BRIDGES OF CENTRAL PARK
---------------------
John Pazmino
NYSkies Astronomy Inc
www.nyskies.org
nyskies@nyskies.org
2003 November 22 initial
2005 April 6 current
Introduction
----------
On Wednesday 19 November 2003 the Science, Industry, and Business
Library, Manhattan, hosted Satinder Puri to discuss the bridges in
Manhattan's Central Park. He's a structural engineer with Arora and
Associates; his talk was part of the continuing series of culture and
science lectures at the Library.
I noted this lecture in my November 2003 'NYC Events' and, yep, I
did attend it. The Library is down the next block from my office, so
it was awfully trivial to squeeze this event into my busy life.
At first, a talk about bridges has nothing to do with astronomy,
but it in fact does. I put the event in 'NYC Events' because we
astronomers frequent the Park both for starviewings and for personal
enjoyment. At the talk, I discovered an other important astronomy
feature of the bridges, this being of global importance.
Second talk
---------
On Wednesday 6 April 2005 Mr Puri gave a return presentation on
the bridges of Central Park. It was virtually the same as the talk
from 2003 with an extra discussion about the 'Gates' exhibit of
February 2005. During his several visits to the Gates he collected new
photos of the bridges. Except for minor editing, my review of Puri's
2003 show is just as good for this one in 2005. The few extra comments
I offer, based on the present talk, are between bumpers '[ ]'.
Central Park
----------
This year of 2003 is the centennial of the enabling law that
created the Park and the opening of a multiyear suite of celebrations
for the Park. Central Park, to keep the history here brief, was built
in a region far north of the developed part of New York in the
antebellum and transbellum era of the 19th century. The boundaries
were at first set at 5th to 8th Avenues, 59th to 106th Streets; a
couple years later, during construction, the northern frontier was
extended to 110th Street.
Puri didn't mention, but it's common urban knowledge, that the
Manhattan street grid was mapped out in 1811, long before the physical
thorofares were laid down. The Park was staked out by geographic
coordinates and the actual streets came much later. New York, the
nucleus of our present cosmopolis, was then almost entirely packed
south of Canal Street with only a couple avenues marked out to the
northern reaches of Manhattan.
To explode one common wrong assumption, Central Park was NOT
merely a fenced in parcel of native countryside, to be reserved from
development. The whole effing place was deliberately sculpted and
molded from human-made plans. The native terrain was shaped over to
conform to this scheme.
Pastoral grounds
--------------
Central Park is the maturation of the European formal gardens,
mixed with psalmic imagery. The intent was to have an eden for the
City within which the visitor was transported to what heaven must have
looked like in the mind of the 19th century: lakes, meadows, sky,
brooks, rocks, and so on. Every original plant, stone, river was
artificially placed just so with little kept from the natural relief.
Even the hills, like Great Hill, were human-built accumulations. {some
features of the natural terrain were incorporated into the human plan
where convenient, to save excess construction.]
Because the Park is narrow, about 3/4 kilometer, but long, about
four kilometers, it was a challenge to create the impression of vast
open spaces. This task was complicated by the necessity to provide
transverse roads linking the present Upper East Side and Upper West
Side of Manhattan. To achieve this eden, surely one of the first
applications of virtual reality!, the designers constructed the
world's first multi-level public park. Various sections are separated
on different platforms up to a dozen meters apart.
To join these levels and to cross the transverses, the Park was
fitted with bridges, the first large scale use of this structure in a
single public place. [In doing so, Central Park is still today the
world's largest single collection of bridges on Earth! The structures
are under continual study by urban planners from all over the world.]
Mr Puri's interest
----------------
Mr Puri started his talk with a rundown of technical terms, like
'span', 'abutment', 'pier', and illustrated them with toys on the
lecture table. These were also included in his handout, which provided
a map of the Park with the bridges marked on it, and some reference
material.
He got into the subject from a friend who clued him to an exhibit
at the Metropolitan Museum of Art. Friend described it as a show about
bridges. Puri went, only to see it was really one of the centennial
shows about Central Park. But some of the pictures did include
bridges, and that sparked his interest as a structural engineer.
When he got home he tickled the web and found an online book
titled, ahem, 'Bridges of Central Park' by Reed, McGee, and Mipaas.
It's yours for download at 'www.greenswardparks.org'. Altho he did
elaborate on several of the bridges, I leave this discourse out here
'coz it's in this book. (The site has several online books about
Central Park and Brooklyn's Prospect Park.) [The book in a regular
printed form is sold at the Urban Center on Madison Av and the Dairy
within the Park.]
