NCEES Practice Exam 112 (lateral)

[*Ananda*]

In the Nov. 2010 Structural Sample Questions and Solutions book, Lateral section problem 112 treats the column axial force in an X brace frame as the support vertical reaction force. Per statics, the axial force in the column is less than the support vertical reaction because the reaction force is distributed to the vertical column and the brace, reducing the load in the column. Can someone explain why the NCEES determination of axial force in the column is correct (p. 189)?

I think the brace frame member forces in Frame A from problem 112 is (just from load E, ignoring any other loading):

Each column: 10 kips / 2 (for two braces assumed to have equal stiffness) x 12/10 (frame geometry) = 6 kips (tension or compression)

Each brace: 10 kips / 2 (for two braces assumed to have equal stiffness) x 15.6/10 (frame geometry) = 7.8 kips (one in tension, the other in compression)

And the summation of the column compression force and the vertical component of the compression brace force is: 6k + 7.8k x 12/15.6 = 12kip which equals the support vertical reaction.

I notice a similar treatment in the NCEES Structural II (November 2007) Problem 450, where their solution ignore the fact that the braces will reduce the column axial force (p. 91).

In contrast, The Kaplan book, Seismic Design Review for the PE Exam by Williams 6th ed. Example 4.5 (on p. 214) calculates a statically accurate "design load value for the column" from an X brace that accounts for the braces.

Also in Williams book, Example 4.4 shows column member forces in a braced frame (p. 204) that are statically correct.

Is there some provision somewhere that states columns in braced frames should be designed with braces assumed tension only? Thanks in advance for any comments.

 

[Ble_PE]

The way I solved this problem is that I assumed (rightly or wrongly) that the braces are tension only X-bracing. If that's the case, then the column would be carrying the entire vertical load because the brace framing into the column in question has buckled under the compression load. This would give you the answer from the solution.

 

[lhpriest]

I think the problem is over simplified to illustrate the "direction of loading provisions" in ASCE 7 12.5.4.

I saw this as a code provision question rather than a structural analysis question. It doesn't make the solution correct, but that was my interpretation.

It would be nice if there were a happy medium between Williams and NCEES. Williams problems tend to be overly complex whereas some of the NCEES stuff seems fairly straightforward, with solutions that are often misleading or incorrect.

 

[*Ananda*]

Thanks for the replies.

Re: tension only bracing, I agree, if one makes that assumption then the maximum axial force in the column per NCEES is correct. But why make this assumption? Tension only bracing is not permitted for SCBF, but is for OCBF (http://www.modernsteel.com/steelinterchange_details.php?id=10). What BF type is to be assumed per question 112 is not given, but either SCBF or OCBF could be used in SDC D, if the OCBF is used in a building less than 35 ft.

My question was posed to confirm whether or not I know the technically correct solution to a brace frame analysis.

Perhaps since brace type is not stated and knowing that maximum axial force in a column comes from a tension only system, that is what should be assumed. Thanks.

 

[colostructural]

Perhaps since brace type is not stated and knowing that maximum axial force in a column comes from a tension only system, that is what should be assumed. Thanks.

I think, we as a whole in the Engineering community, over-think things. I know I am guilty of it, just ask my wife...on second thought...

In order to do a full blown frame analysis we would need to know the column stiffness, beam stiffness, and brace stiffness - otherwise we would just be guessing as to what % of force goes where. When in doubt, keep it simple. That is my motto for this damn exam.