Progressive collapse: norms, calculation and recommendations

2024 Author: Angel Austin | [email protected]. Last modified: 2023-12-17 05:14

The topic of progressive collapses is relevant and mentioned today. Until now, people are horrified by the well-known catastrophe of this kind, which occurred on September 11, 2011 in New York. Millions of people watched on video these tragic events that claimed the lives of 2977 people.

At 8 hours 46 minutes 40 seconds in the direction from the north between the 93rd and 95th floors of the North Tower of the World Trade Center, a terrorist-driven Boeing 767 (Flight 11) crashed. At 09:30:11 between the 78th and 85th floors from the south, the South Tower of the World Trade Center was pierced through by a Boeing 767 (Flight 175) at a speed of 959 km/h.

Progressive collapse (PO) of the South Tower of the World Trade Center occurred 55 minutes and 51 seconds later, at 9 hours 58 minutes, and the North Tower - after 1 hour 41 minutes 51 seconds, at 10 hours 28 minutes. In both skyscrapers, the structural elements holding the floor ceilings, the floor trusses of the impact area were destroyed.

Unfortunately, most POs happen because ofinadequate control of building maintenance. Thanks to the press, we learn about the facts of the collapse of residential entrances, which, unfortunately, are the most frequent.

Note that in the American example, the destruction occurred due to an extraordinary event, and the design of the twin towers met the technical requirements. Accordingly, neither the builders nor the designers had the opportunity to foresee this kind of directed impacts, which produced local destruction, leading to critical chain destruction and, as a result, the collapse of buildings. However, according to statistics, in most cases software occurs under the influence of factors that can be calculated. In addition, scientists and engineers have developed effective methods for calculating the structure of buildings that are less susceptible to such critical damage.

History of the progressive collapse category

The term itself appeared in 1968 after the work of the building commission, which studied the complete destruction of the 22-storey London building "Ronan Point" by a household gas explosion. British designers took this tragedy as a challenge to their professionalism. The scale of the tragedy, which caused dozens of civilian casu alties in peacetime, resonated with society. As a result of engineering surveys in 1970, amendments to the legislation were proposed for parliamentary consideration - a new edition of building codes. The changes were based on the principle of proportionality of the accident to the local impact leading to collapses.

For this, it is the responsibility of the designerswas imputed to the calculation of the progressive collapse. The need for it since 1970 was regulated by law and, accordingly, since then in Britain it has been strictly implemented. Thus, it was normatively established:

1. Even at the design stage, the possibility of dangerous local destruction should be considered.
2. The number of articulated joints is reduced as much as possible, and the degree of continuity for the structure is increased.
3. Building materials with plastic deformation are selected.
4. The design includes elements that are not load-bearing during normal operation, but in case of local destruction, performing (fully or partially) load-bearing functions.

Protection of buildings from progressive collapse is carried out comprehensively, taking into account all these factors. A year ago, a Russian set of rules was developed that regulates compliance with the conditions of survivability of buildings and structures at the stages of their design, reconstruction and overhaul.

Relevance of the problem. Reasons

As evidenced by the software statistics, such a global destruction occurs due to the effects of corrosion, force or deformation nature. Options for such man-made events can be:

1. Groundwater flooding.
2. Erosion of the foundation due to accidents on water lines.
3. Destruction of structural elements due to their overload or due to explosion, collision.
4. Weakening of the structure of materials due to corrosion.
5. Errors in the project when calculating fasteners and load-bearing elements.
6. Explosiongas go fire.

Progressive failure often occurs due to brittle fracture with an increase in the number of microcracks. Obviously, the first case of such destruction, which occurred in 23 AD. e. with the amphitheater of the city of Fidena, described by the historian of Ancient Rome Cornelius Tacitus. The PO that arose on the day of the gladiatorial structures in a crowded building, according to the testimony of this chronicler, took as many lives as a war would. We are talking about several tens of thousands of people.

Let's take a later historical example. Progressive collapse with an increase in the number of microcracks caused the collapse in 1786 of an arch bridge across the River Wye (Great Britain, Herefordshire). Another arch bridge called Lsen-Beneze across the Rhone River (France), built in the 12th century, collapsed so many times due to the adverse effects of the environment and internal degradation so often that in the 17th century it was stopped being restored (different spans of the bridge collapsed 1 time - in 1603, 3 times - in 1605, 1 time - in 1633 and in 1669 - finally).

