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FOREWORD

1. DEVELOPED: MNIITEP (engineers G.I.Shapiro, Yu.A. Eisman) and SIC StADiO (Ph.D. Yu.M. Strugatsky - topic leader)

2. PREPARED for approval and publication by the Department of Advanced Design and Standards of the Moscow City Architecture Committee (engineers Yu.P. Vanyan, Yu.B. Shchipanov)

3. AGREED: TsNIISK named after Kucherenko, TsNIIEP dwellings

4. APPROVED AND INTRODUCED INTO ACTION by the Direction of the Moscow Committee for Architecture and Construction dated 08.24.1999 N 36

1. BASIC PROVISIONS

1. BASIC PROVISIONS

1.1. The structural system of residential panel buildings must be protected from progressive (chain) collapse in the event of local destruction. load-bearing structures in case of emergency impacts not provided for by the conditions of normal operation of buildings (explosions, fires, shock impacts Vehicle etc.). This requirement means that in the event of emergency impacts, local destruction of load-bearing structures is allowed (complete or partial destruction of individual walls within one floor and two adjacent axes of the building), but these primary destruction should not lead to collapse or destruction of structures to which the load is transferred. , previously perceived by elements damaged by emergency impact.

The structural system of the building must ensure its strength and stability in the event of local destruction of load-bearing structures, at least for the time required to evacuate people. The movement of structures and the opening of cracks in them in the emergency situation under consideration is not limited.

1.2 When designing the protection of panel buildings from progressive collapse, two types of intact structural elements should be distinguished. In elements of the first type, the effects of local destruction do not cause a qualitative change in the stress state, but only lead to an increase in stresses and efforts (intact wall diaphragms and floor slabs that are not located above the local destruction). In the elements of the second type (these include structures that have lost their original supports - wall panels and floor slabs located above local destruction), the stress state changes qualitatively in the considered state of the building.

Due to the fact that the elements of the first type under normal operating influences are subjected to loads two to three times lower than the destructive ones, the main design task is to ensure strength and stability. wall panels and floor slabs that have lost their support as a result of local destruction of the walls. Ensuring the stability of these structures, which depends both on the strength of the suspended elements themselves, and on the strength of their bonds with each other and with intact walls, is the main task of protecting buildings from progressive collapse.

1.3. The stability of a building against progressive collapse should be ensured by the most economical means that do not require an increase in the material consumption of prefabricated elements:

- a rational structural and planning solution of the building, taking into account the possibility of the emergence of the emergency situation under consideration; in particular, it is not recommended to use free-standing internal wall pylons connected with other vertical structures only by slabs; the use of free-standing external (end) walls is not allowed;

- constructive measures that promote the development of plastic deformations in prefabricated elements and their joints at ultimate loads;

- rational solution of the system of structural connections, individual nodes and elements of connections and joints of panels.

2. METHODOLOGY OF CALCULATION OF PANEL BUILDINGS FOR RESISTANCE AGAINST PROGRESSIVE CRASH

2.1. The stability of a building against progressive collapse is checked by calculating for a special combination of loads and effects, including permanent and temporary long-term loads, as well as the effect of hypothetical local failures of load-bearing structures.

2.2. Permanent and temporary long-term load should be determined according to SNiP 2.01.07-85 *. In this case, the load combination factors and the reliability factors for loads to permanent and long-term loads should be taken equal to unity.

2.3 The impact of local destruction of load-bearing structures is taken into account by the fact that the design model of the structural system of a building is considered in several versions, each of which corresponds to one of the possible local destruction of structures during emergency impacts.

For panel residential buildings, the destruction (removal) of two intersecting walls of one (any) floor in sections from their vertical joint (in particular, from the corner of the building) to the nearest opening in each wall or to the next vertical joint with wall in a perpendicular direction.

To assess the stability of a building against progressive collapse, it is allowed to consider only the most dangerous calculation schemes of destruction:

local destruction, including destruction of external walls, weakened by doorways of exits to balconies and loggias (schemes 1, 2, 3 in Fig. 1);

local destruction, including the destruction of internal walls, weakly connected with the rest of the vertical structures due to the presence of doorways (see schemes 2, 4, 5 in Fig. 1), due to beam cutting of large-span ceilings (see schemes 2, 4, 5 in Fig. 1) or due to the partial absence of connections through the floors (walls adjacent to the stairwells; Scheme 4 in Fig. 1).

Fig. 1. Fragment of a residential building

Fig. 1. Fragment of a residential building

2.4. When calculating panel buildings for resistance against progressive collapse, the normative resistances of materials are taken in accordance with SNiP 2.03.01-84 * and SNiP II-23-81 *. Design characteristics resistance of materials, determined by dividing the standard resistance by the reliability factors for concrete and reinforced concrete structures, increase by using the reliability factors specified in Table 1. In addition, design resistances multiplied by the coefficients of operating conditions, taking into account the low probability of emergency impacts and intensive growth the strength of concrete in the first period after the construction of the building, as well as the possibility of using reinforcement beyond the yield point of the material.

Table 1

Material	Stress state or material characteristic	Symbol	Reliability factor by material
	Compression
	Stretching

Coefficients of working conditions for concrete are taken according to table 2, for reinforcement of all classes it is introduced single coefficient.

table 2

Factors causing the introduction of the coefficients	Symbol	Condition factor concrete work
1. Concrete structures
2. The increase in the strength of concrete over time, except for concretes of class B50 and higher, concretes based on alumina cement, aluminate and alite Portland cements
3. Prefabricated elements (concrete and reinforced concrete)

The design resistances of rolled steel are taken according to SNiP II-23-81 * taking into account the admissibility of the work of ductile steels beyond the yield point. The coefficient of working conditions for ductile steels is assumed to be 1.1.

2.5. To calculate panel buildings for resistance to progressive collapse, it is recommended to use a spatial design model in the form of a system of plates (with or without openings), interconnected by concentrated bonds, the strength of which is equivalent to the strength of the actual bonds between the panels (Fig. 2, a).

Fig. 2. Computational model of a building with localized destruction

Fig. 2. Computational model of a building with localized destruction

1 - local damage

Such a model should include elements that, under normal operating conditions, are non-bearing, and in the presence of local destruction, they actively participate in the redistribution of the load: external hinged panels, installation ties, etc. The building model should be calculated for all calculated schemes of local destruction of structures selected in accordance with the recommendations of clause 2.3.

