Dead Load on Composite
- The area loads are converted to equivalent line loads by multiplying their magnitudes with the specified load width.
- The load, in addition to the user-specified point and live loads, is applied to a continuous composite beam model and a static analysis is performed.
- Moments and shears are then computed at tenth points of each span.
- The default proportionate share of moment and shear distributed to each beam is:
where Dead Load on Composite is the tributary fraction for dead load on composite, BTrib is the tributary width of the beam under consideration, and Overall Width is the out-to-out width of the bridge. You can modify this calculated value by clicking Analysis Factors on the Analysis screen and then select the Distribution tab.
To distribute all the dead loads on composite equally to all the girders in the bridge, change the dead load distribution factor to a value equal to 1/(number of beams in current span).
Live Load
For the modeled bridge width, IRCH chapter 6-2000, Table 2, Clause 207.4, defines the rules to combine the live loads. Precast/Prestressed Girder allows users either to use these IRC live load rules or use a custom method. If the IRC rule option is selected, the program automatically uses the appropriate live loads and number of design lanes, and generate all the possible load combinations as stated for this particular bridge width. Otherwise, you can select a specific live load and design lane to generate combinations.
| Carriageway Width | Number of lanes for Design purpose | Load Combination | |
|---|---|---|---|
| 1 | Less than 5.3m | 1 | One lane of class A considered to occupy 2.3m. The remaining width of carriageway shall be loaded with 500Kg/m2. |
| 2 | 5.3m and above but less than 9.6m | 2 | One lane of Class 70R OR two lanes of Class A. |
| 3 | 9.6m and Bove but less than 13.1m | 3 | One lane of Class 70R with one lane of Class A OR 3 lanes of Class A. |
| 4 | 13.1m and above but less than 16.6m | 4 | One lane of Class 70R for every two lanes with one lane of Class A for the remaining lanes, if any, or one lane fo Class A for each lane. |
| 5 | 16.6 and above but less then 20.1m | 5 | One lane of Class 70R for every two lanes with one lane of Class A for the remaining lanes, if any, or one lane fo Class A for each lane. |
| 6 | 20.1m and above but less than 23.6 | 6 | One lane of Class 70R for every two lanes with one lane of Class A for the remaining lanes, if any, or one lane fo Class A for each lane. |
Live Load Generation as Per IRC
Based on input geometry, Precast/Prestressed Girder identifies the IRC rules (shown in the table above) applicable based on the computed carriageway width and number of lanes for design purpose. Precast/Prestressed Girder applies all rules for all cases lower than required number of lanes independently and computes the governing moments and shears. So a bridge with a higher number of lanes is analyzed with all relevant IRC rules for cases with lower number of rules as well.
Next, a longitudinal moving load analysis of each vehicle on the bridge is done independently and the resulting Moment and Shear values stored. This is followed by the transverse moment of the trucks from one end of the bridge (for example left to right) to the other along the cross-section (and the other way, from right to left) by following the IRC rules and the spacing between vehicles, curb to wheel spacing criteria, etc. For each position the reaction factor (distribution of live load to transverse beam lines) by the Courbon’s Method is computed or in the end zones the reaction factor is computed using the continuous beam approach prescribed for end zones. Prior to determining the controlling factor, the appropriate multiple lane reduction factor and impact factor is applied to the previously computed moments and shear values at 1/10 points along the span.
Impact Factors for Live Loads
The Impact Factor is computed as per IRC: 6-2000, Clause 211 based on the class of loading.
For Class A or Class B loading, the Impact Factor fraction is computed as 4.5 / (6+L), where L is the length of the span in meters.
For class AA or Class 70 R Loading, if the span is less than 9m, for wheeled vehicles, impact percentage is 25% and for tracked vehicles it is 25% for spans up to 5m and linearly reducing to 10 percent for spans of 9m.
If spans are of 9m or more, for tracked vehicles, impact percentage is 10 percent up to 40m span. For wheeled vehicles, 25 percent for spans up to 12m and 4.5/(6+L) beyond.