introduction of progressive collapse of reinforcement concrete structure
1.1.
General
In the
present era, due to the increasing number of attacks on embassies, commercial
buildings and industrial facilities, considerable effort has been focused on
the consequences of blast loading on existing structures.
One of the
major consequences of bomb attacks, from the structural performance
perspective, is the possibility of progressive collapse that could affect
people and property in entire buildings.
This usually
starts with a localized failure of a primary structural element and
proliferates into a failure that is not proportionate to the local damage
caused by the initiating event, which accordingly leads to partial or total
collapse of the structure. (Yagob et al., 2009)
As for the
European codes: “Progressive collapse is the
spread of local damage, from an initiating event, from element to element
resulting, eventually, in the collapse of an entire structure or a
disproportionately large part of it; also known as disproportionate collapse (Ellingwood
and Leyendecker, 1978).
According to
the Russian norms: “Progressive collapse is a consistent destruction of the bearing
structures of the building (structure) due
to the initial local damage to the individual carriers of structural
components and leading
to the collapse
of the entire
building or substantial part
thereof (STO-008-02495342-2009, 2009).
The different
norms approach the progressive collapse in different ways, but they have in
common some limits for the extent of the damage.
Typically
destruction in such a collapse would extend one structural part, 100 m2 of floor area, or two
stories (Vasilieva, 2013).
Over the past
century, there have been dramatic events that have brought considerable
attention to this phenomenon and alerted professionals to its momentousness.
One of the
first events that brought attention to progressive collapse as an important
factor in structural design was the partial collapse of the Ronan Point
apartment building in East London, England, on May 16th, 1968, due to an
accidental gas explosion on the 18th floor of a 22-story apartment building.
The
accidental explosion in this high-rise building triggered the collapse of the
corner portion of the building along its entire height.
After that, a
number of tragic events of progressive collapse resulting from terrorist acts
have explicitly highlighted the phenomenon (Singh et al, 2015).
Several
buildings, around the world, suffered from progressive collapse which will be
mentioned in chapter four.
That kind of
crash can be initiated by many causes, including design and construction
mistakes and load events that are over design dimensions or are not taken into
account. Such events would include
abnormal loads not usually considered in a project.
Fig.
(1.1) Ronan Point Building after Collapse, 16 May 1968
The potential
abnormal loads that can cause the progressive collapse are categorized like
that:
1.1.1. Pressure Loads:
1) Internal gas explosions.
2) Blast.
3) Wind over pressure.
4) Extreme values of environmental loads.
1.1.2. Impact Loads
5) Aircraft impact.
6) Vehicular collision.
7) Earthquake.
8) Overload due to occupant overuse.
9) Storage of hazardous materials.
1.2.
Types of Progressive Collapse:
Even though
progressive collapse is managed in the design rules and norms as
one event
it can be
divided into several
parts depending on
the reason for
the progressivity. The reason
that causes the progressive collapse depends on the type of structure and the
initiating event.
Five types of
progressive collapse will be described: pancake, zipper, domino, instability,
and section-type destruction.
1.2.1. Pancake-Type Collapse:
When the
capacity of a member carrying vertical load is inadequate it can lead to the
collapse of an entire section of a structure, as shown in Fig. (1.2).
The upper part of
the damaged structure
starts to fall
and accumulate kinetic energy. The impact force due to the
falling part of the structure commonly exceeds the design load of the remaining
structure. If the floor underneath is
not able to resist the impact, the collapse will continue one floor at a time (Starrosek,
2009).
The steps of
a pancake-type progressive collapse are:
- Initial destruction of the construction element carrying vertical
load.
- Changing of the structures potential energy to kinetic energy
until the fall.
- Impact of the destroyed structure to the rest load bearing parts.
- Failure of the vertical load bearing part hit.
- Promotion of the failure in vertical direction.
