reinforcing of steel moment connections with cover...

ELSEVIER PII: S0141-0296(97)00038-2

Engineering Structures, Vol. 20, Nos 4-6, pp. 510-520, 1998 © 1997 Elsevier Science Ltd

All rights rcserved. Printed in Great Britain 0141-0296/98 $19.00 + 0.00

Reinforcing c o n n e c t i o n s

benefits and Michael D. Engelhardt

of steel m o m e n t

with cover plates:

l imitat ions

Department of Civil Engineering, The University of Texas at Austin, Austin, TX 78712, USA

Thomas A. Sabol

Englekirk and Sabol Consulting Engineers, PO Box 77D, Los Angeles, CA 90007, USA

Steel moment connections reinforced with cover plates have received considerable attention in the US since the Northridge earthquake, both in laboratory testing and in new steel moment frame construction. This paper presents experimental data on 12 large scale connection test specimens reinforced with cover plates. Ten of the 12 test specimens showed excellent performance, developing large plastic rotations under cyclic load. Two test specimens failed, however, indicating that cover plated connections are not fool-proof. This type of connection has limitations that must be considered in design and construction. The paper provides a critical assessment of cover plated connections, identifying potential benefits as well as concerns and limitations. Design implications of the experimental data are discussed.

Keywords: seismic resistant steel connections, cyclic loading, fracture

I. Introduction

Since the Northridge earthquake, a great deal of research and testing has been conducted to identify better moment connections for new steel moment frame construction, as well as for repair or upgrading of existing steel moment frames. The majority of these efforts have combined improvements in welding together with modifications to the connection design. In many cases, the modified connection design calls for some type of reinforcement at the connection. In laboratory testing, reinforced connections have often shown dramatic improvements in performance compared to the 'pre-Northridge' connection, and they have been widely implemented in new steel moment frame construction in the west coast of the US. However, while promising better performance, the reinforced connections have also experienced failures in laboratory tests, and introduce potential new problems.

The use of cover plates has been one of the more common connection reinforcing schemes used since the Northridge earthquake. This paper presents data from experiments conducted by the writers on 12 large scale test specimens utilizing cover plates. The experimental data is critically assessed to identify the advantages of cover plated

connections, as well as their potential limitations. Other reinforcing schemes are also briefly discussed. The paper concludes with a discussion of design implications of the experimental data on cover plated connections.

2. Reinforced moment connections

In many of the new moment connection concepts proposed and tested since the Northridge earthquake, improvements in welding are implemented together with some type of reinforcement at the connection, in hopes of achieving large cyclic plastic rotations without connection failure. A variety of methods have been used to reinforce connections. Some possible types of reinforcement are illustrated in Figure 1, and include cover plates, upstanding ribs, side plates, and haunches. Many other variations are also possible. Welding improvements implemented in these connections may include the use of high toughness weld metal, removal of groove weld backing bars, removal of weld tabs, and generally closer adherence to good welding and inspection practices.

The purpose of connection reinforcement, in the most general terms, is to provide a connection that is stronger than the beam. The reinforcement is intended to force the

510

Reinforcing of steel moment connections: M. D. Engelhardt and T. A. Sabol 511

Frame Column

TOp Cover Plate ; -(Tapered)

Flange

~ F a,'re Beam

8otlom Cover Plale (Rectangular)

(a) Coverplate Botlom Upstanding Rib

(b) Upstanding Rib

Frame Colgmn

Side Plates

Fille¢ Pl~le

Beam

(c) Side Plate Figure 1

Frame Column

Top Haunc~ (Allernate)

Botlom Haunch1 from WT

(d) Haunch Examples of reinforced moment connections

plastic hinge away from the face of the column, where premature fractures can occur due to potential weld defects, stress concentrations at weld access holes, stress concentrations due to column flange bending, high levels of restraint and the associated states of triaxial tension, uncer- tain column flange through-thickness properties, etc. The reinforcement reduces stress levels within this vulnerable region near the column face, and forces the large stresses and inelastic strains further into the beam, away from the welds, access holes, triaxial states of tension, etc.

Prior to the Northridge earthquake, a number of investi- gators conducted cyclic loading tests on steel moment connections reinforced either with cover plates or upstanding ribs ~ 3. These early tests showed very high levels of cyclic ductility could be achieved by a reinforced connection. However, these earlier tests were very limited in number and were conducted on relatively small size beams.

Shortly after the Northridge earthquake, a test program was sponsored by the American Institute of Steel Construc- tion (AISC) and a private building owner to quickly identify an improved connection detail for use in new moment frame construction 5,6. "Iqais program, using large scale beams and columns, showed considerable success using connections reinforced wiLth cover plates or ribs. Reinforced moment connections have since been implemented on a number of new steel moment frame structures on the west coast of the US.

One of the most common reinforcing schemes used in cyclic test programs as well as in new moment frame construction is the cover plated connection (Figure la). While showing excellent performance in a number of laboratory tests, cover plated connec, tions have also experienced some failures in laboratory tesl:s and introduce some difficulties in welding and inspection. Successful application of cover plates requires an understanding of the limitations and potential problems that can occur with this connection. In the following section, experiments conducted by the writers on cover plated connections are summarized.

3, .Experimental data on cover plated connections

3.1. Test specimens Since the Northridge earthquake, the writers have conducted a total of 12 cyclic loading tests on large scale specimens constructed with cover plated connections. These tests have been conducted on single cantilever type specimens, without a concrete floor slab. Figure 2 shows the experimental setup. All specimens were subjected to slowly applied cyclic loads at the tip of the cantilever.

