Recommendations for designing for progressive collapse. Progressive collapse of buildings and structures
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The Department of Urban Planning and Architecture of the Ministry of Construction and Housing and Communal Services of the Russian Federation, within its competence, reviewed a letter on the issue of requirements of regulatory and technical documents, and reported the following.
The term “load-bearing structures” is practically not used in regulatory and technical documents, since the definition of load-bearing structures is given in textbooks on structural mechanics and is clear to every designer. The definition of load-bearing capacity is established only in SP 13-102-2003* “Rules for the inspection of load-bearing building structures of buildings and structures” (hereinafter referred to as SP 13-102-2003), which is currently not a valid standardization document. According to SP 13-102-2003*, load-bearing structures are building structures that absorb operational loads and impacts and ensure the spatial stability of the building.
In accordance with the provisions of GOST 27751-2014 “Reliability of building structures and foundations. Basic provisions" calculations for progressive collapse are carried out for buildings and structures of class KS-3, as well as (on a voluntary basis) buildings and structures of class KS-2.
The requirement to account for the progressive collapse of all industrial buildings, established in paragraph 5.1 of SP 56.13330.2011 “SNiP 31-03-2001 “Industrial Buildings” (hereinafter referred to as SP 56.13330.2011), is redundant and contrary to Federal Law No. 384-FZ “ Technical regulations on the safety of buildings and structures. This requirement will be adjusted in 2018 by amending SP 56.13330.2011.
In 2017, SP 296.1325800.2017 “Buildings and structures” was approved. Special impacts" (hereinafter referred to as SP 296.1325800.2017), which comes into force on February 3, 2018 for use on a voluntary basis. This set of rules states that when designing structures, scenarios for the implementation of the most dangerous emergency design situations must be developed and strategies must be developed to prevent the progressive collapse of the structure during local destruction of the structure. Each scenario corresponds to a separate special combination of loads and, in accordance with the instructions of SP 20.13330.2011 “SNiP 2.01.07-85* “Loads and impacts” (hereinafter referred to as SP 20.13330), must include one of the standardized (design) special impacts or one option for local destruction of load-bearing structures for special emergency impacts. The list of scenarios for emergency design situations and the corresponding special impacts is established by the Customer in the design assignment in agreement with the General Designer.
For each scenario, it is necessary to determine the load-bearing elements whose failure entails the progressive collapse of the entire structural system. For these purposes, it is necessary to analyze the operation of the structure under the action of special combinations of loads, in accordance with the instructions of SP 20.13330.
Clause 5.11 of SP 296.1325800.2017 specifies the conditions under which emergency impacts may not be taken into account:
Developed Special technical specifications for the design of a structure;
Scientific and technical support was provided at all stages of the design and construction of the structure, as well as the manufacture of these elements;
The structure was calculated for the design (standardized) special impacts specified in SP 296.1325800.2017, the design assignment and the current regulatory documents;
Additional coefficients of operating conditions have been introduced that reduce the design resistance of these elements and their fastening points (for long-span structures, the specified additional coefficients of operating conditions are given in Appendix B of the specified SP);
Organizational measures were carried out, including in accordance with SP 132.13330.2011 “Ensuring anti-terrorist protection of buildings and structures. General requirements design” and agreed with the customer (see Appendix D of the specified set of rules).
Scientific and technical support is carried out by an organization (organizations) other than those that develop project documentation. Work on scientific and technical support should be carried out by organizations (as a rule, scientific research) with experience in relevant fields and the necessary experimental base.
Document overview
Clarifications are given on the use of regulatory and technical documents when qualifying load-bearing structures. In particular, the following was noted.
The term “load-bearing structures” is practically not used in regulatory and technical documents, since the definition is given in textbooks on structural mechanics and is clear to every designer. A definition is given to the concept of “bearing capacity”.
In accordance with the provisions of GOST 27751-2014 "Reliability of building structures and foundations. Basic provisions", calculations for progressive collapse are carried out for buildings and structures of class KS-3, as well as (on a voluntary basis) buildings and structures of class KS-2.
In 2017, SP 296.1325800.2017 “Buildings and structures. Special impacts” was approved, which comes into force on February 3, 2018 for use on a voluntary basis. When designing structures, scenarios for the implementation of the most dangerous emergency design situations and strategies must be developed to prevent the progressive collapse of the structure during local destruction of the structure. Each scenario corresponds to a different specific load combination. The list of scenarios for emergency design situations and the corresponding special impacts is established by the customer in the design assignment in agreement with the general designer.
The procedure for scientific and technical support of work is explained.
TsNIIPromzdanij MNIITEP
ORGANIZATION STANDARD
PREVENTION
PROGRESSIVE
COLLAPSE OF REINFORCED CONCRETE
MONOLITHIC STRUCTURES
BUILDINGS
Design and calculation
STO-008-02495342-2009
Moscow
2009
Preface
The goals and principles of standardization in the Russian Federation are established by Federal Law No. 184-FZ of December 27, 2002 “On Technical Regulation”, and the rules of development and application are established by GOST R 1.4-2004 “Standardization in the Russian Federation. Organization standards. General provisions".