2.6. In the case of ensuring the plastic work of the structural system in the limiting state, the calculation is recommended to be carried out by the kinematic method of the theory of limiting equilibrium. In this case, it is allowed to check the stability of only the elements located above the local destruction, and the calculation of the building for each selected scheme of local destruction is reduced to the following procedure:

the most probable mechanisms of the progressive (secondary) collapse of building elements that have lost support are set (to set the destruction mechanism means to determine all the destructible connections and find possible generalized displacements () in the direction of the forces in these connections);

for each of the selected mechanisms of progressive collapse, the strengths of all plastically destructible bonds are determined (); there are the resultant external forces applied to individual links of the mechanism, that is, to individual indestructible elements or their parts (), and displacement in the direction of their action ();

the work of internal forces () and external loads () on possible displacements of the considered mechanism is determined

and the equilibrium condition is checked

The specified calculation procedure is detailed in the mandatory Appendix 1 and is applicable only if the requirements of clauses 3.2, 3.3 on ensuring the plastic operation of individual panels and connections between them in the limiting state are met. If the plasticity of any connection is not ensured, its work should not be taken into account (the connection is considered absent). If there are too many such ties and elements that can break down brittle, and their formal exclusion too greatly reduces the estimate of the building's resistance to progressive collapse, either ensure the ductility of the ties, or use another - an elastic design model of the building (see clauses 2.7 and 2.8) ...

2.7. The elastic design model of the building, like the elastic-plastic one, should include the calculated local destruction and allow taking into account the changed nature of the work of the elements that have lost support.

The forces obtained in the elastic calculation in individual elements should be compared with their calculated bearing capacities. In this case, the stability of the building against progressive collapse will be ensured if for any element the condition is met, where and, accordingly, the force in the element, found from the elastic calculation, and its design bearing capacity, found taking into account the instructions in clause 2.4.

2.8. Instead of calculating the resistance against progressive collapse, it is allowed to calculate buildings for a seismic effect equal to 6 points in accordance with SNiP II-7-81 *, taking the necessary coefficients for extrapolation. Based on the results of this calculation, nodes and communications should be designed in accordance with SNiP 2.03.01-84 * and SNiP II-23-81 *.

3. CONSTRUCTION REQUIREMENTS

3.1. To protect large-panel buildings from the progressive collapse of the connection between prefabricated elements, installed based on normal operating or installation loads or for structural reasons, they should be designed taking into account the possibility of emergency local destruction. For effective solution problems of protecting large-panel buildings from progressive collapse, taking into account all design tasks under normal operating and installation conditions, the following communication system is most preferable:

- horizontal longitudinal and transverse connections between floor slabs, providing the necessary strength of the floor disks in tension and shear;

- vertical (interfloor) connections between load-bearing wall panels of one wall pylon, providing the necessary strength of horizontal joints of walls and floors in tension and shear;

- horizontal connections between curtain outer walls and floor discs, ensuring stability and operation against wind and temperature effects of curtain wall panels.

The optimal system of ties does not include the usually used as assembly horizontal ties between wall panels of the same floor; these connections are not always feasible (the possibility of establishing them depends on planning solutions buildings) and, as a rule, are ineffective in conditions other than installation; nevertheless, when using these bonds, they must be designed so that their resistance to progressive collapse is maximum, i.e. in accordance with the requirements of clause 3.2.

3.2. Effective operation of the bonds that prevent progressive collapse is possible only if their plasticity is ensured in the limiting state: it is necessary that after exhaustion bearing capacity the connection was not turned off from work and allowed relatively large absolute deformations (of the order of several millimeters) without destruction.

To ensure the plasticity of the joints of prefabricated elements, their design solutions should include special plastic elements made of plastic sheet or reinforcing steel.

A stretched linear connection between prefabricated elements, as a rule, represents a chain of elements connected in series - an anchor of an embedded part, an embedded part, the connection itself, an embedded part of the second element and its anchor. Due to the random variability of the resistances of the individual elements of this circuit and their connections, the limiting state of the entire joint is determined by the weakest link. Accordingly, the real plasticity of the entire compound depends on which element turns out to be the weakest:

if concrete gouging occurs, in which the embedded part is anchored, then the destruction will be fragile in nature with very insignificant absolute deformations preceding the disconnection of the connection from work (Fig. 3, a);

if one of the welded joints collapses, then, although during high-quality welding, plasticity will manifest itself, due to the small length of the destructible link itself, the absolute deformations preceding the disconnection of the connection from work will be relatively small (Fig. 3, b);

only in the case when the weakest link in the compound is the actual metal bond, the entire compound will exhibit the maximum possible plastic properties (Fig. 3, b).

Fig. 3. Deformation diagram of a stretched linear bond with the destruction of its various elements

Fig. 3. Deformation diagram of a stretched linear bond with the destruction of its various elements

a) - when gouging out anchoring concrete; b) - in case of destruction of welded joints;
c) - upon destruction of a sheet or bar connection

The joints of prefabricated elements that prevent the progressive collapse of panel buildings should be designed unequally, while the element, the limiting state of which provides the greatest plastic deformation of the joint, should be the least strong.

To fulfill this condition, it is recommended to calculate all the elements of the connection, except for the most plastic one, for a force 1.5 times higher than the bearing capacity of the plastic element, for example, anchoring embedded parts and welded joints it is recommended to count on a force 1.5 times greater than the bearing capacity of the connection itself. In this case, the bearing capacity of the connection should be determined in accordance with SNiP II-23-81 * according to the formula

At , . It is necessary to especially monitor the actual exact execution of the design solutions of plastic elements, replacing them with more durable ones is unacceptable.

3.3. The effectiveness of resistance to the progressive collapse of a building requires plastic work in the limiting state, not only of connections, but also of other structural elements. In particular, it is necessary:

overhead lintels, which work as shear links, should be designed so that they fail from bending, and not from the action of a shear force;

the key connections should be designed so that the shear strength of the individual keys is 1.5 times greater than their crush strength.