The test specimens, listed in Table 1, were conducted as part of three different test programs. The specimen designations starting with "AISC" were the first to be conducted following the Northridge earthquake, and are reported in greater detail in Engelhardt and Sabol 5'6. The specimen designated "SAC-4" is reported in greater detail 7. The specimens designated as "NSF-5" to "NSF-7" are not yet reported elsewhere in the literature. Table 1 also reports the member sizes used for the beams and columns, and tensile coupon data for the beams. With the exception of NSF-6, all beams were W36x150, taken from a variety of heats of steel. Beams for the AISC and SAC specimens were of ASTM A36 steel (minimum specified Fy=248Mpa), whereas the beams for the NSF specimens were of ASTM A572 Grade 50 steel (minimum specified Fy = 345 Mpa). All columns were of A572 Grade 50 steel. Note the wide variation of measured yield stress values for the beams. For example, the measured flange yield stress values for the A36 beams varied from 292 to 370 MPa. It is also interest- ing to note that many of the A36 beams had higher measured yield stress values than the A572 Grade 50 beams.

The specimens constructed with the W14 x455 and W14 x 426 columns were designed to provide a very strong column panel zone, not subject to inelastic action. For these specimens, all inelastic action was forced into the beam. The specimens constructed with the lighter W14 x 257 columns (SAC-4 and NSF-6) were designed to permit shear yielding of the column panel zone to evaluate its effect on the performance of the connection.

Specimens AISC-3A and NSF-7 are shown in Figures 3 and 4 to illustrate typical details. Each of the 12 specimens was provided with a tapered cover plate at the top flange. The purpose of the taper was to provide a gradual transition from the reinforced to the unreinforced portions of the beam, and to minimize stress concentrations at the tip of the cover plate. The choice of the tapered cover plate was

I =

Figure2 Test setup

3400 mm

Beam Lateral B r a c e

2135

1350 kN Ram/ ~ and Load Cell'

5 1 2 Reinforcing of steel moment connections: M. D. Engelhardt and T. A. Sabol

Table I Cover-plated connections - member sizes and properties

Specimen Beam size Beam flange strength

F, Fo (MPa) (MPa)

Beam web Column size strength

F, Fo (MPa) (MPa)

AISC-3A W36x150 AISC-3B W36x150 AISC-5A W36x150 AISC-5B W36x150 AISC-7A W36x150 AISC-7B W36x150 AISC-8A W36x150 AISC-8B W36x150 SAC-4 W36x 150 NSF-5 W36x150 N SF-6 W30x 148 N SF-7 W36x 150

294 425 294 425 318 460 370 492 318 460 318 460 311 444 311 444 292 421 296 417 321 445 340 456

320 435 W14x455 320 435 W14x455 375 494 W14x426 380 520 W14x426 375 494 W14x426 375 494 W14x426 343 465 W14x426 343 465 W14×426 329 437 W14x257 310 415 W14x426 334 450 W14x257 360 467 W14x455

®

t W14x455

(A572 Gr. 5 0 )

Figure3 Specimen AISC-3A

I0 3o ° W36xlSO

s i d e s l

BOLTS: l O - 2 6 m r n A490 7 6 r a m HOLES: 2 7 r a m DIA.

I t 16 x 127 x 7fl7

P', lO 3~

46 °

8~

430 ~ 66 / 1 9 t u r n ~,

SECTION A

4 0 5

BACK GOUGE

SECTION B

based on past successful tests on smaller size specimens 2~. Most specimens were provided with a rectangular cover plate at the beam bottom flange. This shape was chosen in consultation with structural steel fabricators and erectors in order to facilitate field construction. The dimensions of the cover plates for all test specimens are listed in Table 2.

When choosing the cross-sectional area of the cover plates, the overall design criterion adopted was that the region of the connection at the face of the column should remain essentially elastic under the maximum bending moments and shear forces developed by the fully yielded and strain hardened beam. It was assumed that the maximum moment that could be generated by the unreinforced beam at the end of the cover plate was on the order of 1.2M~o. M~p represents an estimate of the beam's actual plastic moment, considering possible actual values of Fy considerably in excess of minimum specified values. The 1.2 factor is intended to account for strain hardening that accompanies large cyclic inelastic deformations. The moment of 1.2M~p was then projected from the end of the

cover plate to the face of the column, considering the moment gradient in the beam. This provided the value of moment under which the reinforced connection was to remain essentially elastic. It should be noted that there was considerable uncertainty in the actual implementation of this approach. A major source of uncertainty was the expected value of beam Fy. Variations in the assumed value of Fy can result in significant differences in the size of cover plates. A second source of uncertainty was the acceptable level of maximum stress in the connection at the face of the column. Although the goal was to keep the connection "essentially elastic" at the face of the column, the actual stress limit that should be used to achieve acceptable connection performance is unclear. A third source of uncertainty is predicting stresses in the connection at the face of the column. The actual state of stress in the connection region is complex, and may not be well predicted by simple bending theory. In this regard, there are uncertainties in the amount of moment and shear actually carried by the flange and web connections, stress concentrations across the width


®

25mm ~

W14x455 (A572 Gr. 50',

r,,l

j -

I 50ream NO WELD

4 #

AFTER UT OF r~ ( BEAM FLANGE WELD I0

30 °

10 ~ 355

~ - ~ SECTION A

355

~ : ; 7GLrl 50) BACK GOUGE

SECTION B

Figure 4 Specimen NSF-7

Table2 Details of cover plated connections

Specimen Top cover plate dimensions (thk. x width x length) (rnm)

Bottom cover plate dimensions (thk. x width x length) (mm)

Web connection Electrode for beam flange and cover plate groove welds

AISC-3A AISC-3B AISC-5A AISC-5B AISC-7A AISC-7B AISC-8A AISC-8B SAC-4 NSF-5 NSF-6 NSF-7

19x300x430-ta~ered 19x300x43O-ta3ered 25x300x610-ta 3ered 25x300x610-ta 9ered 19x300x430-ta 9ered 19x300x430-ta 9ered 19x300x430-ta 9ered 19x300x430-ta 3ered 25x300×405-ta3ered 12x300x355-tapered 16x266x355-tapered 12x300x355-tapered

16x355x405-rectangular bolted 1 16×355x405-rectangular bolted 1 25x300x585-tapered bolted ~ 25x300x585-tapered bolted 1 16x355x405-rectangular bolted 1 16×355x405-recta ng ular bolted ~ 16×355x405-rectang ular bolted ~ 16×355x405-rectangular bolted ~ 25x355x405-recta ng ular bolted ~ 12x380x355-recta ng ular welded 2 16x300x355-rectang ular welded 2 12x380x355-recta ng ular welded 2