Fig. (1.2)
- The Stages of the Pancake-Type Progressive Collapse:
(a) The
initial
failure of a
column, (b) Changing of
the structures potential
energy to kinetic energy, (c) Reloading
of the structure below the initial failure, (d) Promotion of the failure (Räty,
2010)
1.2.2. Zipper-Type Collapse:
The loss
of a single
load bearing member
redistributes the force
to the other members situated transverse to the failure direction, as shown in Fig. (1.3). If the resistance of
the remaining members
is exceeded, due
to the extra
load or its dynamic character,
the failure will
be increased. The phases of the zipper-type mechanism are:
- Initial failure of one or a few vertical load bearing members.
- Dynamic increase in
loading to the
remaining members due
to the redistributing of the loads.
- Concentration of forces in load-bearing elements that is similar
in type and function to and adjacent to or in the vicinity of the initially failing elements due to the
combined static and dynamic structural response to that failure.
- Overloading of the remaining members, loaded the most.
- Failure of the
members situated in
a transverse direction
to the falling elements (Starrosek, 2009).
Also for this kind of collapse, the failure of elements may be
connected with any local failure mode, which contains instability (buckling).
Fig. (1.3) - The Stages of Zipper-Type Progressive
Collapse:
(a)
The initial failure of a column, (b) Reloading of the nearest columns, (c) The
progression of the failure (Räty, 2010)
1.2.3. Domino-Type Collapse:
The characteristic of
a domino-type collapse
is the initial
overturning of one element, followed by the unexpectedly
overturning of involved elements, next to the first damaged element of the
structure. If the elements which were
impacted lose their stability overturns the failure is progressing in the
horizontal direction. The phases of a domino-type collapse are:
- Initial overturning of an element.
- The transformation of the structures potential energy to the
kinetic energy due to the turning.
- Impact of the turning element to the next load bearing part.
- Overturning of the load bearing part stroked.
- Leading in a progressive collapse in a horizontal direction.
The height of
the overturning element has to be bigger than the distance to the next element or the elements have to be connected to
each other with
some horizontal load transferring
member, as shown in Fig. (1.4) (Starrosek, 2009).
Fig. (1.4)
- The Stages of Domino-Type Progressive Collapse:
(a) The initial failure and loading of the columns staying next to it, (b) Upheaval of the columns, (c) The promotion
of the failure till the overturning (Räty, 2010)
1.2.4. Instability-Type Collapse:
If the
initial failure occurs in a critical member stabilizing the entire structure a collapse
due to instability can occur, as shown in Fig. (1.5). Instability type collapse´s
initial disruption is minor and critical due to its direction, as a lateral
impact load on bracings, or position, as in the corner of the member
stabilizing the structure. The instability-collapse often occurs in compressed
members where the initial disruption can for example lead to large deformation
and then to collapse. If the initial failure leads to a disproportional
collapse immediately then the progression of the collapse is problematic to
define. The phases of an instability-type collapse mechanism are:
- Initial failure of a stabilizing member.
- Failure of the member transfer stabilizing force to the remaining
members.
- Progressive collapse due
to stability loss
of the member’s
loaded or immediate collapse
due to the
stability loss of
the entire structure (Starrosek, 2009).
Basically this
type of collapse
occurs when is
done the additional
stiffness and brace of the
structural component.
Fig.
(1.5) - The stages of instability-type progressive collapse:
(a) The
primary construction with a bracing truss, (b) The initial failure in the
girder, (c) The loss of steadiness till the lost part of the truss then (d) The
crash till instability. (Räty, 2010)
1.2.5. Section-Type Collapse:
In the
section-type of collapse a beam under a bending moment or a bar under axial
tension is taken into account. When a part of the corresponding cross section
is cut, the inner forces transmitted by that part are redistributed into the
remaining cross section. The corresponding increase in stress at some locations
can be the destruction of further of cross sectional parts and a failure
progression throughout the whole cross section.
A
section-type collapse appears similar to a zipper-type collapse. Actually, the
same list of features applies when the terms “cross sections” and “part of
cross section” are substituted for the terms “structure” and “element”,
respectively (Joshi and Patel, 2012).
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