Eurocode 7 - Part 1 (CEN, 2004) allows the use of pseudo-elastic methods written under the form:
where Em is the design value of the modulus of elasticity.
This method assumes that an estimate of an equivalent and unique Young’s modulus value is possible to represent the ground affected by the load and for the correct level(s) of deformation… All the difficulty lies in the assessment of Em. For the Fort Canning Tunnel in Singapore, Mair (2011)
quotes a value of the undrained modulus Eu for the hard clay matrix Eu ≈ 500 MPa, from back-analysis
of settlements of buildings on rafts. Note that the undrained shear strength of the Singapore clay matrix is cu > 150 kPa.
The value for Eu is consistent with unload-reload moduli from pressuremeter tests and moduli from
plate loading tests in the same clay. Thus, it would be 10 times the “first loading” modulus EM obtained
with the Ménard pressuremeter quoted above, which is not surprising. The elastic calculation (Eqn.5.11) leads to:
sB2= 1,05·2,0·0,66/500 = 0,0023 = 2,8 mm.
This settlement, largely less to the one calculated using the first loading Ménard modulus together with the Ménard empirical formula (Eqn. 5.10), is probably more realistic, as an elastic approach is more appropriate for the reloading phase of the clay (after excavation). The result confirms that the settlement of such a “compensated” foundation can be ignored in practice.
References
Baguelin, F., Jézéquel, J.F., Shields, D.H. (1978). The Pressuremeter and Foundation Engineering. Trans Tech Publications, Clausthal, Germany.
CEN 2002. Eurocode: Basis of structural design. EN 1990: 2002. European Committee for Standardization (CEN): Brussels.
CEN 2004. Eurocode 7: Geotechnical design - Part 1: General rules. EN 1997-1:2004 (E), November 2004, European Committee for Standardization: Brussels.
CEN 2007. Eurocode 7: Geotechnical design - Part 2: Ground investigation and testing. EN1997- 2:2007 (E), March 2007, European Committee for Standardization: Brussels.
Frank R. 1999. Calcul des fondations superficielles et profondes, Presses de l’Ecole des ponts et Techniques de l’ingénieur, Paris.
MELT-Ministère de l'Equipement, du logement et des transports 1993. Règles Techniques de Conception et de Calcul des Fondations des Ouvrages de Génie Civil (in French: Technical Rules for the Design of Foundations of Civil Engineering Structures). Cahier des clauses techniques générales applicables aux marchés publics de travaux, FASCICULE N°62 -Titre V, Textes Officiels N° 93-3 T.O., 182 pages.
Some geotechnical aspects of building design (EN 1997) R. Frank
ANNEX 1
From EN 1997-2 (CEN, 2004): Annex D (informative): A sample analytical method for bearing resistance calculation.
Annex D.3: Undrained conditions
The following symbols are used in Annex D.3
A' = B' L' the design effective foundation area
b the design values of the factor for the inclination of the base, with subscript c (with subscripts q and γ, only used for drained conditions)
B the foundation width
B' the effective foundation width
D the embedment depth
e the eccentricity of the resultant action, with subscripts B and L
i the inclination factor of the load, with subscript c (with subscripts q and γ for drained conditions only)
L the foundation length
L' the effective foundation length
q overburden or surcharge pressure at the level of the foundation base
s the shape factor of the foundation base, with subscript c
(with subscripts q and γ for drained conditions only)
V the vertical load
α the inclination of the foundation base to the horizontal
γ weight density of the soil below the foundation level The notations used are given in Figure D.1.
