Is the program considering negative support negative moment right?

Question:

I was checking the negative moments near the support at node 29 as shown below. It shows that the maximum negative moment occurs when in traffic line lane 2, two concentrated loads are placed in the same span. Traffic line lane 1 shows it correctly by having the two concentrated loads on separate spans.



Per AASHTO LFD code:
3.11.3 Lane Loads on Continuous Spans
For the determination of maximum negative moment
in the design of continuous spans, the lane load shown in
Figure 3.7.6B shall be modified by the addition of a second,
equal weight concentrated load placed in one other
span in the series in such position to produce the maximum
effect. For maximum positive moment, only one
concentrated load shall be used per lane, combined with
as many spans loaded uniformly as are required to produce
maximum moment.

The model is not accurately taking this into account by placing the two concentrated loads in the same span for traffic line lane 1.

Can you clarify on this

Thanks,





Answer:Dear User,

The issue was with the lane definition.
While selecting elements for 'Start Span' information, it is sufficient that only the elements which are the start of the span be checked.
So for example, for lane 1 , and lane 2 , the elements to be checked as '1' , '9' and '27'


This would ensure appropriate application of the negative moment region vehicle placement.

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Question 2:
Shouldn’t we also be selecting the last element on the bridge (element 20) instead of only elements 1, 9, and 27? How would Midas know that the span starts at element 20 (“I” node) when running the vehicles in the opposite direction?

Answer:

As per the image below, irrespective of the direction, Span start, would always be I end of the selected element.
So the span length would be considered from start of element 1 to start of element 3.
Since there is no provision to identify I end and J end separately while defining the lane, this is the general norm adopted by the software.


Creation date: 10/27/2017 7:49 AM ()      Updated: 10/27/2017 5:25 PM ()
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