Dead Loads

As shown in the figure below, the different loads are distributed based on either the tributary width or tributary width fraction. For an interior beam, the tributary width is calculated as half of the spacing to the CL of the beams on each side of the beam under consideration. For an exterior beam, the tributary width extends from the edge of the bridge to half the spacing to the next beam.

Dead Load Due to Deck Self-Weight

The procedure to compute the uniform dead load on precast due to self-weight of deck is as follows:

Dead Load on Composite

The dead loads on composite can either be distributed equally to all beams in the span or be distributed based on tributary fraction. The procedure to compute and distribute the dead load on composite based on tributary fraction is as follows:

1. The area loads are converted to equivalent line loads by multiplying their magnitudes with the specified load width.
2. 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.
3. Moments and shears are then computed at tenth points of each span.
4. 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 the Analysis Factors button on the Analysis tab 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).

Dead Load Due to Supplemental Layer Self-Weight

The self-weight of the supplemental layer is computer based on its thickness, tributary width of the girder under consideration, and the unit weight of the deck concrete. This load is resisted by the composite section and continuous beam model.

Dead Load on Supplemental

The procedure to compute and distribute dead load on supplemental is similar to the procedure for Dead Load on Composite explained above. The only difference is that while the dead load on composite is applied to the continuous composite beam model, the dead load on supplemental is applied to the continuous supplemental section model. (After the supplemental layer hardens, it acts compositely with the composite section, forming what we call the supplemental section).

Live Load

major change from the AASHTO LFD Specifications is that the LRFD Specifications requires significant computing power for calculation of the live load distribution factors. Predefined distribution factors for girders can be used from LRFD Tables 4.6.2.2.2b-1 through 4.6.2.2.2e-1 and 4.6.2.2.3a-1 through 4.6.2.2.3c-1, if the following conditions are satisfied:

If any of the above conditions are not satisfied then the distribution factor has to be computed using a refined method of analysis, typically a finite element or grillage analogy method.

If all the above conditions are met, but any of the spacing conditions in Tables 4.6.2.2.2b-1 through 4.6.2.2.2g-1 and 4.6.2.2.3a-(3c-1) are not met, the distribution factor is computed by the lever rule. This involves summing moments about one support/beam to find the reaction at another support/beam by assuming that the supported component is hinged at interior supports. Also, depending on the skew angle entered by the user on the Geometry tab, Precast/Prestressed Girder automatically applies the provisions of LRFD Table4.6.2.2.2e-1 and Art. 4.6.2.2.3c-1.

When working with Section Type k, i.e., I-girders, if the option for Include Ridge Cross Section Assumption on the Analysis Factors tab on the Analysis tab has been checked, Precast/Prestressed Girder applies the special provision for exterior beams as specified in LRFD Art. 4.6.2.2.2d. The program uses the larger of the table values or LRFD Art. 4.6.2.2.2d.

Permit Vehicle Loading

If Strength II Limit State is selected, the program considers any Permit Vehicle (PV) selected for inclusion in the Live Load envelope results (LRFD Art 3.4.1). If only one permit vehicle is selected the program considers its results in the formation of the live load results. However if multiple Permit Vehicles are selected then the program takes the envelope of these results to form the live load results. If Strength II Limit State is selected and no Permit Vehicle is selected, the program considers the HL93 (Design Lane, Design Tandem and Design Truck) loading for the Strength II Limit State as well. Optionally the user can also select the option to Use Permit Vehicle side by side with Design Loads as specified in the LRFD 2001 Interims (Art 4.6.2.2.4). When this option is selected the program applies the code provisions if the following criteria are met:

In the case where the single lane live load distribution factor is greater than the multi-lane live load distribution factor program computes the final force effect applied to the girder to be the force effect due to permit truck times the single lane distribution factor.

Dynamic Load Allowance Factor for Truck Loads

The dynamic load allowance factor for moment and shear for all components other than deck joints and for all limit states other than fatigue and fracture is 33% (LRFD Art. 3.6.2). 15% is used for fatigue and fracture limit states. The factor shall be taken as (1 + IM/100). For example, dynamic load allowance factor for the design truck is (1 + 33/100) = 1.33.

Multiple Presence Factors

Multiple presence factors are used only when Tables 4.6.2.2.2b-1 through 4.6.2.2.2e-1 and 4.6.2.2.3a-1 through 4.6.2.2.3c-1 cannot be used directly (LRFD Art. 3.6.1.1.2). Precast/Prestressed Girder places as many trucks as possible in the tributary width of the selected girder and calculates the reaction by the lever rule and then reduces it with the appropriate multiple presence factor.