Chapter 3 – Loads and Load effects

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Chapter 3 – Loads and Load effects

Dr. -Ing. Adil Z.Eng. Asmerom

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Loads on Structures

• Classification of loads Area of application: Concentrated, Distributed (UDL) Direction: Vertical (Gravity), Horizontal (Lateral) Response: Static, Dynamic Variation with time: Permanent (Dead), Variable

(Live)

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Loads on Structures

• Classification of loads in Building Codes Permanent (Dead) Variable (Live) Environmental Loads

• Wind• Earthquake (Seismic)• Snow• Rain• Earth pressure

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Wind LoadEBCS -1, 1995

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Wind load

• Wind is air in motion• Structure deflects or stops the wind, converting the

wind’s kinetic energy into potential energy of pressure

• The wind loads that act on a structure result from movement of the air against the obstructing surfaces.

• Wind effects induce forces, vibrations, and in some cases instabilities in the overall structure as well as its non-structural components.

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• Wind velocity increases with the power of the structural height

Wind load

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Wind load

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Wind load• These wind effects depend on:

the wind speed, density of the air, shape of the structure location and geometry of the structure, and vibrational characteristics of the system.

• Wind Forces According to EBSC-1, 1995 wind pressure in this section is valid for rigid

surface only and neglects their resonant vibration

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Wind Load

Two method of analysis is provided the static procedure Only used for structures whose structural properties

do not make them susceptible to dynamic excitation (Cd ≤ 1.2)

A detailed Dynamic Procedure must be used for those structures which are likely to

be susceptible to dynamic excitation (Cd > 1.2) In order to determine Dynamic coefficient Cd , Charts and

figures can be used (EBCS 1-1995 fig 3.7 to 3.13)

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Wind Load

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Wind load (the simple procedure)Wind Pressure: The external and internal wind pressures are

given as: We = qref Ce (ze )cpe

Wi = qref Ce (zi )cpi

• Where: We and Wi are the external and internal pressures; Ce(ze ) and Ce(zi ) are the external and internal exposure

coefficients; Cpe and Cpi are the external and internal pressure coefficients. • The design wind pressure that is used to establish the wind load on

a structure is directly related to reference velocity pressure (qref) and is given by:

refref Vq2

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Wind load• Where: ρ is the density of air andVref is the reference wind velocity to be taken as 22m/s. • The air density is a function of altitude and depends

on the temperature and pressure to be expected in the region during storms. A temperature of 200C has been selected as appropriate for Ethiopia and the variation of mean atmospheric pressure with altitude is given in Table 2.3.(NS)

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• Table 2.1 Values of air density ρ

Site Altitude (m)Above sea level

(kg/m3)

0 1.20

500 1.12

1000 1.06

1500 1.00

2000 0.94

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Wind load• Exposure Coefficient : takes into the account the effects

of terrain, topography and elevation.

Where: KT - the terrain factor Cr(z ) - the roughness coefficient Ct(z ) - the topography coefficient *** Or use table 3.5 from EBCS-1 1995 for Ce(ze ) & Ce(zi )

(See next slide)

)()(71)()()()(

22

zCzCkzCzCzCzCtr

Ttrieee

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Exposure coefficient Ce

Table 3.5 from EBCS-1 1995

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Wind load• Terrain Category: The terrain category attempts to take

into account the effect of the land coverage, and is given below.

The terrain type is classified into 4 groups as follows: • Category I: Lakes with at least 5 km fetch upwind and

smooth flat country without obstacles. • Category II: Farmland with boundary hedges, occasional

small farm structure, houses or trees • Category III: Suburban or industrial areas and

permanent forests. • Category IV: Urban areas in which at least 15% of the

surface is covered with buildings and their average height.

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Wind load• Cr(z ) - the roughness coefficient

Attempts to take into account the effect of the land coverage

Where kT terrain factorzo roughness lengthzmin minimum height

For ground height above 200 m specialist advice is recommended.

min

min

)min()(

200ln)(

ZZforzrCzrC

mZZforzzkzCo

Tr

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Wind load

Terrain Category KT Zo(m) Zmin(m)

I 0.17 0.01 2II 0.19 0.05 4III 0.22 0.3 8IV 0.24 1 16

Terrain factor (KT) can be taken from table or calculated as follows:

Z0= minimum height defined Z0,II= minimum height of category II (0.05)

07.0

,

19.0

IIo

oT z

zk

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Wind load• Ct(z ) - the topography coefficient

It accounts for the increase of mean wind speed over isolated hills and escarpments

Ct(z ) =1 for φ<0.05 Ct(z ) =1+2S φ for 0.05≤ φ <0.3 Ct(z ) =1+0.6 S for φ >0.3

Where: S the orographic location factor, φ the upwind slope H/Lu in the wind direction Le the effective length of the upwind slope, Lu the actual length of the upwind slope in the wind direction Ld the actual length of the downwind slope in the wind direction H the effective height of the feature X the horizontal distance of the site from the top of the crest z the vertical distance from the ground level of the site

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Wind load

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Wind load

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Wind Load• Pressure Coefficient: The shape factor takes

into account the effect of shape of structure on the pressure distribution.

