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THE STRUCTURAL DESIGN OF TALL BUILDINGS, Vol. 5, 1-27 (1996)

SEISMIC DEMAND EVALUATION FOR ATHE NORTHRIDGE EARTHQUAKE4-STORY STEEL FRAME STRUCTURE DAMAGED IN

H. KRAWINKLER AND A. AL-ALIDepartment of Civil Engineering, Stanford U niversity, Stanford, C A 94305-4020, U . S . A .

SUMMARYThis paper summarizes results and conclusions from a case study concerned with th e prediction of seismicdemands and correlation of these demands with connection fractures discovered after the Northridgeearthquake. Two adjacent buildings are used for this purpose. One is a 4-story building that experiencedmany connection fractures in a N-S perimeter frame. The other is a 2-story building tha t did not exhibitvisible connection fractures. The discussion focuses on analytical modeling issues and th e interpretationof analytical results obtained from eight series of static (pushover) and dynamic (time history and spectral)analyses. The analytical models are different in each analysis series, ranging from a simple elastic centerlineanalysis model to inelastic models that incorporate th e contributions of the floor slab to the lateral strengthand stiffness of moment resisting and simple frames. Two of th e models are preliminary attempts to modelthe post-fracture behavior of one of the frame structures.

1. INTRODUCTIONThe 17 January 1994 Northridge earthquake has exposed significant problems in steel momentresisting frame structures with welded connections. In more than 100 buildings, fractures weredetected in and around fuii penetration welds connecting beam flanges to column flanges. Eventhough these fractures did not lead to collapses of buildings, they raise concerns about theseismic safety of steel frame structures and may necessitate a performance evaluation of existingstructures. Fundamental questions concern to what extent analytical predictions of seismicdemand can be used to assess connection performance and what consequences for the safety ofthe structural system may result from fractures connections.Structures damaged in the Northridge earthquake provide an apportunity to address thesequestions. This paper summarizes one of the several case studies of a coordinated program inwhich a number of damaged structures were analysed in detail. The case study discussed focuseson the following specific objectives:

(1) to predict elastic strength and inelastic deformation demands for representative ground(2) to evaluate the correlation between predicted seismic demands and observed connection(3) to assess the sensitivity of demand predictions to different assumptions made in the(4) to provide a preliminary assessment of the consequences of connection fractures;( 5 ) to assist in identifying areas needing future study.

motions;fractures;

analytical model;

CCC 1062-8002/96/010001-270 996 by John Wiley & Sons, Ltd. Received July 1995

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2 H . K R A W I NK L E R A N D A . A L - AL ITwo adjacent buildings are used for this purpose. One of them, a 4-story building with steelmoment resisting frames (MRFs) at the perimeter, experienced many connection fractures inone of the N-S perimeter frames during the N orthridge e arthquake. T he other building, a 2-storysteel M R F structure, did not exhibit visible connection fractures. The em phasis in this paper is o nthe demand predictions for the 4-story M R F in which connection fractures were observed.Eight series of static (pushover) and dynamic (time history and spectral) analyses wereperformed, utilizing four recorded ground motions, an equal hazard spectrum, and ninesimulated records that were generated to represent the ground motions of the Northridgeearthquake at the site of the buildings. The analytical models are different in all eight analysisseries, ranging from a simple elastic centerline analysis mo del t o inelastic models that incor pora tethe contribu tions of the floor slab to the m om ent resisting and simple frames. Tw o of the modelsare preliminary attempts to model the post-fracture behavior of one of the frame structures.This paper summarizes properties of the 4-story building and provides a brief description ofthe observed connection dam age. Then i t focuses on analytical modeling issues for the structureand its elements. The element models discussed are approximate, but they serve the purpose ofproviding information on the sensitivity of the predicted results. Much of the pape r focuses onthe evaluation of analytical results, the sensitivity of results to modeling assumptions and therelevance of the results for an assessment of the likelihood of connection fracture.

