Eurocode 1, Part 1.7

- 1 -ec1p17back.doc

EUROCODE 1, PART 1.7

ACCIDENTAL ACTIONS

BACKGROUND DOCUMENT

FIRST DRAFT January 2005

by

1. Vrouwenvelder

U. Stieffel

G.Harding

NOTATIONS3

0.INTRODUCTION 4

1.GENERAL5

2.ACCIDENTAL ACTIONS5

2.1DEFINITION OF ACCIDENTAL ACTIONS5

2.2ACCIDENTAL VERSUS VARIABLE ACTIONS5

2.3REPRESENTATION OF ACCIDENTAL ACTIONS6

3.DESIGN FOR ACCIDENTAL ACTIONS8

3.1. GENERAL8

3.2.DESIGN FOR UNIDENTIFIED ACCIDENTAL LOADS (Robustness)11

3.3.DESIGN FOR IDENTIFIED ACCIDENTAL LOADS14

4.IMPACT15

4.1.BASICS OF IMPACT ANALYSIS17

4.2.IMPACT FROM VEHICLES20

4.3. IMPACT FROM RAIL TRAFFIC30

4.4SHIP COLLISIONS31

5.EXPLOSIONS37

5.1NATURE OF THE ACTION37

5.2MODEL FOR THE UNCONFINED EXPLOSION39

5.3LOADS MODELS FOR GAS EXPLOSION PRESSURES IN BUILDINGS41

5.4DESIGN EXAMPLE OF A COLUMN IN A BUILDING FOR AN EXPLOSION49

5.5GAS AND FUEL / AIR EXPLOSIONS IN ROAD AND RAIL TUNNELS53

5.6DUST EXPLOSIONS IN ROOMS AND SILOS55

REFERENCES60
NOTATIONS

F=force

Fco=compression strength of colliding object

Fcs=compression strength of the structure

N=number of fatalities per year

Pa=probability of not avoiding a collision, given a ship on collision course

Pc=probability of collision

Pf=probability of failure

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

fs(y)=ship or aircraft position perpendicular to the direction of distribution of initial propagation

k=stiffness

m=mass of colliding object

n=number of cars, ships, per time unit passing a certain point (traffic intensity)

pi=pressure due to explosion

r=distance

t=time

uf=deformation at fracture

vo=initial velocity of colliding object

vr=velocity of colliding object at impact

x,y=coordinates

=angle between rod direction and car direction

s=FORM influence factor

=reliability index

x=probability rate of a ship getting out of control or a car leaving the road per unit distance

α=arctan (d/x)

φdyn=dynamic amplification factor

=normal distribution

=venting parameter

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.

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.

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.

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

Figure 2.2:Typical probability distribution of (a) accidental and (b) variable loads

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
• 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 1Limited consequences

-Consequences class 2Medium consequences

-Consequences class 3Large 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
class 2, lower group
class 3, upper group
class 4 / low rise buildings where only few people are present
most buildings up to 4 stories
most buildings up to 15 stories
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

-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.

3.2 DESIGN FOR UNIDENTIFIED ACCIDENTAL LOADS (Robustness)

3.2.1 Background

The design for unidentified accidental load is presented in Annex A of EN1991-1-7. Rules of this type were developed from the UK Codes of Practice and regulatory requirements introduced in the early seventies following the partial collapse of a block of flats in east London caused by a gas explosion. The rules have changed little over the intervening years. They aim to provide a minimum level of building robustness as a means of safeguarding buildings against a disproportionate extent of collapse following local damage being sustained from an accidental event [3-1].

The rules have proved satisfactory over the past 3 decades. Their efficacy was dramatically demonstrated during the IRA bomb attacks that occurred in the City of London in 1992 and 1993. Although the rules were not intended to safeguard buildings against terrorist attack, the damage sustained by those buildings close to the seat of the explosions that were designed to meet the regulatory requirement relating to disproportionate collapse was found to be far less compared with other buildings that were subjected to a similar level of abuse.

3.2.2 Summary of the method of Annex A

Note that for class 1 there are no special considerations and for class 3 a risk analysis is recommended. So rules are given only for class 2 (both the upper and lower group). A distinction is made between framed structures and load-bearing wall construction.

Class 2, Lower Group, Framed structures:

Horizontal ties should be provided around the perimeter of each floor (and roof) and internally in two right angle directions to tie the columns to the structure (Figure 3.1). Each tie, including its end connections, should be capable of sustaining the following force in [kN]:

-internal ties: Ti = 0.8 (gk +  qk) s L (but > 75kN)(3.1)

-perimeter ties: Tp = 0.4 (gk +  qk) s L (but > 75kN).(3.2)

In here gk and qk are the characteristic values in [kN/m2] of the self weight and imposed load respectively; is the combination factor, s [m] is the spacing of ties and L [m] is the span in the direction of the tie, both in m. Edge columns should be anchored with ties capable of sustaining a tensile load equal to 1% of the vertical design load carried by the column at that level.

Class 2, Lower group, Load-bearing wall construction:

A cellular form of construction should be adopted to facilitate interaction of all components including an appropriate means of anchoring the floor to the walls.

Figure 3.1 - Example of effective horizontal tying of a framed office building.

Class 2 - Upper Group, Framed structures:

Horizontal ties as above;

In addition one of the following measures should be taken:

(a) Effective vertical ties: Columns and walls should be capable of resisting an accidental design tensile force equal to the largest design permanent and variable load reaction applied to the column from any story. Ensuring that upon the notional removal of a supporting column, beam or any nominal section of load-bearing wall, the damage does not exceed 15% of the floor in each of 2 adjacent storeys. The nominal length of load-bearing wall construction referred to above should be taken as a length not exceeding 2.25 H; for an external masonry, timber or steel stud wall, the length measured between vertical lateral supports.

(b) Key elements designed for an accidental design action Ad, = 34 kN/m2.

Class 2 - Upper Group, Load-bearing wall construction.

Rules for horizontal ties similar to those for framed buildings except that the design tensile load in the ties shall be as follows:

For internal ties Ti = kN/m (but > Ft)(3.3)

For perimeter ties Tp = Ft(3.4)

Where Ft = (20 + 4n) kN with a maximum of 60 kN, where n represents the number of storeys; g, q and  have the same meaning as before, and z = 5h or the length of the tie in [m], whichever is smallest.

In vertical direction of the building the following expression is presented:

For vertical tieTv = N(3.5)

but at least 100 kN/m times the length of the wall. In this formula A is the load bearing area of the wall, h is the story height and t is the wall thickness. Load bearing wall construction may be considered effective vertical ties if (in the case of masonry) their thickness is at least 150mm and the height of the wall h < 20 t, where t is wall thickness.