SKY-SCRAPERS AND HOW THEY ARE BUILT
Evolution of the Sky-scraper--Construction--New York's Giant
Buildings--Dimensions.
The sky-scraper is an architectural triumph, but at the same time it
is very much of a commercial enterprise, and it is indigenous,
native-born to American soil. It had its inception here, particularly
in New York and Chicago. The tallest buildings in the world are in New
York. The most notable of these, the Metropolitan Life Insurance
Building with fifty stories towering up to a height of seven hundred
feet and three inches, has been the crowning achievement of
architectural art, the highest building yet erected by man.
How is it possible to erect such building--how is it possible to erect
a sky-scraper at all? A partial answer may be given in one
word--_steel_.
Generally speaking the method of building all these huge structures
is much the same. Massive piers or pillars are erected, inside which
are usually strong steel columns; crosswise from column to column great
girders are placed forming a base for the floor, and then upon the
first pillars are raised other steel columns slightly decreased in
size, upon which girders are again fixed for the next floor; and so
on this process is continued floor after floor. There seems no reason
why buildings should not be reared like this for even a hundred stories,
provided the foundations are laid deep enough and broad enough.
The walls are not really the support of the buildings. The essential
elements are the columns and girders of steel forming the skeleton
framework of the whole. The masonry may assist, but the piers and
girders carry the principal weight. If, therefore, everything depends
upon these piers, which are often of steel and masonry combined, the
immense importance will be seen of basing them upon adequate
foundations. And thus it comes about that to build high we must dig
deep, which fact may be construed as an aphorism to fit more subjects
than the building of sky-scrapers.
To attempt to build a sky-scraper without a suitable foundation would
be tantamount to endeavoring to build a house on a marsh without
draining the marsh,--it would count failure at the very beginning. The
formation depends on the height, the calculated weight the frame work
will carry, the amount of air pressure, the vibrations from the running
of internal machines and several other details of less importance than
those mentioned, but of deep consequence in the aggregate.
Instead of being carried on thick walls spread over a considerable
area of ground, the sky-scrapers are carried wholly on steel columns.
This concentrates many hundred tons of load and develops pressure which
would crush the masonry and cause the structures to penetrate soft
earth almost as a stone sinks in water.
In the first place the weight of the proposed building and contents
is estimated, then the character of the soil determined to a depth of
one hundred feet if necessary. In New York the soil is treacherous and
difficult, there are underground rivers in places and large deposits
of sand so that to get down to rock bottom or pan is often a very hard
undertaking.
Generally speaking the excavations are made to about a depth of thirty
feet. A layer of concrete a foot or two thick is spread over the bottom
of the pit and on it are bedded rows of steel beams set close together.
Across the middle of these beams deep steel girders are placed on which
the columns are erected. The heavy weight is thus spread out by the
beams, girders and concrete so as to cause a reduced uniform pressure
on the soil. Cement is filled in between the beams and girders and
packed around them to seal them thoroughly against moisture; then clean
earth or sand is rammed in up to the column bases and covered with the
concrete of the cellar floor.
In some cases the foundation loads are so numerous that nothing short
of masonry piers on solid rock will safely sustain them. To accomplish
this very strong airtight steel or wooden boxes with flat tops and no
bottoms are set on the pier sites at ground water level and pumped
full of compressed air while men enter them and excavating the soil,
undermine them, so they sink, until they land on the rock and are
filled solid with concrete to form the bases of the foundation piers.
On the average the formation should have a resisting power of two tons
to the square foot, dead load. By dead load is meant the weight of the
steelwork, floors and walls, as distinguished from the office furniture
and occupants which come under the head of living load. Some engineers
take into consideration the pressure of both dead and live loads gauging
the strength of the foundation, but the dead load pressure of 2 tons
to the square foot will do for the reckoning, for as a live load only
exerts a pressure of 60 lbs. to the square foot it may be included in
the former.
