Post-tensioned Masonry Walls Research Articles

Structural behaviour of unbonded post-tensioned masonry walls with threaded rods in clay block

Structural behaviour of unbonded post-tensioned masonry walls with threaded rods in clay block

Simplified approach to predict the flexural strength of self-centering masonry walls

This paper develops a simplified design approach to predict the flexural strength of unbonded post-tensioned masonry walls. The accuracy of different flexural expressions was investigated according to experimental and finite element modelling results. Using an analytical model and considering the stress-strain relationships for unconfined and confined masonry, force displacement curves were developed for eleven tested walls, with and without confinement plates. The developed force–displacement procedure was able to predict the lateral strength, stiffness and post-peak degradation of the behavior of the tested walls. Using a similar analytical procedure, a parametric study was performed to obtain the force-displacement response of walls with different features and to investigate the effect of different parameters including axial stress ratio, length, height and thickness of the wall, on the compression zone length. Multivariate regression analysis was performed to develop an empirical equation to estimate the compression zone length in unbonded post-tensioned walls. According to the results, the wall length and axial stress ratio were found to be the most significant factors affecting the compression zone length. Depending on the configuration of the wall, the compression zone length varied between 6.7% to 28% of the wall length. The proposed equation for compression zone length was then incorporated into the flexural analysis of post-tensioned masonry walls and validated against experimental results and finite element results. Comparing the prediction from Masonry Standards Joint Committee, MSJC (2013), and the proposed method reveals that ignoring the elongation of PT bars in strength prediction resulted in a considerable underestimation of the strength. Using the non-iterative proposed approach significantly improved the prediction.

In-plane flexural strength of unbonded post-tensioned concrete masonry walls

The target of this paper is to develop a design equation to predict the in-plane flexural strength of unbonded post-tensioned masonry walls (PT-MWs). Using validated finite element models, a parametric study was performed to investigate the effect of different parameters on the wall rotation and compression zone length. Multivariate regression analysis was performed to develop an equation to estimate the rotation of the unbonded PT-MWs at peak strength. Using the drift capacity of the walls and the proposed equation, a design expression and a related step-by-step design method have been developed to estimate the flexural strength of unbonded PT-MWs. The accuracy of the procedure was examined using experimental and finite element model results. Ignoring the elongation of the PT bars in the strength prediction resulted in an underestimation of about 40%, while using the proposed approach the prediction improved significantly. It was found that the wall length and axial stress ratio are the most influential factors contributing to the rotation and compression zone length of unbonded PT-MWs. According to the results presented in this study it is recommended to limit the axial stress ratio to a value of 0.15. In addition, limiting the maximum spacing between PT bars to a distance of six times the wall thickness is recommended to prevent local shear failure and vertical splitting cracking.

Experimental Investigation of In-Plane Cyclic Response of Unbonded Posttensioned Masonry Walls

AbstractThis paper reports on an experimental study on four unbonded posttensioned masonry walls (PT-MWs). All walls had identical thickness, height, and length of 190, 2,000, and 1,400 mm, respectively, and were constructed using concrete masonry units (CMUs) and mortar type N, and were fully grouted. Different horizontal spacing values of 400, 600, and 1,200 mm were used between the posttensioning bars in the walls. Only Wall W4 had horizontal bonded reinforcement, located in the fourth and seventh courses. Two different levels of posttensioning force corresponding to an average posttensioning compressive axial stress on the masonry of 1.35 and 2.7 MPa were applied to the walls. Different initial posttension stresses in the bars ranging from 0.32 to 0.63 of the yield stress of each bar were applied to the walls. The walls were subjected to incrementally increasing in-plane lateral displacement cyclic load applied to the top of each wall. The experimental results including damage pattern, force displacem...

Strength and Seismic Performance Factors of Posttensioned Masonry Walls

In this chapter, the behavior of PT-MWs is investigated using a database of tested walls. The accuracy of ignoring elongation of PT bars which is considered in the current masonry standard joint committee (MSJC in Building code requirements for masonry structures, ACI 530/ASCE 5, TMS 402, American Concrete Institute, Detroit, 2013) code in evaluating the strength of PT-MWs is studied using the available test results. Using the experimental results, the structural response parameters including ductility, response modification factor and displacement amplification factor are determined for different types of walls including fully grouted, partially grouted, ungrouted walls, walls with confinement plates, walls with supplemental mild steel and walls with an opening.

