TALAT Lecture 2204: Design Philosophy

TALAT Lecture 2204

Design Philosophy
30 pages, 10 Figures

Advanced Level

prepared by Steinar Lundberg, Hydro Aluminium Structures, Karmoy

Objectives:

− to establish an understanding of the requirements on load bearing structures with
respect to safety against failure
− to introduce the design analysis process with methods of verification and partial
safety factors
− to describe the characteristic of loads and load combinations on structures
− to introduce the subject of load and resistance factors in the verification methods
− to describe the basic structural design properties of aluminium alloys vs. steel

Prerequisites:

− background and experience in structural engineering and design calculations
− basic understanding of the physical and mechanical properties of aluminium

REVICED NOVEMBER 1997 in connection with the Leonardo da Vinci project:
TAS/WP 1 by Steinar Lundberg.

Date of Issue: 1994
 EAA - European Aluminium Association

2204 Design Philosophy

Contents

2204 Design Philosophy.........................................................................................2
2204.01 Notations ................................................................................................... 3
2204.02 Introduction and Definition ..................................................................... 4
2204.03 Requirements on the Load Carrying Structure..................................... 5
Specification of Requirements.................................................................................5
Requirements on Safety Against Failure..................................................................5
Requirements on the Serviceability of Structures in Normal Use ...........................7
Limit States ..............................................................................................................8
Economic Considerations on the Formulation of Requirements .............................9
2204.04 The Design Analysis Process .................................................................... 9
Introduction............................................................................................................10
Methods of Verification.........................................................................................11
The Load and Resistance Design Factor Method ..................................................12
Method of Allowable Stresses ...............................................................................13
2204.05 Loads and Load Factors......................................................................... 14
Introduction............................................................................................................14
Classification of Loads ..........................................................................................14
Characteristic Loads, Normal Loads and Long-Term Loads .................................16
Load Combinations, Design Value of the Load.....................................................17
Example............................................................................................................. 20
Loads on Buildings, Bridges and Hydraulic Structures .........................................22
2204.06 Resistance and Resistance Factors ........................................................ 22
Assumptions Concerning Strength Properties .......................................................22
Models of analysis .................................................................................................24
2204.07 Design Criteria ........................................................................................ 25
The load and resistance factor method...................................................................25
Method of allowable stresses .................................................................................25
2204.08 Aluminium Alloys as a Structural Material ........................................... 26
2204.09 References/Literature .............................................................................. 30
2204.10 List of Figures............................................................................................ 30

TALAT 2204 2

2204.01 Notations

E - Youngs modulus of elasticity
F - load
Fd - design load
Fk - characterictic load
Gk - characteristic value of permanent load
Qk - characteristic value of variable load
Qak - characteristic value of accidental load
R - resistance
Rm - tensile strength of test specimen
Rp0,2 - 0,2 proof strength of test specimen
S - load effect
Sk - characteristic value of snow load
Wk - characteristic value of wind load
f - strength
fd - design strength
fk - characteristic strength
l - dimension
ld - deisgn dimension
lk - characteristic dimension
s - safety factor
tq - duration of variable loads
ttot - service life of structure
δ - deflection
γF - load factor
γG - load factor for permenent loads
γM - resistance factors
γQ - load factor for variable loads
γQi - load factor for variable loads except the
main variable load
η - factor for transforming strength of test specimen
to strength of structure
ηd - relative duration
σ - stress
σall - allowable stress
σy - yield stress
ψ - reduction factor for loads in combination

TALAT 2204 3

2204.02 Introduction and Definition

The procedure, starting from general information concerning the use, location etc. of the
structure to be built, and leading to complete design documents sufficient for the
manufacture and erection/installation, is referred to as the design procedure.

The course of the design procedure naturally depends on the type of structure, the
purchaser, future proprietor etc. In most cases the various phases within the procedure
may in general be described in the following manner.
• The purchaser initiates the project and provides the conditions and general
requirements.
• The design engineer formulates the conditions and requirements in technical
terms guided by the regulations given by authorities.
• The designer selects the structural system and materials in cooperation with the
purchaser and based on a preliminary design analysis.
• The designer performs the design analysis which includes dimensioning by
structural analysis, preparation of drawings, specifications and descriptions.
• The design documents are approved by the purchaser, authorities and, possibly,
by a responsible designer.
• The manufacture and erection/installation can be commenced supervised by the
purchaser, the authority and the designer.

It should be pointed out that all phases of the procedure are of importance in order to
arrive at an adequate design implying good quality and acceptable economy. There may
be a tendency to underestimate the responsibilities of the designer in the early stage of
the procedure.

In summary, the objectives of the design procedure are:
• to produce design documents (drawings, descriptions, specifications etc.)
suitable as a basis for fabrication of the structure,
• to verify that the documents are in agreement with the purchaser's requirements
according to the given design conditions and valid regulations, and
• to ensure, as far as possible, that the documents specify a structure satisfactory
from an economical point of view.

TALAT 2204 4

2204.03 Requirements on the Load Carrying Structure

• Specification of requirements
• Requirements on safety against failure
• Requirements on the serviceability of structures in normal use
• Limit state
• Safety classes
• Economic considerations on the formulation of requirements

Specification of Requirements

Requirements here and in the following sections denote expressions of expectations
defined by the purchaser, future proprietor, utilizers, authorities, etc. concerning the
function of the structure. The requirements may to some extent be varied with respect to
the balance between quality level and cost.

The requirements on a load carrying structure may be specified as follows:
• requirements on safety against failure,
• requirements on serviceability in normal use,
• requirements on durability.

Requirements on Safety Against Failure

The concept of failure may imply anything from destruction of a structural element to
collapse of the entire structural system. The cause of a failure may be of various kinds
and can be classified in three categories:

1. Unfavourable combinations of factors affecting the resistance.
An unfavourable combination of critical parameters has occurred. These parameters
may be interpreted as loads, strength of the material, dimensions, imperfections and
minor damages. They possess values which may be extreme, but do not deviate
significantly from normal conditions.

