Actions on structures.
Actions on structures are divided into permanent, variable and accidental. The correct design of the structures will take them into account, since their function is to support them.
The aim is to obtain a reliable structure, reliability being understood as a low percentage of failures.
The relationship between reliability and percentage of failures is related by means of a formula: Ψ(β)=1-Pf; being the reliability “Ψ(β)” and the percentage of failures “Pf”.
Types of actions structures.
- The permanent actions in a structure are those produced by the building's own weight, due to its materials: the structure itself, the carpentry, the cladding, the enclosures, the fillings and the fixed equipment. They are called permanent because they do not move. Their values are taken using reference tables.
- The variable actions can be: overloads of use, snow or wind. As its name indicates, they change over time, which is why they are calculated using approximate values in Kn/m2. Depending on the use of the building the values change.
In overloads due to snow, the geographical location of our building comes into play. Coefficients are used for this, being that of Murcia at 40 0.2 Kn/m2 and that of Avila at 1113 masl 1kn/m2. Therefore, the influence of height and the possibilities of snow when increasing the bearing capacity of the structure can be observed. These data therefore depend on the altitude, and the climatic zone where it has to be built and the altitude at which it is located.
Wind overloads are taken into account along their main axes. The action of the wind is measured with the expression: qe = qb Ce Cp, where qb is the dynamic pressure of the wind, Ce is the expansion coefficient and Cp is the pressure coefficient. The dynamic wind pressure varies according to the formula qb=0.5 δ Vb2; where "δ" is the air density, whose value is 1.25 Kg/m3 and "Vb" is the wind speed, which is taken from the following map, in which there are three zones A, B and C.
The exposure coefficient Ce varies with the height and the degree of harshness of the environment, although in urban buildings of up to 8 floors the value 2 can be taken.
The dynamic wind pressure, "qb" is also taken from the map, and depends on the area.
Accidental action is caused by earthquakes. In the south of Spain there is an important seismic zone due to the meeting of the Eurasian plate with the African plate. That is why seismic actions are taken into account since the probability of earthquakes is high.
An earthquake is caused by sliding between tectonic plates. The released energy is transmitted through the so-called "P" waves from the hypocenter in the lithosphere, to the surface, where the energy is released producing "s" waves, which are the ones that cause the damage. Since the "p" waves produce compression of the ground, and the "s" waves, the area of the surface closest to the hypocenter is called the epicenter.
The standards to be taken into account in earthquake actions are: NCSE-02. Building and EHE-08 Annex 10. The ductility of the structure is important, which is why in an earthquake-resistant structure it is best to have many plastic hinges. This better resists the torsional movements produced by the “s” waves of the earthquake.
Building design and actions on structures.
It should be noted that the design of the building influences the resistance to earthquakes, the symmetries and volume variations can prevent that while one part enters into resonance with the earthquake, another supports that part. In addition, options such as load-bearing walls better withstand twisting. What would avoid disproportionate sections in the pillars.
There are other actions on structures. As the reduction of loads of use or those caused by parapets and railings