為何橋要起得高過地面?

橋要起得高過水面或地面並不是因為什麼工程上的特別原因…

這是因為生活上實際環境的需要…

例如在某道路上建築一行車天橋，是為了疏導交通；建築一行人天橋，是為了人、車分路，保障行人安全…

試問把橋建築在地面上，又如何能達到上述目的呢？(不如索性唔起天橋，以免架床疊屋)

反而是工程上為了達到把橋穩固地架在空中，而要作出適當的設計，如用料、承托力等…

I hope you don't mind if I answer your question in English, I can type faster this way:

Let me start with how engineers classuify "Bridges". Actually there are 2 types of bridges - "Bridge Overpass" and "Bridge Underpass". Both are considered as "Grade Separation".

Overpass - You have an existing roadway and you need to build a bridge to fly over and across the existing roadway. Therefore, you need to build bridge bents (橋墩) to "fly over" the existing roadway (e.g. 加士居道天橋)。

Underpass - You have an existing roadway and you have physical limitations and do not want to "fly over", then you have another option, which is trenching underneath the existing roadway and build an underpass to cross the existing road surface. But you will end up having the existing roadway supported by a bridge at grade and the new roadway is depressed to go under.

So, to answer your question: are all bridges need to be above see level or existing road surface? The answer is no, not necessary.

When you take a side view of a bridge (such as Ching Ma Bridge), you may notice that the profile of the bridge is slightly concave up - center portion is higher than the two bridge approaches. There are many reasons for it, such as drainage, load calculations, aesthetics etc.

I hope I touched the topics that you are interested, if not, feel free to ask again.
圖片參考：http://hk.yimg.com/i/icon/16/1.gif

你這個問題真是問得十分奇怪，若果橋不高於地面的話，還哪裏叫做「橋」呢？

橋是為了便利交通，使兩個地方直接連結起來，而不需要捌彎迂迴才能到達目的地。
也為了增加更多可用空間，正如高樓向高空發展一樣。
至於跨海越洋的天橋就更不用說了。

Thats the main purpose of bridge - to fly over a location that allow the traffic flow to continue to flow under the bridge.