E7OT-4 E70T-4 E70TG-K23 E7OTG-K23 E70T-7 E70T-7 E70T-7 E7OT-7 E71T-8 E71T-8 E71T-8 E71T-8

(1) Fully tensioned high strength bolts combined with supplemental welds between the shear tab and beam web to develop 20% of the nominal plastic moment of the beam web. (2) Beam web welded directly to column using complete joint penetration groove weld. (3) For AISC-5A and AISC-5B, E70TG-K2 was used for the top flange and cover plate and for the bottom flange. Bottom cover plate was welded to column using E70T-7.

of the beam flange groove: welds when continuity plates are or are not provided, the influence of multi-axial states of stress, etc. Consequently, depending on the assumptions made, a wide variety of =over plate sizes can result from this seemingly simple de,;ign approach.

In choosing the dimensions of the cover plates using the general design philosophy discussed above, two different approaches were followed. In all the AISC specimens, as well as in SAC-4, the cross-sectional area of the cover plate was taken to be in the range of about 75-100% of the beam flange area. These specimens were all provided with a bolted web connection (Table 2). On the other hand, for specimens NSF-5 to NSF-7, smaller cover plates were used,

with cross-sectional areas on the order of about 50% of the beam flange area. To compensate for the smaller area of cover plates on the NSF specimens, the beam web was welded directly to the column flange. The welded web was assumed to be more effective in transferring bending moment at the connection than the more traditional bolted web connection j,3 and, therefore, permitted smaller cover plates on the NSF specimens. This was believed to be a preferable connection design, as it avoided the very large groove welds required for thick cover plates.

In all cases, both the cover plates and beam flanges were groove welded to the column flange. The cover plates, in turn, were fillet welded to the beam flanges (Figure 3).

514 Reinforcing of steel moment connections: M. D. Engelhardt and 7-. A. Sabol

The length of cover plate was generally chosen to permit placement of a sufficient length of fillet weld to develop at least the yield strength of the cover plates. Note that a variety of lengths were used (Table 2). The longest cover plates were on specimens AISC-5A and 5B, whereas the shortest cover plates were on the NSF specimens.

3.2. Welding and fabrication details All the specimens were welded by the self-shielded flux- cored arc welding (FCAW) process, using a variety of different electrodes. Table 2 lists electrodes used for the groove welds connecting the beam flanges and cover plates to the column flanges. The electrodes are denoted by stan- dard designations of the American Welding Society 8. One of the key differences among the electrodes listed in Table 2 is the level of notch toughness provided by the deposited weld metal. The E70T-4 electrode used on specimens AISC-3A and 3B provides no specified level of toughness. The measurements made on samples of weld metal taken from test specimens showed very low levels of toughness for the E70T-4 welds, with Charpy V-Notch values in the range of 5-23 J (4-17 ft-lbs) at room temperature 9. The E70T-4 electrode was widely used in west coast US prac- tice prior to the Northridge earthquake. The E70T-7 electrode, used for several test specimens, also provided no specified level of toughness. Weld metal samples extracted from the specimens showed Charpy V-Notch values of about 25 J (19 ft-lbs) at room temperature. Finally, the E70TG-K2 and E71T-8 electrodes were specifically chosen to provide high levels of toughness. Both electrodes provide a minimum specified Charpy V-Notch value of 27 J at

-29°C (20 ft-lbs at -20F). Among the 12 test specimens, different approaches were

taken in the welding and fabrication sequence at the connection. All the specimens were constructed by large com- mercial structural steel fabricators experienced in west coast building construction. Even though the specimens were completely constructed in fabrication shops, the construction operations were separated into typical shop and field operations in order to simulate typical construction practices. All "shop" welds were made by fabrication shop welders, using welding positions typical to shop welding. More specifically, all shop welds were made with the specimen oriented to permit fiat position welding. For "field" welds, the specimens were oriented to simulate field welding positions. Field welders from steel erection companies were brought to the fabrication shops to make the "field" welds on the specimens. These welders were more experienced in a wider variety of welding positions, as typically required in field welding.

For all specimens, the initial steps in the fabrication called for shop welding of the shear tab, continuity plates, and bottom cover plate to the column. The bottom cover plate was shop welded to the column to permit it to serve as a seat for the beam during later field erection. In all cases, the bottom cover plate was shop welded to the column using a double bevel groove weld with backgouging of the root, as shown in Figure 5. This groove weld con- figuration was chosen to minimize weld volume and the associated shrinkage and distortion. The bottom cover plate groove weld was ultrasonically tested in the shop. In the case of the AISC specimens, the top cover plate was shop welded to the top beam flange, and the combination of beam flange and cover plate were beveled to form one large groove. For the SAC and NSF specimens, the top cover

Column Flange ~ 3 ~ o o m m

1 r ~ - ~ t f

'P/2i' "backgou e

r~ ...._.~ ._~ ~.,,,~_.~._ t p "~ "~ ~5._oBottom Cover Plate

tp/2 4~5o ~ g

Figure 5 Typical groove weld details at bottom flange

plate was not shop welded to the beam flange, as described below.

For field erection, the following sequence was used for the AISC specimens. First, the beam was set on the bottom cover plate. The web bolts were installed and fully tensioned. The beam bottom flange was then groove welded to the column flange. For this groove weld, the bottom cover plate served as backing (Figure 5). Next, the top flange and cover plate were welded to the column, using a single large groove weld, as illustrated in Figure 6a. Finally, the bottom cover plate was fillet welded to the bottom flange of the beam. Upon completion, all field groove welds were ultrasonically tested.

