Seismic Loading 3

[franck]

Hi guys,

I am a spacecraft designer and I need to take into account a seismic loading to size a structure that is holding my spacecraft.
Basically I have an article "Proposed Canadian Code Provisions for seismic design of elements of structures, non-structural components, and equipment". In that article, I have the following equation to derive my design force:

Design_force=0.3*Fa*Sa(0.3)*Ie*Sp*Structure_Weight.

With

*Fa (acceleration-based site coefficient)
*Sa(0.3) spectral response acceleration for a duration of 0.2 second
*Ie (earthquake importance factor)
*Sp = force factor

Unfortunately the following do not make sense to me

1-In the article, I have for Toronto: Sa=0.28 and Fa=2 but what are the units (0.28g2/Hz and 2g's???)?

2-What value should I use for Ie? Used in the equation but no value in that article

3-Actually I am in Ottawa, are those value still applicable.

Best regards,

Franck

 

[092961]

I'm not a structural engineer, but I am involved in designing frame-mounted equipment that is sometimes installed in seismic zones. We recently purchased software called STAAD.pro to perform structural analysis including seismic loading. The problem is that, at least in California, the frame must be analyzed member by member with each one having a different capable limit based on whether it's in compression or tension. This software goes through the structure, member by member and identifies whether it capably handles the loading conditions or not. Check out

http://www.reiworld.com

. Now, for the UBC code in the US, there is a process for "Nonbuilding Structures" which probably applies to your spacecraft structure. This simplifies the analysis greatly, but there are still some key steps. I'd recommend picking up a copy of "Seismic Design of Building Structures" by Michael R. Lindeburg, available from Professional Publications.
I believe Sa is spectral acceleration, which is how responsive the soil is to a siesmic event and its units are ft/sec^2 or m/sec^2. Otherwise the "factors" are generally unitless. The importance factor, I, is a function of how critical it is that the structure survive a seismic event. I would use 1.0 unless there is a potential for hazardous emissions in the event of failure. You'll need to include wind loading as well, which in some cases can exceed seismic loading.
I've probably only raised more questions, so hopefully at least I've provided a reference you can use...

 

[franck]

Thanks 092961! Your response is really helpful.

Franck

 

[VoyageofDiscovery]

Hi franck,

Comparing apples with apples wrt Sa(0.2), where 0.2 is period in seconds, 0.28g in Toronto would be 0.67g in Ottawa. This value however is for an exceedance of 2%/50years or approx 1 in 2500 year. You should check if this is appropriate.

Fa is a function of underlying soil conditions and period.

Importance factor is a function of the outcome of collapse. I wouldn't know anything wrt launch pads.

HTH

VOD

 

[tomirvine]

Where is the launch site?

NASA-HDBK-7005, Dynamic and Environmental Criteria, gives a shock response spectrum for seismic loads at Vandenberg. This spectrum could be applied to a finite element model of the spacecraft.

Tom Irvine

http://www.vibrationdata.com

 

[franck]

Thanks Tom and VOD for your tips!

Tom, I am not too concerned about the Launch Site since the Rocket quasi-static (or sine) environment should cover the seismic event even if we are actually launching from Vandenberg (California). I am just concerned with the support, which will hold the satellite, when we are going to test it environmentally here in Ottawa.

VOD, I am not sure that I understand the concept of exceedance of 2% over 50 years. Does it mean that I have 2% of chance to exceed 0.67g in 50 years?

Best regards,

Franck

 

[VoyageofDiscovery]

franck,

Yes.

As structural engineers we design for things that will probably occur within an accepted level of risk. This level of risk is determined by the Owner for specific projects or, for Code established projects, the Code specifies the level of risk through probability.

For example, typical bridge structures in Canada are designed to stay open to traffic for a design earthquake of 10%/50years. The intensity of this design earthquake will be less than a 2%/50years. That is why I questioned the 0.28g for Toronto.

It is best you have a structural engineer size your support structure.

Regards

VOD

 

[franck]

Thanks VOD for the clarification!