Figure 7.13. Box caisson subject to uplift forces hf

Notes on Example 7.3

O The partial factors given here are taken from EN 1997-1 Annex A4 and provide no margin on destabilizing permanent actions. (Different values are specified in the UK National Annex, as discussed under © below.)

© The design is (just) verified against uplift according to EN 1997-1. The equivalent lumped factor of safety of 1.13 would probably be regarded by most engineers as too low.

© The UK National Annex to BS EN 1997-1 increases the partial factor on destabilizing permanent actions to 1.1 and UPL is no longer avoided. The depth of ballast must be increased to meet the requirements of the UK National Annex.

© An acceptable design is achieved by increasing the depth of ballast to 3.9m. The traditional factor of safety is then 1.26, which is a more acceptable level of reliability.

© The Eurocodes allow floods to be treated as accidental design conditions, for which the partial factors reduce to 1.0. So, although the destabilizing water uplift is increased by the flood, a depth of ballast of 3.9m still produces a satisfactory design against uplift.

Example 7.3 Box caisson Verification of stability against uplift (UPL)

Design situation

Consider a box caisson of width B = 15m and height H = 7m which is to form a crossing over a river. The depth of water in the river is hw = 5.7 m. The caisson is to be filled with ballast, with characteristic weight density kN

Yk = 18-, up to a height hf = 2.2m. The characteristic weight density of m kN kN water is yw = 9.81-and of reinforced concrete Yck = 25-(as per EN

1991-1-1). The thickness of the caisson's walls, roof, and floor are assumed to be t = 0.4m.


The characteristic destabilizing water uplift beneath the caisson is:

The characteristic self-weight of the structure is approximately:

The characteristic self-weight of the ballast (ignoring the volume taken up by the caisson's walls) is approximately:

Effects of actions - permanent and transient design situations According to EN 1997-1, partial factors on destabilizing and stabilizing permanent actions are yg dst = 1 and yg stb = 0.9 respectively.®

The design destabilizing vertical action is Vd dst = Yg dst * ^Gk = 838.8-

m kN

The design stabilizing vertical action is Vd stb = Yg stb * WGk = 839.8-

The degree of utilization is

The design is unacceptable ifthe degree of utilization is > 100% The traditional lumped factor of safety for this design is F =

The degree of utilization is

0 0

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