EUROCODE 1, PART 1.7 ACCIDENTAL ACTIONS BACKGROUND DOCUMENT FIRST DRAFT ...

62 Pages·2007·0.76 MB·English

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EUROCODE 1, PART 1.7

ACCIDENTAL ACTIONS

BACKGROUND DOCUMENT

FIRST DRAFT January 2005

by

A. Vrouwenvelder
U. Stieffel
G. Harding
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NOTATIONS 3

0. INTRODUCTION 4

1. GENERAL 5

2. ACCIDENTAL ACTIONS 5
2.1 DEFINITION OF ACCIDENTAL ACTIONS 5
2.2 ACCIDENTAL VERSUS VARIABLE ACTIONS 5
2.3 REPRESENTATION OF ACCIDENTAL ACTIONS 6

3. DESIGN FOR ACCIDENTAL ACTIONS 8
3.1. GENERAL 8
3.2. DESIGN FOR UNIDENTIFIED ACCIDENTAL LOADS (Robustness) 11
3.3. DESIGN FOR IDENTIFIED ACCIDENTAL LOADS 14

4. IMPACT 15
4.1. BASICS OF IMPACT ANALYSIS 17
4.2. IMPACT FROM VEHICLES 20
4.3. IMPACT FROM RAIL TRAFFIC 30
4.4 SHIP COLLISIONS 31

5. EXPLOSIONS 37
5.1 NATURE OF THE ACTION 37
5.2 MODEL FOR THE UNCONFINED EXPLOSION 39
5.3 LOADS MODELS FOR GAS EXPLOSION PRESSURES IN BUILDINGS 41
5.4 DESIGN EXAMPLE OF A COLUMN IN A BUILDING FOR AN EXPLOSION 49
5.5 GAS AND FUEL / AIR EXPLOSIONS IN ROAD AND RAIL TUNNELS 53
5.6 DUST EXPLOSIONS IN ROOMS AND SILOS 55

REFERENCES 60
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NOTATIONS

F = force
F = compression strength of colliding object
co
F = compression strength of the structure
cs
N = number of fatalities per year
P = probability of not avoiding a collision, given a ship on collision course
a
P = probability of collision
c
P = probability of failure
f
P(d|f) = probability of a person being killed, given structural failure
T = period of time under consideration, mostly a year

a = deceleration
b = typical dimension of structural object
d = distance from structure to the road
f (y) = ship or aircraft position perpendicular to the direction of distribution of initial
s
propagation
k = stiffness
m = mass of colliding object
n = number of cars, ships, per time unit passing a certain point (traffic intensity)
p = pressure due to explosion
i
r = distance
t = time
u = deformation at fracture
f
v = initial velocity of colliding object
o
v = velocity of colliding object at impact
r
x,y = coordinates
α = angle between rod direction and car direction
α = FORM influence factor
s
β = reliability index
λ(x) = probability rate of a ship getting out of control or a car leaving the road
per unit distance
α = arctan (d/x)
φ = dynamic amplification factor
dyn
Φ = normal distribution
φ = venting parameter
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1. INTRODUCTION

This background document provides explanatory material in support of the draft of Eurocode 1,
Part 1.7, Accidental Actions, dated September, 2004. The document is intended for a better
understanding of the numbers and rules given in the code. It is envisaged to be of help in setting
up National Annexes in the various member states, in applying the document for the design of
new structures and for formulating corrections and future improvements.

Since the design philosophy for accidental actions differs from the design philosophy for
permanent and variable actions, it has been unavoidable to include design principles to a
limited extent. It should be noted, however, that later on it should be considered if certain
parts ought to be transferred from EN 1991-Part 1.7 to EN 1990.

The present version of this background document is a revision of the background document for
the ENV.
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2. ACCIDENTAL ACTIONS

2.1 Definition of Accidental Actions

Accidental actions in the Eurocode system are defined as actions with low probability,
severe consequences of failure and usually of short duration. Typical examples are fire,
explosion, earthquake, impact, floods, avalanches, landslides, and so on

Next to these identified accidental actions, structural members may got damaged for a
variety of less identifiable reasons like human errors in design and construction, improper
use, exposure to aggressive agencies, failure of equipment, terrorist attacks and so on.