3.4. The cross-section of all types of ties listed in clause 3.1 should be determined by calculating the operational, installation or emergency effects considered here, but not less than the following values ​​required to ensure the perception of tensile forces:

for horizontal ties located in the floors along the length of the building extended in the plan - 15 kN (1.5 tc) per 1 m of the building width;

for horizontal ties located in the ceilings perpendicular to the length of a building extended in the plan, as well as for horizontal ties in buildings with a compact plan - 10 kN (1.0 tf) per 1 m of the building length; for horizontal ties between concrete and reinforced concrete hinged external panels with floor disks - at least 10 kN (1 tf) per 1 m of wall length;

for vertical interfloor links, the optimal design solution of which provides for the use of parts for lifting panels (lifting loops, pins, etc.) - no less than the strength of the corresponding part for lifting;
with other design solutions, at least 25 kN (2.5 tf) per 1 m of the wall width.

APPENDIX 1 (mandatory). METHODOLOGY FOR CALCULATING STABILITY AGAINST PROGRESSIVE COLLAPSE OF PANEL BUILDINGS CROSS-WALL SYSTEM

ANNEX 1
(required)

1. Methodology for calculating buildings with non-bearing longitudinal outer walls made of light non-concrete materials

1. For buildings with load-bearing transverse and internal longitudinal walls and non-load-bearing non-concrete longitudinal external walls, the risk of local destruction is determined only by its location on the building plan and does not depend on its location along its height. The most dangerous and, therefore, calculated local damage are:

destruction of the panel of the end transverse wall adjacent to the corner of the building;

destruction of the panel of the inner transverse wall, carrying the load from hinged loggias or balconies and, moreover, weakened by doorways.

The number of calculated local destruction of the indicated types in each specific case is determined individually, depending on the features of the building plan and the adopted constructive solutions... With a unified solution of prefabricated elements and connections between them and a relatively simple building plan, one can confine oneself to considering two or three dangerous local destruction.

For each selected local failure, it is necessary to consider all the mechanisms of progressive collapse indicated in clauses 2-5 and check the equilibrium condition

Where, - respectively, the work of internal forces () and external loads () on possible displacements of the mechanism under consideration:

2. The first mechanism of progressive collapse is characterized by the simultaneous downward displacement of all wall panels (or their individual parts) located above the local collapse (Fig. 4). Such a displacement is possible when the shear bonds between the longitudinal and transverse walls are destroyed (Fig. 4, a) or when the overhead bulkheads and floor slabs are destroyed (Fig. 4, b, c).

Fig. 4. A variant of the mechanism of progressive destruction of type I

Fig. 4. A variant of the mechanism of progressive destruction of type I

When assessing the possibility of simultaneous collapse of structures of all floors, the equilibrium condition (1) is replaced by the condition

Where and - respectively, the work of internal and external forces on the movements of the elements of one floor; the floors are separated by the bottom surface of the slab, which refers to the floor above the slab.

If the floor slabs are not inserted into the longitudinal load-bearing walls, collapse is prevented only by the shear bonds between the panels of the destroyed transverse wall and the longitudinal wall (Fig. 4, a). In this case, equilibrium condition (2) is equivalent to the requirement

Where is the strength of the shear bonds in the vertical joint between the longitudinal and transverse walls; , - respectively, the weight of the transverse wall panel and the load on it from the loggia; , - respectively, the weight of the panels of the outer walls adjoining from both sides to the destroyed transverse wall; , - uniformly distributed load on floor slabs; ,,, - dimensions of floor slabs resting on the destroyed wall.

If floor slabs are inserted into longitudinal and transverse walls (platform joints), then they form an almost indestructible shear bond between them. In this case, only those types of type I collapse mechanism are considered that are possible when the transverse wall is weakened by openings (see Fig. 4, b, c). In this case, condition (2) takes the form

where, - respectively, the work of internal and external forces on the movements of individual parts of the panel of the inner wall; , - respectively, the work of internal and external forces applied to the floor slabs; - the work of external forces applied to the outer panels.

The work is determined by the resistance to bending of the above and below the opening lintels and in the general case is determined by the ratio

where,,, - respectively, the bending strength of the left and right support sections of the upper and lower lintels, and is the span of the lintels.

If the transverse wall is separated from the longitudinal doorway and there is no connection between them, then = 0. If the connection between the transverse and longitudinal walls is carried out by a jumper - "flag" (see Fig. 4, c), then the strength of the support section () is determined by the strength of the horizontal linear connection (); in this case, the strength of the shear bond in accordance with the recommendations of clause 4 must satisfy the condition

The work is determined by the weight of the collapsing part of the inner wall panel, (where is the weight of the entire panel, 0< <1) и приложенной к ней вертикальной нагрузкой от навесной лоджии()

The work of external and internal forces applied to floor slabs, initially supported on three sides, is determined by their plastic fracture according to the scheme shown in Fig. 4, b, c, and is calculated by the formulas

The span of the th slab in the direction of the longitudinal walls and the span in the direction transverse to the building; , - bending moments taken by the th floor slab when it is bent along the beam pattern, respectively, along the spans and when the lower fibers (upper fibers) are stretched; - the width of the doorway in the inner wall (see Fig. 4, b, c); - binding the opening to the inner end.

If the floor is made of beam slabs, then inequality (9) is assumed to be

The work of forces due to the weight of the outer panels adjacent to the damaged wall on the left and right (and) is approximately calculated as follows:

The fulfillment of requirement (4) is a prerequisite for preventing the progressive collapse of the building, with relatively small displacements (less than 10 cm) of structures that have lost support. If it is fulfilled, you should proceed to checking the additional conditions set forth in clauses 3-5.

If condition (4) is not met, two options are possible:

the first is to achieve its implementation by strengthening (or redistributing) the reinforcement of the lintels of the inner walls and floor slabs;

the second is to switch to other constructive methods of protection against progressive collapse, allowing very large displacements (tens of centimeters) of elements that have lost support and require, accordingly, to perform the calculation according to the deformed scheme (see clause 6).

3. The mechanism of progressive collapse of the second type is characterized by the simultaneous rotation of each wall panel, located above the local destruction, around its center of rotation (Fig. 5). Such a displacement requires the destruction of the stretched bonds of these panels with an intact wall (in Fig. 5, a), the destruction of the shear bonds of the wall panels with floor slabs in horizontal joints (in Fig. 5) and plastic fracture of the floor slabs, initially supported on three sides, along the diagram shown in Fig. 5, d.