The columns carry the entire weights including dead and live loads and
the wind pressure, into the footings, these again distributing the
loads on the soil. The aim is to have an equal pressure per square
foot of soil at the same time, for all footings, thus insuring an even
settlement. The skeleton construction now almost wholly consists of
wrought steel. At first cast-iron and wrought-iron were used but it
was found they corroded too quickly.
There are two classes of steel construction, the cage and the skeleton.
In the cage construction the frame is strengthened for wind stresses
and the walls act as curtains. In the skeleton, the frame carries only
the vertical loads and depends upon the walls for its wind bracing.
It has been found that the wind pressure is about 30 lbs. for every
square foot of exposed surface.
The steel columns reach from the foundation to the top, riveted together
by plates and may be extended to an indefinite height. In fact there
is no engineering limit to the height.
The outside walls of the sky-scraper vary in thickness with the height
of the building and also vary in accordance with the particular kind
of construction, whether cage or skeleton. If of the cage variety, the
walls, as has been said, act as curtains and consequently they are
thinner than in the skeleton type of construction. In the latter case
the walls have to resist the wind pressure unsupported by the steel
frame and therefore they must be of a sufficient width. Brick and
terra-cotta blocks are used for construction generally.
Terra-cotta blocks are also much used in the flooring, and for this
purpose have several advantages over other materials; they are
absolutely fire-proof, they weigh less per cubic foot than any other
kind of fire-proof flooring and they are almost sound-proof. They do
equally well for flat and arched floors.
It is of the utmost importance that the sky-scraper be absolutely
fire-proof from bottom to top. These great buzzing hives of industry
house at one time several thousand human beings and a panic would
entail a fearful calamity, and, moreover, their height places the upper
stories beyond reach of a water-tower and the pumping engines of the
street.
The sky-scrapers of to-day are as fireproof as human ingenuity and
skill can make them, and this is saying much; in fact, it means that
they cannot burn. Of course fires can break out in rooms and apartments
in the manufacturing of chemicals or testing experiments, etc., but
these are easily confined to narrow limits and readily extinguished
with the apparatus at hand. Steel columns will not burn, but if exposed
to heat of sufficient degree they will warp and bend and probably
collapse, therefore they should be protected by heat resisting agents.
Nothing can be better than terra-cotta and concrete for this purpose.
When terra-cotta blocks are used they should be at least 2 inches thick
with an air space running through them. Columns are also fire-proofed
by wrapping expanded metal or other metal lathing around them and
plastering.
Then a furring system is put on and another layer of metal, lathing
and plastering. This if well done is probably safer than the layer of
hollow tile.
The floor beams should be entirely covered with terra-cotta blocks or
concrete, so that no part of them is left exposed. As most office
trimmings are of wood care should be taken that all electric wires are
well insulated. Faulty installation of dynamos, motors and other
apparatus is frequently the cause of office fires.
The lighting of a sky-scraper is a most elaborate arrangement. Some
of them use as many lights as would well supply a good sized town. The
Singer Building in New York has 15,000 incandescent lamps and it is
safe to say the Metropolitan Life Insurance Building has more than
twice this number as the floor area of the latter is 2-1/2 times as
great. The engines and dynamos are in the basement and so fixed that
their vibrations do not affect the building. As space is always limited
in the basements of sky-scrapers direct connected engines and dynamos
are generally installed instead of belt connected and the boilers
operated under a high steam pressure. Besides delivering steam to the
engines the boilers also supply it to a variety of auxiliary pumps,
as boiler-feed, fire-pump, blow-off, tank-pump and pump for forcing
water through the building.
The heating arrangement of such a vast area as is covered by the floor
space of a sky-scraper has been a very difficult problem but it has
been solved so that the occupant of the twentieth story can receive
an equal degree of heat with the one on the ground floor. Both hot
water and steam are utilized. Hot water heating, however, is preferable
to steam, as it gives a much steadier heat. The radiators arc
proportioned to give an average temperature of 65 degrees F. in each
room during the winter months. There are automatic regulating devices
attached to the radiators, so if the temperature rises above or falls
below a certain point the steam or hot water is automatically turned
on or off. Some buildings are heated by the exhaust steam from the
engines but most have boilers solely for the purpose.
