Basis of design: EN 1990

Handling uncertainty

Nothing in the design of a structure is known exactly: not the actions, not the strength of the material, not the geometry, not the accuracy of the model.

EN 1990 gives a system for this: it does not calculate with “certain” values but with characteristic values and partial factors, which handle the uncertainty separately on each side.

• The action is increased by a factor.
• The resistance is reduced by another.
• The two are compared.

The limit states

Ultimate limit state

The state whose occurrence means collapse or a similarly serious event. The safety margin is large here, because the consequence is severe.

It covers strength, stability, overturning, sliding and failure of the connections.

Serviceability limit state

The state that threatens normal use, the appearance of the structure or the integrity of the finishes: excessive deflection, vibration, cracking.

The factors are milder here, because the consequence is less serious — but in a thin-walled structure this is often the stricter condition. Serviceability criteria.

Load combinations

Actions do not reach their maxima at the same time: it is improbable that record snow, record wind and a house full of people will coincide.

So the standard works with combination factors: one action appears at its maximum, the others at a reduced value. Several combinations have to be examined and the governing one taken. Partial factors and load combinations.

The reliability level

The standard scales safety to the severity of the consequences: a hospital or a building holding many people is designed to a higher reliability level than an agricultural building.

The design working life

Every structure is assigned a design working life, and the actions and durability requirements follow from it. Design working life.

Why this is worth understanding as a non-engineer

Because it shows that a structure does not carry everything but carries specified actions, chosen on a probabilistic basis, with a given safety margin. That is what makes it intelligible why it matters what goes on the floor, and why the designer has to be asked before an alteration.

The point

EN 1990 is the grammar of structural design. Understand it and the other standards are only vocabulary.