Partial safety concept

The partial safety concept is a concept for the stability calculation of buildings and a result of the European standardization for the building industry . According to the Eurocode, the symbol for the partial safety factor in static calculations is the Greek letter gamma " γ ".

history

In order to reduce trade barriers and to harmonize tenders, a uniform European standard should be achieved. Therefore the nationally different technical rules had to be adjusted. This became the Eurocodes , the first generation of European standards in the 1980s. At the beginning of the work on the standards, there was no agreement as to which of the various national concepts available at the time should be used as a template for a European building standardization concept. A consistent concept that was new to everyone, the partial safety concept, was the only solution. The required safety level for a structure or component should be achieved by carefully examining all influencing factors and assigning partial safety factors to them - hence the name partial safety concept.

An explanation

A load (action) acts on a component, the size of which is statistically distributed, for example with a normal distribution or another statistical distribution, or is assumed in this way. The stress has a mean value or other characteristic value and scatters around this with a standard deviation . The resistance of the component to withstand this stress is also statistically distributed. It also has a mean and a standard deviation.

The two distributions overlap with a small amount of intersection. The component would fail in the area of ​​this intersection. In the construction industry, a very small failure probability of around 1 · 10 ⁻⁶ is generally acceptable, which means that one out of 1 million similar and similarly loaded components will fail. Therefore, the mean values ​​of stress and resistance must be so far apart that the intersection is so small that it corresponds to this desired low probability of failure. In this case one would have security 1, because the borderline case of minimum security has just been reached.

You also need a higher level of security. This is achieved by multiplying the actions or the stresses with partial safety factors (and thus increasing them) or dividing the resistances with other partial safety factors (and thus reducing them). Every action and every resistance has its own safety factor. Even with these partial safety factors, the failure probability must still be less than 1 · 10 ⁻⁶ , and this is where the verification consists. The partial safety factors must be determined for each influencing factor according to their statistical spread and according to the possible accuracy of their determination.

New norms

As part of European standardization and harmonization, a number of new, fundamental standards have already been adopted or submitted as drafts that are based on the semi-probabilistic partial safety concept. The earlier standards were based on a global (deterministic) security concept. This change has a significant impact on the dimensioning of structures and components, as the new procedure results in different calculation algorithms.

The new versions of various German standards already belong to the new generation of standards and are based on the partial safety concept, for example:

• DIN 1045-1 Structures made of concrete, reinforced concrete and prestressed concrete : dimensioning and construction draft
• DIN 1052 design, calculation and dimensioning of wooden structures
• DIN 1054 Permissible load on the subsoil
• DIN 1055 effects on structures
• DIN 18800 steel construction

EN 1990 “Fundamentals of Structural Planning” defines principles and requirements for the structural safety, serviceability and durability of structures, describes the fundamentals of structural design and provides information on the reliability requirements to be applied. This defines the basis for the European basic standards in construction EN 1991 to EN 1998.

trouble

The starting point for the new concept is widely accepted, but the difficulties are in the details.

The definition of the characteristic values ​​of parameters is of central importance for verifying the load-bearing capacity according to the partial safety concept. In general, the characteristic value of parameters should be a careful estimate of the mean value. However, the characteristic quantities and mean values ​​are often not known well enough because there are not enough samples for a statistical evaluation.

The unfavorable effects should be increased by certain partial safety factors - but it is not always clear which effects are favorable and which are unfavorable, for example with earth pressure .

Usually you have to prove different load cases with different degrees of safety. You have to make it plausible that you have to consider different material parameters in different load cases.

As with almost every major change in the standard, a design based on the current semi-probabilistic partial safety concept will differ from a design based on the old deterministic partial safety concept . Often, newer design methods allow more complex approaches that allow higher utilization and thus allow a higher load, but mostly the safety standards are also increased due to damage cases and also due to increasing safety requirements, which means that evidence of existing structures according to current standards is no longer possible it is possible.

Individual evidence

1. ↑ Construction standards committee (NABau) in DIN: DIN EN 1990 . In: DIN Construction Standards Committee (publisher): www.beuth.de . German version EN 1990: 2002 + A1: 2005 + A1: 2005 / AC: 2010. Beuth, Berlin 2011, p. 112 .