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seismic design

B

A Lesson in

ErtqukeEnineerinBy Paul erling Oyen, P.e.

BUILDINg 1 O ThE gLENDaLE hIgh SChOOLcampus (inGlendale, Cali.) is not unlike other high school buildings in thearea. Built in the late 1960s, the two-story brick structure houses

administrative oces and the school library. However, one ele-ment that will set it apart rom others o its type is the exposedseismic bracing that will become an identiying characteristic othe building, once a seismic retrot is completed this summer.

The roo is a cast-in-place slab over steel beams and tapered steelgirders. The roo girders sit on second-story reinorced concreteand masonry bearing walls, and the second foor is a reinorcedconcrete pan-joist system supported by rst-story reinorced con-crete columns. There are no structural walls in the rst story andno rst-story columns at the corners, allowing the second-story

walls to cantilever one ull bay. The rst foor is a reinorced con-crete slab-on-grade, and the oundation o the structure is com-prised o 18-in-diameter, 30-t-deep, drilled piers in groups o twoto our at each column. Grade beams tie the pile caps together.

Following the 1994 Northridge Earthquake, a review o thecampus identied Building 1 as seismically decient, because othe sot story condition in the rst story. Above the rst storys

nonductile concrete moment rames are ull perimeter concreteand masonry shear walls with ew openings. The second-storysystem is much stier than the rst story. In the event o a signi-cant seismic event, displacement and damage would be localizedto the rst story and prevent energy rom dissipating throughoutthe structure. Non-ductile detailing exacerbates the condition.

Typical o 1960s construction, the existing concrete columnshave non-seismic reinorcement ties that are not spaced closelyenough to provide sucient concrete connement. Additionally,some o the existing concrete rame beams are shear critical underlateral loading. These details prevent the building rom perormingductility and dissipating energy during strong seismic shaking. Assuch, the building would most likely not survive the shaking o acode-level earthquake.

Steel bracing picks up the seismic slack for a concrete high school

building in southern California.

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Elimintin Defciency

To bring the building into the post-Northridge world, the Glen-dale Unied School District initially approached structural engineer-ing rm Simpson Gumpertz & Heger with the task o eliminatingthe sot-story deciency. The obvious solution was to add concreteshear walls to the rst story. However, this option proved unaccept-

able, as the walls would prevent natural light rom entering the build-ing, and the overall result would not be aesthetically pleasing.

Another option we explored or eliminating the sot story wasto use conventional steel braces to stien the rst story. Thissolution required W14x311 braces or stiness, adding severaldrilled piers to existing pier groups, and making dicult connec-tions between the braces and the existing structure. This provedto be ineasible as well, as connections with the code-requiredoverstrength orces could not be made to the existing pile caps.

hndlin Displcement

We eventually settled on a displacement-based design approachusing ASCE 41, Seismic Rehabilitation of Existing Buildings. Thisapproach didnt eliminate the sot story but rather improved the

structures ability to saely handle the expected large displacementsat the rst story. The method determines the deormation demandsat the rst story rom the design-level earthquake. The structuralperormance is predicted by comparing computed inelastic deor-

mations to acceptable deormation limit states. The design objectiveor a seismic rehabilitation is to make the imposed seismic deor-mations stay within the limits. This can be done by decreasing thedeormations (stiening the structure) or increasing the limits (add-ing deormation ductility). Both were done to Building 1.

We reduced the displacement demands on the structure by stien-ing the rst story, though not enough to eliminate the code-denedsot story. This was accomplished by adding steel bracesHSS8x8x5/8

with two -in. x 5-in. side platesat each o the end bays (eight total).The weight o each brace rame was reduced rom 8,000 lb to 1,500 lb.Accounting or the additional steel in the column jackets, the total steelweight went rom 64,000 lb to 30,000 lb.

We increased the buildings displacement capacity by selectivelycutting existing interior column reinorcement bars, jacketing all rst-

eht st bcsHSS8x8x5/8 wth two 3-. x 5-. sd ptspovdssmc focmt fo gd Hh Schoo. ims: Cots SgH

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story columns with grout-lled steel shells,and incorporating riction dampers in eachnew steel brace.

Though counterintuitive, cutting exist-ing reinorcement bars in the concrete col-umns protects the concrete rame beamsrom catastrophic shear ailure. The weak-ened columns act like a use. Under lateralloading, the columns will now hinge plasti-

cally prior to exceeding the shear capacityo the beams. The grout-lled steel shellsprovide connement at these plastic hingelocations to considerably increase the rota-

tional capacity o columns, thus increasingthe displacement capacity o the structure.

Dul unctionlityIn this design the riction dampers have

two unctions. They allow the braces todeorm axially or the ull design target dis-placement o the structure without buckling,and they limit the amount o orce that can be

imparted on the oundation and the connec-tion to the existing structure. Friction dampersare essentially two plates o steel sandwichedtogether by pretensioned bolts in long slottedholes. The tension in the bolts and the aying

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surace between the plates is calibrated toallow the plates to slip under a specied load.

The riction dampers in this rehabilitation aredesigned to slip at 200 kips, providing +/- 4.5in. o displacement capacity. They are attachedto the top o the existing concrete columns bythrough-bolted steel gusset plates. Each bracesits on new concrete pile caps over drilled piersat each corner. These pile caps are tied to theexisting pile caps with new grade beams, andno additional oundation work was necessary.

Again, the rehabilitation design was per-ormed to the ASCE 41 standard, which

uses the concept o perormance-basedengineering. The perormance objective orthis building is the Basic Saety Objective,

which corresponds to the implied peror-mance o a code-based design or an ordinarybuilding. We perormed nonlinear pushoveranalysis to evaluate the seismic demandsusing CSIs PERFORM-3D. We modeledexisting concrete moment rames with steelshells as nonlinear rame elements and braces

with riction dampers as elastic perectlyplastic axial springs with a yield orce equalto the specied 200-kip slip load. Second-

story shear walls were elastic shell elements,and the orces in the walls showed that theyremain elastic under seismic loads.

Paul Erling Oyen is Staff II Structures withSimpson Gumpertz and Heger, Inc.

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