Sample Section: 3 Design principles

3.1 Actions

3.1.1 Types of action

BS EN 199014 distinguishes between permanent actions (dead weight of structure, finishes, permanent fixtures and fixed partitions) and variable actions (every other type of action). Actions involving exceptional conditions of the structure or its exposure (including fire, explosion, impact or local failure) are also defined but dealt with separately under ‘accidental design situations’.

3.1.2 Characteristic values of actions

For permanent actions it is usually acceptable to use an average weight as the characteristic value or, if a range of weights is specified, the highest value in the range. For variable actions, an upper characteristic value appropriate to the design life (e.g. once in 50 years) is normally used. However, in ULS load cases where actions can have a favourable effect (e.g. tile weight resisting overturning), the lowest value should be used for permanent loads if a range of weights is specified. Other adjustments for favourable effects are made by means of the partial load factors (Table 3.1).

The values to be used in calculations are:

• characteristic dead load16

• characteristic imposed load16

• characteristic snow load32

• characteristic wind load33

Note a Values for unfavourable effects are for main values. Values for favourable effects are for actions which relieve some of the load on a member or tend to stabilise a member or structure. For the design of elements in a foundation, e.g. timber piles, see Clause A1.3.1(5) of BS EN 1990, approach 1 b The combined check is an optional alternative to separate calculations for equilibrium and strength verifications when both have to be carried out. However, if it is employed then it must also be verified that setting γG to 1.00 for both the favourable and unfavourable parts of the permanent load does not produce a less favourable effect.

Derived/adapted from Tables NA.A1.2(A) and NA.A1.2(B) of NA to BS EN 1990

3.1.3 Design values of actions

For fundamental ULS the characteristic values of actions are converted to design values by the partial load factors shown in Table 3.1. Additional factors applicable to snow and wind loads are given in the relevant codes. For accidental design situations the load factors for permanent and variable actions are set at 1.0. For SLS the load factor is 1.0, unless the load is favourable (e.g. an imposed load on the second span of a two-span joist) in which case a value of 0.0 should be used.

3.1.4 Action combinations

Where more than one variable action acts simultaneously, a leading variable action is chosen, and the others are reduced by specified combination factors. In cases where the leading variable action is not obvious, each action should be checked in turn to determine the combination which produces the worst effect. Other factors are used to determine the design value of accidental load combinations, and the average value of variable loads in the calculation of creep deformation. All these factors, normally referred to as ‘psi’ factors, are shown in Table 3.2. For examples of their use, see Section 3.2.

a The roof imposed load need not be applied at the same time as wind/snow loads, particularly for Category H roofs (Clause 3.3.2(1) of BS EN 1991-1-1).

from Table A1.1 of NA to BS EN 1990

Clause 3.3.1 of BS EN 1991-1-1 states that when imposed loads act simultaneously with other variable actions (e.g. wind or snow) the total imposed loads on the floors of multi-storey buildings shall be considered as one variable action. For timber structures this requirement is applicable principally to the instantaneous load case (wind + snow + floor imposed) and to the short-term load case (snow + floor imposed). In addition, Clause 6.3.1.2 states than for building ‘‘categories A to D, for columns and walls, the total imposed loads from several storeys may be multiplied by a reduction factor α n .’’ α n is applicable to buildings with four or more storeys, and its values should be taken from the National Annex to BS EN 1991-1-1. For timber, the medium-term load case (floor imposed) may be critical, and since this does not involve other variable actions the design value of the individual imposed loads from two or more floors may be calculated as Qk,1 + Σi>1ψ0, Qk,i. This value may be reduced by α n if and where it is applicable.

3.2 Limit states

This Manual adopts the limit state principle and the partial factor format common to all Eurocodes and defined in BS EN 1990.

3.2.1 Ultimate limit states (ULS)

3.2.1.1 Definitions

ULS are breached by loss of equilibrium or material failure. All relevant states should be checked.

• equilibrium:

• uplift of whole or part (e.g. roof) of structure

• overturning

• sliding

The stability of the foundations should be checked in accordance with BS EN 1997-134.

• strength:

• material rupture in bending, tension, compression or shear

• stability failure (e.g. buckling of columns, lateral torsional buckling of beams, stresses induced by sway in portals, diagonal bracing in roofs or walls)

• connection failure — yield in fasteners or metal-work, or timber failure by embedment, shear, splitting, fastener pull-out or head pull-through

Where time-dependent deformations (deflections of members or slip in connections) cause a significant redistribution of effects in an assembly or framework, the strength of the members should be considered both before and after creep deformation has occurred.

Fundamental design situations occur during the construction phase and in normal use. Accidental design situations occur at or following an exceptional event such as fire, exceptional snowdrift, impact or explosion. All relevant design situations should be considered.

3.2.1.2 Load cases

The load timber can resist depends on the duration of the load. Therefore, a duration should be assigned to every action (Table 2.2). When several actions occur simultaneously, a separate load case should be considered for each load duration represented, unless it is obvious that it will not govern the design. The duration of each load case should be taken as the shortest load duration of any load in the load case.