The building's least attractive side, from the south
The steel cross-bracing is common, although occasionally missing. Why is that?
Edit: An anonymous source gave the following answer to the “why” above.
Regarding the cross-beams at the back of the Park Row building, and your question as to why some buildings don’t have them: its all a question of bulk and engineering. Obviously the cross-beams create a “box” while still allowing an air shaft for more windows: this serves both the engineering and commercial needs.
All buildings must handle the horizontal load generated by a maximum value of wind stress, which becomes much greater as the height increases. Pre-war buildings used steel behind the brick or stone facade, but probably still relied on the sheer weight of the masonry component. My guess is that the tools used to calculate those stresses were not terribly sophisticated, so out of an excess of caution they added cross-beams. Take a look at the back side of the Woolworth building: it too has them.
By contrast, the Twin Towers were a marvel of engineering precisely because they defied the wind: two straight towers, 110 stories, with no setbacks and no visible bracing beams (either externally or internally). The trick was to closely space the exterior columns, so that they acted like tubes, to handle both vertical and horizontal loads. This solved the dual load problem for tall buildings in a very elegant fashion, because it allowed for lightweight floors with no internal columns except for the central core.
You might find it interesting to research the design philosophy of the architect (Minoro Yamasaki) and his earlier buildings–much like Roebling, he first tried out his signature concept out of town. Say what you may about why these design decisions contributed to the buildings’ collapse on 9/11; but at the time they were built, they represented a very dramatic and novel way to solve the problem. By contrast, the Hancock Tower in Chicago relies on giant exterior cross beams.
According to wikipedia, this was the tallest office building in the world for 9 years at the turn of the century. I also recommend the contemporary perspectives quoted on wikipedia; some of them are rather funny.
