Using influence surfaces for load optimization

[Anonymous]

Hello everyone, some things confuse me about influence surfaces and load optimizations.

Here is my 3D bridge model:

I plan to separately design the beams for each span for minimal reinforcements. The plan is for the beams to be designed to handle the worst case of design traffic considering an AASHTO LRFD 7 specifications for a design truck, design tandem, and a special permit vehicle. I believe load optimization finds the worst case for you, which I hope is correct. This brings me to my main question:

When I choose an influence surface, do I have to do it for every single beam I design, or will one selection do? Is it correct to pick the beam I am currently designing for the influence surface generation?

After the influence surface generation, I noticed that the load optimization has different options available.

My own country (based on AASHTO)'s codes require the checking of the worst of:

1. Design truck + lane load

2. Design tandem + lane load

3. Two design trucks with 90% load + lane load

However, I am having a few problems.

First, my country's design truck and tandem are a bit different from AAHSTO's, but only through the numerical values. I thought I could just define them as custom permit vehicles, but it appears that Load Optimization uses the default anyway, and the custom vehicles have errors where one axle went missing. So I was wondering if I should just edit the numbers on the Structural Loads and if that would still be the optimal.

Also, I do not know if the two 90% design trucks are not happening because the program does not account for them or because they are exceeded by other load combinations. I have ticked the box that said double design truck, so I think it was accounted for, but I want to make sure.

Lastly, is this something to worry about?

I get the feeling that only one of the 5 beam elements is being optimized for. Also, I do not know what to make of the other options. What does it mean when I tick/untick overhang? Finally, what do the options in the drop downs mean?

 

Thank you so much, any input will help.

[dave_geeves · Accepted solution]

Hi,

Thanks for such a detailed post regarding your questions, I now know a little more about your structure and what you are trying to achieve - but It would still have been useful to have a copy of your structure to investigate. 

I am in the UK so have good experience in BS codes and Eurocodes but have limited experience in LRFD codes as I've never been able to afford them - especially as they change each year.  I have some knowledge of the traffic loading in  (pure) LRFD but I do not know how to apply variations of this code.

With regards to your first question - your selection of influences to consider is entirely based upon engineering judgement.  If you are unsure of the structural behaviour then you will need to select each point along each girder.  This can be done by setting the pick mode to "Longitudinal Beam" and selecting each girder in each span in your structure. which will automatically define an influence point at each node in your bridge deck.  When the load optimisation is run it will  probably create several hundred load case, compilations and envelopes, which, when solved, will give all the results that your need. So yes, you do need to create the influence for all longitudinal girders - not just one - and obtain the envelope for each design beam.

My preferred method is to create the loads for one property set at a time.  In other words, define influences for just internal beams span 1, analyse them with load optimisation, solve these load cases and then transfer the envelope results to that design beam.  This can then be repeated for the other design beams.

With regard to the variations from AASHTO LRFD in design trucks I am not sure if this can be done.  It may be useful if you explain which code you are using so that the developers can assess if this is possible.  The application of the load optimisation is very code specific and the rules related to variation of standard codes may not be possible but sometimes there are workarounds which can be used.  Your idea of just increasing the axle loads in the loads generated is a sound solution, as the variation of the design truck wheel loads will probably not make too much difference to the optimal load positions and I would generally be happy to use this - but it is probably a lot of work.

Sorry I cant be more constructive on this but (I hope my explanations have helped.  Another thing to point out is that if you select he "Help" button on a data form, and then select the Field help it will provide the reasons and data required in each of the data fields.

I hope this has been useful, and if this has answered your question then please mark my reply as a solution so that others can benefit.  Thanks.

Kind regards

Dave Geeves