Expert: Arshad Khan - 6/26/2008

Question
QUESTION: hello Mr. Archad,
when a bridge is made over a river, the bridge stands over an arc made of specific elements, and when an object reaches the center of mass of the bridge, a force is created (F), but because of the arc which is tangent to the bridge at G(center of mass) then (F) will be seperated into (F1) and (F2), 1st force goes through 1st way of the arc and the 2nd goes through 2nd way, and that in order not to let the force (F) go into the river so the bridge will be broken, IS THAT RIGHT??
I HOPE YOU UNDERSTAND MY QUESTION
thanks for listening.

ANSWER: Hi Mohammad,

I do not quite understand your question, but in principle so long as the arch bridge has good supports that do not move, then yes the load at the top of the arch will be transmitted a axial loads through the arch, back into the supports.

---------- FOLLOW-UP ----------

QUESTION: hello,
well what I want to know actually the role of the arc over a river, does it has the same role as a rod? well i mean that when a force(F) exerted by an object then absolutley (F) will be directed down ward(to the river) and we can't put rods over a deep river then we put an arc, then (F) will be directed along the arc which leads to ground, then (F) will be reacted by normal reaction(N) while in the river (N) won't be foud so the bridge will fall.is that true?
I HOPE YOU UNDERSTAND MY QUESTION
i know its kind of tiredness for you,
thanks for listening
I hope i'll mail in future

Answer
Hi Mohammad,

I am sorry, but your question is still not clear- what do you mean by rod? Do you mean a beam or are you refering to the reinforcing steel (rebar)?

Basically the atrch will not behave like a beam and the rebar inside the arch will mainly be in compression.