Civil Engineering Reference
In-Depth Information
4.1.1 Mechanism of Failure
To assess the safety of the building and devise efficient repair schemes and
procedures, it was necessary to determine the cause of excessive lateral
deformations undergone by the structure and the internal stresses in the structural
elements. Preliminary structural analysis using the finite-element method
indicated that the structural deformations of PG-1 were primarily caused by a
large lateral pressure applied to the north wall of the building. The analysis
consisted of a three-dimensional model of the structure, subjected to a pressure
distribution acting on the north wall. The actual magnitude and shape of the
pressure could not be determined using traditional earth pressure theories, due to
the uncertainty about (1) the initial magnitude of backfill compaction stresses in
the retained soil, (2) the construction sequence, and (3) the magnitude of
relaxation and redistribution of earth pressure due to structural movements.
Accordingly, a simplified triangular (hydrostatic) pressure distribution was
utilized, and its magnitude gradually increased until the measured and calculated
building movements were matched. The final magnitude of the pressure
corresponded to an equivalent lateral earth pressure coefficient,
k
, equal to 2,
which includes the effect of hydrostatic pressure, compaction stresses, and friction
at the interface between the soil and wall. This coefficient is substantially larger
than the active or at rest earth pressure coefficients calculated using classical earth
pressure theories.
4.1.2 Objectives of the Instrumentation and Monitoring Program
At the onset of the project, the safety and stability of the structure were
questionable, and the rate of movement was unknown. In the absence of definitive
knowledge on the amount of movement, the structure was in immediate need of
stabilization and additional strengthening. Failed and severely cracked columns
were strengthened using 100-150 mm (4-6 in) reinforced concrete jackets. The
design of the stabilization scheme against additional lateral movement was based
on estimates of earth pressure obtained from the finite element analysis discussed
earlier. On the other hand, the possibility of relaxation of earth pressure due to the
large movements and the potential redistribution of the forces within the elements
of the concrete structure warranted more investigation into the behavior of the
building in an effort to minimize the cost of structural retrofit. Thus,
instrumentation and monitoring of the building became a viable and economical
alternative.
The monitoring program was carried out for four and a half years to measure
the building movements, assess the safety of the building, and determine the need
for additional strengthening and stabilization. Data from the monitoring program
revealed that the behavior of the building involved a complex soil-structure caused
by large thermal movements of the building since the building elements were
