column stiffness factor

[pattontom]

It is said that bigger columns attract greater earthquake forces. Imagine a column that is size 0.3x0.3 mtr versus one size 0.6x0.6 meter and 3 meters height each. The earthquake would produce the same displacement and the 0.6x0.6 meter column would be stiff hence attract greater seismic forces. This is the basis of the captive or short column above window effect. Now. When you guys design columns. How much do you take this into account? Are you so conscious of this that you put much more ties in columns for bigger columns to counter the stiffness factor during earthquake? I'm asking this because I have columns size 0.4x0.5. I wonder whether to make it 0.5x0.5 or even 0.6x0.6. Is the stiff factor linear or exponential to the column sizes? Although extension shear analysis can show it. Any rule of thumb estimate of the relationship of stiffness factor and shear for seismic lateral movement?

 

[pattontom]

This is what I was saying:

http://www.iitk.ac.in/nicee/EQTips/EQTip05.pdf

"In the straight vertical position, the columns carry no horizontal earthquake force through them. But, when forced to bend, they develop internal forces. The larger is the relative horizontal displacement u between the top and bottom of the column, the larger this internal
force in columns. Also, the stiffer the columns are (i.e., bigger is the column size), larger is this force. For this reason, these internal forces in the columns are called stiffness forces. In fact, the stiffness force in a column is the column stiffness times the relative displacement between its ends."

So to make the columns sized 0.4x0.5 meter to become 0.5x0.5 or even 0.6x0.6 would attract greater earthquake forces? But how much exactly? Any rule of thumb you use?

 

[structSU10]

The load distributes in a number of ways to your LFRS, and with EQ i doubt there are any good rules of thumb as total load is based upon your structures natural frequency and the magnitude of the EQ. The diaphragm or collectors that tie together elements of the LFRS affect the load in individual members, so a rigid diaphragm will transmit much different loads than a semi-rigid or flexible diaphragm.

Following the code and using ELF procedure or a modal analysis in an FEM program is a good way to get member forces necessary for design.

Also note a structures response to an EQ is a complicated situation, and one should have an understanding of what 'makes sense' load wise before trusting a program to handle it for you. With an EQ a certain amount of energy is input into the system, which must be dissapated through yielding, cracking, etc within the structure, so strength is not the only consideration - ductility of the overall system is also important - which is what codes have certain detailing requirements to help ensure proper detailing for energy dissapation.

 

[pattontom]

I know about capacity design, special moment frames and base shear and EQ dynamic response. All I'm asking is given the same building. Would it hurt to make the columns bigger (overdesigned a bit), this would have minimal construction cost. But bigger columns is said to attract bigger seismic force. It has to do with ground displacement. Just to give a clear idea. Supposed you have a 1 meter column and reaching the clouds (ignoring wind, euler force etc.). And the ground moves 10 meters. The column won't break, it would just deflect. Now imagine the same 1 meter column only in 2 storey building and the ground moves 10 meters. It can shred to smithereens. So say your column size is only 0.4x0.4, would it hurt to make it bigger like 0.6x0.6 meter. But it is said to attract bigger forces like I explained and as the following article illustrates graphically.

http://www.iitk.ac.in/nicee/EQTips/EQTip05.pdf

Any quick rule of thumb to know how big you can overdesign columns without the seismic disadvantage of being more stiff?

 

[oldrunner]

When you use larger columns - properly reinforced - then because they are stiffer and being stiffer, they will attract more shear forces - that is, if you are just depending upon a collection of columns. Actually you should probably take a course in seismic design of buildings. I know that there is a good book concerning earthquakes: "Design of Multistory Reinforced Concrete Buildings for Earthquake Motions" by John Blume,Nathan Newmark and Leo Corning which was written in the 60'S but its still being used today. Most structural engineers can still manually analyze a building and distribute the lateral forces per the stiffnesses of the columns and shear walls.

If you don't understand what you are doing - but asking good questions, then it's a good idea to consult with a structural engineer. Making things just stronger or stiffer or something else can sometimes lead to problems and unintended consequences.

 

[pattontom]

I'm asking because to even improve further or lessen joint shear.. the joint area has to be bigger.. and this can happen either with bigger column or deeper beams.... but it's better to have bigger columns to lessen the loads.

You know that in designing building, first the beams have to designed, then the joint area to minimize joint shear and then the column sizes and the foundations. So you have flexibility of getting bigger column sizes for better joint shear resistance.

 

[hokie66]

Suggest you get some tutorial in the strong column, weak beam concept. In a major seismic event, you want the beams to absorb the energy as they fail ductilely. Column failure is likely to be much more catastrophic.

 

[pattontom]

The strong column-weak beam and ductility thing is the first thing I learnt. To make the column stronger than beam.. the joints have to be strong so plastic hinges can be formed in the beam instead of column.. i know all of that and understood everything in the following excellent article on seismic design of special moment frames...

http://fris2.nist.gov/NEHRPClearinghouse/NIST/PB2009102037.pdf