ANTONIO ROSSELLÓ PULIDO BÀRBARA QUETGLAS FLORIT
1. Dades de la població beneficiària El projecte s’adreça a menors,
Longitudinal / braking load can be generated from two locations in the program. First is the generation from Longitudinal / Braking load case dialog. Secondly it can be generated along with Live load generation. When longitudinal load is generated from its own load case, only one longitudinal load case is generated by the program. When the longitudinal load is generated along with the live load generation, program generates same number of LF/BR cases as it generates live load cases. For this situation, every LF/BR case is generated for same number of live load lanes loaded as in the live load case. For example, if program generates 5 live load cases, it will generate 5 LF/BR cases. If the first live load case generated is based on two lanes loaded simultaneously, then first LF/BR case will also have two lanes loaded.
As per AASHTO Standard Art. 3.9, the longitudinal load as a percent of live load is, C = 5% of lane load
As per AASHTO LRFD Art. 3.6.4., the braking force as a percent of live load is, The braking force shall be taken as greater of:
C = 25% of the axle weights of the design truck/tandem, or C = 5% of the design truck + lane load, or
5% of the design tandem + lane load
When lane load is used in computing the LF/BR force, the uniform load intensity is multiplied by a contributing length to compute the total force. This total force is then used to compute the LF/BR percent (C) above. RC-PIER assumes that average of the two spans adjacent to the pier as the default contributing length. However, there may be cases when a different length may be adequate. User must adjust this length in that case.
In LRFD mode, when a truck load is considered for generation, all axles of the truck are considered in computing the total load even if the span is too short to accommodate the entire length of the truck.
Program also provides an option in which the total live load values for lane and truck can be specified rather than selecting a truck or lane load. In AASHTO Standard since only lane is used, user can specify only lane load total. In AASHTO LRFD, both lane as well as truck total live load per lane have to be specified. In that case, program uses the value input as total for one loaded lane.
Once the percent C is computed, program then determines the total longitudinal load and then computes the bearing loads. In this computation, it uses the live load reduction factor / multiple presence factor. In computing bearing/girder and cap loads it assumes that the total longitudinal load is applied at 6 ft above the deck in the direction of stationing. For LRFD California the user can input the height above deck, the default value in this case is considered 0. By default the LF and BR load cases are reversible in RC-PIER so this load gets considered in both directions.
The total longitudinal load is distributed among all the bearings/girder locations as bearing loads. The moment about the X axis is applied as cap load and the moment computed about Z axis is applied as a force couple in Y direction at exterior bearings. If there are two bearing lines, half of the force is distributed on bearings in each bearing line. For skewed bridges, RC-PIER resolves the load into X and Z components resulting in bearing loads in both X and Z directions of the pier.
This generation internally accounts for the pier view direction and then generates loads as per the view direction. In case of skewed bridges, RC-PIER calculates the appropriate force and resolves it into X and Z components appropriately. The moments generated about X axis are applied as cap load whereas the moments generated about Z axis is applied as couple on the exterior bearings/girders. If the pier is non-integral and has two bearing lines then each line will resist half of the moment which is then divided by the distance between the first and the last bearing to generate the force in Y direction.
Figure TH-25 Auto Longitudinal/Braking Load Generation
Auto Centrifugal Load Generation
Centrifugal load can be generated from two locations in the program. First is the generation from Centrifugal load case dialog. Secondly it can be generated along with Live load generation. When centrifugal load is generated from its own load case, only one load case is generated by the program. When the centrifugal load is generated along with the live load generation, program generates same number of CF/CE cases as it generates live load cases. For this case, every CF/CE case is generated for same number of live load lanes loaded as in the live load case. For example, if program generates 5 live load cases, it will generate 5 CF/CE cases. If the first live load case generated is based on two lanes loaded simultaneously, then first CF/CE case will also have two lanes loaded.
As per AASHTO Standard Art. 3.10, the centrifugal load as a percent of live load is: In US units:
C = (6.68 S2)/R where:
C = the centrifugal force in percentage of Live Load, without impact. S = the design speed in miles per hour.
R = the radius of curve in ft.
X
LFZ
Z LFX
In SI units:
C = (0.79 S2)/R where:
C = Centrifugal force in percentage of Live Load, without impact S = Design speed in km/hr
R = Radius of curve in m
As per AASHTO LRFD Art. 3.6.3 and equation 3.6.3-1, the centrifugal force as a factor of live load is, C = 4/3 v2/gR
where:
C = Centrifugal force as a factor of the total axle weight of design truck or tandem V = highway design speed ft/sec (m/sec)
g = gravitational acceleration 32.2 ft/sec2 (9.8 m/sec2) R = the radius of curve in ft. (m)
In this generation, all the axles of a truck are considered for generation even if the span is too short to accommodate the entire length of the truck.
Program also provides an option by which the total live load values for truck can be specified rather than selecting a truck from the library. When total truck load is specified, program uses the value input as total truck load per lane. Once the percent (or factor) C is computed, program then determines the total centrifugal load and then computes the bearing/girder loads. In computing bearing/girder and cap loads it assumes that the total longitudinal load is applied at 6 ft above the deck in the direction of stationing. The load direction is as per user specified.
For LRFD California the user can input the height above deck, the default value in this case is considered 0. The curvature of the bridge alignment defines the direction of the centrifugal force on the bridge. If the pier view direction is upstation and the bridge curvature is such that the center of the curve is on the right side of the
alignment looking upstation, a user should specify to apply the load in – X direction. See Figure TH-26 a, b, c, and d for four possible cases considering curvature and the pier view direction.
The total centrifugal load is distributed among all the bearings/girder as bearing/girder loads. The moment about the X axis is applied at cap load and the moment computed about Z axis is applied as a force couple in Y direction at exterior bearings. If the pier is non-integral and there are two bearing lines, half of the force is distributed on bearings in each bearing line. For skewed bridges, RC-PIER resolves the load into X and Z resulting in bearing loads in both X and Z directions of the pier.
This generation internally accounts for the skew in selected pier view direction and then generates loads as per the view direction. If the pier view direction is switched, previously generated loads are converted and generation direction of load is also adjusted.
Figure TH-26 Centrifugal Load and its Direction Pier View Upstation, Bridge Curves Right, X -
Pier View Upstation, Bridge Curves Left, X +
Pier View Downstation, Bridge Curves Right, X +
Pier View Downstation, Bridge Curves Left, X -