Category Archives: Engineering

The Freedom Tower – From Construction to Completion

Overview

One World Trade Center - Freedom Tower photographed from Broadway
One World Trade Center – Freedom Tower looking south from Broadway. Photo: SMS Photos of a Lifetime ©

From tragedy to triumph, a tower soars 104 stories, 1,776 feet high, representing the year the Declaration of Independence was signed.

One World Trade Center (AKA The Freedom Tower) opened to businesses on November 3, 2014, and the three-story observatory, which opened on May 29, 2015, invites visitors to a spectacular view of the New York skyline.

Skidmore, Owings & Merrill, famous for designing some of the most notable modern tall buildings in the world, were the primary architects, under the supervision of designer David Childs. The firm, also known as SOM, was the architect of the Burj Khalifa and Chicago’s Willis Tower (formerly the Sears Tower).

The Preliminaries

Soon after the destruction of the original World Trade Center, the Lower Manhattan Development Corporation initiated proposals for the reconstruction of a new tower, as well as a plan to memorialize the victims of the September 11 attacks. 

When the public rejected the first round of designs, a second, more open competition took place in December 2002, in which a blueprint by Daniel Libeskind was selected as the winner. This design went through many revisions, mainly because of disagreements with developer Larry Silverstein, who held the lease to the World Trade Center at that time.

Construction began on April 27, 2006, but not after continuous delays and ongoing bureaucracy, including disputes between the Port Authority of New York and New Jersey and the developer Tishman Realty & Construction. The Tishman construction firm was famous for its participation in building some of the tallest buildings in New York City, including the original World Trade Center complex and the John Hancock Center in Chicago. John Tishman died on February 6, 2016.

Security Preparations

Bird's eye view of "ground zero" after the 9/11 attacks and before construction of the Freedom Tower
Bird’s eye view of “ground zero” after the 9/11 attacks and before the construction of the Freedom Tower. Photo: SMS – Photos of a Lifetime ©

No doubt that security was a prominent concern in the design and construction of this tower, and terrorism was indeed a major consideration. 

No one was more concerned than the NYPD, and after many debates and delays, the final proposal for the Freedom Tower 11-Year was approved and shown to the public on June 28, 2005, with a 187-foot base of concrete added.

Additionally, the building had installed stainless steel panels and blast-resistant glass. The Freedom Tower is designed to withstand earthquakes and has an elaborate security facility integrated within it.

In addition to 24×7 monitoring, there is a high-tech security system that includes video analysis in which computers would alert security personnel to abnormal situations automatically.

There are additional security apparatuses that have been installed, but their actual function has not been made public. What is known is that there are radiation detectors abound in lower Manhattan and the NYPD Hercules Team is ready at a moment’s notice.

Building the Skyscraper

Foundation of One World Trade Center
Construction of the foundation of One World Trade Center. Photo:  SMS – Photos of a Lifetime ©

On November 18, 2006, 400 cubic yards of concrete were poured onto the building’s foundation.

On December 17, 2006, a ceremony was held in Battery Park City, with the public invited to sign a 30-foot (9.1 m) steel beam. The beam was welded onto the building’s base on December 19, 2006. Construction was slow but continuous. 

In 2012, workers installed the steel framework at the top of the tower to support the 408-foot spire. The spire was fabricated as 16 separate sections at a factory near Montreal, Quebec, and was transported by barge to New York City in mid-November of that year. 

On May 10, 2013, the final component of the skyscraper’s spire was installed, making the building, including its spire, reach a total height of 1,776 feet, representing the date of the Declaration of Independence.

Negotiating the Wind Forces

Optimizing One  World Trade Center for high winds was unique as the tower’s design included a geometrical shape that helps reduce exposure to wind loads.

Additionally, the core has reinforced concrete which provides the main support against resistance to the wind forces and other forces of nature.

The Observatory

View from Freedom Tower Observatory
View from Freedom Tower Observatory of downtown Manhattan taken on opening day. Photo:  SMS – Photos of a Lifetime ©

The One World Trade Center observatory opened on May 29, 2015, and is currently the highest of the four observatories in the city at  1,268 feet.