Standard information
1. DEVELOPED AND INTRODUCED working group composed of: Doctor of Technical Sciences, Prof. Granev V.V., engineer Kelasev N.G., engineer Rosenblum A.Ya. - topic manager, (JSC TsNIIPromzdanii), engineer. Shapiro G.I. (SUE "MNIITEP"), Doctor of Technical Sciences, Prof. Zalesov A.S.
3. APPROVED AND ENTERED INTO EFFECT by order of the General Director of OJSC “TsNIIPromzdaniy” dated September 7, 2009 No. 20.
4. INTRODUCED FOR THE FIRST TIME
Withpossession
STO-008-02495342-2009
ORGANIZATION STANDARD
PREVENTING PROGRESSIVE COLLAPSE
REINFORCED CONCRETE MONOLITHIC BUILDING STRUCTURES
Design and calculation
Date of introduction - 09/07/2009
Introduction
Progressive collapse ( collapse progressive ) denotes the sequential destruction of the load-bearing building structures of a building (structure), caused by initial local damage to individual load-bearing structural elements and leading to the collapse of the entire building or a significant part of it.
Initial local damage to the structural elements of a building is possible in emergency situations (gas explosions, terrorist attacks, vehicle collisions, defects in design, construction or reconstruction, etc.) that are not provided for by the conditions of normal operation of the building.
In the load-bearing system of a building, destruction of individual load-bearing structural elements in an emergency is allowed, but these destructions should not lead to progressive collapse, i.e. to the destruction of adjacent structural elements to which the load is transferred, previously perceived by elements destroyed as a result of an emergency.
When developing the standard, the provisions of SNiP 2.01.07-85* “Loads and impacts” (ed. 2003), SNiP 52-01-03 “Concrete and reinforced concrete structures. Basic provisions", SP 52-101-2003 "Concrete and reinforced concrete structures without prestressing reinforcement" and STO 36554501-014-2008 "Reliability of building structures and foundations. Basic provisions".
1 area of use
1.1 This organization standard establishes the rules for the design of reinforced concrete monolithic structures of residential, public and industrial buildings that are subject to protection from progressive collapse in emergency situations.
1.2 Objects, the destruction of which can lead to large social, environmental and economic losses and the design of which must ensure the prevention of progressive collapse, include:
a) residential buildings with a height of more than 10 floors;
b) public buildings* with occupancy of 200 people. and more simultaneously within a block limited by expansion joints, including:
Educational purposes;
Health and Social Services;
Service (trade, food, household and public services, communications, transport, sanitary services);
Cultural and leisure activities and religious rituals (physical education and sports, cultural, educational and religious organizations, entertainment and leisure and entertainment organizations);
Administrative and other purposes (government bodies of the Russian Federation, constituent entities of the Russian Federation and local self-government, offices, archives, research, design and engineering organizations, financial institutions, judicial institutions and the prosecutor's office, editorial and publishing organizations);
For temporary stay (hotels, sanatoriums, hostels, etc.).
c) production and auxiliary buildings housing 200 people. and more simultaneously within a block limited by expansion joints.
*) The classification of public buildings by purpose is given in SNiP 2.08.02-89*"Public buildings and structures" and SNiP 05/31/2003"Public administrative buildings".
1.3 Life support facilities for cities and towns, as well as particularly dangerous, technically complex and unique facilities **) should be designed in accordance with special technical conditions.
**) The classification of especially dangerous, technically complex and unique objects is given in the Town Planning Code of the Russian Federation, Art. 48 1.
1.4 In relation to a specific object, the requirement to prevent progressive collapse in emergency situations is accepted in accordance with the design assignment, agreed upon in the prescribed manner and approved by the customer and/or investor.
2 Terms and definitions
2.1 Progressive collapse - sequential destruction of the load-bearing structures of a building (structure), caused by initial local damage to individual load-bearing structural elements and leading to the collapse of the entire building or a significant part of it (two or more spans and two or more floors).
2.2 Normal operation of the building - operation in accordance with the conditions provided for by SNiP 2.01.07-85 and SNiP 52-01-03.
2.3 The primary structural system of a building is a system adopted for the conditions of normal operation of the building.
2.4 Secondary structural system of a building - a primary structural system modified by eliminating one vertical load-bearing structural element (columns, pilasters, section of wall) within one floor.
3 Basic provisions
3.1 The structural system of the building should not be subject to progressive collapse in the event of local destruction of individual structural elements in emergency situations not provided for by the conditions of normal operation of the building. This means that under a special combination of loads, local destruction of individual elements of the building’s structural system is allowed, but these destructions should not lead to the destruction of other structural elements of the modified (secondary) structural system.
