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[whitespace] Milpitas Tilt-Up
Photograph by George Sakkestad

What Goes Up: Construction workers raise the walls on a new tilt-up in Milpitas; an older tilt-up (inset) suffered a partial collapse during the 1994 Northridge quake.

Tilting at Danger

Loose soils and cheap construction clash in Silicon Valley's "Golden Triangle." Look up. Is your tilt-up still standing ... up?

By Gil Davis

IT COULD BE the biggest threat to Silicon Valley's future. But this menace isn't a busted stock option, a hostile takeover or a crashing Nasdaq. In fact, despite the fact that it affects everything from warehouses and office buildings to research labs and superstores, this menace almost never appears in the news.

It's the "tilt-up" building, an ever-present symbol of the valley's economic prosperity and the design of choice for quick, inexpensive and hassle-free construction in Silicon Valley for more than 30 years.

So-named because its concrete walls are cast on-site and lifted up to their vertical position by a crane, the tilt-up is Silicon Valley's ubiquitous style of commercial construction.

But because the walls are held vertical by a precarious connection to the roof, structural engineers say, structures built in the tilt-up style are among the most dangerous to occupants in the event of an earthquake.

"Tilt-up construction is very vulnerable to damage and somewhat vulnerable to collapse in a strong earthquake," warns Ronald Hamburger, president of the Structural Engineers Association of California. "It's primary seismic liability, which exists even in the current building code, is its roof-to-wall connections. They can break and allow the tilt-up's concrete walls to fall to the ground. Also possible is the collapse of sections of roof beams and roof panels."

To make matters worse, seismologists say, tilt-ups have proliferated in some of the most unstable areas of the valley floor.

Tilt-up construction exploded in the northern flatlands of Mountain View, Sunnyvale, Santa Clara and San Jose in the 1970s and 1980s. This decade, tilt-up construction has centered on the so-called Golden Triangle, bounded by Highways 880, 237 and 101 and extending north into the baylands of Milpitas.

But all these areas, where San Francisco Bay meets the valley floor, are susceptible to liquefaction during an earthquake. The Association of Bay Area Governments predicts the Golden Triangle will be especially susceptible due to its high water table. The combination of stiff buildings built atop undulating land could prove deadly when the next Big One hits.

The United States Geological Survey says there's a 67 percent chance that the San Francisco Bay Area will suffer a magnitude 7 or above earthquake between 1990 and 2020. (The Loma Prieta quake of 1989 was a 6.9.) But even a moderate earthquake, like the 6.7-magnitude quake that struck Northridge in 1994, could cause hundreds of Silicon Valley's most densely populated buildings to literally shake apart.

Humble Beginnings

THE TILT-UP concept was originally intended as a simple, windowless warehouse that housed only a few personnel. Since then, tilt-ups have been extended to all sorts of configurations such as two-story structures with minimal walls and many windows.

A key factor in whether a tilt-up will survive an earthquake is how well the roof is attached to the tops of the heavy walls.

The walls of a tilt-up are formed on-site, after the floor slab is poured. The flat slab becomes the bottom of a mold for the walls. The form for each wall section is typically 10 to 12 inches thick, 25 feet wide and some 30 feet tall. After workers install reinforcing steel in the form, they pour concrete into the wall segment and allow it to dry. Then a crane lifts the wall sections into a vertical position one by one and they're propped up by wooden braces. The structure becomes rigid when workers fasten the roof to the tops of the walls.

"Picture the seismic waves coming through a building site," explains Don Cushing of ALX Engineering, "Just as a sailboat's mast will begin swinging wildly back and forth, the concrete walls will try to do the same. What keeps the walls from falling to the ground is the strength of the connection between the wall and roof."

If the ground undulates quickly enough, the top of a tilt-up's wall could easily experience the same force as the earth's gravity, which is called 1G, he says. If the concrete wall panel weighs 50,000 pounds, then the wall-to-roof connection must be strong enough to hold the panel's weight of 50,000 pounds, which is equivalent to 1G of force. Of course, this static force will become even larger as the dynamic forces of an earthquake begin to move things around.

House of Cards

THE FIRST dramatic warning about the seismic deficiencies of tilt-ups came during the 1964 Alaskan earthquake (magnitude 8.4) in which three of the five bays of an Elmendorf Air Force Base warehouse fell to the ground.

Since then, improvements to the Uniform Building Code have been largely reactive.

