Sa vs Cs

[palvarez83]

So both Sa and Cs are plotted against period (T). The graps look very similar (except for very small period and Cs incorporates R and Ie). I undestand Cs is used to find base shear.

However, what is Sa used for at all? I understand it is the "elastic" response coefficient, but I haven't run into many problems where it is actually used. Could one of ya'll explain where i would used Sa?

Thanks in advance.

 

[Lungshen]

Nope. It has to do with how a structure respond to the seismic acceleration based on its fundamental period. There may be questions on the exam asking you to find Sa based on the Periods T given. So know figure 11.4-1 and section 11.4.5 of ASCE 7 well. From Fig 11.4-1 you can tell that longer the period, acceleration slows down. I think it has more to do with explaining the concept than actually used to solve for any force. As far as what is it use to calculate anything like base shear, I can't think of any either. If you post the question in the structural section....those guys can probably answer it better than me.

I am structural and passed CA seismic with ease on first try (maybe 20 hours, one week of reading and work some problems) and to be honest I don't understand the concept 100% and how they derive those equations in the code. In real life practice, all you need to know is how to come up with the base shear based on the site specific information, and how to throw/break those force into the building and design the lateral system for it.

The CA seisimic is the basic of the basics compare to the national SE lateral. With average 3 mins a question, a lot of stuff on the exam are just looking up the table, pull some R value. Chug and plug into the equation straight forward enough, It doesn't ask you to design a lateral system like in the national exam, it only ask you to derive those forces. The most rigorus type of calculation is the vertical distribution and the horizontal distribution of the forces. Make sure you know ASCE 7 section 12.8.3 and 12.8.4 very well and do enough practice to solve the forces quick.

 

[palvarez83]

Thanks.

 

[ptatohed]

Sa = Spectral Acceleration = Max acceleration experienced by a SDOF at the structure's natural period, T = ft/s2

Cs = Seismic Response Coefficient = Used to calculate Seismic Base Shear, V = Unitless

Also: Sa = w x Sv and w2 x Sd

[w = Angular Natural Frequency (rad/sec), Sv = Spectral Velocity (in/sec), Sd = Spectral Displacement (ft)]

 

[palvarez83]

Sa = Spectral Acceleration = Max acceleration experienced by a SDOF at the structure's natural period, T = ft/s2

Cs = Seismic Response Coefficient = Used to calculate Seismic Base Shear, V = Unitless

Also: Sa = w x Sv and w2 x Sd

[w = Angular Natural Frequency (rad/sec), Sv = Spectral Velocity (in/sec), Sd = Spectral Displacement (ft)]

Thanks. Any idea why velocity is in in/sec instead of ft/sec? Seems to me like the units of length for Sa , Sv , and Sd should be consistent.

 

[ptatohed]

Sa = Spectral Acceleration = Max acceleration experienced by a SDOF at the structure's natural period, T = ft/s2

Cs = Seismic Response Coefficient = Used to calculate Seismic Base Shear, V = Unitless

Also: Sa = w x Sv and w2 x Sd

[w = Angular Natural Frequency (rad/sec), Sv = Spectral Velocity (in/sec), Sd = Spectral Displacement (ft)]

Thanks. Any idea why velocity is in in/sec instead of ft/sec? Seems to me like the units of length for Sa , Sv , and Sd should be consistent.

Yeah, you're right. I was just giving the most common units seen for each. But you can use inches or feet just as long as any math operations performed use consistent units.

 

[chrshyd]

So both Sa and Cs are plotted against period (T). The graps look very similar (except for very small period and Cs incorporates R and Ie). I undestand Cs is used to find base shear.

However, what is Sa used for at all? I understand it is the "elastic" response coefficient, but I haven't run into many problems where it is actually used. Could one of ya'll explain where i would used Sa?

Thanks in advance

My 2 cents are: Sa is used inderictly when calculating Cs-max = (Sd1/T)*(1/R/I) = Sa/(R/I) while Cs = Sds/(R/I).