This paper discusses the coefficient of effective wall-length of mud wall examined statistically using the data in cyclic loading tests reported in late years. The coefficient of effective wall-length is called “Kabe-bairitsu” in Japan. The Kabe-bairitsu of Japanese mud wall is 0.5 in the Enforcement Ordinance of the Building Standards Law. The mud wall fabricated by the specification in the notice No. 1543 of the Ministry of Land, Infrastructure and Transport in 2003 is the Kabe-bairitsu of 1.0 or 1.5. In many experiments of mud wall carried out recently, the numerical values of Kabe-bairitsu seem to be considerably high. Therefore the Kabe-bairitsu may be reevaluated.
The 135 experimental data of mud wall is gathered in the paper published from 1990 to 2015. The mud wall taken here is plastered in both sides and the width of 1.8 meters. The experimental condition is not necessarily same. The plastering work has the regional characteristics. The place to dig clay is different. The specification of mud wall is various in the thickness of wall, the space distance of bamboo lathing and so on. The wooden frame of mud wall is various in the dimension of column, the dimension of Nuki, the connection method of column and sill, and so on. Moreover, there are three methods to prevent from being able to pull column up in the experiment. With disregard to these conditions, the average Kabe-bairitsu of 135 test data was 2.2 and the coefficient of variation was 0.25. The Kabe-bairitsu was not remarkably affected by the difference in each condition.
The relationships between the Kabe-bairitsu of mud wall and the decision factors are investigated by scatter diagrams. There are four factors: (1) yield strength Py, (2) product of ultimate strength Pu by 0.2/Ds, where Ds is the structural characteristics factor, (3) product of maximum strength Pmax by 2/3, (4) strength of specific deformation P1/120 or P1/150. The factors (1) and (2) determined the Kabe-bairitsu in 95% of test specimens.
The main observations from this statistical investigation on the 135 experimental data were as follows:
1. More than 95% of Kabe-bairitsu of 135 experimental mud walls were beyond 1.0. With disregard to the conditions of experiments and the specification of specimens, the average Kabe-bairitsu of 135 mud walls was 2.2 and the coefficient of variation was 0.25.
2. The coefficient of variation of Kabe-bairitsu evaluated by each condition or specification was almost less than 0.30. If the difference of condition and specification is disregarded and only the statistical dispersion of 0.061 in the 135 data is considered, the Kabe-bairitsu of 135 mud walls will be 2.4. When a reduction coefficient of 0.75 is adopted as the safety factor, the Kabe-bairitsu is 1.8. Also the confidence limit value of 95% in the Kabe-bairitsu of 135 was 2.1. When the reduction coefficient of 0.75 is multiplied by 2.1, the Kabe-bairitsu becomes 1.6. Even if the dispersion is considered, the Kabe-bairitsu seems to be more than 1.0 or 1.5.
3. The decision factor of the Kabe-bairitsu of mud wall was the yield strength Py or the product of ultimate strength Pu by 0.2/Ds.
4. The correlation between the horizontal rigidity and the Kabe-bairitsu of mud wall was relatively small.
5. The place to dig clay affected the Kabe-bairitsu of mud wall, but the influence of other variations in fabrication of mud walls was not so clear.
6. The mean of the structural characteristics factor Ds was 0.31, but the coefficient of variation was large relative to the other wooden load-bearing wall.
