31 Dec 93
1-ft-wide strip is usually sufficient. Strip selection
d. Rigid links. Rigid links are short members at
should also consider blockouts that cause discontinu-
joints that are stiffened to represent the behavior of
ity or member property reduction in the structural
wide supports. They are used to approximate real
framing. Some strips will have tributary load. An
behavior at the intersections of thick concrete mem-
example of these strips/sections occurs at the culvert
bers. The length of the rigid link is generally half the
valve well. The walls of the well can be assumed to
distance between the joint and the face of the sup-
act as a plate fixed against rotation on three sides
porting concrete. The length can be extended by half
with reactions on upstream and downstream sections
the length of a fillet if present. A link should have a
and the base slab. Wall reactions are distributed
stiffness of at least ten times greater than that of the
throughout the width of the upstream and downstream
intersecting flexible member. In regions of complex
sections. Vertical forces are assumed to be resisted
geometry, finite element runs can be used to calibrate
by the foundation below the wall with no transfer to
the length of rigid links. For application of rigid
adjacent sections. The wall plate must be designed to
links, see Figure A-5.
transfer the load in the assumed direction (see
(1) The foundation can usually be modeled by
Strips in three-dimensional monoliths will have unbal-
elastic springs (both vertical and horizontal) for soil
anced vertical loads due to the foundation pressure
or piles. Pile springs are attached to the base slab
gradient. Strip equilibrium is achieved by using
centerline by rigid links which model the eccentricity
to the foundation. The length of the link is generally
half the thickness of the base slab.
referred to as shear transfer. Shears should be
applied such that moments are not introduced into the
(2) Hydrostatic uplift is modeled as a load.
external stability of the monolith. Shear transfer also
Hydrostatic uplift reduces bearing pressures, which
provides redundancy in the monolith which is
affects frame response.
required to distribute the effects of small discontinu-
ities (from blockouts/voids) in the structural framing.
(3) For soil-founded locks, assumed shapes of
This permits the designer to ignore small voids in the
bearing pressures can be modeled as a load in lieu of
frame analysis. Generally within a monolith, thick
using foundation springs (see paragraph 6-3a). Sup-
reinforced concrete members establish shear transfer
ports are still required to provide stable boundary
without special details.
conditions, but each support reaction should be zero.
Note that displacements of the soil must be compati-
(3) A monolith may require several strip analyses
ble with the deflections of the structure in order to
and parametric studies in order for the designer to
accurately model the soil-structure interaction (see
understand its behavior. Selecting a strip and inter-
paragraph 6-3b). The use of foundation springs
pretation of analysis results are challenging tasks and
accommodates this requirement and is the preferred
should be assigned to more experienced engineers.
method of analysis.
c. Frame member. Generally, framing is
(4) Pile foundation analyses should include hori-
modeled along member centerlines except for the
zontal base shears, particularly if battered piles are
very deep member that forms the culvert roof in
present. Horizontal base shear on battered piles
monoliths such as the gate monoliths. This member
creates a vertical component of force that will load
is modeled near the top of the culvert and intersects
the U-frame. Torsional moments on a monolith
the culvert walls at their centerlines (see Figure A-4).
create horizontal base shears and should be evaluated.
The block above the culvert can be modeled as a
rigid body if its span-to-depth ratio is 1 to 1 or lower.
(5) Due to uneven distributions of foundation
If the member above the culvert is relatively thin, it
bearing pressures, differential settlement between
behaves like a typical frame member. Member sec-
monoliths and within a monolith should be consid-
tion properties are computed using member gross
ered. Within a monolith, usually the base slab is
constructed first, the subsequent load from buildup of