HistoryLesson May/June 2020ff...the toppling force. Accordingly, the members need only ac-count for...

The marvel of, and controversy surrounding, New York City’s famous Citicorp Center—the gleam-ing aluminum-sheathed tower with the 45-degree-

angle crown—began with St. Peter’s Lutheran Church. The church had long occupied the southeast corner of Lexing-ton Avenue and 54th Street in Manhattan, and in the early 1970s when the church agreed to allow banking giant Citi-corp to build a tower within its air rights, it did so with a few conditions. The bank was required to demolish the old church and build a new one on the same site. And the new skyscraper could not intrude on the new church in any way.

The consequence of this arrangement yielded a landmark of engineering design—as well as a fascinating story about potential structural de� ciencies that highlights the ethical re-

sponsibilities of all who call them-selves professional engineers.

The 59-story, 915 ft tower—now called Citigroup Center—was not only distinct for its architecture but also for its structural engineer-ing. The presence of the church re-quired radical rethinking. Instead of columns coming down to meet the foundation at the corners, archi-tect Hugh Stubbins Jr. and struc-tural engineer William J. LeMes-surier opted to plant them in the center of each side of the site, and then raise the actual building 114 ft atop the columns. The effect looks like a giant skyscraper placed on stilts, with the building corners cantilevering by 72 ft.

LeMessurier, the founder of Cambridge, Massachusetts-based LeMessurier Consultants, famous-ly conceived the structural sup-port system on a napkin in a Greek restaurant. “The thought process went like this,” he told an audience of engineering students at the Mas-sachusetts Institute of Technolo-gy (MIT) in 1995 in a presentation that can be found on YouTube. “If we’re gonna have a big column at the bottom, why don’t we just run the big column all the way up and pick up the loads of gravity com-ing down in increments and guide it down to the center column [via]

diagonal[s] in compression?”His system called for 48 diagonal braces, resembling

chevrons, organized in six eight-story sections that would carry the load onto the mast columns on the sides of the building. Toward the bottom of the elevated structure, a two-story truss would transfer horizontal shear loads to the core, while the columns would carry the gravity and wind forces to the ground.

Any tall-building engineer must consider the impact of wind. The main winds that concerned LeMessurier were per-pendicular or face winds—the winds that hit a building straight on its side and can cause a tower to sway side to side in a process known as vortex shedding. “If you have a � uid moving by an object that is symmetrical, you will get turbulence developing, W

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[46 ] C i v i l E n g i n e e r i n g M A Y / J U N E 2 0 2 0

.HI S TO RY LE S S ON.

Taking Responsibility: The Citicorp Center

The bulk of the 59-story, 915 ft tower—now called Citigroup

Center—sits 114 ft aboveground atop four massive columns.

The innovative design created corners that cantilever by 72 ft to avoid a church on the site.

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alternating on each side,” he said in his presentation, “and the time intervals of those turbulences forming—which are called vortices—they’re like tornadoes up and down the building.”

LeMessurier’s solution was a tuned mass damper, a giant concrete box in the top �oors—�oors originally planned to house high-end condos—that would counteract the motion of wind on the building. The damper—29 ft square, 8 ft thick, and weighing 400 tons—�oated on pressurized oil bearings and connected to linear gas springs recharged with nitrogen gas.

The Citicorp Center opened in 1977 to great acclaim, and LeMessurier’s tuned mass damper began to draw interest from designers of other skyscrapers. But in June 1978, Princeton University engineering student Diane Hartley wrote an un-dergraduate thesis challenging how well the building would do facing quartering winds, the winds that strike a building on its corners. She got in touch with LeMessurier’s of�ce and was reas-sured that the building was �ne. But word of Hartley’s inquiry got back to LeMessurier, and he de-cided to analyze the effects of the quartering winds himself.

He found what he described in his presentation as a “very pecu-liar behavior.” In the face of quar-tering winds, tension on half the diagonal beams doubled, while tension on the other half dropped to zero. In total, the tension on the building from quartering winds increased 40 percent.

