EUROCODE 1, PART 1.7 ACCIDENTAL ACTIONS …

1 - 1 - EUROCODE 1, PART ACCIDENTAL ACTIONS background document first DRAFT January 2005 by A. Vrouwenvelder U. Stieffel G. Harding - 2 - NOTATIONS 3 0. INTRODUCTION 4 1. GENERAL 5 2. ACCIDENTAL ACTIONS 5 DEFINITION OF ACCIDENTAL ACTIONS 5 ACCIDENTAL VERSUS VARIABLE ACTIONS 5 REPRESENTATION OF ACCIDENTAL ACTIONS 6 3.

2 DESIGN FOR ACCIDENTAL ACTIONS 8 GENERAL 8 DESIGN FOR UNIDENTIFIED ACCIDENTAL LOADS (Robustness) 11 DESIGN FOR IDENTIFIED ACCIDENTAL LOADS 14 4. IMPACT 15 BASICS OF IMPACT ANALYSIS 17 IMPACT FROM VEHICLES 20 IMPACT FROM RAIL TRAFFIC 30 SHIP COLLISIONS 31 5. EXPLOSIONS 37 NATURE OF THE action 37 MODEL FOR THE UNCONFINED EXPLOSION 39 LOADS MODELS FOR GAS EXPLOSION PRESSURES IN BUILDINGS 41 DESIGN EXAMPLE OF A COLUMN IN A BUILDING FOR AN EXPLOSION 49 GAS AND FUEL / AIR EXPLOSIONS IN ROAD AND RAIL TUNNELS 53 DUST EXPLOSIONS IN ROOMS AND SILOS 55 REFERENCES 60 - 3 - 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)

3 = 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 = arctan (d/x) dyn = dynamic amplification factor = normal distribution = venting parameter - 4 - 1.

4 INTRODUCTION This background document provides explanatory material in support of the draft of EUROCODE 1, Part , 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 to EN 1990. The present version of this background document is a revision of the background document for the ENV. - 5 - 2.

5 ACCIDENTAL ACTIONS 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.

6 Local damage may be acceptable and non-structural measures ( 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. ACCIDENTAL versus variable ACTIONS Figure 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.

7 If the load is present, it normally will take only a short time, varying from a few seconds (explosions) to some days (floods). Figure shows a typical probability distribution for the one year maximum of the loads. ACCIDENTAL loads have a probability of 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.

8 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. - 6 - 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.

9 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.

10 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. - 7 - Figure : Typical time characteristics of (a) ACCIDENTAL and (b) variable load Figure : Typical probability distribution of (a) ACCIDENTAL and (b) variable loads (a) Force Force (b) time time f(x) f(x) 98% 2% x = load x = load (b) (a) - 8 - 3.