3.2 Prevention of progressive collapse of the building should be ensured:
A rational design and planning solution for the building, taking into account the likelihood of an emergency;
Constructive measures that increase the static indeterminacy of the system;
The use of design solutions that ensure the development of plastic (inelastic) deformations in load-bearing structural elements and their connections;
The necessary strength of load-bearing structural elements and stability of the system for the conditions of normal operation of the building and for cases of local destruction of individual structural elements of the building.
3.3 When designing a building, along with calculations for normal operation, there must be:
Static calculations of the modified structural systems of the building with structural elements removed as a result of the accident (secondary structural systems) and, accordingly, modified design schemes for the action of a special combination of loads were carried out. The calculation of the foundations should be made only according to the bearing capacity for the conditions provided for in clause 2.3. SNiP 2.02.01-83*;
Stability margins of secondary structural systems have been established, and if they are insufficient, the cross-sectional dimensions of the elements have been increased or the structural and planning solution of the building has been changed;
The required class of concrete and reinforcement of structural elements were determined together with the calculation results for normal operating conditions.
3.4 As a hypothetical local destruction, one should consider the destruction within one (each) floor of the building of one (each) column (pylon) or a limited section of walls in turn.
3.5 The conditions for ensuring the prevention of progressive collapse of the secondary structural systems of the building are:
Non-exceeding in structural elements of the values of forces (stresses) determined at load values according to , in relation to the forces (stresses) in them determined at the limiting values of the characteristics of materials using the appropriate reliability factors;
Preventing a decrease in the system stability margin in relation to the reliability coefficient for stability γ s = 1.3.
In this case, the reliability coefficient for liability should be taken equal to γ n = 1.0, unless otherwise provided in the design specifications.
Movements, opening of cracks and deformations of elements are not limited.
4 Constructive and planning solutions
A rational structural and planning solution for a building from the point of view of preventing progressive collapse is a structural system that ensures, when a separate (any) vertical load-bearing structural element of the building is removed, that the structures above the retired element are converted into a “suspended” system capable of transferring loads to the remaining vertical structures.
To create such a structural system, the following should be provided:
Monolithic coupling of floor structures with reinforced concrete vertical structures (columns, pilasters, external and internal walls, staircase railings, ventilation shafts, etc.);
Reinforced concrete monolithic belts along the perimeter of the floors, combined with the floor structures and performing the functions of over-window lintels;
Reinforced concrete monolithic parapets combined with covering structures;
Reinforced concrete walls in the upper floors of a building or reinforced concrete beams in the roof, connecting columns (pilasters) with each other and with other vertical reinforced concrete structures (walls, staircase railings, ventilation shafts, etc.);
Openings in reinforced concrete walls do not reach the entire height of the floor, leaving, as a rule, sections of blank walls above the openings.
5 Loads
5.1 Calculation of secondary structural systems to prevent progressive collapse should be carried out for a special combination of loads, including standard values of permanent and long-term live loads, with a combination coefficient equal to Ψ = 1,0.
5.2 Constant loads should include the own weight of load-bearing reinforced concrete structures, the weight of building parts (floors, partitions, suspended ceilings and communications, curtain and self-supporting walls, etc.) and lateral pressure from the weight of the soil and the weight of the road surface and sidewalks.
5.3 Long-term temporary loads include:
Reduced loads from people and equipment according to table. 3 SNiP 2.01.07-85*;
35% of the total standard load from vehicles;
50% of the full standard snow load.
5.4 All loads should be considered as static with a load safety factor γ f = 1,0.
6 Characteristics of concrete and reinforcement
6.1 When calculating reinforced concrete structural elements to prevent progressive collapse, the following should be taken into account:
a) calculated values of concrete resistance to axial compression, equal to their standard values, multiplied for structures concreted in a vertical position by the operating condition coefficient γ b 3 = 0,9;
b) calculated values of concrete resistance to axial tension, used when calculating the action of transverse forces and the local action of loads, equal to their standard values, divided by the reliability coefficient for concrete γ n = 1,15;
c) calculated values of the tensile strength of longitudinal reinforcement of structures equal to their standard values;
d) calculated values of the resistance of longitudinal reinforcement of structures to compression, equal to the standard values of tensile resistance, with the exception of reinforcement of class A500, for which R s= 469 MPa (4700 kgf/cm 2), and class B 500 reinforcement, for which R s= 430 MPa (4400 kgf/cm2);
e) calculated values of the tensile resistance of transverse reinforcement of structures, equal to their standard values, multiplied by the operating condition coefficient γ s 1 = 0,8;
f) standard values of resistance of concrete and reinforcement, as well as values of the modulus of elasticity of reinforcementE sand initial modulus of elasticity of concreteEbaccording to SP 52-101-2003.
7 Calculation
7.1 Calculation of secondary structural systems of a building to prevent progressive collapse should be carried out separately for each (one) local destruction.
It is allowed to calculate only the most dangerous cases of destruction, which can be schemes with the destruction of vertical load-bearing structural elements in turn:
a) having the largest cargo area;
b) located at the edge of the ceiling;
c) located in the corner,
and extend the results of these calculations to other parts of the structural system.