It should be noted that modern urban planning technologies, unfortunately, have not deactivated the progressive collapse of buildings and structures. Sad statistics continue into the 21st century:

1. 1999-08-09 - terrorist attack - an explosion of 350 kg of TNT, which brought down two entrances of a nine-story building on the street. Guryanov (Moscow) and led to the death of 106 people.
2. 2002-02-07 - domestic gas explosion withepicenter on the 7th floor of the landing of a nine-story building on Dvinskaya Street (St. Petersburg), which led to the death of two people.
3. 14.02.2004 - the collapse of the roof of the Transvaal Park with an area of about 5 thousand m22, which led to the death of 28 people.
4. 2007-13-10 - domestic gas explosion in the house on the street. Mandrykovskaya (Dnepropetrovsk) destroyed the third entrance of a residential building and led to the death of 23 people.
5. 27.02.2012 - A gas explosion initiated by a suicide collapsed the entrance to the house on N. Ostrovsky Street, ten people were killed.
6. 20.12.2015 - gas explosion in the house on the street. Cosmonauts (Volgograd), 3 apartments destroyed, one person died.

Regulations

Before considering the problem, it would be logical to get acquainted with the regulatory documents that consider it and organize appropriate prevention. Protection of buildings and structures from progressive collapse in the Russian Federation is regulated by regulatory documents, the list of which is presented below:

1. Manual for the design of residential buildings. Issue. 3. Structures of residential buildings (to SNiP 2.08.01-85). - TsNIIEP housing. - M. -1986.

2. GOST 27751-88 Reliability of building structures and foundations. Basic provisions for the calculation. - 1988

3. GOST 27.002-89 “Reliability in engineering. Basic concepts. Terms and Definitions". - 1989

4. Recommendations for preventing progressive collapse of large-panel buildings. - M.: GUP NIATs. - 1999

5. MGSN 3.01-01 "Residential buildings", - 2001, paragraphs 3.3, 3.6,3.24.

6. NP-031-01 Design Code for Seismic-Resistant Nuclear Power Plants, 2001

7. Recommendations for the protection of residential frame buildings in emergency situations. - M.: GUP NIATs. - 2002

8. Recommendations for the protection of buildings with load-bearing brick walls in emergency situations. - M.: GUP NIATs. - 2002

9. Recommendations for the protection of monolithic residential buildings from progressive collapse. - M.: GUP NIATs. - 2005

10. MGSN 4.19-05 Multifunctional high-rise buildings and complexes. - 2005 paragraphs 6.25, 14.28, Annex 6.1.

Recently, the problem of software has found more complete coverage in the latest domestic regulatory sources. Any construction documentation for buildings with a normal and increased level of responsibility must necessarily take into account the requirements of the set of rules (SP) 385.1325800.2018, which regulates the protection of buildings from progressive destruction.

Software and bearing capacity of buildings

According to paragraph 4.1 of these rules, the customer has the right to initially require the inclusion in the design of the building (structure) under construction of additional elements that increase the bearing capacity of the structure.

The same joint venture "Calculation for progressive collapse" most fully presents in two options for designing protection against software during major repairs. The first - in the case of overhaul of buildings and structures of an increased level of responsibility and the second - for the same objects of a normal level of responsibility. In the first case, the bearing capacity increases by a factor ofsecond.

The main condition for compliance with the requirements of software protection is compliance with the condition of exceeding the bearing capacity of structural elements and their connections over the forces leading to local collapses in these structural elements and connections. If any design does not meet this requirement, then it should either be strengthened or replaced.

If we are talking about the reconstruction of buildings (structures), then first they must be technically inspected in accordance with GOST 31937, and only then the reconstruction itself is carried out as a whole, or within the boundaries of expansion joints (depending on the chosen reconstruction strategy).

Sector of local destruction

Diagnosing the survivability of buildings in relation to software, planners at the design stage detail its possible sources - points of local destruction. Each such destruction is considered by them separately and spatially. In particular, the calculation for progressive collapse considered by us begins with the forecast of local destruction sectors in the design of load-bearing structures:

• for buildings and structures up to 75 m high, they are limited to a circle with a diameter of at least 6 m;
• for buildings and structures from 75 m to 200 m in height - a circle with a diameter of at least 10 m;
• for buildings and structures over 200 m in height - a circle with a diameter of at least 11.5 m.

