One of the tallest buildings in an earthquake hot zone had to balance safety and style.
On engineer Leonard Joseph’s computer screen, the Wilshire Grand was an apparition of white lines f loating calmly in black space.
Then Joseph clicked his mouse, and the 73-story tower began to move, slowly at first, then more violently, as a simulated earthquake, magnitude 7.8, shook its foundation.
The skyscraper bowed, swayed and wobbled. Joseph was incredulous. “It’s like those inflatable figures on the roadside,” he remembered thinking.
If the tower were to dance like that, he realized, it would never stand. The more it bent, the more the gravity load would increase the bending, and down this billion-dollar hotel and office project would fall.
Joseph knew the computer had amplified the movements 50-fold to make the trouble spots obvious, a 150-foot bend being more conspicuous than a three-foot bend.
Even so, he found the images disturbing, a reminder of the risk of raising a skyscraper in Southern California.
The structure at the corner of Wilshire Boulevard and Figueroa Street is one of the tallest ever built in a seismic hot zone. Its design has undergone the most sophisticated earthquake modeling performed on a building in Southern California. But even that has its limits. “Earthquake design is a fuzzy proposition,” said Joseph. “You can’t ask an engineer to guarantee that a building will never collapse in an earthquake. That is not fair ....
“You can ask that it will behave as well as possible, meeting at least the code requirements. Even that’s a heavy responsibility.”
By the fall of 2014, the challenge of making the Wilshire Grand stand up to fierce ground movements had consumed Joseph for two years.
Early tests showed that the tower needed special bracing at three points to prevent catastrophic failure, but there was another problem.
On the top floor, an earthquake could deliver a whiplash up to 4gs of acceleration, more than space shuttle astronauts experienced during launch.
The results doomed the architect’s original vision for the top of this soaring edifice: a filigree of steel encased in glass and topped by a spire. Rising 300 feet above the tower, the features — too tall, too light — would never survive those top-floor forces.
On that point, there was no room for debate.
“There are some things you can’t negotiate. You can’t negotiate with God or Isaac Newton,” Joseph said.
If every building is an act of defiance against the laws of physics, then a skyscraper is a brazen assault. Vertical forces push down, and lateral forces push sideways, each capable of damaging if not toppling the structure.
Before leading a team of engineers who designed structural elements of the Wilshire Grand, Joseph hadhelped shape some of the world’s most distinguished skyscrapers: the Petronas Towers in Malaysia, Taipei 101 in Taiwan and Shanghai Tower in China.
Los Angeles’ tower, however, proved to be in a class by itself.
Architect David Martin wanted large windows in every room, which required a relatively new style of construction using a concrete core.
To make space for an adjoining plaza, he pushed the tower to a corner of the site, limiting the size of the foundation.
To increase energy efficiency, he gave the skyscraper two narrow sides and two broad ones, like a domino standing on end.
The result was a slender, airy design whose purpose was to be a beautiful hotel, not a fortress against earthquakes.
The engineers were left with the job of having to fortify it.
The Wilshire Grand’s design had to have the right combination of structural elements to keep the building erect when pushed down by gravity or pushed sideways by windstorms and earthquakes, the principal forces that lead to failure.
In 1884, William Le Baron Jenney designed the 10-story Home Insurance Building in Chicago using steel columns and beams instead of bricks and mortar to support the building. For most of the next century, steel girders angled like jungle gyms above American streets.
But steel-frame buildings lose their efficiency at about 60 stories.
In the 1970s, a new technique allowed buildings to shoot skyward. In place of the jungle gym, buildings were held aloft by perimeter columns. With its twin towers standing 110 stories, New York’s World Trade Center was the nation’s grandest example.
The perimeter columns had one drawback, however. They obstructed views.
In the 1990s, advances in concrete technology had led to the conception of a high-rise as two interdependent structures: A concrete core, rising the height of the tower, serves as the central support for a skyscraper built around it. Exterior columns are still necessary, but they are much smaller.
High-rises with narrow concrete cores can be additionally supported with structural elements known as outriggers: braces that form giant triangles with horizontal and diagonal members extending from the core to the perimeter columns.
Together, the outriggers and columns act like ski poles for the concrete core, helping to resist vertical and lateral forces.
The style met Martin’s requirements for the Wilshire Grand. Thirty outriggers, positioned between the 28th and 31st floors, the 53rd and 59th floors and the 70th and 73rd floors, extended from the core.
But that didn’t mean the tower could survive earthquakes.
To engineer Marty Hudson, earthquakes are like fingerprints. No two are alike, which makes it impossible to design a building as unusual as the Wilshire Grand from the equations found in building codes.