7.2 As the initial one, one should take the design scheme adopted when calculating the primary structural system of the building for normal operation conditions, and transform it into a secondary system by eliminating one by one the vertical load-bearing structural elements for the most dangerous cases of destruction. In this case, it is recommended to include in the work structural elements that are usually not taken into account when calculating the primary system.
7.3 As one excluded vertical load-bearing structure, a column (pylon) or a section of load-bearing walls intersecting or adjacent at an angle should be taken. The total length of these wall sections is measured from the intersection or junction to the nearest opening in each wall or to the junction with a wall in a different direction, but not more than 7 m.
7.4 Vertical structures of the system should be considered rigidly clamped at the level of the top of the foundations.
7.5 Static calculation of the secondary system should be carried out as an elastic system using certified software packages (SCAD, Lyra, STARK - ES, etc.) taking into account geometric and physical nonlinearity. It is allowed to carry out calculations taking into account only geometric nonlinearity.
When calculating taking into account geometric and physical nonlinearity, the stiffness of sections of structural elements should be taken in accordance with the instructions of SP 52-101-2003, taking into account the duration of the loads and the presence or absence of cracks.
When calculating taking into account only geometric nonlinearity, the stiffness of sections B of structural elements should be determined as the product of the modulus of proportionality E pr at the moment of inertia of the reinforced concrete section Jb.
Proportionality module E pr should be taken:
when determining efforts - E pr = 0,6E b E pr = E b for vertical elements;
When calculating stability - E pr = 0,4E b for horizontal elements and E pr = 0,6E b for vertical elements
7.6 Calculation of sections of structural elements should be carried out in accordance with the Allowance for forces determined as a result of static calculations, assuming them to be short-term.
7.7 As a result of the calculation of the primary and secondary structural systems, the forces (stresses) in the structural elements are determined, the resulting concrete class and reinforcement of the elements and their joints are assigned, and the stability margin of the frame is established, and if it is insufficient, the cross-sectional dimensions of the elements are increased or the structural design of the building is changed.
8 Design requirements
8.1 The design of elements and their connections should be carried out in accordance with the Manualand SP 52-103-2007.
8.2 The class of concrete and reinforcement of structural elements should be assigned to the highest level based on a comparison of the calculation results for the conditions of normal operation of the building and to prevent progressive collapse.
8.3 When reinforcing structural elements, special attention should be paid to the reliability of the anchorage of the reinforcement, especially at the intersections of structural elements. The lengths of anchorage and overlap of reinforcing bars must be increased by 20% relative to the required ones.
8.4 Longitudinal reinforcement of structural elements must be continuous. The cross-sectional area of the longitudinal reinforcement (separately lower and separately upper) of beamless floor slabs and beams of beam floors must be at least μ s,min= 0.2% of the cross-sectional area of the element.
8.5 Longitudinal reinforcement of vertical load-bearing structural elements must withstand a tensile force of at least 10 kN (1 tf) for each square meter of the load area of this structural element.
An example of calculating a building frame to prevent progressive collapse *)
*) Compiled by Eng. A.P. Blackie
The building of a hotel and office complex of variable number of floors ( and ). The largest number of above-ground floors is 14, underground - 1. Maximum size in plan 47.5 × 39.8 m. Located in the Moscow region. Wind district IB, snow region III.
The building is framed with a central staircase-elevator core and two side staircases. The strength, stability and rigidity of the building frame is ensured by floor discs and a system of columns and walls embedded in the foundation.
The main grid of columns is 7.5×7.2 m. Columns square section from 400×400 to 700×700 mm. Beamless ceiling 200 mm thick with capitals.
Frame structures (columns, floors), foundations, stairs, walls of staircases, elevator and communication shafts, external walls of the underground and 11th (technical) floors, partially, internal walls - monolithic reinforced concrete. Concrete class B30, longitudinal working reinforcement class A500C.
To prevent progressive collapse in an emergency, special structural elements are provided (reinforced concrete walls along the perimeter of the technical XI floors, wall along axis 11 starting from XII floor and up to the covering, wall along axis 1 starting from X floors and up to the covering), providing, along with the structural elements necessary for the functioning of the building during normal operation, the transformation of structures into a “suspended” system above the columns along the perimeter of the building that were hypothetically removed as a result of an emergency and, partially, the middle ones. The zones around part of the middle columns, which do not turn into “suspended” systems when these columns are destroyed in the event of an emergency impact on them, are additionally reinforced, if necessary (see below).
The design diagram of the building is adopted in the form of a spatial system of columns and walls embedded in the foundation, united by floors and stairs (). The calculation was made using the software package SCAD Office 11.3.
According to the level of responsibility, the building is classified as Level I (increased). The reliability coefficient for liability is assumed to be γ n= 1.1 for the main load combination.
The building frame was calculated for the main combination of loads for the operation stage (primary structural system) and for a special combination of loads to prevent progressive collapse (secondary structural systems).
