اثر مقیاس در رفتار سازه بنایی تحت فشرده سازی

This paper discusses the effect of scale or size effect on the structural behaviour of masonry under compression. Small scale modelling has been employed as a means to understanding masonry structural behaviour over the years, because testing prototype masonry structures is both costly and difficult to control in terms of instrumentation. An experimental programme involving tests at four scales namely prototype, half, fourth and sixth scale were undertaken under compression with a view to understanding how compressive action affects masonry structural behaviour over the range of scales under consideration. The results show that masonry compressive strength is increased as the scale is reduced but the stiffness is not significantly affected.

Small scale masonry model testing has been carried out for many decades. Early researchers in this area include [1], [2] and [3]. Tests by these authors has established that it is possible to model masonry behaviour at reduced scales but not the strength and stiffness, this has also been reported by Refs. [4] and [5]. Recent interest in masonry modelling has arisen because of the need to assess and maybe strengthen existing historic masonry structures like bridges and buildings. For instance there are over 40,000 masonry arch bridges in the UK. Most of these bridges are over 100 years old, while some are as old as 500 years. Increasing traffic speeds and weights have made assessing both the ultimate and serviceability requirements of these bridges necessary. Furthermore there is also the need to understand the structural behaviour of masonry structures under extreme natural events like windstorms, floods, earthquakes etc., since some of these events, like flooding have become recurrent actions posing danger to thousands of people inhabiting or working in masonry structures. For example in January 2005, up to £250 m worth of damage was caused by flooding due to very heavy rainfall in Carlisle, England [6]. In Iran an earthquake in Bam killed about 35,000 people and flattened about 90% percent of the city’s mainly masonry houses [7]. Because of the issues associated with the cost implications of full size masonry tests, coupled with the danger of instrumentation destruction at failure, repeatability and difficult boundary conditions it has become increasingly necessary to carry out such tests at reduced scales. One aspect of this study is reported in this paper, looking at masonry behaviour under compressive loading at prototype, 1/2, 1/4 and 1/6 model scales. This paper primarily aims to compare masonry structural behaviour at prototype and small scales.

The results of the masonry tests at different scales has shown that the strength of masonry triplet in compression was higher than the prototype in the fourth and sixth model scales but the similar to the prototype in the half scale. The same pattern was also repeated in the tests of the unit strengths, therefore indicating the strong influence of the unit in determining the masonry properties. There is evidence of anisotropy of strength in clay brick masonry possibly due to the manufacturing process of extruded clay units which therefore makes the direction of loading on a cut model brick important. It can then be suggested that the increase in model unit strength, could be attributed to two factors, namely; strength anisotropy and size effect in quasi-brittle materials like clay bricks. It was found that triplet stiffnesses in the four scales were identical to each other and no scale effect was observed. The prototype masonry and model stiffness were in good agreement with the prototype and model mortar stiffness respectively. This may be attributed to the way the specimens were constructed, which has effectively cancelled any differential compaction of the joints due to the different weight of the unit bricks in the various scales. The good agreement in the stiffness of the four scales may be evidence of the importance of the mortar bed in determining the stiffness properties of masonry. This finding is very significant bearing in mind that model tests by other researchers showed a much softer model response to the prototype under axial compression. The findings here are important for testing the properties and understanding the structural behaviour of prefabricated masonry panels under compression. Because the results show that the stiffness of prototype and model scale masonry are similar for masonry constructed while the units are laid on their sides as in prefabricated masonry panels. Therefore using the conventional formulae for calculating masonry stiffness from the unit and mortar strength stiffness might lead to over estimating the masonry stiffness for such prefabricated masonry panels.