2. Unforeseen loads.
An event (explosion, fire, ship impact etc.) not considered in the design has appeared as
a single occurrence with such a magnitude that the consequence was failure of the
structure. The load may either be of a character entirely different from those
considered in the design, or it may be of the same character but of a magnitude not
foreseen.

TALAT 2204 5

3. Gross errors.
A gross error has been committed in the design work, material production, or
construction. A gross error implies that the structure has received some material or
geometrical property of a character entirely different from what was intended.

The requirement on safety against failure means that the structure shall be designed and
fabricated in such a way that the probability of failure becomes sufficiently low. The
concept "sufficiently low" also implies that the probability has to be lower the more
serious the consequences would be of a failure happening.

The measures to be taken to ensure a sufficiently low probability of failure should in
principle be adapted to all categories mentioned above.

When the cause of a failure is attributed to the first category, the risk of failure can be
sufficiently reduced at the design level by choosing sufficiently large factors of safety,
which can also be dependent with regard to the consequences of a possible failure.

The measures which can be taken against failure occurring because of an unforeseen
load are more difficult to quantify. Some loads of that kind may be known to a certain
degree through experience from earlier incidents. This is, for instance, the case with
loads arising as a consequence of a collision or an explosion. Other kinds of loads may
be possible but so far unknown. A reasonable step may be to design a structure with
respect to a few known loads of the kinds mentioned above and further assume that it
will also be able to resist other types of loads of a similar category. As a complement, or
an alternative, it is possible to select a structure of such a type and perform a detailed
design in such a way that the carrying system becomes highly insensitive to local
damage, which may arise from loads of the kinds mentioned. Unforeseen loads may, for
example, be caused by impact of various kinds, flood and earthquake. The character of
these loads implies that the probability of their occurrence is small. Therefore, they need
to be considered only for those types of structures where the consequence of a possible
failure may be expected to be very serious. Structures of a vital importance should thus,
if possible, be designed according to damage tolerance criteria.

Gross errors can, for example, be caused by the designer in miscalculating a wall
thickness by a factor of 2, or in the manufacture of a metal structure by forgetting to
define the characteristics of a welded joint or a similar operation. Such errors can not be
compensated for by choosing a larger safety factor in the design analysis. Measures to be
taken to decrease the frequency of gross errors are:
• improved training and information,
• improved organization at the building site,
• more effective quality control.

In summary it may be stated that the measures which can be taken in order to keep the
probability of failure at a low level do not only apply to the choice of safety factors but
include also training, information, organization and quality control.

TALAT 2204 6

Requirements on the Serviceability of Structures in Normal Use

If a load carrying structural member is, in normal use, subjected to damage or causes
damage to other members and, if the damage is unacceptable, the function or
serviceability of the structural member can be considered to be unsatisfactory. The
damage may be permanent or occasional. The word damage is used here in a wider
sense and can be the cause of, for instance, some kind of inconvenience.

Examples of permanent damages may be open cracks in the structural member, cracks
in other building components, e.g. partition walls, and disturbing permanent deflections
of beams. If such damage has occurred and involves inconvenience, it will continue to
bring the same or about the same inconvenience until repaired. In this case the
requirements given and the measures taken to avoid the inconveniences should be aimed
at reducing the risk of generation of the damage. In principle, the problem is equivalent
to that concerning safety against failure. Even if no well-defined limit exists between
these cases, the risk which can be accepted for a minor damage to occur to the structure
in normal use, is normally higher, however, than the acceptable risk of failure. This
implies that it is, in general, only necessary to consider causes of damage corresponding
to the category in the preceding paragraph.

Examples of occasional damages are occasional large deflections of beams and
occasional vibrations. The inconvenience of such damages will only appear during those
periods when the load or other actions occur which cause the damage. The requirements
and measures to reduce the inconveniences should, in this case, be concentrated to the
duration of the damage. Vibrations of a certain intensity may be acceptable from a
comfort point of view if they appear infrequently and only during short periods of time.
On the other hand vibrations of the same intensity may be entirely unacceptable if they
are effective during longer periods.

The requirements on the serviceability of a structure in normal use apply, in most cases,
to deformations including oscillations and vibrations (considered as time dependent
deformations). The inconveniences resulting from large deformations can be the
following: they
• can cause damage to other building components,
• may convey a feeling of discomfort to people in the building,
• can disturb and impair the function of machines, instruments and similar
objects supported by the structure,
• may be disturbing from an aesthetic point of view.

Further cases of damage or poor function in normal use may refer to
• abrasion,
• leakage, e.g. in liquid tanks,
• surface finish, e.g. roughness or discoloration etc.

TALAT 2204 7

It is not possible to express generally valid requirements concerning the function of a
structure in normal use by numerical values. The requirements which should be
formulated are too much dependent on the situation to which the requirement applies.
Usually, the future proprietor/utilizer may establish the requirements after consultation
with the design engineer. Moreover, the requirements must be expressed with due
regard to the situation. A requirement concerning limitations of the deformations can
thus be formulated in one of the following ways:
• limitation of absolute values of displacements,
• limitation of the mutual displacements between the nodes of e.g. a frame
system,
• limitation of the deflection of a structural component (e.g. a beam) in
proportion to the span,
• limitation of the angular deformation of a structural component.

Specific recommendations are given in the different national codes concerning
limitation of angular deformations in order to avoid damage in adjacent building
components. Furthermore, recommendations are provided concerning the bending
stiffness of beams required to guarantee that deflections do not cause discomfort for
people walking on a floor or over a bridge, or that the structure is not operationable at
this deflection (crane beams).

