EARTHQUAKE: THE LONG ROAD BACK : Lessons from the Past Hold Up Well : Caltrans Gets Confirmation of Its Quake-Proofing Remedies as Most Retrofitted Overpasses Withstand 6.6 Jolt

Engineers combing through collapsed freeways in the wake of Monday’s Northridge earthquake say they knew how to prevent the destruction but simply ran out of time before disaster struck.

Although the last three major quakes--Sylmar, Whittier Narrows and Loma Prieta--all revealed significant deficiencies in freeway overpass construction, they also taught Caltrans and industrial engineers important lessons about quake-proofing vital arteries, and those lessons seem to have been learned well, experts said this week.

All of the 122 Southern California overpasses that had been retrofitted survived the magnitude 6.6 temblor and, with one exception, the 11 that collapsed were on the list for future retrofitting.

The exception was the Mission/Gothic interchange on the Simi Valley Freeway (California 118), close to the quake’s epicenter. “If Caltrans had known it was sitting on an active fault, it would have been on the list too,” USC engineer James Moore said.

The overpass collapses were traced to well-known mechanisms and “the retrofit remedies that have been and are being installed could almost certainly have saved all of these bridges,” said structural engineer Nigel Priestly of UC San Diego. All the bridges in Los Angeles were scheduled to have been retrofitted by the end of next year.

Nearly all of the bridge failures in Monday’s temblor could be traced to fracturing of the shorter columns supporting the roadbeds. Studies of collapses in previous quakes have shown that the rigidity and brittleness of the shortest columns limited their ability to flex like the taller pillars on the same spans. That appears to be the cause of the failures in Monday’s quake as well.

Scientists say that fracturing can be prevented by jacketing the columns in steel. This has been the major thrust of retrofitting in the last five years.

The retrofitting now being done is based on knowledge gained during the three previous large quakes. The 1971 magnitude 6.4 Sylmar quake showed engineers that bridge spans could slide off their supporting columns if the spans were not physically joined together. Since the temblor Caltrans has installed hinge joints--a series of steel cables that hold the spans together--on all bridges in the state, a task that was completed in 1989. Those have worked successfully.

In the 5.9 Whittier Narrows quake in 1987, engineers were stunned when the supporting column on a ramp connecting Interstate 5 and Interstate 605 burst under compression from the quake. That triggered research on so-called single-column bridges, most of them on- and off-ramps, in which each span is supported by a single column in the center. That research led to the technique of jacketing the columns in steel and concrete for extra strength.

Close on the heels of Whittier Narrows, the magnitude 7.1 Loma Prieta temblor in 1989 put the spotlight on overpasses with supporting columns of different lengths.

The relationship between the two types of columns is roughly similar to that of the reed and the oak tree, Priestly said.

In a windstorm, the reed escapes damage by bending with the wind. It can be bent nearly parallel to the ground without being damaged because of its flexibility, thereby riding out the fiercest storms. But the oak tree resists bending, and thus absorbs the full force of the wind, eventually shattering.

A similar situation occurs on freeway overpasses. Tall pillars bend with the sideways movement caused by a quake, but shorter, more brittle ones resist bending. Because all the pillars are linked by the bridge, however, the bending of the tall pillars causes most of the force to be transferred to the short pillars, which are not designed to absorb such forces.

Therefore, they shatter, losing their load-bearing ability and causing the overpasses to fall. That seems to have happened at nearly every overpass that failed in Monday’s temblor, Priestly said, including the interchange of Interstate 5 and the Antelope Valley Freeway (California 14) in Sylmar.

Some geologists have speculated that unusual vertical acceleration of the ground at that interchange caused the pillars to punch through the overpass, “but that is not our reading of what happened there,” Priestly said. “The short column adjacent to the south abutment attracted a very high shear force. It had little transverse (sideways) reinforcement, so it failed, lost its load-bearing capacity and dragged the rest of the structure down.”

The solution to the problem is simple, experts say, and is virtually the same solution as that for single-column bridges. After the 1987 Whittier quake, engineers began jacketing the columns in steel-reinforced concrete to bolster them against cracking. By preventing large chunks from shearing off, the jacketing renders the short columns more flexible, allowing them to bend as much as five inches, compared to a maximum of about one inch before retrofitting.