In the case under consideration, condition (2) takes the form

where,,, is the same as the quantities,,, in (4), and is the work of the resistance forces of the connections (and) of the wall panels that have lost their support, with intact structures. Individual terms from (12) are calculated as follows:

where,, - the distance from the center of rotation to the line of action of the forces and and the force of gravity (see Fig. 5);

Calculated by formulas (8) with a corresponding replacement of the superscript, and

Here all quantities have the same meaning as in (9); the value is calculated by the formula (11).

The fulfillment of condition (12) should be achieved primarily by increasing the shear bonds (), since an increase in the strength of the stretched bond () is not always possible (Fig. 5, b), and sometimes it is impractical: if a transverse wall is attached to the longitudinal wall only with on the one hand, then to take this connection into account in the calculation, it is necessary to evaluate the bending strength of the longitudinal wall from its plane (see Fig. 5, c).

Fig. 5. Mechanism of progressive destruction of type II

Fig. 5. Mechanism of progressive destruction of type II

4. In addition to the non-collapse conditions (4) and (12), it is necessary to assess the possibility of collapse of only one floor slabs located directly above the knocked-out panel of the transverse wall and initially supported on three sides (third mechanism).

In order for these plates not to collapse, it is enough to fulfill the condition

where is the strength of the shear connection between the curtain panel and the transverse wall (Fig. 6); in the formula (16) is taken by calculation, but not more than the value.

If relation (16) is not met, this means that the slabs must be attached to the upper transverse wall with ties that perceive tension (Fig. 6). Then condition (16) is replaced by the following:

Where is the work of the tensile forces. This work is calculated by the formula

Number of links; - the coordinate determined by the line of action of the resultant reaction of the considered bonds on the assumption that they all have reached their limiting value -.

Fig. 6. The scheme of the collapse of floor slabs

Fig. 6. The scheme of the collapse of floor slabs

If the floors are made of beam slabs, condition (16) is not met (); therefore, in this case, the setting of connections of the type under consideration is mandatory. Moreover, their strength is determined by the magnitude of the support reactions of each beam slab.

5. The fourth collapse mechanism provides for the movement of structures of only one floor, located directly above the knocked-out panel of the transverse wall (Fig. 7). This mechanism involves a combination of the translational movement of the transverse wall (as in the first mechanism) with the fracture of the plates, characteristic of the second mechanism (see Fig. 5, c, d). Such a mechanism is possible only when the transverse wall is weakened by door or window openings.

The condition for the impossibility of the formation of a mechanism of the type under consideration

where is the work of the tensile forces of vertical ties of the type and;

Where is the number of bonds of the sixth type; , - ultimate efforts in bonds of the sixth and fifth type; - displacements in the direction of the -th bond of the fifth type, they are defined as the difference between the displacements of the point of attachment of the bond to the plate and the point of attachment of the bond to the transverse wall panel.

If, in the absence of constraints of the sixth type (= 0), condition (19) is not satisfied, it is not recommended to achieve its fulfillment by strengthening the constraints of the fifth type - this is uneconomical, since these constraints, as follows from equation (20), work unevenly. In this case, the most rational solution is to put connections of the sixth type and form inter-floor connections.

6. If, with local destruction of the inner transverse wall, it is not possible to ensure the fulfillment of condition (4), that is, it is not possible to prevent progressive collapse according to the first scheme (see Fig. 4), it is recommended to use special bonds of floor slabs to ensure their effective resistance to progressive collapse in case of large deflections as elements of the hanging system (Fig. 8). Such a technique usually turns out to be expedient and necessary in case of local destruction of a transverse wall, significantly removed from the rest of the bearing walls and connected with them only by beam floor slabs or weakly reinforced large-span slabs, initially supported on three sides.

Fig. 7. Scheme of collapse of structures of one floor

Fig. 7. Scheme of collapse of structures of one floor

Fig. 8. Work of floor slabs as elements of a hanging system

Fig. 8. Work of floor slabs as elements of a hanging system

The requirements that the ties and slabs forming the hanging system must satisfy follow from the calculation according to the deformed scheme (see Fig.8b): a chain of series-connected elements (link - plate - link - plate - link) must include a very plastic link, which would provide a total elongation of the chain of the order of several percent (naturally, any cracks in the slabs are allowed). To fulfill this condition, it is necessary that

Where is the linear load on the destroyed wall from each floor

Linear bearing capacity of the weakest link in the hanging chain; - the calculated relative elongation of a slab with a smaller span (more precisely, the relative increase in the distance between the points of joining of this slab with other slabs); - deflection at which equilibrium is achieved; , are the minimum and maximum spans, respectively.

Relations (21) were obtained from the assumption that, due to the random variability of the resistance of materials, the maximum possible elongation is realized only in one slab. Thus, in the case for, it follows from (21) that and.

The maximum possible relative elongation of a slab significantly depends on the design solution of its reinforcement and connections between the slabs, on the ratio of the strengths of individual elements, on their plasticity, on the strength of the connection of these elements; theoretically, it is not possible to determine this value in the general case, and therefore it is recommended to evaluate each specific constructive solution experimentally.

2. Methodology for calculating buildings with external walls made of concrete or reinforced concrete panels

7. For the design of buildings with reinforced concrete outer walls, the same basic types of progressive collapse mechanisms should be used as for buildings with non-bearing outer walls made of lightweight non-concrete materials. In this case, however, it must be borne in mind that the formation of these mechanisms requires the destruction of not only internal wall panels and floor slabs, but also external wall panels, which in this case must be included in the work, even if they are designed as hinged.

External wall panels with an opening, regardless of the type of general progressive collapse mechanism, work in a skewed manner like rectangular frames (Fig. 9). In this case, if the floor slabs are inserted into the outer walls, then they are also involved in the work, and the nature of their destruction changes - to the main plastic hinges shown in Figs. 4 and 5, hinges associated with a break in the outer edge of the slab are added (Fig. 10 ). When checking the possibility of collapse of some floor slabs (see clause 10), these hinges are absent.

In order to take into account the resistance of the outer walls to progressive collapse and the additional resistance of the floor slabs associated with them, it is necessary to calculate the work of the corresponding internal forces () according to paragraph 14 and use it when checking the equilibrium conditions specified in paragraph 15.