A somewhat different sequence was used for field erection of the NSF specimens. For these specimens, the beam was first set on the bottom cover plate, as above. Erection bolts were then installed in the shear tab to hold the beam during welding. The beam bottom flange was then groove welded to the column, as was the case for the AISC specimens (Figure 5). Next, the beam top flange was groove welded to the column. This weld was then permitted to cool and was ultrasonically tested. Next, the beam web was groove welded to the column flange, with the shear tab acting as backing for the weld. The bottom cover plate was then fillet welded to the bottom beam flange. Next, the top cover plate was placed on the top of the beam flange and was groove welded to the column, as illustrated in Figure 6b. Finally, the top cover plate was fillet welded to the top beam flange. After all welding was complete, the top cover plate groove weld and the beam bottom flange groove weld were ultrasonically tested.

Compared to the AISC specimens, the welding sequence for the NSF specimens was chosen to provide smaller groove welds with less shrinkage and restraint, and to permit better ultrasonic testing of the top flange groove weld. For specimen SAC-4, the welding details and sequence were identical to the NSF specimens, with the exception that SAC-4 had a bolted rather than a welded web.

For all specimens, weld tabs were used for the groove welds, and were removed after welding was completed. The bottom flange detail used no backing bar for the groove weld, eliminating the need for backing bar removal. A backing bar was used for the top flange groove welds and was left in place after welding was complete. In the case of the SAC and NSF specimens, a fillet weld was placed between the backing bar and the column flange. From a


Column Flange

% %

/ 10 mm 300

~Top Cover Plate

\Beam Top Flange

..... ~ F~ ( AfterFlangeUTweldof Beam Column Flange ~ r~10mm30 v

10 rnm 300 ~ w ' \ ' G

W K,~ ~ T 0 p Cover Plate

~ ~ - J ~ B e a m Top Flange

81/

(a) (b) Figure 6 Typical groove weld details at top flange: (a) detail used for AISC specimens; (b) detail used for SAC and NSF specimens

theoretical perspective, this fillet weld helps reduce the notch effect of a left in place backing bar.

3.3. Test results Each of the specimens was subject to slowly applied cyclic displacements at the tip of the cantilever. Specimens were tested until connection failure occurred or until limitations of the test setup were reached. The test results are summarized in Table 3. This table lists the maximum plastic rotation achieved by the specimen in the test, and briefly describes the performance of the specimen. For a number of specimens, the maximum plastic rotation listed in the table is followed by a (+) superscript. For these cases, testing was stopped due to limitations in the test setup. These specimens could have likely achieved higher plastic rotations had the testing lL)een continued.

Of the 12 specimens tested, 10 showed excellent performance, achieving high levels of plastic rotation. Two specimens, AISC-3A and AISC-5B, showed poor performance, experiencing brittle: failures at low levels of plastic rotation. The typical response of the successful specimens is described below, followed by a description of the failed specimens.

3.4. Description of successful specimens The successful cover plated specimens each showed similar patterns of behavior. Yielding, as observed by whitewash flaking, was typically concentrated in the region of the beam near the tip of the cover plates. The cover plated region of the connection adjacent to the column showed little or no yielding, as intended by design. Local flange and web buckling in the beara typically commenced at plastic rotation levels of about 0.01-0.015 radian, and became increasingly more pronounced at higher levels of plastic rotation. Local buckling was generally accompanied by a twisting of the beam, and resulted in a gradual reduction in specimen capacity. In most cases, the local flange buckling was accompanied by a gradual tearing of the fillet welds connecting the cover plates to the beam flange. In no case, however, did this tearing of the fillet welds have an adverse effect on the sl:rength of the specimen. For most

Table 3 Test results for cover plated connections

Specimen Maximum plastic rotation (rad)

Description of failure and additional comments

AISC-3A 0.015

AISC-3B 0.025

AISC-5A 0.025 AISC-5B 0.005

AISC-7A 0.035 ÷

AISC-7B 0.050 + AISC-8A 0.035 ÷ AISC-8B 0.035 ÷ SAC-4 0.037 +

NSF-5 0.033 ÷ NSF-6 0.038 ÷

NSF-7 0.038 ÷

brittle fracture through top flange and cover plate groove weld gradual deterioration in strength due to beam local buckling, fol lowed by gradual tearing of beam bottom flange at end of cover plate same as AISC-3B brittle fracture at beam bottom flange connection; fracture is contained within the column flange base metal gradual deterioration in strength due to beam local buckling, and due to gradual tearing of fi l let welds connecting cover plates to beam flanges; test terminated to avoid damage to test setup; specimen still had substantial strength at end of test same as AISC-7A same as AISC-7A same as AISC-7A same as AISC-7A; significant yielding occurred in column panel zone same as AISC-7A gradual tearing of fillet welds connecting cover plates to beam flanges; test terminated due to reaching maximum displacement capacity of test setup; strength of specimen still increasing at end of test; column panel zone yielding dominated inelastic response same as AISC-7A

516 Reinforcing of steel moment connections: M. D. Engelhardt and 7-. A. Sabol

5000

4000

301)0

2000

~ 1000

~ 0

~ -1000 0

-2000

-3000

4"Yt ' '

0.03 0.04

Figure 7

-40OO

-5000

-0.04 -0.03 -0.02 -0.01 0 0.01 0.02

Plastic Rotation (rad)

Hysteretic response of specimen NSF-7

Figure8 Specimen NSF-7 after testing

specimens, testing was stopped after twisting of the beam became sufficiently severe to risk damage to the test setup. For two of the specimens, AISC-3B and AISC-5A, gradual tearing occurred in the beam flanges at the ends of the cover plates. This tearing, however, occurred after numerous cycles of inelastic loading at high levels of plastic rotation. Figure 7 shows the hysteretic response of specimen NSF- 7, a typical successful connection. Figure 8 is a photograph of this specimen at the end of testing. For specimens SAC- 4 and NSF-6, substantial yielding and inelastic deformation occurred in the column panel zones. This panel zone deformation had no apparent detrimental effect on these connections, and contributed to the overall ductility of these specimens.

