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Lower bound limit analysis of unreinforced masonry shear walls

Abstract This paper describes a new technique for computing the lower bound limit loads in unreinforced masonry shear walls under conditions of plane strain. From a macroscopic point of view, masonry displays similar behaviour to jointed rock or reinforced earth, which have already been successfully modelled using the lower bound theorem. The overall behaviour of the masonry shear wall is controlled by the mechanical properties of the intact unit (brick/block) and the discontinuities or joints, as well as the relative positions and orientation of the joint sets. As a result, masonry needs to be treated as an anisotropic, inhomogeneous material. In order to make use of the lower bound theorem of classical plasticity, two basic assumptions have to be made. Firstly, the material exhibits perfect plasticity, and obeys an associated flow rule without strain hardening or softening. Secondly, the body is assumed to undergo only small deformation at the limit load, and so the geometric description of the body at collapse remains unchanged. Both of these assumptions are reasonable in the case of unreinforced masonry shear walls. In the present paper, the yield surfaces of the intact brick units and of the head and bed joints are expressed separately. By using a Mohr–Coulomb approximation of the yield surfaces, the proposed numerical procedure computes a statically admissible stress field via linear programming and finite elements. The stress field is modelled using linear three-noded triangular elements and allowing statically admissible stress discontinuities at the edges of each triangle. By imposing equilibrium, yield criterion and stress boundary conditions, an expression of the collapse load is formed, which can be maximized subject to a large number of linear constraints on the nodal stresses. Because all the requirements are met for a statically admissible stress field, the solution obtained is a rigorous lower bound on the actual collapse load. The numerical solutions obtained from the lower bound formulation are compared with available experimental and finite element results from the literature. The lower bound approach developed in the present paper is shown to give good approximations to the ultimate collapse load for the two examples presented.

CONCENTRATED LOADS ON HOLLOW CONCRETE MASONRY

Masonry design codes typically allow enhancement of masonry strength beneath a concentrated load due to the strengthening effect of the more lightly stressed surrounding material. This has been verified for solid masonry, but its applicability to face-shell bedded hollow masonry is open to question since failure occurs by splitting of the webs, rather than by vertical cracking. This study investigated the behavior of face-shell bedded hollow concrete masonry subjected to in-plane concentrated loads. A total of 49 wallettes, some plain and others with one- and two-course bond beams, were constructed and subjected to either concentric or eccentric concentrted loads through various-sized loading plates. All wallettes failed consistently by local web-splitting, in a manner similar to hollow masonry subjected to uniaxial compression. The testing program is described together with details of mechanisms of failure, dispersion of load through bond beams, influence of bond beam grout strength, and potential strength enhancement. General implications for the design of hollow masonry subjected to concentrated loads are also discussed, and it is shown that strength enhancement occurs on the basis of loaded length rather than loaded area.

The Significance of Temperature Based Approach Over the Energy Based Approaches in the Buildings Thermal Assessment

Abstract The design of low energy buildings requires accurate thermal simulation software to assess the heating and cooling loads. Such designs should sustain thermal comfort for occupants and promote less energy usage over the life time of any building. One of the house energy rating used in Australia is AccuRate, star rating tool to assess and compare the thermal performance of various buildings where the heating and cooling loads are calculated based on fixed operational temperatures between 20 °C to 25 °C to sustain thermal comfort for the occupants. However, these fixed settings for the time and temperatures considerably increase the heating and cooling loads. On the other hand the adaptive thermal model applies a broader range of weather conditions, interacts with the occupants and promotes low energy solutions to maintain thermal comfort. This can be achieved by natural ventilation (opening window/doors), suitable clothes, shading and low energy heating/cooling solutions for the occupied spaces (rooms). These activities will save significant amount of operating energy what can to be taken into account to predict energy consumption for a building. Most of the buildings thermal assessment tools depend on energy-based approaches to predict the thermal performance of any building e.g. AccuRate in Australia. This approach encourages the use of energy to maintain thermal comfort. This paper describes the advantages of a temperature-based approach to assess the building’s thermal performance (using an adaptive thermal comfort model) over energy based approach (AccuRate Software used in Australia). The temperature-based approach was validated and compared with the energy-based approach using four full scale housing test modules located in Newcastle, Australia (Cavity Brick (CB), Insulated Cavity Brick (InsCB), Insulated Brick Veneer (InsBV) and Insulated Reverse Brick Veneer (InsRBV)) subjected to a range of seasonal conditions in a moderate climate. The time required for heating and/or cooling using the adaptive thermal comfort approach and AccuRate predictions were estimated. Significant savings (of about 50 %) in energy consumption in minimising the time required for heating and cooling were achieved by using the adaptive thermal comfort model.

SUBSTRUCTURING TECHNIQUE IN NONLINEAR ANALYSIS OF BRICK MASONRY SUBJECTED TO CONCENTRATED LOAD

Abstract A multi-level substructuring technique and a mesh grading scheme are used in the nonlinear finite element analysis of brick masonry subjected to in-plane concentrated loads. Masonry structures are ideally suited for solution using these techniques since masonry consists of a regular assemblage of bricks and joints in a repetitive pattern. Large wall panels can therefore be modelled without the need for excessive computer storage requirements. It is shown that the dual application of these techniques is highly efficient and leads to significant savings in costs. The possibility of modelling the elastic region of brick masonry as an isotropic continuum using similar techniques is also considered.