2 . PROPERTIES OF 4-STORY CASE STUDY BUILDINGA plan view of the 4-story building, which is rectangular with plan dimensions of 11 1 ft x 63 ft,is shown in Figure 1. The structural system consists of complete perimeter MRFs and interiorsimple frames with the beam and column sections shown in Figures 2 and 3. The M R F a t the

tN

Figure 1 . Plan view of 4-story building

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S T EE L F R A M E S E IS M IC D E M A N D E V A L U A T I O N 3+Story Bldg., MRF on L ine D (NS) 4-Story Bldg. , Simple Frame on Llne C (NS)

(a) (b)Figure 2. Sections of M R F o n lin e D (a) and simple frame on l ine C (b). (N-S directio n, 4-story building)

$-Story Bldg., HRF on Llne 1 ( EW ) 4 -S to r y Bldg., Slmplo Frame on Line 2 (EW)W 1 6 X 31 W 1 6 X 2 6 W 1 8 X 4 6 W 1 6 X 2 6

W 1 8 X 3 5 W1 8 X 3 5 W 2 1 X 57 W 2 l X 5 0rn fi 3

W 1 8 X 3 5 5 W 1 8 X 3 5 5 W 2 1 X 6 2 5 W 2 l X S OW 1 8 X 35 W 1 8 X 35 W 2 1 X 5 7 W 2 l X 5 0

b?

TB5 _ f - - CB'MV' 400'/' / / I / '

Figure 3. Sections of M R F o n li ne 1 (a) and simple frame on line 2 (b). (E-W direction, 4-story building)

east side is located 8 ft from the end of the building in or der t o avoid fo undation interferencewith an adjacent building.The columns of the MRFs are embedded into grade beams and anchored to the top of thepile cap and may be considered as fixed at the base. The foundation system comprises drilledconcrete piers with pile caps, grade beams and tie beams. Typical drilled piers are 2 4 diameterand 34ft long. One to three piers are provided at column locations. A 26" x 28" concretegrade beam is used at the gr ound floor with reinforcing bars welded t o th e columns. The columns

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4 H . K R A W I N K L E R A N D A . AL-ALIof the simple frames are connected to individual column footings with base plates andanchor bolts and are considered as pinned at the ends. All columns are made of A-572 grade50 steel.All beams and girders are made of A-36 steel. The girder-to-column connections utilize fullpenetration groove welds at girder flanges and shear tab plates with A325 bolts at the girderweb. A36 continuity plates, a,, less than girder flange thickness, are used opposite girder flangeswith full penetration groove welds to the column. Shear doubler plates are not needed in anyof the joints.The floor construction is steel framing with a 6p thick slab ( 3 p light weight concrete on 3composite metal deck) at floors and roofs. The exterior wall system is thin set brick veneersupported on a metal stud wall.The building was designed according to the 1988 UBC. Th e structural system qualified asa regular structure with an R , of 12 as defined in the UBC. Roofs were designed for 20 psf liveload, and floors were designed for 80 psf live load plus 20 psf for partitio ns. At the time of th eearthquake, the building had not yet been occupied. Thus, almost no live loads and only afraction of the design partition load of 20 psf were present.

3 . DAMAGE DESCRIPTIONIn the 4-story building, a total of 14 bottom flange fractures and two top flange fracturesin beam-flange to column-flange connections and one shear-tab to column-flange connectionfailure were detected. Th e to p a nd bot tom beam-flange to column-flange connection fracturesmay typically be described as partial or complete separation between the weld and thecolumn flange, o r fractures within the weld material. There were many additional cases in whichcracks were detected th roug h u ltrasonic insp ection, but the severity of these cracks was difficultto judge.All but one of the severely fractured beam flange connections were in the N-S momentframe on Line D (see Figure 1). The locations of these fractured connections are shown inFigure 4. The black portion of the fracture symbol indicates the location(s) of the fracture(bottom flange, or both flanges). No fracture was observed in the N-S moment frame on LineA, and only one fracture was observed in the E-W frame on Line 1. Th us it appears th at theN-S frame on Line D was subjected t o much more severe deformations than the other mom entframes.

