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RE: Golf Course Bridge

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Bill Cain wrote:

. > Of course, since the EIT after your signature on a prior posts indicates 
. > that you have probably been educated since advent of the frequent use of 
. > the computer for structural analysis, you may not have learned influence
. > lines. Refer to most any text on indeterminate analysis such as Norris 
. > and Wilbur or Kinney for a discussion of this technique. It will give you 
. > a much better feel for the structure than reams of computer printouts.


When I last taught structural analysis, I decided not to use the term 
"influence lines," because horror stories about how hard influence lines were 
had frightened the students.  (Probably the difficulty of influence lines 
was due to the teachers' lack of experience in creating them.)  Instead, 
after we had finished shear and moment diagrams, I gave (amid groans) the 
students an assignment to calculate the shear and moment at each 1/10th point 
for a 1 kip load as the load moved across the beam from 1/10th point to 
1/10th point and to put the results in a table.  Before the next class 
period, one of the students stopped by my office and asked if we were going 
to study influence lines; that she had heard that they were "really hard."  I 
told her that she just did.

The derivation for maximum moment due to a series of concentrated moving 
loads is/should be derived in any elementary strength of materials or 
elementary structural analysis text book and it is, essentially, as Bill 
describes, i.e., for a simply supported beam, the centerline of the span is 
placed half way between the resultant of the concentrated loads and the 
concentrated load below which maximum moment will occur.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

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