Is the program considering negative support negative moment right?
Question 1:
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.
As Per AASHTO LFD code:
3.11.3 Lane Loads on Continuous Spans
For the determination of the 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 a 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.
Answer:
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.
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.
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.
As Per AASHTO LFD code:
3.11.3 Lane Loads on Continuous Spans
For the determination of the 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 a 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.
Answer:
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.
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.
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