Eccentric Beam-Column Connections

Beam-column connections are critical regions in reinforced concrete (RC) moment-resisting frame
structures designed to resist strong earthquakes. Many experimental and analytical research studies have been conducted to investigate the behavior of RC beam-column connections subjected to seismic loading and to establish guidelines for design. From these studies, several key design parameters governing the behavior of RC beam- column connections have been identified, such as the relative column-versus-beam flexural strengths at the connection (including slab effects), confinement of the joint core, joint shear stress, and anchorage of reinforcement in the connection region. Joint ACI-ASCE Committee 352, Joints and Connections in Monolithic Concrete Structures, has integrated the available research results into a state- of-the-art report entitled, “Recommendations for Design of Beam-Column Connections in Monolithic Reinforced Concrete Structures (ACI 352R-02).”1 Many of the research findings are also reflected in seismic design provisions found in Chapter 21 of ACI 318-05, Special Provisions for Seismic Design.2
When an RC beam-column connection is subjected to lateral earthquake loading, the beam tension T and compression C forces from bending are transmitted to the joint at the beam-column interfaces, producing relatively large joint shear forces. In an eccentric connection,
Eccentric Beam-Column Connections
Performance and design of joints subjected to seismic lateral load reversals
the column centerline is offset in plan from the beam centerline (by an eccentricity e, as indicated in Fig. 1), thereby concentrating the joint shear toward one side of the joint. Due to this eccentricity, the transmitted forces can also introduce torsion into the joint, adding to the shear stresses. ACI 352R-02 addresses eccentric RC beam-column connections to only a limited extent. The provisions for eccentric connections are based on information about their poor joint shear performance in practice3 and on two research studies,4,5 where a total of six experimental subassemblies were tested to assess the effect of eccentricity on joint behavior. The design approach taken in Section 4.3 of ACI 352R-02 is simply to place additional restrictions on the permissible joint shear force in connections where the eccentricity between the beam and column centerlines e exceeds 1/8 of the width of the column bc . Section 21.5.3 of ACI 318-05 has adopted a similar approach, with provisions for assessing the design joint shear strength of eccentric connections that are in some cases even more restrictive than those in ACI 352R-02. Codes and guidelines for seismic design of RC structures elsewhere in the world (New Zealand and Japan, for example) use comparable approaches to address eccentric joints.
Section A.1 of ACI 352R-02 further notes that the effect of eccentric beams on joints is an area in need of additional
BY JAMES M. LAFAVE, JOHN F. BONACCI, BURCU BURAK, AND MYOUNGSU SHIN
Concrete international / SEPTEMBER 2005 59
research. This is particularly so in light of how common this type of connection is in exterior RC building frames, where beams often frame into a column with flush outside beam and column faces (as in a building that exhibited noticeable joint damage in a recent strong earthquake6). In response to this need, Joint ACI-ASCE Committee 352 appointed a task group to investigate, review, and summarize all research currently available on the subject of eccentric RC beam-column connections and to propose design recommendations compatible with ACI 352R-02, as appropriate. This article is the product of those efforts.
SUMMARY OF AVAILABLE RESEARCH Eleven research studies on eccentric RC beam-column
connections subjected to reverse-cyclic lateral loading were found in the literature, comprising nearly 40 test specimens (experimental subassemblies). The tests included 22 interior (cruciform) connections4,5,7-14 and 15 corner connections.15,16 A brief summary of many of these testing programs can be found elsewhere.17 Five of the interior connections were edge connections with a floor slab and transverse beam on one side only (see Fig. 2).9,10,14 The rest of the interior connections did not have floor slabs or transverse beams.4,5,7,8,11-13 Of the corner connections, four out of five in one study had spread-ended (tapered width) beams to minimize eccentricity at the joint,15 while all 10 in the other study were so eccentric that the beam often framed into a perpendicular girder rather than directly into the column.16 Modest axial column compression loads were applied to some specimens prior to testing.4,5,8,10-12,14
It was noted in most of the studies that eccentric connections had somewhat lower joint shear strengths and an earlier onset of strength degradation (less distortion capacity) than similar concentric connections. Some researchers proposed addressing this by using a modified (reduced) effective joint width in design for eccentric connections.5,8,9,11,15 Eccentric connections were
