Hiner Problem #19

[H2OEngineer]

On the last part of the problem I am having trouble understanding the sign change in computing the Max Force on Wall-2

The Base Shear is in the E-W direction. When evaluating the accidental torsion ey2 a counter clockwise rotation about the CR occurs. This causes an additive force to the Base Shear on Wall-2 as illustrated in solution sketch. The computed Mt2 is negative due to the negative -5.0 ft of accidental torsion.

My question is : Where does the second negative sign in the solution equation come from to make the equation a positive 1500 kips?

 

[H2OEngineer]

Correction the solution is not a positive 1500 kips as I orginally stated, I should have said the Torsional Shear becomes positive as a result of the second negative sign and thus the Torsional Shear is added to the Direct Shear. I do not see where the second negative sign comes from to create the positive Torsional Shear.

 

[ptatohed]

Can you please clarify what you mean by "the second negative sign"? Are you referring to the -50 ft?

 

[H2OEngineer]

Yes.

The solution for Max F on Wall 2 uses Mt2 with ey as -5.0 ft and yielding -1500 kip'-ft for Mt2. The second negative sign shown in the solution for Max F on Wall 2 is on d using -50ft. When evaluating the moment with ey2 equaling -5.0 ft the rotation provides an additive force to Wall 2, as shown in the sketch for ey2. It seems to me the solution for the torsional shear should be subtracted from the direct shear in either case of using Mt1 or Mt2.

Mt1 is positive, but the rotation provides a subtractive force to Wall 2. Mt2 is negative, but provides an additive force to Wall 2.

Can you help explain where I am thinking incorrectly?

 

[ptatohed]

Yes.

The solution for Max F on Wall 2 uses Mt2 with ey as -5.0 ft and yielding -1500 kip'-ft for Mt2. The second negative sign shown in the solution for Max F on Wall 2 is on d using -50ft. When evaluating the moment with ey2 equaling -5.0 ft the rotation provides an additive force to Wall 2, as shown in the sketch for ey2. It seems to me the solution for the torsional shear should be subtracted from the direct shear in either case of using Mt1 or Mt2.

Mt1 is positive, but the rotation provides a subtractive force to Wall 2. Mt2 is negative, but provides an additive force to Wall 2.

Can you help explain where I am thinking incorrectly?

The neg 50' (d2) is simply the distance from the Center of Rigidity (CR) to Wall 2, using your established x,y coordinate system. So, you used the Southwest (bottom, left) corner as your 0,0, right? Thus, measuring from center down to Wall 2, you get -50'. Measuring from center up to Wall 1, you get +50'. Makes sense?

This same logic is used in solving for Max FA, FB, and FC in the Northsouth direction. dA (-59.7') is negative because Wall A is west (left) of CR but dB and dC are positive because Walls B and C are right (East) of CR. I'm not sure about your edition but in my older SDR Workbook 2011 edition, instead of putting the negative sign before the 59.7', for some reason Hiner put it before the Torsional shear formula (but really there should be a plus sign before the Torsional Shear and the dA should be negative).

Does that help?

 

[H2OEngineer]

Excellent, yes that clarifies it for me. I had originally thought it was based on the origin to CR, but talked myself out of it based in part on the moved sign you noted with mac force on Wall A.

Thank you again!

Steven

 

[H2OEngineer]

ptatohed

Apologies for resurrecting this question again but it is still not sitting right with me.

Your comment above stated:

So, you used the Southwest (bottom, left) corner as your 0,0, right? Thus, measuring from center down to Wall 2, you get -50'. Measuring from center up to Wall 1, you get +50'. Makes sense?

Looking at the following Hiner Problem #20, the very last part-D is also looking at the E-W direction. In this problem there are two walls (Wall -D and Wall-E) that would be measured down from the point of CR. Wall-C would measured up from the point of CR. The solution does not seem to apply a negative sign to any of the three walls based the orientation to the point of CR. The solution does apply a negative sign to Wall-C due it having a counter force against the shear direction.

So I am still confused on the rule of when to apply a negative sign to the parameter "d".

 

[ptatohed]

ptatohed

Apologies for resurrecting this question again but it is still not sitting right with me.

Your comment above stated:

So, you used the Southwest (bottom, left) corner as your 0,0, right? Thus, measuring from center down to Wall 2, you get -50'. Measuring from center up to Wall 1, you get +50'. Makes sense?

Looking at the following Hiner Problem #20, the very last part-D is also looking at the E-W direction. In this problem there are two walls (Wall -D and Wall-E) that would be measured down from the point of CR. Wall-C would measured up from the point of CR. The solution does not seem to apply a negative sign to any of the three walls based the orientation to the point of CR. The solution does apply a negative sign to Wall-C due it having a counter force against the shear direction.

So I am still confused on the rule of when to apply a negative sign to the parameter "d".

Yeah, good question H2O. I see your point. The signage for part D is exactly the opposite of what I would have thought for Walls C, D, and E. I think it has something to do with the boxed Note on page 2-54 but I don't want to risk telling you the wrong thing any more! So, here is my advice. 1.) I can honestly tell you that Hiner's Part 2 Example Problems 1 - 21 are, for the most part, beyond the scope of the exam. I strongly urge you to just know Hiner's and Ibrahim's multiple choice problems inside and out, and you will do just fine on the exam. 2.) Try to e-mail Hiner and ask your question to him. 3.) I know it may sound weird but if you call or e-mail Dr. Ibrahim and ask him your question (yes, even if it is from Hiner's book), I am certain he'd be happy to answer your question.

Good luck and please do share when you get your answer.

 

[H2OEngineer]

Thank you for all your help on the Seismic Exam material ptatohed. I feel I did very well on the exam this morning!

I am now on to studying surveying. I have Mahallati´s book for surveying, although I am only into the first chapter (just started this afternoon after the seismic exam) I am thinking I may need an additional study guide book. Got any recommendations?

 

[ptatohed]

Thank you for all your help on the Seismic Exam material ptatohed. I feel I did very well on the exam this morning!

I am now on to studying surveying. I have Mahallati´s book for surveying, although I am only into the first chapter (just started this afternoon after the seismic exam) I am thinking I may need an additional study guide book. Got any recommendations?

Good job H2OE. Let us know how you do when you get your results.

When is your Survey exam?

Well, you saw this thread already, right? http://engineerboards.com/index.php?showtopic=25246

I'd recommend hitting the Caltrans free LSIT/LS material (hit only the things relevant to the CA-Survey exam). http://www.dot.ca.gov/hq/row/landsurveys/LSITWorkbook/WorkbookTOC.html

Let me know if you have any questions. Survey is my favorite.

PM me your e-mail address, I might have a few more things for you.

Good luck.

 

[H2OEngineer]

Hi Ptatohed, will pm you now.