Load Combination Generation

[Serhiy2]

Good day,

I'm sure that the questions which I'm going to ask here was discussed before but I couldn't find the right post so I apologize in advance.

I'm working on a moment frame design where I have different snow, wind and crane load cases. I'm analyzing it in RISA and need to come up with all possible load combinations. In other words I need to consider Sa+Wb+Cc where S=snow, W=wind, C=crane and a,b and c are indexes that can be equal to 1, 2, 3 irrespective of one another.

Risa's load combination generator doesn't have this ability and doing it by hand is time consuming. I'm wondering if there is a spreadsheet or other tools available which would do this work automatically.

I worked few years ago on a project in Autodesk Robot and its load combination generator has the ability to generate load cases with different basic load cases.

 

[jayrod12]

Technically risa just takes a spreadsheet input. If you could automate it in excel, you could just copy paste it into risa.

 

[Serhiy2]

Excel is what I ended up doing it through. It took me 2 hours to come up with 360 load cases. I remember doing more complex problem in Autodesk Robot and the generator they have embedded did all this in seconds. Too bad Risa doesn't have it.

I guess I need to learn to write macro in excel...

 

[jayrod12]

Perhaps this might help you in the future since you've already gone through the long way. You can save your LC into the generator

Custom Load Combination in Risa 3D

 

[DaveAtkins]

360 load cases??? That seems extreme. Can't you reduce the number of basic load cases, as well as load combinations? Use engineering judgment to pick the worst cases?

DaveAtkins

 

[jayrod12]

Actually that's a good point, based on your description of the problem, you had 3 different loads types and 3 different values for each type. I would expect 27 load cases then (3^3)?

 

[Serhiy2]

I had three different load types (snow, wind, crane) with 3 different values for each except for wind load I had 4 values. This resulted in 36 permutations. I also extracted 10 ULS load cases from NBCC which I wanted to use for every permutation so all together 360. But you are right, for serviceability load cases I had 36. I'm sure some of my load cases repeat but because I was able to do it in excel semi-automatically, I didn't spend more time checking which load cases can be removed.

Regarding the engineering judgement, I could have done but decided to put all of it in Risa so I don't miss anything. Also, my model is not simple enough for me to go by the judgement only.

Thanks for the feedback!