• The external pressure coefficients Cpe for buildings and individual parts of building depend on the size of the loaded area A. They are given for loaded area A of 1m2 and 10m2 in the relevant tables for the appropriate building configuration as cpe,1 and cpe,10, respectively. For areas between 1m2 and 10m2, values are obtained by linear interpolation. That is:

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• Cpe = Cpe,1 for A≤1m2

• Cpe = Cpe,1 +( Cpe,10 – Cpe,1)log10A for 1m2<A<10m2

• Cpe = Cpe,10 for A≥10m2

Wind Load (Ext. Pressure Coeff.)

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Wind Load (on building face)

• Values of external pressure coefficients for different cases are given in Table A.1 to Table A.5 of EBCS-1, 1995.

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• It accounts for the variation in dynamic pressure in different zones of the structure due to

• Its geometry• Area and • proximity to other structures

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• Reference height and wind pressure profile


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• Reference height and wind pressure profile


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EXTERNAL PRESSURE COEFFICIENTS FOR VERTICAL WALL.

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Wind Load (on flat roofs)• Flat roofs are defined as having a slope (α) of –5°< α < 5° • The roof should be divided into zones as shown in Figure

below.

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Wind Load (on flat roofs)

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Wind Load (on flat roofs)• External pressure coefficients for flat roof

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Wind Load (on Monopitch roofs)• The roof, including protruding parts, should be

divided into zones as shown in Figure below and NS• The reference height Ze should be taken equal to h.

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Wind Load (on Monopitch roofs)

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Wind Load (on Monopitch roofs)• External pressure coefficients for Monopitch roof ϴ =0º

and ϴ =180º

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Wind Load (on Monopitch roofs)• External pressure coefficients for Monopitch roof ϴ =90º

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Wind Load (on Duo pitch roofs)• The roof, including protruding parts, should be


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Wind Load (on Duo pitch roofs)

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Wind Load (on Duo pitch roofs)• External pressure coefficients for Duopitch roof ϴ =0º

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Wind Load (on Duo pitch roofs)• External pressure coefficients for Monopitch roof ϴ =90º

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Wind Load (on Hipped roofs)• The roof, including protruding parts, should be


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Wind Load (on Hipped pitch roofs)

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Wind Load (on Hipped pitch roofs)• External pressure coefficients for Monopitch roof ϴ =0º

and ϴ =0º

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Wind LoadInternal pressure Coefficient Cpi• Internal and external pressures shall be considered to

act at the same time. The worst combination of external and internal pressures shall be considered for every combination of possible openings and other leakage paths.

• The internal pressure coefficient Cpi for building w/o internal partition is a function of opening ratio m defined as

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• For closed buildings with internal partitions and opening windows the extreme values :

Cpi = 0.8 and Cpi = -0.5

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Wind loadNet pressure: the difference of the pressures (external and internal) on each surface due account of their signs.

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Local effects of wind pressure

• Wind around a corner

Images from FEMA Multi Hazard Seminar

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• Uplift on roof



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Earthquake LoadEBCS -8, 1995

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Earthquake Load An earthquake is the vibration of Earth

produced by the rapid release of accumulated energy in elastically strained rocks. It is the earth’s natural means of releasing stress. Energy released radiates in all directions

from its source, the focus; Energy propagates in the form of seismic

waves;

•Tectonic Earthquakes: occur when rocks in the earth's crust break due to geological forces created by movement of tectonic plates. • Volcanic Earthquakes: occur in conjunction with volcanic activity. • Collapse Earthquakes: are small earthquakes in underground mines, • Explosion Earthquakes: result from the explosion of nuclear and chemical devices. * About 90% of all earthquakes result from tectonic events, primarily movements on the faults

Types of Earthquakes

Basic principles of conceptual design

• The guiding principle in conceptual design against seismic hazard are: structural simplicity uniformity and symmetry bidirectional resistance and stiffness torsional resistance and stiffness diaphragmatic action at storey level adequate foundation

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Consequence of structural regularity on seismic design EBCS 8, 1995