Effects of Tendon Spacing on In-Plane Behavior of Posttensioned Masonry Walls

AbstractIn-plane behavior of unbonded posttensioned clay brick masonry walls are investigated using detailed finite-element (FE) models. FE models of three walls having different aspect ratios of 1.50, 1.00, and 0.67 are developed. Effects of tendon spacing on in-plane behavior of these walls are investigated using different tendon spacings ranging from 0.61 m (2 ft) to 3.81 m (12.5 ft). In addition, the effects of having different horizontal reinforcement ratios of 0%, 0.1%, and 0.5% on the in-plane behavior of the walls were investigated. The stresses in the tendons are calculated using different approaches, and then the flexural strength of each wall is calculated and compared with those obtained from the numerical analysis. The simulation results show that for spacing between tendons up to 2 m (6.6 ft), ignoring the elongation in the tendon in calculating the flexural strength results in too conservative design. For these walls, it is more appropriate to use equations provided by the Masonry Standard ...

Influence of imperfections on the out-of-plane flexural strength of post-tensioned masonry walls

The influence of imperfections on the response of simply-supported post-tensioned masonry (PTM) walls subject to out-of-plane loading is investigated. Two types of imperfections of the masonry and/or tendons are studied: a linear misalignment and an initial deflection defined using a sinusoid. Walls with a range of slenderness ratios are considered, in addition to walls with restrained and unrestrained tendons. By means of finite element studies, imperfections were found to affect the nominal flexural strength of the walls by as much as 10% for a linear imperfection and 20% for an initial deflection. However, the tolerance limits on the placement of masonry and post-tensioning tendons, coupled with the conservatism implicit in the nominal flexural strength formulas developed for design use in the US, appear to be sufficient to offset those differences, thus precluding consideration of accidental eccentricity for PTM walls.

Cyclic Out-of-Plane Behavior of Slender Clay Brick Masonry Walls Seismically Strengthened Using Posttensioning

AbstractEquations for the design of a posttensioned seismic retrofit of unreinforced clay brick masonry walls are discussed, and results from an associated experimental program are reported. A total of eight full-scale multiwythe vintage solid clay brick masonry walls were subjected to uniformly distributed one-directional and reverse cyclic out-of-plane loading, of which two walls were tested as built and six walls were seismically retrofitted using unbonded posttensioning. Test wall configurations and constituent masonry materials were selected to replicate typical characteristics of historic clay brick masonry walls. The test walls were seismically retrofitted by applying varying magnitudes of posttensioning using a single tendon inserted into a cavity located at the center of each test wall. Several aspects pertaining to the seismic behavior of posttensioned masonry walls were investigated, including damage patterns, force-displacement behavior, tendon stress variation, wall secant stiffness, hysteret...

Finite-Element Models for Slender, Posttensioned Masonry Walls Loaded Out-of-Plane

Nonlinear finite-element models were developed, verified, and used to simulate the measured response of twelve 3.54 m (11.6 ft) tall posttensioned masonry walls with 810×100 mm (32×4 in.) cross sections that were tested to failure under uniform lateral loading in a laboratory. Variables included masonry type, wall slenderness, degree of tendon restraint, and prestress magnitude. The force-displacement relations obtained from the experimental tests of the walls were simulated using models developed by using an extended version of the program Dynamic Response Analysis of Inelastic Two-Dimensional Structures (DRAIN-2DX), and the results illustrate the influence of tensile strength, masonry type, magnitude of prestress, and tendon restraint on wall response to out-of-plane lateral loading. The computations indicate that load-displacement response of posttensioned masonry (PTM) walls is influenced by masonry tensile strength and that the stiffness and number of tendon restraints have a direct effect on peak ...

Seismic Performance of Face Loaded Unreinforced Masonry Walls Retrofitted Using Post-Tensioning

Out-of-plane flexural testing of three full-scale unreinforced masonry (URM) walls seismically retrofitted using post-tensioning is reported. The selected wall configurations were representative of common URM walls that were vulnerable to out-of-plane failure, and imitated heritage URM construction by using salvaged clay brick masonry and ASTM type O mortar. Varying levels of pre-compression were applied to the test walls using a single mechanically restrained tendon inserted into a cavity at the centre of each test wall. Three different types of tendons were used for post-tensioning of the test walls, being threaded mild steel bar and sheathed greased seven wire strands (with tensile yield strengths of 1300 and 1675 MPa). Behaviour of the post-tensioned URM walls was compared to the response of a non retrofitted URM wall, with the out-of-plane flexural strength of the post-tensioned masonry walls observed to range from 2.9 to 10.3 times the strength of the non retrofitted URM wall. Several aspects pertaining to the seismic behaviour of post-tensioned masonry walls were investigated, including tendon stress variation, damage patterns, force displacement behaviour, initial stiffness and displacement capacity. Test results were compared with equations developed in previous studies, and it was established that the walls that were post-tensioned using seven-wire strands had measured strengths that compared favourably with predicted values, whereas the wall that was post-tensioned using mild steel bar had failed at a lower measured strength than the predicted value.

