ASCE 7 Code question 3

[DWHA]

I am trying to determine if a building is classified as rigid or flexable for the wind gust factor in the ASCE 7-02. It states that if the fundamental frequency is greater than or equal to 1 hz, then it is rigid.

I have never determined the fundamental frequency before, does everyone do this everytime? Or are 99% of the buildings in 1 class or the other, and it is only in certain cases do you have check this?

FYI I am doing an analysis of a pre-engineered building.

Thanks in advance for everyones help

 

[JedClampett]

I never think about it and use the .85 allowed.

 

[DaveAtkins]

I'm with JedClampett. BUT--I have often wondered if I am correct. I have no idea if all these buildings I work on are rigid or flexible. I can't imagine calculating the fundamental frequency for each project. Not enough time or budget!

DaveAtkins

 

[DWHA]

I found in the commentary that it states if a building height/lenght < 4 then it is rigid as a general guide.

 

[Taro]

It's not hard to calculate the frequency. The earthquake provisions in Chapter 9 give equations for the approximate fundamental period of the building. The natural frequency is just the inverse of the period.

 

[UcfSE]

I wouldn't use the earthquake equations to find the period. The assumed damping used in deriving those will not be the same as for wind effects, so you will be off.

 

[WillisV]

There are equations to approximate the natural frequency for wind effects in the Commentary to Chapter 6 of ASCE 7-05 - see pages 293-294 equations C6-14 etc.

 

[msquared48]

Interesting...

That means that about 99% of all the structures I have ever worked on were rigid as "T" is generally in the range of .2 to .3, and the inverse is greater than 1.

That being said, then how could you logically ever have a "flexible" diaphragm condition with a "rigid" structure?

Mike McCann
McCann Engineering

 

[Lion06]

I believe (someone please correct me if I am wrong) that you can have a flexible diaphragm with a rigid structure. The distinction lies somewhere in the ratio of diaphragm deflection to story drift.

 

[msquared48]

Structural EIT:

I'm playing semantics here.

Nevertheless, I thought it an interesting contradiction. In that rigidity is the inverse of deflection, and being flexible is just over the fence of being rigid, they are both deflection related. So, how can you logically have a flexible diaphragm, but have the overall structure be rigid? To me, it is an oxymoron.

It seems like the definitions should intertwine a bit here for more congruency. The problem here is in the code definitions. The buildings are still standing.

Mike McCann
McCann Engineering

 

[DTJ]

For most single story structures you probably will be rigid, but on process towers, open structures etc. you need to look at these individually. I have had braced frame structures because of their height be classified as flexible and end up with Gust factors approaching 1.

 

[Taro]

UcfSE, damping affects the amplitude of oscillation but does not affect the frequency/period unless the damping ratio is over 20%. Most practical structures have design damping ratios less than 5%. The level of damping is explicitly considered in the gust effect calculations. I typically use a value of about 2%.

 

[slickdeals]

I have gotten a response from Dr. Kishor Mehta that the building should be classified as flexible or rigid only based on frequency and no rule of thumb as given in ASCE 7-02's commentary.

A flexible building can have a rigid diaphragm. The way the flexibility of a building is defined is different from the way a flexibility of a diaphragm is defined. As stated in the commentary, a flexible building is more susceptible to gust effects and hence a different approach to calculating the G value. It is a rather complex equation and I don't know if indeed it is worth the time. I design buildings in Miami and strangely no building department has ever posed a question on flexible vs rigid building and its associated G value. I don't know enough to say whether results from the analysis/testing will be drastic for a building designed using a G of 0.85 vs. actually calculating it.

It should be noted that if the building is flexible, then not only the "G" value changes but also the eccentricity (Section 6.5.12.3).

HTH