The talk was illustrated with computer slides of original design
plans, early photographs and paintings, and his own photos taken this
summer and fall. He stopped every so often to do some demo with his
tabletop models.
The bridges
---------
Bear in mind that the Park was built in the mid 1800s with no
additional bridges added since then. All the specimina in the Park
today are the prime furniture, with accumulated repairs and restoring.
Hence, a study of these bridges is a timewarp into engineering of a
century and a half ago, all taken in by a weekend of strolls thru the
Park. Mr Puri did such walks to take his pictures and inspect many of
the structures. [One notable exception is Capstow bridge near 59th St
and 5th Av. It was a wood bridge that decayed in the late 1800s and
was replaced, still in the 19th century, by the present stone bridge.]
He noted that none of the bridges represent any marvels of
engineering, they all being built according to the ordinary and
routine methods of that time. However, there could be a latent feature
in two of them, which would brand Central Park as the world's first
application of an incredible pontifical technique!
By the way, 'pontifex' is usually associated with the Papacy, but
it is nothing but the Latin word for 'bridge maker'. The early Church,
taking over the lapsed Roman government, looked after the bridges in
Rome. The Pope, by office, was the 'pontifex maximus', the greatest of
the bridge makers.
All of us visiting Central Park see at least a couple bridges in
the parts we habituate. Yet the number of bridges is immense for such
a small real estate: forty-four! Or about thirteen per square kilometer.
Actually, Puri pointed out, there were three more which were
demolished in the 1930s as part of Park modifications. The one I
previously knew of was pulled down to make room for the present zoo.
[Peri found several more bridges, not in the 'Bridges' book, so the
number is closer to 50.]
Bridge structures
---------------
The concept of a bridge goes back to the caveman era, when it was
desired to cross a stream by dumping loose rocks into it and walking
across their tops. This is a rock pile bridge, not very fancy but
workable. The stream continues its course thru gaps among the rocks;
there is no attempt to dike up the stream. Central Park HAS a rock
pile bridge! It is closed due to the slippery irregular 'deck'.
A step up, also from caveman times, is a rock slab bridge. This
consists of purposefully placed flat plates of rock overlapping across
a waterway. The deck is far smoother and safer to walk on. Due to the
limit on slab size to hand and the instability of stacking them too
high or long, this bridge is confined to short spans. Yes, there are
four rock slab bridges in Central Park.
All the other bridges are 'regular' structures demonstrating some
elevated intelligence, the sort of artifact on Mars that marks that
planet as the abode of life. They are a mix of wood, masonry, and iron
bridges. The most common shape is the arch, at least superficially. I
say that because some arch bridges are really girder bridges with an
arch-shaped facades.
Purists, Mr Puri explained, insist that a bridge must cross water,
not other land, else it's just an overpass. Puri and most engineers
aren't so fussy. At long as it's a structure to separate traffic on
two levels so there's no interference, it's a bridge. I forget the
count, but the Central Park bridges cross either water (brook, lake,
&c) or land (roads and paths).
Function of the bridges
---------------------
The Central Park bridges were designed for the traffic extant in
the early to mid 1800s, except for railroads. No trains were ever
allowed in the Park. The heaviest loading would be a horse-hauled
carriage. Other traffic was foot or horseback. There were not even
bicycles or skates back then. Motor vehicles were an unimaginable
component of traffic, remaining so until the dawn of the 20th century.
For this loading, the bridges vary widely in strength from
carrying only a footpath with plank deck and lacework railings, to
very massive affairs with heavy parapets and deep deck. These latter
proved adequate for light motor traffic today, but heavy vehicles like
trucks are still banned.
The spans, the clear passage under the bridge between its side
walls, abutments, or piers, range from a couple meters to something
over twenty. The present day visitor may assume that the longer
bridges were newly built, so much do they resemble contemporary
highway overpasses.
Motor traffic took its toll on the bridges thru collision of the
magnitude inconceivable to the pontifices. The bridges are duly
repaired in one of two ways. The first is to use as much as practical
prime materials, like stone from the original quarries and hand-hewed
tooling. The other, for cost, durability, safety, is to employ modern
materials while preserving the ka of the bridge.
