LL Response
The live load response is calculated based on influence line loading algorithms. The truck type and the specification type dictate the loading algorithm that is used. Several truck and lane types are defined in CIP RC/PT Girder. These include Fixed Axle, Variable Axle, Notional, Special Enveloping, Cooper Train, Lane Load with Rider, and Lane without Rider. The following algorithms are used for each truck type.
LFD
| Fixed Axle Truck | The truck is moved forward and backward over the entire length of the bridge. All axles are included in the response. |
| Variable Axle Truck | The truck is moved forward and backward over the entire length of the bridge. A special algorithm is used that determines the axle spacing (within the given range) that causes the maximum response. Only one axle spacing can be variable. |
| Notional Loading | If this flag is set, the axles that reduce the maximum response (have opposite effect) are ignored. In effect, when determining the maximum response, the negative influence line ordinates are ignored, and visa versa; otherwise the same as fixed or variable axle truck. |
| Special Enveloping | This is not applicable to LFD. |
| Cooper Train | A uniform load trailing the axles is added, and is extended for the entire length of the bridge. |
| Lane Load | LFD lane load includes a shear and a moment rider. The lane load is placed on positive or negative patches of the influence line. The rider is placed as a concentrated load at the maximum ordinate of the influence line. Only for negative moment loading, two riders are placed on the two highest ordinates of the influence line. |
LRFD
| Fixed Axle Truck | The truck is moved forward and backward over the entire length of the bridge. All axles are included in the response. |
| Variable Axle Truck | The truck is moved forward and backward over the entire length of the bridge. A special algorithm is used that determines the axle spacing (within the given range) that causes the maximum response. Only one axle spacing can be variable. |
| Notional Loading | If this flag is set, the axles that reduce the maximum response (have opposite effect) are ignored. In effect, when determining the maximum response, the negative influence line ordinates are ignored, and visa versa; otherwise the same as fixed or variable axle truck. |
| Special Enveloping | This is only applicable to LRFD. In this case, the truck response is only applicable to vertical reactions and negative moments. It is ignored for all other response types. Note that this truck is defined as two fixed axle trucks with variable axle spacing from the code specified value to a large value extending beyond the entire bridge length. |
| Cooper Train | A uniform load trailing the axles is added, and is extended for the entire length of the bridge. |
| Lane Load | LRFD lane load does not include a shear or moment rider; therefore, the riders will be ignored. The lane load is placed on positive or negative patches of the influence line. |
| Impact | The impact factor can be specified by the user or calculated by the program according to the specifications. The user-specified impact factor can be only one value for the entire bridge. The program-calculated values are based on the code equations, i.e., LFD 3.8.2, LRFD=0.3. |
| Distribution Factors | The distribution factors can be specified by the user or calculated by the program based on the code equations. These values are input as number of wheel lines under LFD, and as number of lanes for LRFD. For the user-specified distribution factors, any number of locations along the length can be specified, and CIP RC/PT Girder will use linear interpolation to obtain the distribution factors for each POI. |
LFD
The distribution factor for LFD for whole width design is the edge-to-edge width divided by 7.0 ft. For Slab bridges, it is the reciprocal of the distribution width from LFD 3.24.3.2. For single girder, the value for interior girder is S/7, and for exterior it is We/7. Single girder distribution factors follow the LFD specification equations, except single lane distribution factor for shear at exterior girder is calculated by lever-rule.
LRFD
The distribution factor for LRFD follows the code equations, LRFD 4.6.2.2.2. These are applicable to single girder analysis. If the number of cells are more than 8, then 8 is used in the equations. If the number of cells is 0, then distribution factor is calculated as zero. Other limitations are ignored. For whole-width analysis, the distribution factor is calculated as the summation of all the distribution factors for all girders. For slab cross sections, the distribution factor is calculated as the reciprocal of the distribution width per LRFD 4.6.2.3. CIP RC/PT Girder does not adjust the shear or moment values for the skew correction factor.
| Side-by-Side | The side-by-side loading is based on the LRFD specifications for Strength 2, as per LRFD 4.6.2.2.5. CIP RC/PT Girder implementation includes this loading for both LFD and LRFD. CIP RC/PT Girderhas the capability to define an associated design live load along with each primary live load. If the side-by-side loading is specified, CIP RC/PT Girder calculates the response as: |
| Live Load Response |
The truck and lane responses are combined to obtain the total response due to any give live load. Each live load can have any number of trucks and only one lane load. |
LFD
The live load response is calculated as the envelope of all truck and lane responses. In the case of side-by-side live loads, the live load response is calculated first, and then enveloped with the lane. Impact factor is applied to all truck and lane loads.
LRFD
The live load response is calculated as the combination of the truck and lane, using the truck response with distribution factor and impact, and the lane with distribution factor, but without impact. If the special enveloping flag is set for the truck, then 90% of the lane response is added to the truck response. If the side-by-side flag is set for the live load, then after obtaining the entire live load response for the primary live load and the associated live load, they are combined as shown in previous section.
| Deflection | All live load deflection responses are calculated by loading deflection influence lines. The shear distribution factor is applied to the vertical deflection, and the moment distribution factor is applied to rotations. |
| Forces | Live load forces (shear, axial, moment) are obtained by loading the force influence lines and applying the appropriate impact and distribution factors. |
| Stresses | All live load stresses are obtained by calculating the force enveloped (axial and moment) and calculating the stresses from them by using the cross section properties at the POI. Stresses are calculated based on maximum and minimum axial and moment and corresponding values. |
| Reactions | All live load reactions are calculated by loading reaction influence lines. Shear distribution factors are used for vertical reactions, and moment distribution factors are used for moment reactions. |