steel moment frames part 1

Seismic Design Factors for Steel MomentFrames with Masonry Infills: Part 1

Shiv Shanker Ravichandran,a) and Richard E. Klingner,b) M.EERI

In this two-part work, seismic behavior and design of steel moment frameswith masonry infills are investigated systematically. In this first part, the infillstrength ratio (the ratio of the story shear strength of infills to the story shearstrength of the bare frame) is shown to have a fundamental effect on the seismicbehavior of an infilled frame. This fundamental effect is demonstrated usingpushover analysis of an example steel moment frame with masonry infills in uni-formly infilled and open ground story configurations. In general, infill strengthratios greater than about 0.35 are associated with progressive deterioration ofseismic performance, leading to story mechanisms concentrated in the lower stor-ies. Greater infill strength ratios can also lead to local shear failures in framemembers. [DOI: 10.1193/1.4000060]

INTRODUCTION

An infilled frame, shown in Figure 1, is a structural system in which a frame of steel orreinforced concrete is filled with a panel of another material, such as concrete, clay, or auto-claved aerated concrete (AAC) masonry. The infill is generally a partition wall that also canserve as a thermal, acoustical or fire barrier. Infills can be located in a frame in many ways,ranging from placement in all bays and stories of the frame (Figure 1a), to random placement(Figure 1d). The open ground story system of Figure 1b is system popular in many parts ofthe world, as it leaves the ground story available for a lobby or parking.

Under lateral loads, infills act as compression struts extending between diagonally oppo-site corners of the infilled bay, as shown in Figure 2. The infill stiffens the frame, significantlyreducing drift under lateral loads from service level winds or earthquakes. The infill alsostrengthens the frame, and if properly designed may be able to reduce damage and probabilityof collapse under strong earthquakes.

PAST EXPERIENCE WITH INFILLED FRAMES DURING EARTHQUAKES

Evidence abounds regarding the beneficial effect of infills on seismic behavior of frames.In numerous instances, infilled frames have withstood an earthquake that collapsed otherwisesimilar frames without infills. However, infills are also known to adversely affect the beha-vior of the frame during earthquakes. This is probably because infills are commonly used indeveloping countries, where steel moment frames are rare. Hamburger (2006) discusses the

Earthquake Spectra, Volume 28, No. 3, pages 11891204, August 2012; 2012, Earthquake Engineering Research Institute

a) Design Engineer, Leslie E. Robertson Associates Consulting Engineers (India) Pvt. Ltd., Mumbai - 400028;former Graduate Research Assistant, Department of Civil, Architectural and Environmental Engineering,The University of Texas at Austin, TX 78712

b) L. P. Gilvin Professor in Civil Engineering, The University of Texas at Austin, TX 78712

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superior structural performance and fire resistance of steel-frame buildings with clay brick orhollow clay tile masonry infills, compared with that of other structural systems during the1906 San Francisco earthquake. Murty (2000) notes the excellent performance of reinforcedconcrete frames with masonry infills in India during moderate earthquakes.

However, infilled frames are also known to adversely affect the behavior of the frameduring earthquakes. Saatcioglu (2001) notes the poor performance of reinforced concrete

Figure 1. Example locations of infills in the panels of a frame.

Figure 2. Structural action of infills.

1190 S. S. RAVICHANDRAN AND R. E. KLINGNER

infilled frames, the primary structural system used in Turkey, during the Kocaeli earthquakeof 1999. Being stiffer, the infills attracted higher seismic loads, which could only be with-stood as long as the infills remained elastic. After the infills started to degrade, the remainingframes did not have enough lateral load resistance or inelastic deformation capacity, andoften collapsed. Of particular concern is the tendency of infilled frames to form adverse fail-ure mechanisms, such as those shown in Figure 3a, that lead to premature structural collapse.Such failure mechanisms are generally a result of irregular placement of infills leading to aweak story that has fewer infills than adjacent stories. This results in concentration of inelas-tic deformations at those stories. Even when infills are placed in a regular manner over theheight of the frame, Dolsek (2001) observes that weak-story failure mechanism can stilloccur. Irregular placement of infills in plan also leads to torsional effects. Throughoutthe world, a particularly common irregularity in the placement of infills is the open groundstory (Figure 1b). As shown on the right side Figure 3a, inelastic story drifts tend to be con-centrated at the open ground story, leading to collapse there. This is extensively presented inHumar (2001), Scawthorn (2000), Saatcioglu (2001), Christopoulus (2005), and elsewhere.Apart from failure mechanisms resulting from weak stories, infills can also cause local shearfailures in adjacent frame members. Development of strut action in the infill requires that thehorizontal and vertical components of the compressive force be resisted by the boundingcolumns and beams, respectively (Figure 3b). This can result in local shear failure inframe members, as observed in lab specimens of Al-Chaar (2002).