For multi-storey, large-span buildings, local damage is considered in the form of damage to any of the supporting structures. In this case, the zone of local destruction should be localized by the structure and in no case should it develop into software.

SP "Protection of buildings from progressive collapse" provides for preventive measures to prevent global destruction of this kind:

• taking into account the maximum number of probable local destruction;
• use of materials and structures prone to plastic deformation
• increasing the static indeterminacy (SN) of the structure (increasing the level of its non-sparseness, reducing the number of hinged elements).

Forcibly using a special term, let's explain it. CH-systems - a complex characteristic of the interaction of the building structure and the forces applied to it. In other words, in SN systems, in contrast to statically determined ones, the distribution of forces depends not only on the external forces applied to buildings (structures), but also on the distribution of these forces on structural elements, which, in turn, are characterized by elastic moduli.

It is the operating load-bearing structural elements (the so-called connections) under local influences that prevent the transformation of an integral statically indeterminate system into a geometrically changeable one (the latter implies the possibility of software). Thus, it is the bonds that make progressive collapse impossible. Building codes - that's what should take into account and regulate the prevention of software.

Briefly about normative documentation

You are obviously wondering whichsoftware regulatory documentation is the most advanced in the world. It should be recognized that, despite the domestic developments of recent years, the consideration of software counteraction today is most detailed (relevance - 2016) in the American standards UFC 4-023-03 and GSA.

The fact is that they take into account the latest building materials, as well as various building designs. At the same time, the Russian collection E TKP 45-3.02-108-2008 was compiled on the basis of recommendations written in the 2000s regarding reinforced concrete structures.

Note the clear progress of the Russian regulatory documentation in recent years and the obvious efforts to streamline the existing disparate and numerous sources of norms. However, it will be fair to say about the shortcomings. Take at least the normative documentation. Experts note that today different sources of domestic regulatory documentation are often contradictory and also contain flaws. Here are just a few examples:

1. In GOST 27751-88, clause 1.10, “Regulation” goes at the level of “any structural element”. (Allow me, we need to be specific, because we are talking about human lives!)
2. STO 36554501-024-2010 "Ensuring the safety of large-span structures …" (It is erroneously stated in paragraph D.3 that the choice of software calculation should be determined by special technical conditions. Such logic is absurd).
3. In SNiP 31-06-2009 "Public Buildings and Structures" in paragraph 5.40 it is mentioned that the design should "consider design situationsof a terrorist nature." (But this is a dead end. Suppose the designers check the local destruction of a column on one floor, but the terrorists put explosives under two columns. In the same place - paragraph 9.8 - again the regulation goes at the level of "any structural element.)
4. STO-008-02495342-2009 “Reinforced Concrete Building Software Prevention”. (The document is criticized. In principle, neither the dynamics of software nor plastic deformations are considered.)

Obviously, this list can be continued. The progress of the construction industry, which has significantly accelerated in recent years, has led to the obsolescence of most of the existing regulatory documents regulating the field of software. Obviously, the effective prevention of progressive collapse will soon require adaptation to domestic realities of the already generalized foreign experience. This refers to the US standards UFC 4-023-03 and GSA, which contain not vague, but very clearly formulated requirements for the structures and materials of specific types of buildings.

Unfortunately, many domestic experts consider the joint venture “Protection of buildings from software …”, joint venture “Buildings and structures. Special Impacts).

High-Rise Software Recommendation Features

In particular, it regulates the progressive collapse for high-rise buildings considered by us. The peculiarity of the calculation of software for high-rise buildings is determined by a wider step in the location of walls or columns. At the same time, the general design, in the event of an emergency impact, allows local collapse of load-bearing elements, but only within one floor,without further chain continuation of this destruction. The collection of rules contains recommendations regarding the design and construction of new, as well as the verification and reconstruction of already constructed high-rise buildings and structures. (For reference, the height criterion is a height of more than 75 m, which is equivalent to a 25-story building.)

Calculation by the limit equilibrium method

The design of a high-rise building is calculated based on the assumption that under the influence of local destruction it is transformed into a state conditionally called "limit states of the first group". Let's explain this term. The limiting state is called such a state of the structure when it ceases to resist destruction or is damaged (undergoes deformation). In total, two groups of limit states are distinguished. The first is conditionally called the state of complete operational unsuitability. The second is called the state of damage, which allows partial exploitation.

