The final version of Working Stress Design was the ACI 318 1963 Code（Taiwanese Civil 401-56）, but the Ultimate Strength Design had been discussed for several years. In 1955, the united convention of ASCE and ACI proposed the USD method, and they defined the primitive factor of safety for this design method. In 1971, the ACI 318 1971 Code adopted USD to replace WSD completely, and the primitive factor of safety was shown as following formula.
In this definition, ACI seems to make an obvious fault, and it's the dead load must be considered not only the numerator but also the denominator. Why? When an engineer designs a building, he doesn't just calculate the live load. However, nobody cares about this fault, so that everybody has forgotten it. Actually, the factors of dead load 1.2 and live load 1.6 are the wrong combination, so that the factor of safety produced a unreasonable result to lead the buildings lighter too much.
The best definition of factor is not the possibility, but the ratio of dead load and live load. However, the primary problem is the engineer cannot know the final results before he starts to calculate the building. Besides, in order to avoid somebody misunderstanding the building weight, I design a simple building example of the 12 floors and 36m building (non-stairs) by the USD method. Actually, the building is the prototype of the Building of Ching-Min Huang Office, and I intend to raise the height of the 1st floor and consider the roof as a coffee house. Both of width and length are 15m, and the thicknesses of floor and wall are 15cm and 12cm, respectively. There are 12 columns which section is 80cm x 80cm, and the beam section are 30cm x 50cm and 40cm x 65cm. In addition, the windows and holes are approximately 30% of the walls which are not including 3 shear walls. About decorating the building, the beams and columns are approximately 2% of its weight, and the plates and walls are approximately 5% of its weight.
Can these factors be defined by and mathematical method? No, this problem cannot be solved, and the primary reason is that the unknown coefficients are more than the controlled equations. Even if the buildings are always the similar forms, the dead loads of structures shall be interdependent with the factor of dead loads. Hence, whatever the factor of dead load is, the buildings shall generate different results. It means any value of factor is possible, so that the load combination is not the only one solution.
References
In this definition, ACI seems to make an obvious fault, and it's the dead load must be considered not only the numerator but also the denominator. Why? When an engineer designs a building, he doesn't just calculate the live load. However, nobody cares about this fault, so that everybody has forgotten it. Actually, the factors of dead load 1.2 and live load 1.6 are the wrong combination, so that the factor of safety produced a unreasonable result to lead the buildings lighter too much.
The best definition of factor is not the possibility, but the ratio of dead load and live load. However, the primary problem is the engineer cannot know the final results before he starts to calculate the building. Besides, in order to avoid somebody misunderstanding the building weight, I design a simple building example of the 12 floors and 36m building (non-stairs) by the USD method. Actually, the building is the prototype of the Building of Ching-Min Huang Office, and I intend to raise the height of the 1st floor and consider the roof as a coffee house. Both of width and length are 15m, and the thicknesses of floor and wall are 15cm and 12cm, respectively. There are 12 columns which section is 80cm x 80cm, and the beam section are 30cm x 50cm and 40cm x 65cm. In addition, the windows and holes are approximately 30% of the walls which are not including 3 shear walls. About decorating the building, the beams and columns are approximately 2% of its weight, and the plates and walls are approximately 5% of its weight.
Can these factors be defined by and mathematical method? No, this problem cannot be solved, and the primary reason is that the unknown coefficients are more than the controlled equations. Even if the buildings are always the similar forms, the dead loads of structures shall be interdependent with the factor of dead loads. Hence, whatever the factor of dead load is, the buildings shall generate different results. It means any value of factor is possible, so that the load combination is not the only one solution.
References
- N. J. Everard & J. L. Tanner；譯：李寬材 & 羅慕麟（1974）鋼筋混凝土四百題，五南出版社。
- 國立編譯館（1974）鋼筋混凝土，復興書局。
