Follow-up - NCEES SE Practice Exam - Lateral Questions 124 & 125

[HerkEng]

Sorry, I know this was just asked by another (colleague asked on my behalf). Here is my more precise question.

I have a question regarding lateral questions 124 and 125 of the NCEES SE Practice Exam (copyright 2017, first printing - errata not applicable). Both question involve steel SCBFs and section F2 6c (1).

Question 124 provides an amplified seismic brace force or 175 kips. The solution to 124 goes on to calculate the expected yield strength per F2 6c (1) (a) as 104.8 kips (ASD) or 157.1 kips (LRFD). The final answer being (C) reflecting these calculated values.

Question 125 provides a table of computer calculated axial forces for various members, including the member in question, M4, as 71 kips tension. The diagram indicates the table values correspond to applied E_h loads. The solution to goes on to note that the values should be the lesser of F2 6c (1) (a) and F2 6c (1) (b). The solution then goes on to calculate the expected yield strength per F2 6c (1) (a) as 217 kips (offering only the LRFD solution without noting so!). The final answer being (D) reflecting the calculated expected yield strength.

Why is the answer to 125 not (A), 71 kips per the table values? And would the answer to 124 be different if the stated amplified seismic brace force was less than the calculated expected yield strength? What the heck is going on here?

 

[TheBigGuy]

The required tensile strength is the lesser of the following:

a)  RyFyAg (assuming LRFD)

b)  The maximum load effect, indicated by analysis, that can be transferred to the brace by the system.

See F2.3 for what constitutes an "analysis."  The key word there is "amplified."  The 100 kips applied to the system is not the amplified seismic load.

 

[RyanSE]

What about 142? If you amplify the loads given you would still get a result lower than 217.