Hudson was asked to create simulated earthquakes to test the tower design.
Working with data prepared by the California Geological Survey and the Southern California Earthquake Center, he began by cataloging nearly 100 local faults, poring over analyses of their geometry, type, slip rate and maximum possible magnitude.
Hudson studied how waves of energy, generated by earthquakes ranging from magnitude 4 to the low 8s, moved through the earth across Southern California. From that, he extrapolated how the earth movements would translate into shaking at the corner of Wilshire Boulevard and Figueroa Street.
The goal was to evaluate the greatest jolt that the building could experience.
Hudson then needed to understand how that energy would play out, second by second, as the earth
moved. So he turned to records of actual earthquakes around the world that came from faults similar to those in Southern California and were transmitted through comparable soil conditions.
Data in hand, the next step was to test the information against the Wilshire Grand’s specifications.
Engineers turned to their computers, entering 112,500 lines of information that included such details as the size and location of the beams, columns and walls, along with their strengths, springiness and behaviors when overloaded.
Then they began running a program that pitted Hudson’s earthquakes against the building.
The computations were so complicated that the computer needed nearly three days to run the simulations. The results provided visual representations of the building’s movements and numeric spreadsheets that pinpointed failings.
The team scrutinized the data. Blue numbers meant that a brace or a wall had survived the shaking. Red numbers were trouble.
The tests helped the engineers refine the size and depth of the foundation, which would need to resist as much as 13.2 million pounds of force pulling up and 25 million pounds of force pushing down on each of the 20 perimeter columns as the tower swayed during an earthquake.
The numbers also pointed out a major problem. Strained by the force of Hudson’s earthquakes, the outriggers jammed into the core, delivering more stress than the concrete could absorb. The inside walls between the elevators and stairwells were failing. Joseph saw wide cracks forming in the core.
Looking for solutions, engineers settled ona device known as a buckling-restrained brace. It consisted of a long steel bar encased in a steel box filled with grout. When a building moves, the steel box allows the bar to compress or stretch like taffy without buckling.
Joseph replaced each of the original wide-flange diagonal braces with one or more buckling res trained braces.
He ran new tests, and the core survived. The Wilshire Grand would have 170 of these braces.
Joseph wasn’t finished. He kept returning to the animation.
The 7.8 earthquake — derived from the one that struck Tabas, Iran, in 1978 — turned the skyscraper into a snake with broad undulations coursing throughout the structure. He knew the building could sway up to 8 feet in an earthquake, but these cobra-like movements were different.
Much as harmonics, overlapping vibrations, arise from a plucked guitar string, multiple vibrations occur in a building that has been shaken by an earthquake. These vibrations are waves of movement that travel up and down the structure.
Because of the height of the Wilshire Grand, it can produce more than 200 of these harmonics, jiggling that is caused and compounded by the speed and duration of the seismic waves.
Movement at the base of the tower could amplify into a roller coaster ride at the top. With possible accelerations of 4gs, engineers worried that the crown and spire might buckle or even land in the street “like a Hollywood production,” Joseph said.
Removing those architectural elements was out of the question.
Luminous by day, illuminated by night, the sail-like crown was the building’s hood ornament, a distinctive mark in the city’s skyline. As an aesthetic decision — to show off its musculature — the sail was surrounded by glass.
Architect Martin wanted it to look delicate and lacy with long, Aframe diagonals. He had hoped that its light weight would enable it to withstand strong lateral forces. A magazine editor looked at drawings for the concept and said it looked like the Eiffel Tower, and the analogy stuck.
But Joseph knew that this Eiffel Tower would be unsafe. He had hoped that the reinforced outriggers would solve the problem by controlling the movement of the tower. They didn’t. Engineers considered anchoring the sail to the building with long cables that would allow a gentle rocking. But further tests showed the sail would rock so violently that it would damage the concrete core.
A redesign of the sail into a shorter feature offered no advantage, structurally or financially.
Skeptics talked of eliminating the sail entirely, especially as its cost started to rise.
Martin insisted that it remain. But he had to compromise. The sail had to be sturdier, less light and airy.
Engineers refigured his Eiffel Tower into a 500-ton complex of wide-flange braces, ranging from 22 to 44 feet in length, crisscrossing like a cat’s cradle. “We decided to go with brute force,” Joseph said.
For Martin, the solution meant that the Wilshire Grand would retain its soaring prominence. Not flat-topped like the city’s other high-rises, it could join City Hall as Los Angeles’ other crowned edifice, adapted to the precarious reality of Southern California.
FOR MORE THAN two years, Leonard Joseph has been consumed by the challenge of making the skyscraper stand up to Southern California’s fierce quakes.