3.5. Description of failed specimens Of the 12 cover plated specimens, two showed poor behavior. The first of these was specimen AISC-3A. Figure

i I

l l ' t l i l l / i ! L

i T

i L

-0.02 .0.01 0.01

Plastic Rotation (rad)

I

I

L i

0.02

i

0.00 0.04

5000

401)0

31)1)0

2000

1000

~ o :

• , -2000 Fracture i t '

Flange We -3000

-4t~0 I l

-5000 -0.04 41.03

Figure 9 Hysteretic response of specimen AISC-3A

9 shows the hysteretic response of AISC-3A. This specimen sustained only a few inelastic cycles of loading, prior to experiencing a brittle fracture at the top flange weld. The fracture passed completely through the groove weld connecting the top flange and top cover plate to the column. The fracture was through the weld metal, near the face of the column. Interestingly, specimen AISC-3B, which showed very good performance, was nearly identical to the failed specimen AISC-3A. Both specimens AISC-3A and 3B were constructed using identical member sizes and heats of steel, identical connection details, and identical welding electrodes. The only apparent difference was that a different welder was used on AISC-3A than on AISC-3B. Both specimens, however, passed two independent ultrasonic tests. Further, the fracture surface on failed specimen AISC-3A showed no evident workmanship defects.

A rather detailed metallurgical investigation was conducted to understand the causes of failure and why these two nearly identical specimens showed such drastically different levels of ductility 9. This study, while not completely conclusive, established that a primary difference between AISC-3A and AISC-3B was in the toughness of the weld metal. Even though the E70T-4 electrode was used for both specimens, the two different welders used different welding procedures, apparently resulting in a substantial difference in weld metal toughness. The successful AISC-3B was welded in accordance with a written Welding Procedure Specification, and tests on the weld metal showed Charpy V-Notch values of about 23 J (17 ft-lbs) at room temperature. In contrast, tests indicated Charpy V-Notch values for the weld metal in the failed AISC-3A were likely to be about 5 J (3.8 ft-lbs) at room temperature. This extraordi- narily low level of toughness appeared to have been a decis- ive factor in the fracture of the AISC-3A top flange weld. Data collected during fabrication of AISC-3A suggested the welder likely exceeded voltage and current limits specified on the Welding Procedure Specification, resulting in the very low weld metal toughness. Note that low toughness cannot be detected by ultrasonic testing.

The failure of specimen AISC-3A suggests two important lessons. First, weld metal toughness likely plays a very important role in connection performance. The use of cover plates and the associated stress reduction on the groove welds does not preclude the need for weld metal toughness. On the contrary, high toughness may be particularly important in cover plated connections because of the increased effective thickness of the beam flange and the associated restraint and triaxial tension that may result. The second lesson from AISC-3A suggests failure of a welder to adhere to a properly formulated welding procedure can result in weld metal deficiencies, i.e. low toughness, that cannot be detected by routine nondestructive test methods. The E70T-4 electrode may have been particularly vulnerable to this effect due to its inherent low toughness. Such problems may be avoided by better in-process inspection, in which adherence to the welding procedure specification is verified at the time of welding. The use of higher toughness electrodes would likely also be highly beneficial. Even if a reduction of toughness is suffered due to improper welding parameters, a substantially higher level of toughness would still remain.

The second cover plated specimen to experience failure was AISC-5B. After only a few inelastic cycles of loading, a brittle fracture occurred at the bottom flange, causing a complete separation of the beam bottom flange and cover

Reinforcing of steel moment connections: M. D. Engelhardt and 7". A. Sabol 517

plate from the column flange. The fracture was completely contained within the column flange, immediately adjacent to the groove weld. Interestingly, a companion specimen, AISC-5A, showed excellent performance. Specimens AISC-5A and AISC-5B were constructed with the same member sizes, the same connection detail and the same welding electrodes. Different heats of material, however, were used for these two specimens. The beam of AISC-5B had a somewhat higher yield strength than the beam for AISC-5A (Table 1). Note., that both AISC-5A and 5B were welded with a high toughness electrode at the top flange and cover plate and at the bottom flange (Table 2). How- ever, for both specimens, the groove weld connecting the bottom cover plate to the column flange was made with the lower toughness E70T-7 electrode.

The fracture surface within the column flange of the failed AISC-5B had a flat appearance. This suggested that deficiencies in the colunm flange through thickness properties may have played an important role in the failure. Sub- sequently, detailed fractographic and metallographic exam- inations were conducted en AISC-5B ~0 to better understand the causes of failure. This study suggested that several factors may have combined to cause the failure:

• The beam of specimen AISC-5B had a higher yield strength than specimen AISC-5A, and a higher yield strength, in fact, than any other cover plated connection tested (Table 1 ). This generated very high levels of stress on the groove welds and surrounding base metal regions.

• The cover plates for specimens AISC-5A and 5B were heavier and longer than for any other specimen (Table 2). With plastic hinge formation occurring at the tips of the cover plates, the long length of the cover plates substantially increases the bending moment at the face of the column.

• Fracture initiation occurred in the column flange, in the heat affected zone (HAZ) adjacent to the groove weld. Comparison of AISC-5A and AISC-5B showed a much larger HAZ in the column of AISC-5B, suggesting a much higher heat input was used for welding. This high heat input may have resulted in a low toughness in the column HAZ.

• Dissection of specimen AISC-5B showed the presence of an additional crack that was not originally visible. This crack was located within the bottom flange groove weld. The crack ran horizontally near the junction of the beam bottom flange groove weld and the bottom cover plate groove weld. This crack may have been caused by a stress concentration at the junction of the cover plate and flange, combined with inadequate toughness of the E70T- 7 weld metal. This crack did not propagate into the column flange and, therefore, was not the initiation site for the final brittle fracture in the column flange. However, it was surmised 1° that a stress raiser extending beyond the tip of the crack into the column flange contributed to the initiation of the fi'acture within the column flange.

There are several lessons that might be implied by the failure of specimen AISC-5B. First, excessive reinforcement may be detrimental to the connection. Very long cover plates result in a large amplification of moment from the hinge location to the face of the column, negating much of the stress reduction intended by the cover plates. Further,

very thick cover plates may result in very high restraint at the face of the column. This increases welding residual stresses and also increases the degree of triaxial tension at the face of the column, thereby promoting more brittle behavior. Thus, whereas some degree of reinforcement benefits the connection, the use of excessively heavy and long reinforcement may be detrimental.

