Introduction and Statement of the Problem
The 22 February 2011 Christchurch earthquake resulted in a building stock with wide spread seismic damage. Accordingly, amendments were made to the Building Act 2004, requiring territorial authorities to conduct seismic assessments of all non-residential and multi-unit, multi-storey residential buildings within five years (2013-2018). Then, the building owners are required to strengthen or demolish any earthquake-prone structures within 15 years (2018-2033). Therefore, a sudden demand on the assessment and retrofit of existing RC structures emerged, which is usually not taught in engineering schools. The existing literature on these is mostly academic and distant from practical applications. Among these retrofit methods in literature, calculation of strength hierarchy at an RC beam column joint is especially important in order to determine the failures at an RC frame structure. In this work, the existing method for evaluation of strength hierarchy for beam column joints used in the past experimental work at University of Canterbury is adapted and improved for use in multi-storey buildings. Application of the method has been confirmed using a pre 1970s and a modern construction.
Benefits of the method
- Application can be done in a matter of hours.
- The engineer understands the structural behavior and its parameters.
- Manual calculations, a spread sheet software and basic reinforced concrete knowledge is required.
- Until the 1st failure of the system occurs, the portal frame gives very accurate estimations since the system is essentially elastic, which is the most important value for the hierarchy of strength assessment.
- Can give a good guidance on what type of retrofit option is viable for the given structural system.
Steps of Strength Hierarchy Assessment Method:
Portal frame analysis to represent the demand at each beam-column joint (axial force, Ni, and moment, Mi) under increasing Fi levels
Beam capacity represented as constant moment on N-M diagram
Axial force N and bending moment M interaction diagrams for the considered columns
Joint shear capacity represented as a function on N-M diagram
Column shear capacity represented as moment on N-M interaction diagram.
The hierarchy of strength evaluated at considered joint’s column N-M interaction diagram. The order of member failures, or hierarchy of strength, are designated with the given numbers.
Example Application: Vulnerable Pre 1970s Test Specimen
The summarized method has been applied using a previous experimental study (Pampanin et al. 2007; Pampanin et al. 2004). The considered test specimen is a 2D RC frame without transverse reinforcement at beam column joint panel zones. The type of detailing used in this specimen simulated a vulnerable beam column joint with beam bars bent 180 degree into the joint panel zone, typical of pre 1970s. The testing of this specimen confirmed the damage vulnerability of beam column joints without any transverse reinforcement. The external beam column joints at the 1st and 2nd floor levels showed joint shear failure. On the other hand, internal joints did not suffer any shear failure except for column plastic hinging at the 1st and 2nd floor levels. This experimental damage and associated capacity values have been validated with the application of the reported assessment method. Some of results for this application are shown below.
Example Application: Modern Structure
In order to validate the applicability of the method for modern buildings, a ten storey building has been used. The building is a typical building designed according to NZS 1170.5 (Bull and Brunsdon 2008). The hierarchy of strength assessment confirmed the beam sway mechanism, the assumed mechanism in modern capacity design. The beams at external beam column joints of the 1st floor level are expected to reach their ultimate moment carrying capacity at 1150-1200 kN base shear. Considering the design base shear for this structure is approximately 1102 kN, the method seems to give quite accurate estimations. For reference, the strength hierarchy comparison of the 1st floor external beam column joint is shown below.
Strength hierarchy comparison for the external beam column joint at 1st floor level of the ten storey modern building example (Redbook building by Bull and Brunsdon 2008)
A simple method to evaluate the hierarchy of member capacities (or hierarchy of strength) at beam column joints of RC multi-storey structures has been reported. The method has been validated using two example structures. One of these examples was a tested RC frame that simulated vulnerable pre 1970s construction practice. The other example was a 10 storey RC frame system designed according to modern design codes and capacity design principles. The results of these assessments confirmed that the method can realistically estimate the failure mode of the RC frame structures. In addition, the global base shear corresponding to the identified failures can be easily computed as part of the developed method.
The author would like to express his gratitude to UC Quake Centre for funding this project. Suggestions and comments given by the project’s steering team members Robert Finch (UCQC), Des Bull (Holmes Consulting Group), Carl Ashby (Opus) and Richard Sharpe (BECA) are appreciated. Last but not least, the author is grateful for the background information, sources and comments provided by Umut Akguzel (Ramboll UK Ltd.) and Stefano Pampanin (University of Canterbury).