Good News for Tall, Concrete Cores

05/16/2007 Engineering News Record

Tests on Simplified Link Beams Provide Case for Improved Constructibility and Performance

By Nadine M. Post

Surprisingly positive results of performance tests on simplified, reinforced concrete link beams—the most congested part of a shear wall core—have opened the door to more-constructible towers in seismic zones. But engineers suggest the research will have an even greater impact on seismic design of tall buildings, for it offers proof that performance-based engineering can provide better quality than “prescriptive” design.


Beams span openings into core.
The research represents “a major step,” says Ron Klemencic, president of Magnusson Klemencic Associates, Seattle, and the “brainchild” of the tests at the University of California, Los Angeles. Even with only half the tests completed, engineers are a step closer to being able to design buildings in seismic zones to a specific level of performance, he suggests.

The purpose of the research, sponsored by a $200,000 grant from the Pankow Foundation and donated services and materials from MKA and a team of contractors led by Webcor Concrete, San Mateo, Calif., is to test a more-constructible link beam, developed by MKA with a simpler reinforcing-steel configuration (see top drawing, opposite). “The amount of cracking and degree of damage to the beam is much lower than we expected,” says Klemencic.

John Wallace, director of UCLA’s Structural/Earthquake Engineering Research Laboratory and the engineer performing the tests, says the results, even after only three of eight tests, are already “providing more comfort to design engineers.”

Nic Delli Quadri, until a month ago chief of the engineering bureau of the Los Angeles Dept. of Building & Safety and now a consultant to the department, witnessed the May 4 test along with Klemencic. “The expectation was that you’d have chunks of concrete spalling out of the link beam,” he says. “That didn’t happen. Now, engineers can better predict what is going to happen and have more confidence in link beams performing as fuses.” Delli Quadri says that all the tall buildings in seismic L.A. are coming in for review as performance-based designs.

Core wall-link, or “coupling,” beams, which span doorway openings to the elevator lobby or exit stairs, are the most critical and highly stressed components in the structure, say sources. Through computer modeling and quake simulation, engineers are able to calculate the level of stress in the elements. The tests put real data behind the modeling and simulaton, says Klemencic.

Currently, the American Concrete Institute’s model code provides engineers with only one way to detail the beam, Klemencic says. The result is a highly congested and difficult-to-build geometry. Considering the tight rebar-placing tolerances, “it is very difficult [and time- consuming] to properly construct the beams in the field,” he says. That tends to have a significant negative impact on the construction schedule.

The less-congested detail is proposed for the 2008 edition of ACI 318. The detail, appropriate for residential or commercial construction of any height, eliminates some steel hoops around the diagonal rebar. It can slice a day or two off the time it takes to build a floor. “Different engineers have different approaches to the link beam and the surrounding rebar,” says Chris Plue, Webcor’s director of construction. “The best designs can save you a day per floor.”

Webcor has already accomplished this on two performance-based concrete frames in San Francisco, designed by MKA using the same simplified detail used in the test. The 64-story One Rincon Hill project, currently 56 stories, has a three-day-per-floor cycle instead of four, says Plue.

With the simplified link-beam rebar, workers install the core-wall rebar in one day, instead of two or three, depending on the number of openings, Plue adds.

In addition to testing new reinforcing steel arrangements, the research is focused on the kind of incremental damage the beams undergo as they are tested for events of different magnitude. For fairly frequent quakes, every 35 or 40 years, “we found to date that the cracks are very small and no repair is needed,” says Wallace. “They are hard to find.”

The research has also found that if the beam is “pushed” farther to simulate the 500-year or 2,500-year event, damage is two to three times less than currently implied by the code. “We are pushing these to a 10% rotation without structural loss,” says Wallace.

In all, eight configurations will be tested by the fall. The first four represented two beams detailed for residential and two for commercial cores. One of each was detailed according to the current code; the other according to the proposed code with simplified rebar. That allowed a performance comparison to be made.

For the rest of the tests on the simplified detail, rebar will be reduced by half in an attempt to provide just enough to meet the prescriptive code requirement, says Klemencic. The intent is to present a rationale to reduce the amount of rebar required by the code. Wallace, whose report will be issued next year, says it may be possible to make link beams even more constructible.

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