Loads

[Anonymous]

Hi,

I'm working in a footbridge project. I'll study the dynamic effects caused by a pedestrian loads in the bridge. So, i have some doubts in order to calculate the modal parameters of the structure:

The truss structure was modeled as a truss structure, the access ramps and deck as a shell.

I have to study some different situations in order to evaluate the frequency of structure. 
(discharged bridge and charged bridge or another situation)

- what the difference in calculation between using a uniform load or a (FE) uniform as mentioned  in prt sc?

When I use the "surface object" the frequency is the same,  considering the discharged bridge or charged bridge. I consider a certain % of the load case (SOB P)  to simulate this different situations.

When I use the "(FE) uniform" , ROBOT updates the frequency to different values according to different situations. 

Because this, i can't understand what is the correct frequency. 

Any ideia to solve this problem?

Best Regards,

Diogo
 

[Pawel.Pulak]

"Surface on object" is the type of load which was dedicated only to claddings - not to meshed panels.

I have written "was" because several versions ago FE loads were enabled also for claddings - it was then that "Surface on object" loads were hidden in the interface and made available only from the table of loads.

Can you attach your model to check what type of panels (meshed or not meshed) were loaded by you using "Surface on object" and "(FE) uniform" loads?

Regards,

[Anonymous]

Hi,

I can understand your explanation, but I can't understand the difference in calculations. 

I want to estimate the natural frequency of structure, but i can't understand what is the correct. For this, I'll send you a model.

Thanks,

Best Regards,

Diogo 

MODEL: https://feupload.fe.up.pt/get/91q4cI9ktBsuXsI

[Pawel.Pulak · Accepted solution]

As I have written before: ""Surface on object" is the type of load which was dedicated only to claddings - not to meshed panels."

The panel calculation model used in your structure is "Shell", i.e. the meshed one, with load applied to finite elements and not cladding-like distribution:

"surface on object" loads are not applied to such panels.

You would be able to see it if you would not set static load cases as "auxiliary" ones. After switching off "auxiliary" for cases 1to3 and comparing sums of load and reactions for "surface on object" loads and for "(FE) uniform" loads it can be clearly seen: 

For  "surface on object" loads if the loads were not applied they were also not converted to masses so there was no difference in vibration frequencies when mass from live loads (case 3) was considered or not (because it was always zero).

Moreover generally frequencies for "surface on object" loads were significantly higher than for  "(FE) uniform" loads - because as visible on the screen capture above a lot of load was "lost" also from cases 1 and 2.

The improvement request was registered to report at least some verification warning when  "surface on object" loads are defined on meshed panels but not applied.

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Regards,

[Anonymous]

Thank you for your help.

Best Regards,
Diogo