3. METODOLOGÍA DE TRABAJO
3.10. Objetivo Específico 1
3.10.1. Distribución espacial de la densidad poblacional
3.1.4.1 Future Paving Allowance
Vehicular bridges without a bituminous overlay shall be designed for a future paving load of 30 pounds per square foot to account for a 2½” bituminous concrete overlay.
Structures with a bituminous overlay shall not be designed for any additional loads to account for future additional paving.
3.1.4.2 Remain-in-place Forms
Bridges where the use of remain-in-place forms is allowed shall be designed for the additional dead load of the form and the concrete in the valleys.
3.1.4.3 Temperature Effects
For bridges where the effects of temperature cause loads on members, the temperature range for cold climates shall be used as outlined in the AASHTO Specifications.
3.2 EARTH PRESSURES 3.2.1 General
For determination of lateral earth pressures, methods prescribed in the AASHTO Specifications may be used. For broken back slopes, walls supporting high fills, or other special loading conditions, proper design methods should be selected by the designer and submitted for approval during the preliminary stage of design.
In the design of substructure units such as abutments, wingwalls and retaining walls, the lateral earth pressure shall never be assumed to be less than 33 pounds per square foot per linear foot of wall.
3.2.2 Design Assumptions 3.2.2.1 Unit Weight of Soil
The unit weight of soil over the footing shall be assumed to be 125 pounds per cubic foot unless specific conditions require it to be otherwise.
3.2.2.2 Coefficient of Friction
The coefficient of friction between the bottom of footing and the soil beneath it shall be a maximum of six tenths (0.6) for all except special cases.
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ANUAL 3.2.2.3 Footing KeysUse of a key in the footing to develop passive pressure against sliding is not normally recommended; if proposed, the designer shall submit studies to the Department for approval during the preliminary stage of design.
3.2.3 Surcharge
Earth pressures due to surcharge loads shall be taken into consideration. A minimum surcharge loading equivalent to two feet of soil shall be used.
3.2.4 Unbalanced Loads
Concrete tunnels with skewed end sections, integral or semi-integral abutments, rigid frames, box culverts, etc., will require special consideration in the design and sequence of backfilling in order to prevent cracking due to unbalanced loading.
The designer shall specify backfilling requirements where the effects of unbalanced backfill loading will produce a detrimental effect.
3.2.5 Box Culverts
For the design of box culverts, the lateral pressure due to earth adjacent to and above the box shall be computed for earth at rest. It may be assumed to be 0.45 times the corresponding vertical earth pressure, unless specified by the designer to be otherwise. 3.3 DISTRIBUTION OF LOADS
3.3.1 Distribution of Loads to Superstructure Components 3.3.1.1 Dead Loads and Composite Dead Loads
All composite dead loads shall be distributed to all beams equally.
For precast adjacent box beam bridges, all dead loads shall be distributed to all beams equally. If multiple beam types are required in the same cross section, the distribution of loads must take into account the stiffness of each beam. The distribution of dead loads for this condition shall be as follows:
(
1 2 3 n)
k Total k I I I I I x DL DL + + + + = K where: = kDL Dead load on beam ‘k’ =
Total
DL Total deal loads applied to the superstructure parapet excluding beam weight (parapets, wearing surface, railings, sidewalks, etc.)
k
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(
I1 +I2 +I3 +K+In)
= Total moments of inertia of all beams 3.3.1.2 Live LoadsThe distribution of loads to superstructure components shall be in accordance with AASHTO Specifications.
For precast adjacent box beam bridges, all live loads shall be distributed to all beams equally. If multiple beam types are required in the same cross section, the distribution of loads must take into account the stiffness of each beam. The distribution of live loads for this condition shall be as follows:
1. A distribution factor should be calculated according to the AASHTO Specifications assuming the entire bridge cross section is made up of the predominant beam in the cross section.