Limit States

The requirements on the load carrying function of a structure apply to both safety
against failure and to serviceability in normal use. These two requirements are, at least
in some cases, quite different in nature and should thus be separated in their
formulation. This can be achieved by performing the design analysis at two limit states
with regard to the function of the structure:
• ultimate limit state, which is a state where the structure is at the limit of
failure,
• serviceability limit state, which is a state where the structure is at the limit of
not satisfying the requirements for normal use.

The implication of the limit states is illustrated in Figure 2204.03.01, which shows the
deflection versus load for a simply supported beam. The serviceability limit state and
the ultimate limit state are indicated by their upper limits.

The limit states are thus conceivable states of the structure. The requirements
concerning safety against failure are, in principle, formulated such that the probability
that any of the possible ultimate limit states is exceeded is satisfactorily low. The
requirements with regard to serviceability in normal use are established in a
corresponding way, or such that the time during which the limit is exceeded, will be
satisfactorily short.

TALAT 2204 8

q

Ultimate limit state
Requirements concerning safety
δ against failure:
Probability of exceeding the
q ultimate ultimate limit state shall be
limit state satisfactorily low.

Serviceability limit state
serviceability limit state Requirements concerning
servivability under normal use:
The time during which the limit
is exceeded shall be
self weight satisfactorily short.
δ

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Training in Aluminium Application Technologies
Limit States 2204.03.01

Economic Considerations on the Formulation of Requirements

Some of the requirements which apply to a structure - in particular those concerning the
safety against failure - constitute the requirements of the society. They are given in the
national codes and standards and should be regarded as minimum requirements.
Therefore, they cannot be modified in an alleviating direction.

The remaining requirements are given by the purchaser/future proprietor and utilizer
(tenant). This means, that in the early phase of the design procedure, the cost of future
maintenance and repair during the service life of the structure have been determined to a
certain degree. There are thus good reasons to consider, at an early stage, the
formulation of the requirements from an economic point of view.

2204.04 The Design Analysis Process

• Introduction
• Methods of verification
• The load and resistance factor method
• Method of allowable stresses

TALAT 2204 9

Introduction

After the formulation of requirements follows the selection of systems and materials. At
this point the design analysis begins, which involves a detailed determination of
dimensions and strength of structural components. The methods of analysis can often be
decided by the designer himself. It is essential that the verification of the structure, with
the chosen dimensions and the properties of the materials selected, satisfies the
requirements established. The procedure can be described according to Figure
2204.04.01 for a simple case. With the assumptions stated concerning loads, dimensions
and material properties, calculation models are applied which provide the load effect S
(Solicitation, in ENV 1999-1-1, called E) and carrying capacity R (Resistance). The load
effect may be expressed as a section quantity (e.g. a bending moment in a beam) caused
by the load.

The resistance is the capacity of the structure to resist a load effect of the same kind (the
capacity of the beam to transfer a moment). The verification implies that the resistance
R has to be higher than the load effect S.

1. Formulation of requirements
2. Selection of system and material
3. Design analysis
Requirements

Model Load
Loads
for S effect S

Verification YES
Dimensions
R>S

Material Model Resistance
properties for R R NO

Assumptions on loads, dimensions and material properties, and calculation
models provide the load effect (solicitation) S and resistance R.
The load effect may be expressed as section quantity, e.g. bending moment
caused by the load.
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Schematic Description of the Design Analysis Process 2204.04.01

The case described concerns safety against failure, but the procedure of verification that
the requirements on the serviceability of the structure in normal use are satisfied will in
principle be the same. In many cases the procedure is more complicated. Several
different kinds of load effects and resistance (e.g. normal forces and bending moment)
may act at the same time. The verification analysis provides an answer, yes or no. In
case the answer is no, the procedure has to be repeated with updated dimensions and
material properties.

TALAT 2204 10

Applications of the design analysis process will be found in lecture 2204.07 (Design
Criteria).

Methods of Verification

The quantities which describe the load effect S and the resistance R (e.g. load values F,
strength values f and dimensions l) are stochastic variables which can be represented in
a simplified manner by frequency curves according to Figure 2204.04.02.

The verification consists of demonstrating that the resistance R is greater than the load
effect S. This can be done by use of a number of methods, listed in historical order:
• The safety factor method (method of allowable stresses)
• The load factor method with one single load factor (often used in plastic
design)
• The load and resistance factor design method (method of partial coefficients),
• Probabilistic methods

Frequency Diagram illustrating schematically
the Method of Partial Coefficients

Resistance R
Frequency

Load effect S Sk and Rk are characteristic
Sk Rk values of load effect and
resistance.
Sd and Rd are design values.
S d < Rd

Rk
γfSk <
γnγm

Frequency Diagram Illustrating Schematically
alu
2204.04.02
Training in Aluminium Application Technologies the Method of Partial Coefficients

The first method has been used earlier, and is still being used in design codes in many
countries but it is being replaced by the third method.

Probabilistic methods have to be based on statistical data for loads, strength properties
etc. which, so far, are available only on a very limited scale. The methods are, therefore,
only used in very special cases.

TALAT 2204 11

The load and resistance factor method and the method of allowable stresses are briefly
described below. A more comprehensive discussion of the methods will be found in
chapters 2204.05 and 2204.06 and in the lecture series 2400 (Fatigue).

The Load and Resistance Design Factor Method

The load and resistance factor method (often called the method of partial coefficients) is
a verification method which is accepted in many countries. In the following, the method
is described as it is applied in the Eurocodes. The formulation is very similar to that
used in the different national codes and standards.

The basis is formed by the so called characteristic values:
Fk for loads (called «actions» in Eurocodes)
fk for strength
lk for dimensions where, in most cases, lk is equal to the
nominal value, i.e. the value given in drawings and
descriptions.

