Question 19

Posted in: Beams

Judging from last weeks performance quite a few of you have forgotten  how to draw BMD. So the next few questions will focus on this  issue.

The question focusses on  a 3 span  bridge with a  drop-in span in the middle similar to the Ness Bridge in Inverness Scotland, check out the Happy pontist blog: a  great  resource from  civil engineering  students.

Please identify the INCORRECT BMDs


HINT. The  trick  with  such  'drop-in' structures is that the double pin in the middle isolates the effects of load ( moment) from  either side. That is to say that a load on the  first span  cannot produce any moment on the middle or  third span.UPDATE 15/7/13. Judging  from the number of incorrect answers this  question  has  received I will be leaving it  up  for  a couple of  days  more.

If you  draw the deflected shape for the case where there load is at any point between the first  support and the adjacent pin, you  should see that  the middle span(between pins ) is always straight just  rotated slightly)

Try the question again and attempt to draw the deflected shapes.

SOLUTION: 17/6/13. A,C,D are the INCORRECT BMDs (this is what  you were asked to submit)  the  rest of them (B,E and F) are the correct BMD for the structure.

Posted in: Beams


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