almost always observed to experience greater stiffness degradation in the joint than comparable concentric connections due to extensive joint cracking at the flush face of the column and beams. This damage was typically noted in conjunction with larger joint shear deformations and/or larger joint hoop strains near the flush side of the joint.4,5,8,13,15 However, differences in joint hoop strains between the flush exterior side of a joint and the offset interior side were much less when a floor slab and transverse beam were present.9,10 The small differences seen in such cases were actually quite similar to those noted in concentric edge connections with floor slabs and transverse beams.17
e
bb
hc
bc
Fig. 1: (a) Plan view; and (b) isometric view of typical eccentric RC beam-column connection (floor slab not shown)
Fig. 2: Eccentric RC beam-column edge connection test with floor slab and transverse beam
(a) (b)
60 SEPTEMBER 2005 / Concrete international
The relatively flexible joints observed with eccentric RC beam-column connections were found to contribute about 1/6 to 1/4 of the overall story displacements at low to moderate subassembly drifts (approximately 1 to 2%). The contribution increased to about 1/3 to 1/2 of the overall subassembly drift at larger displacements (when significant joint damage had occurred).4,5,8,9,12,14,15
Finally, the presence of a floor slab was observed to add to the total possible joint shear demand in an eccentric connection, but it also appears that the slab (along with the transverse beam, when present) effectively
reduced the connection eccentricity and aided in the resistance mechanisms.9,10 Both eccentric and concentric edge connections typically exhibited greater effective slab widths than those commonly prescribed for use in design.17
EFFECTIVE JOINT WIDTHS To better evaluate the joint shear capacity of eccentric
RC beam-column connections, a detailed examination was conducted of all 16 eccentric connection subassemblies found in the literature that were reported to have failed
Specimen (1)
bj,est / (bb+bc)/2
(10) Joh, Goto,
and Shibata4 JX0-B5 5.9 11.8 0.25 66.0 8.03 1.36 1.05 1.09 0.91
Raffaelle and Wight5
1 10 14 0.14 146 13.5 1.35 1.12 1.08 1.12
2 7 14 0.25 94.6 9.02 1.29 0.99 1.03 0.86
3 7.5 14 0.23 106 8.54 1.14 0.89 0.91 0.79
4 7.5 14 0.23 92.7 10.4 1.39 1.09 1.11 0.97
Teng and Zhou8
S3 7.9 15.7 0.25 161 15.9 2.02 1.64 1.61 1.34
S6 7.9 15.7 0.25 87.9 12.5 1.56 1.36 1.25 1.04
Shin and LaFave9
1* 11 18 0.19 145 14.1 1.29 1.09 1.03 0.97
2* 7 18 0.31 146 13.0 1.85 1.45 1.48 1.04
Burak and Wight10
2-S* 10 21 0.26 194 15.4 1.54 1.27 1.23 0.99
3-S* 10 21 0.26 186 17.1 1.71 1.42 1.37 1.10
Goto and Joh11 UM-60 7.9 17.7 0.13 175 20.7 1.59 1.81 1.27 1.62
UM-125 7.9 17.7 0.28 148 17.3 2.20 1.79 1.76 1.35
Kamimura, Takimoto, and
NN.2 7.1 13.8 0.16 94.2 11.0 1.17 1.13 0.93 1.06
Kusuhara et al.13
JE-55 7.1 12.6 0.17 104 12.6 1.53 1.35 1.22 1.28
JE-55S 7.1 12.6 0.17 105 12.7 1.54 1.36 1.23 1.29
Average 1.53 1.30 1.23 1.11
Standard deviation 0.30 0.28 0.24 0.22
Range 1.14
to 1.62
Note: 1 in. = 25.4 mm; 1 kip = 4.45 kN * Indicates specimens with floor slab and transverse beam on one side of joint
TABLE 1: EFFECTIVE JOINT WIDTHS FOR ECCENTRIC RC BEAM-COLUMN CONNECTIONS
Concrete international / SEPTEMBER 2005 61
due to joint shear.4,5,8-13 They are listed in numbered Columns 1 to 4 of Table 1 along with their beam width bb, column width bc, and normalized eccentricity e/bc values (see also Fig. 1). These specimens were all interior connections (most of which had one beam face flush with the exterior face of the column) and had well- distributed and well-detailed joint hoop reinforcement. Four of the specimens had a floor slab and transverse beam on one side of the joint, as noted below the table. During testing, a few of the specimens underwent beam plastic hinging in conjunction with joint shear failure. The joint shear strength for each specimen was considered to be equal to the maximum shear force applied to the joint during the test (Vj,m) and is given in Column 5 of Table 1. The method used to determine the maximum joint shear force from the maximum applied column shear force during testing is provided elsewhere.17
The estimated effective joint width bj,est, shown in Column 6 for each specimen, was calculated from the experimentally-determined maximum joint shear force as
(1)
where ƒ′c is the actual measured concrete compressive strength (psi) and hc is the column depth (in.), with bj,est in inches, Vj,m in lb, and √ƒ′c in psi. The joint shear stress factor specified by ACI 352R-02 and ACI 318-05 (γn = 12) is used to place eccentric connections on an equal basis for comparison with similar concentric connections. (If Eq. (1) were used with Vj,m in N, ƒ′c in MPa, and hc and bj,est in mm, then γn would be 1.0.) For the 16 specimens, ƒ′c ranged from about 2800 to 5600 psi (19 to 39 MPa), and the column cross-sectional aspect ratio (hc /bc) ranged from 0.50 to 1.00.