The calculation of Fk and fk is indicated in chapter 2204.05 and chapter 2204.06. From
the characteristic values the design values are deduced:

Fd = γF Fk for loads (4.1)

fk
fd = for strength (4.2)
γM

ld = lk + ∆l for dimensions (4.3)

γF and γM are called partial coefficients. The partial coefficient γF for load is in the
following referred to as the load factor, and the partial coefficient γM is named resistance
factors. ∆l is an additive quantity by which deviations from the ideal dimensions are
considered. In most cases ∆l can be set to zero. The partial coefficients are discussed in
more detail in 2204.05 and 2204.07.

The design values are used in the calculation models for load effect and resistance and
provide the design criteria.

R(fd,1d) ≥ S (Fd,1d) (4.4)

The load and resistance factor method is illustrated in Figure 2204.04.02. Since the load
factor can be given different values for different kinds of loads a more consistent design

TALAT 2204 12

for a low risk of failure can be attained. For example, γF = 1.1 is adopted for gravity
loads and 1.5 for environmental loads, such as snow and wind loads in load
combinations see 2204.05.

Method of Allowable Stresses

In some design codes the scatter in loads, resistance etc. is covered by one single safety
factor s. The verification consists of demonstrating that

σ ≤ σall (4.5)

where σ is the stress determined from the loads and, for instance when designing against
yield failure (plastic deformations),

σy
σ all = (4.6)
s

The safety factor s may vary within rather wide limits (1.3 - 3.5) depending on what
elements of uncertainity have to be considered. In design against buckling, safety factors
to the order of magnitude 10 are found in older codes. It should be noted, however, that
the analysis in this course provides lower limit values of the carrying capacity, for
instance with respect to buckling and a safety factor of the order of 1.5 to 2 would be
appropriate.

The Method of Allowable Stress

Frequency Frequency
Dead load
Resistance

s = 1.5 - 1.7 Resistance
Live load s S Sy
sy Small risk
of failure

Stress Stress
Sy Sy
S < Sall= S < Sall =
s s

Frequency Diagrams Illustrating Schematically
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2204.04.03
Training in Aluminium Application Technologies the Method of Allowable Stress

TALAT 2204 13

2204.05 Loads and Load Factors

• Introduction
• Classification of loads
• Characteristic loads, normal loads and long-term loads
• Load combinations, design value of the load
− Examples
• Loads on buildings, bridges and hydraulic structures

Introduction

The following discussion on loads is, primarily, applicable to the construction sector,
i.e. for buildings, bridge and hydraulic constructions, and for scaffoldings in installation
and erection, cranes, masts, power-line pylons, lighting posts and similar load carrying
structures.

The discussion will, however, be of interest also to design engineers working with other
types of structures such as cisterns, pressure vessels, tanks, transportation vehicles etc.

Classification of Loads

Loads are in the present publication used as a common name for effects due to forces
and deformations. A force effect is primarily caused by external forces on a structure,
while the deformation effect is primarily caused by a forced displacement, e.g. a support
settlement, change of temperature or humidity.

Loads may be classified with respect to their variation with time as
• permanent load approximately constant in time
• variable load
- static load
- dynamic load which causes additional forces due to acceleration
including resonance
- fatigue load load with so many load cycles that fatigue failure can
occur
• accidental load e.g. impact, explosion

Loads can also be classified with respect to variation in space
• fixed load the load distribution over the structure is uniquely
defined

TALAT 2204 14

• free load has an arbitrary distribution over the structure within
possible limits

The duration tq of variable loads (Figure 2204.05.01) is the time during which the
magnitude of the load amounts to at least the value q within the service life ttot of the
structure. The relative duration is defined as

ηq = tq/ ttot (5.1)

It is assumed that the variations of the load are similar during the entire service life ttot.
The reduction factor Ψ, which defines a normal load value of ΨQk, is derived from the
relative duration ηq.

In ENV 1991-1 the Ψ - factor (combination value) is divided into 3 factors:

Ψ0 = coefficient for combination value of a variable load
Ψ1 = coefficient for frequent value of a variable load
Ψ2 = coefficient for quasi-permanent value of a variable load

The combination values (Ψ0 ) are associated with the use of combinations of loads, to
take account of a reduced probability of simultaneous occurence of the most
unfavourable values of several independent loads.

The frequent value (Ψ1) is determined such that the total time, within a chosen period of
time, during which it is exceeded for a specified part, or the frequency with which it is
exceeded, is limited to a given value. The part of the chosen period of time or the
frequency should be chosen with due regard to the type of construction works
considered and the purpose of the calculations. Unless other values are specified the part
may be chosen to be 0,05 or the frequency to be 300 per year for ordinary buildings.

The quasi-permanent value (Ψ2) is so determined that the total time, within a chosen
period of time, during which it is exceeded is a cinsiderable part of the chosen period of
time. The part of the chosen period of time may be chosen to be 0,5. The quasi-
parmanent value may also be determined as athe value averaged over the chosen period
of time.

These representative values and the characteristic value are used to define the design
values of the loads and the combination of loads. The combination values are used for
the verification of ultimate limit states and irreversible serviceability limit states. The
frequent values and quasi-permanent values are used for the verification of ultimate
limit states involving accidental loads and for the verification of reversible serviceability
limit states. The quasi-permanent values are also used for the calculation of long term
effects of serviceablilty limit states.

TALAT 2204 15

In structures subjected to fatigue loading, the load range, the load level, and the number
of load cycles are usually of importance. (For the design of aluminium alloys structures
with regard to fatigue see lecture 2400).

Duration tq = ∑ ti during service time ttot
Intensity t1 t2 t3 ti tq
Relative duration ηq =
ttot
q
It is normally assumed that the
variations of the load are similar during
the entire service life.

The reduction factor ψ, which defines the
ttot normal load value ψQk , is derived from
the relative duration ηq .

