Maximum Limit on Building Natural Period

[ilyas415]

Hi all,

I was recently part of a ~65 m tall steel building design where (owing to the unusual geometry) the fundamental period of the building is about 3.5 seconds.

Lateral resistance is provided by a combination of moment frames and bracing.

American code: f= 10/#storeys = 10/16 = 0.625 Hz = 1.60 seconds
Eurocode (adjusted): f= 0.7*46/h = 0.50 Hz = ~2.00 seconds
Japanese code: T= 0.03*h = 1.95 seconds

As you can tell from the above empirical approximations, the building period is much longer than expected. We've checked wind loads and peak wind accelerations, but they are all within acceptable limits and criteria.

Nevertheless we are concered that the building will be too flexible. The design will be handed on to another party for further development, but we don't want them to reduce the current amount of bracing.

Q1). Does anyone have any experience with setting limits on the building natural period?

Q2). I also wonder if anyone has any bad experiences with buildings with unusually long periods?

Thank you.

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Ilyas

 

[Trenno]

Ilyas,

Obviously a few fundamental things dictate period - namely stiffness and mass. You either have an exceptionally flexible or heavy building.

I assume this is a commercial tower? I've seen some engineers revise the floor loading to more "realistic" loads in lieu of the code values, in addition adopting SLS fixity stiffness conditions (e.g. rigid steel connections compared to pinned).

To my knowledge there are no strict / codified limits to building periods.

I found this paper a few years ago which I thought was quite interesting. It collated fundamental period data from >400 tall buildings in China.

CTBUH Paper

 

[HTURKAK]

Q1). Does anyone have any experience with setting limits on the building natural period?


A: The codes do not set limits on the building natural period explicitly but limits the min. dynamic base shear (computed using the procedure RSA etc. ) to be not less than (ASCE) 100% of the equivalent lateral force (ELF) seismic base shear.
Some other codes require 85% of the ELF base shear.
This limitation IMO, implicitly dictates the upper limit for the natural period with scaling requirement of the spectrum.


Q2). I also wonder if anyone has any bad experiences with buildings with unusually long periods?

A: Allowable Story Drifts is another threshold ( ASCE Table 12.12-1 ).. In general, freshers try first with very flexible SFRS to utilize bigger period and relevant small spectrum coeff. and then eventually come back to starting point which the drifts do not comply with requirements.
My opinion...









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[ilyas415]

@Trenno / @HTURKAK,

Thank you for your great replies!

@Trenno,

This is an office building. The majority of connections are considered as moment connections except for the secondary beams. For secondary beams, usually we just consider them as rigid connections only when considering floor vibration analysis. But thank you very much for the useful reference! It gives a good indication at least where the vast majority of expected building periods should be based on actual buildings. This is a very useful starting point.

@HTURKAK,

Right, we are also scaling the RSA base shear to 100% of the ELF seismic base shear for this case. Regardless, the regions seismicity is so low that the final seismic loads are also very low (< 2%g). If I remember, the building interstorey drifts were within code criteria, but I think it is worth having another look as you mentioned. Thank you.



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Ilyas

 

[JoshPlumSE]

Tall buildings (with high natural periods) usually perform BETTER in seismic events than shorter building. Essentially, they "ride" the wave of the earthquake, flexing back and forth and don't see the full brunt of the earthquake the way a short building would.

Also, a 65 meter building would be approximately 210 feet, right? So, a 16 story building would have a story height of 13.3 ft. okay. Though I should point out the 0.1 N equation from ASCE 7 (eqn 12-8-8) only applies to structures less than 12 stories.

Ta = Ct*(h_n)^x = Something like 0.02*(210)^0.75 = 1.1 seconds. (For "all other" systems).
Ta = .028*(210)^0.8 = 2.0 seconds (for steel moment frames)

Then the max allowed to be used would be Cu * 1.1 seconds. So, that would be something between 1.5 seconds and 1.9 seconds for "all other systems". Or, between 2.8 and 3.5 seconds for "steel moment resisting frames". Which is pretty close to what you actually got.....