ICAO doc9157 part2 recommend many standard of taxiway system, including width, separation, slope,etc.
How about height of the object above taxiway, like passenger bridge or rail track? Is there any ICAO doc recommendation or major aviation authorities' rule(like FAA) about the vertical clearance of taxiway?
Bridges over taxiways are not a common airport feature. The FAA does have standards for wingspan clearance on taxiways, but not heights. Airport information will tell pilots if there are wingspan or height limits on certain taxiways, and ATC will know which aircraft are or are not able to take certain routes.
For example, Denver International Airport has a bridge over one taxiway. This provides 42 feet of tail clearance, which leaves 3.5 feet of clearance for the A320 family (except the A318). These are the typical aircraft that use this taxiway. A 737 classic would have 5.5 feet of clearance but the 737 NG and A318 would only have 1 foot of clearance. Since the 42 foot number is only in the center of the taxiway, and accounting for other conditions, 1 foot of clearance does not leave much margin for error. So for regular operations, the A320 and 737 classic are the largest planes that will pass under the bridge.
The bridge at Gatwick Airport provides 22m of clearance, enough for a 747 (19.4m high) but not an A380 (24m high).
This suggests that about 10% margin is enough clearance for taxiway bridges. The FAA requires that taxiways have 20% margin plus 10 feet to objects. While wingtip clearance has many variables that require larger margins, tail height will not change much.
In effect, the FAA's Airplane Design Group specifications impose a lower bound on how high bridges over taxiways must be if a taxiway is to accommodate aircraft of a given Design Group. The Airplane Design Group imposes an upper limit on both wingspan and tail height for airplanes in each group, so an airport designing a taxiway to accommodate a given group would need to make sure the clearance above the taxiway at least exceeded the upper limit of the tail height for that group.
Here are the tail height limits for each group. Each design group goes up to, but does not include the upper limit.
Source: Table 1-1 of FAA Advisory Circular 150/5300-13 (PDF), located on page 15 of the PDF.
To get a feel for what these limits allow, here are some examples of aircraft in each design group:
I am not, however, sure by exactly how much the clearance underneath the bridge must exceed the ADG tail height standard. I'd expect there's some guidance for this, but I wasn't able to find it in a quick search of that document. If anyone does find that, feel free to edit it in here.