Table 1 also contains ratios of the estimated effective joint width bj,est to the effective joint widths computed following ACI 318-05 (bj,318) and ACI 352R-02 (bj,352). Per ACI 318-05, bj,318 = bb + 2x, where x is the smaller distance between the beam and column edges. Most of the connections tabulated are one-sided (flush) eccentric connections where the bj,318 value is by definition simply equal to the beam width (bb). Per ACI 352R-02, bj,352 = bb + Σmhc /2, where m is 0.3 when e is greater than bc /8 and m is 0.5 otherwise. The ACI 318-05 effective joint width definition could be conservative (simply representing the width of the eccentric beams in most of the tabulated cases). Such a narrowly defined joint region might be considered as effectively confined on at least two opposite vertical faces, a case where γn is typically taken as 15 in design (1.25 if N and mm units are used). The b ′j,est values used in the table were computed using Eq. (1) with γn = 15. Finally, the estimated experimental effective joint
width of each specimen was compared to the basic effective joint width definition currently used by ACI 352R-02 for concentric connections, namely bj = (bb + bc)/2.
The ACI 318-05 approach (see Column 7 of the table) systematically underestimates the joint shear strength of eccentric connections (by an average of more than 50%), especially in cases with floor slabs and transverse beams. The ACI 352R-02 effective joint width definition (tabulated in Column 8) was found to be about 30% conservative on average. A comparison of Column 9 to Column 7 in the table indicates that the computed b ′j,est values were closer to the bj,318 values than were the bj,est values. Finally, the simple effective joint width (see Column 10 in the table) equal to the average of the beam and column widths matched the estimated experimental value fairly well on average and had the smallest standard deviation of all the approaches examined. This effective joint width definition worked especially well for cases with floor slabs and transverse beams. While it was not conservative for all cases without slabs, in only one specimen did it give a nominal joint shear strength prediction slightly less than 85% of the experimentally determined value.
RECOMMENDATIONS Based on the preceding information, it is recommended
that an effective joint width (bj) equal to the average of the beam and column widths [(bb + bc )/2] can be used to estimate the joint shear strength of eccentric RC beam-column connections for design. This recommendation should be considered in conjunction with the other design provisions of ACI 352R-02 such as using a design yield stress multiplier of at least 1.25 and including the contribution to joint shear forces (“demand”) of slab reinforcing steel within an appropriate effective tension flange width.
Acknowledgments The authors would like to thank fellow members of Joint ACI-ASCE
Committee 352, Joints and Connections in Monolithic Concrete
Structures, for their constructive comments and suggestions
regarding the subject of this article.
References 1. Joint ACI-ASCE Committee 352, “Recommendations for Design
of Beam-Column Connections in Monolithic Reinforced Concrete
Structures (ACI 352R-02),” American Concrete Institute, Farmington
Hills, MI, 2002, 37 pp.
2. ACI Committee 318, “Building Code Requirements for Structural
Concrete (ACI 318-05) and Commentary (ACI 318R-05),” American
Concrete Institute, Farmington Hills, MI, 2005, 430 pp.
3. Ohno, K., and Shibata, T., “On the Damage to the Hakodate
College by the Tokachioki Earthquake, 1968,” Proceedings of the U.S.-
Japan Seminar of Earthquake Engineering with Emphasis on the Safety
of School Buildings, Sendai, 1970, pp. 129-144.
62 SEPTEMBER 2005 / Concrete international
4. Joh, O.; Goto, Y.; and Shibata, T., “Behavior of Reinforced
Concrete Beam-Column Joints with Eccentricity,” Design of Beam-Column
Joints for Seismic Resistance, SP-123, James O. Jirsa, ed., American
Concrete Institute, Farmington Hills, MI, 1991, pp. 317-357.
5. Raffaelle, G.S., and Wight, J.K., “Reinforced Concrete Eccentric
Beam-Column Connections Subjected to Earthquake-Type Loading,”
ACI Structural Journal, V. 92, No. 1, Jan.-Feb. 1995, pp. 45-55.
6. Hirosawa, M.; Akiyama, T.; Kondo, T.; and Zhou, J., “Damages
to Beam-to-Column Joint Panels of RC Buildings Caused by the 1995
Hyogo-Ken Nanbu Earthquake and the Analysis,” Proceedings of the
12th World Conference on Earthquake Engineering, Paper No. 1321,
Auckland, New Zealand, 2000.