Experimental Evaluation of Static Cyclic In-Plane Shear Behavior of Unreinforced Masonry Walls Strengthened with NSM FRP Strips

AbstractAn experimental study was conducted to assess the effectiveness of strengthening unreinforced-masonry (URM) shear panels with near surface-mounted (NSM) fiber-reinforced polymer (FRP) strips. A total of 23 wall panels (5 URM and 18 reinforced) were subjected to vertical precompression combined with either monotonic or increasing reversing cycles of in-plane lateral displacement under fixed-fixed boundary conditions. Two wall aspect ratios (height/length) and six different reinforcement schemes were tested. The experimental program was designed to produce diagonal cracking in the URM specimens and hence investigate the effectiveness of the various reinforcement schemes in controlling this failure mode. This was achieved for the aspect ratio 1 wall panels. The study revealed that the FRP strengthening was effective in improving the ultimate load, displacement capacity, ductility, and energy dissipation compared with the URM response. For the aspect ratio 0.5 panels, base sliding failures dominated t...

A statistical study on the combined effects of wall thermal mass and thermal resistance on internal air temperatures

Statistical analyses are important for real-world validation of theoretical model predictions. In this article, a statistical analysis of real data shows empirically how thermal resistance, thermal mass, building design, season and external air temperature collectively affect indoor air temperature. A simple, four-point, diurnal, temperature-by-time profile is used to summarise daily thermal performance and is used as the response variable for the analysis of performance. The findings from the statistical analysis imply that, at least for moderate climates, the best performing construction/design will be one in which insulation and thermal mass arrangements can be dynamically altered to suit weather and season.

Reducing the Energy Consumption of Existing Residential Buildings, for Climate Change and Scarce Resource Scenarios in 2050

A pilot study of energy efficiency measures, or retrofit actions, was carried out for a single-story detached (single-family) house. It used climate downscaling to project the climatic conditions of a region, and building simulation techniques with two thermal comfort approaches for scenarios of “Climate Change” and “Scarce Resources” in the year 2050. This study was the first stage of a research program to find cost-effective retrofit actions to lower greenhouse gas (GHG) emissions for existing Australian houses in a temperate climate. The pilot study ranked retrofit actions that were cost-effective in reducing the heating and cooling energy usage of a house. These actions included removing carpet from a concrete floor for added thermal mass, and adding external shading with deciduous trees to lower summer radiation from the northern windows (in the southern hemisphere). Also, the alternative thermal comfort approach showed that occupants had more control to lower their energy usage than the standard Australian approach.

Earthquake shear load in load-bearing masonry buildings

Abstract Load-bearing masonry buildings are composed of concrete slabs and supporting masonry walls, and in Australia usually incorporate slip joints in the interfaces between walls and slabs for serviceability reasons. The apparent conflicting requirements of the slip joints, to slip under long-term loads and to transmit short-term loading from wind and earthquake actions, together with the lack of attention to the design of slip joints, make slip in the joints likely to occur when the building is subjected to a lateral loading. Joint slip leads to a redistribution of loading in the building wall system, resulting in wall shear loads different from that determined using the current standard elastic design procedures. Redistribution of loading may be also caused by softening of the masonry. This paper has assessed the realistic response of various load-bearing masonry buildings, including the potential for slip in the joints and softening of the masonry, and evaluated the level of wall shear load deviations resulting from the application of current design procedures. Analyses of the buildings, using non-linear static and dynamic finite element models, have indicated that joint slip may occur in buildings subjected to earthquake loading. The application of current elastic design procedures, which do not account for loading redistribution, resulted in major wall shear load deviations, such as overestimations of up to +75% and underestimations of up to-62%. It was therefore concluded that improvements in the current design procedures are required for a more accurate evaluation of wall shear loads in load-bearing masonry buildings subjected to the lateral loading.

The importance of air movement in warmer temperatures: a novel SET* house case study

ABSTRACT Surface temperatures increased rapidly in the last 100 years by 1 K (Kelvin), and could increase by a further 1.4 K in just 35 years, challenging building designers to provide comfort while minimizing carbon emissions. Ways to do this are with more effective indoor ventilation and lighter clothing for high temperatures and humidity, but some thermal simulation and rating systems do not consider these aspects. This paper reports on a novel simulation case study that estimated the heating and cooling energy used in a home in a Warm Temperate climate under a changing climate with a case-study method that used (1) CSIRO’s Climate Futures online tool; (2) the Australian Nationwide House Energy Ratings Scheme (NatHERS) AccuRate simulation and ratings tool; (3) a special CSIRO humidity research engine and (4) alternative Standard Effective Temperature (SET*) comfort approaches. The results showed that (A) one SET* approach with air movement, changed clothing and occupant acclimatization saved over 95% of the NatHERS residential heating and cooling energy, and should be included in NatHERS; and (B) residential retrofits or occupant education is needed for warming temperatures.

Adaptation the Use of CFD Modelling for Building Thermal Simulation

This paper demonstrate the possibility of using CFD simulation alone to determine the internal air temperature of buildings for long periods (one year), without the assistance of any additional software, with fast computing times and an acceptable degree of accuracy for the simulation results. An experiment on CFD simulations were carried out to examine the accuracy of CFD simulation to predict building internal air temperature for a complete house in Perth, Australia. Real data recorded inside a house were compared with CFD modeling results to find the precision of the CFD simulation after CFD adaptation process applied in this research. This study is an attempt to use CFD alone to calculate the buildings internal air temperatures for extended periods (months, years). Using CFD simulations with extended periods have some problems, for example; lengthy computing times, discrepancies in peak temperature time and the internal air temperatures inside the model keep rising with time. Performing CFD analysis after applying the measures to adapt the use of CFD modeling resulted in faster computing times, with 1% of the computing time compared to that for a 1 minute time step, and with 90% of the results lying within 3°C of the real (observed) data. The overall results from CFD simulations with an average accuracy of 92% compared with the real data recorded inside the house.