Seismic Question

[Adrock]

You have a uniform load supported by 3 different columns of different lengths say 15ft (1st), 10ft (2nd) and 5ft (3rd). All 3 are founded on the same soil and are identical except for the lengths. During an earthquake which column is more likely to fail?

 

[kevo_55]

Adrock,

I'm not sure if you're giving all of the info in order to solve the question.

I do know that with all things the same the less slender a member is (with the same end conditions), the more load it will recieve in a lateral force situation. Kaplan has a really good example of this in one of their books.

I hope this helps!

 

[EnvEngineer]

The want you to understand the effect of height on stiffness, look at the basic column equation and they have a height element, given all the same I believe the taller will fail first. Figure the load on the taller structure have more moment on the column than the short stiff column.

 

[outatime2002]

You have a uniform load supported by 3 different columns of different lengths say 15ft (1st), 10ft (2nd) and 5ft (3rd). All 3 are founded on the same soil and are identical except for the lengths. During an earthquake which column is more likely to fail?

The shorter column is most likely to fail (assuming it is shear critical). It is the stiffest element in the system, therefore it will resist the largest lateral force of the three columns. A real world application that best proves this is the damage to the reinforced concrete bridges from the 1994 Northridge Earthquake. The shortest bridge columns failed in shear because (1) they received the highest lateral forces and (2) they had inadequate transverse/shear reinforcement. Today, we see far better seismic detailing in new R/C construction because we learned this valuable lesson from the 1994 quake.

I hope this helps.

 

[Adrock]

Another detail would be all 3 columns are the same distance apart in this order 15', 10' and 5'. Yes it makes sense now. Thanks!

 

[civilist]

You have a uniform load supported by 3 different columns of different lengths say 15ft (1st), 10ft (2nd) and 5ft (3rd). All 3 are founded on the same soil and are identical except for the lengths. During an earthquake which column is more likely to fail?

Even assuming all the material (and the end connections) are the same, I don't think you have enough information. For example, you need to know the actual local ground motion and period of each structure, and if there is resonance between the seismic event and the structure, that structure will likely fail first.

 

[dastuff]

I'd say the tallest would fail first from bending. The vertical capacity of your column decreases quickly w/r/t height. There is a chance that shear could fail too, but they all should have the same shear running through them since they all (presumably) have the same force running through them.

This is not 100% true though since as civilist said the height of the column will affect the frequency and all that good stuff so the seismic force running through each will be different. But given no additional info I'd say that a 15'-0" cantilevered load is worse off than a 5'-0" cantilevered load.