8. In order to take into account the resistance of the outer wall to progressive collapse, it is necessary to calculate the work of internal forces during the destruction of the panels of the outer walls of a typical floor (). Since, during local destruction of the inner wall, two panels of the outer wall (or one two-module) resist the progressive collapse on each floor, the value is generally considered as the sum of the terms

The amount of work () depends on the ratio of the geometric dimensions of the panel and the reinforcement of its lintels and walls, as well as on the presence of an opening for the balcony door in it. In the general case, any outer panel can be considered as a frame that collapses due to the formation of four plastic hinges in it (see Fig. 9.b, c), so that

In this case, the limiting bending moments acting in the corner joints (for example, in the upper left corner) are determined as the smallest of the two values ​​of the bearing capacities for bending of the lintel and the wall that form this angle.

Fig. 9. Work of elements of external walls

Fig. 9. Work of elements of external walls

Fig. 10. Floor Slab Operation in Buildings with Reinforced Concrete Exterior Walls

Fig. 10. Floor Slab Operation in Buildings with Reinforced Concrete Exterior Walls

In the event of local destruction of the transverse wall adjacent to the corner of the building, the outer wall panel may collapse according to the rotation pattern of the hard disk (see Fig. 9, a); in this case, the work of internal forces will be determined by the strength of the shear bond of this panel with the overlying overlap () and the stretched bond with the adjacent facade panel ()

Of the two possible values ​​determined by formulas (23) and (24), in
further calculations take into account less.

9. To take into account the resistance of the outer wall to progressive collapse, first of all, it is necessary to make sure that it "carries itself", that is, to check the condition

In which the work of external forces is determined by the formula (11).

In cases where condition (25) is not met (), the entire further calculation is carried out in the same way as for buildings with longitudinal curtain walls made of light non-concrete materials - according to the recommendations of clauses 25 with the only difference that in all ratios the work is replaced size. If condition (25) is fulfilled, then further calculation is determined by the constructive solution of the conjugation of the floor slabs and the outer longitudinal wall.

If the floor slabs are not inserted into the outer wall, it is necessary that the strength of the connection between the inner panel of the transverse wall and the panels of the outer walls during their mutual displacement () satisfy the condition

In this case, the check for the possibility of progressive collapse is carried out sequentially according to the recommendations of paragraphs 8-11 with the following minor changes:

in relations (4) and (12), work is replaced by the value -;

in formulas (16), (17) it is assumed that;

in formula (19) is taken.

If the floor slabs are inserted into the outer wall, then the shear bond between the inner transverse and longitudinal outer walls may not be installed (= 0), and only conditions (4) and (12) are checked to assess the protection of the building from progressive collapse.

The Department of Urban Planning and Architecture of the Ministry of Construction, Housing and Utilities of the Russian Federation, within its competence, considered a letter on the issue of the requirements of regulatory and technical documents, and the following is reported.

The term "load-bearing structures" is practically not used in normative and technical documents, since the definition of load-bearing structures is given in textbooks on structural mechanics and is understandable for every designer. The definition of the bearing capacity is established only in SP 13-102-2003 * "Rules for the examination of load-bearing building structures buildings and structures "(hereinafter - SP 13-102-2003), which is currently not valid documents on standardization. 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 ”calculation for progressive collapse is carried out for buildings and structures of the KS-3 class, as well as (on a voluntary basis) buildings and structures of the KS-2 class.

The requirement on the need to calculate 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 - 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 amended in 2018 by amending SP 56.13330.2011.

In 2017, SP 296.1325800.2017 “Buildings and structures. Special Impacts "(hereinafter - SP 296.1325800.2017), which comes into force on February 3, 2018 for use on a voluntary basis. This set of rules indicates that when designing structures, scenarios for the implementation of the most dangerous emergency design situations should be developed and strategies should 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 Actions ”(hereinafter referred to as SP 20.13330), must include one of the standardized (design) special actions or one variant of local destruction of load-bearing structures for emergency special actions. The list of scenarios of emergency design situations and the corresponding special effects 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, the failure of which entails a 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 indicates the conditions under which it is allowed not to take into account emergency effects:

Special technical conditions for the design of the structure have been developed;

Scientific and technical support was carried out at all stages of the design and construction of the structure, as well as the manufacture of these elements;

The calculation of the structure for the action of design (standardized) special impacts specified in SP 296.1325800.2017, design assignment and current regulatory documents was carried out;

Additional coefficients of operating conditions have been introduced, which reduce the design resistance of these elements and their attachment points (for large-span structures, the indicated additional coefficients of operating conditions are given in Appendix B of the specified joint venture);

Organizational measures were taken, including in accordance with SP 132.13330.2011 “Ensuring anti-terrorist security of buildings and structures. General design requirements ", 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, research) with experience in the relevant fields and the necessary experimental base.

Document overview

Clarifications are given on the application of normative and technical documents in the qualification of load-bearing structures. In particular, the following was noted.

The term "supporting structures" is practically not used in normative and technical documents, since the definition is given in textbooks on structural mechanics and is understandable for every designer. The definition of the concept of "bearing capacity" is given.

In accordance with the provisions of GOST 27751-2014 "Reliability of building structures and foundations. Basic provisions" calculation for progressive collapse is 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 application on a voluntary basis. When designing structures, scenarios for the implementation of the most dangerous emergency design situations and strategies should 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 of emergency design situations and the corresponding special effects is established by the customer in the design assignment in agreement with the general designer.

The procedure for scientific and technical support of works has been clarified.

Published: March 8, 2008

Measures to protect against progressive collapse

6.1.1. High-rise buildings should be protected from progressive collapse in the event of local destruction of load-bearing structures as a result of emergency emergencies (ES).

The latter include:

Natural emergencies - dangerous meteorological phenomena, the formation of sinkholes and sinkholes in the foundations of buildings;

Anthropogenic (including man-made) emergencies - explosions outside or inside a building, fires, accidents or significant damage to supporting 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 nodes 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 emergencies situations. M., 2002).

6.1.3. The calculation of the stability of the building must be made for a special combination of loads, including permanent and long-term loads with the following possible schemes of local destruction:

Destruction (removal) of two-intersecting wall (any) floors at sections of other intersections (in particular, cornering) of the nearest openings in each wall or the next intersection with another wall with a length of less than 10 m, which corresponds to the damage to structures up to 80 square meters;

Destruction (removal) of columns (pylons) or columns (pylons) adjacent to adjacent sections of walls located on one (any) storey of the area of ​​local destruction;

Collapse of the overlap section of one floor on the area of ​​local destruction.

