TM 5-818-1 / AFM 88-3, Chap. 7
CHAPTER 5
SETTLEMENT ANALYSES
structure and its foundation, and the distribution of load
5-1.
Settlement problems.
within the building. In conventional settlement analyses
a. Significant aspects of the settlement of
of the type described in this manual, the stiffness of the
structures are total settlement-magnitude of downward
building and foundation are not considered. Tolerable
movements- and differential settlement-difference in
angular distortions are listed in table 5-2, and empirical
settlements at different locations in the structure. Table
correlations that may be used to estimate probable
5-1 lists conditions that cause settlements which occur
angular distortions based on calculated maximum
during construction and result in only minor problems
settlements are summarized in table 5-3. Because of
and postconstruction settlements which occur after a
the natural variability of soils, differential settlement will
structure is completed or after critical features are
occur though total settlements are calculated to be
completed. Differential settlements distort a structure. A
uniform. An indirect means for controlling differential
structure can generally tolerate large uniform, or nearly
settlement is to limit total settlement to 3 inches for
uniform, settlements.
structures on clay and to 1 1/2 inches for structures on
b. Differential settlement can have a number
sand.
of undesirable results:
(1) Tilting is unsightly. A tilt of 1/250 can
be distinguished by the unaided eye.
5-2.
Loads causing settlement.
(2) Moderate
differential
settlement
a. Loads causing settlement always include
causes cracking and architectural damage.
With
the estimated dead load and a portion or all of the live
increasing differential settlement, doors and windows
load. For office buildings, about 50 percent of the
may become distorted and not open and close properly.
estimated building live load may be assumed to cause
Larger differential settlements may cause floors and
settlement. For heavily loaded warehouses and similar
stairways to become uneven and treacherous and
structures, the full live load should be used.
windows to shatter. At this point, the usefulness of the
b. For many purposes, settlements need be
building has been seriously impaired.
computed only for the maximum dead load plus
(3) Severe differential settlements may
settlement-causing live load. Occasionally, settlements
impair structural integrity and make structures
occurring during a part of the construction period must
susceptible to collapse during an earthquake or other
be computed.
This may require additional stress
major vibration.
computations for partial loading conditions.
(4) If a structure settles relative to the
c. Loads
that
are
less
than
the
surrounding ground, or the ground settles relative to the
preconsolidation stress cause minor settlements
structure, entryways may be disrupted, and utility lines
because only recompression of soil occurs.
The
may be damaged where they enter the structure.
increment of loading that exceeds the preconsolidation
c. Even if settlements are uniform or nearly
stress causes relatively large settlements and occurs
so, large total settlements can also result in problems:
along the virgin compression portion of laboratory
(1) Sites located near a river, lake, or
consolidation curves.
A careful estimate of
ocean may flood during periods of high water.
preconsolidation stresses is essential for settlement
(2) Surface drainage may be disrupted. If
analyses. Means for estimating such stresses are given
water ponds around and beneath structures, they may
in chapter 3.
become inaccessible and subject to mildew and wood
rot.
5-3.
Stress computations.
d. Experience provides a basis for estimating
a. One of the first steps in a settlement
the magnitudes of differential settlement that cause
analysis is computation of effective overburden stresses
in the soil before and after loading. The initial stress p1'
stucco, and brick facing. Differential settlement is best
at any depth is equal to the effective weight of overlying
expressed as the angular distortion in radians between
soils and may be determined by multiplying the effective
two points.
Angular distortion,
which always
unit weight of the soil by its thickness. It is customary to
accompanies settlement of a building, is determined by
construct a load-depth diagram by plotting
the uniformity of foundation soils, the stiffness of the
5-1
Previous Page