The failure of AISC-5B appears to further confirm the importance of weld metal toughness and the use of proper welding procedures. The use of the lower toughness E70T- 7 electrode for the bottom cover plate may have contributed to the weld metal crack, which in turn may have contributed to the final fracture in the column.

The failure of AISC-5B also suggests that base metal factors may have an important influence on connection performance. The beam material for AISC-5B was specified as A36 steel, with a minimum specified yield strength of 250 MPa. The actual yield strength was 370 MPa in the flange, i.e. 50% higher than the nominal minimum specified value. This high value of beam yield stress was clearly a factor in this failure. Better control on the upper limit of yield stress for structural steel would likely permit more reliable connection performance. Further, the nature and location of the fracture within the column flange suggests that additional information is needed on the toughness and through-thickness strength and ductility of column flanges.

A final issue of concern raised by the failure of this test specimen is the possible presence of a stress concentration at the junction of the beam flange and cover plate. As noted above, a crack was observed at this location at the bottom flange of AISC-5B. However, it is unclear how severe this stress concentration is and under what circumstances it may become a significant factor in connection performance. The space between the cover plate and the beam flange may be viewed as a notch in a fracture mechanics analysis. If the principal stress in the flange and cover plate is considered to be oriented parallel to the flange, then this notch should not be particularly significant, since it is oriented parallel to the stress flow. However, if stresses develop in this region that run perpendicular to the notch, then crack growth is possible. Stresses that tend to open this crack may develop, for example, due to differential bending curvature between the beam and the cover plate or possibly due to tensile stresses in the adjoining column flange. AISC-5B is the only specimen in the program to have experienced such a failure. A section cut through the bottom flange of AISC- 5A did not show the presence of a crack in the weld, as was found in AISC-5W °. Sections were also cut through several other cover plated connections with no evidence of a crack at the flange-cover plate junction. Furthermore, other tests on cover plated connections, conducted prior to the Northridge earthquake 2~, showed no evidence of this phenomenon. Consequently, it is unclear if the crack found in the groove weld of AISC-5B preceded and contributed to the column flange fracture, or if it occurred as a result of the column flange fracture. In any event, the possible presence of a stress concentration at the flange-cover plate junction and its implications toward connection performance is an issue that merits further study. The potential for a notch at the beam flange-cover plate junction also further emphasizes the importance of using high toughness weld metal for attaching both the cover plate and the beam flange to the column.

518 Reinforcing of steel moment connections: M. D. Engelhardt and 1". A. Sabol

4. Further assessment of cover plated connections

4.1. Potential advantages The experimental data presented above, combined with earlier data on cover plated connections 2-~ suggest that cover plated connections provide a number of potentially important advantages for seismic resistant steel construction. The data also suggest several issues of concern with this type of connection. Some of the important potential advantages are as follows:

• Cover plated connections can provide highly ductile response and significantly better performance than the previous 'pre-Northridge' connection. In the experiments presented above, 10 of the 12 tests showed excellent inelastic deformation capacity, developing large levels of plastic rotation capacity under cyclic loading. Experi- ments on the pre-Northridge welded flange-bolted web detail 7, r a-a3 showed rather consistently poor performance, with failures commonly occurring with little or no plastic rotation.

• Compared to a number of other reinforcement options, cover plated connections appear to be less costly. A cost analysis was conducted for the connection details investi- gated in the AISC sponsored test program shortly after the Northridge earthquake TM. This analysis compared the cost of cover plates, ribs and side plates for a W36x150 beam, and showed that the cover plates were the least costly of the three options. More recently, a detailed cost study was conducted for an actual 25-storey steel office building in Salt Lake City, Utah tS. This study compared the cost of a number of different connection types for a W36×194 girder, including cover plates, ribs, haunches, side plates, and others. This study again showed that cover plates were the least costly of the reinforced connections.

4.2. Issues of concern As noted above, cover plated connections can develop out- standing levels of ductility, and appear to be one of the least costly options for connection reinforcement. However, as demonstrated by the failure of two laboratory specimens, these connections are not fool-proof. There are several issues of concern and some potential limitations of cover plated connections that should be recognized and considered in design and in future research:

• The fact that two of the 12 test specimens experienced early brittle failures suggests some concern on the overall reliability of cover plated connections. Some of the factors contributing to the laboratory failures are understood and can be avoided in the future. For example, the failure of AISC-3A was likely to be caused by very low toughness weld metal. The use of higher toughness weld metal should alleviate this problem. However, the failure of AISC-5B was more complex, with no single simple cause. Overall, while cover plated connections appear to be a substantial improvement over the "pre-Northridge" connection, further testing and analysis is needed to better understand their behavior and limitations, and to better quantify their reliability.

• The data suggests that the performance of cover plated

connections may worsen when the cover plates become very long. As discussed above, very long cover plates largely negate any potential benefits of the plates, due to the large moment amplification from the tips of the plates to the face of the column. Long cover plates will be particularly detrimental on short span girders with a high moment gradient.

• Similarly, very thick plates may cause problems. The large groove weld needed for the combined thickness of a thick cover plate and flange results in very high shrinkage and restraint. Furthermore, thick cover plates may generate high degrees of triaxial tension near the face of the column that can promote more brittle behavior. In the view of the writers, the design of the NSF test specimens may be somewhat preferable to the AISC and SAC specimens. The NSF specimens used relatively thin cover plates (about one-half of the beam flange thickness) combined with a fully welded web. Concerns about restraint and triaxial tension also suggest that the use of cover plated connections should be approached with great caution for beam sizes heavier than those already tested.