Time

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Training in Aluminium Applicat ion Technologies
Variation of Load with Time 2204.05.01

Characteristic Loads, Normal Loads and Long-Term Loads

According to most national codes, loads are defined as follows:
• the characteristic value Gk of a permanent load shall be assumed to be the mean
value.
• the characteristic value Qk of a variable load shall be a value with the probability 0.02
of being exceeded at least once during one year.
• the normal value ΨiQk of a variable load shall be determined considering the relative
duration ηq = tq/ttot,
• characteristic value Qak of an accidental load shall be determined with respect to the
nature of the load.

Further below it is indicated where Gk, Qk, Ψi and Qak for normal loads on buildings,
bridges and hydraulic structures are defined. If the characteristic value is not available in
a load standard, the value of Qk may in principle be estimated by use of the following
procedure (determination of Gk usually does not present a problem).

1. Several observations, about 50, of the yearly maximum load are available. Fit a
reasonable distribution function FQ to measured values and determine Qk from the
condition FQ = 0.98.

2. A smaller number of observations are available. The problem consists of finding a
conservative distribution. A lognormal distribution function complies with this

TALAT 2204 16

requirement in most cases, and for such a distribution, Qk can be determined by
computing:
a) the mean value µ of log χ
b) the standard deviation σ of log χi
c) log Qk = µ + 2.05σ, or Qk = exp(µ + 2.05 σ), where 2.05 = Θ-1(0.98) and Θ is the
distribution function of the standardized normal distribution.

3) No observations of the yearly maximum load are available. In this case it is in
principle not possible to determine Qk. The situation is not unusual, however, and it
is thus often necessary to make an estimate of Qk.
a) Compare with other similar loads for which Qk is known.
b) Guess the mean m and the standard deviation s. Adopt Qk = exp(logm + 2.05σ)
where δ = s/m, compare 2) above. It is normally easier to make a reasonable guess
of m and s than to guess directly the 98 per cent fractile.
c) Assume Qk to be equal to the physical upper limit of the load. It is sometimes
possible to indicate an upper limit. For instance, a reservoir or a tank can only be
filled to its capacity.

Load Combinations, Design Value of the Load

For each critical load case, the design values of the effects of loads should be
determined by combining the values of loads which occur simultaneously, as follows:

a) Persistent and transient situations: Design values of the dominant variable loads
and the combination design values of other loads.

b) Accidental situations: Design values of permanent loads together with the frequent
value of the dominant variable load and the quasi-permanent values of other
variable loads and the design value of one accidental load.

Seismic situations: Characteristic values of the permanent loads together with the quasi-
permanent values of the other variable loads and the design value of the seismic loads.

When the dominant load is not obvious, each variable load should be considered in turn
as the dominant load.

TALAT 2204 17

Design situation Permanent Single variable actions Qd Accidental actions
actions Gd or seismic actions
Ad
Dominant Others
Persistent and γG Gk (γP Pk) γQ1 Qk1 γQi Ψ0i Qki
transient
Accidental γGA Gk (γPA Pk) Ψ11 Qk1 Ψ2i Qki γA Ak or Ad
Seismic Gk Ψ2i Qki γI AEd

In general, the design value of the loads is a load combination as follows:

∑γ
j ≥1
Gj ⋅ Gkj + γ Q1 ⋅ Qk 1 + ∑ γ Qi ⋅ Ψ0i ⋅ Qki
i >1

where γGj = partial factor for permanent load j
Gkj = characteristic value of a permanent loads
γQi = partial factor for for variable load i
Qk1 = characteristic value of the variable load 1
Qki = characteristic value of the variable load i
Ψ0i = combination coefficients
γP = partial factor for prestressing loads
Pk = characteristic value of prestressing load

In the relevant load cases, those permanent actions that increase the effect of the
variable actions (i.e. produce unfavourable effects) shall be represented by their upper
design values, those that decrease the effect of the variable actions (i.e. produce
favourable effects) by their lower design values.

Where the results of a verification may be very sensitive to variations of the magnitude
of a permanent action from place to place in the structure, the unfavourable and the
favourable parts of this action shall be considered as individual actions. This applies in
particular to the verification of static equilibrium.

For building structures, the partial factors according to ENV 1991-1 for ultimate limit
states in the persistent, transient and accidental design situations are given in table
below. The values have been based on theoretical considerations, experience and back
calculations on existing designs.

TALAT 2204 18

Situations
1)
Case Action Symbol P/T A
Case A Loss of Permanent actions: self weight of
static equilibrium; structural and non-structural compo-
strength of nents, permanent actions caused by
structural material ground, ground-water and free water
or ground - unfavourable γGsup 1,10 1,00
insignificant - favourable γGinf 0,90 1,00

Variable actions
- unfavourable γQ 1,50 1,00

Accidental actions γA 1,00

Case B Failure of Permanent actions (see above)
structure or struc - unfavourable γGsup 1,35 1,00
tural elements, - favourable γGinf 1,00 1,00
including those of
the footing, piles, Variable actions
basement walls - unfavourable γQ 1,50 1,00
etc., governed by
strength of Accidental actions γA 1,00
structural material
Case C Failure in Permanent actions (see above)
the ground - unfavourable γGsup 1,00 1,00
- favourable γGinf 1,00 1,00

Variable actions
- unfavourable γQ 1,00 1,00

Accidental actions γA 1,00
P: Persistent situation T: Transient situation A: Accidental situation

1) The design should be verified for each case A, B and C separately as relevant

Recommended Ψ factors for buildings according to ENV 1991-1 are given in the table
below. In ENV 1991-1 the values are boxed. For other applications see relevant parts of
ENV 1991.