To assess the stability of the publication against progressive destruction, it is allowed to consider unnecessarily 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 failure according to the limiting states of the first group with the design resistances of materials (concrete and reinforcement) equal to the standard values.

In this case, the magnitude of deformations and the width of crack opening in structures are not regulated.

6.1.5. Permanent 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, while the load combination factors and load safety factors are taken to be equal to one.

6.1.6. For the design of buildings against progressive collapse, a spatial design model should be used, which can take into account elements that are not 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 should reflect all the schemes of localized destruction specified in paragraphs. 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 bearing capacity of columns, girders, diaphragms, floor disks and joints of structures; creation of non-linearity and continuity of reinforcement of slabs, increasing the plastic properties of connections between structures, inclusion of non-bearing elements in the work of the spatial system.

Effective operation of the links preventing progressive failure is possible by ensuring their plasticity in the limiting state, so that, after the carrier has been exhausted, the communication should not be switched off from work and allowed without destruction, the necessary deformations.

6.1.8. In high-rise buildings, preference should be given to monolithic and precast-monolithic floors, which must be reliably connected to the vertical supporting structures of the building by ties.

The ties connecting the ceilings with columns, crossbars, diaphragms and walls must keep the floor from falling (in case of its destruction) to the underlying floor. The ties must be calculated for the standard weight of half of the span with the floor and other structural elements located on it.

From: zina, & nbsp

In recent years, the danger of terrorist acts has been growing in the world, and the geography and scale of terrorism are expanding.

Terrorists usually pursue political, religious, nationalistic, selfish or other goals and are aimed at intimidating people, society, and government bodies. Terrorist attacks usually kill innocent people and cause social, material or environmental damage.

In contrast to emergencies of man-made and natural origin, terrorist acts refer to emergencies caused by deliberate unlawful actions with the malicious intent of various criminal groups or individuals. Therefore, such emergencies cannot be attributed to random events, but their forecast is possible. These events are predicted with the help of information obtained through various channels, including undercover, as well as game methods (like antagonistic zero-sum games).

In 1998, the law "On the fight against terrorism" was adopted, which entrusted the Ministry of Internal Affairs of the Russian Federation with the task of preventing, detecting and suppressing crimes of a terrorist nature.

The targets of terrorist attacks are usually potentially dangerous industries. crowded places (especially in confined spaces), transport facilities, public and administrative buildings, as well as multi-storey residential buildings.

As means of terror, explosive devices, combustible mixtures, potent poisonous substances (SDYAV), poisonous, radioactive substances and bacterial aerosols can be used. In this case, explosive devices can be disguised as various household products.

The result of a terrorist attack can be an explosion, fire, contamination of the territory, air, water or food, epidemics, etc.

Terrorist attacks are known that were committed without the use of special means, but by deliberately releasing an energy potential or an active component from existing energy networks (for example, gas pipelines) or storage tanks for chemically hazardous substances.

It is advisable to provide technical and constructive measures to counter terrorist attacks in buildings even at the stage of its design, because it is more difficult to carry out these measures in already existing buildings.

When considering a set of anti-terrorist measures, proceed from the following general principles:

Identification of the most vulnerable places in the building and its life support systems, restriction or complete exclusion of access of unauthorized persons to these places;

use in the life support systems of buildings of equipment equipped with safety elements that exclude unauthorized start (stop) or intentional damage to equipment;

in providing observation (monitoring) and control over the situation inside and outside the building;

the use of modern means and systems for recognizing the presence of dangers and threats;

the use of automatic protection means that ensure the operation of the corresponding units and devices when dangers are recognized;

the presence of an emergency power supply, as well as an alarm system and warning people about the dangers that have arisen;

availability of developed instructions for the behavior of people in extreme situations.




Each object of interest to terrorists has some vulnerabilities. In residential and public buildings, these are basements, elevator shafts, technical floors, air intakes of ventilation systems.




For example, ventilation and central air conditioning systems and elevator roofs are the most vulnerable to chemical or biological terrorism. In the first case, hazardous substances in the form of gas or aerosols that have entered the air intake device spread through the air duct network to the premises at a high speed, in the second, when the elevator moves, a powerful air flow is created and the substance spreads through the floors, the son-in-law enters the premises.




When the outside (atmospheric) air is contaminated, it is advisable inside the building to provide for the possibility of creating a backwater (overpressure) using the supply ventilation system (provided that the air intake device is outside the contaminated zone).

In general, in order to reduce the effectiveness of a terrorist attack using ventilation and air conditioning systems in a building, the following requirements should be taken into account in their design:




• place air intakes in places that are relatively hard to reach and hidden for unauthorized persons and equip them with strong grilles; ensure the standard or increased tightness of the air duct network (according to SNiP), the device of bypasses and the equipping of the network sections with automatic dampers to turn off the contaminated areas and change the direction of air flows;

  provide a system of sensors for detecting toxic substances near the air intake, at the outlet from the fans, at the inlet and outlet of the central air conditioner;

  provide access control to the technical floors of the building, to viewing hatches, fans, filters, pumps, irrigation chambers, power supply devices, etc.

  when integrating all life support systems of a building into a single dispatching computer system, which is typical for "smart" buildings, in addition to emergency power supply of this system, provide information protection of computer programs from unauthorized access and attempts to hack via a telephone line or from the Internet.

Currently, one of the urgent tasks related to protective technologies is the creation of effective and inexpensive means of detecting a wide range of chemical and biological substances in the air, as well as methods for their neutralization.

1. Progressive collapse

   In its initial ideology, the method of calculated limit states was not guided by the analysis of emergency situations, which were considered outrageous and excluded from consideration on the grounds that the limit states of the first group precede an accident and their avoidance, in theory, prevents the occurrence of an accident.