2. The distribution factor for each beam should be calculated based on the relative stiffness of the two adjacent beams as follows:
(
k 1 k k 1)
k k I I I I x 3 x DF DF + − + + = where: kDF = Distribution factor for beam ‘k’
DF = Distribution factor as described above in 1
k
l = Moment of inertia of beam ‘k’
1 k
l − = Moment of inertia of beam to the left of beam ‘k’
1 k
l + = Moment of inertia of beam to the right of beam ‘k’
3. Since all beams of a common type should be designed and detailed in a similar fashion, the maximum distribution factor for each beam type should be used for the design of all common beams.
3.3.2 Distribution of Loads from Superstructure to Substructure Components 3.3.2.1 Dead Load Reactions
3.3.2.1.1 General
The dead load reactions from the design of the superstructure may be used for the design of substructure components.
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ANUAL 3.3.2.1.2 Abutments and Solid Wall PiersFor the design of earth retaining abutments and solid wall piers, the dead load reactions may be distributed evenly over the entire length of the substructure stem.
This procedure is sufficiently exact for most design purposes. However, in the design of abutments where the height of the stem is less than the beam spacing measured along the abutment, considerable judgment should be exercised in the establishment of a reasonable width over which each reaction is distributed. 3.3.2.1.3 Other Piers
For the design of hammer head, bent type and single column piers supporting discrete longitudinal composite members on bearings, the dead load reactions should be applied as concentrated loads at the bearing locations.
For the design of hammer head and bent type piers supporting precast concrete adjacent box beams on bearings, the dead load reactions may be distributed evenly to the top of the pier.
3.3.2.2 Live Load Reactions 3.3.2.2.1 General
The vehicle live load reactions from the design of the superstructure shall not be used for the design of the substructure components. These reactions are based upon the maximum conditions for one member. The use of these loadings would result in a substructure design with an unrealistic loading condition. The governing live load reactions for design should be determined from the vehicle loadings described in Article 3.1.3. When applying the truck load, only one truck per lane should be utilized. The concentrated load for shear should be included for the lane load. The live loads should be increased for the impact effect in accordance with the AASHTO Specifications. The live loads may be reduced due to multiple lanes being loaded simultaneously in accordance with the AASHTO Specifications.
3.3.2.2.2 Abutments and Solid Wall Piers
For the design of earth retaining abutments and solid wall piers, the live load reaction is calculated using a vehicle lane reaction. The vehicle lane reaction is determined by positioning the vehicle live load longitudinally on the structure to obtain the maximum reaction at the support location. The vehicle lane reaction should be increased for the number of design traffic lanes and distributed over the entire length of the substructure stem to obtain a load per unit length.
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This procedure is sufficiently exact for most design purposes. However, in the design of abutments where the height of the stem is less than the beam spacing measured along the abutment, considerable judgment should be exercised in the establishment of a reasonable width over which each reaction is distributed. 3.3.2.2.3 Other Piers
For the design of hammer head, bent type and single column piers supporting discrete longitudinal composite members on bearings, the reactions at the bearings are calculated using the vehicle lane reaction. The vehicle lane reaction is determined by positioning the vehicle live load longitudinally on the structure to obtain the maximum reaction at the support location. To determine the live load reactions at the bearings, the vehicle lane reactions should be placed within the design traffic lanes and then distributed to the members assuming the slab between the members to be simply supported. The vehicle lane reactions should be positioned within the design traffic lanes in accordance with AASHTO Specifications. The design traffic lanes and the vehicle lane reactions within the lanes should be arranged to produce reactions that result in extreme force effects on the component under consideration.
For the design of hammer head and bent type piers supporting precast concrete adjacent box beams on bearings, the vehicle lane reactions may be distributed directly to the pier without any transverse distribution. The vehicle lane reactions should be positioned within the design traffic lanes in accordance with AASHTO Specifications. The design traffic lanes and the vehicle lane reactions within the lanes should be arranged to produce extreme force effects on the component under consideration.