The seismic behaviour of small reinforced concrete beam-column knee joints
DOI:
https://doi.org/10.5459/bnzsee.31.4.215-245Abstract
The majority of research into beam-column knee joints has been conducted with monotonic loading. Many of these joints failed to reach their member moment capacity, especially under opening moments, while a few cyclic knee joint tests have been completed in the United States this decade. This paper describes the cyclic testing of 8 small knee joints designed to the 1995 New Zealand Concrete Standard. In addition two joints designed and detailed to the 1965 N.Z. Concrete Code were also tested. Joints with U-bar anchorages performed better than joints with standard 90 degree hook details on beam and column bars. The current Concrete Standard (NZS3101:1995) designs usually attained their nominal moment capacity in both directions up to and including ductility 4 displacements, but subsequently strengths fell off at higher ductilities. Joints with extra diagonal bars across the inner comer were able to sustain their nominal member strengths to higher ductility levels, especially under opening moments. A maximum horizontal joint shear stress of 0.12 f’c, for knee joints, in ductile frame buildings is recommended, where this limit is 60% of the current NZS3101:1995 Standard recommendation. An approximate 25% degradation of the joint shear stress occurred as displacement ductility factors increased from 1 to 8. The 1960's designed joints behaved poorly, as expected, with joint shear and anchorage failures occurring, in both moment directions, at strength levels below the beam's nominal strength. A maximum joint shear stress of only 0.072 f’c was reached and this fell to about a third of that stress between displacement ductility factors of 1 and 4 under closing moments.
References
Nilsson, I.H.E. (1973), Reinforced Concrete Comers and Joints Subjected to Bending Moment, Document D7: 1973, National Swedish Building Research.
Mayfield, B., Kong F-K., Bennison, A. and Twiston-Davies, J.C.D. (1971), Comer Joint Details in Structural Lightweight Concrete, Title No. 68-37, ACI Journal, 68-5, pp 366-372.
Mayfield, B., Kong, F-K. and Bennison, A. (1972), Strength and Stiffness of Lightweight Concrete Comers, Title No. 69-38, ACI Journal, 69-7, pp 420-427.
Skettrup, E., Strabo, J., Anderson, N.H. and Brondum- Nielsen, T. (1984), Concrete Frame Comers, Title No. 81-46, ACI Journal, 81, Nov.-Dec., pp 587-593.
Megget, L.M. (1994) The strength of small reinforced concrete beam-column knee joints under opening moments, Australasian Structural Engineering Conference, Sydney, Australia, 21-23 September, Vol. 2, pp1125-1131.
NZS3101 (1982), Code of Practice for the design of concrete structures, Standards Association of New Zealand, 127pp.
Bari, A.A. (1989), Reinforced Concrete Comers subjected to opening bending moments, University of Auckland, Dept. of Civil Engineering Master of Engineering project report, supervised by RC Fenwick, February, 86pp.
Priestley, M.J.N. (1993), Assessment and design of joints for single level bridges with circular columns, Structural Systems Research Project, Report No. SSRP 93/02, Dept. of Applied Mechanics and Engineering Sciences, University of California, San Diego, La Jolla, February, 62pp.
Mazzoni, S., Moehle, J.P. and Thewalt, C.R. (1991), Cyclic response of RC beam-column knee joints- test and retrofit, Report No. UCB/EERC-91/14, EERC and Dept. of Civil Engineering, Berkeley, California, 18pp.
NZS3101 (1995), Concrete Structures Standard, Part 1, The design of concrete structures, Standards New Zealand, 256pp.
Nilsson, I.H.E. and Losberg, A. (1971), A discussion of the paper "Opportunities in bond research" by ACI Committee 408, Proc. ACI Journal, Vol. 68, No. 5, pp 393-396.
Cote, P.A. and Wallace, J.W. (1994), A study of reinforced concrete knee joints subjected to cyclic lateral loading, Report No. CU/CEE 94/04, Dept. of Civil Engineering, Clarkson University, Potsdam, New York, 143pp.
ACI Committee 318 (1991), Building Code Requirements for Reinforced Concrete, American Concrete Institute, Detroit, ML
ACI-ASCE Committee 352 (1991), Recommendations for Design of Beam-Column Joints in Monolithic Reinforced Concrete Structures, (ACI-ASCE 352R), American Concrete Institute, Detroit, MI.
McConnell, S.W. and Wallace, J.W. (1994), The use of T-headed bars in reinforced concrete knee joints subjected to cyclic lateral loading, Report No. CU/CEE-94/10, Dept. of Civil Engineering, Clarkson University, New York, 44pp.
McConnell, S.W. and Wallace, J.W. (1994), A study of the cyclic behavior of reinforced concrete knee joints, Report No. CU/CEE-94/11, Structural Engineering Mechanics and Materials, Clarkson University, New York.
Wallace, J.W., McConnell, S.W. and Gupta, P. (1996), Cyclic behavior of RC beam-column joints constructed using conventional and headed reinforcement, 11th World Conference on Earthquake Engineering, CD-ROM Paper No. 655, Acapulco, Mexico, 8pp.
NZSS1900: Chap.9.3 (1964), New Zealand Standard Model Building Bylaw, Design and Construction, Division 9.3, Concrete, NZ Standards Institute, July, 60pp.
Park, R. and Paulay, T. (1975), Reinforced Concrete Structures, Published by John Wiley & Sons, 786pp DOI: https://doi.org/10.1002/9780470172834
Cheung, P.C., Paulay, T. and Park, R. (1991), Some possible revisions to the seismic provisions of the NZ Concrete Design Code for moment resisting frames, Proc. Pacific Conference on Earthquake Engineering, Auckland, Vol. 2, pp 79-91.
Paulay, T. and Priestley, M.J.N. (1992), Seismic Design o f Reinforced Concrete and Masonry Buildings, Published by John Wiley & Sons, New York, 744pp. DOI: https://doi.org/10.1002/9780470172841
Ingham, J.M., Priestley, M.J.N. and Seible, F. (I994), Seismic performance of bridge knee joints - Volume 1, Rectangular column/cap beam experimental results, Report No. SSRP-94/12 Structural Systems Research Project, University of California, San Diego, La Jolla, 280pp.
Priestley, M.J.N. (1996), Displacement-based seismic assessment of existing reinforced concrete buildings, NZNSEE Bulletin, Vol. 29, No. 4, pp 256-272.
Park, R. (1997), A static force-based procedure for the seismic assessment of existing reinforced concrete moment resisting frames, Bull. NZNSEE, Vol. 30, No. 3, September, pp 213-226.
Megget, L.M. (1998), The seismic behaviour of small reinforced concrete knee joints with anchor plates, IPENZ Annual Conference Proceedings, "The Sustainable City", 12-16 February, Auckland, Vol. 3, pp 1-6.
Megget, L.M. (1998), The seismic behaviour of small reinforced concrete beam-column knee joints, Auckland Uniservices Ltd., Report No. 6262.00, prepared for the New Zealand Earthquake Commission Research Foundation, January, 114pp.
