Determine Cs

hank89 · Apr 3, 2012

Here is problem #450 from the old NCEES Structural II sample questions, please determine Cs:

Ss= 1.0 Fa=1.1 Sds=0.733

S1=0.2 Fv=2.0 Sd1=0.267

Site Class= D

SDC= D

I=1.0

Roof Height =14.5ft

Ord Conc. Braced Frame R=3.25

Ta=0.02*14.5^0.75 = 0.149sec

To=0.2SD1/Sds=0.073 sec

Ts=SD1/Sds= 0.364 sec

To<Ta<Ts therefore Sa=Sds and 12.8-2 controls Cs=Sds/(R/I)=.733/3.25=0.266

However the NCEES solution uses 12.8-3 Cs=SD1/(T*(R/I)=0.267/(.149*3.25)=0.551

Why are they using 12.8-3 if Ta is less than Ts????????????

Thanks!

pe_structural_II_errata1.pdf

dakota_79 · Apr 3, 2012

They don't use Cs = 0.551, per your attachment. They just check it as Csmax in following ASCE7 in more of a cookbook fashion (instead of intuiting it from your logic, which I also use when I calc it, btw, so I see your confusion).

Notice on page 11, 2/3 of the way down, they show Cs = 0.225 < Csmax = 0.551, therefore use Cs = 0.225.

hank89 · Apr 3, 2012

Ahhhh!!!!...In the NCEES booklet I have they used Cs=0.558...and they corrected it in the ERRATA I had attached....thanks!!!!

dakota_79 · Apr 3, 2012

Where'd you find these examples, anyway? wish i would've had them.

hank89 · Apr 4, 2012

I purchased the Struct I and II example problems from NCEES a few years ago when I planned to take the SE exams but never did. If my scanner was working I'd scan them and email them to you.

JDstruc · Oct 23, 2013

hank89 said:
Here is problem #450 from the old NCEES Structural II sample questions, please determine Cs:

Ss= 1.0 Fa=1.1 Sds=0.733

S1=0.2 Fv=2.0 Sd1=0.267

Site Class= D

SDC= D

I=1.0

Roof Height =14.5ft

Ord Conc. Braced Frame R=3.25

Ta=0.02*14.5^0.75 = 0.149sec

To=0.2SD1/Sds=0.073 sec

Ts=SD1/Sds= 0.364 sec

To<Ta<Ts therefore Sa=Sds and 12.8-2 controls Cs=Sds/(R/I)=.733/3.25=0.266

However the NCEES solution uses 12.8-3 Cs=SD1/(T*(R/I)=0.267/(.149*3.25)=0.551

Why are they using 12.8-3 if Ta is less than Ts????????????

Thanks!

Thank you SO MUCH for attaching that old NCEES Structural II errata. I've noticed that this thing is filled with errors, especially on those particular problems....I've been searching for this damn errata for a while now. Thank you! BUMP to any other Structural guys who are also supplementing their National NCEES 16-Hour Structural exam by studing this old Structural II book. This errata is a MUST! Its attached in the first post. Thank you again.