28-02-2013, 04:39 PM
Analysis, Design and Rating of Reinforced Concrete Box Culverts
Analysis, Design.pdf (Size: 769.13 KB / Downloads: 201)
Introduction
BRASS-CULVERT™ designs, analyzes, and/or rates one-, two-, three-, or four-cell reinforced concrete
box culverts with prismatic members (precast or cast-in-place) with or without a bottom slab. All cells
are assumed the same size, and the clear opening dimensions remain constant during the design process.
The wall and slab thicknesses may be specified and/or sized by the program. Analyses and designs may
accommodate either detailing with continuous or simply-supported slab-wall details.
The program designs the box culvert by either Allowable Stress Design (ASD) or Load Factor Design
(LFD) using the AASHTO Standard Specifications for Highway Bridges or by Load and Resistance
Factor (LRFD) using the AASHTO LRFD Bridge Design Specifications. LRFR analyses are available
using the AASHTO Manual For Bridge Evaluation. Alternatively, BRASS-CULVERT™ reviews a
known design or existing system and performs specification and resistance checking. LRFR, LFD and
ASD rating factors for flexure and shear are computed for both the design and review modes. All
applicable specification checks are computed /reviewed.
Units
The input and/or output data may be in either US or SI units. Input is read in the user-defined units
system and then converted to US units (in-lb system) for computation. If the user requests SI output, then
the output is converted to SI. All computations are performed with US units (in-lb system).
Reinforcement bar sizes may be entered in US standard, SI, Canadian, or EU sizes. The various metric
sizes are available if 'SI' is selected as the input method. The computations are performed on a unit-width
basis, i.e., one-foot. The output from SI is reported for a one-meter section. Hence, the unit conversion
factors should convert the results (typically action, e.g., moment) but also the unit-width basis.
Intermediate and detailed output files are written in US units only.
Cell Layout
BRASS-CULVERT™ can model culverts with one to four cells. As illustrated below, physical joints are
denoted by circles and members by numbers along the lines connecting the circles. If no haunches are
used in the analysis, one plane frame element is used to model each member. The culvert structure is
modeled using the walls and slabs of the cell layout. Left and right sides of the members are as denoted
with L and R, respectively. The start and end of members are indicated by the direction of arrows (points
toward the end). The sign convention for loads and actions are described later.
Boundary Conditions and Continuity
The nodes along the bottom slab are all restrained against vertical displacement. The right-most node is
also restrained against horizontal displacement (pinned). The upper-right node is also restrained against
horizontal displacement. See the figure above.
Span Length
The length of each member is based upon the adjoining member centerlines. The design span length is
explained in the output. The user must define the span length for the reinforcement configuration. No
correction is made for skew distribution factors.
Moments at the geometric centers, moments at faces of support, or moments within fillets (haunches)
built monolithic with the member and support may be selected for member design. BRASSCULVERT
™ automatically computes the moments at these locations based on the user-defined
preference. The shear is computed at a distance equal to the effective depth away from the support face.
Load Modeling
The loading applied to the top slab is uniformly distributed over the entire bottom slab. A more refined
assumption appears impractical due to the lack of precise soil information for each site. This load
distribution is used for all loadings, i.e., dead load and unit live loads used in the development of
influence lines. Therefore, the influence lines for actions include both the load effects of the unit load
applied to the top of the box and the unit load uniformly distributed on the bottom. The details about the
influence lines are addressed later.
Self Weight
The culvert self-weight is automatically computed based on the wall thicknesses. The top-slab weight is
applied to the top of the box. The wall weight and the top slab weight are applied to the bottom slab
(upward). The bottom slab weight is not applied in the model because its load is directly resisted by the
soil. The wall weight is not included in the fixed-end-action computations for axial forces. Hence, the
wall axial force is the same at the top and bottom of the wall.