The load values are given in table. 1 and 2.
Table 1
|
Place |
Vertical loads tf/m² (without dead weight) |
||||||||
|
regulatory |
settlement |
||||||||
|
permanent |
temporary |
basic combination |
special combination |
||||||
|
full |
incl. duration |
permanent |
temporary for |
||||||
|
overlap |
frame |
||||||||
|
full |
lasts |
full |
duration |
||||||
Overlap |
0,15+0,45+0,04 = 0,64 (floor, partitions, suspension) |
0,07 |
0,18+0,50+0,05 = 0,73 |
0,24 |
0,09 |
0,12 |
0,09 |
0,64+0,07 = 0,71 |
|
|
Cover exp. |
0.39 (roof, suspension) |
0.13 (snow) |
0,07 |
0,48 |
snow bag |
0,09 |
0,20 |
0,09 |
0,39+0,07 = 0,46 |
The load from the external walls is assumed to beqn = 0,4 tf/m² walls and q p= 0.56 tf/m² wall.
table 2
|
No. n/n |
Load application location |
Type of calculation |
Combinations of calculated vertical loads (without dead weight), tf/m² *) |
|
|
basic |
special |
|||
|
on floors |
(0.73 + 0.12) 1.1 = 0.94 |
0,71 |
||
|
overlap calculation |
(0.73 + 0.24) 1.1 = 1.07 |
0,71 |
||
|
For the coating in use |
calculation of foundation, columns and frame |
(0.48 + 0.2) 1.1 = 0.75 |
0,46 |
|
|
coverage calculation |
(0.48 + snow) 1.1 |
0,46 |
||
|
from the walls |
calculation of all structures |
0,56∙1,1 = 0,62 |
0,40 |
|
*) - the values of all loads, except for walls, are given per m² of flooring and covering, and from walls - per m² of wall.
The values of the calculated resistances of reinforcement and concrete are given in table. 3.
Table 3
|
Type of design |
Force and nature of reinforcement |
Design resistance of reinforcement, kgf/cm² for a combination of loads |
Design resistance of concrete, kgf/cm² for load combinations |
||
|
main |
special |
main |
special |
||
|
Overlap |
R s = 4430 |
R sn = 5100 |
Compression R b = 173 |
Compression R bn = 224 |
|
|
Transverse reinforcement class A240 |
R sw = 1730 |
R sn γ s 1 = 2450·0.8 = 1960 |
Stretching R bt = 11.7 |
Stretching |
|
|
Columns, pilasters walls |
Compression of longitudinal reinforcement class A500C |
R sc = 4080 |
R s = 4700 |
compression Rb· γ b3 = 173·0.9 = 156 |
compression Rbn· γ b3 = 224·0.9 = 202 |
|
Tension of longitudinal reinforcement class A500C |
R s = 4430 |
R sn = 5100 |
|||
Table 4
|
Frame element |
Initial modulus of elasticity of concrete E b × 10 -6 tf/m² |
Deformation modulus Epr when calculating tf/m² × 10 -6 |
||
|
forces and reinforcement of elements |
sustainability |
|||
|
for the main load combination |
for a special combination of loads |
|||
|
Floor slabs |
3,31 |
3.31 0.6 = 2.0 |
3.31·0.2 = 0.66 |
3.31 0.4 = 1.3 |
|
Beams |
3,31 |
3.31 0.6 = 2.0 |
3.31·0.2 = 0.66 |
3.31 0.4 = 1.3 |
|
Columns |
3,31 |
3,31 |
3.31 0.3 = 1.0 |
3.31 0.6 = 2.0 |
|
Walls |
3,31 |
3,31 |
3.31 0.3 = 1.0 |
3.31 0.6 = 2.0 |
The deformation moduli of reinforced concrete structures are taken according to table. 4.
When calculating secondary structural systems for a special combination of loads, cases of excluding, in turn, the middle column No. 14, the outer column No. 21 and the corner column No. 23 are considered. I and XIII floors (see,)
Calculations have shown that, in comparison with the primary structural system, when excluding the columns indicated in turn, the margin of general stability of the building frame practically does not change, but there is an obvious redistribution of forces in the structures.
Some results of calculations of the primary and secondary systems when removing column No. 14 are presented in Table. 5 and 6 and in Fig. 5÷8.
