Safety first for skyscrapers
Uncle Sam’s scientists, armed with every known precision test instrument, have set out to answer the much-debated question of “How safe is a tall building?”
By JN Miller
Another official government investigation is getting underway in Washington. The investigators going to work now are scientists, and their problem is to find out whether skyscrapers — including the 10 and 20-story skyscrapers of the average American city — are safe.
One key question the probing scientists will seek an answer for is: Just what is the effect of terrific winds on skyscrapers — winds that often make the tallest buildings sway?
In charge of Uncle Sam’s investigation, the only government skyscraper probe ever attempted, is the US Bureau of Standards at Washington, with a capable staff of technical experts equipped with unique instruments.
That every city in the nation will be interested vitally in the probe is shown by the following government statistics: There are about 4780 buildings 10 stories or more in height located in 131 of 173 cities with 50,000 or more population. Of these, about 15 structures are taller than 500 feet, while three others more than 800 feet high have just been completed.
Dr HL Dryden, expert on wind velocities for the Bureau of Standards, is the man in charge of this program. The very first step, now underway, is the making of a miniature model of the titanic Empire State Building. The model will be tested under various conditions of wind velocity.
“The miniature building, which will be about five feet high and made of aluminum plate,” Dr Dryden says, “is being made from a complete set of blueprints very kindly furnished us by the architect’s office at the Empire State Building. Of course, the model won’t have the exact surfaces and window arrangements of the actual building. But the general story and structural arrangements are being simulated as closely as possible.”
Dr Dryden tells just how Uncle Sam will go about checking up his model tests with the actual measurements being conducted on the full-size structure:
“When our model is completed,” he says, “we will place it in our 10-foot wind tunnel at the Bureau of Standards and measure pressure conditions and wind speed at the same places on the model’s surface that are being measured on the building itself — 35th, 55th and 75th floor levels.
“Experience in wind tests has taught us that the corresponding pressures and speeds ought to be substantially the same. If this is so, builders of great skyscrapers of the future will not need to go to the expense, trouble and labor of putting wind and sway measuring instruments in the buildings at strategic points. For models will do the work quicker, cheaper and more efficiently.
“However,” Dr Dryden continues, “in this connection, it should be borne in mind that our government is not testing the strength of the model under examination, but is simply measuring the effect and force of the wind on the model. We can simulate wind conditions in our wind tunnel up to 70 miles an hour.
“We shall fasten the model securely in the wind tunnel and measure the pressure at a number of widely distributed pressure holes for different wind speeds and wind directions. Or, we shall attach the model to suitable balances and measure the overturning moment and its point of application.”
Inside the Empire State Building
Within the real Empire State Building itself instruments called extensometers have been attached to the four corners of a number of columns, and also to portal beams. They will make possible the reading of bending and overturning stresses in the columns, and of bending stresses in the portal guides.
Under the building code of the city of New York, the structure was designed to withstand a wind pressure of 20 pounds per square foot above the sixth-floor level. The code does not require that wind pressures below the 100-foot level, about the sixth floor, be figured.
The force of the wind will obviously be greatest at the very top of the tower which has been generally referred to as a mooring mast for airships. Whether it will actually be practicable to use this tower as a mast for dirigibles is a question that has been disputed among members of the engineering profession. Some authorities also claim that it would be very difficult if not impossible to tie an airship to the mast because of currents of air set up by the towering Empire State Building itself and by lesser surrounding structures.
The field of research into which the scientists at the Bureau of Standards will be taken by their tests with the model of the Empire State Building in the wind tunnel is little explored. Scientists and engineers have only a vague idea of the effect of wind forces on big buildings. By rough estimate, they can tell that it is possible for a large building to be subject to tremendous pressure by wind.
For example, they can calculate that the largest side of the Empire State Building forms a huge and effective blockade with an area of about 200,000 square feet in the face of any wind. Then, just for the sake of estimating, they may figure the momentum of a huge block of air, having a cross-sectional area of 200,000 square feet and a depth of 500 feet, moving against the building.
This block of air weighs about 5000 tons, the same as a train made up of 50, loaded, 100-ton coal gondolas. So the momentum of this air at any speed is the same as that of the coal train. Just imagine a 50-car coal train rushing head-on into the Empire State Building at about a 40 or 50-mile-an-hour clip and you will get some idea of the forces a good storm can throw against the structure.
An instrument especially designed and built for the US Coast and Geodetic Survey is being used for measuring the swaying of America’s highest skyscraper. This device is known as a “vertical collimator.” It is located at the foot of the fire stair at the sixth floor and enables the trained observer to sight on an illuminated target at the 85th floor.
This tremendous vertical length of sight provides an excellent opportunity for observing the sway of the structure in any direction. The slightest sway, even to a quarter of an inch, may be measured accurately through this unique device.
Records of the windiest days in the week as observed at the building are being kept and compared with those of the United States Weather Station, which is located at a lower level in a neighboring building.
Sway measuring instruments have shown that a test building in Columbus, Ohio, moved less than a quarter of an inch in a 30-mile wind. But many older skyscrapers are known to sway considerably.
In addition to the measurement of wind strain and velocity, the government is making an important study of structural beams and girders of the kind that join the skeletons of high buildings. For instance, a mammoth testing machine, with a capacity of 10,000,000 pounds, is used to measure the effect of tremendous forces brought to bear on huge pieces of structural steel. Other apparatus measures the strength of small steel pieces.
It is a rather curious fact that Uncle Sam’s initial study of the effect of winds upon skyscrapers was largely inspired by the knowledge gained by an extensive study of the effect of high-speed wind velocities on airplanes in the 10-foot wind tunnel at the Bureau of Standards. Having given the nation’s military and commercial aeronautical interests much practical information through this type of research, the government scientists reasoned, naturally enough, that the wind tunnel system might be used to advantage in studying tall buildings.
However, as explained by Dr Dryden, it has taken many months to convince America’s construction engineers that tests with models could reap worthwhile results, so far as their own immediate interests were concerned. Today the engineers have offered their advice and cooperation in the new government project.