TALAT 2204 19

Action Ψ0 Ψ1 Ψ2
Imposed loads in buildings 1)
category A: domestic, residential 0,7 0,5 0,3
category B: offices 0,7 0,5 0,3
category C: congregation areas 0,7 0,7 0,6
category D: shopping 0,7 0,7 0,6
category E: storage 1,0 0,9 0,8
Traffic loads in buildings
category F: vehicle weight: ≤ 30kN 0.7 0,7 0,6
category G : 3OkN < vehicle weight ≤ 160kN 0,7 0,5 0,3
category H: roofs 0 0 0
Snow loads on buildings 0,6 0,2 0
Wind loads on buildings 0,6 0,5 0
Temperature (non-fire) in buildings 3) 0,6 0,5 0)

1) For combination of imposed loads in multistorey buildings, see ENV 1991-2-1.
2) Modification for snow loads for different geogaphical regions may be required.
3) See ENV 1991-2-5.

The combination of actions to be considered for serviceability limit states depends
on the nature of the effect of actions being checked, e.g. irreversible, reversible or
long term. Three combinations designated by the representative value of the
dominant action are given in the following table.

Combination Permanent Variable actions Qd
actions Dominant Others
Gd
Characteristic (rare) Gk (Pk) Qk1 Ψ0i Qki
Frequent Gk (Pk) Ψ11 Qk1 Ψ1i Qki
Quasi-permanent Gk (Pk) Ψ21 Qk1 Ψ2i Qki
For serviceability limit states, the partial factors (serviceability) γG and γQ are taken as
1,0 except where specified otherwise.

Example

Indicate the load combinations in the ultimate limit state which have to be considered in
the design analysis of the tank roof in Figure 2204.05.03. The roof shall be designed for
gravity load G, snow load S and wind load W. (Other loads may occur but are not
included for the sake of simplicity).

TALAT 2204 20

Example of Load Combinations
gravity load G snow load S
In principle:

γgG + 1.5 S + ψW snow is principal load
ψ = 0 or 0,6
γgG + 1.5 W + ψS wind is principal load
ψ = 0 or 0,6
γg = 1,10 or 0,90
wind load W
Actual load combinations:

1,10 G + 1,5 S snow is principal load
-0,90 G + 1,5 W wind is principal load

alu Conical Roof subjected to Gravity, 2204.05.03
Training in Aluminium Application Technologies Snow and Wind Loads

In general, four different alternatives must be investigated:
1. γgGk + 1,5Wk wind is the principal load
2. γgGk + 1,5Wk + 0,6Sk wind is the principal load
3. γgGk + 1,5Sk snow is the principal load
4. γgGk + 1,5Sk + 0,6Wk snow is the principal load

The load factor γg may, according to ENV 1991-1, assume the values 0,90 and 1,10,
respectively. Due to symmetry, only one wind direction has to be investigated, but the
wind load may have two different distributions, corresponding to two load cases.

The snow load also provides two load cases, either a uniform or a triangular distribution
over the roof surface.

The four alternatives thus result in a large number of possible load combinations. Many
of these are not critical, however, and may be sorted out at an early stage.

The cistern roof will probably be dimensioned by either

a) 1,10 Gk + 1,50 Sk or

b) 0,90 Gk - 1,50 Wk

Since two snow and two wind load cases must be examined, a) and b) will result in four
load combinations.

TALAT 2204 21

In an actual situation where Gk, Sk and Wk are known, the number of load combinations
may often be further reduced, which implies that an individual structural element
normally needs to be examined only for one or a couple of load combinations.

In certain types of structures, e.g. an unsymmetrical framework truss, a general
application of the rules for selection of design load combinations leads to an
overwhelming number of load cases, most of which are critical only for some elements.
It should be noted, however, that the designer is free to perform an analysis on the safe
side which, in many cases, will lead to a drastic reduction of the load combinations
which must be considered. The increase in weight, for instance, that results is often
marginal.

Loads on Buildings, Bridges and Hydraulic Structures

Frequently occurring loads on buildings, bridges and hydraulic structures are given in
national or international specifications. Loads on overhead cranes are stated by the
suppliers. Loads on power-line pylons are chosen according to special standards, etc.

2204.06 Resistance and Resistance Factors

• Assumptions concerning strength properties
• Models of analysis

Assumptions Concerning Strength Properties

The material strength properties are the yield and ultimate strength limits in
compression and tension, the modulus of elasticity and the shear modulus. Other
material properties related to strength are Poisson's ratio, fatigue strength, fracture
toughness, creep properties and thermal expansion.

The requirements for design analysis of a structure indicate the strength class of the
material to be used. In the analysis, then, various kinds of strength values are introduced
which apply to the strength class selected. The strength values introduced in the design
analysis are sometimes based on results from tests performed in advance. The producer
of the material certifies that the strength properties are according to the requirements
specified. Alternatively, the strength properties are checked at the delivery.

TALAT 2204 22

The procedure used to verify that the strength of the material meets the given
requirements normally includes tests with special test specimens and a specified
procedure. In certain cases the results of these tests cannot be considered to be directly
representative for the strength of the material in the actual structure and, thus, have to be
corrected. This may be performed by dividing the strength values obtained in the tests
by a number η, normally greater than 1, such that:

1
f structure = f testspecimen (6.1)
η

The factor η should not be mistaken for the reduction factor with respect to buckling.
For metals, the value of η should be close to one.

The characteristic value of strength fk should be interpreted as a condition for the
analysis which refers to the expected results of actual or imagined tests. It thus applies
to the strength of the test specimen and not to that of the actual construction. The
characteristic value is defined somewhat differently for different materials.

The design value for strength should, naturally, be valid for the material of the structure.
This means a certain deviation from the basic presentation in 2204.04 in such a manner
that the coefficient η should be entered into the equation below which translates
characteristic values into design values. With this modification the formula for
computation of the design value fd from the characteristic value fk becomes

fk
fd = (6.2)
ηγ M

The value of η depends on factors quite different for different materials, and no
generally valid figures can be given. For metals, η = 1.0 may be used and η may,
therefore, be omitted in the above equation.