Table 5
|
No. Column No. 4) |
Estimated total area of longitudinal reinforcement of columns, cm 2 |
|||||||
|
with primary structural system 1) |
when removing column No. 14 on I floor 2) |
when removing column No. 14 on the XIII floor 2) |
resulting |
|||||
|
1st floor |
XIII floor 3) |
1st floor |
XIII floor |
1st floor |
XIII floor |
1st floor |
XIII floor |
|
|
13 |
Introduction The loss of their strength properties by individual load-bearing elements of the frame may entail the sequential inclusion of an increasing number of load-bearing structures into the collapse zone - a “domino” effect will arise. Progressive or avalanche collapse is the collapse of building structures (or parts of it two or more floors high) that have lost support as a result of local destruction of any floor. A related term is survivability - the ability of a technical device, structure, means or system to perform its basic functions, despite the damage received, or adapting to new conditions. IN modern world the risk of avalanche-like destruction is significant, therefore there is a need for accurate calculation algorithms, new reliable and economically feasible methods for structural strengthening of the load-bearing frame of a building, clear legislative regulation of design and calculations taking into account possible extreme impacts. Goal of the work The purpose of the work is to review modern Russian and foreign publications related to the subject of calculations for progressive collapse in linear and nonlinear formulation of the problem, analysis of Russian legislation regarding the survivability of load-bearing structures; identifying the most likely causes of progressive building collapse. Causes of progressive collapse When developing design solutions, it is necessary to take into account not only the standard operating conditions of the structure, but also possible emergency situations. Progressive collapse can occur as a result of emergency situations or man-made impacts, divided into force, deformation and corrosion. Possible man-made causes of local damage may be:
An example is the collapse of a 9-story large-panel building on March 6, 1982 in Volgodonsk. The cause of the complete collapse of a large-panel residential building was poor-quality sealing with a freezing mortar of a horizontal groove formed in connection with the replacement of the base panel. At the moment of thawing of the solution, the wall panel lost stability, as a result of which all 9 floors of the large-panel building collapsed.
At all stages of the life cycle of a structure (research, design, construction, operation, dismantling), errors are made that can lead to progressive collapse. Emergency situations that can cause an avalanche-like collapse of a building are:
In addition, the risk of collapse cannot be completely excluded due to the heterogeneity of strength and other technical properties of building materials, uncertainty of system requirements, and the impossibility of ideally modeling the system even using all the capabilities of modern software systems. The most common forms of failure of metal structures are loss of stability and brittle failure, which occurs due to the uncontrolled development of microcracks in the material. The progressive collapse of the entire bridge structure can begin with one microcrack in the metal of the supporting structures, which means it is necessary to study the strength properties of materials from the point of view of reliability theory. History of the study of progressive collapse The starting point for the study of progressive collapse can be considered the sixteenth of May 1968: in London, due to a domestic gas explosion, the twenty-two-story house Ronan Point was completely destroyed, see Figure 1. The accident killed 22 people. The partial collapse of Ronan Point led to major changes in legislation, the first of which was the Fifth Amendment to the UK Building Regulations (Part A) in 1970, dealing with disproportionate collapse. The amendment contained requirements according to which the building should not be subject to destruction disproportionate to the accident, in other words, it required to prevent the progressive collapse of buildings. Figure 1. Destruction of the Ronan Point house Most famous case progressive structural collapse is the destruction of the World Trade Center in New York, which occurred on September 11, 2011 as a result of terrorist attack. The destruction of the World Trade Center had catastrophic consequences: 2,751 people became victims. The deliberate collision with the Boeing 767-222 was not the first terrorist attack that occurred at the World Trade Center: on February 26, 1993, a car loaded with 680 kg of explosives exploded in the underground parking lot of the North Tower, killing more than a thousand people: six were killed, more than a thousand were injured. . Due to the high strength of the building's frame, there was no destruction of the load-bearing structures in 1993. The problem of progressive collapse has not escaped Russia either. In modern Russia, the most common cause of accidents that can lead to progressive collapse is the explosion of domestic gas, which occurred due to the negligence of users. Already in 2013, gasification in Russia amounted to 65.3%, which means that for most residential buildings the risk of progressive collapse is significant. Examples of such accidents include:
In 2016, there were already more than five major accidents associated with household gas explosions. The largest accidents in Russia were:
Thousands of people became victims of such disasters, but these tragedies could have been avoided. Review of Russian regulatory documentation regarding design for progressive collapse Obviously, taking into account a possible emergency situation will entail a significant increase in the cost of design and construction, which is why only a few developers agree to it voluntarily. Consequently, clear regulatory documentation is required that strictly regulates the need and composition of the calculation. Most of the modern foreign standards are focused not on preventing significant destruction, but on ensuring the safety of people and the possibility of their timely evacuation. Unfortunately, at present there is practically no such documentation in Russia. Only strict recommendations on the composition and calculation algorithm can prevent the catastrophic consequences of possible emergency situations. A significant gap in Russian legislation in the field of construction is the lack of clear regulatory documents regulating the design of buildings taking into account resistance to progressive collapse and establishing requirements for the calculation of the load-bearing frame of the building. The document of the highest legal force in the field of ensuring the survivability of building structures is Federal Law No. 384-FZ. Article 16.6 states the need for calculations for buildings and structures of a high level of responsibility, which, in accordance with the Urban Planning Code, include technically complex, especially dangerous and unique objects. The list of buildings subject to calculation is most fully specified in GOST 27751-2014. Reliability of building structures and foundations. Basic provisions (clause 5.2.6) calculations are required for buildings of class KS-3 and KS-2, subject to large crowds of people, the list of which is indicated in Appendix B. Thus, from July 1, 2015, calculations are required for most public and residential buildings. Although accounting for progressive collapse is required for an increasing number of buildings, there is still no clear calculation algorithm or specific recommendations for choosing an accident zone. Similarly, questions arise regarding the choice of the required number of destructible load-bearing elements. All these issues are covered in a wide range of design recommendations issued by MNIITEP and NIIZHB in the 2000s, standards of organizations, but none of these documents has legislative force. The most significant gap exists in the area of calculations of steel frames to ensure their survivability. Existing documentation (MDS 20-2.2008; STO 36554501-024-2010) relates only to long-span structures. The regulatory documentation states the need to assess the survivability of the load-bearing frame for all reinforced concrete monolithic buildings (clause 6.2.1. SP 52-103-2007), but does not provide any methodological instructions, other than the recommendation to perform calculations using the finite element method using software certified in Russia complexes (clause 6.3.7.). Many software packages have a built-in module for calculating progressive collapse; however, the calculation results have not yet been confirmed and require additional experimental justification. The developers of the SCAD and Lira software systems offer their own calculation methods (see Figure 2), however, the reliability of the results obtained has not yet been confirmed and requires research in this direction.