STEEL MOMENT FRAME USED FOR STUDY

This work uses an example steel moment frame in ATC-63 (2008) to study behavior ofsteel moment frames with masonry infills. It is a four-story special steel moment frame(R 8), shown in Figure 4. It is one of the four perimeter frames providing lateral supportto a four-story building. The frame is designed for Seismic Design Category D and base shearof 309 kips according to the requirements of ASCE7-05 (ASCE 2005) and AISC (2005). Thebeams have reduced beam sections (RBS) with 45% reduction in the flanges and are located

Figure 3. (a) Story mechanisms due to irregular placement of infills. (b) Shear forces on framemembers due to infill strut action.

SEISMIC DESIGN FACTORS FOR STEEL MOMENT FRAMESWITHMASONRY INFILLS: PART 1 1191

15 in. from the face of column flanges. The beam-column connections conform to strongcolumn-weak beam concept. From now on in this work, this steel moment frame is referredto as the ATC-63 steel moment frame.

FUNDAMENTAL IMPORTANCE OF INFILL STRENGTH RATIO

Preliminary pushover analyses of the ATC-63 steel moment frame with uniform infillsand open ground story infills indicated that the failure mechanism of the infilled framedepends in general on the ratio of the story shear strength of the infills to the story shearstrength of the bare frame. In this work, this ratio is termed the infill strength ratio. Infillconfigurations whose total lateral strength in a particular story exceeds about 35% of thelateral strength of the bare frame in that story are observed to provoke story mechanismsin the frame and local shear failures in frame members. The infill strength ratio is similarto the term in FEMA (2000) for infills, but is the inverse of it.

STORY SHEAR STRENGTH OF BARE FRAME

The story shear strength of the bare frame is computed using Equation 1, considering astory mechanism that leads to hinges at the top and bottom of columns at that story (Figure 5).If axial loads in columns are significant, then their effect on plastic moment capacity ofcolumn sections should be considered.

EQ-TARGET;temp:intralink-;e1;41;166Fstorymechanism X

Mph

(1)

where

Fstorymechanism = shear strength of the story under consideration

Figure 4. Four-story steel moment frame from ATC-63 (2008; ATC-63 steel moment frame).


Mp = plastic moment capacity of columns at the story under considerationh = height of the story under consideration

STORY SHEAR STRENGTH OF INFILLS

The story shear strength of infills is calculated as the total expected lateral shear strengthof all infill panels at that story.

ANALYTICAL MODELING OF STEEL MOMENT FRAMESWITH MASONRY INFILLS

The analytical model of the ATC-63 steel moment frame with masonry infills is devel-oped in OpenSees (OpenSees 2006), as shown in Figure 6.

Figure 5. Story mechanism used to calculate story shear strength of bare frame.

Figure 6. (a) Analytical model of ATC-63 steel moment frame (b) Idealization of infills asequivalent struts.


ANALYTICAL MODELING OF STEEL MOMENT FRAME COMPONENTS

Frame members are modeled using elastic beam-column elements and expected locationsof plastic hinges are modelled using point plasticity springs. The moment-rotation propertiesof the W-sections are computed using Lignos (2007), which uses the bilinear Ibarra-Krawinkler hysteretic model (Ibarra 2005) to represent hysteretic behavior of plastic hingesin steel frame members. The behavior of panel zones in beam-column joints is modeled usingthe Joint2D element in OpenSees, based on Lowes (2003). The rotational flexibility of thepanel zone is computed by calculating the shearing deformation for a unit moment imposedon the panel, and its strength is calculated as outlined in Section J10 of AISC (2005) and inKrawinkler (1978).