   The introduction of a two-tier approach to the design of earthquake-resistant structures, as well as an analysis of the actual causes of accidents, have shaken this paradigm. In particular, recently there has been a clear tendency to design with protection against progressive destruction. The term "progressive collapse" and the wording of the problem of protecting panel buildings from it appeared in 1968 in a report by the commission investigating the causes of the famous accident of the 22-storey Ronan Point apartment building in London. This dramatic event began with a gas explosion in one of the apartments on the 18th floor, caused by a gas stove leak. The outer panels of the building were designed to withstand only wind pressure, and after collapsing on one floor, the ability to transfer vertical loads from the upper floors was lost. Debris from floors from the 18th to 22nd floors fell on the floor of the 17th floor, which gave rise to a chain of overlapped failures, since the debris load exceeded the carrying capacity of one floor. The result was that an entire corner of the building above and below the explosion site collapsed.

   The Ronan Point Building has complied with all building codes and has been found to be free from manufacturing defects. But the progressive collapse was inevitable, since the design scheme was similar to a house of cards, that is, it had no way to redistribute the load on individual subsystems and thereby localize the failure.

   A new wave of activity was caused by collapses caused by terrorist attacks on a high-rise building in Oklahoma City and on the towers of the World Trade Center in New York, and in our case - the destruction of the cover over a water park in Moscow. Numerous public appearances, often unqualified, gave rise to rumors, doubts and unrealistic demands. Even in the publications of professionals, there are references to some myths related to the supposedly absolute survivability of old-design buildings in which people can be, or, conversely, to complete disregard for the possibility of an emergency and the need for an absolute guarantee of the indestructibility of objects.

   Normative documents for the design of supporting structures in an explicit form say nothing about the need to check structures for survivability, that is, about the need to monitor the situation after the failure of any of the parts or subsystems of the supporting frame. True, usually the norms contain a reference to GOST 27751-88, where in paragraph 1.10 it is said that when calculating structures, an emergency design situation should be considered that occurs immediately after the failure of any structural element. But the link itself is very vague, and the wording of the GOST is inaccurate, since it can hardly be assumed that the designer is obliged to ensure the existence of the object after the failure of any structural element. It is enough to imagine any dome covering with a destroyed support ring or a bridge with a collapsed support to require the closure of almost all temples and the cessation of traffic on all bridges.

   Obviously, for some structures, survivability must be achieved by the simultaneous use of three types of protection: a sufficient reserve of the bearing capacity of some structural elements, the exclusion of progressive destruction due to the failure of other structural elements and a complex of protective anti-terrorist measures.

   Obviously, it is required to concretize the indication of GOST 27751-88, for example, supplementing it with the requirement that the formulations of failure states be contained in the design standards for buildings and structures of a particular type. In fact, this is what they do, for example, when designing the structures of power transmission lines, where the rules indicate a list of emergency modes. The ideology of designing a nuclear power facility is similar, where, in particular, it is fundamentally important to use concepts such as design basis and beyond design basis accident.

   Protection of buildings in emergency design situations should be provided in advance and is determined by the relevant design standards; for load-bearing elements, it is implemented, in particular, in the form of creating the necessary reserves of bearing capacity to prevent destruction. Protection of buildings in beyond design-basis situations is focused not on preventing destruction, but on ensuring the safety of people and the possibility of their evacuation, on the implementation of the necessary margin of time, etc.

   Assessment of the possibility of progressive destruction and the development of measures to prevent it poses the following unconventional tasks for the designers:

   determination of the list of starting emergency actions causing local destruction;

   development of a methodology for calculating complex multi-element structures for sudden destruction of one or more load-bearing elements;

   establishment of criteria for failure of load-bearing elements overloaded as a result of emergency impact;

   development of constructive measures to protect and mitigate the consequences of emergency impact.

   The scientific solution to many of these problems, and in particular to their normative formulation, has most often not yet begun, although there are some pioneering developments here. As the analysis of emergency situations shows, the most frequent initiating events leading to beyond design basis accidents are local emergency impacts on individual structures of one building: explosions, fires, karst sinkholes, collisions of vehicles, defects in structures and materials, failures of building engineering systems, incompetent reconstruction, etc. etc. These are random, generally unpredictable effects, the parameters of which are very difficult to determine.

   Our general construction standards do not provide data on the values ​​of emergency impacts; such information is fragmentarily present in regulatory documents of a different type. It seems that it would be useful to have a normative document that would provide the rules for determining loads for such massive emergency situations as impacts during collisions of vehicles, falling loads, industrial explosions, etc. Data on some of these loads are contained in chapter of Eurocode-1, many of them are traditionally taken into account in the design of nuclear power facilities.

   It was also suggested that instead of real beyond design basis emergency actions, consider their conditional analogs or local damages already caused by them. In particular, the guidelines provide the following list of such initiating events:

   • the formation of a karst sinkhole with a diameter of 6 m, located anywhere under the foundation;

   damage to the floor with a total area of ​​up to 40 m 2 ;

   destruction of two intersecting walls in the area from their conjugation (including from the corner) to the nearest opening or to the next intersection, but at a length of no more than 3 m;

   destruction of any of the walls of the outer wall or the inner wall between two doorways;

   the appearance within one floor of a horizontal load on vertical elements (on the rods, a concentrated force of 3.5 t, on the walls and diaphragms of 1 t / m 2 ).

   This list also indirectly indicates that small structures, the dimensions of which are comparable to the size of "local" damage, does not make sense to check for the possibility of progressive destruction. Therefore, it is advisable to establish some criteria for the selection of objects of analysis, and here it is advisable to have a classification of buildings and structures according to the following criteria:

   objects of class 1, in the design of which it is allowed not to take into account the possibility of emergencies;

   objects of class 2, in which all structures can be protected from accidental damage by non-constructive security measures and therefore it is unnecessary to check them for progressive destruction;

   objects of class 3, some structural elements of which cannot be protected from accidental damage, which will require checking for progressive destruction.

   Naturally, this classification cannot be invariant with respect to the list of initiating events, therefore, most likely, it should be presented in the design codes for buildings and structures of a certain type. There, perhaps, a list of initial situations that can give rise to a process of progressive destruction should be indicated.

   It can be assumed that the probability of coincidence of the initial event initiating a chain of failures with extreme values ​​of temporary loads is negligible. In particular, this provision is reflected in the so-called "single failure principle", which is used in the General Provisions for the Safety of Nuclear

   stations (OPB-88/97), where it is declared that it is possible to limit ourselves to considering cases of only one failure of a technical system or only one personnel error.

