Estimation of Fundamental Period of Vibration of a Structure 4

[techiestruc]

Hello!

I'm just confused regarding the appropriate approach on how to obtain the fundamental period of a structure using computer software. I am aware that there are approximate methods / formulas provided on many design codes (i.e.,IBC and ASCE) but I want to be clear to myself that I am certain of what I am doing when I use these programs.

In determining the period of the structure, do I have to include Live Loads or I only use DLs and structure selfweight?

For seismic design, I use the Effective Seismic Weight (see ASCE 7-05 Sec. 12.7.2) in obtaining the period of the structure? Am i correct? [Note: Eff. Seismic Weight includes portion of LLs for areas used for storage + snow load. Both of which are not DLs.]

Is the fundamental period to be used in seismic design different from the period for wind design? or can I use the period I obtained from seismic and apply to wind?

Please state your references if you have any. Just a noobie here.
Thanks!

 

[Agent666]

Using the effective seismic weight is correct. Basically you are using the overall average weight of the structure when the earthquake happens. This is usually defined in codes as you have noted and may include a proportion of live load or other loads.

If the weight/mass is the same and distributed the same under a wind event then the fundamental period will obviously be the same. Only if the mass or distribution of mass differs or different effective section properties are required to be considered between wind and seismic events will the period differ.

 

[techiestruc]

Thanks for the fast response Agent666.

So what you're saying is that if my structure happens to be a wind-governing case (seismic usually ignored) for industrial facilities, then I still have to utilize the provision in seismic design in ASCE determining the effective seismic weight to obtain the fundamental period of the structure?

 

[DAR5052]

The fundamental period of the structure can be calculated for wind loads using the commentary in ASCE 7-05 (Section C6.5.8). I had the same question before and found this article published in Structuremag that cleared up a lot of questions. Hope that helps...

http://www.structuremag.org/article.aspx?articleID=686

 

[techiestruc]

Thanks DAR5052. I've already read that journal. I just want to clarify myself on the applicability of fundamental period as calculated by structural analysis programs with load combinations following the ASCE seismic provisions (i.e., D + 0.25L + S to be used as mass to obtain the period). Do I have to provide another LC to compute for the period for wind or the LC provided above will also be applicable for wind load cases?

In this case I apply the provision for period determination by obtaining it through a "properly substantiated analysis" but I don't know what load to apply.

 

[WillisV]

Use of the effective seismic weight definition is a reasonable assumption for mass calculation for fundamental period calculation for both seismic and wind design.