The 1971 San Fernando earthquake (magnitude 6.4) inspired numerous code improvements enacted in 1973 and 1976. Before the improvements, building codes allowed the roof-to-wall connection to consist of a 4-inch-thick wood ledger that was attached horizontally near the top of the inside of the concrete wall. Nails were driven through the plywood roof into the ledger at specified intervals, like every three to six inches.

That situation was fine when the ground remained still, but when a tremor shook the ground even moderately, the nails tended to pull through the plywood or the ledger split, separating the roof from the wall.

The 1976 revision to the building code required steel connectors for the critical roof-to-wall joints. Tilt-ups built in the late 1970s, 1980s and much of the 1990s are equipped with the safer steel connectors.

But those, too, proved inadequate during the 1994 Northridge quake, when even the steel joints were found to be too weak to resist even a moderate seismic event.

Northridge left more than 400 tilt-ups--out of 1,200 existing in the San Fernando Valley--with a partial roof or exterior wall collapse, according to a City of Los Angeles report. No one was killed by falling debris largely because the quake took place before normal working hours.

Especially troublesome was the fact that the more stringent requirements established in 1976 didn't prevent 25 percent of the supposedly stronger tilt-ups from experiencing the characteristic roof-connection failure which precedes collapse.

Once again, structural engineers went back to the drawing boards. In 1997 the building code was revised to require both stronger steel connectors and stronger seismic demands on any buildings near an active earthquake fault. Also part of the new regulations were higher seismic design forces for structures located on soft soils that can amplify an earthquake's ground motions up to 7.25 times what is experienced on bedrock.

The new regulations took effect July 1. But since there is no mandatory retrofit program to make older tilt-ups comply with current codes, structural engineers are increasingly dubious about the earthquake survivability of tilt-ups built before this year.

Tilt-Up
Photograph by George Sakkestad

An older tilt-up suffered a partial collapse during the 1994 Northridge quake.

Surfing Soil

A LARGE QUAKE temporarily turns the earth into something between a solid and a liquid as waves literally fan out across the region. A single point of ground can initially rise and fall as much as 3 feet while lurching back and forth horizontally up to 6 feet in a second or two.

Liquefaction occurs in loose, water-saturated sands as they are shaken by an earthquake. The water portion of the equation is routinely provided by San Francisco Bay, which lifts the water table as close as 7 feet beneath a building's slab.

The Association of Bay Area Governments expects vast areas of Silicon Valley to be susceptible to moderate to high levels of liquefaction during a major quake.

Major liquefaction along Coyote Creek, which runs between north San Jose and Milpitas, was first recorded following the 1906 San Francisco earthquake, says Tom Holzer of the USGS in Menlo Park.

Holzer says a second major problem for the Bay Area is the propensity of bay muds to amplify earthquake motions beyond what would be experienced on nearby bedrock.

Amplifications of up to 3.5 times were measured during the Loma Prieta earthquake at Treasure Island. Amplifications of 7.25 times were recorded during the Northridge earthquake, which led to the collapse of the Interstate 10 freeway at La Cienega Blvd.

Together, liquefaction and amplification could deliver a deadly one-two punch to the Valley's buildings that are constructed on soft or water-saturated soils.

Tilting at Windmills

LOCAL BUILDING officials don't keep statistics on tilt-ups, but Zone Four, a private engineering firm specializing in tilt-up retrofits, estimates about 5,000 are standing in Santa Clara County.

Sheila Lee, building official with the City of Santa Clara, says most of her city's tilt-ups were constructed in the 1970s and don't comply with current building codes.

"There is a potential for some damage, but we don't know how much," she says. "This city doesn't require owners to bring their tilt-ups up to code, and I'm not aware of any local city that does."

Lee couldn't recall seeing a seismic retrofit for a tilt-up since she started her current job six months ago.

Sunnyvale building official Ali Fatapour says Sunnyvale had "tons" of tilt-ups built during the construction boom of the 1980s, and then again in the last years of the 90s.

"During the last few years we're starting to see some more applications for tilt-up buildings and tilt-up retrofits," he says. "We've had at least one seismic upgrade every month for the last three years. The majority of these retrofits are for tilt-up construction."

But, Fatapour adds, the city is not driving the effort. "We can't require building owners to seismically upgrade their buildings unless they change the occupancy [or the use] of the building, or substantially add or remodel the building," Fatapour says.

These strengthenings are motivated by the fact that some high-tech firms realize they can't risk losing their facilities and equipment, which often have a value far exceeding that of the building in which they're located.

