3.6. Impacts on buildings and structures
Under the impact understand the dynamic contact interaction of different bodies. Emergency shock loads on buildings and structures, as well as seismic and emergency explosive ones, are attributed to special impacts, since they are characterized by high intensity and rare frequency. According to various estimates, losses from accidental attacks range from tens to hundreds of millions of dollars per year, in addition, they are often accompanied by loss of life.
Emergency impacts can take place in various fields of technology, including in the industrial, civil, energy, transport and other construction sectors. Practice shows that the increase in the number and variety of types of emergency impact impacts is 10-30% per year.
A blow can occur, for example, when erecting a multi-storey building or a multi-tier or high structure in the event of an emergency break in a sling, with a careless fixing of the load, an incautious turn of the crane, etc.
The constructions of the built-in floors of single- and multi-storey industrial buildings with vertical technological process under the fall of loads carried by bridge or overhead cranes, which are usually equipped with such buildings, are subject to emergency influences.
Horizontal strikes are subjected to columns of garages, pillars of industrial and transport trestles, street lamp posts for vehicle collisions, bridge and moorage support in bulk ships, etc.
Given the severity of the consequences, a special group of structures undergoing a whole range of emergency impact impacts are unique and costly objects such as nuclear power plants and offshore oil and gas facilities (platforms).
Protective covers of nuclear power plants - the most critical elements of these structures - are calculated for the crash of an airplane in an air crash, impacts by objects carried by hurricanes, as well as on internal shock effects arising from accidents in reactor support systems and when loading nuclear fuel.
Offshore oil production platforms are subjected to impacts from ships, ice floes, icebergs, and falling elements of drilling and production equipment.
There are two more extensive groups of crashes: the first is strikes caused by the spread of equipment and structures during internal industrial explosions, the second - the blows caused by the collapse of structures or equipment in earthquakes or external explosions.
Finally, a constant source of accidental attacks is the construction of prefabricated elements, which has prevailed for many years in our country and is becoming increasingly popular abroad. Vertical impacts by mounted elements, resulting from malfunctions in installation equipment, usually lead to a progressive collapse of the erected building or part thereof. Horizontal impacts by precast elements on the erected part, caused by unsuccessful maneuvering of the crane are also very common.
Most of the accidental impacts are caused by bodies (drums) of considerable mass (up to several tons) with relatively low speeds (m/s, tens of m/s). From the standpoint of the reaction of the design, this is distinguished by emergency strikes from the high-velocity bombardments and projectiles that have been studied by military engineers through protective structures, since it allows one to use methods based on the representations of classical dynamics. At the same time, the requirements for the safety of structures and structures experiencing emergency shocks are close to the requirements for impacts on structures where significant plastic deformations can be tolerated. The main difference here is that in the case of impacts it is necessary to take into account not only common, but local (local) deformations and damages that can cause the design failure. The failure in the latter case is understood as the penetration of the structure (Figure 3.2, a) or the deflection of its part (Figure 3.2, b), which occurs in reinforced concrete slabs and shells in as a result of the appearance of a stretching reflected from the lower surface of the wave. The punctured concrete fragment can act as a secondary impactor and pose a threat to people and equipment under construction.
Fig. 3.2. Scheme of local limit states when hitting the plate: a - punching; b - splitting off part of the construction
The safety margin for the general action of the impact is determined by the ratio according to inequality (3.2); the maximum movement is determined from the dynamic calculation. With the local action of impact on a reinforced concrete slab or a shell, the corresponding condition is as follows
where b - the thickness of the structure; bmin - minimum (threshold) thickness of the structure, in which there is still no break-off. The value of bmin is found from the relationships:
where ; R - cube strength of concrete; d - diameter of the contact zone (drummer); M s is the mass of the drummer; V 0 - speed of the impactor at the time of contact with the structure; N - coefficient, depending on the shape of the bow of the striker.
The permissible thickness of the structure is assumed to be greater than that defined by conditions (3.4) and (3.5).
A special case is the definition of the security boundary of reinforced concrete columns subjected to impacts in the lower part (from vehicles):
where F md (t) is the maximum dynamic axial force in a compressed inclined strip between cracks (Figure 3.3); Fred - the ultimate force corresponding to the beginning of destruction of concrete in the compressed zone.
Fig. 3.3. Scheme of cracking of a reinforced concrete column with impact in its lower part
The determination of the safety boundary can also be carried out on the basis of a risk analysis. The risk of failure of a structure or structure is usually determined by the formula
where P 1 - the probability of an emergency impact load determined by Poisson's law; P 2 - the probability of failure of the design in question; P 3 - the probability of achieving at least one limit state (general, local) in the given construction, which can be is taken equal to one if the limiting state has come, and to zero, if not.
Protection against accidental impacts, as well as other special dynamic effects, can be passive and active.
Passive shock protection reduces to strengthening the elements to perceive additional forces caused by shock dynamic load. Constructive schemes of elements and buildings as a whole do not fundamentally change.
If the gain is economically inefficient (given the low probability of accidental impacts) or impossible for other reasons, it is advisable to use active shock protection methods aimed at repaying the impact energy. In this case, additional elements can be introduced into the structural design of the element or structure or it can be changed. In Fig. 3.4, а is an example of active shock protection of support platforms in the form of thin-walled metal cylinders (barrels) reinforced on the ground by crossing metal strings. In Fig. 3.4, b, a variant is proposed in which the metal membrane plays the role of the trap of the falling cargo piercing the shelf of the reinforced concrete slab. Fastening the membrane to the overlap strands of mild steel play the role of elements that absorb the impact energy.
Fig. 3.4. Examples of active impact protection of structures:
1 - protective element; 2 - support; 3 - monolithic concrete; 4 - prefabricated reinforced concrete floor panel; 5- heavy; 6 - metal membrane
The decision to choose the type of protection is based on a feasibility study in each case.
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