As originally conceived, the building was still strong enough to weather even this increase in tension. But, as he began to dig into the construction of his tower, he learned that potentially trou-bling changes had been made. Most notably, those meaty diag-onal beams that were transferring loads down to the columns were not welded, a departure from what was called for in the design speci�cations. The steel contrac-tor, Bethlehem Steel, had offered Citicorp a $250,000 credit to re-place the welds with bolts. Bolts were cheaper and easier to install.

As Joe Morgenstern wrote in 1995 in the New Yorker, which broke the incredible story of Citi-corp Center’s eventual retrofit, “The choice of bolted joints was technically sound and profession-ally correct. Even the failure of his associates to �ag [LeMessurier] on the design change was justi�able; had every decision on the site in

Manhattan waited for approval from Cambridge, the build-ing would never have been �nished.”

LeMessurier told the MIT audience that he had a “bit of a worry” at the news about the bolts, but he didn’t panic. A week or so later he went to the New York of�ce of his com-pany and went through the shop drawings. There he discov-ered another problem.

LeMessurier’s New York staff had wrestled with how the building’s innovative design would meet the city’s build-ing code. According to an article published by the AIA Trust (“LeMessurier Stands Tall: A Case Study in Professional Ethics,” Spring 2013) and written by Michael J. Vardaro, “The force of wind against the building creates tension in the members as they resist the wind’s efforts to topple the building. Since the build-ing has weight, that weight (dead load) naturally counteracts

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An analysis of a structure based on Citicorp Center conducted by Dat Duthinh, Ph.D., A.M.ASCE, and his coauthors shows that for face winds (a), the peak across-wind

overturning moment is about 12 percent higher than the peak along-wind overturning moment. For corner winds (b), the along- and across-wind overturning moments are,

respectively, about 20 percent and 50 percent lower than their face-wind counterparts.

S O U R C E : “ W I N D E F F E C T S O N A T A L L B U I L D I N G W I T H S Q U A R E C R O S S - S E C T I O N A N D M I D - S I D E B A S E C O L U M N S : D A T A B A S E -

A S S I S T E D D E S I G N A P P R O A C H ,” J O U R N A L O F S T R U C T U R A L E N G I N E E R I N G , A S C E , 2 0 1 9 , 0 6 0 1 9 0 0 1 - 1 , R E S T O N , V I R G I N I A

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the toppling force. Accordingly, the members need only ac-count for the difference between the force resulting from the dead load (compressive) and the toppling (tensile) force.”

But how would the New York City building code account for these diagonal braces, his New York colleagues had debat-ed. If they were viewed as columns, the building code called for subtracting only three-quarters of the dead load in calculating the required strength of the building’s structural elements. The New York of�ce of LeMessurier Consulting decided the diago-nal braces functioned as trusses, which weren’t mentioned in the code. Considering them as trusses allowed engineers to subtract the full dead load in their calculation, a decision that meant few-er bolts could be used to secure the diagonal members.

Growing more concerned that his building might truly be vulnerable, LeMessurier returned to the Boundary Layer Wind Tunnel Laboratory (BLWTL) in London, Ontario, Canada, which years earlier had conducted the design’s original wind tun-nel tests. (The BLWTL was named a Historic Civil Engineer-ing Landmark by ASCE last year.) In late July, the lab’s results seemed to con�rm the engineer’s suspicions: the building was at risk. LeMessurier retreated to his summer home in Maine to crunch the numbers and determine how great the risk was.

“What I wanted to know was when was this building go-ing to fall down?” LeMessurier said at MIT. Under what cir-cumstances? He reached two conclusions. If the mass tuned damper worked, it would be enormously helpful in protecting the building. Citicorp Center would be susceptible to failure during a 55-year-return-period storm event. “But if the damp-er failed because a storm knocked the power out, then you had to rely on that structure by itself. The return period to failure was sixteen years.” That meant every year there would be a 1 in 16 chance that a storm would produce winds strong enough to strike the building’s corners in such a way as to bring it down.