By introducing the partial coefficient γM, uncertainties in the strength of the material are
taken into consideration as caused by:
− the normal scatter of the material strength,
− the variability of the factor or function η which translates the strength of test
specimens into strength of the structure.

For practical reasons other factors not directly related to the strength of the material are
taken into consideration by γM. Such factors are:
− deviations of dimensions and geometry from the nominal values assumed in
the design analysis, if such deviations are not considered elsewhere,
− unreliability of the model of analysis, if kept within reasonable limits.

TALAT 2204 23

The partial coefficients used in ENV 1999-1-1 is different for resistance of members and
connections. They are, however, boxed values.

Resistance of class 1 cross sections: γM1 = 1,10
Resistance of class 2 or 3 cross sections: γM1 = 1,10
Resistance of class 4 cross sections: γM1 = 1,10
Resistance of member to buckling: γM1 = 1,10
Resistance of net section at bolts holes: γM2 = 1,25

Resistance of bolted connections: γMb = 1,25
Resistance of riveted connections: γMr = 1,25
Resistance of pin connections: γMp = 1,25
Resistance of welded connections: γMw = 1,25
Slip resistance connections:
- ultimate limit state: γMs,ult = 1,25
- serviceability limit state γMs,ser = 1,10
Adhesive bonded connections: γMa ≥ 3,0

Models of analysis

The calculations used in the design are based on models by means of which the behavior
of the structure is described. The models of analysis may be more or less complicated
and provide a more or less accurate description of the function of the structure. Often a
model giving a higher accuracy turns out to be more complicated. In certain cases the
nature of the problem demands a more sophisticated model, e.g. for stress analysis in
structures subjected to fatigue. Usually, there is an option, however, between different
models and the choice has to be made on an economic basis, which applies to the cost of
material/construction in relation to the cost of the design analysis.

Models of analysis should be considered as approximate descriptions of the function of
a structure. Even the most advanced models are thus subject to some uncertainties. With
regard to this fact numerical values of coefficients etc. should be chosen in such a way
that the model gives results on the safe side. But it is often not feasible to enter such
values of the coefficients that the results are conservative in all conceivable cases.

Probabilistic aspects may be introduced, choosing the strength coefficients in such a
way that the model gives results on the unsafe side only in a small fraction of the cases.
This fraction should not exceed 5 per cent. The resulting resistance may thus be
interpreted as a characteristic value.

TALAT 2204 24

2204.07 Design Criteria

The load and resistance factor method

The load and resistance factor method is briefly described in 2204.04. The method is
applied in many design specifications and is sometimes referred to as the method of
partial coefficients. According to this method the characteristic values of loads and
resistance are first determined. Then the design values are obtained by:
− multiplying the the characteristic values of the loads by the load factor γF,
− dividing the characteristic values of the resistance by the resistance factors
γM,.

The design analysis should verify that the stresses caused by design loads σSd (or section
forces MSd) are smaller than the design value of the resistance expressed in terms of the
same quantity (σRd, or MRd), i.e.

σSd < σRd (7.1)

where σSd = stress caused by the load: ΣγG Gk + ΣγQi Ψ0i Qki

fk
σ Rd = (7.2)
γM

fk = characteristic strength, refering to a limit state
γM = resistance factor considering uncertainties in the material parameters and
tolerances for dimensions.

Method of allowable stresses

A safety factor should consider the unreliability of load assumptions as well as the
unreliability of resistance values. Since uncertainties of the methods of analysis are
included in the estimation of the resistance, a moderately low safety factor may be
chosen, normally 1.5 for normal types of loading.

TALAT 2204 25

The allowable stress σall is thus determined as

fk
σ all = (7.3)
s

where fk = the resistance according to this course.
s = safety factor, normally 1.5.

The allowable stress shall be higher than the stress determined from loads without load
factors i.e.

σ < σall (7.4)

2204.08 Aluminium Alloys as a Structural Material

Most of the structural aluminium alloys have relatively high strength compared to the
modulus of elasticity. A comparison between different aluminium alloys and tempers
and some other materials shows the table in Figure 2204.08.01.

Strength (Rp0.2) and Modulus of Elasticity (E)
for Some Metals

Material Rp0.2 E E/Rp0.2 • Aluminium has high strength
compared to modulus of
AA 5083-0 125 70000 560 elasticity, especially strain
AA 5083-H321 220 70000 318 hardened and heat treated
AA 6082-T6 270 70000 259 alloys
AA 7108-T6 360 70000 194
• Steel structures are often
St 42 260 210000 808 designed in the
St 52 360 210000 583 ultimate limit state

Concrete C45 28 28000 994 • Aluminium structures are
Timber 20 9000 450 mostly designed in the
serviceability state (deflections)

alu Strength (Rp0.2) and Modulus of Elasticity (E)
2204.08.01
for Some Metals

This effect is especially clear when the aluminium alloy is strain-hardened or heat-
treated. Structural aluminium alloys have roughly twice the strength of steel compared
to the modulus of elasticity.

TALAT 2204 26

Steel designers often use the strength of the material when designing a steel structure,
and than check if the deflection is within the requirement.

When designing an aluminium alloy structure, it will often be the deflection criteria
which is governing. The design procedure will for that reason be designing according to
the deflection criteria or stability and than check the stress or the bearing capacity of the
structure.

Comparing steel and aluminium alloy members in tension with the same elastic strain,
the steel member will have 3 times the stress of the aluminium alloy member, see
Figure 2204.08.02.

