The ,"new TACOMA NARROWS BRIDGE"...Why are there no,"grated decks", to allow airflow from strong wind ?

Question

The first,"TACOMA NARROWS BRIDGE", just "two lanes", known around the entire world now as"GALLOPPING GERDIE".
The man who had the money to ,"insure", the first bridge, put the money in his pocket, as he was so sure the engineers had designed the perfect bridge.
Well obviously they hadn't.
The engineers did not design a way for the ,"strong winds", which are common in the ,"NARROWS STRAIGHT", to ,"flow through the decks". So the bridge went down in the first major storm it faced.
The new," NARROWS BRIDGE", has no ,"grated decks", or whatever.
I spoke to one of the, "foremen", on the westside, you know, just mentioned it to him.
I was making a delivery.
Anyway, he assured me, it would be a real big ,"boo-boo", (that was his terminology) and that ,"Engineers", had been designing the bride for many years...
So who can tell me...WHERE IS THE AIRFLOW, supposed to go, if not through the DECKS of the new BRIDGE???
Preferrably someone who knows about engineering.
T.Y.

Answer 1

First - the original 1940 edition of the bridge (Galloping Gertie) was specifically designed to be narrow and elegant, reflecting the theme of the times in terms of both architecture and engineering. To this end, the footings for the towers were narrower and the span itself was narrow (two lanes) and slender (the steel "deck" was basically a solid plate welded between two solid steel beams, making the shape of an upside-down "U"). The beams were only eight feet high! Asphalt was laid over the metal to give a smooth driving surface.

The problem is that the winds hit the "solid" side beams, and set up a resonance in the structure, eventually leading to its collapse. The narrowness of the tower supports and the length of the span were also contributing factors.

In 1950, the "current" bridge was built. It's side plates are more like modern trusses, with huge triangular holes to allow air passage through the sides and beneath the steel surface holding up the paving. In addition, the towers were given a wider stance with more "meaty" vertical members. This effectively de-tuned the system ... hence it's still in operation more than fifty years later.

The "new" bridge - set to open in 2007, if you believe the WSDOT propaganda - is very similar to the 1950's version. Again, a truss-like side member supports a horizontal steel surface which in turn supports the paved vehicle surface. Towers and footings are sturdy and widely spaced, leading to a low likelihood of repeating the earlier resonance-related failure.

To perhaps answer your question even more fully - the air is passing through the Tacoma Narrows strait, not traveling from the water to the sky (or vice versa). The plane that has to allow air unrestricted passage is the side panels (or, in the modern bridges, the trusses) ... not the "deck" which underlies the asphalt.

The overall stiffness of the structure has to be considered - allowing airflow through a grated floor amy seem like a good idea, but it compromises both lateral (side to side) and vertical stiffness in such a way that it was not chosen as the "best" alternative. Different choices might have been made if the deisgn was not a "suspension" bridge ...

Answer 2

On a suspension type bridge, up and down motion due to aerodynamic lift and side swaying due to wind forces are not the problem. The suspension cables already have some give in those directions, so that is not the danger. The danger of collapse comes when the deck section starts twisting. (Technically, it was not resonance but a phenomenon called flutter that caused the original bridge to collapse.) As long as the bridge span's cross section is designed rigidly enough against torsion, the bridge will be safe.

Answer 3

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