LeMessurier quickly sounded the alarm, meeting with Stubbins, the project’s insurers and their lawyers, the leaders of Citicorp, and ultimately city building of�cials. The insur-ers, in particular, “thought I was nutty,” LeMessurier recalled in his MIT presentation. “No one in his right mind calls up and says, ‘My building is going to fall down,’ when there’s no visible evidence of the stress.”

But he succeeded in convincing everyone that a remedy needed to be completed immediately; this was in August 1978, during the middle of hurricane season. Fortunately, the diagonal braces were quite accessible in the building. The connections were made above the �oor, so it was relatively painless to install steel plates—2 in. thick and 6 ft long—to function as bandages on both sides of these joints. “You just open them up, get material, and weld,” LeMessurier said. “You could build all the strength in the joints you want.”

On LeMessurier’s recommendation, another prominent engineer, Leslie E. Robertson, P.E., S.E., Dist.M.ASCE, the lead structural engineer of the World Trade Center twin towers, was brought in to oversee the pending retro�t. LeMessurier estimated the repairs could be done for about $1 million ($4 million today); in the end, the total cost was closer to $8 mil-lion ($32 million today). But once Citicorp signed off, work proceeded swiftly and without incident.

In his presentation to MIT, LeMessurier emphasized his

sense of accountability to uphold the profession’s highest standards as a professional engineer. “If you’ve got a license from the state and you’re going to use that license to hold yourself out as a professional, you have a responsibility be-yond yourself,” he said. “If you see something that is a social risk, you must do something.”

This quote is the crux of the legacy of LeMessurier’s ethi-cal conduct. Citicorp Center was a double win. LeMessurier designed a daring structural solution for an iconic New York skyscraper. Then, when �aws were discovered—�aws that were not caused by LeMessurier—the engineer courageously stepped forward, putting his career and reputation at risk, to blow the whistle on his design. The changes to the building were made, no lives were lost, and LeMessurier, who died in 2007, has been widely upheld throughout the profession for exhibiting the fortitude to do the right thing.

The industry has been largely laudatory in its praise of LeMessurier. While still a student, Chet E. Robinson, P.E., M.ASCE, wrote in a paper that won ASCE’s 1999 Daniel W. Mead Prize for Students that “LeMessurier acted honestly and ethically by admitting that he had made a mistake and then �xing that mistake. LeMessurier’s reputation was not harmed. It was enhanced through the admiration he received for doing what was right” (“Ethics: A Design Responsibil-ity,” Civil Engineering, January 2000, pages 66–67).

Though he did take accountability, LeMessurier later ac-knowledged his mistake was “in designing a structure that was innovative, and I didn’t check and dog people carrying it through carefully enough” (“Critics Grade Citicorp Confes-sion,” Engineering News-Record [ENR], November 20, 1995).

With the bene�t of hindsight, other engineers are less con-vinced that the merits of LeMessurier’s actions are quite as clear-cut as they are commonly portrayed. LeMessurier had been concerned that the corner winds, compared with the face winds, could double the stresses in some of the diagonal braces, and that would cause the structure to collapse in a windstorm. Dat Duthinh, Ph.D., A.M.ASCE, a research structural engineer at the National Institute of Standards and Technology (NIST), disagrees with LeMessurier’s �ndings—against what he ad-mits was his own bias at the beginning toward LeMessurier and his expertise. “LeMessurier put himself forward as a para-gon of ethics, and I just wanted to con�rm that,” says Duthinh.

NIST, which Duthinh says maintains one of the world’s largest databases of wind tunnel tests, has a new method for analyzing wind measurements on buildings. “We have de-veloped a technique that takes all the measurements from the wind tunnel test … directly to calculate the stresses and the forces on the building at any moment in time,” he says.