Technically, the calculation is made by modeling the non-linear stiffness characteristics of a high-rise building structure by a system of differential equations. The calculation of a high-rise building is based on the construction of a spatial model, which takes into account non-bearing elements, but capable of taking on the redistribution of efforts under local influences. In this case, the stiffness characteristics of the structural elements adjacent to the fracture site are taken into account. The calculation model itself is calculated many times, each time taking into account a specificlocal destruction. This method allows you to achieve the most reliable results. At the same time, in the built model, the factor of reducing excess material costs is considered.

How is a spatial model analyzed? On the one hand, the forces in structural elements are equated to the maximum possible, which can be sustained by them. It is believed that the progressive collapse of high-rise buildings becomes impossible when the forces are less than the bearing capacity of the structure. If the strength requirements are not met, then the bearing capacity of the building must be strengthened by additional or reinforced load-bearing elements.

Ultimate forces in elements are determined differently: for the long-term part of the effort and the short-term part.

Kinematic method

If the structure of a high-rise building is plastically deformed, then the kinematic method becomes relevant for software calculation. In this case, the calculation of the building is carried out as follows:

1. The most possible variants of software are considered, and for them a set of destructible bonds is determined, and possible displacements in the resulting plastic hinges are calculated. (A plastic hinge is a section of a beam or other structural element in which plastic deformation occurs under the influence of forces.)
2. Calculation for progressive collapse considers the ultimate forces that any structural element can withstand, including plastic hinges.
3. As a result - internal forces determined by strengthstructures must exceed external loads. Such a check is carried out both within the same floor and throughout the structure. In the latter case, the possibility of simultaneous collapse of the floors is being investigated.

If the material from which the structural element is made is not capable of plastic deformation, then this element is simply not taken into account in the calculations.

Study of possible software development after local destruction

Progressive collapse guidelines advise designers to explore four typical software development scenarios:

1. Simultaneously, all vertical structures located above the local destruction are shifted down.
2. Simultaneous rotation around its axis of all structural parts located at levels above the local destruction. The destruction of bonds is considered, since overlaps and vertical bonds are shifted in the complex.
3. A vertical structure was knocked out, and a partial collapse of the ceiling above it occurred.
4. Only structures above the floor above are displaced.

SP "Progressive collapse protection" mainly provides for the prevention of the development of these four scenarios.

Modular building software recommendations

In case of volume-block (modular) construction, a significant part of the processes is carried out in the factory. Installation is also facilitated by the fact that the blocks have a certain volume. Therefore, the modules that make up the structure are obviously made of materials that are not very susceptible to destruction. Corrosion of materials is prevented by their multilayer coating with protective special compositions, the use of galvanized steel.

In the joint venture we are considering, the progressive collapse for block-modular buildings has its own characteristics. For this type of buildings, attention is paid to such structural elements as the junctions of blocks considered to neighboring blocks. The control criterion is the bearing capacity of these nodes, thanks to which the building as a whole resists local destruction and withstands the forces attributable to them due to its bearing capacity.

Progressive collapse of block structure buildings can also occur due to local damage to the block that performs load-bearing functions. In order to resist this, the subsequent compensation of the redistribution of efforts from the destroyed block to the neighboring blocks is important. This state of affairs should be facilitated by a significant bearing capacity and ability to plastic deformation of nodal interconnections, on the one hand, and high-quality factory installation of blocks reinforced with reinforcement, on the other.

Calculation of a building for progressive collapse is carried out by the limit equilibrium method, as well as the finite element method. Since we considered the limit equilibrium method earlier, we will describe the second method in more detail.

The finite element method is widely used in solid mechanics to calculate deformations. Its essence lies in solving a system of differential equations. Then the solution area (depending ondifferent coefficients) is divided into a number of segments, each of which is examined for optimality.

Based on the selected coefficients for variable differential equations, the optimal bearing elements are determined.

Recommendations for Solid Building Software

Calculation for the progressive collapse of monolithic buildings also proceeds from the fact that local destruction of vertical load-bearing structures, if they occur, should not go beyond one floor. Violation of the integrity of two intersecting walls (from the corner to the nearest opening), separate columns, alternating columns with adjoining wall sections are considered as such local destruction.