The sanitary system is another important feature. The supplying of
water for wash-stands, the dispositions of wastes and the flushing of
lavatories tax all the skill of the mechanical engineer. Several of
these mighty buildings call for upwards of a thousand lavatories.
In considering the sky-scraper we should not forget the role played
by the electric elevator. Without it these buildings would be
practically useless, as far as the upper stories are concerned. The
labor of stair climbing would leave them untenanted. No one would be
willing to climb ten, twenty or thirty flights and tackle a day's work
after the exertion of doing so. To climb to the fiftieth story in such
a manner would be well-nigh impossible or only possible by relays, and
after one would arrive at the top he would be so physically exhausted
that both mental and manual endeavor would be out of the question.
Therefore the elevator is as necessary to the skyscraper as are doors
and windows. Indeed were it not for the introduction of the elevator
the business sections of our large cities would still consist of the
five and six story structures of our father's time instead of the
towering edifices which now lift their heads among the clouds.
Regarded less than half a century ago as an unnecessary luxury the
elevator to-day is an imperative necessity. Sky-scrapers are equipped
with both express and local elevators. The express elevators do not
stop until about the tenth floor is reached. They run at a speed of
about ten feet per second. There are two types of elevators in general
use, one lifting the car by cables from the top, and the other with
a hydraulic plunger acting directly upon the bottom of the car. The
former are operated either by electric motors or hydraulic cylinders
and the latter by hydraulic rams, the cylinders extending the full
height of the building into the ground.
America is pre-eminently the land of the sky-scraper, but England and
France to a degree are following along the same lines, though nothing
as yet has been erected on the other side of the water to equal the
towering triumphs of architectural art on this side. In no country in
the world is space at such a premium as in New York City, therefore,
New York _per se_ may be regarded as the true home of the tall building,
although Chicago is not very much behind the Metropolis in this respect.
As figures are more eloquent than words in description the following
data of the two giant structures of the Western World may be
interesting.
The Singer Building at the corner of Broadway and Liberty Street, New
York City, has a total height from the basement floor to the top of
the flagstaff of 742 feet; the height from street to roof is 612 feet,
1 inch. There are 41 stories. The weight of the steel in the entire
building is 9,200 tons. It has 16 elevators, 5 steam engines, 5 dynamos,
5 boilers and 28 steam pumps. The length of the steam and water piping
is 5 miles. The cubical contents of the building comprise 66,950,000
cubic feet, there are 411,000 square feet of floor area or about 9-1/2
acres. The weight of the tower is 18,300 tons. Little danger from a
collapse will be apprehended when it is learned that the columns are
securely bolted and caissons which have been sunk to rock-bed 80 feet
below the curb.
The other campanile which has excited the wonder and admiration of the
world is the colossal pile known as the Metropolitan Building. This
occupies the entire square or block as we call it from 23rd St. to
24th St. and from Madison to Fourth Avenue. It is 700 feet and 3 inches
above the sidewalk and has 50 stories. The main building which has a
frontage of 200 feet by 425 feet is ten stories in height. It is built
in the early Italian renaissance style the materials being steel and
marble. The Campanile is carried up in the same style and is also of
marble. It stands on a base measuring 75 by 83 feet and the
architectural treatment is chaste, though severe, but eminently
agreeable to the stupendous proportions of the structure. The tower
is quite different from that of the Singer Building. It has twelve
wall and eight interior columns connected at every fourth floor by
diagonal braces; these columns carry 1,800 pounds to the linear foot.
The wind pressure calculated at the rate of 30 lbs. to the square foot
is enormous and is provided for by deep wall girders and knee braces
which transfer the strain to the columns and to the foundation. The
average cross section of the tower is 75 by 85 feet, the floor space
of the entire building is 1,080,000 square feet or about 25 acres.
The tower of this surpassing cloud-piercing structure can be seen for
many miles from the surrounding country and from the bay it looks like
a giant sentinel in white watching the mighty city at its feet and
proclaiming the ceaseless activity and progress of the Western World.