Total Base Shear-- Tall Bldg vs Small

[NS4U]

let me preface this post with saying, I am speaking VERY generally, so please do not harp on minor nuances. Assume two building weigh EXACTLY the same, but one is tall and one is short.

My question pertains to the calculation of base shear for seismic purposes.

the short building is the stiffer structure, it has a low period (high natural freq.)... this will mean that I need to design for a higher base shear, per the code.

The tall bldg, has a higher period (lower natural frequency) and the codes say this building would be designed for a smaller base shear.


Now looking at this from a dynamics point of view, isn't EQ excitation primarily low frequency? meaning, for the tall bldg, the EQ will excite more modes and put more energy into the tall building because the tall bldg has a low natural freq. If so, why is the taller bldg designed for the smaller base shear.

For the short, stiff building, with a very high frequency I envision it just rigidly going back and forth with the ground with very little displacement because it has such a high natural frequency.

I'm sure I'm missing something here, could someone please explain?

Thanks

 

[msquared48]

In the smaller structure, more of the total energy is absorbed by the structure through the lateral resisting system, whatever it is, like the Oak tree.

Whereas the taller building is more like a reed absorbing energy not only through the lateral resisting system of the structure, but also through more lateral movement than the smapper building. This difference in movement allowed is the difference in energy delivered to the two systems that the buildings each have to take.

Also, remember that with the taller builidng, the higher modes are more likely to be seen than the shorter one.

Mike McCann
MMC Engineering

 

[frv]

At the heart of the issue is ductility.

Current design practice requires that the force caused by seismic accelerations must be resisted either by strength OR ductility. NOT both. That is to say, the more ductile a building (in this case, the taller building), the less force you need to account for in the SFRS.

It is important to mention that buildings aren't designed to be serviceable after an earthquake. Seismic design is akin to designing a car for a head-on collision. It's not meant to be pretty; just save lives.

 

[jheidt2543]

Mike:

Is another way of saying it, assuming both buildings experiance the same earthquake, they both receive the same engergy input, but they respond differently based on their weight, stiffness and bracing system?

 

[JStephen]

"isn't EQ excitation primarily low frequency?"

I think that's the flaw in the reasoning. The excitation (and response) varies with frequency, but that is accounted for in the spectrum used or assumed. You're calculating it one way, then arbitrarily assuming it's different, then wondering why the calculation doesn't match the assumption.

 

[NS4U]

jheidt,
essentially that is what I'm getting at. Two structures, same mass, same excitiation, DIFFERENT stiffness. My question is why is the stiffer stucture designed for a higher force?


JStephen,

I agree, the excitation and response magnitudes vary with frequency... but, as I understand it, the highest magnitude of excitation in EQs occurs at low frequency.

Therefore, if your structure has a low natural frequency, won't this cause more stress in the structure becuase it will deform (respond) more? If so, why do codes have you design for a lower force for more flexible structures?

 

[jtx]

Energy dissipation...

The shorter structure does not dissipate the energy of an earthquake like a taller structure. The short bldg will tend to absorb a lot of the energy, therefore increasing the stress the bldg will see. The taller, FLEXIBLE, bldg not only absorbs a fraction of the energy, it can dissipate it through lateral translation of the bldg, thus reducing the stress on the members.

That in part is why we use the R value when computing the base shear coefficient

 

[haynewp]

I was going to say the same as jtsouflias. I will just add that earthquakes are of course a dynamic event so energy dissipation is able to be utilized.