Michael C. Griffith

EVALUATION OF OUT-OF-PLANE STABILITY OF UNREINFORCED MASONRY WALLS SUBJECTED TO SEISMIC EXCITATION

The paper presents a systematic assessment of a simplified procedure to evaluate the response of unreinforced masonry walls subjected to out-of-plane seismic excitation. The nonlinear force-displacement response of a wall is idealised by means of a suitable tri-linear curve. The meaningful parameters characterising the walls and different ground motions were combined for a total of 1248 case studies. For each combination of para-meters, a nonlinear SDOF dynamic time-history analysis was performed, and the results were taken as the reference for a simplified “equivalent stiffness” approach. It is shown how a suitably accurate prediction of collapse can be made by using appropriate stiffness values and elastic response spectra. Among the most relevant results for applications, it appears that initial stiffness (and therefore initial period) is not crucial in determining the occurrence of collapse. Instead, collapse depends primarily on the second and third branches of the tri-linear force-displacement relationship, i.e. on maximum strength and ultimate displacement capacity. It is shown how these latter parameters axe only mode-rately sensitive to material mechanical parameters which are usually affected by strong uncertainty when assessing an existing building, namely the elastic modulus E and the compressive strength of masonry.

Performance of Unreinforced Masonry Buildings during the 2010 Darfield (Christchurch, NZ) Earthquake

Abstract The 2010 Darfield earthquake caused extensive damage to a number of unreinforced masonry buildings. While this damage to important heritage buildings was the largest natural disaster to occur in New Zealand since the 1931 Hawke’s Bay earthquake, the damage was consistent with projections for the scale of this earthquake, and indeed even greater damage might have been expected. In general, the nature of damage was consistent with observations previously made on the seismic performance of unreinforced masonry buildings in large earthquakes, with aspects such as toppled chimneys and parapets, failure of gables and poorly secured face-loaded walls, and in-plane damage to masonry frames all being extensively documented. This report on the performance of the unreinforced masonry buildings in the 2010 Darfield earthquake provides details on typical building characteristics, a review of damage statistics obtained by interrogating the building assessment database that was compiled in association with post-earthquake building inspections, and a review of the characteristic failure modes that were observed. It was observed that structures that had been seismically retrofitted appeared to perform well, with further study now required to better document the successful performance of these retrofit solutions.

Time-history analysis of URM walls in out-of-plane flexure

Abstract This paper introduces a single degree-of-freedom analytical model which has been used by the authors to investigate the out-of-plane performance of unreinforced masonry walls in vertical one-way bending subject to seismic actions. The objective of the modelling is to reduce costs on physical experimentation by supplementing and extending the limited experimental database on dynamic wall behaviour with analytical data. The procedure necessitates sufficient physical testing of specimens with representative boundary conditions to enable the analytical model to be calibrated against experimental results. A computer program which requires minimal data preparation effort and computational time has been written to conduct time–history analysis on the model. Importantly, key parameters have been incorporated in the program to facilitate calibration of the model. Details in developing representative tri-linear elastic force-displacement relationships and non-linear damping relationships to define the model are described in the paper. The developed model has been verified by comparing the computed acceleration and displacement response time-histories with results obtained from an extensive series of shaking table experiments. Seven of such time-history comparisons are shown in the paper to demonstrate the accuracy of the analytical model.

The Tension Stiffening Mechanism in Reinforced Concrete Prisms

Tension stiffening is an important phenomenon in reinforced concrete because it controls not only deflections but also crack spacings, crack widths and the formation of multiple cracks. It is now common practice to study the effects of tension stiffening in concentrically loaded prisms, which is the subject of this paper, and use these behaviours as guidance for the effects of tension stiffening in reinforced concrete beams. As tension stiffening is a mechanism for stress transfer between the concrete and reinforcement, the interface bond stress-slip (τ–δ) properties are of utmost importance. In this paper, partial interaction theory is used to develop generic closed form solutions for crack spacings and widths, the load to cause primary, secondary cracks and subsequent cracks. Four different types of interface bond characteristics (τ–δ) are considered: a linear ascending bond slip which is useful at serviceability; a linear descending bond slip which is useful at the ultimate limit state; a nonlinear bond slip characteristic which closely resembles material bond slip behavior at all limits; and the CEB-FIP Model Code 90 (CEB 1992).