Figure 2. Display of calculation results when using the “Progressive Collapse” module of the SCAD PC
These recommendations are similar in terms of the algorithm for calculating building structures; significant differences appear only in terms of recommendations for measures of structural strengthening of the frame, which is associated with significant differences in the operation of the frame made of stone and metal materials. According to all modern regulations all that is required is calculation for the first group of limit states, determination maximum movements and no bending is required. The selection of the most dangerous element from the point of view of destruction is carried out by analyzing the design diagram and calculation results for several emergency scenarios. There are no instructions in the regulatory documentation regarding the need to take into account the nonlinear operation of structures, which can have a strong impact on the correctness of the calculation results, since with progressive destruction, structural elements often have significant displacements in modulus, which can lead to significant changes in the operation of structures. Thus, it can be argued that active work is currently underway in Russia to develop a regulatory framework for calculating progressive collapse, the range of buildings and structures that require taking into account a possible accident is constantly expanding, in addition, more and more high-rise buildings are being built, for which the probability of avalanche collapse is especially important. This means that it can be argued that, in order to achieve accurate results, the calculation algorithm and software will be constantly improved. The relevance of studying progressive collapse is confirmed by the wide attention of modern scientists to the issues of ensuring the strength and survivability of building structures under conditions of extreme influences, the work of engineering structures in the elastic-plastic stage. Now in Russia and the CIS countries, design institutes such as MNIITEP, NIIBZH, NIISK are dealing with this issue. The result of many years of work of the institutes MNIITEP and NIIBZH are recommendations issued in the 2000s for the protection of various types of buildings from avalanche collapse. NIISC specialists have developed DBN V.2.2-24.2009 “Design of high-rise and civil buildings”, containing a methodology for calculating a high-rise building for progressive collapse; in Ukraine, the methodology is advisory in nature. Review of the works of modern scientists dealing with the issue of progressive collapse Many authors have studied Russian and foreign legislative framework. Reviews can be found in V.Yu. Gracheva, T.A. Vershinina, A.A. Puzatkina; J.S. Dzhumagulova and A.K. Stamalieva, A.V. Perelmuter, and in. Scientists argue that further work is required on the regulatory framework: its clarification and expansion. In addition to research institutes, individual scientists have also made a huge contribution to the development of research into the problem of progressive collapse. IN. Almazov developed a classification of types of progressive collapse, gave recommendations on the calculation algorithm, and proposed cost-effective options for the structural strengthening of buildings; the scientist studied the dynamic effect of progressive collapse using the example of multi-story reinforced concrete frames when one of the load-bearing columns of the first floor was removed. He proposed a method for calculating the dynamism coefficient depending on the number of storeys of the frame, which allows solving the problem in a static formulation. No less pressing than the issue of legislative regulation of calculations and design is the issue of a generally accepted approach to ensuring the strength of the building frame under extreme influences. It is impossible to accurately predict the location and magnitude of an extreme load; similarly, defects in the installation and manufacture of building structures, deviations in the properties of materials are unpredictable - all this not only complicates modeling, but also makes absolutely accurate calculations impossible. In this regard, many authors deal with the issues of constructive solutions that help preserve the structural integrity of the building, predicting the most likely emergency situations and their consequences. Computer calculation of the model for avalanche destruction is complicated by the impossibility of using the finite element method due to the lack of accurate data on the behavior of the structure during progressive collapse and sufficient experience in constructing structural complex models and interpreting the calculation results. Research is needed to develop an improved methodology for assessing the vulnerability of structural systems and improving them to mitigate progressive collapse under various hazard scenarios. Engineers need design and calculation methods that can prevent the potential for progressive building collapse. The development of such methods is actively carried out by many scientists. In emergency situations, materials operate beyond the stage of elastic deformation; it is also necessary to take into account significant movements occurring in load-bearing structures. Significant deformations in modulus can lead to a redistribution of loads, and therefore a change in the entire design scheme. Thus, when calculating for progressive collapse, it is necessary to take into account the geometric and physical nonlinearities of the load-bearing frame of the building. Work is underway in this area. The constant improvement of computer technology makes it possible to build more and more detailed models of structures and contributes to the ever wider spread of solving problems in a nonlinear formulation. Assessing the correctness of calculation models, checking the results of computer calculations, and the art of interpreting the results obtained is one of the central problems not