Wind is chaotic, Duthinh explains. “In the wind tun-nel you keep everything the same. You measure the pressure that the wind exerts on the roof or on the walls of the build-ing. But that pressure does not remain the same. It goes all over the place. There are all kinds of turbulence that happens. Wind effects on buildings is even more complicated than wind effects on an airplane.”

Modern technology permits a far more exact analysis of the data that wind tunnels can generate than was possible in LeMessurier’s era. “They had wind tunnel tests in 1975. They

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could rotate a building, which is equivalent to changing the direction of the wind,” Duthinh explains. “But they could not have hundreds—if not thousands—of pressure sensors all over the building wall or roof,” as researchers may have today.

In reconsidering the Citicorp Center case, Duthinh and his colleagues found that they disagreed with LeMessurier’s assess-ment of the danger that quarter winds posed. “Face winds do dominate the designs, just as the building code had said,” he ex-plains. “We didn’t want to go into the structural problems be-cause we don’t know exactly the details of how it was designed. However, if the conclusion is that face winds are more demand-ing structurally than quarter winds, then if you design your building properly for face winds, it follows that your design will be okay for quarter winds. You design for what is most demand-ing. So the details of the connection don’t matter.”

What accounted for LeMessurier’s apparent error in calcu-lations? Duthinh speculates that LeMessurier may have been overly conservative in measuring movement peaks on both sides of the corners and combining them, even though those peaks would not occur on both sides simultaneously.

Further, questions remain about the of� cial account. The New Yorker article implies that in 1978 LeMessurier calculat-

ed the impact of quarter winds on the building as if this was being done for the � rst time. But in an article by architecture professor Eugene Kremer (“Re-Examining the Citicorp Case: Ethical Para-gon or Chimera,” CrossCurrents, Fall 2002, Vol. 52, No. 3.), Robert J. McNamara, LeMessurier’s managing principal on the Citicorp Center project, is quoted as saying that as far back as 1970, when conceptual designs on the tower began, “the effects of quartering wind were originally studied by Bill LeMessurier.” And LeMessurier “concluded that the quartering wind did not govern the design and need not be further considered,” McNamara wrote.

And in the ENR article, Stanley Goldstein, a se-nior engineer in the New York of� ce of LeMessurier Consultants, claimed that LeMessurier had known all along about the change from welds to bolts.

The New Yorker article references a Citicorp press release that was released to the public that was tech-nically correct but arguably misleading. It indicat-ed that the changes were being made because the BLWTL had told LeMessurier that the probable wind speeds affecting the building might be a bit faster than its 1973 estimates. This was, technically speak-ing, true—but it was not the reason the building was being retro� tted. LeMessurier told the MIT students that he and Citicorp had to “cook up a line of bull” for the public. He said he didn’t want to cause a panic that a major Manhattan skyscraper was in danger of falling. New York newspapers went on strike just as the retro� t was getting under way, so the complete story was hidden until the New Yorker’s article.

Meanwhile, in August and early September 1978, city of� cials were planning an emergency response to be imple-mented if Citicorp Center did indeed collapse during hur-ricane season. Police were ready to evacuate 10 blocks of the densest real estate in America, and the Red Cross had 2,500 volunteers on standby.

Duthinh contends that LeMessurier could have been more forthright about releasing important documents, including the BLWTL’s wind tunnel report and an internal report about the � asco titled Project SERENE. And he is certain LeMessurier was simply wrong in the reasons that motivated him to action.

But taking action was nevertheless the right decision. “The loads he thought [his building] had to withstand were much too high, and he deemed his building unsafe,” Duthinh says. “And he went and � xed it.

“He acted out of an abundance of caution, and he did the ethically correct thing.” —T.R. WITCHER

T.R. Witcher is a contributing editor to Civil Engineering.

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Winds were studied not only as they struck Citicorp Center but also as they “cornered” around such near-

by towers as the Lipstick and Chrysler Buildings.

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