ETL 1110-2-355
31 Dec 93
The uniform pressure distribution for concentric
e. Flotation. A flotation analysis using unfac-
loading is based upon the assumption that the base
tored loads should be performed on each monolith
slab of the monolith behaves as a rigid base. To
that can be dewatered. For further information, see
compute the pressure distribution, the sum of all
ETL 1110-2-307. Drag loads will not be used to
vertical forces acting on the base of the monolith is
resist uplift (flotation) due to the varying nature of
distributed equally across the monolith bottom. An
drag loads. If insufficient capacity exists to keep the
example of this computation is given in Figure A-2.
monolith from floating, the monolith can be held
This uniform pressure is modified to account for
down with anchors, heels, or more concrete mass, or
loads eccentric to the centroid of the monolith base.
improved foundation drainage systems can be added
An example of this computation is given in Fig-
to the monolith.
ure A-3. The stepped pressure distribution is an
f. Differential settlement. Differential settle-
approximation based upon observed data from instru-
ment occurs between adjacent monoliths due to the
mented U-frame locks founded on soil. In this distri-
difference in size and weight of the monoliths as well
bution, the pressure beneath the lock wall is increased
as differing foundation conditions beneath each mon-
to a set percentage of the pressure beneath the
olith. Differential settlement should be held to the
remainder of the monolith. These two pressures are
practical minimum possible. There are several ways
modified proportionately until the total pressure
to handle this problem, including use of keys, dowels,
equals the sum of the vertical forces. Observed data
and construction sequencing. Keys can be formed
from Port Allen Lock suggest the amount of increase
between adjacent monoliths. Dowels can be added
should be 75 percent. An example of this computa-
between the base slabs of adjacent monoliths. The
tion is given in Figure A-2. This stepped base pres-
construction sequence for adjacent monoliths can be
sure is modified to account for loads eccentric to the
specified such that the heavier monolith is partially
centroid of the monolith base. An example of this
placed prior to placement of the lighter monolith.
computation is given in Figure A-3. For further
The magnitude of the forces being carried by dowels
information on the amount of increase to use, see
or keys is difficult to predict, but the designer must
Sherman (1968) and (1972).
try to account for these forces by some rational
method.
b. Location of resultant analysis. A resultant
location analysis using unfactored loads should be
performed on each two-dimensional and three-
7. Structural Analysis
dimensional monolith. The analysis consists of deter-
mining the location of the resultant of all loading in
Once the design criteria have been established, all
relation to the kern of the monolith base. The resul-
reasonable load cases have been identified, and the
tant location for usual load cases should be the mid-
initial foundation parameters have been established,
dle third of the base. The resultant location for the
the analysis of the structure may be performed. The
unusual load cases should be the middle half of the
structural analysis is necessary for ensuring that the
base. The resultant location for the extreme cases
wall and slab thicknesses are sufficient and for deter-
should be within the base. Usually the location of
mining the reinforcement requirements of the struc-
the resultant is not a problem.
ture. Before performing an analysis of a U-frame
lock, the designer must decide whether each monolith
behaves in a two-dimensional or a three-dimensional
c. Sliding analysis. A sliding analysis using
manner. The method of analysis must also be
unfactored loads should be performed in accordance
selected, which can be a frame analysis, a finite
with ETL 1110-2-256. The CASE computer program
element analysis, hand calculations, or a combination
CSLIDE can perform this analysis (Pace 1987).
of these. These decisions are based on experience
and good engineering judgment. A parametric study
d. Bearing analysis. A bearing pressure analy-
which bounds the extremes of behavior of a structure
sis should be performed using unfactored loads. The
can also be used as a tool to ensure adequacy of a
foundation capacity should be developed taking into
structure. As a result of these analyses, the designer
account such items as soil type and stratification.
can then determine final member sizes and
The computed bearing pressures must be less than the
reinforcement.
foundation capacity.
A-12
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