Basis of design

Serviceability limit states are defined as:

States that correspond to conditions beyond which specified service requirements for a structure or structural member are no longer met

Verification of serviceability involves checking that design effects of actions (e.g. settlements) do not exceed their corresponding design limiting values (i.e. limiting settlements).

Verification of serviceability is expressed in Eurocode 7 by the inequality:

Ed < Cd [EN 1990 exp (6.13)] & [EN 1997-1 exp (2.10)]

in which Ed = the design effects of actions and Cd = the limiting design value of the relevant serviceability criterion.

Examples of situations where serviceability is a concern are shown in Figure 8.1 overleaf. From left to right, these include: (top) the settlement, s, of a pad footing due to its self-weight and any imposed loads must not exceed the project-specific limiting settlement and the differential settlement, As, of foundations for a framed structure must be within specified limits; (middle) the horizontal deflection, 5, of a retaining wall due to unbalanced earth pressures must be within specified limits and vibration due to machinery must not cause discomfort to neighbours; and (bottom) the rate at which water is pumped from a basement must be sufficient to prevent heave underneath the basement and the capacity of the pump, on the downstream side of a dam, must be sufficient to remove water flowing underneath the dam.

Figure 8.1. Examples of serviceability limit states

8.1.1 Effects of actions

'Effects of actions' (or 'action effects') is a general term denoting internal forces, moments, stresses, and strains in structural members - plus the deflection and rotation of the whole structure. [en 1990 ยง1.5.3.2]

For serviceability limit states, the effects of actions are the various forms of foundation movement shown in Figure 8.2: settlement (s), rotation (0), angular strain (a), and tilt (rn) - as well as differential settlement (5s), relative deflection (A), and relative rotation or angular distortion (P). The ratio A/L

is known as the deflection ratio. Most of these terms are already in widespread use in geotechnical practice.1

Figure 8.2. Definition of various forms of foundation movement

8.1.2 Limiting serviceability criteria

Annex H of EN 1997-1 also provides guidance on allowable deformations in open-framed structures, infilled frames, and load bearing and continuous brick walls (see table below). These guidelines apply to normal structures, not to extraordinary structures or to structures subject to markedly nonuniform loading.

Movement

Maximum movement to avoid limit state

0 0

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