For those building tallest skyscraper in a seismic hot zone, earthquake simulations led to big changes

New Wilshire Grand's engineer has been consumed by making the tower stand up to fierce ground movements

Plans for L.A.'s tallest skyscraper have been tested against 11 of the most severe quakes possible

On engineer Leonard Joseph's computer screen, the New Wilshire Grand was an apparition of white lines floating 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.

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Joseph was incredulous.

"It's like those inflatable figures on the roadside," he remembers 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, particularly one as slender and tall as the New Wilshire Grand. The animation was so sobering that he keeps it under wraps, lest the simulation go viral and raise unfounded fears about the tower's integrity.

Under construction at the corner of Wilshire Boulevard and Figueroa Street, the New Wilshire Grand will be the tallest structure built in a seismic hot zone when completed in 2017. 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, who works for the firm Thornton Tomasetti. "You can't ask an engineer to guarantee that a building will never collapse in an earthquake. That is not fair, and that is not the deal that society has made with the construction world.

“ You can't ask an engineer to guarantee that a building will never collapse in an earthquake. ” - Leonard Joseph Share this quote

"You can ask that it will behave as well as possible, meeting at least the code requirements. Even that's a heavy responsibility."

For two years, Joseph has been consumed by the challenge of making the New Wilshire Grand stand up to fierce ground movements. The forces the tower generates and must withstand in an earthquake are greater than anything he has faced.

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 this point, there was no room for debate.

"When we work with lawyers, developers and some architects, the impression is that everything is negotiable," Joseph said. "That with enough money, anything can be built.

"Perhaps that's how it works in a real estate deal, but there are some things you can't negotiate. You can't negotiate with God or Isaac Newton."

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.

Working with the principal engineers at the firm Brandow & Johnston, Joseph led a team that designed the structural elements of the New Wilshire Grand tower. At 63, he has helped shape some of the world's most distinguished skyscrapers: the Petronas Towers in Malaysia, Taipei 101 in Taiwan and Shanghai Tower in China.

The New Wilshire Grand, however, proved to be in a class by itself, presenting engineers with unprecedented challenges.

Architect David Martin wanted large windows in every room, which required a relatively new style of construction using a concrete core. Martin designed a core just 33 feet wide along its narrowest side for a building 1,100 feet tall.

To make space for an adjoining plaza, Martin pushed the tower to a corner of the site, limiting the size of the foundation.

Just 33 feet wide at its narrowest, the concrete core of the New Wilshire Grand, at the corner of Wilshire Boulevard and Figueroa Street, will provide crucial support for the 1,100-foot-tall hotel and office building.

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.

Consider a tall, narrow chest of drawers. Standing still, it presses into the rug and leaves an impression in the nap. Pushed slowly, it slides across the floor. Pushed abruptly, it tips over.

The New 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.

Mankind's earliest engineers had a relatively easy time by comparison. Real estate was plentiful, so ziggurats could rely on broad foundations. But as cities became crowded, foundations grew smaller.

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.

The evolution of building design

But steel-frame buildings lose their efficiency at about 60 stories. Above that, the columns have to become larger and more closely spaced, cutting into valuable real estate.

In the 1970s, a new technique allowed buildings to shoot skyward. In place of the jungle gym, buildings were held aloft by perimeter columns, a technique known as a tube system. 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 — chemical additives that made the material stronger and easier to deliver hundreds of feet above the street — 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 New 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.

Outriggers

To engineer Marty Hudson, earthquakes are like fingerprints. No two are alike, which makes it impossible to design a building as unusual as the New Wilshire Grand from the equations found in building codes.

Hudson, of the geotechnical firm AMEC, 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, their type, their 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 maximum acceleration — 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 that came from faults similar to those in Southern California and were transmitted through comparable soil conditions.

With the help of an independent review board, Hudson culled through 3,551 recordings of 173 earthquakes taken by 1,456 monitoring stations around the world.

He came up with 11, the best representation of the most severe earthquakes the building would experience.

Data in hand, the next step was to test the information against the New Wilshire Grand's specifications.

With 900 hotel rooms and accompanying office space, the tower will be built around a concrete core that will rise 841 feet, 6 inches. Its walls are 4 feet thick at the base and 2 feet near the top. The entire building will weigh 300 million pounds (330 million fully loaded with guests, their luggage and stocked mini-bars).

Physics 101

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 considered adding more concrete to the walls, but that would crowd the elevator shafts. Placing steel plates inside the walls would slow the construction and raise costs.

One option remained, a device known as a buckling-restrained brace. It consists 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 of the outriggers with one or more buckling-restrained braces.

He ran new tests, and the core survived. The New Wilshire Grand will have 170 of these braces.

Buckling-restrained braces (BRBs)

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 New 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. It towered above the pool, the bar and the hotel's other outdoor amenities on the roof, and 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, A-frame 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.

Underlying support

Engineers considered anchoring the sail to the building with long cables that would allow a gentle rocking. But further tests showed that 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 — more complex than he had wanted — meant that the New 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.

Sources: Leonard Joseph, Thornton Tomasetti; Tammy Jow, Joseph Varholick, Carey McLeod and Noel Moreno, A.C. Martin; Steve Carroll, Schuff Steel; Ian Aiken, SIE Inc.; Nippon Steel Engineering USA; G.G. Schierle, USC School of Architecture; Dave Eichten, Pankow.

Credits: Graphics reporting by Thomas Curwen | Graphics by Lorena Iñiguez Elebee | Production by Armand Emamdjomeh | Design by Armand Emamdjomeh and Lily Mihalik

thomas.curwen@latimes.com

Twitter: @tcurwen