Cấu tạo thép tòa nhà - P3

Every building consists of a load bearing frame, usually created by reinforced con-crete, or by steel, or by the combination of both areas with usual earthquake activity. Although the load bearing frame is not visible after the completion of the construc

1.3 Frame loadings

The frame of a building is designed to constantly cope with all gravitational loads (self weight, walls, floors, cars, furniture, humans etc) It should also be capable to periodically cope with loads such as wind pressure and snow and even take into account possible thermal effects

In every building such as the above, there are constant (dead) and moving (live) loads which are imposed upon the frame Live loads are usually smaller than dead loads, e.g 3 people and all furniture

of the sitting room weight less than 1cm2 of a slab whistle a car weights almost the same as only one beam

Apart from coping with usual loadings, in areas with intense earthquake activity the

frame should be designed with a surplus of bearing strength distributed in such a

way that in the rare but crucial moments of an extreme earthquake to ensure the stability of the building and avoid extensive damages

The most characteristic attribute of an element is its mass The type of forces imposed upon an element depends on the force field that the element is surrounded from Earth’s gravitational field creates the force of gravity commonly referred as weight What is more, during an earthquake there are horizontal accelerations that create the horizontal forces of the earthquake These forces are imposed upon the building and create the loadings of the frame These loads can be categorized as gravitational loads, wind loads and earthquake loads

1.3.1 GRAVITATIONAL LOADS

The loads in a building are divided into permanent (dead) and moving (live) loads The dead loads consist of the self weight of the reinforced concrete structural elements, the weight of walls and the weight of all coverings and coatings As live loads we consider all loads from humans, furniture, vehicles etc

Dead Loads

The density and corresponding specific weight of the materials used in construction are as follows:

Reinforced concrete ρ=2.50t/m3 (ε=25.0 kN/ m3) Lightweight regulating concrete ρ=0.80t/m3 (ε=8.0 kN/m3) Sand mixture ρ=2.00t/m3 (ε=20.0 kN/m3)

The total dead loads of one m2 for the slab in the above picture is

g=0.15*2.50+0.04*0.8+0.02*2.0+0.02*2.7=0.5t Summarizing, the self weight of one cm2 of an ordinary slab is 0.5t (weight 5.0 kN)

Single wall ρ=0.21t/ m2 (ε=2.1 kN/m2) Double wall ρ=0.36t/ m2 (ε=3.6 kN/m2)

A wall with 1.0m length, 2.85m height and 10cm width has a mass of 0.6t (weight 6.0 kN)

Live loads

Loads from people:

Regular load of humans ρ=0.20t/m2 (ε=2.0 kN/m2)

The live load of one m2 of a house is 0.2t (weight 2.0 kN)

The loads from snow are usually smaller from the live loads of people and it varies between 0.6 and 1.5kN/m2

In order to assume an approximate value of the total loads imposed on a building we can calculate 1 m2 of a slab (1m x 1m)

In 1m2 of floor, the permanent (dead) loads are 0.5t (weight 5.0kN) and the moving (live) loads are 0.2t (weight 2.0kN) However if we also include in the calculation the loads imposed by beams, columns, walls and coverings, the total dead loads (self weight of the structure) are more than 10kN/m2, whistle the live loads remain unchanged ( 2kN/m2) What is more, in a certain time period during the total life span of the building, the possible extensive permanent loads will reach 100% of the predicted, whistle the total extensive live loads will not go over 30% of the predicted Therefore, it is obvious that the dead loads are much more than the calculated live loads This is a major drawback of reinforced concrete structures since there are excessive dead loads required in order to support significantly smaller live loads

The maximum live loads of a house are approximately 20% in proportion to the dead loads

In a random time instance, e.g during an earthquake, the extensive live loads are approximately 6% in proportion to the dead loads

1.3.2 SEISMIC LOADS

The determination of the influence of an earthquake upon a building is always performed by taking into account that the supposed earthquake will impose a horizontal ground acceleration of A = a * g The Greek earthquake region is divided into 3 seismic zones I,II,III Each of the zones is assigned a different value of the ‘a’ coefficient: For zone I the coefficient is 0.16, for zone II is 0.24 and for zone III the coefficient is 0.36

A mass M gives a gravitational force of W At the moment of the earthquake, there is also a horizontal seismic force H, which is calculated as a percentage of the W force

This percentage usually varies between 0.0 and 0.5 However, during intense earthquakes, it can go over the value of 1

The value of ε is directly proportional to the value of the coefficient ‘a’, of the horizontal seismic acceleration It is also influenced by factors such as the type of soil, the geometry of the frame and the mass concentration in the building

In general, the shape of the distribution diagram of seismic accelerations resembles a triangular distribution

The triangular distribution of seismic accelerations

In the above building, situated in zone II, the average seismic acceleration has a value of 0.12g (ε0=0.12) whistle the resultant seismic force Fs, has a value of 1400kN This force is theoretically imposed approximately on the 2/3 of the total

height of the building

1.3.3 Wind Loads

If the same building was situated in an area of frequent intense winds, the average pressure from these winds would be 1.5kN/m2 and the resultant force from wind pressures would be 400kN, in proportion to the total design loads of 12000kN (the percentage of the wind pressures in proportion to the total gravitational loads is 0.03)

The theoretical point of imposing the resultant wind force is approximately the middle of the height of the building Therefore, in the particular building, the influence of wind

pressures is approximately 4 times lower than the influence of seismic accelerations

Buildings made out of reinforced concrete have a high self weight, both during the design and the functioning mode, and is therefore less influenced by winds, in comparison to structures made out of wood or steel

In earthquake resistant buildings, it is assumed that there is either an influence from winds or an influence of seismic forces Therefore, we assume that there is no simultaneous influence of both wind and seismic forces The influence of an earthquake upon a building is significantly higher than the influence of wind and that is why earthquake resistant buildings are designed to cope only with seismic and not wind forces Especially throughout the Greek earthquake region, the influence of wind pressures is significantly lower than the influence of seismic forces even in the zone with the lower intensity of earthquakes