Seismic Retrofitting of Realistic Beam–Column Joints with Shear Failure Using FRP Sheets and FRP Anchors
Publication: Journal of Composites for Construction
Volume 28, Issue 4
Abstract
One of the common deficiencies of pre-70s reinforced concrete (RC) moment frames is the shear failure of beam–column (BC) joints. In addition to that, joints with high tension demand have limited shear capacity to resist seismic actions, according to the New Zealand (NZ) seismic assessment guidelines. Fiber-reinforced polymers (FRP) have been widely used in research and practice for seismic retrofitting of BC joints prone to shear failure. However, in many cases, the tests were conducted on two-dimensional BC joints, which does not necessarily represent the limitations structural engineers face in practice, such as the presence of the diaphragm slab, transverse beams, and access limitations to the joint panel. The American Concrete Institute guidelines for FRP strengthening only mention that strengthening joints is possible but do not provide design guidance. The European fib Bulletin 90 provides design guidance but assumes that the joint panel is always available to install FRP, which is rarely the case in real structures. We have proposed a design method for shear strengthening of beam–column joints using FRP sheets and FRP anchors based on the adaptation of aforementioned design documents, published research, and first principles of engineering. The intention is for this case study to inspire other engineers who face the same issue and to encourage further research in this area.
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Data Availability Statement
No data, models, or codes were generated or used during the study.
Acknowledgments
The authors appreciate the professionalism and experience provided by the FRP contractor, FRP Solutions, and especially Mikey Kennedy and Ryan Cudby.
Notation
The following symbols are used in this paper:
- Adowel
- cross-sectional area of the anchor dowel;
- Ag
- gross area of the member section;
- Aj
- effective horizontal joint area;
- Ajh
- total area of horizontal joint shear reinforcement parallel to the direction being considered;
- As
- area of the anchor fan;
- b
- substrate width;
- bf
- FRP width;
- bj
- effective width of the joint;
- d
- effective depth of the beam;
- d0
- diameter of the anchor hole;
- Ea
- modulus of elasticity of the dowel;
- Ef
- modulus of elasticity of the FRP fabric;
- Fmax
- maximum force that an FRP sheet bonded to a concrete block can resist in accordance with fib Bulletin 90;
- fcm
- mean compressive strength of concrete;
- concrete compressive strength;
- ffbk
- 95 percentile design debonding stress for end peeling failure;
- ffbk,IC
- IC debonding failure stress;
- fy
- yield strength of steel;
- fyt
- yield strength of the joint shear reinforcement;
- fu
- tensile/ultimate strength of steel;
- Gfd
- fracture energy or the energy necessary to fracture a thin top layer of concrete and initiate the debonding;
- hb
- beam section depth;
- hc
- depth of the column section;
- hef
- embedment depth;
- hj
- height of the joint panel;
- jd
- lever arm;
- kb
- factor accounting for the ratio of FRP width to substrate width (bf/b);
- kj
- coefficient for calculating the principal tension stress limit in a beam–column joint;
- le
- minimum bond length;
- axial load demand (combined gravity and seismic);
- Nfr
- fiber rupture force capacity;
- Nsd
- fan-to-sheet failure mode force capacity;
- average combined pullout and concrete cone capacity of FRP anchors;
- average concrete cone capacity of FRP anchors;
- nf
- number of fabric layers;
- tf
- thickness of a single layer of fabric;
- tf,h
- total thickness of the FRP sheet;
- Vj
- nominal joint shear strength;
- Vjh
- horizontal shear demand of the joint;
- Vp,jh
- probable horizontal joint shear strength;
- Vsb
- shear bond strength of the epoxy resin;
- vp,jh
- probable horizontal joint shear stress capacity;
- vp,jh,c
- principal compression stress;
- vp,jh,t
- principal tension stress;
- wb
- width of the beam;
- α
- fanning angle in degrees (half angle);
- αj
- coefficient relating the total and effective areas of joint transverse reinforcement;
- βl
- multiplier if the bond length is shorter than the minimum bond length;
- γ
- accounts for the effects of transverse beams and transverse reinforcement of the joint;
- average failure strain of the dowel;
- design strain of the FRP;
- lower bound tensile strain of steel; and
- ϕo
- overstrength factor.
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Journal of Composites for Construction
Volume 28 • Issue 4 • August 2024
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© 2024 American Society of Civil Engineers.
History
Received: Jul 24, 2023
Accepted: Feb 21, 2024
Published online: May 13, 2024
Published in print: Aug 1, 2024
Discussion open until: Oct 13, 2024
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