Design Situation and Load Combination

  this article is a continuation of the previous one about Action on Structure.

Limit State Design

Nowadays, as many international building codes do, Eurocodes follow the Limit State Design philosophy for structural designs, whereby the structure remains fit for usage during its designed lifetime by staying within the acceptable limit of safety and serviceability requirements. The two principal limit states for which the structure needs to be designed are the ultimate limit state, ULS, and the serviceability limit state, SLS.

According to EN 1990 clause 3.3(1)P, The Ultimate limit state is associated with collapse or other forms of structural failure, the safety of the people, and the structures. Whereas the Serviceability limit state [EN1990 clause 3.4(1)P], corresponds to the condition beyond which specified service requirement[ maximum compressive stress, crack width, and deflection] for structural or non-structural members are no met. Likewise, SLS concerns about the function of the construction work, the comfort of the people, and appearance.

The ULS shall be verified for the following condition [EN 1990 clause 6.4.1(1) P]:

1. EQU: Loss of static equilibrium (Stability) of the structure or any part of it when considered as a rigid body.

where,

          2. STR: Correspondes to internal failure or excessive deformation of the structure or its structural members, including footings, piles, basement walls, etc. STR generally concerns the strength of the materials of a structure, or the stability of its members.

                                                                 where,

 

3. GEO: Failure or excessive deformation of the ground or foundations on which the structure lays on.

Other conditions covered in general terms here are:

4. FAT: concerned with fatigue failure of the structure or its structural members.
5. UPL: concerns to loss of equilibrium of the structure or the ground due to uplift by water pressure
6. HYD: hydraulic heave, internal erosion and piping in the ground caused by hydraulic gradients.

       ULS Design Situations

According to EN1990, clause.3.2(2)P, The design situations under which ULS design checks may be performed are:

1. Persistent Design Situation, which concerns the conditions of normal use
2. Transient Design Situations, which concern temporary conditions applicable to the structure.
3. Accidental Design Situations, which concern exceptional conditions applicable to the structure or to its exposure including fire, explosion,  impact or the consequences of localized failure.
4. Seismic Design Situations, which concern conditions applicable to the structure when subjected to earthquakes and other seismic events.

The checking for serviceability limit state [SLS], should be based on criteria concerning:

      I. Deformations: (As adeflections) which affect the appearance of the building, the comfort of users, or the way the structure (including machines or services within it functions;
      II.  Deformations: which cause damage to finishes. cracking of plaster or non-structural members.
      III. Vibrations: which cause discomfort to people, or which limit the functional effectiveness of the building.

   Note: A distinction shall be made between reversible and irreversible serviceability limit state 

 Load Combinations

Load Combinations are linear multiple of load cases and are used when designing structures in agreeing with building codes. Each load case within the load combination is multiplied by a load factor, adjusting the weighting of that load type in combination with others. It is important to ensure you are correctly applying the relevant design load combinations to ensure the minimum design requirements of your local design code are being met.

 EN1990 Load Combinations

 

Combinations of actions for Persistent or Transient design situations

Based on the EN 1990 clause 6.4.2.1. equation 6.10.

 

where,                                     

    “To be combined with” can be used as both operations, one-time as an addition and the other time as substruction.

Variable load reduction factors, EN 1990 Table A1.1.

EN1990 also provides alternatively for STR and GEO limit states on equation 6.10a and 6.10b, the less favorable of the two following expressions:

Equation 6.10a [ the only difference from equation 6.10 is that a reduction factor ψ0,1 leading variable].

Equation 6.10b [ the only difference from equation 6.10 is because of a reduction factor for unfavorable permanent actions].

 

Combinations of actions for Accidental design situations

EN 1990 clause 6.4.3.3(2), defines the general load combination for accidental design situations.

The choice between ψ1,1Qk,1 or ψ2,1Qk,1 should be related to the relevant accidental design situation whether the action is impact, fire or survival after an accidental event or situation.

Combinations of actions for Seismic design situations

EN 1990 clause 6.4.3.4(2), defines the general load combination for accidental design situations.

In the end, It is important to ensure one is correctly applying the relevant design load combinations to guarantee the minimum design requirements of your local design code are being met.