Edgar Rodriguez, building official for Milpitas, says there's no doubt that tilt-ups built in his city prior to 1976 are very susceptible to failure because they didn't have an adequate connection between the roof and walls.

But in Milpitas' current construction boom, which has lasted several years, a number of its new buildings are tilt-ups designed to a much higher standard.

"The corporations is this area are very cognizant of the earthquake threat," he says. "Several companies have upgraded the seismic resistance of buildings because they can't afford too much downtime."

Rodriguez says that some areas of Milpitas may experience liquefaction during an earthquake. When one of those areas is proposed for a building site, a professional soils engineer may recommend a very thick slab with a lot of heavy reinforcement to support the building.

Black Box

IT'S IMPORTANT to note that just because a particular tilt-up was constructed prior to July 1, 1999, doesn't mean it will necessarily collapse or come close to a major failure in a moderate or major earthquake, says Dave McCormick, chair of the Structural Engineers of Northern California Existing Buildings Committee.

"Some tilt-ups have already been seismically upgraded to more stringent requirements or were overdesigned in the first place," he says. "A particular building's survivability depends on the characteristics of a particular tremor, the type of land it's built on, and many other factors."

Joseph Sutton of DASSE Design Inc., in San Francisco, says it's his experience that the Silicon companies that take the long-range view are more likely to design their structures more conservatively in the first place and avoid designing to minimum code requirements. "Less-seismically resistant buildings are more likely to be constructed by speculators who want the cheapest possible construction," he says.

Spying a business opening, Zone Four of San Leandro specializes in tilt-up earthquake retrofits.

An example of a Zone Four installation can be seen inside the REI store in San Carlos just west of Highway 101 at Brittan Avenue. A glance up at the ceiling reveals scores of meaty connectors placed symmetrically on both sides of the large beams across the entire roof. They're designed to hold the roof together when the walls start to wobble.

One look at REI's up-to-date installation and even nonengineers can readily see the weaknesses of older tilt-up buildings, explains Zone Four president Rob Lucey.

For example, Lucey says pre-1997 tilt-up construction often used smallish rods on only one side of the large beams where they connect with the heavy walls. Smaller beams are held at right angles to the major beams using U-shaped hangers that can allow the beams to drop to the floor if they come out of the holder during shaking. In other places, the ends of the beams are held together with straps and a few bolts. Bolt holes are often drilled too big in an effort to speed installation. Sloppy installation can greatly weaken the connection's holding capacity and reliability.

The cost of upgrading only the roof, which includes the important roof-to-wall and beam-to-beam connections, starts at about $2-$3 per square foot. Construction problems can increase the price, including difficulty gaining access to beams hidden behind false ceilings.

The retrofit of the REI store was structurally engineered by Ralph Teyssier of Simpson, Gumpertz & Heger of San Francisco.

Asked how much the REI store had been strengthened by the owner-funded retrofit, Teyssier says, "There's almost no comparison between the building's seismic resistance when comparing the 'before' and 'after' conditions. It's like going from 0 to 60 miles per hour."

'Life Safety'

EVEN IN technology-conscious Silicon Valley, most residents are woefully uninformed when it comes to seismic safety: most assume that if a building's door is open, the structure must be safe.

Municipal building codes only have a goal of "life safety," which means that occupants will hopefully be able to live through a harrowing 'quake and be carried out alive.

The building code's claim to promote life safety is based on the state of the art at the time of building design. Newer buildings have caused relatively few deaths in California. Some of it has been luck, but in general, newer buildings have fulfilled the code objective of life safety.

Pre-1999 tilt-ups still have the potential for life-safety problems, more so than some other structures, McCormick says.

Over the years, a given building can be seen as weaker and weaker in terms of what's currently known about seismic safety. And since the State of California has no mandatory retrofit program, except for unreinforced masonry, older structures seem less safe with the passage of time.

San Jose code inspector Amal Sinha says the Loma Prieta and Northridge quakes prompted more and more people to become concerned about the seismic problems seen in various types of construction.

"Whenever there's a major temblor, people are less likely to argue with us about the need to bring their buildings into compliance with current building codes," Sinha says. "Even people who are proposing new construction are more willing to listen to us."

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From the October 14-20, 1999 issue of Metro, Silicon Valley's Weekly Newspaper.

Copyright © 1999 Metro Publishing Inc. Metroactive is affiliated with the Boulevards Network.

For more information about the San Jose/Silicon Valley area, visit sanjose.com.




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