Static structures (e.g. – buildings) – Machine or implement
Patent
1987-03-09
1989-03-14
Murtagh, John E.
Static structures (e.g., buildings)
Machine or implement
52740, E04C 503, E01D 1100
Patent
active
048115410
DESCRIPTION:
BRIEF SUMMARY
The invention, according to claim 1, refers to a concrete reinforcement bar that is a threaded bar, and especially a spreader bar.
Threaded bars of that kind have been disclosed in German Pat. No. 17 84 630. Said threaded bar has hot-rolled, helically extending ribs situated on two opposite sides of the bar periphery and forming part of a thread. The ribs each extend in full height only over about one third of the bar periphery and are inclined against each other by their front faces that convert into the smooth bar surface of the cross section of the core. The ribs, which are rolled on the circular core cross section, have a trapezoidal cross section with slightly rounded flutes.
Threaded bars of that kind are used, for instance, as loose reinforcement bars in steel concrete structures, or as spread bars in prestressed concrete structures, or also, for instance, in cables of cable-staid bridges.
An important criterion for the excellence of the threaded bar, especially in case of loose reinforcement bars, is the fatigue strength which depends not only on the quality of the steel but also especially on the transition between the ribs and the bar core. Due to the trapezoidal cross section of the ribs in the already known threaded bar, a notch effect appears in the transition of the ribs to the pin cores whereby the fatigue strength is reduced. This is a deficiency, especially for the normal loose reinforcement in steel concrete, since it gives steel concrete structural parts in which alternative loads have a very large proportion in the total load of the threaded bar and the reinforcement bars are accordingly exposed to strong tension fluctuations. Especially when the tension fluctuations are positive and negative load alternations or bending moments in which cases the reinforcements act alternatively as traction and pressure reinforcements, as it substantially occurs in live loads in bridge beams, the possible stress reversal between ribs and bar cores is reduced. Such positive and negative load alternations are especially critical in the field of sleeve joints in which both threaded bars are screwed to each other. Due to the shape of the threaded ribs and of the corresponding screw thread in the sleeves, the bars and the sleeves must be made with relatively greater tolerance, whereby the sleeve joint has relatively great play. In the load alternations involved, the reinforcement on said sleeve joints becomes weakened.
If the threaded bars are used as spread bars, then this deficiency is not so serious since the threaded bar is always under tension. The tension oscillations, due to load alternation, more or less make up only a small fragment of the whole tension.
However, when those threaded bars are used, especially in cables, tension oscillations can appear, depending on the intended use, that require a maximum fatigue strength from the spreader bars also. Bending moments are to be especially mentioned in the area of the point of support of the threaded bars.
Problems also arise in the manufacture of the already known threaded bar. Thus, on account of the trapezoidal cross section of the thread ribs, the rolls with which the threaded pins are produced can only be made by hand. Besides, the narrow rounding on the flutes between thread ribs and bar cores forbid the use of mechanized cutting tools. The relatively high thread ribs, with steep sides, are difficult to roll and during the rolling operation are fully filled with material only when quite a specific narrow temperature range is maintained. This requires long experience of the rolling mill personnel and rejected material must be expected. Different ways have been sought to reduce said problems in the rolling operation. One threaded bar resulting therefrom, which also is used in the practice, has been described in U.S. Pat. No. 4,056,911.
Depressions are dug between the thread ribs in this threaded bar, in the circular core cross section on both sides of the interface of top and bottom rolls, a longitudinal rib resulting between said depressions into
REFERENCES:
patent: 3376686 (1968-04-01), Salm
patent: 3561185 (1971-02-01), Finsterwalder
patent: 3782839 (1974-01-01), Majewski
patent: 4056911 (1977-11-01), Tani
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