Abstract:
The incorporation of rocking foundation is remarkably effective construction alternative
for safeguarding the superstructure from significant damage caused by severe lateral
forces during intense earthquakes.Even though, the mechanism of rocking foundations
and its beneficial effects are widely documented, generalized design guidelines and its
applicability on the Reinforced Concrete (RC) framed buildings are yet to be explored.
Hence, The scope of this thesis work aims to to quantify the seismic force and displacement
demands for RC frame buildings on the explicit and combined effects of the rocking
foundation and superstructure behaviour with respect to key parameters addressing the
seismic force and displacement demands. The objective of this study is to demonstrate the
advantageous impact of using rocking foundations on the seismic performance of Reinforced
Concrete (RC) framed building. This will be accomplished by comparing the performance
of buildings that have conventionally designed foundations, rocking foundations and fixed
base counterparts. Rocking at the foundation level is achieved by under proportioning the
footings by considering the reduced earthquake loads for footing design.
The present research comprises of two distinct objectives, where for RC framed building
without shear wall solely the supporting foundations are allowed for rocking. However, for
RC building with shear wall solely the foundation supporting shear wall is allowed to rock.
Within the OpenSees framework superstructural elements are model as fiber-based modelling
with distributed plasticity whereas substructural elements and soil are modelled using Beam
on Nonlinear Winkler Foundation (BNWF) modelling. The observations made from eigen
analysis indicates the period lengthening for the both structural configurations considered.
For RC framed buildings without shear wall, nonlinear static pushover assessments showed
that permitting the foundation rocking increases yield and peak displacement by about
9% to 34% without substantial reduction in the strength. Also, the plastic displacement
capacity increases as the rocking effect increases. This shows that rocking the foundations
in a structure is advantageous for its overall seismic performance.
According to the nonlinear dynamic time history analyses, seismic moment transferred
from the column to the foundation decreases by 20% to 50%. Due to reduction in the
peak roof acceleration and increasing settlement at the base of the foundation with
increasing effect of rocking, reduced seismic moment is noticed at the base of the structural
members. Similar responses are noticed for the buildings where only the shear wall
foundation is allowed to rock. It is found that the foundation of a shear wall can be
designed by taking into account 40% of the earthquake loads for zone V design level
and 60% of the loads for zone II design level without encountering excessive settlements
beyond permissible limits as per Indian standards. From the hysteric responses for
the shear wall foundation rocking, it is evident for very strong impact seismic motion,
conventionally designed footings tends to experience higher flexural displacement along
with higher seismic force demands and settlement demands too. This suggests that an
overdesigned footing may not always be beneficial for the superstructure. From the fragility assessment it is observed that the probability of exceeding 25mm settlement increases
with increase in foundation rocking regardless of soil type. However, the probability at
the collapse prevention level of 60mm is not considerably influenced for the foundation
proportions while transitioning from conventional footings to moderate rocking footings.
This implies that reducing the dimensions of the foundation may not necessarily result
in reasonable settlement limitations being exceeded. The most favourable conditions
for foundation rocking is observed to be dense and very dense sand than medium dense sand.