Dead Load on Composite

The procedure to distribute the superimposed dead load on composite and on supplemental based on tributary fraction is: First, the uniform surface load is multiplied by the overall width of the bridge to obtain the total dead load acting on the bridge. Then, a continuous beam analysis is performed to compute moments and shears at tenth points at each span. The default proportionate share of moment and shear distributed to each beam is:

where:

DL - Comp Trib Frac = Tributary fraction for dead load on composite

Btrib = Tributary width of beam under consideration

Overall Width = Out-to-out width of the bridge.

Note: You can modify this value by selecting the Analysis Factors button on the Analysis tab.

Pedestrian/Sidewalk Loads

The pedestrian or sidewalk load effects are first computed for the entire bridge based on composite action and then distributed to each girder as a ratio of the tributary width fraction.

Live Load

The axle load distribution factor can be computed by the program or entered manually by the user. To manually input the axle load distribution factor, go to the Analysis tab and click the Analysis Factors button. Then, on the Distribution Factors tab, select the Manual option and input the desired values in the fields for moment and shear for the appropriate group. The axle load distribution factor is half the wheel load distribution factor.

To have the program calculate the axle load distribution factor, select the Computed option. The program will then calculate the live wheel load distribution according to one of the following cases:

Case	D.F. Calculation	AASHTO LFD Reference
Number of beams = any type of selection	D.F. = (number of lanes) (lane load reduction factor)	Art. 312
Interior Longitudinal Stringer, one lane	D.F. = S/7 or as simple span* if S exceeds 10 ft	Table 3.23.1
Interior Longitudinal Stringer, 2 or more lanes	D.F. = S/5.5 or as simple span* if S exceeds 14 feet	Table 3.23.1
Exterior Longitudinal Stringer	D.F. = As simple span*.	Art. 3.23.2.3.1.2
Interior Spread Box Girder, Interior Open Box Beam	D.F. = AASHTO LFD Eq. 3-33	Art. 3.28.1
Exterior Spread Box Girder, Exterior Open Box Beam	D.F. = As simple span* and not less than 2 x NL/Nb	Art. 3.28.2
Multi-beams	D.F. = S/D (see below for values of K used)	Art. 3.23.4.3

Note: For calculating distribution factors on a simple span (i.e. lever rule) basis, the deck is assumed to act as a simple span between beam centerlines. The distribution factor is calculated first with one lane loaded, then for two lanes loaded, and so on until the maximum number of lanes permitted is loaded. Each of these distribution factors is multiplied by the corresponding lane load reduction factor (Art. 3.12.1). The maximum of these calculated values is used for the computed distribution factor.

Longitudinal Stringer

AASHTO Girder Stringer, Rectangular Stringer, Special Longitudinal Stringer

Spread Box Beam

Spread Box Girder

Multi-Beam

Adjacent Box Beam, K = 1.0

Channel Beam, K = 2.2

Double Tee, K = 2.2

Non-voided Rect. Beam, K = 0.7

Rect. Beam w/ Round Voids, K = 0.8

The figure below illustrates the model for determining the distribution factor for an exterior beam using the simple span (i.e., lever rule) assumption explained above. The first wheel is placed at a distance from the curb equal to 2 ft.

If using loading Group IA, the distribution factor is calculated for only one lane. As specified in Art. 3.5.1, Group IA only needs to be considered for loads less than H20.