SEISMICALLY CHIC

One of the tallest build­ings in an earthquake hot zone had to bal­ance safety and style.

Los Angeles Times - - WILSHIRE GRAND -

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.

Mel Mel­con Los An­ge­les Times

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.

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