Gust Effect Factor - Rigid Building Assumption

[abusementpark]

Are there any universally accepted rules of thumb for when it is safe to assume your structure is rigid without trying to calculate a period to prove it? For instance, I have seen in one place where it is recommended that any building less than 50 feet in height can be assumed rigid regardless of the system.

When do you feel it is necessary to calculate the period to check if the structure is rigid?

 

[ToadJones]

I cant wait to hear the answers here.

I have been irritated by this factor for years.

I had a old timer I worked with once "steel flexible concrete rigid"

I seriously think he used that as his go-by

 

[ishvaaag]

For starters reading a previous forum entry...

http://www.eng-tips.com/viewthread.cfm?qid=30135

 

[ishvaaag]

Another good one...

http://www.nd.edu/~nathaz/journals/(1998)On_the_formulation_of_ASCE7-95_gust_effect_factor.pdf

 

[ishvaaag]

Further Interesting Info...

http://www.sdlal.com/new files/C-CS-BuildingPeriods-Jacobs-June08.pdf

 

[asixth]

I thought it was for buildings with a fundamental frequency less than 1Hz. Going by the rule of thumb of frequency ~=10/(number of stories) this would be buildings greater than 10 stories.

 

[FiniteElemet]

IBC 2009 1613.6.1 adds to the end of ASCE 7 Section 12.3.1.1
Diaphragms constructed of wood structural panels or untapped steel decking may be assumed flexible provided...
1) Toppings of concrete or similar materials on WSP diaphragms are nonstructural and no greater than 1.5 in thick.
2) each line of vertical elements of SFRS complies with allowable story drift of Table 12.12-1.
3) vertical elements of SFRS are light-frame walls sheathed with WSP or steel sheets.
4) portions of WSP diaphragms that cantilever beyond vertical elements of LFRS are designed in accordance with SDPWS 4.2.5.2

 

[FiniteElemet]

Also read ASCE 7 Section 12.3.1.1, 12.3.1.2, 12.3.1.3

 

[abusementpark]

ishvaag - The thread and technical paper you linked are interesting, but do not really address the question at hand, which is in regard to safe rules of thumb to avoid calculating the building period to prove that a structure is rigid. I've read the third link you posted, which is the article about building periods. It is actually the source of the 50 ft rule of thumb I referenced in the original post. Do most engineers agree with this rule?

asixth - Interesting rule of thumb.

FiniteElemet - I'm not sure what assumptions about diaphragm behavior apply to the structure as a whole.

 

[abusementpark]

Anyone??

I just want to know what others have been doing.

I promise I won't tell the authorities if you have been just summarily assuming everything is rigid.

 

[DST148]

abusementpark> We typically do ten to twelve story high building structures in steel, concrete, and masonry. This is what we have followed in the past for rigid structure assumption.
Steel and concrete moment frames: about ten story high
Steel, concrete, and masonry - braced frames or shear walls: about fifteen story high

 

[rowingengineer]

I don't know your code, but for the Australian code I have always consider static analysis for wind to be appropriate up to 50m high building.

http://www.nceng.com.au/

"A safe structure will be the one whose weakest link is never overloaded by the greatest force to which the structure is subjected” Petroski 1992

 

[WillisV]

abusementpark - the 50' you mention is not really a "rule of thumb", it is based on plotting the ASCE commentary equations for wind periods vs. flexibility factors as shown in Figure 3 of the article you referenced. In any event, it takes about 10 seconds to calculate the period of a building using the approximate frequency calculations in the ASCE commentary for wind design, so I see no reason not to do it for all buildings.

 

[ToadJones]

Willis, are you referring to Equations C6-14 thru C6-25?

 

[WillisV]

@Toad - Yes.

 

[ToadJones]

do you simply take the period to be 1/n?

 

[WillisV]

@Toad. Yes, by definition frequency is inverse of period. Read the third link by ishvaag.

 

[ToadJones]

I must have deleted the end of my previous post...let me try again.

I was meaning to ask about Steel Braced frames. I didn't see anything in the commentary about them. I was wondering what you do for them.
do you simply take the period to be 1/n1 (from C6-17 or C6-18?)