Tendon Stress in Unbonded Posttensioned Masonry Walls at Nominal In-Plane Strength

Accurate estimation of stress in unbonded tendons at the nominal strength limit state is essential for calculating the in-plane flexural strength of posttensioned masonry walls. These walls gain their lateral strength and desirable seismic properties through the utilization of vertical unbonded posttensioning. Unlike walls with bonded tendons where the tendon stress can be found through strain compatibility with the surrounding masonry, the stress in unbonded tendons results from wall deformation between the two tendon anchorages. International masonry codes, including ASCE 5-05 (Masonry Standards Joint Committee), provide varying equations for calculating tendon stress at the nominal strength limit state but in no case have these equations been verified for in-plane loading. A finite-element model validated against large-scale in-plane structural testing of posttensioned concrete masonry walls is used to demonstrate the accuracy of existing code equations. A revised equation is then shown to provide a more accurate prediction of tendon stress for walls loaded in-plane and is recommended for inclusion in future masonry code revisions.

Behavior of Slender, Posttensioned Masonry Walls under Transverse Loading

A conceptual model of the response of slender, posttensioned masonry walls to uniformly distributed, transverse loading is introduced and verified through an experimental program. To validate the model, 12 simply supported 3.54 m (11.6 ft) tall walls with 810×100 mm (32×4 in.) cross sections were tested under monotonically increasing transverse (i.e., out-of-plane lateral) loads. Six walls were built using cored clay brick, and the remaining six using hollow concrete block. The walls were posttensioned using threaded steel bars with six walls featuring “unrestrained” tendons (i.e., neither grout nor mechanical devices were used to restrain the bars), while the other six had “restrained” bars (i.e., mechanical devices were used to restrain the bars relative to the masonry), and three magnitudes of effective prestress were investigated. Initial response to transverse loads was linear, but cracking was observed over a broad range of loading. The response of the wall specimens is traced through crack propagat...

Direct Displacement-Based Seismic Design of Unbonded Post-Tensioned Masonry Walls

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Assessing Creep and Shrinkage Losses in Post-Tensioned Concrete Masonry

Prestress losses due to creep, shrinkage, and steel relaxation lead to a reduction in structural efficiency in post-tensioned concrete masonry walls. This paper investigates the magnitude of losses that can be expected from medium-weight concrete masonry units. The findings from two series of creep and shrinkage experiments are presented and compared with test data from other researchers and with values stipulated in international standards. A creep coefficient of 3.0 is found, a value that is in agreement with the Australian, British, and Canadian standards. Shrinkage strains of 1000 and 600 microstrain for grouted and ungrouted walls, respectively, were obtained under laboratory conditions. Prestress force losses due to creep and shrinkage are shown to be significant and consistent with the recommendations made in the U.S. code.

Structural Testing of Enhanced Post-Tensioned Concrete Masonry Walls

Structural Testing of Enhanced Post-Tensioned Concrete Masonry Walls

Prestressed concrete masonry walls subjected to uniform out-of-plane loading

Two series of post-tensioned concrete masonry walls subjected to uniform lateral loading were tested to investigate their flexural strength behaviour. Each series of walls consisted of four full-scale prestressed specimens, with varying levels of prestressing force, and one reinforced specimen. Of particular interest were the load–deflection curves, initial cracking loads, wall stiffness, crack patterns, and ultimate failure loads. An air bag test apparatus was used for applying lateral uniform pressures to the specimens. Results of this experimental investigation showed that, for a given wall thickness, increased prestressing force increases the cracking load, initial wall stiffness, and ultimate failure load. The results have established a linear relationship between increased prestressing force and initial cracking load, initial wall stiffness, and ultimate failure load. The proposed model, which takes into account changes in wall stiffness after initial cracking of the wall, accurately predicts wall behaviour. Key words: masonry, prestressed, walls, strength, behaviour, uniform, pressure, experimental, analytical.