ANALYTICAL MODELING OF INFILLS: EQUIVALENT STRUT APPROACH

Infills are idealized as equivalent struts (Figure 6b; Klingner 1978, Madan 1997, Negro1997, Combescure 2000, Crisafulli 2000, Dolsek 2001, Dolsek 2002, Dolsek 2008a, 2008b).This approach requires much less computational effort than micro-modeling approaches suchas the finite element method, yet still provides reasonable accuracy, thereby allowing easyanalytical representation of multi-story, multi-bay frames. In the equivalent-strut approach,the infill is represented as a combination of two compression-only truss elements, each actingindependently. Each equivalent strut element is assigned an appropriate hysteretic force-deformation relationship, generally including a descending post-peak strength, in-cycledegradation, and pinching. In this work, the Ibarra-Krawinkler hysteretic model (Ibarra2005) with pinched hysteretic rules is used for this purpose.

INFILL CASES CONSIDERED FOR STUDY

The infill cases to be used with the ATC-63 steel moment frame are selected to representa broad range of infill strength ratios. In doing so, in any story that is infilled, infills areconsidered present in all the bays, as shown in Figure 1a and 1b. Accordingly, three infillcases are selected: the first, with an infill strength ratio below 0.35, the second, with a ratio ofabout 0.5; and the third, with a ratio of about 1. For the first case, infills are chosen to be madeof Autoclaved Aerated Concrete (AAC) because of its low strength. In particular, the firstcase can represent an infill made of 8-in. thick Class 4 AAC units. The second and third casescorrespond more naturally to conventional masonry, with its higher strength. In particular,the second and third cases can represent clay masonry with specified compressivestrengths of 4 ksi and thicknesses of 8 in. and 12 in., respectively. These are summarizedin Table 1.

Table 1. Infill cases considered for evaluation

InfillCase Infill Material

Thickness ofInfill (in.)

Specified CompressiveStrength (psi)

1 AAC, Class 4 8 5802 Conventional masonry (clay) 8 40003 Conventional masonry (clay) 12 4000


Starting with the ATC-63 steel moment frame, and infilling all bays with the infill casespresented in Table 1, results in three infilled steel moment frames for the uniformly infilledconfiguration (Figure 1a) and three more for the open ground story configuration (Figure 1b).This total of six infilled steel moment frames is summarized in Table 2, along with theterminology used to describe each of these infilled frames in the rest of this work.

PROPERTIES OF AAC INFILL

The stiffness, strength and hysteretic force-deformation behavior of the 8-in. thick Class4 AAC infill is modeled as described in Ravichandran (2009) using the Ibarra-Krawinklerhysteretic model (Ibarra 2005). Based on the preliminary observation that behavior ofthe frame and its failure mechanism deteriorate with increasing infill strength, the strengthof the AAC infill for this evaluation is conservatively taken as the maximum shear strength,97.4 kip, obtained for the AAC infill in the infilled frame specimen described inRavichandran (2009). That specimen also had 8-in. thick Class 4 AAC infill.

PROPERTIES OF CONVENTIONAL MASONRY INFILL

By Table 1 of MSJC (2008a), for conventional masonry the specified strength f 0m of 4 ksican be achieved using ASTM C270 Type S mortar and clay masonry units with a testedstrength of 11.5 ksi. The corresponding actual strength of masonry, f m, is assumed as6 ksi. As for the AAC infill, the hysteretic force-deformation behavior of conventionalmasonry infill is modeled using the Ibarra-Krawinkler hysteretic model. The initial stiffnessis calculated using draft MSJC infill provisions, which are based on Flanagan (2001). Thelateral strength of the conventional masonry infill is calculated as one standard deviation lessthan the mean strength predicted by Flanagan (2001). The peak strain is assumed to bereached at a lateral deflection of 1 in. as recommended by Flanagan (2001). The residualbranch of the monotonic curve is modeled using recommendations by Flanagan (1999)such that the strength drops to 75% of the peak strength at a deformation 1.5 times thatat peak strength. The values of pinching parameters kf and kd for the Ibarra-Krawinkler hys-teretic model that best represent the behavior of the AAC infill in the infilled frame specimenof Ravichandran (2009) are 0.15 and 0.5, respectively and these are used for conventionalmasonry infills as well.