In the Eurocode system, fire and earthquake are dealt with in specific parts. The document
EN 1991-1-7 deals primarily with impact and explosion. In addition, the document also gives
general guidelines how to deal with identified and unidentified accidental actions in general.
Because of the nature of accidental loads the design approach may be different from normal
loads. Local damage may be acceptable and non-structural measures (e.g. sprinkler
installations or vent openings) may prove to be more cost effective than structural ones.

The scope of EN 1991-1-7 gives no attention to events, which are generally denoted as
accidents, like persons falling through windows or roofs. The reason is that they have no
damaging potential for the structural system.

2.2 Accidental versus variable actions

Figure 2.1 shows the typical difference between a variable and an accidental load as far as the
time characteristics are concerned. The variable load is nearly always present, although its
value may be small for a substantial part of the time. However, serious non-zero values will in
most cases (wind, snow, traffic) occur many times during the design life of the structure. A
typical accidental load, on the other hand, will most probably not occur during the working life of
the structure. If the load is present, it normally will take only a short time, varying from a few
seconds (explosions) to some days (floods).

Figure 2.2 shows a typical probability distribution for the one year maximum of the loads.
Accidental loads have a probability of 0.98 per year or more to be zero. Variable loads as wind
and traffic have zero probability to be absolutely zero. For snow and earthquake intermediate
values may occur. Note that only in a limited number of cases the probability of occurrence of
an accidental action and the probability distribution of its magnitude can be determined from
statistics. As a result design values in practice often are to some extend nominal values.

For some actions in the category variable actions, abnormal values may occur that are not
sufficiently taken care of by the normal check of component failure. Special structures may
therefore need a check for such abnormal loads. Examples are snow loads in some alpine
areas and ice loads on masts and towers. The corresponding safety checks may follow the
principles described for accidental situations, even if the loads are not classified as accidental
actions according to the present standard.
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2.3 Representation of accidental actions

Actions on structures can usually be represented as static loads and structural response is
usually preformed using a linear elastic analysis. Accidental actions, however, are in general
more complex. For instance in the case of impact the action is a truck with random elastic
plastic mechanical and geometrical properties that hits a structure at a random angle and
velocity. An explosion is a pressure wave where the pressure interacts with the response of the
structure.

Nevertheless, for structures where the consequences of failure may be considered as limited,
there is a need for simplified design rules. The first simplification is that accidental loads are
considered as a dynamic force or even as a static equivalent force. Chapter 1 of EN 1991-1-7
gives the following relevant definitions for these quantities:

A dynamic force is a force that varies in time and which may cause significant
dynamic effects in the structure; in the case of impact the dynamic forces represents
the forces at the point of impact.

A static equivalent force is an alternative representation for a dynamic force and
includes the dynamic response of the structure.

In the case of a dynamic force one may start a dynamic analysis, provided that the time
dependent behaviour of the load is given. Alternatively one may use dynamic amplification
factors as specified in the code for a number of design situations. When a static equivalent force
is considered no further dynamic considerations are required.

According to chapter 2, clause (2), impact actions indeed are to be considered as free actions,
but the set of locations where the forces may apply is nevertheless restricted. Information is
presented in Section 4.

Note that in Annex A even further simplified representations of the accidental loads are
presented. The forces presented in this Annex can directly be applied to dimension floors,
columns and connections between them. These forces are of a prescriptive nature and a direct
relation with physical entities like impact and explosions should be considered as marginal.
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Force

(a)

time

Force

(b)

time

Figure 2.1: Typical time characteristics of (a) accidental and (b) variable load

f(x)

(a)
98%
2%

x = load

f(x)
(b)

x = load

Figure 2.2: Typical probability distribution of (a) accidental and (b) variable loads
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3. DESIGN FOR ACCIDENTAL ACTIONS

3.1 General

3.1.1 Identified and unidentified accidental design situations

According to EN 1990, the term "design situation" is defined to mean the circumstances in
which the structure may be required to fulfil its function. The selected design situations have
to be sufficiently severe and varied enough to encompass all conditions that can reasonably
be foreseen to occur during the execution and use of the structure.

