According the definition of Mark, (L) and (R) are marks.
According the definition of Obstruction, all columns are obstructions.
I don't think that is any ambiguity, but I would suggest improving the phrasing to give better service to the competitors making explicit mention of the marks.

Why was it felt necessary to tell a boat how to go under a bridge?  Was it to expressly invoke the Mark definition?  Otherwise, those columns are obstructions whether called out or not.

If, as in some races here on SF Bay, some spans should NOT be transited, I wonder if it would been clearer either to define them as restricted or simply say "a boat shall pass to the north of column 2 and to the south of column 5, for example.  This would make them passing marks.  Column 4 would be an obstruction still.

Michael, it was a made-up scenario based upon wording I've seen in several race documents .. not a precise quote but pretty close.  Here in the Chesapeake, we have the occasional race that goes through the Chesapeake Bay Bridge, most of which does not have the clearance (clearance-heights for each span is not easily found). 

Finally, because there is a gradient of clearances, smaller boats competing under handicap rules with significantly shorter mast-heights could cross in the shallows to avoid adverse currents.  

I think all the above contribute to the reasons the specific spans are stated.

We have a similar race which passes thru several bridges, some with fairly narrow passages.  Light air and adverse current are a big issue when passing thru bridges, so we added the following language to promote safety and avoid unnecessary contortions (and protests!)...  "For safety, bridge foundations and fender systems may be touched, secured-to, or fended-off from without penalty."

Furthermore, we even allow motoring thru bridges...  "For safety, while transiting the primary navigational passage of a bridge and in no other bridge span or passage, yachts may motor at a steady course and speed, not to exceed four (4) knots, from the time the bow is two (2) boat lengths from the nearest fender until the yacht clears the fenders on the opposite side."  Admittedly, this is a somewhat goofy race, but it seems to work.

Okay, my comment was hasty, and in the middle of another urgent task.
As I think of it "go through one of these spans" does seem the most direct way to phrase it. It is, in effect, a gate.

Michael and Juan, below exemplifies where I think there might be a little bit of ambiguity with “pass through one of” language regarding the middle columns being marks or obstructions.

When a boat is "passing through one of" the spans, the columns on either side bound that span ..and a boat "shall pass through one of" the spans. Consider the Alt scenario below to examine the question.   Would we still consider L2 through R1 obstructions?

Absolutely agree, if the columns could be defined, an SI stating "pass between the L and R columns" would be much cleaner.

Well, that really brings it into focus.  If you start getting into the business of defining columns as marks, some spans will have marks and some will not, leading to odd tactical choices perhaps.

Maybe the best thing to do is define the forbidden spans as restricted. "For safety, a boat shall not pass through any span other than the one of the center three spans of the Smackemup Bridge"

Having a little fun here:
Does "shall pass through one of the spans" require the boats to pass through the same selected span going and coming?

This reminds me of a race I set up where one of the marks was "shall pass under the shadow of the Golden Gate Bridge"
Well... it was foggy that day, so.....

I think Juan's got the simple straightforward answer. 

I don't see any ambiguity. 

Following Angelo's photo and post, if the spans outside L and R are not navigable then L and R are continuing obstructions at which rule 19, and not rule 18 will apply.

How about an incident when an outside span is navigable by a small boat with a low mast, but not by a boat with a tall mast?

Is there any problem with just taking it case by case, so in some incidents an inside boat not entitled to mark-room will be able safely to go outside the column/obstruction/mark, and rule 18 will apply, and in others, she will be too tall, and won't be able to go outside,  so rule 19 applies?

I don't think we should be striving after consistency for consistency's sake.

Michael Moradzadeh
Said Created: Today 16:57

Was that intended as a touch and go into the shadow without passing under the bridge?

Or was it just meant to be some elegant verbiage meaning go under the bridge? 

Definitely a few/several spans outside of the middle 3 are clearly navigable .. even for a J/105 with a 50' bridge clearance.  The problem is that I've never been able to find a document that describes the clearance at MHT/MLT .. so when you're cruising around .. one takes their chances.  At some point I need to take a laser range finder and chart it .. as the pilings have depth markings for reference.

Angelo's Alternate Scenario.

No fair.  You're changing the problem. 

I think the original wording referring to spans (implicitly  making L and R marks) is easier to visualise and understand than trying to explicitly designated the columns as marks.

Now you are introducing complex structures instead of single columns.

I think the issue remains whether the spaces between L2 and C1, and C2 and R1 are navigable.

Bear in mind we should be thinking about the columns/structures and the bridge deck above as an obstruction.

These double structures usually have all sorts of diagonal bracing between the piles, so are obviously all one structure, and one obstruction,

OTOH if there is navigable water between L2 and C1 and C2 and R1, then what I think you have is multiple gates:  boats must pass through either L1/L2 or R1/R2.

Depending on the size of the L2C1 and C2R1 structures, they may or may not be continuing obstructions and rule 18 may or may not apply.

Obviously enough, as complexity increases,  so does the risk of ambiguous or ineffective SI.

John, a series of gates was what I was getting at … that the “through one of the spans” might be equivalent to 3 gates with shared marks (using that wording)

The idea behind my alternative was to imagine  slowly pulling the gates apart to illustrate the point.  

The idea I hoped to explore was that the original wording might cause confusion in the minds of racers on the status of the interior columns. 

On the original scenario, with that wording and single columns or structures between the spans, I don't think there is any ambiguity. 

Boats are required to pass through  a gate between L and R.   Boats may pass CL and CR on either side.

Once you get to bits of navigable water between the spans you want boats to pass through,  you're going to need something more in your SI.

I agree John, that looking at it as we are, the middle columns in the original are obstructions. That said, I think stating it that way might lead to some confusion on the water in the minds of some racers. 

If possible to define them clearly, I think it would be better define the outside ends and use them to define the gate directly. 

Reminds me of the round the island race, isle of sheppey:
7. | The course
7.1 | The race will be sailed in a clockwise direction around the Isle of Sheppey, starting and finishing from IOS Sailing Club, Sheerness. Competitors are advised to acquaint themselves with the course, as there are several underwater obstructions, shoals and areas of foul ground.
7.2 | The Kingsferry Bridge: competitors will be permitted to negotiate the bridge by wading, paddling, or sailing under any of the side spans. Competitors are advised to walk their craft in a capsized position underneath the mainland side of the bridge. No special arrangements will be made for multihulls. There may be marshals in the immediate vicinity of the bridge who can be called upon to give assistance. If there is a queue at the bridge, competitors should wait in turn.
7.3 | Sailing through the centre lifting section of the Kingsferry Bridge is NOT permitted, and may result in disqualification without hearing. However, in exceptional cases, special dispensation may be granted by the OOD in advance, permitting use of the centre span due to the nature and/or restrictions of the specific craft entered (ie. fixed keel). In such cases, the OOD will factor in a discretionary time penalty taking this into account.