7. Lawrance, G.M.; Beattie, G.J.; and Jacks, D.H., “The Cyclic Load
Performance of an Eccentric Beam-Column Joint (Report 91-25126),”
Central Laboratories, Lower Hutt, New Zealand, 1991, 81 pp.
8. Teng, S., and Zhou, H., “Eccentric Reinforced Concrete
Beam-Column Joints Subjected to Cyclic Loading,” ACI Structural
Journal, V. 100, No. 2, Mar.-Apr. 2003, pp. 139-148.
9. Shin, M., and LaFave, J.M., “Seismic Performance of Reinforced
Concrete Eccentric Beam-Column Connections with Floor Slabs,”
ACI Structural Journal, V. 101, No. 3, May-June 2004, pp. 403-412.
10. Burak, B., and Wight, J.K., “Seismic Behavior of Eccentric
RC Beam-Column-Slab Connections Under Sequential Loading in
Two Principal Directions,” Innovations in Design with Emphasis on
Seismic, Wind and Environmental Loading; Quality Control and
Innovation in Materials/Hot-Weather Concreting, SP-209, V.M.
Malhotra, ed., American Concrete Institute, Farmington Hills, MI,
2002, pp. 863-880.
11. Goto, Y., and Joh, O., “Shear Resistance of RC Interior
Eccentric Beam-Column Joints,” Proceedings of the 13th World
Conference on Earthquake Engineering, Paper No. 649, Vancouver,
BC, Canada, 2004, 13 pp.
12. Kamimura, T.; Takimoto, H.; and Tanaka, S., “Mechanical
Behavior of Reinforced Concrete Beam-Column Assemblages with
Eccentricity,” Proceedings of the 13th World Conference on Earth-
quake Engineering, Paper No. 4, Vancouver, BC, Canada, 2004, 10 pp.
13. Kusuhara, F.; Azukawa, K.; Shiohara, H.; and Otani, S., “Tests
of Reinforced Concrete Interior Beam-Column Joint Subassemblage
with Eccentric Beams,” Proceedings of the 13th World Conference on
Earthquake Engineering, Paper No. 185, Vancouver, BC, Canada, 2004,
14 pp.
Eccentric Reinforced Concrete Beam-Column-Slab Connections Under
Earthquake Loading,” Proceedings of the 13th World Conference on
Earthquake Engineering, Paper No. 2150, Vancouver, BC, Canada, 2004,
14 pp.
15. Chen, C.C., and Chen, G.K., “Cyclic Behavior of Reinforced
Concrete Eccentric Beam-Column Corner Joints Connecting
Spread-Ended Beams,” ACI Structural Journal, V. 96, No. 3, May-
June 1999, pp. 443-449.
16. Vollum, R.L., and Newman, J.B., “Towards the Design of
Reinforced Concrete Eccentric Beam-Column Joints,” Magazine of
Concrete Research, V. 51, No. 6, Dec. 1999, pp. 397-407.
17. Shin, M., and LaFave, J.M., “Reinforced Concrete Edge
ACI member James M. LaFave, PE, is an Associate Professor of Civil Engineering at the University of Illinois at Urbana- Champaign. He is the Chair of Joint ACI-ASCE Committee 352, Joints and Connections in Monolithic Concrete Structures, and is a member of ACI Committees 439, Steel Reinforcement, and E 802, Teaching Methods and Educational
Materials. His research interests include earthquake-resistant design of reinforced concrete structures and durability of structural concrete.
John F. Bonacci, FACI, is an Associate Professor of Civil Engineering at the University of Toronto, Ontario, Canada. He is a member and past Chair of Joint ACI-ASCE Committee 352, Joints and Connections in Monolithic Concrete Structures, and is a member of ACI Committees 374, Performance-Based Seismic Design of Concrete Buildings; 318-D, Flexure
and Axial Loads: Beams, Slabs, and Columns; and Joint ACI-ASCE Committee 445, Shear and Torsion.
ACI member Burcu Burak is a PhD candidate in civil engineering at the University of Michigan, Ann Arbor, MI. She is an associate member of Joint ACI-ASCE Committee 352, Joints and Connections in Monolithic Concrete Structures. Her research interests include earthquake- resistant design, analysis, and rehabilitation of reinforced concrete and fiber-reinforced
composite structures.
ACI member Myoungsu Shin is an Assistant Professor of Industrial and Engineering Technology at Morehead State University, Morehead, KY. He received his PhD in civil engineering from the University of Illinois at Urbana-Champaign. He is an associate member of Joint ACI-ASCE Committee 352, Joints and Connections in Monolithic Concrete Structures. His research interests include
earthquake-resistant design of reinforced concrete structures.
Beam-Column-Slab Connections Subjected to Earthquake Loading,”
Magazine of Concrete Research, V. 56, No. 5, June 2004, pp. 273-291.
Received and reviewed under Institute publication policies.