There are three floors, including exhibits and a restaurant.

The most convenient way to purchase tickets would be to purchase them online.

 

Surrounding Area

World Trade Cener North Tower Reflecting Pool
World Trade Center North Tower Reflecting Pool. Photo: Wikimedia Public Domain

Visitors who come to the Freedom Tower should also visit the 911 Memorial, which is a tribute to the 3,000 people who were lost, including the first responders.

The memorial contains the footprints of the former Twin Towers. It has continuous running water over two one-acre pools, one for each of the towers, called “Reflecting Absence“, signifying the physical void left by those who were lost. 

Skyscraper Wind Forces and How to Overwide Them

NYC Midtown Skyline Western View
NYC Midtown Skyline Looking West from Long Island City. Photo: Photos of a Lifetime

Overview

Skyscrapers are defined as being at least 330 feet (100 meters) high with supertalls classified as 984 feet (300 meters) and megatalls at 1,968 feet (600 meters) or higher.

There is no doubt that these structures are a marvel of modern engineering. From the Burj Khalifa in Dubai to the Shanghai Tower in China and the famous Empire State Building in New York, they stand as a testament to human ingenuity and perseverance. For architects and engineers, the challenge to design them is complex.

From the foundation to the roof, they are carefully planned and executed which might take years to complete before even one brick is laid down. Considerations towards building codes, structural stability, aesthetics, and of course economics are primary factors to be studied.

Exploring the making of skyscrapers is an exciting journey for anyone interested in how tall buildings are constructed and is a credit to the dedication of the architects and engineers who build them.

Designing the Structure 

3D rendering of a modern building with construction using CAD software
3D rendering with construction specifications using CAD software

Economics always comes into play, so whatever the planned design is, it must be within the developer’s budget. Architects use computer-aided design (CAD) software to create 3D models of a building.

Usually created on networked desktop computers, CAD is used primarily for analyzing and optimizing a building’s design. Above is an example of how the software is used.

Of course with skyscrapers, there may be hundreds of CAD diagrams that would be needed. The software includes the building codes that they must follow.

Building the Foundation 

Foundation of One World Trade Center
Foundation of One World Trade Center 4/6/2008. Photo: Photos of a Lifetime

It should go without saying that structural stability is of the utmost importance and the foundation is the first step in helping to buttress the building from the forces of nature to which these buildings may be subject. 

Beginning with the foundation, engineers must determine if the soil below the building is strong enough to support the structure. A good example is in New York City where there is solid bedrock that makes it perfect for the construction of skyscrapers. 

Rebar at construction site on Long Island
Rebar ready for pouring of concrete at the construction site. Photo SS

Steel and concrete are the most commonly used materials for foundations. Concrete is strong under compression, but not as strong under tension, and in its pure form, it is unsuitable to withstand the stresses of the wind forces and vibrations,

To compensate for this lack of tensile strength, workers pour a liquid concrete mixture into a wire mesh steel frame, called rebar or reinforcing bar, which strengthens the tension component of the concrete. Together, the product is known as reinforced concrete and forms a strong solid foundation to support any tall building. 

Enter the Forces of Nature

Burj Khalifa
Burj Khalifa, Dubai, UAE – Tallest Building in the World. Photo by Wael Hneini on Unsplash

The engineers must consider the wind loads (wind force) that hit the buildings which can cause them to sway. The higher the building is, the more wind it will be subject to. Skyscrapers will sway from strong winds and can easily move several feet in either direction.

To reinforce the structure to withstand these winds, there are several options that engineers will use.

 

Strong Internal Cores

Empire State Building
The Empire State Building employs the core concept along with its setbacks. Photo by Ben Dumond on Unsplash.

One of the most popular methods for mitigating wind forces is the ability to build strong cores in the center of the building. Usually constructed around the elevators are solid steel and/or concrete trusses, braced by steel beams.

All the tall buildings of the 20th century have this method and it is still going strong into this century, but usually, there are more obstacles to the wind added, especially if the building is of the super or mega tall variety.