Recommendations for protection against progressive collapse prescribe to consider a spatial model, which, in addition to bearing, includes other elements that can redistribute bearing functions.

The modeling takes into account:

• monolithic connection of load-bearing elements (external and internal walls, columns, ventilation shafts, stairwells, pilasters);
• monolithic reinforced concrete belts covering the floors, which are lintels located above the windows.;
• monolithic reinforced concrete parapets connected with floors;
• elements connected to columns: reinforced concrete beams, stairwell railings, walls;
• openings in walls not exceeding a floor in height.

In addition, for a monolithic building, the design values must be observed:

• resistanceconcrete axial compression:
• resistance of concrete to axial tension;
• resistance of longitudinal reinforcement to axial compression;
• resistance of longitudinal reinforcement to tension;

Design requirements

Protection of buildings and structures from progressive collapse is based on the provision of the dynamics of the development of the influence of various local destructions on the overall structure of the building (structure). Currently, the software of frames of large-span high-rise buildings of various geometry is being studied especially actively both at the stage of their design and during restoration after they received local damage. Collections of recommendations and rules are being developed, binding standards are being approved.

It should be mentioned that the joint venture “Protection against progressive collapse”, which we repeatedly mentioned, as a normative set of rules, was compiled jointly by the Research Institute Center “Construction” and the Federal South-Western State University, taking into account federal laws No. 184-FZ and No. 384 -FZ regulating technical regulation and security measures in this case. It is adapted for regulation:

• construction of buildings (structures) of a normal level of responsibility and an increased level;
• reconstruction of buildings (structures) of a normal level of responsibility and an increased level;
• overhaul of buildings (structures) with a high level of responsibility.

The JV under consideration regulates:

• used building materials and their characteristics;
• possible loads and their effects onbuildings (structures);
• characteristics of calculation models;
• Destructive anti-software measures.

Features of computer calculation

As we have repeatedly mentioned, protection against progressive collapse involves computer modeling by finite element and limit equilibrium methods. It is useful to know that specialized software packages STADIO, ANSYS, SCAD, Nastran act as a tool for modeling by the limit state method. In this case, a full-fledged model is created, since thanks to the mentioned method, almost complete correspondence of the model to the dynamics of the building's response to local damage is achieved.

The kinematic method uses the same programs, but it is less formalized and requires the performer to build a personal calculation method.

As a result of kinematic calculations:

• determine structural elements that lose their integrity;
• the structural elements themselves are combined into equivalent groups;
• calculates the amount of construction work for each group;
• determine the most dangerous places of local destruction that can cause software;
• destruction is predicted, allowing early planning for restoration work.

Conclusion

Our time is distinguished by the emergence of an increasing number of high-rise residential and office buildings. In recent years, there has been an increase in public interest in the problems of improving reliability.industrial and residential buildings. In particular, not the last place is occupied by the question: “How can a progressive collapse be most guaranteed to be prevented?” And this is not accidental, because such accidents bring the most significant material losses and cause deep negative social consequences. After all, such accidents can take hundreds, and even thousands of lives.

Research is underway in three directions:

• development of ideal connections between structural elements;
• creating structural elements for maximum reliability;
• optimally obstructive overall design of buildings (structures).

Design offices, special construction and research companies do not turn their research into know-how, the latter are published and summarized. And this is understandable, because the problem of software is not only constructive, but also socially significant. However, the regulations still need to be improved. In addition, the disparate experience of specialists in the field of diagnostics of possible software should first be standardized and updated, and then transformed into practical preventive diagnostics, carried out on a planned, regular and non-commercial basis.

Obviously, now the calculation of the software should become more accessible and easier for the owners of residential and industrial assets in the procedure. After all, there is the problem of aging housing stock, and in such accidents we are talking about the loss of human lives.

A well-established system of preliminary payments for software, if it is legally justified and actually launched, would become an effective tool to prevent new tragedies.

Perhaps timely prevention can prevent such software as the collapse of the entrance of a residential building on December 31, 2018 in Magnitogorsk, which killed 39 people. Normatively, it is necessary to establish a list of situations when, not only necessarily, but also urgently, it is necessary to carry out a calculation for a progressive collapse. The need for such a calculation is especially urgent when the owner of the apartment decides to redevelop, often unaware that it affects the load-bearing structural elements. It was this uncontrolled violation that caused the aforementioned software.