressure:

wsqe IqCCP

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)

UBC (20-1)


Wind Load ExampleExample: 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)

P=0.00256CSV2 V: mph


Pwindward = 20.48 psf

Pleeward = 12.80 psf

Ptotal = 33.28 psf

P=0.00256CSV2

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)

wsqe IqCCP UBC (20-1)

Wind Load Example


Pwindward = 15.56 psf

Pleeward = 9.73 psf

Ptotal = 25.29 psf

wsqe IqCCP

Wind Load Example


Wind Load Example


Earthquake loads are necessary to consider in earthquake prone regions.

Earthquake waves are of two types:

Body waves

Surface waves

Earthquake LoadEarthquake 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.



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



Shallow EQ of depth, say, 15-20km are far more dangerous thandeep EQ of depth, say, 150-200km.



Earthquake LoadFactors 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.


Earthquake LoadFactors effecting earthquake response of structures


Earthquake LoadResponse Modification Factor


EQ generally have short periods which may match the natural period of the low rise buildings, say 10 to 20 stories which causesresonance results in serious damages. The possibility of resonance for high rise buildings is low due to longer time periods.

Earthquake LoadNatural Time period of structures


According to UBC 97 design base shear :

V = CVIW/RT

V = total base shear

CV = Seismic coefficient

I = Importance factor

W = Total seismic dead load

R = Response factor depends on type of structural system

T =Elastic fundamental period of vibration.

Earthquake Load UBC 97


The coefficient I has assigned values of 1.0 to 1.25, depending on building use.

Cv is obtained from table whose value depends on seismic zone and type of soil on which a structure is build.

T = Ct hn¾

Ct = 0.035 for steel moment resisting frame



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

V=Ft + Fx

• Concentrated force Ft at top shall determined by:

Ft = 0.07 T V• Ft need not exceed 0.25V and may be taken as 0 if T is 0.7 or

less.

• Force Fx at each level including level n:



Earthquake Load UBC 97Distribution of EQ Load


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 that an event will be exceeded at least once in the n years is

Pn= 1-( 1-1/R)n

Probability of Exceedence of the event in any one year is the inverse of the Mean Return Period = 1/R

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.

Mean Return Period


P50=1-( 1-1/95)50

=1- 0.589

= 0.41 or 41%

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?

There is 41 percent chances that the structure will be exposed to a windstorm exceeding 150mph.

Mean Return Period


P50=1-( 1-1/95)50

=1- 0.589

= 0.41 or 41%

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?

There is 41 percent chances that the structure will be exposed to an earthquake exceeding 0.4g

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:

P50=1 - ( 1 - 1/475)50

=1- 0.90

= 0.01 or 10%

Mean Return Period


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.

Impact Load


Load Combinations

1. 1.0D + 1.0L

2. 0.75D + 0.75L + 0.75W

3. 0.75D + 0.75L + 0.75E

D = dead load

L = Live load

W = Wind load

E = Earthquake load

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. 1.0D + 1.0L

2. 1.0D + 1.0L + 1.0W

3. 1.0D + 1.0L + 1.0E

Load CombinationsASD Load combinations


LRFD Load Combinations 1. 1.4D

2. 1.2D + 1.6L + 0.5(Lr or S or R)

3. 1.2D + 1.6(Lr or S or R) + (0.5L or 0.8W)

4. 1.2D +1.3W + 0.5L + 0.5(Lr or S or R)

5. 1.2D ± 1.0E + 0.5L + 0.2S

6. 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

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 .

LRFD Load Combinations

Load Combinations


Example: increase in dead load on the ground floor due brickslying on the roof for the construction of the first floor


Load Combinations


Why negative sign in equation (6) ?

It accounts for the stability of structures due to lateral loadings.


Load Combinations


The stabilizing effect of gravity is reduced and the destabilizing effect of lateral load due to wind or earthquake is increased tohave the worse situation


Load Combinations


Load CombinationsExample: 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


Load Combinations1) 1.4D

1.4(311 plf) = 435 plf

2) 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


Load Combinations

5) 1.2D ± 1.0E + 0.5L + 0.2S1.2(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 plfb) 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.


Thank You!

lecture -3 design loads

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