Table 2. Infilled steel moment frames used for study

Infill Material

Infill Configuration

Uniformly Infilled Frames Open Ground Story Frames

8-in. thick Class 4 AAC AAC uniformly infilled frame AAC open ground story frame8-in. thick conventionalmasonry

8-in. thick conventional masonryuniformly infilled frame

8-in. thick conventional masonryopen ground story frame

12-in. thick conventionalmasonry

12-in. thick conventional masonryuniformly infilled frame

12-in. thick conventional masonryopen ground story frame


SUMMARY OF INFILL STRENGTH RATIOS FOR THE INFILL CASES

The story shear strength of the ATC-63 steel moment frame at the bottom story is com-puted using Equation 1 corresponding to expected yield strength of steel (55 ksi) as 2137 kip.Table 3 provides a summary of the infill strength ratios at the bottom stories of the ATC-63steel moment frame for the infill cases.

PUSHOVER ANALYSIS OF UNIFORMLY INFILLED FRAMES

Pushover analysis is performed on the analytical models of the ATC-63 steel momentframe as well as the uniformly infilled frames listed in Table 2. The lateral load profile com-puted using ASCE7-05 for the ATC-63 steel moment frame is used for the uniformly infilledframes as well so that the results obtained from the pushover analysis would be comparablefor all the frames. Figure 7 presents the pushover curves. In Table 4, the stiffness, strengthand ductility of the uniformly infilled frames are compared with that of the ATC-63 steel

Table 3. Infill strength ratio for different infill cases at the bottom story of the ATC-63steel moment frame

InfillCase Infill Type

InfillThickness

(in.)

Strength of FourInfill Panels ata Story (kip)

Infill strengthRatio at Bottom

Story

1 AAC, Class 4 8 390 0.182 Conventional masonry 8 1448 0.683 Conventional masonry 12 2172 1.01

Figure 7. Pushover curve for the uniformly infilled frames.


moment frame. The ductility is calculated as recommended by ATC-63 (2008). In general,increasing infill strength ratio progressively increases initial stiffness and strength whiledecreasing the ductility.

FAILURE MECHANISM AND DISPLACED PROFILE FROMPUSHOVER ANALYSIS

In Figure 8 are compared the displaced profiles of the ATC-63 steel moment frame andthe uniformly infilled frames at the ultimate roof drift ratio of the ATC-63 steel momentframe, determined as prescribed by ATC-63 (2008). From the displaced profile of theuniformly infilled frames, the failure mechanism of the frames can be deduced. The pushoverfailure mechanism of the AAC uniformly infilled frame is same as that of the bare frame. Thefailure mechanism of the 8-in. thick conventional masonry uniformly infilled frame is con-centrated in the bottom three stories. For the 12-in. thick conventional masonry uniformly

Table 4. Stiffness, strength and ductility of the uniformly infilled frames from pushoveranalysis

Infill Type

Initial Stiffnesswith Respect toATC-63 SteelMoment Frame

Strength Dividedby Design Base

ShearDisplacementDuctility

Bare frame 1.0 4.0 6.2AAC, 8-in. thick 2.7 5.3 4.1Conventional masonry, 8-in. thick 10.1 8.1 2.6Conventional masonry, 12-in. thick 12.5 10.5 2.3

Figure 8. Displacement profile of the uniformly infilled frames from pushover analyses.


infilled frame, the failure mechanism is limited to the bottom two stories. This is corroboratedby Figures 9 and 10, in which concentration of the failure mechanism in the lower stories ofthe frame is seen to increase the plastic rotation demand in hinges of beams and columns inthese stories compared to the ATC-63 steel moment frame. Further studies using pushoveranalysis indicated that an infill strength ratio of 0.35 marks the onset of change in failuremechanism compared to the bare frame for the ATC-63 steel moment frame in uniformlyinfilled configuration. Also, pushover analysis with stronger infills indicated that a weakground story mechanism would form at an infill strength ratio of about 2.0.

