Experimental Investigation of Progressive Collapse of Steel Frames
34
Nonlinear 3-D finite element models were developed
using nonlinear software to conduct the analyses of steel
frames under progressive collapse. For progressive colla-
pse analysis, a nonlinear static analysis method employs
a stepwise increment of amplified vertical loads which
can be referred to as a vertical pushover analysis. The
force redistribution within the steel frames under pro-
gressive collapse was investigated in this study and the
failure modes were predicted. Progressive collapse is of
particular concern since it may be disproportionate, i.e.,
the collapse is out of proportion to initial local failure.
After the progressive and disproportionate collapse of the
Ronan Point apartment tower [3], prevention of progres-
sive collapse became one of the main concerns of struc-
tural engineers, and code-writing bodies and governmen-
tal user agencies attempted to develop design guidelines
and criteria that would reduce or eliminate the suscep-
tibility of buildings to this form of failure. These efforts
tended to focus on improving redundancy and alternate
load paths, to ensure that loss of any single component
would not lead to a general collapse. Improved local
resistance for critical components and improved conti-
nuity and interconnection throughout the building (which
can improve both redundancy and local resistance) can
be more effective than improved redundancy in many
instances. Through an appropriate combination of im-
proved redundancy, local resistance and interconnection,
it would be possible to greatly reduce the susceptibility
of buildings to disproportionate collapse [2].
Astaneh (2002) [4] investigated the strength of a ty-
pical steel building and floor system to resist progressive
collapse in the event of removal of a column. They tested
a specimen of size 60 ft by 20 ft one-story steel building
with steel deck and concrete slab floor and wide flange
beams and columns. The connections were either stan-
dard shear tab or bolted seat angle under bottom flange
and a bolted single angle on one side of the web. It was
observed that after removing the middle perimeter
column, the catenary action of the steel deck and girders
was able to redistribute the load of removed column to
other columns. The floor was able to resist the design
dead load and live load without collapse. Damage to the
system was primarily in the form of cracking of floor
slab, tension yielding of the steel corrugated deck in the
vicinity of collapsed column, bolt failure in the seat
connections of the collapsed column and yielding of the
web of the girders acting in a catenary configuration.
Astaneh (2003) [5] carried out an experimental in-
vestigation of the viability of steel cable-based systems
to prevent progressive collapse of buildings. The tests
were conducted on a full-scale specimen of a one-story
building. One side of the floor of the specimen had steel
cables placed within the floor representing new construc-
tion and the other side had cables placed outside as a
measure of retrofit of the existing building. The author
claimed that the test results showed that the system could
economically and efficiently prevent progressive collapse
of the floor in the event of removing one of the exterior
columns.
Gravity load collapse of a reinforced concrete frame
was studied by Moehle and Elwood [6,7]. Their studies
found that residual axial capacity could prevent collapse
of a building although shear failure in a concrete column
had occurred. A formula using a shear-friction model
was suggested to simulate additional axial load capacity
after shear capacity was exhausted [6,7]. The study [6]
also considered residual capacity of adjacent elements in
analyses after a component fails and leads to redistri-
bution of the applied loads.
Most flat plate buildings are prone to progressive col-
lapse (Hawkins (1979)) [8]. Punching shear failure in a
flat plate building was often observed even before
yielding of the bottom reinforcement of slabs occurred.
Mitchell and Cook (1984) [9] found that punching shear
failure at exterior columns had a low possibility of
leading to progressive collapse. However, unless conti-
nuous bottom reinforcement through a column or good
anchorage was provided, tension membrane by slabs was
not effective and could lead to catastrophic failure.
Proper detailing of slab reinforcement at a column sup-
port enabled a damaged slab to hang from its support.
Therefore, well detailed flat slab buildings were capable
of resisting additional loads even after punching failure
at a support region occurred.
A study by Malvar (2005) [10] reported behavior un-
der blast load and suggested appropriate retrofit schemes.
Both specific local resistance and alternate load paths
were considered to rehabilitate the building. Although
the suggested retrofit schemes were not necessarily appli-
cable to all concrete buildings, the fundamental retrofit
concepts to prevent progressive collapse were defined.
The study recommended that exterior frames can be re-
habilitated by providing specific local resistance using
steel jacketing or wrapping with fiber material. Interior
frames can be supported by developing load paths using
adjacent components.
There is a lack of full-scale tests reported in the litera-
ture on progressive collapse of steel frames because they
are quite expensive and time-consuming. Therefore, nu-
merous small-scale tests were reported in the literature
on progressive collapse. The small-scale tests can be con-
ducted in labs and the result is considerable savings in
time and money. Efforts to develop comprehensive and
progressive collapse-resistant specifications have been
hindered by a lack of both experimental and analytical
information about progressive collapse, leading to the
current investigation. On the experimental front, the rate
of loading and the scale of the problem, i.e., which in-
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