Stress-Strain Diagram for Steel (St52)
and Aluminium Alloy (AA6082-T6)
δ (MPa)
St 52

300

260

AA 6082 T6
200

Structure in tension AA 6082 - T6 St 52

100 Factor against yielding 3,1 1,3
87
Factor against fracture 3,6 2,0 - 2,4

ε (%)
0,124 0,3 0,5

alu Stress-Strain Diagram for Steel (St52) 2204.08.02
Training in Aluminium Application Technologies and Aluminium Alloy (AA6082-T6)

The stress in an aluminium alloy structure designed according to the deflection criteria
is very often low. A steel structure will usually be designed according to strength
criteria. Figure 2204.08.02 shows stress strain curves for an aluminium alloy member of
6082-T6-alloy and a steel member of St 52. The example shows different stress in the
members for the same strain, caused by the difference in the modulus of elasticity.

A structure or member in tension designed according to the deflection criteria will
usually be in this situation. The safety against yielding and fracture will in this example
be:

AA 6082 - T6 St 52
Factor against yielding 3,1 1,3


TALAT 2204 27

Comparing members in steel and aluminium in bending, the shape of the members will
be different. At the same deflection, the strain will be different. In Figure 2204.08.03
this is illustrated for a 6,0 m long beam with a distributed load of 11,6 kN/m and a
deflection of l/250. For this example we will have the following factors against yielding
and fracture:

AA 6082-T6 St 52


Because of the relatively low modulus of elasticity of aluminium alloys compared to
their strength, the safety of designing an aluminium alloy structure to the deflection
criteria, is very high and usually higher than a steel structure.

Stress and strain for beams made of
St 52 or AA 6082-T6 with the same deflection and
a weight reduction for the aluminium beam of 40 %
q = 11,6 kN/m
δ (MPa)
St 52
300

AA 6082 T6 δ = L/250
200 6000

161
0,077

100

67 Structure in Bending AA 6082 - T6 St 52
0,096

ε (%) Factor against fracture 4,6 3,2 - 3,9
0,124 0,3 0,5

alu The Stress and Strain for Beams Made of
2204.08.03
Training in Aluminium Application Technologies St 52 or AA 6082-T6

The deflection of members in bending are dependent on the modulus of elasticity (E)
and on the moment of inertia (I) together with the load and the span. With the same span
and load, it will be the product E • I which will determine the deflection.

To get the same deflection of steel and aluminium alloy beams in bending, the moment
of inertia of the aluminium alloy beam must be three times that of steel. If the increase
in the moment of inertia is to be done only by increasing the thicknesses of the web and
flanges the aluminium alloy beam will have the same weight as the steel beam. To save
weight the aluminium alloy beams in bending have to be higher. An example will
illustrate this:

An aluminium alloy beam shall have the same deflection as an IPE 240 steel beam. The
moment of inertia of the IPE 240-beam is 38,9 · 106 mm4 about the strong axis. The

TALAT 2204 28

weight of this beam is 30,7 kg/m. The aluminium alloy beam must have a moment of
inertia of 116,7 · 106 mm4 to get the same deflection.

If the height of the aluminium alloy beam shall be 240 mm, this will be satisfied by an I-
beam of I240 x 240 x 12 x 18,3, which has a moment of inertia of I = 116,6 · 106 mm4
and a weight of 30,3 kg/m (approximately the same weight as the steel beam). If the
height of the aluminium alloy beam can be 300 mm, the deflection criteria will be
satisfied by an I300 x 200 x 6 x 12,9 which has a moment of inertia of 116,7 · 106 mm4
and a weight of 18,4 kg/m which is a weight saving of 40%.

An I330 x 200 x 6 x 10 will have a moment of inertia of 117,3 · 106 mm4 and a weight
of 15,8 kg/m which give a weight saving of 49%.

These three different aluminium alloy beams will give the same deflection as an IPE 240
steel beam. It will be the shape and stability of the beam which will determined the
weight of the beam. Figure 2204.08.04 shows the beams and the weight savings.

Comparison between four beams which will give the same deflection
Steel Aluminium Aluminium Aluminium
Alloy Alloy Alloy
t

w
h

b

Moment of inertia
in mm4 38,9 E 6 116,6 E6 116,7 E6 117,3 E6
EI (N/mm2) 8,17 E12 8,16 E12 8,17 E12 8,21 E12
h (mm) 240 240 300 330
b (mm) 120 240 200 200
t (mm) 9,8 18,3 12,9 10
w (mm) 6,2 12 6 6
g (kg/m) 30,7 30,3 18,4 15,8

alu Comparison between four beams which will give the
2204.08.04
Training in Aluminium Application Technologies same deflection

TALAT 2204 29

2204.09 References/Literature

[1] : Lars Østlund. Handboken Bygg (in Swedish)

[2] : CEN/TC 250/SC 1: ENV 1991-1. Eurocode 1 Basis of design and actions on
structures. Part 1: Basis of design. 1994

[3] : CEN/TC 250/SC 9: ENV 1999-1-1. Eurocode 9 Design of aluminium structures.
Part 1-1. General rules. 1997

2204.10 List of Figures

Figure No. Figure Title (Overhead)
2204.03.01 Limit States
2204.03.02 Safety Classes
2204.04.01 Schematic Description of the Design Analysis Process
2204.04.02 Frequency Diagram Illustrating Schematically the Method of Partial
Coefficients
2204.04.03 Frequency Diagrams Illustrating Schematically the Method of Allowable
Stress
2204.05.01 Variation of Load With Time
2204.05.02 Recommended Load Factors
2204.05.03 Conical Roof Subjected to Gravity, Snow and Wind Loads
2204.06.01 Values of n as Functions of the Safety Class
2204.08.01 Strength (Rp0.2) and Modulus of Elasticity (E) for Some Metals
2204.08.02 Stress-Strain Diagram for Steel (St52) and Aluminium Alloy (AA6082-
T6)
2204.08.03 The Stress and Strain for Beams Made of St 52 or AA 6082-T6
2204.08.04 Comparison Between Four Beams Which Will Give the Same Deflection

TALAT 2204 30

TALAT Lecture 2204: Design Philosophy

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TALAT Lecture 2204: Design Philosophy