* Fundamental period < 2 seconds

REGULARITY SIMPLIFICATIONBEHAVIOR FACTORPLAN ELEVATIO

NMODE

LANALYSI

S

Yes Yes Planar Static* Basic

Yes No Planar Static* Increased

No Yes Spatial Static* Basic

No No Spatial

Dynamic

Increased 55

Regularity in Plan

• symmetric in plan w.r.t. 2 orthogonal directions• compact plan configuration (no H, I, X shapes) Re-

entrants in one direction <25%• In-plane stiffness of floors sufficiently large compared

to stiffness of vertical elements• Under the equivalent static seismic force, max.

displacement in the direction of seismic force does not exceed avg. displacement by 20%

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Regularity in elevation

• All lateral load resisting systems run without interruption from foundation to top

• Both lateral stiffness & mass of story's remain constant or reduce gradually without abrupt changes

• ratio of actual storey resistance to required resistance should not vary disproportionately between adjacent storeys.

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Regularity in elevation (contd.)

when setbacks are present:

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Earthquake Analysis of Linear Systems

• Equivalent static analysis (ESA)• Dynamic analysis

Response history analysis (RHA) or (THA) Response spectrum analysis (RSA)

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EBCS 8 Elastic Design Spectra

0

0.5

1

1.5

2

2.5

3

0 0.5 1 1.5 2 2.5 3Spec

tral a

ccel

erat

ion

/ Gro

und

acce

lera

tion:

b

o

Period T (sec)

Normalized Elastic Response Spectra

soil class A

soil class B

soil class C

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Equivalent Static Analysis, EBCS 8, 1995

• Base shear force, Fb = Sd(T1) W• Fundamental period, T1 = C1 H3/4 ; T1 = 2

• Sd (T1) = abg• Distribution of lateral force

• Accidental torsion, eli = ±0.05 Li

• Torsional effects in individual elements, d=1+0.6 x/Le

d

btjj

iitbi FTFand

hWhWFFF 107.0)(

Rayleigh coefficient

Not explicitly shown

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jj

jj

uFum

T

2

1 2

Design spectrum coefficients

a= aoI bedrock acceleration ao=0.1, 0.07, 0.05, 0.03 acceleration ratio

I = 1.4, 1.2, 1.0, 0.8 importance factor

S = 1.0, 1.2, 1.5 site coefficient

g= gokD kR kW 0.70 behavior factor

5.22.13/2

1

TS response factor

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Seismic Hazard map of Ethiopia

0.1g0.05gao =

bedrock acceleration coefficient ao

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Importance factors for buildingsImporta

nce category

BuildingsImporta

nce factor

IBldgs whose during EQ is

vital importance, e.g. hospitals, fire stations, ...

1.4

IIBldgs whose collapse

results in serious consequence, e.g. schools,

assembly halls, 1.2

IIIordinary buildings not

belonging to other categories

1.0

IVBldgs of minor importance

for public safety, e.g. agricaltural bldgs., etc.

0.864

Comparing acceleration coefficients

0

0.5

1

1.5

2

2.5

3

0 0.5 1 1.5 2 2.5 3

Res

pons

e fac

tor

Peroid T (sec)

Comparison of response factor ordinates for class C soil

Design Spectra

Response factor

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in ESA and A/g in design spectrum are not identical

Subsoil classification

Subsoil

classDescription

Site coeff.

S

A

Rock vs 800 m/s in the top 5m

and stiff clay deposits vs 400 m/s at 10m depth

1.0

Bmedium dense sand, gravel or medium stiff clays vs 200 m/s

at 10m depth1.2

C

Loose cohesionless soil deposits with or without some

soft cohesive layers vs < 200 m/s in the uppermost

20m

1.5where vs is shear wave velocity

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Torsional effects

• Spatial (3D) model, accidental torsional effectsMli = eli Fi

where Mli torsional moment at storey ieli accidental eccentricity of storey mass i

(eli = ± 0.05 Li)Fi horizontal force acting at storey i

• Planar (2D) models: amplify the action effects in individual load resisting elements with a factor d

• If torsional irregularity exists, increase eli by

(d=1+0.6 x/Le)

670.3

2.1

2

max

avg

A

Scaling of results

• when base shear determined from procedures of RSA < ESA: base shear shall be increased to the following

%age of the values determined from ESA• 100% for irregular buildings• 90% for regular buildings

deflections, member forces and moments increased proportionally

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Combination of components of seismic action

• Horizontal components shall be considered to act non- concurrently

• Vertical components amounting to 70% of the horizontal components shall be taken into account for: horizontal (or nearly) members spanning 20 m horizontal (or nearly) cantilever components horizontal (or nearly) prestressed components beam supporting columns

analysis is made on a partial model consisting of the element under consideration and adjacent elements

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Determination of displacements

