MIL-HDBK-1003/11
machine characteristics which may be considered as approximate rather than
precise. In many engineering problems which defy an exact analysis, a safe
design may be assured by the use of a greater factor of safety. In the
field of machinery foundation design, this approach may ensure structural
adequacy but not necessarily dynamic stability. Normal differences between
the predicted and actual characteristics of the soil and machinery may have
adverse effect upon the characteristics which might destroy or wipe out
expected design margins. The foundation characteristics may be further
affected by deviations in actual construction from the details specified by
the foundation designer. The designer should include provisions for field
testing and adjustment of foundation mass in cases where design studies
indicate a possible deficiency in design margins. This may be accomplished
by making the bottom base slab extend outside the main foundation block.
The dynamic stability may then be checked experimentally by placing bagged
sand at various points around the unit upon the base slab extension while
the engine-generator is operating. When optimum dynamic equilibrium is thus
determined, sand may be replaced with equivalent mass concrete anchored to
the main foundation block and base slab extension.
5.4.13.3 Minimum Requirements. Soil borings should extend no less than 50
ft (15 m) below the bottom of unit foundations, unless rock will be
encountered at shallower depth. From these boring, allowable soil bearing
pressures engine-generator the need for piles can be determined. Foundation
design should be governed by the following:
a) The entire foundation bearing surface should be at the same
elevation. Steps or cascades at support level should be avoided.
b) The unit foundation support level should be carried at least
30 in (762 mm) below any trenches or basement floor levels which are
adjacent to the unit. This may be reduced to 18 inches (457.2 mm) for 750
kW or smaller units.
c) Minimum static load design reinforcement is 2/10 of one
percent of the cross-sectional area vertically and horizontally for all
foundations. Minimum reinforcement for dynamic loads shall be at least 3 to
5 times this requirement. Usually, the entire foundation block is
considered to be affected by dynamic loads. For larger or not well balanced
units, reinforcing should be designed substantially heavier.
d) If bearing level is solid rock, such that thee is a minimum
depth of 5 ft (1.5 m) of rock, cover the bearing surface with a 12-inch
(304.8 mm) layer of sand for a cushion.
e) Great care should be taken to avoid excessive or unequal
settlements. Generally, the soil at elevations upon which unit foundations
will bear directly should be capable of supporting a minimum uniform load of
3,000 psf (14,646 kg/m2) without excessive settlement. The soil, at
elevations lower than the bearing level for depths at least equal to unit
block lengths, should be of uniform quality without layers or pockets of
weak soils. If the quality of soil remains in doubt, even after a
comprehensive soil investigation, then consider the use of piles, piers, or
caissons.
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