EFFECT OF INFILLS ON AXIAL FORCES IN COLUMNS

For the uniformly infilled steel moment frames, axial forces in exterior and interior col-umns were monitored during the pushover analysis, and are presented in Figure 11. Com-pared to the bare frame, the axial forces in exterior columns of the AAC uniformly infilledframe increase progressively from top to bottom. Because the infills in all stories crushed atabout the same time during the pushover analysis, the differences at each story between themaximum axial force in a particular column line of the ATC-63 steel moment frame and thesame column line in the AAC uniformly infilled frame, are multiples of 40 kips (the verticalcomponent of the axial strength of the equivalent strut). In Column Line 1 (Figure 12), thedifference in maximum axial force between the ATC-63 steel moment frame and AAC

Figure 9. Plastic rotation in beams during pushover analysis of the uniformly infilled frames.


uniformly infilled frame was equal to 40 kips at the fourth story, 80 kips at the third story, andso on down to the bottom story. For Column Line 5, in axial compression, the vertical com-ponents in the equivalent struts cascade down the columns from top of the third story.Accordingly, while the column axial force in the fourth story was nearly the same forthe AAC uniformly infilled frame as for the ATC-63 steel moment frame, they differ byabout 40 kip at the third story, 80 kip at the second story, and 120 kip at the groundstory. In interior column lines, in contrast, AAC infills do not cause significant changesin axial forces compared to the bare frame. This is because the vertical components ofthe forces in the equivalent struts framing into opposite sides of an interior column tendto neutralize each other, except at the top story. Because of the relatively weak AAC infills,axial forces in the columns of the ATC-63 bare frame do not increase significantly dueto uniform AAC infilling. The maximum increase in axial force (160 kips in tension and120 kips in compression) are only 4.8% and 3.6%, respectively, of the concentric axial capa-city of the ground story column. These increases do not significantly reduce the momentcapacity or ductility of the columns, and consideration of their effect on the moment-rotationbehavior of columns will not significantly change the results for the AAC uniformlyinfilled frame.

The trend in variation of column axial forces in the conventional masonry uniformlyinfilled frames is similar to that observed in the case of the AAC uniformly infilledframe, although some differences are apparent. In contrast to the case of AAC uniformlyinfilled frame, however, the increase between two adjacent stories relative to the ATC-63steel moment frame is consistently less than the vertical component of the axial strength

Figure 10. Plastic rotation in columns during pushover analysis of the uniformly infilled frames.


Figure 12. Equivalent struts active during pushover analysis of the uniformly infilled frames andillustration of non-simulated collapse modes in infilled frames due to simultaneous shear failurein frame members.

Figure 11. Variation in axial forces of columns during pushover analysis of the uniformlyinfilled frames.


of the infill. The first reason for this is that the infills did not crush completely at the upperstories, and those upper stories did not participate in the failure mechanism of the frame.The second reason is that axial force in the columns due to frame action is generallyless than in the ATC-63 steel moment frame, because the failure mechanism of theframe does not involve beam hinging at all stories. The maximum increase in axial forcerelative to the ATC-63 steel moment frame in the ground story of Column Line 1, isabout 500 kip and 800 kip for the 8-in. and 12-in thick conventional masonry uniformlyinfilled frames, respectively. These are 17% and 27%, respectively, of the axial capacityof the ground story columns. Such large increases in axial force may affect the momentcapacity or ductility of the columns, and consideration of its effect on the moment-rotationbehavior of columns is likely to exacerbate the results for the conventional masonry uni-formly infilled frames.

NON-SIMULATED COLLAPSE MODES IN INFILLED FRAMES

When the infill acts as a diagonal strut, it causes local shear forces in frame members,which add to the shear forces due to frame action (Figure 3b). If the combined shears exceedthe shear capacity, the frame member can fail locally in shear. This possibility could not bedirectly considered in the analysis, for the following two reasons:

1. Failure due to shear is not included in the analytical model used for frame members.2. Equivalent struts representing infills connect to the center of beam-column joints,

and transfer their axial forces as point loads to the frame.