Corner Softening

Taipei 101 in Tiawan China
Taipei 101 in Taiwan China uses corner softening to dampen the winds

This is a style that softens the edges of tall buildings to reduce the vortices (strong winds) that these structures are subject to.

The 1,667-foot Taipei 101 in Taiwan uses this method which is very effective in controlling high winds.

But that’s not all Taipei 101 uses against wind vortices as we will see below.

 

Setbacks

New York Telephone Building NYC 1926
Lower Manhattan’s NY Telephone Building was one of the first to employ the setback method. Photo: ©Joseph H. Sachs 1926

In 1916, due to the effect that tall buildings were having over the streets of Manhattan, specifically the 555-foot tall Equitable Building that was completed a year before, a new zoning law was introduced that would force developers to apply setbacks to all tall buildings.

New York City’s Empire State Building and Chrysler Building are excellent examples of the esthetic beauty of the art deco structures with the required setbacks.

This is why all NYC art deco buildings of the time had these, which unexpectedly caught on from an esthetic viewpoint and many structures throughout the United States and the world followed suit. Not only were setbacks desirable but, from an engineering point of view, they helped to diminish the harsh winds that tall buildings were subjected to.

Twisting

Shangai Tower
Shanghai Tower (right). Photo by qi xna on Unsplash

Spiraling skyscrapers are becoming more popular. Not just for their aesthetic appeal, but also because of their ability to reduce wind vortexes by up to 24%.

For the Shangai Tower, this resulted in a reduction of $58 million that the developers did not have to add to buttress the building.

Even more, aesthetically pleasing is the 1,417-foot tall Diamond Tower in Jeddah, Saudi Arabia. If that’s not intriguing enough, there is the 1,273-foot Dubai Tower that not only twists but also rotates 360 degrees. Built by Italian/Israeli architect David Fisher of Dynamic Architecture, the building gives spectators and residents alike an ever-changing view of the Dubai skyline.

Tubular System

Willis Tower Chicago
Willis Tower in Chicago uses a tubular system to mitigate the wind. Photo: ©SMS

The Willis Tower in Chicago is an excellent example of buildings that employ the tubular method for addressing wind forces.

This super tall consists of a collection of nine tubes supporting each other, subsequently, buttressing the building to fight off the winds more than if it was just one straight up-and-down structure.

Additionally, since they level off at different heights, the wind forces are inherently disrupted.

 

Cutout

A less used method but efficient nonetheless. The wind is allowed to pass through specific areas of a building. which reduces the wind loads on the building. Below is an animation demonstrating how the building negotiates the wind forces using cutouts on various floors.

When employing the cutout wind method, other methods of wind optimization are used along with it.  New York’s 432 Park Ave makes use of this system. Additionally, the building uses the tuned mass damper system as described in the next section.

Tuned Mass Damper (TMD) (AKA harmonic absorber)

Damper illustration for Taipei 101
Tuned Mass Damper illustration for Taipei 101

Large, heavy dampers, usually near the top of the building compensate for building vibrations, such as high winds. Similar to a pendulum that sways back and forth, these dampers will move against the wind thereby stabilizing the building.

In technical terms, the mass damper is designed to work in harmony with the oscillation frequency of the building from the wind, thereby reducing the overall sway of the structure. 

Combination of the Above

Many times, multiple of these strategies are used to tackle the vortices, and in so doing, can be very effective in taming the wind. 

Constructing the Superstructure 

The superstructure is the actual building, more specifically, it is the framework that connects the foundation to the roof, and this is where the CAD model is pertinent. The CAD model will help to identify the best location for the support columns, known as the centroid. 

Engineers will select the most economical material and size of steel for these columns. They must also ensure that the columns are spaced far enough to resist both wind and potential seismic forces. Consideration will go into the type of concrete material and the size of the floor slabs, based on how much weight the slabs must support.

They must also consider the thickness of the slabs based on the amount of deflection allowed by the building codes, but we’ll leave the details of these considerations to another article that provides the specifics of these components.

Summary

The engineering behind the making of skyscrapers is a complex and lengthy journey. When engineers design a supertall, they must consider many different aspects of the project. They must select materials that can withstand the forces of nature such as high winds, heavy precipitation, and even earthquakes.

