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The Power of Concrete in Post-9/11 Construction

September 11, 2022

Concrete.

This ancient building material, in use for thousands of years, gained new respect after the horrors of 9/11.

Skyscraper design has evolved over the decades, but until the World Trade Center (WTC) collapse, the trend had been toward the use of lighter, airier materials to construct our towers in the sky. Tenants and therefore building owners preferred glass skins and open floorplans, with an emphasis on environmental friendliness. Lightweight, high-tech fireproofing materials protected critical support elements.

But how did such construction fare when challenged with the fire and mayhem of September 11, 2001 in the WTC? The upgraded fireproofing materials present were dislodged by the force of the airplane impacts, and so did not perform to specification. Fire-weakened steel support beams sagged, setting in motion a cascade of other material failures which led to the destruction of all 3 buildings forming the WTC complex.

Suddenly, lightweight and airy was out. Massively supportive and naturally fireproof was in. Concrete fit the bill.

Today, new concrete mixes as well as new structural engineering concepts are being incorporated into the construction of modern tall building designs. These and other innovations are examples of how the industry is benefitting from the increased role of technology in construction and its management. 

Traditional concrete is reinforced with steel rebar. The steel strengthens the concrete, and the concrete offers natural fire protection to the steel. (Concrete maintains most of its mechanical properties, without undergoing physical or chemical changes, when exposed to heat from fire.) New ‘ultra high-performance fiber-reinforced concrete’ adds blast resistance to the medium’s strengths as a building material. Tiny steel microfibers are embedded into the mix and result in a concrete that resists cracking. This adds strength to structures that might be subject to blast explosions, including terrorist attacks. Buildings vulnerable to damage from earthquakes also benefit from this new form of concrete. Normal concrete can withstand compression pressure of 3,000 to 6,000 pounds per square inch (psi). Advanced concrete can tolerate up to 30,000 psi.

In the WTC collapse, vertical steel beams softened and lost their ability to support their dependent loads. The new WTC towers, now under construction, use different engineering. Instead of support beams, the weight of the entire structure will be borne by walls 3-feet-thick. These concrete support “spines” run from the ground to the very top floor, and are centered in the building. Without traditional support columns, all of the building’s weight will serve to compress these spines, reducing lateral load resistance for wind and earthquake disturbances as added benefits.

To aid evacuation in the event of fire, the stairways in the new construction have been significantly broadened. Additionally, air pressure within the stairwells will be maintained at a higher level to discourage smoke from accumulating there during an emergency.

Going forward, security in design will remain a prominent concern for architects, construction professionals and others -- all a result of the tragedy of September 11. Concrete may have existed for millennia, but it continues to find new roles to play.


Works cited:

BuilderSpace. (2021, July 18). What kind of glass is used in high-rise buildings? BuilderSpace. Retrieved September 11, 2022, from https://www.builderspace.com/what-kind-of-glass-is-used-in-high-rise-buildings#:~:text=High%2Drise%20buildings%20use%20heat,forces%20from%20the%20strong%20winds.

FAQs - NIST WTC Towers investigation. NIST. (2022, April 5). Retrieved September 11, 2022, from https://www.nist.gov/world-trade-center-investigation/study-faqs/wtc-towers-investigation#:~:text=The%20subsequent%20unusually%20large%20number,inward%20on%20the%20perimeter%20columns.

Public Broadcasting Service. (2011, September 6). How has Skyscraper Design changed since 9/11? PBS. Retrieved September 11, 2022, from https://www.pbs.org/newshour/show/how-has-skyscraper-design-changed-since-9-11

Scientific, G. (2022, February 2). The history of concrete. Giatec Scientific Inc. Retrieved September 11, 2022, from https://www.giatecscientific.com/education/the-history-of-concrete/#:~:text=6500BC%20%E2%80%93%20UAE%3A%20The%20earliest%20recordings,straw%20to%20bind%20dried%20bricks.

Shih-Ho Chao, Associate Professor of Structural Engineering and Applied Mechanics. (2021, December 22). How building design changed after 9/11. The Conversation. Retrieved September 11, 2022, from https://theconversation.com/how-building-design-changed-after-9-11-64580


Capitol Tech wishes peace and healing to all of those affected by the awful tragedy of September 11th, 2001. As society moves forward and develops new technology to prevent such a disaster from ever happening again, we still hold close the memories of every innocent  life lost on that September day, and will continue to ensure that their legacies are never forgotten.