   But from the low probability of the initial event realization, it follows that the behavior of the structure is subject to verification, on which only constant loads and a long part of temporary ones act, and it is important to assess the relative level of the structure load in such a state. So, in industrial buildings, the forces in the columns caused by constant and long-term loads rarely exceed 15-20%, the main contribution to the load is determined by the action of loads from overhead cranes. Therefore, the destruction of the column (for example, as a result of a terrorist act) may not lead to the collapse of the entire building, since the spatial junctions are capable of carrying a twenty percent load. In office, residential and public buildings, the efforts from the dead weight of the supporting and enclosing structures, as well as from the action of the long-term part of the payloads, amount to 70-80% of the level of the bearing capacity, and here it is already difficult to expect the preservation of the building in the event of failure of any of the main columns. Therefore, the words from the article “During the war years, the retreating fascist troops, trying to destroy our industrial potential, blew up a column of a huge workshop, and, looking back, they saw with surprise that it did not fall ... Now, from TV screens, we are convinced that that if one column falls, then the whole building must fall. If this is so, then such a building should be located away from people with a sentry at the gate, who would not let anyone in, except for the authors of the project. "

2. The events of September 11, 2001 in the Pentagon building

The purpose of the Pentagon building is office. Floor area - 122 600 m 2 ... The total area of ​​the building is 613,000 m 2 .

The building is five-storey and has the shape of a pentagon (see Fig. 87). Internally, the building is divided into buildings, forming five concentric rings, designated A-E starting from the inner ring. In the upper three floors, the rings of the building are separated by light spaces. Between the second and third rings there is a passage known as the AE-passage.

The structural system of the building, including the covering, is made of monolithic reinforced concrete structures. Concrete is normal heavy.

Figure 87 General plan of the Pentagon building

The height of the building is 19.74 m. The height of the 1st floor is 4.30 m. The width of the outer ring "E" of the building is 18.288 m. The structural scheme is a complete frame-connected frame. The columns of the 1st floor of the building are square, with a cross-section of 0.53 × 0.53 m with spiral reinforcement (Fig. 89).

The slabs consist of slabs, girders and a system of beams supported by columns. Monolithic joist floors are made using main and secondary beams.

Figure 88 Cross section of a building (multiply by 0.3048 to convert feet to meters)

Beams and slabs have double reinforcement in the support sections and single reinforcement in the span sections. Stretched reinforcement of span and support sections is connected by inclined rods.

Most of the columns are square, as shown in rice. 5.12. Overall dimensions will vary from 0.53 mx 0.53 m on the ground floor to 0.35 mx 0.35 m on the fifth floor. The supporting columns have spiral reinforcement.

The length of the columns on the 1st floor is 4.3 m. Heavy concrete on granite aggregate. The diameter of the rods of the longitudinal working reinforcement is 20 mm.

The fire resistance limit of such columns is more than 180 minutes in terms of loss of bearing capacity (> R180).

The floor slabs of the Pentagon building are reinforced concrete, monolithic, with a section height of 140 mm, have double reinforcement in the support sections and single reinforcement in the span sections (Fig. 90). Stretched reinforcement of span and support sections is connected by inclined rods. The slabs are located on beams with a cross section of 0.35 × 0.51 m and a length of 3 m.

Figure 89 Reinforced concrete column of the outer ring of the Pentagon building

Figure 90 Construction of the floor slab of the Pentagon building

Beams with a span of 3 or 6 m, sometimes 4.6 m. The main beam with a section of 0.4 × 0.6 m overlaps a span of 6.1 m parallel to the outer walls and serves as a support for the secondary beam in the middle.

American Airlines Flight 77 took off from Washington DC for Los Angeles on September 11, 2001 at 8:20 am. It carried 58 passengers and four crew members. At approximately 8:54 am, the hijackers hijacked the plane.

At 9:37 am, Flight 77, traveling at 530 mph, collided with the Pentagon building. All on board the aircraft were killed and a large number of civil and military personnel of the Pentagon.

According to eyewitness reports and other information, the Boeing 757 flew at a very low altitude before colliding with the Pentagon building. At a distance of approximately 97 meters from the western facade of the Pentagon building, it flew only a few feet from the ground. The aircraft hit the first floor of the building, at an angle of approximately 42 ° to the outer facade of the building (Fig. 91).

The collision of the aircraft with the building in question led to the emergence and development of emergency situations in the form of combined special effects of the "blow - explosion - fire" type.

The first special impact - an aircraft strike - destroyed and damaged a number of structural elements of the 1st floor of the building. The main impact was taken by the supporting elements of the building - reinforced concrete columns.

The wreckage of the aircraft entered the building (Fig. 92). From the destroyed tanks of the aircraft, located in its wings, fuel was thrown into the impact zone inside the building.

This led to the emergence of a second special effect on the structure of the building - an explosion of a mixture of fuel and air. The explosion destroyed and damaged another part. structural elements of the building.

Figure 92 Scheme of damage to the structures of the Pentagon building on the path of the aircraft wreckage after it

Collisions with a building

After the impact and explosion inside the building, in the affected area, a third special impact arises and develops - a fire. The fire covers part of the premises in the path of the aircraft wreckage.

The Pentagon building in the first minutes of CHE 42 despite significant damage to structures in the first three rings of the building (Fig. 92), in general, it retained its stability.

However, 19 minutes after the start of the combined special impact of the "blow - explosion - fire" type, a progressive collapse of the outer ring of the Pentagon building occurred in the "CHE IEF 43 "(Fig. 94).

42 Combined Special Impact (CHE) is an emergency situation associated with the emergence and development of several types of special impacts on an object in various combinations and sequences. The English version of the name “combined hazardous effect” - CHE is used as an abbreviation for this concept.

43 The main special impacts of a technogenic nature on construction objects: hit( I), explosion( E), fire ( F)

etc.

Figure 93 View of the facade of the outer ring of the Pentagon building in the first minutes after the impact of the aircraft and the explosion of fuel

(progressive collapse of structures has not yet occurred)

Figure 94 Progressive collapse of the outer ring structures of the Pentagon building during the events of September 11, 2001

Thus, similar to the behavior of the WTC towers in New York during the events of September 11, 2001, despite the fact that the ability to resist the fire of the main load-bearing structures of the Pentagon building (fire resistance limit for loss of bearing capacity) exceeded 180 minutes, the progressive collapse of the outer ring structures the Pentagon building on September 11, 2001 happened much faster - 19 minutes after the start of the terrorist attack.