only of calculations for progressive collapse, but of all construction as a whole. Design and research institutes and developers of modern calculation programs take part in working on these problems, which contributes to the constant improvement of software systems. Analysis of the capabilities of the finite element method, examples of calculations of building models and new computational algorithms are also reflected in the works of Russian and foreign scientists. Conclusion Due to the ever-increasing number of accidents causing disproportionate destruction of buildings, there is a need for accurate calculation algorithms, new reliable and economically feasible methods for structural strengthening of the load-bearing frame of a building, clear legislative regulation of design and calculations taking into account possible extreme impacts. The work provided the history of the emergence and development of the problem of progressive collapse of buildings, a review of modern Russian and foreign publications related to the subject of calculations for progressive collapse in linear and nonlinear formulation of the problem, and an analysis of Russian legislation relating to the survivability of load-bearing structures. The most likely causes of the progressive collapse of buildings were also analyzed. Bibliography:
Measures to protect against progressive collapse 6.1.1. High-rise buildings must be protected from progressive collapse in the event of local destruction of load-bearing structures as a result of emergency situations (ES). The latter include: Natural emergencies – dangerous meteorological phenomena, the formation of karst sinkholes and failures in the foundations of buildings; Anthropogenic (including man-made) emergencies - explosions outside or inside a building, fires, accidents or significant damage to load-bearing structures due to defects in materials, poor quality work, etc. 6.1.2. The stability of a building against progressive collapse should be checked by calculation and ensured by constructive measures that promote the development of plastic deformations in load-bearing structures and their units under extreme loads (Recommendations for the protection of residential buildings of wall structural systems in emergency situations. M., 2000. Recommendations for the protection of residential frame buildings in emergency situations situations. M., 2002). 6.1.3. Calculation of the stability of a building must be carried out for a special combination of loads, including permanent and long-term loads with the following possible local failure patterns: Destruction (removal) of two intersecting walls of one (any) floor in the area at least their intersection (in particular, from the side of the building) of the nearest openings in each wall or to the next intersection with another wall no more than 10 m long, which corresponds to damage to structures in a circle with an area of up to 80 m 2 (area of local destruction); Destruction (removal) of columns (pylons) or columns (pylons) with adjacent sections of walls located on one (any) floor of the area of local destruction; Collapse of a section of the floor of one floor in the area of local destruction. To assess the stability of a building against progressive collapse, it is allowed to consider only the most dangerous schemes of local destruction. 6.1.4. Checking the stability of a building against progressive collapse includes the calculation of load-bearing structures in places of local destruction according to the limit states of the first group with the calculated resistances of materials (concrete and reinforcement) equal to standard values. At the same time, the magnitude of deformations and the width of cracks in structures are not regulated. 6.1.5. Constant and temporary long-term loads when calculating the stability of a building against progressive collapse should be taken according to Table 5.1 of these standards. In this case, the coefficients of load combinations and the reliability factors for loads are taken equal to unity. 6.1.6. To calculate buildings against progressive collapse, a spatial calculation model should be used, which can take into account elements that are non-load-bearing under normal operating conditions, and in the presence of local influences actively participate in the redistribution of the load. The design model of the building must reflect all the patterns of local destruction indicated in paragraph. 6.1.3. 6.1.7. The main means of protecting buildings from progressive collapse is reserving the strength of load-bearing elements, ensuring the load-bearing capacity of columns, crossbars, diaphragms, floor disks and structural joints; creating continuous and continuous reinforcement of floors, increasing the plastic properties of connections between structures, including non-load-bearing elements in the work of the spatial system. Effective operation of connections that prevent progressive collapse is possible by ensuring their plasticity in the limiting state, so that after the load-bearing capacity is exhausted, the connection does not turn off from operation and allows the necessary deformations without destruction. To fulfill this requirement, the connections must be made of plastic sheet or reinforcing steel, and the anchorage strength of the connections must be greater eefforts that cause their fluidity. 6.1.8. In high-rise buildings, preference should be given to monolithic and prefabricated monolithic floors, which must be reliably connected to the vertical load-bearing structures of the building with connections. The connections connecting the floors with columns, crossbars, diaphragms and walls must keep the floor from falling (in the event of its destruction) to the underlying floor. The connections must be designed for the standard weight of half the span of the floor with the floor and other structural elements located on it. From: zina,  Loading...You might be interested:
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