The footings were designed as inverted "T" shaped grade beams. These inverted
"T" shaped footings were sized and reinforced to provide the strength required
to carry the superstructure. The structural system for the superstructure is
load-bearing masonry walls and open-web joists for the partial second floor
and the roof. This structural system was selected as the most economical in
light of the aesthetic constraints on the exterior finishes imposed by the
facility master plan.
The masonry walls are not flexible in terms of accommodating differential
settlements. The wall footings, however, were designed to be rigid enough to
bridge over or otherwise resist differential settlements in the event that the
footing lost support for a span of 10 ft such as at a building corner.
Considering the potential footing subgrade problems, the inverted "T" beam
footing size and reinforcing were selected so as to provide the strength and
effective moments of inertia required to resist footing deflections. This
design limits the differential settlements experienced by the masonry walls to
a tolerable range. The floor slab was reinforced both in the top and in the
bottom of the slab. It was cast directly on the surface of the compacted
fill, and was set on and tied to the inverted "T" shaped grade beams at the
edges. This was done to ensure that the floor slab would continue to settle
at the same rate as the building walls, and to prevent the slab from remaining
somewhat buoyant while the building footings and superstructure settled.
To control the quality of the compacted fill work and to monitor the
consolidation of the soft, silty clay stratum, 15 settlement plates were
installed on the existing ground surface to monitor the settlements during the
compacted structural fill work. The settlement plates were observed weekly
throughout the two-month compacted fill placement, and monthly during the
18-month facility construction period. After construction, settlement
readings were made at a lesser frequency ranging from three months to yearly
by March 1981. Observations were made by reading elevations on steel plates
and pipes placed on original grade.
Four pneumatic piezometers, consisting of wellpoint and pore water pressure
sensing elements, were installed after completion of the compacted structural
fill to evaluate the pore water pressures developed in the clay material from
the fill and building loads. The piezometer readings showed that the pore
water pressures dissipated slowly, and were leveling off at a rate which was
consistent with the rate of settlement. They provided a good correlation
between the actual and predicted rate of settlement.
The construction of this project began in August 1976. The average settlement
which had occurred by the completion of the compacted fill work was about 2
inches. Construction of the building was completed in June 1978. The average
rate settlement measured after completion of the buildings was about 3 inches
per year, about 16% higher than predicted by analytical results. From the
beginning of the project to March 1981, the average and maximum settlements
were about 1.3 and 1.5 ft respectively. No wall cracks or structural distress
of the buildings have occurred to this date.
Designing structural foundations to accommodate the effects of large
settlements is a real challenge to engineers. The risk of foundation failure
or building damage can be minimized if the foundation system is designed in
such a way that it can promote a uniform settlement over the building area.