Figure 4. Location of fractured connec tions in N-S M R F on line D

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STEEL FRAME SEISMIC DEMAND EVALUATION 5

4. ANALYTICAL MODELING ISSUESTwo- and three-dimensional elastic analyses and two-dimensional inelastic static (pushover) anddynamic (time history) analyses were performed. In all analyses, the following dead loads wereapplied a s inital conditions.

Uniformly distributed floors load = 79 psf (including 10 psf partitions)Uniformly distributed roof load = 74 psf (including 6 psf for penthouse)

Weight of exterior walls = 16 psf over exterior surfaceThe loads were distributed to the beams a nd girders according to well established load pa thassumptions. N o live loads w ere applied, since the building was not occupied a t the time of theearthquake.

4 .1 . Computer models of 4-story structureAn elastic model for the complete structure was formulated for three-dimensional analysis,which was needed t o evaluate torsion al effects caused by asymm etry in the N-S direction. Noinelastic three-dimensional analyses were performed. The great majortiy of the analyses weretwo-dimensional, using the computer program DRAIN-2DX. For this purpose, the structure is

modeled as a series of two-dimensional frames linked together at the floor levels by rigid links.Two frames each are used in the N-S and E-W directions, one representing an exterior perimeterM R F and o ne representing an interior simple frame, as shown in Figure 5. In the N-S directionthe frame on line D is chosen as the MRF, since it has a larger tributory floor area than theframe on Line A and is expected to be subjected to larger seismic demands. Torsionaleffects are accounted for in an approximate manner by assigning seismic masses equal totributary masses. The accuracy of this assumption will be discussed later.Th e incorporation of the simple frames is necessary in order to m odel their contributions tostrength and stiffness. Even if all simple beam connections are modeled as hinges, the simpleframes contribute t o strength an d stiffness because the columns tha t are part of the orthog onalMRFs are fixed at the base. Moreover, in several of the analysis models the contributions ofthe simple connections to strength and stiffness are also modeled.The gravity loads are distributed to the individual frames, including the simple frames,according to tributary areas. This is necessary in order to transfer P-delta effects from the

0 0 0 @ 0 0 0 @MRF on Line D Simple Frame o n Line C

Figure 5. Computer model for two-dimensional analysis (N-S direction)

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6 H . K R A W I NK L E R A N D A . A L - A LICOLUMN

JOINTII

BEAM

. SPRING

SPRING

Figure 6 . Element modeling at beam-to-column joint

interior of the structure through the diaphragms (rigid links) to the perimeter MRFs, while stillmaintaining realistic axial loads in the M R F columns.Th e total seismically effective weight for the s tructu res includes all uniformly distributed deadloads, the exterior walls, an d a 10psf partition load on each floor. The to tal seismically effectiveweights for the com puter m odels are a s follows:

N-S: W, = W, = W4 = 356 kKoo f = 312 kF o t =

Koo f = 27 3 kF o , =

1380 k (58% of total weight because of asymmetry)E-W: W, = W, = W4 = 308 k

1197 k (half of total weight)The modeling of elements at a beam-column joi nt is illustrated in Figure 6. Colu mns are

modeled as stan dar d beam-column elements with the AISC Plastic Design M-P interactiondiagram . Since only in very few cases did th e axial beam colum n load exceed O.15Py, he columnstrength was usually equal to M,. Beam s are modeled as linear elastic elements w ith a very largestrength. Plastic hinge rotations at the ends of the beams are modeled by means of rotationalsprings which are given a very large elastic stiffness and a strength eq ual to the bending strength ofthe beam a t the connection. This mod eling facilitates modifications of beam strength p roperties atthe connection and the incorporation of a consistent strain hardening stiffness (as a fraction of6EI/L of the beam). Joint panel zone shear deformations are modeled by means of a rotationalspring connected to scissors rotating ab ou t the center of the joint. T his mode l will be dicussed inSection 4.2.4.2. Modeling of element strength and stifn ess properties

Variou s elastic an d inelastic models are used in the analyses to investigate the effects of differentelement strength and stiffness representations on the predicted seismic demands. T he properties ofthe beam elements, beam springs, and joint elements are varied using the element mod els discussednext. Th e models discussed are approx imate, but are deemed adeq uate for the pu rpose of the study.

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