Lane Supports (Negative Moments at Interior Piers)

Assign the elements constituting a multiple span being considered for moving load analysis using traffic lane loads.

When a support is defined by the function, midas Civil automatically places concentrated loads of equivalent magnitudes to the uniformly distributed loads on the spans each side of the support in addition to the uniformly distributed loads. The concentrated loads are located so as to produce the maximum negative moment due to the traffic lane loads in accordance with the AASHTO Standard, AASHTO LRFD and PennDOT.

AASHTO LRFD Design Load

This function also automatically considers the requirement of the effect of 90% of two design trucks spaced at 50 ft and 90% of the design lane load for negative moment in accordance with AASHTO LRFD when specified. The requirement of 14 ft distance between the 32 kip axles is also maintained.

PennDOT Design Load (PHL-93TRK)

This function considers the Pennsylvania DOT requirements. For the negative moment between points of dead load contraflexure, the effect of two design trucks spaced a minimum of 50 ft. between the lead axle of one truck and the rear axle of the other truck, combined with the effect of the design lane load; the distance between the 32 kips axles of each truck shall be taken as 14 ft.. The two design trucks are placed in adjacent spans to produce maximum force effects. (PUB 15M (5-12) 3.6.1.3.1.)

PennDOT Tandem Load (PHL-93TDM)

For the negative moment between points of dead load contraflexure, the effect of two tandems with axle weights of 25 kips spaced from 26 ft. to 40 ft. apart, combined with the effect of the design lane load. The two tandems should be placed in adjacent spans to produce maximum force effects. (PUB 15M (5-12) 3.6.1.3.1.)

AASHTO legal load (The Manual for Bridge Evaluation)

This function also automatically considers the requirement of the effect of a lane load of 0.2 klf combined with two AASHTO Type3-3 vehicles or state legal loads multiplied by 0.75 spaced at 30 ft for negative moment in accordance with the Manual for Bridge Evaluation by AASHTO .

In addition, for span lengths greater than 200ft, critical load effects shall be created by the AASHTO Type3-3 vehicles or state legal loads multiplied by 0.75 and combined with a lane load of 0.2klf.

Each State DOT legal load

This function also automatically considers the requirement of the effect of a lane load of 0.2 klf for both negative moment and for span lengths greater than 200ft in accordance with DOT Legal vehicles when specified using the LRFR method.

From the Main Menu select Load > Moving Load > Lane Supports - Negative Moments at Interior Piers.

When AASHTO LRFD is selected

Auto Input

AASHTO LRFD specifies that additional loads are imposed in the support regions when calculating negative moments between points of contraflexure under a uniform load on all spans. In case of a three-span continuous bridge, the regions marked in red are the negative moment zones as shown in the figure below.

Girder Group: : Select Structure Group(s), which contain main girder elements and click in order to add the group in the list. Negative moment zones will be calculated by applying a uniform load on all spans.

When the selected Girder Group consists of beam elements

Lane Support will be automatically assigned in the negative moment zones. If the moment diagram due to a uniform load is displayed as shown in the figure below, Lane Support will be assigned to the element no. 7 (j-end), 8~9 (i & j-end) and 10 (i-end).

The fact that a uniform load is applied in the gravity direction (GCS Z-direction) to find the negative moment zones, the local z-axis of the beam elements in the Girder Group must coincide with the GCS Z-axis.

When the selected Girder Group consists of plate elements

Lane Support will be automatically assigned to the element if negative moment occurs at any node (I, J, K or L) of the plate element due to the automatically placed unit uniform load.

For plate elements, negative moment zones are found on the basis of 'Mxx'. So the moments due to vertical loads must be 'Mxx'. In order for this to happen, the local x-axis of the plate elements in the Girder Group must coincide with the direction of the bridge axis (in the direction of the traffic).

Display the Lane Support input

If lane supports are defined using the Auto Input option, the Lane Support can be checked in the post-processing only. Since negative moment zones under a uniform load can be found only after performing analysis, the negative moment zones cannot be checked in the pre-processing.

User Input

Options

Add: Enter the selected elements to be part of continuous beam elements.

Delete: Select and delete previously entered continuous beam elements.

Element Type

Select the element type for the elements being designated to become the continuous beam.

Support Position

Locate the support in the case of beam elements.

Both: supports located at both ends of the beam element

End-i: support located at i-end (start of the element)

End-j: support located at j-end (end of the element)

Select support positions to calculate negative moment

In case of a beam element, specify the Element Type and Support Position. Then select the corresponding elements and click . The positions of negative moment calculation will be displayed as shown in the figure below.

Note

Lane Support is applied to consider following conditions as per AASHTO Standard and AASHTO LRFD.

[3.6.1.3 Application of Design Vehicular Live Loads]

...,the extreme force effect shall be taken as the lager of the following:

- For both negative moment between points of contraflexure under a uniform load on all spans, and reaction at interior piers only, 90 percent of the effect of two design trucks spaced a minimum of 50.0 FT between the lead axle of one truck and the rear axle of the other truck, combined with 90 percent of the effect of the design lane load. The distance between the 32.0 KIP axles of each truck shall be taken as 14.0 FT.

When midas Civil envelops maximum and minimum results, two design truck scenario is considered for the negative moment and reaction only. In other words, although the maximum shear or axial force effects are found under two truck loading, midas Civil will disregard them and present the force effects from a single truck scenario. For example, if a single truck results in the maximum axial force -240 kips and double trucks result in the maximum axial force of -450 kips, midas Civil will report -240 kips in the min/max envelope results.

When AASHTO Standard is selected