natural frequency limit of free-standing structure
natural frequency limit of free-standing structure
(OP)
Does anyone have any comments/ advice on minimum natural frequency limits when desiging a free-standing carport or gazabo etc cantilevered from the base. I have been adopting 1.0 as a minimum but would like to see what other people are using.
Thanks.
Thanks.






RE: natural frequency limit of free-standing structure
Mike Halloran
Pembroke Pines, FL, USA
RE: natural frequency limit of free-standing structure
RE: natural frequency limit of free-standing structure
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: natural frequency limit of free-standing structure
Are you concerned that 10s seems too much? Keep in mind that the natural frequency is in Hertz, not seconds. Perhaps you're thinking of the fundamental period (T) which is measured in seconds. Since the period is the inverse of the frequency, the period for this particular example is 0.09s.
RE: natural frequency limit of free-standing structure
RE: natural frequency limit of free-standing structure
I vaguely remember seeing 2 Hz for a much taller building.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: natural frequency limit of free-standing structure
Thanks everyone for your responses. Let me try to clear up my question… The structure I am looking at is made up of two frames about 8 metres apart. Each frame has one single column that is cantilevered from the base. The base fixity of the two columns provides the only lateral stability for the structure. The structure is to be built in a cyclonic region in Australia however it is very well shielded.
I am more concerned about excitation caused by people shaking the structure than excitation caused by wind. I have modelled a single 2D frame in Microstran (the structural modelling software our office uses) and performed a dynamic analysis. The dynamic analysis calculates the natural frequency of the structure. The natural frequency depends on the steel sections selected and the gravity loads on the structure - additional loading i.e. wind does not come into it. To meet the structural design actions and deflection limits under wind loading and dead and live loading a nominal section like 75x75x3 square hollow section is more than sufficient. A 75x75x3 SHS gives the structure a natural frequency of just less than 1Hz (my gut feel is that this it a bit too low). However, to achieve around 7 Hz the member would have to be a 250x250x9 SHS (sorry, I am not familiar US sections, the first 2 numbers are the outside dimensions and the 3rd number is the wall thickness in millimetres).
To me it seems like extreme overkill to use a section that is more than 30 times stronger and 10 times heavier to increase the natural frequency to a value around 7 Hz.
I hope this makes sense. Thanks Adam.
RE: natural frequency limit of free-standing structure
There is also the issue of incidental loading to consider. E.g., if you go with the 75mm tube, what happens the first time it gets hit by a car?
Mike Halloran
Pembroke Pines, FL, USA
RE: natural frequency limit of free-standing structure
RE: natural frequency limit of free-standing structure
Good grief! My tongue got tied on that one! You're absolutely right, I assumed that wngadam needed the period for seismic calculations such as used in ASCE 7, so I did that calculation, coming up with 0.09s. But then I converted it to natural frequency of 10Hz but got my wording wrong. Sorry for the confusion.
Adam, this sounds like a servicability issue. UcfSE alluded to the kid-effect and Mike mentioned damage by a car. Unless you know for sure that you have an environmental condition to consider (i.e. this is a gazebo located in a public playground where kids WILL climb on the thing, an attractive nuisance), then you design to what you know. I think you'll drive yourself crazy trying to design for all of the "what if's" that could occur. I doesn't seem fair and reasonable for you to be responsible for the irresponsible actions of others (i.e. someone shaking the structure back and forth).
You mentioned that you considered wind loading on the structure. Being in Australia, don't you guys have seismic loading also? The structure sounds like a flat roof canopy car port supported on two columns (inverted pendulum). I could see the seismic loading governing over the wind load, depending on the weight and vertical profile of the roof structure. If so, then that would give you the justification for the stiffness of 7Hz.
RE: natural frequency limit of free-standing structure