• Displacements induced by the design seismic action:

ds = de / gd

where ds = displacement due to design seismic actionde = displacement from linear analysis based

on design spectrum (shall also include torsional effects)

gd = displacement behavior factor

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Safety Verifications

1. Ultimate limit statessafety against collapse (ULS) is ensured if resistance, ductility, equilibrium, foundation stability and seismic joint conditions are met

a. Resistance condition Design action effects design resistance; Ed Rd

Second order effects:

if 0.10 no need to consider0.1 < 0.2 consider 2nd order effects by amplifying results by a factor 1/(1- )

shall not exceed 0.25

hVdP

tot

rtotInterstorey drift sensitivity coeff.,

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Safety Verifications (contd.)b. Ductility condition

check that the structural elements and the structure as a whole posses adequate ductility

specific material related requirements shall be satisfied

c. Equilibrium condition bldg. should be stable against overturning and sliding additional SLS verification for bldgs with sensitive equipments

d. Resistance of horizontal diaphragms Horizontal diaphragms & bracings shall have sufficient over-

strength in transmitting lateral loads The above requirements are satisfied if the diaphragms can

resist 1.3 times forces obtained from analysis

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Safety Verifications (contd.)e. Resistance of foundation

Verification of foundations according to EBCS 7. Action effects based on capacity design consideration, but shall

not exceed that of linear behavior with g =1. If the action effects are determined using g ≥ 0.7, no capacity

design consideration is needed

f. Seismic joint condition To check that there is no collision with adjacent structures

• Distance between potential points of impact < max. ds

• When floor elevations of adjacent bldgs are the same the max. separation distance referred above can be reduced by a factor of 0.7

• If shear (bumper) wall provided on the perimeter of the bldgs no separation distance needed, a 40 mm separation can be used for the rest of the bldg.

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Safety Verifications

2. Serviceability limit stateslimitation of damage requirement (SLS) is satisfied if, under the design seismic action, the interstorey drifts dr are limited to:

a. For bldgs having non-structural elements of brittle materials attached to the structuredr ≤ 0.01 h

b. For bldgs having non-structural elements fixed in a way not to interfere with structural deformations dr ≤ 0.015 hwhere h is the storey height

c. Additional SLS verification may be req’d for important bldgs containing sensitive equipments

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Design provisions for concrete buildings

• Design Concepts: EQ resistant concrete bldgs shall provide adequate energy

dissipation capacity Overall ductile behavior is ensured if the ductility demand is

spread over a large number of elements. Ductile mode of failure (flexure) should precede brittle failure modes (shear)

With regard to required dissipation capacity, three ductility classes are set to provide appropriate amount of ductility

Different behavior factors g are used for each ductility class In seismic zones 1 & 2 design load combinations according to

EBCS 2 provisions with an appropriate g.

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Ductility Classes• Depending on the required hysteretic dissipation

capacity DC”L” (low ductility)

• structures designed and dimensioned according to EBCS 2• concrete class C 20

DC”M” (medium ductility)• specific provisions for design and detailing to ensure inelastic

behavior of the structure without brittle failure• concrete class C 25

DC”H” (high ductility)• special provisions for design and detailing to ensure stable

mechanisms with large dissipation of hysteretic energy • concrete class C 25

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Behavior factors g• g = gokD kR kW 0.70

go = basic value of structural response, depends on structure type

Structural type go

Frame system 0.20

Dual system

Frame equivalent 0.20Wall eqv with coupled walls

0.20

Wall eqv with uncoupled walls

0.20

Wall system

with coupled walls 0.20with uncoupled walls 0.25

Core system 0.30Inverted pendulum system 0.50

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Behavior factors g (contd.)

• kD = factor reflecting ductility class kD = 1.0 for DC”H”, kD = 1.5 for DC”M”, kD = 2.0 for DC”L”.

• kR = factor reflecting structural regularity in elevation kR = 1.00 for regular structures kR = 1.25 for irregular structures

• kW = factor reflecting the prevailing failure mode in structural systems with walls 1.00 for frame and frame equivalent systems (2.5 – 0.5 ao) 1.0 for wall, wall equivalent systems and

core systems78

Design criteria for concrete bldgs

• local resistance criterion all critical regions shall have adequate resistance second order effects shall be taken into account

• capacity design criterion brittle or undesirable failure mechanisms shall be

prevented, (e.g. shear failure of elements, BCJ failure, or yielding of foundation)

plastic hinges shall be distributed throughout the structure, only in beams and not in columns except at the base of the bldg. (refer to the next slide)

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Choice of ductile mode failure

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Chapter 3 – Loads and Load effects

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Transcript of Chapter 3 – Loads and Load effects