The phrase "which can reasonably be foreseen" is somewhat ambiguous in the case of
accidental situations, the characteristics of which are that they cannot easily be foreseen in
detail or maybe not at all. In particular this holds for accidental actions. IN EN 1991-1-7
therefore a distinction is made between so-called identified and unidentified actions. The
identified actions may be analysed using classical (advanced) structural analysis. For the
unidentified actions more general robustness requirements (e.g. prescribed tying forces)
have been introduced. Note that for low safety class structures the design may be confined
to these robustness requirements only, as they also work positively for the identifiable
causes.

3.1.2 Objective of design for accidental actions / Acceptance of localised damage

The objective of the design is to reduce risks at an economical acceptable price. Risk may
be defined as the danger that undesired events represent. Risk is expressed in terms of the
probability and consequences of undesired events. Thus, risk-reducing measures consist of
probability reducing and consequence reducing measures, including contingency plans in
the event of an accident. Risk reducing measures should be given high priority in design for
accidental actions, and also be taken into account in design. No structure can be expected
to resist all actions that could arise due to an extreme cause, but there is to be a reasonable
probability that it will not be damaged to an extent disproportionate to the original cause.

As a result of this principle, local failure (which in most cases may be identified as a
component failure) may be accepted in accidental design situations, provided that it does not
lead to a system failure. The consequence is that redundancy and non-linear effects both
regarding material behaviour and geometry play a much larger role in design to mitigate
accidental actions than in the case of variable actions. The same is true for a design that
allows large energy absorption.

3.1.3 Design Strategies

Design with respect to accidental actions may pursue one or more as appropriate of the
following strategies, which may be mixed in the same building design:

• Preventing the action occurring or reducing the probability and/or magnitude of the
action to a reasonable level. (The limited effect of this strategy must be recognised; it
depends on factors which, over the life span of the structure, are commonly outside
the control of the structural design process)
• Protecting the structure against the action (e.g. by traffic bollards)
• Designing in such a way that neither the whole structure nor an important part thereof
will collapse if a local failure (single element failure) should occur
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• Designing key elements, on which the structure would be particularly reliant, with
special care, and in relevant cases for appropriate accidental actions
• Applying prescriptive design/detailing rules which provide in normal circumstances
an acceptably robust structure (e. g. tri-orthogonal tying for resistance to explosions,
or minimum level of ductility of structural elements subject to impact)

3.1.4 Consequences classes

Design for accidental situations is in particular implemented to avoid structural catastrophes. As
a consequence, design for accidental design situations needs to be included only for structures
for which a collapse may cause particularly large consequences in terms of injury to humans,
damage to the environment or economic losses for the society. Exempted are thus in particular
low-rise buildings, where, compared to high-rise buildings, both the probability of the occurrence
of an accidental action and the consequences are small. Nevertheless, protective measures like
fire isolation of steel members and design measures like favouring ductile design in earthquake
areas are relevant also for low-rise buildings.

A convenient measure to decide what structures are to be designed for accidental situations
is to arrange structures or structural components in categories according to the
consequences of an accident. Eurocode 1991 Part 1.7 arranges structures in the following
categories based on consequences of a failure:

- Consequences class 1 Limited consequences
- Consequences class 2 Medium consequences
- Consequences class 3 Large consequences

Less important individual structural members or sub-systems may be placed in a lower
safety category than the overall structural system.

Examples of placing structures in safety categories are shown in the informative annex A.
Table 3.1 illustrates the concept of the categorization.

Table 3.1. Safety categories suggested in draft for EC 1 Part 1.7

Consequences class Example structures
class 1 low rise buildings where only few people are present
class 2, lower group most buildings up to 4 stories
class 3, upper group most buildings up to 15 stories
class 4 high rise building, grand stands etc.

Reliability differentiation is also discussed in EN 1990, Section 2.2. As argued there, there
may be various reasons for reliability differentiation, and the choice of categories or classes
may to some extent depend on particular needs.

Not only the appropriate measures but also the appropriate method of analysis may depend
on the safety category, e.g. in the following manner:

- Consequences class 1: no specific consideration of accidental actions

- Consequences class 2: depending on the specific circumstances of the structure in
question: a simplified analysis by static equivalent load models for identified
accidental loads and/or by applying prescriptive design/detailing rules
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- Consequences class 3: extensive study of accident scenarios and using dynamic
analyses and non-linear analyses if appropriate

It is up to member states to decide what is considered as an appropriate strategy in the
various cases.