After Loma Prieta, Caltrans identified 860 freeway and highway bridges in the state that required retrofitting of the short columns;456 of them are in the Los Angeles area, according to James Roberts, the agency’s chief bridge engineer.

Of those 456, 122 bridges have been retrofitted--including all single-column bridges--and work is in progress on another 43, he said. Contracts for the remainder will be let this year, he said, and “by the end of 1995 they should all be finished.”

It costs an average of $500,000 to retrofit each bridge and requires about a year of work, he said.

Caltrans has received high marks from academic engineers for its methodical approach to retrofitting the bridges. At the beginning of the process, Caltrans assessed each bridge’s age, design, traffic load and proximity to faults. Those with the highest ratings received first priority. The overpasses on the Santa Monica Freeway were of intermediate priority and, in fact, contracts for their retrofitting were scheduled to be issued in the next couple of months. The others that collapsed had lower priorities.

“Caltrans has acted very responsibly within the constraints of available resources,” said USC’s Moore, one of the state’s few structural engineers who does not receive funding from the agency.

About the only criticism that has been directed against the agency involves the collapsed ramp at the Interstate 5/California 14 interchange. The interchange was under construction at the time of the Sylmar quake, when a different ramp collapsed and was subsequently rebuilt. The ramp that collapsed Monday was completed under the old quake guidelines in force when construction began in 1971.

Some critics, such as Assemblyman Richard Katz (D-Sylmar), charged this week that Caltrans should have held off on construction of the interchange until new guidelines were issued. But they were not developed until the end of 1973, Roberts said, and there would have been howls of outrage if the freeway had been delayed that long.

Roberts said retrofitting of the collapsed interchange could have been completed before Monday’s quake had it not been held up for more than a year to obtain special equipment to deal with geological irregularities under the interchange.

“There are some pretty terrible geologic formations down there (under the footings),” he said.

FREEWAY FAILURES

If there is a reassuring element to the roadway failures, it is that there were no surprises. Engineers suspected certain elements of the L.A. freeway system were at risk, and it was those elements that indeed failed. And retrofitting techniques can eliminate many of the potential trouble spots.

Short Columns the Main Culprits

Engineers believe most failures can be traced to less flexible short supports.

* In a quake, long columns survive because they sway slightly.

* Short columns, unable to bend, absorb horizontal energy produced by longer columns and blow out.

The Solution

Jacketing short columns actually makes them more flexible, allowing them to bend up to five times as much. It reduces the force they must absorb and makes them less likely to break.

REPLACING THE I-10

Parts of the Southland’s busiest road--the Santa Monica Freeway--will have to be replaced from the ground up. A look at what has to be done:

1. Damaged freeway demolished and removed.

2. Columns poured, reinforced with steel

3. Girders go up by crane, providing a skeleton for roadway

4. Plywood fitted over girders as forms for roadway’s lower slab

5. Concrete poured; plywood removed

6. Plywood forms placed over lower slab; concrete for road surface is poured

Quick Fix

Officials are considering erecting temporary bridges on the Santa Monica and Simi Valley freeways. The temporary structures would allow a partial flow of traffic on one side of the roadway while workers construct the permanent ones.

Bolstering the Bridge

By wrapping older columns in steel casings, engineers can stop older columns from splitting during a quake. Among the other methods used to make older bridges more resilient in a quake:

Original Concrete Column

Steel Rebar

Polystyrene Wrap

3/8” Steel Casing Pressure Grouted to Existing Column

Hinges: Allow slight shifting

Buffers: Function as shock absorbers

Cable: Straps keep joints from coming apart

Footings: Bases enlarged and piling driven deeper (Tie-Down Rods Drilled 65 Feet Into ground)

KEY FEATURE IN NEW BRIDGES

When the I-10 is rebuilt, its new columns will feature spiral steel rods to hug the components together.

Old Method: Before 1971, steel support bars had no side-to-side support

New Method: Columns are now made with steel rod wrapped around vertical steel supports