• The welding and fabrication sequence used for a cover plated connection may significantly affect problems associated with weld shrinkage and also affect the ability to ultrasonically test the welds. For example, the detail used at the top flange of the AISC specimens (Figure 6a) called for a single large groove weld for the combined thickness of the flange and cover plate. This large weld, in addition to causing potential problems due to shrinkage and restraint, is also difficult to inspect ultrasonically. The interface between the cover plate and flange blocks transmission of the ultrasonic wave, and increases the difficulty of inspection. In contrast, the detail used at the top flange of the NSF specimens (Figure 6b) called for the flange and cover plate to be welded and inspected separately. This resulted in a much smaller groove weld with less shrinkage. Further, the cover plate was groove welded to the column prior to being fillet welded to the beam flange, thereby minimizing shrinkage restraint. Finally, the welding sequence for the NSF specimens permitted more thorough ultrasonic testing, since the beam flange and cover plate groove welds could be tested separately. In general, the welding and fabrication sequence chosen for a cover plated connection should carefully consider shrinkage and restraint as well as inspectability. Consultation with a welding specialist may be beneficial in this regard.

• As discussed above, an investigation into the failure of specimen 5B raised concerns about the presence of a stress concentration in the groove weld at the interface of the cover plate and beam flange. Further analysis and testing is needed to better understand this problem. The use of high toughness weld metal may be helpful in miti- gating potentially detrimental effects of this stress concentration.

• Two of the test specimens (SAC-4 and NSF-6) experienced significant inelastic deformation in their column panel zones. The panel zone yielding had no apparent detrimental effects on these specimens. On the contrary, the panel zone contributed to the energy dissipation and ductility of these specimens. However, in a recent test of a cover plated connection with a weak panel zone, poor performance was reported ~7. Failure was reported to have occurred by a fracture within the column that appeared to initiate at the flange/web fillet of the column, after


large shear deformations occurred i n t h e columffp~inel zone. This data suggests large inelastic shear deformations of the panel zone may be detrimental. Clearly, further work is required to better define the most appropriate degree of panel zone participation in developing inelastic deformations. In the mean time, it may be pru- dent to develop designs which force most of the inelastic action into the beam. Note that panel zone effects are a concern for many connection types, not just cover plated connections.

4.3. Design implications The limited data available on cover plated connections does not warrant the development of detailed design guidelines. Further testing and analysis are needed to better understand the behavior of the connection before explicit design rec- ommendations can be dew,loped with confidence. Nonethe- less, from the currently available data, it is possible to infer some potentially importanl: design issues. Based on the data presented in this paper, combined with the judgment of the writers, the following suggestions are provided for the design of cover plated moment connections:

• Designs which combine an all-welded beam web connection with relatively thin and short cover plates appear preferable to designs w!ith a bolted web and very heavy and long cover plates. In general, excessively thick and long cover plates may be detrimental. The design of the NSF test specimens, with a welded web and a cover plate thickness equal to about one-half the beam flange thickness, appears to be a reasonable model.

• Use of high toughness weld metal for the flange and cover plate groove welds appears beneficial. The high toughness self-shielded FCAW electrodes used in the test specimens provided a minimum specified Charpy V- Notch toughness of 27 J at -29°C (20 ft-lbs at -20 F). Two different electrodes were used with this toughness specification; the E70TG-K2 and the E71T-8. Although these two electrodes have identical toughness specifi- cations, recent data suggest that the E71T-8 electrode actually provides higher toughness than the E70TG-K2, and may be preferable 1~'.

• Cover plated connections require the placement of one weld on top of another. The bottom flange detail shown in Figure 5 calls for the beam bottom flange field groove weld to be placed over the shop groove weld of the cover plate. Similarly, the top flange detail shown in Figure 6b calls for the field cover plate groove weld to be placed over a previously deposited field groove weld for the beam top flange. Recent experiences with cover plated connections indicate th~.t a loss of toughness can occur when one weld overlays another, and different electrodes are used for the two welds j9. Low toughness can occur in the weld metal mixture, even though each electrode, when used alone, provides high toughness. Thus, when- ever one weld overlays another, the same electrode should be used for both welds. Alternatively, if different electrodes are used, Charpy V-notch tests should be conducted on the weld mixture to assure that adequate toughness is provided.

• Proper welding practices, including enforcement of an appropriate welding procedure specification, should be emphasized.

• The welding sequence used to construct the connection

stio~faldbe eh0sen to minimize shrinkage and restraint and to permit maximum access for ultrasonic testing of groove welds. Consideration of shrinkage and restraint becomes increasingly critical as flange and cover plate thickness increase. The use of cover plates for beams heavier than those which have already been successfully tested should be approached with great caution. The degree of triaxial tension developed near the face of the column will likely increase for very thick beam flanges and cover plates. For these cases, other connection options may be preferable to cover plates. In any case, the use of cover plates for heavier beam sections should not be attempted without some verification testing.

5. Other connection options

In addition to cover plates, designers should also consider other connection options. Figure 1 illustrates some other types of reinforcement. The upstanding ribs as well as haunches have shown good performance in a limited number of tests 5-7,12,j3. Ribs and haunches avoid the very large groove welds required for cover plated connections and the associated potential problems of high shrinkage, restraint, and triaxiality. However, ribs and haunches introduce their own unique welding and inspection difficulties that must also be carefully considered in their design and fabrication. Further, available cost data suggest these options may be significantly more costly than cover plates 14'j5. Double haunch connections, while providing perhaps the greatest degree of connection reinforcement, appear to be particularly costly ~5 as well as taking up a substantial amount of space.

The writers have previously tested a side plate connection similar to that illustrated in Figure l d 5'6. The side plates were intended to avoid potential problems with column flange through-thickness properties by avoiding welds to the column flange. These connections proved extremely costly and showed rather poor performance. However, a modification of this concept employing somewhat different details has shown successful performance. This improved side plate detail, which is a proprietary design ~6, represents another useful connection option.

In addition to the reinforced connections discussed above, a number of other concepts have shown good performance in laboratory testing. In the view of the writers, one of the most promising details is the "dogbone" connection in which portions of the beam flange are trimmed away in the region adjacent to the beam-to-column connection. The result is similar to reinforcement, i.e. the connection is stronger than the beam. The dogbone may avoid many of the potential difficulties of reinforcement and may be less costly 15.