Using data from the analysis, however, the local shear failures in frame members can beaddressed as non-simulated collapse modes (ATC-63 2008). This was done for infilledframes as explained here. The axial forces in equivalent struts representing infills were mon-itored during the analysis. At each instant of the analysis, the appropriate component of theaxial force in the equivalent struts is added to shear force in frame members due to frameaction alone. This gives the total shear force in frame members due to frame action plusframe-infill interaction forces.

At any particular story, the forces in equivalent struts are similar. Shears in the interiorcolumns of a particular story are nearly identical, and the shear in the interior columns alwaysexceeded the shear in the exterior columns. Shears in beams of interior bays at a particularstory were nearly identical, as were shears in beams of both exterior bays at a particular story.At any particular story, because the forces in the equivalent struts and shear forces in interiorcolumns are nearly uniform, a shear failure in Column Line 3 is considered to indicate a shearfailure in Column Line 2 and Column Line 4 as well. Similarly, a shear failure in the beam ofone of the interior bays is considered shear failure in beams of all interior bays, and a shearfailure in the beam of one of the exterior bays is considered shear failure in beams of bothexterior bays. Shear failure in such a large number of structural members can significantlyimpair the stability of the structure, and can effectively be treated as the failure of the entirestructure.

From this, it was detected that the local shear capacity of frame members was exceededonly for the 12-in. thick conventional masonry uniformly infilled frame in the columns ofbottom three stories thereby reducing its ductility from 2.3 to 1.3. Therefore, stronger infillscan lead to local shear failure in frame members due to frame-infill interaction forces.


PUSHOVER ANALYSIS FOR OPEN GROUND STORY FRAMES

Pushover analysis is performed on the analytical models of the open ground story frameslisted in Table 2, in the same manner as was done for the uniformly infilled frames. Resultsfor the open ground story frames are similar to those of the uniformly infilled frames.However, the formation of hinges in columns progresses faster towards the ground storyfor the open ground story configuration reaching it at an infill strength ratio of about 1.0rather than 2.0 with the uniformly infilled configuration. Also, for all infill cases occurrenceof local shear failure in frame members did not occur for the open ground story frames.

SUMMARY AND CONCLUSIONS

An important parameter defining behavior of infilled frames is shown to be the ratio ofstory shear strength of infills to the story shear strength of bare frame. This ratio is termed theinfill strength ratio.

The effect of infill strength ratio is demonstrated using pushover analysis of an examplesteel moment frame with masonry infills. From the results of those analyses, it can be saidthat for uniformly infilled frames:

1. If the infill strength ratio is less than about 0.35, the presence of infills doesnot change the failure mechanism, which involves hinging in beams and at columnbases.

2. When the infill strength ratio reaches about 0.35, the presence of the infill begins tochange the failure mechanism of the frame, from hinging in beams and at columnbases, to story mechanisms involving column hinging at multiple levels of the lowerstories.

3. When the infill strength ratio reaches about 1.0, infills significantly change the fail-ure mechanism of the frame, concentrating it in the bottom half of the frame.

4. When infill strength ratio reaches about 2.0, infills consistently lead to ground storymechanisms.

Results from pushover analysis of the example steel moment frame with an open groundstory configuration are similar to those with uniformly infilled configuration except that theformation of hinges in columns progresses faster towards the ground story reaching it at aninfill strength ratio about 1.0 rather than about 2.0 for the uniformly infilled configuration.Stronger infills also lead to local shear failure in frame members due to frame-infill inter-action forces.

In Part 2 of this work, archetypical steel moment frames with masonry infills of differentinfill strength ratios are evaluated using the ATC-63 methodology (ATC-63 2008) to deter-mine seismic design factors and design guidelines for steel moment frames with masonryinfills.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge the financial and technical assistance provided by theAutoclaved Aerated Concrete Products Association, Xella AAC, Xella Mexicana and Trus-tone AAC for the work described in this paper.


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(Received 16 March 2010; accepted 8 October 2011)


steel moment frames part 1

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