Paso 3. Estimación de las fuentes de incertidumbre estándar

2.7.1 Testing of Steel Reinforcement

Reinforcing steel is routinely sampled and tested for tensile strength, during production, to maintain quality. The size, shape, position of the gauges and method of testing of small sample pieces of steel is given in EN 10002 (2001). The tensile test gives values of Young’s modulus, limit of proportionality, yield stress or proof stress, percentage elongation and ultimate stress. Occasionally, when failures occur in practice, samples of steel are taken from a structure to assess the strength.

Methods of destructive testing fusion welded joints and weld metal in steel are given in BS EN 288-3 (1992). Welds of vital importance should be subject to non- destructive tests. The defects that can occur in welds are; slag inclusions, porosity, lack of penetration and sidewall fusion, liquation, solidification, hydrogen cracking, lamellar tearing and brittle fracture.

2.7.2 Testing of Hardened Concrete

Concrete supplied, or mixed on a site, is routinely tested by crushing standard cubes or cylinders when they have hardened. The method of manufacture and testing of the specimens is laid down in BS EN 12390-1 (2000). Occasionally some of the cylinders are split to provide the tensile strength of concrete. Rarely, after a structure is complete, doubt may be cast on the strength of the concrete. Cores can be cut from the concrete and tested as cylinders but this is expensive and may not be practical (EN 12504-1, 1999).

Alternatively, non-destructive tests may be used e.g. ultrasonic pulse velocity method (EN 12504-4, 1998). This consists in passing ultrasonic pulses through concrete from a transmitting transducer to a receiving transducer. Velocities range from 3 to 5 km/s and the higher the velocity, higher the strength of concrete. An alternative is the rebound hardness test (EN 12504-2, 1999) where the amount of rebound increases with the strength of concrete. This test is not so accurate but gives a rapid assessment before other methods are considered. The ultrasonic test is the more reliable and gives information over the thickness of the specimen whereas the rebound test give only surface values. Further details are given in Neville (1995).

2.7.3 Testing of Structures (EN 1990, Annex D)

Occasionally, new methods of construction are suggested and there may be some doubt as to the validity of the assumptions of behaviour of the structure. Alterna- tively if the structure collapses, there may be some dispute as to the strength of a 30
_{Chapter 2 / Mechanical Properties of Reinforced Concrete}

component, or member, of the structure. In such cases, testing of components, or part of the structure, may be necessary. However it is generally expensive because of the accuracy required, cost of material, cost of fabrication, necessity to repeat tests to allow for variations and to report accurately. Tests are described as;

(a) acceptance tests – non-destructive for confirming structural performance, (b) strength tests – used to confirm the calculated capacity of a component or

structure,

(c) tests to failure – to determine the real mode of failure and the true capacity of a specimen,

(d) check tests – where the component assembly is designed on the basis of tests. Structures which are unconventional, and/or methods of design which are unusual or not fully validated by research, should be subject to acceptance tests. Essentially, these consist of loading the structure to ensure that it has adequate strength to support, e.g.

1,0 ðtest dead load) þ 1,15 ðremainder of dead load) þ 1,25 ðimposed load):

REFERENCES AND FURTHER READING

ACI Committee 209 (1972) Shrinkage and creep in concrete. ACI Bib. No. 10.

BS EN 288-3 (1992) Specification and approval of welding procedures for metallic materials. Welding procedure tests for the arc welding of steels, BSI.

BS 4449 (1997) Carbon steels for the reinforcement of concrete, BSI.

BS EN 206-1 (2000) Concrete Pt 1. Specification, performance, production and conformity, BSI. BS EN 12390-1 (2000) Testing hardened concrete, BSI.

EN 12504-1 (1999) Testing concrete in structures- Part 1: Cored specimens – Taking examining and testing in compression. BSI.

EN 12504-4 (1998) Testing concrete in structures – Part 4: Determination of ultrasonic pulse velocity, BSI.

EN 12504-2 (1999) Testing concrete in structures – Part 2: Non-destructive testing – determination of rebound number, BSI.

EN 10002 (2001) Methods of testing of metallic materials, BSI. EN 1990 (2002) Basis of structural design.

Grundy, P. (1985) Fatigue limit design for steel structures. Civ. Eng. Trans., ICE, Australia, CE27, No. 1.

Millard, S.G. and Johnson, R.P. (1984) Shear transfer across cracks in reinforced concrete due to aggregate interlock and dowel action, Mag. Conc. Res., Vol. 36, No 126, March 9–21. Munse, W.H. (1984) Fatigue of welded structures, Welding Res. Council, New York. Neville, A.M. (1995) Properties of Concrete, Longman, London.

Timoshenko, S. (1946) Strength of Materials Pt II. Van Nostrand, New York.

C h a p t e r

3

/ Actions

3.1 INTRODUCTION

Actions are a set of forces (loads) acting on a structure, or/and deformations produced by temperature, settlement or earthquakes.

Ideally, loads applied to a structure during its working life should be analysed statistically and a characteristic load determined. The characteristic load might then be defined as the load above which no more than 5 per cent of the loads exceed, as shown in Fig. 3.1. However data is not available and the characteristic value of an action is given as a mean value, an upper or lower value or a nominal value. Guidance on values is given in EN 1990 (2002), Pts. 1-7.

Actions are classified by: (1) Variation in time:

(a) permanent actions (G), e.g. self-weight, fittings, ancillaries and fixed equipment,

(b) variable actions (Q), e.g. imposed loads, wind actions or snow loads, (c) accidental actions (A), e.g. explosions or impact from vehicles. (2) Spacial variation:

(a) fixed actions, e.g. self-weight for structures sensitive to self-weight,

(b) free actions which result in different arrangement of actions, e.g. movable imposed loads, wind actions and snow loads.

These actions are now described in more detail.