They must also ensure that the building is structurally sound. In today’s skyscrapers, you can rest assured that the hard work and dedication that was put into each of these buildings by the architects, engineers, and construction workers make these buildings sound and secure. 

 

7 Buildings that Use Cantilever Architecture

Citicorp Tower looking up
Citicorp Tower, NYC. Photo: Wikimedia CC

In the 19th century, with the advent of structural steel, engineers began using cantilevers to construct taller buildings. This type of architecture is primarily used when there isn’t enough space on one side of a structure for its foundation. Engineers have to build the foundation out from one side and then use beams that extended from it to support the weight. 

This construction style is eye-catching and certainly more daring than other methods of building. It also requires serious engineering skills, as well as a detailed understanding of how much weight the beams can bear without giving way. Indeed, the correct structural engineering is imperative as just a small miscalculation in the production of steel and concrete can result in catastrophe.

‍If you live in a big city, you might have noticed that more buildings are being built with these overhangs. This is especially true for cities where space is at a premium, such as New York City.  In this article, we are going to take office building construction to a whole new level – the use of cantilevers!

The Citicorp Center, New York City

Citicorp Tower cantilevers
Citicorp Tower cantilevers. Photo: Wikimedia CC

​If there was ever a building that emphasised cantilever design it would be the Citicorp Center in midtown Manhattan. Completed in 1977, the 59-story, 915-foot-high skyscraper sits on three stilts with an internal core at the center.

The building is ​structurally sound now; however, thanks to an observant doctoral student at Princeton University, a discrepancy was discovered when wind forces hit particular angles of the building.

In 1978, Diane Hartley, who was writing a structural engineering thesis, found that the engineer’s calculations did not match hers, which was disturbing since it indicated a possibly dangerous situation.

Should a strong wind ​happen​ to hit the building’s corners, the possibility of the building toppling over had a chance of collapse. It was a one-in-16 chance, but the danger still existed.

Hartly proceeded to notify William LeMessurier, who was the chief structural engineer. He checked her math and realized she was correct.

Quietly, LeMessurier proceeded to correct the issue. In ​coordination with the NYP​​D, an evacuation plan was enacted, which covered a ten-block radius. An evacuation almost materialized as a hurricane was forecast to be heading towards NYC. Hurricane Ella was on its way but moved away from the city at the eleventh hour.

Interestingly enough, word did not get out about this until 1995 when it was published in the New Yorker Magazine.

In addition to the skyscraper’s unusual cantilevered design, the developers used their ingenuity to build a large solar panel at the top of the structure; hence, the slanted roof at the top points south. But the idea never materialized, the slanted roof remains as an esthetic addition to the building.

The Rotterdam Tower

De_Rotterdam Tower showing cantilevered construction
Photo: Wikipedia-CC

This intriguing building is located in the Netherlands and is part of the Erasmus Bridge Complex. It is a mixed-use building that houses offices, a hotel, and apartments. The building has a cantilever design, which is why the residents can enjoy a gorgeous view of the river

The architects designed the building so that it extends out over the river and almost touches the bridge. They also designed it so that it is taller on one side. The weight of this building is distributed between its central core and its cantilever, which is why it can be so tall without the ground beneath it being affected.

Statoil Regional and International Offices

Statoil is an energy producer in Norway and the 57th largest company in the world. Norwegian architects A-Lab designed a 117,000-square-meter commercial building complex that fits into the picturesque shoreline of Fornebu in perfect harmony.

Additionally, this architectural expression injects new energy into the nearby park and commercial area and was a key challenge in their design. Of course, it is the overhangs that make the building stand out. They stretch up to 100 feet in many directions.

Marina Bay Sands Hotel

The Marina Bay Sands Hotel is considered one of the most impressive hotels in the world. It is a massive construction project that began in 2003 and was completed in 2011. The project was a collaboration between the Las Vegas Sands Corporation and the Singapore government and was built on the site of a former shipyard. The hotel has three 55-story towers. but in addition to these buildings, it has a sky park that is cantilevered over all three towers.