PARTIAL-INTERACTION ANALYSIS OF COMPOSITE BEAM/COLUMN MEMBERS*

ABSTRACT Members in which two or more elements are bolted together to form composite structures that are stronger, stiffer, and more ductile than the sum of the individual elements are now commonly used in practice, not only for new structures but also, more recently, for the retrofitting of existing structures. The shear connectors that are used to tie the elements together rely on slip, that is partial interaction, to transfer the longitudinal shear. This makes the behavior of composite structures complex, so that most composite structures are designed assuming no slip, full interaction, which gives an upper bound to the strength and stiffness. In order to determine the true strength, to estimate the amount of slip to prevent fracture of shear connectors due to excessive slip, and to ensure that plated reinforced concrete columns can achieve their required seismic ductility, it is necessary to allow for slip in the mathematical model. In this paper, the classic linear-elastic partial-interaction theory for composite steel and concrete beams is extended to allow for axial forces, as occur in plated reinforced concrete columns and prestressed composite beams, and also to allow for boundary conditions associated with plastic hinges. Furthermore, a set of generic parameters that govern the fundamental response of the composite members is developed that can be used for the eventual formation of design guides and procedures. *Communicated by P. Pedersen.

Out-of-Plane Behavior of One-Way Spanning Unreinforced Masonry Walls

An analytical model is developed to describe the out-of-plane response of one-way spanning unreinforced masonry (URM) walls by investigating the effects of various parameters. Horizontal crack height, masonry compressive strength, and diaphragm support stiffness properties are assumed as variables, and sensitivity analyses are performed to study the influence of these parameters on the cracked wall characteristic behavior. The parametric studies show that crack height significantly influences wall stability by affecting both the instability displacement and the wall lateral resistance. The reduction in cracked wall lateral resistance and in the instability displacement caused by finite masonry compressive strength is shown to be significantly amplified by the applied overburden. A study using the typical configuration of flexible diaphragms and URM walls indicates that the wall top support flexibility does not significantly influence cracked wall out-of-plane response. An existing simplified wall behavioral model is improved, and a procedure is proposed for calculation of the wall out-of-plane response envelope.

Concrete Component of the Rotational Ductility of Reinforced Concrete Flexural Members

Reinforced concrete flexural members inherently rely on member ductility to ensure a safe design by allowing for: redistribution of applied stress resultants; quantification of drift for determining magnified moments; and for the absorption of seismic, blast and impact energy. Structural engineers have recognised that much of the member rotation is concentrated in a small region referred to as the plastic hinge and because of the complexity of the problem this has been quantified mainly through testing. In this paper, a new plastic hinge approach that is based on well established shear-friction theory is postulated. The generic behaviour of this novel shear-friction hinge is shown to agree with that exhibited in tests. Furthermore, the shear-friction hinge explains the mechanics of the benefits of confinement, such as that due to FRP encasement or steel stirrups, on the rotational capacity of RC members.

In-Plane Shear Improvement of Unreinforced Masonry Wall Panels Using NSM CFRP Strips

AbstractThe large number of earthquake-prone vintage unreinforced masonry (URM) buildings in many seismically active parts of the world results in a need for minimally invasive and cost-effective strengthening techniques to enhance the poor earthquake performance of such buildings. The objective of the research reported here was to investigate the applicability of using near-surface-mounted (NSM) carbon fiber–reinforced polymer (CFRP) strips as a retrofitting technique for improving the in-plane shear strength and displacement capacity of multi-leaf URM walls constructed using solid clay brick masonry. The use of this technique for repairing earthquake damaged URM walls was also investigated. Ten multileaf wall panels measuring approximately 1,200×1,200  mm were constructed using recycled vintage solid clay bricks and retrofitted using NSM CFRP strips with varying reinforcement ratios. These panels were loaded in diagonal compression, and the results were compared with those obtained from testing of nomin...

Simulation of retrofitted unreinforced concrete masonry unit walls under blast loading

This paper describes an investigation into the effectiveness of using spray-on nano-particle reinforced polymer and aluminium foam as new types of retrofit material to prevent the breaching and collapse of unreinforced concrete masonry walls subjected to blast over a whole range of dynamic and impulsive regimes. Material models from the LS-DYNA material library were used to model the behaviors of each of the materials and its interface for retrofitted and unretrofitted masonry walls. Available test data were used to validate the numerical models. Using the validated LS-DYNA numerical models, the pressure-impulse diagrams for retrofitted concrete masonry walls were constructed. The efficiency of using these retrofits to strengthen the unreinforced concrete masonry unit (CMU) walls under various pressures and impulses was investigated using pressure-impulse diagrams. Comparisons were made to find the most efficient retrofits for masonry walls against blasts.

Behaviour of Steel Plated RC Columns Subject to Lateral Loading

The main focus of this paper is to describe the behaviour of RC columns that are retrofitted with an alternative technique to “jacketing” or wrapping. This new technique consists of attaching steel plates to the flexural faces of a concrete column using bolts. It is envisaged that this technique would be suitable primarily for columns having rectangular cross-sections and in situations where lateral loading induces predominately a single plane of bending (as opposed to biaxial bending). Effectiveness of this new technique has been demonstrated by experimental testing and numerical simulations. This paper studies the mechanism of the new retrofit scheme, how it works, and the behaviour of columns retrofitted using such a scheme, as well as the important parameters that affect the response of the retrofitted columns. This study forms the basis for the design of the plate retrofitting system.