Figure D.1 — Notations
D.3 Undrained conditions
The design bearing resistance may be calculated from:
R/A' = (+2) cu bc sc ic + q (D.1)
with the dimensionless factors for:
o the inclination of the foundation base: bc = 1 – 2α / (π + 2);
o the shape of the foundation:
sc = 1 + 0,2(B'/L') for a rectangular shape;
ANNEX 2
From EN 1997-2 (CEN, 2004): Annex E (informative): E.2 Example of a method to calculate the
settlements for spread foundations
The following is an example of a method to calculate the settlement s, of spread foundations using a semi-empirical method developed for MPM tests. The example was published by the French Ministère
de l‘Equipement du Logement et des Transport (1993). For additional information and examples, see
EN1997-2, Annex X, §X.3.2.
0 0 0 2 9 9 a d c v d c B λ B αλ B s q σ E B E where Bo is a reference width of 0,6 m;B is the width of the foundation; d, c are shape factors given in Table E.2;
is a rheological factor given in Table E.3;
Ec is the weighted value of EM immediately below the foundation;
Ed is the harmonic mean of EM in all layers up to 8B below the foundation;
v0 is the total (initial) vertical stress at the level of the foundation base;
q is the design normal pressure applied on the foundation.
Table E.2 — The shape coefficients, c, d, for settlement of spread foundations
L/B Circle Square 2 3 5 20 d c 1 1 1,12 1,1 1,53 1,2 1,78 1,3 2,14 1.4 2.65 1,5
Table E.3 — Correlations for deriving the coefficient for spread foundations
Type of ground Description EM/pLM
Peat 1 Clay Over-consolidated Normally consolidated Remoulded 16 9–16 7–9 1 0,67 0,5 Silt Over-consolidated Normally consolidated >14 5–14 0,67 0,5 Sand >12 5–12 0,5 0,33
Sand and gravel >10
6–10 0,33 0,25 Rock Extensively fractured Unaltered 0,33 0,5
CHAPTER 6
FIRE RESISTANCE ACCORDING TO EN 1992-1-2
Caroline MORIN and Fabienne ROBERT
Fire resistance according to EN 1992-1-2 C. Morin and F .Robert
6.1
Introduction
Fire is a definite danger to any construction and needs to be prevented and fought by all possible means. The fire may occur anywhere, in any session and in any phase in the lifetime of a building (construction, service, refurbishment or demolition).
The aim of this chapter is to give a general overview of the fire design according to Eurocodes (EN
1990, EN 1991-1-2 and EN 1992-1-2) through the example out of the concrete building. The fire load- bearing capacity of three concrete members (a column, a beam and a slab) will be determined. The global analysis of the overall structure is not covered.
EN 1990 concerns the basis of structural design. EN 1991-1-2 describes the thermal and mechanical actions for the structural design of building exposed to fire. EN 1992-1-2 describes the principles, requirements and rules for the structural design of concrete buildings for the accidental situation of fire exposure, including the safety requirements, design procedure and design aids. EN 1991-1-2 and EN
1992-1-2 are intended to be used in conjunction with EN 1991-1-1 and EN 1992-1-1.
In this chapter, the prescriptive approach is adopted (in opposite to the performance-based code), i.e. it uses nominal fires to generate thermal actions like the standard temperature-time curve (EN 1991-
1-2, Sec-3).
Fire resistance is defined as “..the ability of a structure, a part of a structure or a member to fulfil its
required functions (load bearing function and/or fire spreading function) for a specified load level, for a specified fire exposure and for a specified period of time…”.
The methods given in EN 1992-1-2 are applicable since concrete materials used in the building are normal weight concrete materials with strength class lass then the limit strength class C90/105. In this chapter the different methods given in EN 1992-1-2, Sec-4 are used:
o tabulated data (EN 1992-1-2, Sec 5);
o simplified calculation methods (EN 1992-1-2, Sec-4);
EN 1992-1-2 gives alternative procedures, values and recommendations for classes with notes indicating where national choices have to be made. Therefore the National Standard implementing
EN 1992-1-2 should have a National Annex containing the Eurocode all Nationally Determined Parameters to be used for the design of buildings, and where required and applicable, for civil engineering works to be built in the relevant country. For this example, the French National Annex has been selected.