Snow-proof Conditon and Safety-Factorof theLeg Structure of Steel Tower under Snow Cover : Rationalzation of f he Snow-Proof Construeton

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概要

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• In the snowy districts of Japan, structures completely buried under snow-cover are dragged down by the “subsidence force” of snow layer. In extreme cases, these structures are damaged and broken down. The legs of steel towers of transmission lines are also not an exception (Photo. 1).<BR>The snow-proof structure of the tower leg, which is used in the current design, originated from these bitter experiences, but it has been based on a mere guess-work and not upon the results of scientific observation. Therefore, there are two debatable points in the problem of designing such a structure : appropriateness and evenness of the safty-factors of the individual beam of this structure (Photo. 2).<BR>Recently, experimental studies with regard to the “subsidence force” have been carried out, in fairly systematic way, by many researchers in this country, but have not yet succeeded in clarifying the force distribution on such a complicated structure as the leg of steel towers.<BR>The anthor derived mathematical formulas (8) (11) of the “snow-proof condition” for the beams of tower leg against the subsidence force, assuming that the steel tower was an assemblage of the “beampolygons” formed by the free sections of the beams within the four sheets of tower-surfaces; subsidence forces acting on each individually enclosed polygon were independent of each other, directly proportional to the area of each of these polygons; and that the subsidence for each polygon was divided into two “components which act perpendicularly as a wedge upon the two beams, between which the lowest corners of the polygon was held. (These two beams are named” subject beam “and” conjugate beam “because there is a conjugate relation.) <BR>Definition of the” snow-proof modulus “ : Σ<SUB>a</SUB>≡<I>Z<SUB>a</SUB>/q</I>·<I>a<SUP>2</SUP></I>·<I>b</I> sin β…… (8) <BR>Definition of the” critical snow-proof modulus : Σ<I><SUB>o</SUB></I>≡γ/2<I>K</I>·σ<I><SUB>max</SUB></I>… (9) <BR>Snow-proof condition for the beams : Σ<I><SUB>o</SUB></I>≤Σ<I><SUB>a</SUB></I>… (10) <BR>Safety-factor of the beams : Σ<I><SUB>a</SUB></I>/Σ<I><SUB>o</SUB></I>… (11) <BR>Where, σ<I><SUB>max</SUB></I> : the tensile strength of steel; <I>Z</I><SUB>a</SUB> and <I>a</I> : the section modulus and the length of the subject beam; <I>b</I> and β : the length and the inclination of the conjugate beam; γ : the subsidence force per unit area of tower surface ; <I>K</I> : a constant determined from the acting manner of the force component and the supporting manner of both ends of the beam; <I>q</I> : =1 when the beam polygon is triangular, =2 when rhombic.<BR>Physical meaning of these formulas are that the value Σ<I><SUB>a</SUB></I> is a degreeless value, calculable only from the design of the leg structure (3), and is considered to be a value representing its security against the subsidence force (11); and that when the value of snow-proof modulus Σ<I><SUB>a</SUB></I> is larger than the value Σ<I><SUB>o</SUB></I>the beam is free from the snow damage (10).<BR>It was found that the value of Σ<I><SUB>o</SUB></I> was about 0.5-1.0×10<SUP>-6</SUP>, using the formula (10) and the result of damage-investigation on 1357 beams, which constituted the leg structures of 33 steel towers in HOKKAIDO and in the neighbourhood of SHIOZAWA-MACHI in the period 1954-1955.<BR>The beams of the investigated towers were numbered as Fig. 1. The values of the snow-proof modulus : Σ<I><SUB>a</SUB></I> for these becams were calculated as Table 1, which was plotted in the (Σ<I><SUB>a</SUB></I>, <I>h</I>) diagram as Fig 2.

著者

• 荘田 幹夫

  鉄道技術研究所, 防災研究室 塩沢雪実験所

関連論文

• A Model Test of the Sloping Snow-cover
• Snow-proof Conditon and Safety-Factorof theLeg Structure of Steel Tower under Snow Cover : Rationalzation of f he Snow-Proof Construeton

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