Despite close to a century of hazard from motor traffic, none of
the bridges was totaled by it. Except for the three removed as
mentioned above, Central Park has every one of its prime bridges. [Not
quite. Gapstow bridge, spanning the Pond at the southeast corner of
the Park, was built in the 1890s. It replaces the prime all-wood
structure that deteriorated beyond repair.]
The overall care of the bridges crossing or carrying motorways is
done by the City's Department of Transportation. By arrangement with
the Parks Department, it also looks after other bridges as needed..
Pastoral considerations
---------------------
Pastoral in the religious sense of a peaceful ideal natural
setting. Or that promoted by some strands of environmentalism. Every
sightline in Central Park was supposed to land on some article of
beauty and aestheticism. Altho it may seem hatstand to include an
artificial bridge as a part of a natural setting, the pontifices rose
to the mission to build structures not only of functionality but ones
of supreme pleasure to behold. Today, after countless other bridges
were constructed thruout the world, the bridges of Central Park still
earn lavish accolade for their sheer beauty.
When the Park was built, there was no artificial nighttime
illumination, only pathetic gas lamps here and there. The bridges (and
every thing else in the Park) depended entirely on natural daylight.
Today, with the ability to illuminate edifices by electric lighting,
the bridges remain luminous only by nature. There is minor incidential
illumination from nearby new roadway lighting, but nothing specific
like floodlights.
Hence, the surfaces -- top, facades, underbelly -- of the bridges
were made to fire or dance under sun and moon light. They are heavily
textured by rough stone, geometric patterns, structural elements. The
intent was to give the visitor 'eye candy' along with the trees,
lawns, hills, and other eden-like features.
There have been concessions to safety and utility. The underpasses
of many bridges were fitted with lighting to relieve the darkness
that can prevail, even in bright daylight. Many bridges were made into
corridors for modern utilities such as electric, signal, fiberoptic,
telephone. In such cases the utility lines were buried in the deck or
hidden along the parapet. There are a couple instances where a conduit
was crudely nailed to the facade, where its line slashes across the
bridge motif.
An incredible pontifical feature?
------------------------------
In the talk and in the handouts, two bridges (I forget which) were
described as made from reinforced concrete. Reinforced concrete is a
staple material for modern structures, including bridges. It consists
of a mat, cage, grill of steel rods laid out like a skeleton in the
structure. Around them is placed regular concrete, tamped and shaken
to grip the rods. The concrete and steel make a solid strong bond
together and act as a single mass within the structure.
The rods also help join concrete elements, like beams and columns.
They also make the structure resist against tension and torsion;
concrete alone can withstand neither.
My recollection of history is that reinforced concrete came into
use in the early 20th century. In fact, one major example of a seminal
application was the New York underground transit system.
Allowing that the description of the two Central Park bridges is
correct, than perhaps my history is mistaken. If so, all is well.
On the other hand, if the history is accurate, as it is related in
engineering litterature, then Mr Puri's account, taken from the
'Bridges' book, is astounding! It would document the use of an
incredibly modern and sophisticated technique long before any one
realized. Note that steel itself, for the rods, was not an engineering
material until the 1880s, pioneered in the first New York skyscrapers.
In such a situation, the bridges of Central Park take on a
fetching fascination. Perhaps they are the world's first use of
reinforced concrete, then not to be tried again for the next half
century!
Astronomy of the bridges
----------------------
Among the uglier fixtures in the Great Beyond is grotesque
nighttime illumination of structures. Part of the reason is the lust
to rid the landscape of the night phase of the diurnal cycle. The
usual results are dismal to the max.
The collateral damage is that photons from the lamps cascade and
fountain into the air, illuminating it to veil the night sky from
view. This is one major example of reckless light pollution or
luminous graffiti.
Light pollution advocates can do themselfs genuine good to spend
time in Central Park and examine its bridges. They can then appreciate
how, in the era of routine dark skies (altho there was plenty of raw
noxious industrial air pollution!) the natural sun and moon light was
exploited.
They can (digitally or chemically) photograph the bridges under
various daylighting to show that, indeed, the absence of artificial
lighting can be a massive blessing to their hometown. They can recount
the heavy use of Central Park was a starviewing site, smack in the
middle of the 21st century City for its paucity of obnoxious lights.
(There ARE some!). Where is the American Urban Star Fest held each
year? Fill in the blanks: C _ _ _ _ _ _ P _ _ _ . They can explain
back home about the theme of Central Park, that humankind can live in
harmony with nature, not fight against it.
Think about that when you muse at that overpass over the knoll
while sunbathing in Central Park.