As new connection options are developed, and as new industry standards for moment connections are developed, very thorough and extensive test programs are essential for establishing the capabilities, limitations, and reliability of any proposed connection. A small number of tests is insuf- ficient for this purpose. For example, in the case of cover plated connections, two out of twelve test specimens showed unsatisfactory performance. Had only a small number of tests been conducted, all might have been successful. The result would have been considerably less information on the limitations of this connection. Test programs using

520 Reinforcing of steel moment connections: M. D. Engelhardt and T. A. Sabol

full scale member sizes and the full range of material strengths anticipated in actual construction are essential.

6. C o n c l u s i o n s

Moment connections reinforced with cover plates can provide very high levels of cyclic ductility. Out of 12 large scale cover plated moment connections tested by the writers, 10 showed excellent performance. Further, cost data suggest that cover plated connections are among the most economical of a number of different reinforcement concepts. The failure of two test specimens, however, indicates that cover plated connections are not fool-proof. This type of connection has limitations that must be considered in design and construction.

Cover plated connections are just one of a number of different moment connection concepts available to designers. Many other connection types have shown promising performance in laboratory testing. Designers must carefully weigh the options based on the latest available test and cost data when choosing a moment connection. No single connection type is likely to be appropriate for all conditions. Ultimately, as new industry standards are developed for steel moment connections, all proposed connection concepts, including cover plated connections, should undergo very extensive testing, analysis, and scrutiny. Indiscrimi- nate application of connection designs based on limited testing must be avoided in order to avoid repeating the mis- takes of the past.

A c k n o w l e d g e m e n t s

The writers gratefully acknowledge financial support provided by the following organizations for the investigations reported herein: the American Institute of Steel Construc- tion Inc., the National Science Foundation (Grant nos CMS-9358186 and CMS-9416287), the SAC Joint Venture, and the J. Paul Getty Trust. The writers also gratefully acknowledge generous donations of materials and fabrication from the Structural Shape Producers Council, PDM Strocal, Inc., the Herrick Corporation, W&W Steel Com- pany, and the Lincoln Electric Company. The opinions expressed in this paper are those of the writers, and do not necessarily reflect the views of the organizations noted above.

R e f e r e n c e s 1 Tsai, K.C. and Popov, E.P. 'Steel beam-column joints in seismic

moment resisting frames', Report no. UCB/EERC-88/19, Earthquake

Engineering Research Center, University of California, Berkeley, CA, 1988

2 Engelhardt, M. D. and Popov, E. P. 'Behavior of long links in eccen- trically braced frames', Report no. UCB/EERC-89/OI, Earthquake Engineering Research Center, University of California, Berkeley, CA, 1989

3 Tsai, K.C. and Popov, E.P. 'Seismic design of steel beam-to-box column connections', in Proc. Structures Congr. '93, ASCE, 1993

4 Joanes, L. J. 'Investigation of groove welds for seismic steel moment connections', Masters Report, Department of Civil Engineering, The University of Texas, Austin, TX, 1992

5 Engelhardt, M. D. and Sabol, T. A. 'Testing of welded steel moment connections in response to the Northridge earthquake', Northridge Steel Update 1, American Institute of Steel Construction, 1994

6 Engelhardt, M.D. and Sabol, T.A. 'An overview of the AISC Northridge moment connection test program', in Proc. 1995 Nat. Steel Construct. Conf., American Institute of Steel Construction, 1995

7 Shuey, B. D., Engelhardt, M. D. and Sabol, T. A. 'Testing of repair concepts for damaged steel moment connections', Report to the SAC Joint Venture, Ferguson Structural Engineering Laboratory, The Uni- versity of Texas, Austin, TX, 1996

8 Structural Welding Code - Steel, AWS D1.1-94, American Welding Society, Miami, FL, 1994

9 Rajan, V. B. 'Report on welds from SMRF connections tested at the University of Texas at Austin', Lincoln Electric Company, Cleve- land, 1995

10 'Evaluation of two moment resisting frame connectors utilizing a cover-plate design', British Steel Report no. SL/EM/R/S1198/18/95/D, 1995

11 Engelhardt, M. D. and Husain, A. S. 'Cyclic loading performance of welded flange - bolted web connections', J. Struct. Engng, ASCE 1993, 119 (12), 3537-3550

12 Uang, C. M. and Bondad, D. "Static cycle testing of pre-Northridge and haunch repaired steel moment connections', Report no. SSRP- 96/02, Division of Structural Engineering, University of California, San Diego, CA, 1996

13 Popov, E. P., Blondet, M., Stepanov, L. and Stojadinovic, B. 'Full- scale steel beam-column connection tests', Report to the SAC Joint Venture, Department of Civil Engineering, University of California, Berkeley, CA, 1996

14 The Herrick Corporation, "Cost comparisons - University of Texas at Austin test specimens', Northridge Steel Update 1, American Insti- tute of Steel Construction, 1994

15 Engelhardt, M. D., Winneberger, T., Zekany, A. and Potyraj, T. 'The dogbone connection - Part 11', Modern Steel Construction, 1996

16 Uang, C. M. and Latham, C.T. 'Cyclic testing of full-scale MNH- SMRF ru moment connections', Report no. TR-95/01, Structural Sys- tems Research, University of California, San Diego, CA, 1995

17 Obeid, K. 'Steel moment frame connections: shear in the panel zone', NEWS - Structural Engineers Association of Northern California, San Francisco, CA, 1996

18 Tide, R. H. R. 'Stability of weld metal subjected to cyclic static and seismic loading', US-Japan Seminar on Innovations in Stabili~ Con- cepts and Methods for Seismic Design in Structural Steel, Honol- ulu, 1996

19 Wolfe, J., Nienberg, M., Manmohan, D., Halle, J. and Quintana, M. 'Welding alert, cover plated moment frame connections', NEWS - Structural Engineers Association of Northern California, San Franci- sco, CA, 1996

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