Study of the Behaviour of Precast Connections under Progressive Collapse Scenario by Digesh Dilipkumar Joshi

Guided by: Dr. P. V. Patel With Synopsis and CD 11EXTPHDE80

ABSTRACT:
Progressive collapse is a situation where local failure of a primary structural component leads
to the collapse of adjoining members, which in turn leads to spread of collapse. Progressive
collapse of building structures is initiated when one or more vertical load carrying members are
seriously damaged or collapsed due to extreme loading imposed by earthquake, flood,
explosion, vehicle impact etc. As a result, a substantial part of the structure may collapse,
causing greater damage to the structure than the initial impact. Progressive collapse of any
structure causes catastrophic failure of structure and substantial loss of human lives as well as
natural resources. In order to reduce risk of progressive collapse, it is necessary to design and
detail the structure to develop alternate load path in the event of extreme loading.
In precast concrete construction, the components of structures are produced in controlled
environment, transported and individual precast elements are connected appropriately at site.
Because of better quality control and faster rate of construction, the precast concrete
construction is being adopted world-wide including India. However, in precast concrete
construction, connections are the critical elements of the structure, because in past, major
collapse of precast building took place because of connection failure. Therefore, it is very
important to study the performance of precast beam column assembly under a progressive
collapse scenario. Behaviour of precast beam column assemblies under progressive collapse
condition is not reported in greater detail in literature. National Institute of Standard and
Technology (NIST), U. S. Department of Commerce has presented report on experimental and
computational study of two precast concrete moment-frame assemblies under a column removal
scenario, in September 2015. During last couple of years, few more investigators have reported
results of experimental and numerical studies conducted on different precast beam column
connections under progressive collapse scenario.
In the present research work, behaviour of different precast connections are evaluated under
progressive collapse scenario by conducting experiments and performing numerical analysis.
Experimental studies are conducted on sixteen reduced (1/3rd) scale precast test specimens,
which are constructed by providing wet and dry connections. Each test specimen represents
beam column assembly, consists of two span beam and three columns with removed middle
column, which in turn represent column removal scenario. Connections are provided at beam column junction for eight precast test specimen, while for remaining eight precast test
specimens, connections are provided away from the beam column junction i.e. within beam
length, to avoid any discontinuity at beam column junction. Test specimens are extracted from
6-storey symmetrical precast building having overall plan dimensions of 16 meter × 12 meter.
As, perimeter frames of any building are exposed to higher risk of occurrence of any undesired
event due to ease of accessibility, the prototype of test specimens is assumed to be located at
the middle of the perimeter frame in longer direction subjected to column loss at bottom storey.
Two monolithic test specimens are also prepared to compare behaviour of precast connections
with that of monolithic connection under column removal scenario.
Monotonic vertical load is applied at the top of the removed middle column with the help of
hydraulic jack till the complete failure of specimen takes place. Response of test specimens are
evaluated in terms of ultimate load carrying capacity, load versus vertical deflection at the
location of removed middle column, deflection profile of the specimen measured along the
beam length, crack formation and failure propagation of test specimens. For some of the test
specimens, strain measurement is also carried out at selected locations on concrete surface as
well as on steel reinforcement bars.
Numerical analysis using finite elements for monolithic and precast specimens is carried out
using ABAQUS software for validation of results obtained from experimental studies. Finite
Element (FE) models are developed for precast connections under column removal scenario
and analysis is carried out by considering material and geometrical nonlinearities. Concrete
damage plasticity model is used for nonlinear FE analysis. Concrete part of specimen is
modelled as 8-node linear brick elements (C3D8R) with reduced integration. 2-node three
dimensional truss elements (T3D2) are used for modelling of steel reinforcement bars. The
numerical analysis results in terms of ultimate load carrying capacity, deflection profile of
specimen at specified load values and failure modes are compared with that observed during
experimental studies.
From the experimental results, it is concluded that precast connections provided away from the
beam column junction, specifically wet connections, where individual elements are connected
by overlapping or welding of projecting reinforcement bars, can be used as an alternative of
monolithic connection. Behaviour of such precast connections under progressive collapse scenario will be similar to that of monolithic connection. Further, from the experimental results,
it is observed that, precast wet connections performs superior as compared to precast dry
connections under progressive collapse scenario. Results of experimental studies also indicates
that, behaviour of precast wet connections is further enhanced though inclusion of
polypropylene fibres with cast-in-place micro concrete and by providing additional lateral
confinement reinforcement within connection region. Experimental results on precast dry
connections considered for the study, suggests that precast dry connections, constructed by
welding steel plates, performs superior as compared to those constructed using bolting. From
the results of numerical analysis, it is found that, Finite Element (FE) models developed for
precast connections under progressive collapse scenario, using concrete damage plasticity
model are about to capture the behaviour of specimen during initial phase up to flexural action
and Compressive Arch Action (CAA). Stress contours and scalar stiffness degradation contours
obtained from numerical analysis closely matches with the failure pattern of specimen observed
during experimental studies, which indicates close agreement of experimental and numerical
analysis results. Present study can be useful in developing guidelines for better performance of
precast beam column assemblies during progressive collapse scenario.