Non Linear moving load case

[s3ndy]

Dear All,

I have some quiries on the SAP moving load and non linear analysis case. Can SAP do non linear moving load analysis case? I have a steel frame with bracing member subject to traffic loading. I assign the bracing as a tension member only and run non linear analysis. But in SAP,what I find is that, we can't set the load comb. as non linear case and moving load case at the same time. That's why under moving load case, I still find some bracing still take compression force and can't be designed for since they are slender.

Many thanks

sendy

 

[IJR]

s3ndy

What kind of bracing is that? Floor bracing or main vertical bracing? Is it possible for you to make manual calcs before using software?

Define any analysis case that will induce loads in your bracing as nonlinear static, if you aim at tension only MODELLING(uppercase is intentional). Thus if wind will go into bracings, then WIND must be in nonlinear static case. If floor weight will go into bracings then floor weight must be in a nonlinear static case. This can be very tricky when you model in 3D because some other loads which you did not declare as nonlinear(including easy to ignore type like self weight of bracings) will sneak into bracings and cause the compression you did not expect.

sometimes you may even need to do different runs, one specially for bracings.

Combinations is an arithmetical work(you add, envelope results etc) and can even be specified after analyses is complete. It is for this reason that combinations can not be linear or nonlinear or anything besides arithmetic.

hope this helps

respects
ijr

 

[s3ndy]

Hi Ijr,

Thanks for the explanation. It s vertical bracing system. Say for wind or floor weight, I can run as nonlinear static case. But how about moving load? Traffic loading (moving load) will go into my bracings. Can I set the moving load as non linear static case as they are two different cases in SAP. I couldn’t select moving load case the same time as non linear case. It seems that I have to choose one of them. That’s why under moving load case, my bracings were under compression and it failed due to slenderness.

 

[IJR]

s3ndy

I think you are going to have to use a simpler model. I havent checked if moving load can be nonlinear, but I guess it is likely NOT to be nonlinear, because the computations needed inside SAP2000 will be too much for the software(nonlinear means several iterations and rewriting of all those matrices after every single iteration).

How about finding stiffness kx(spring) of the bracing system consisting of only one brace(those that will take tension only) and then using this spring in your bigger model? Then you can do your static linear analysis and use the spring reaction as a load to the bracing system.

Tell us how it goes.

respects
ijr

 

[s3ndy]

Hi IJR,

thanks a lot fot the helpful advice...I will try and let you know then..

thank you

s3ndy