Designed by Israeli architect Moshe Safdie,  the hotel has 2,500 rooms and a lobby that crosses the entire three buildings just like the sky park above.

Marina Bay Sands Hotel Sinagpore

Marina Bay Sands Hotel by architect Moshe Safdie. Photo by Julien de Salaberry on Unsplash

Building the Hotel

One of the most interesting aspects of the construction of the hotel was that developers used an unusual design that allowed them to build upwards while keeping the foundations stable.

This was necessary because Singapore is located on a floodplain, and it is impossible to build foundations below ground level, so the engineers designed the foundation so that the bottom of the hotel would be constructed on a metal mesh, which would be anchored to the ground. The mesh would keep the foundation stable while allowing sand and water to flow freely through it. The foundation is built in modular sections, which can be raised and lowered as necessary. The builders also used a system of shuttles to transport construction materials to the upper floors of the hotel, as well as the rooftop.

Lessons Learned from MBS’s Construction

As we have seen, the construction of the Marina Bay Sands Hotel was a challenge. It is rare for the ground to shift so dramatically in an area where there is no flooding, and it is even more unusual for builders to build on top of a metal foundation. Although this construction project was unique, it still provides some important lessons for other builders.

The first is that challenges are an inevitable part of construction, and there are always several factors that have to be taken into account. The second is that challenges should not be seen as a reason to abandon the project. When building on the water, the builders of the Marina Bay Sands had to be flexible, and ready to make adjustments at any time. If they had been too rigid, they may not have been able to proceed with the project at all.

One Vanderbilt – New York City

Vanderbilt Office Building under construction
Vanderbilt Office Building under construction. Photo: ©SMS

With space so much at a premium in this city, the only way to build is up, and even then, it might not be enough to encompass the amount of office space that the developers envisioned for the Vanderbilt tower.

Located across from Grand Central Terminal, it is the fourth tallest building in NYC, rising 1,401 feet above the ground. On the south and west sides, it is cantilevered over Vanderbilt Ave. and 42nd Street respectively, and this overhang starts at only approximately 50 feet up and then supports the rest of the superstructure. There is an observatory at the top, which is the 5th observatory in Manhattan. 

Other skyscrapers with noticeable cantilevered construction in New York include Central Park Tower and the Citicorp Headquarters, displayed above. 

J. P. Morgan Chase Headquarters – New York City

Steel Cantilever at Chase Bank Headquarters
Steel Cantilever at Chase Bank Headquarters Under Construction. Photo: ©SMS

Also known as 270 Park Ave., this 1,388-foot-tall, 70-story, 2.5 million-square-foot super tower is located between Park and Madison Avenues, and 47th and 48th Streets.

This massive building will be supported, in part by steel cantilever columns that protrude diagonally out on the eastern and western sides of the building.

Interestingly, the building is replacing the former Union Carbide 52- story tower (later bought by Chase) that was previously there. The building was completely demolished, which made it the largest intentionally demolished building in the world.

The new Chase headquarters will have zero carbon emissions and will be 100% powered by New York hydropower in upstate NY, which produces electricity completely from flowing water.

No doubt, this will be one of New York’s most advanced skyscrapers.

Frank Gehry’s Chiat/Day Building

Binoculars Building, Los Angeles
Binoculars Building, Los Angeles, CA. Photo: Wikimedia CC

This building is a former office building in Los Angeles, California that was converted into a mixed-use building. It is now home to a variety of businesses, as well as the famous advertising agency Chiat/Day.

Designed by notable architect Frank Gehry, this building with a cantilever on one side so that it could house all of the businesses. They designed the cantilever so that it wouldn’t cause damage to the building’s foundations.

The building’s cantilever also allowed designers to create an interesting façade. They were able to extend the second floor out so that it creates a terrace, which is accessible from the sidewalk.

Summing Up

The cantilever is an interesting architectural feature that many people likely do not think about as they walk under these overhangs, but it is a complex engineering solution that isn’t suitable for every project; however, in these examples, it works brilliantly.

While they may be pretty to look at, they also serve a critical function, which makes them a necessity. While the specific structural design of each cantilever will vary depending on the building type, design, and geographic location, the overall concept is the same.