ETL 1110-2-355
31 Dec 93
the structure being damaged lead one to grouping the
appropriate load factors is simplified when internal
load cases into three different categories: usual,
member results are categorized as dead and live
unusual, and extreme. A usual load case is one that
loads. Alternatively, analysis can be performed with
affects the structure for extended periods on a recur-
factored loads which directly yields internal member
ring basis. Such load cases permit no reduced load
reactions for member design. Analysis with factored
factors to the structure components. An unusual load
loads causes the points of inflection to shift and the
case is one that the structure sees occasionally and/or
foundation pressure distribution to change when com-
for short periods. Such load cases have minor reduc-
pared with an analysis with service loads.
tions to load factors on the structure components. An
extreme load case is one that might happen only once
6. Foundation Analysis
or twice to the structure. Such load cases have major
reductions to the load factors on the structure
6-1. Determination of type of foundation. The
components.
selection of the type of foundation is probably the
most critical aspect of the design of a U-frame lock
by combining the individual loads together. Usually
because of cost considerations and the overall behav-
the controlling load cases will be those with the
ior of the structure. Since this decision will have a
greatest vertical or lateral loads. However, some
significant impact on the project cost, the determina-
cases may control that do not have the highest verti-
tion of the type of foundation should be made in the
cal or lateral load but for which the combination of
feasibility phase of the project. A thorough sub-
lateral and vertical loads is more severe. In addition,
surface investigation and testing program should be
for monoliths subject to dewatering, the dewatered
undertaken to define the soil strengths and param-
case may require that additional means be provided to
eters. A soil foundation is usually more economical
resist uplift or that limits be placed upon the maxi-
if special measures (deeper excavation, elaborate
mum pool elevations at which the monolith may be
pressure relief system, etc.) are not required. The
dewatered. Both maximum and minimum coeffi-
soil foundation has to be able to satisfy stability
cients of lateral soil pressure can be used in the fac-
requirements for sliding and overturning, as well as
tored stress analysis to bracket the actual lateral soil
resisting uplift (flotation) and earthquake forces.
pressures. Also, both uniform and stepped bearing
Included in evaluating soil-founded versus a pile-
pressure distributions are used in the factored stress
founded lock should be the consideration of differ-
analysis to bracket the actual base pressure distribu-
ential settlements between monoliths. If a soil
tion. By combining the above loads with the varying
foundation is not feasible because of site conditions,
foundation pressure distributions and limiting lateral
then a pile foundation is required. When considering
soil pressure distributions, the number of possible
a pile foundation, all types of piles should be consid-
loading combinations soon becomes astronomical.
ered and the most feasible and economical types of
All of these combinations need not be analyzed.
piles should be chosen based on strength, geotechni-
Sensitivity studies, engineering judgment, and other
cal conditions, availability of material, and construc-
rational methods should be used to select a reasonable
tion limitations. In order to make these comparisons
number of cases to analyze. It is important to note
and the comparison to the soil foundation alternative,
that for pile-founded structures, some load cases may
pile quantities should be computed based on assumed
not be critical for the pile foundation design but could
lateral and vertical pile capacities and the minimum
control the concrete monolith design. All load cases
pile spacing that is expected, taking into consideration
used on pile-founded structures should be analyzed
the fact that the density of piles may need to be
after the final pile layout has been developed so that
higher in some areas of the structure than others.
proper pile forces are included in the concrete mono-
This quantity computation should be performed on
lith design.
one of the more massive monoliths and on one of the
chamber monoliths, and the results should then be
5-4. Application of load factors. Analysis with
extrapolated out for the entire lock. If this initial
comparison shows the pile foundation to be more
unfactored (service) loads is used for foundation
economical or approximately the same as the soil
design and can be performed for the ultimate strength
foundation, then the designer should proceed with a
design of reinforced concrete. For frame analysis
more detailed analysis of the layout using the most
with service loads, internal member reaction results
economical type of pile. A rigid base analysis can be
are factored for member design. Application of
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