What Is a Double Threaded Bolt?
In the event of an earthquake or typhoon, the columns of a post-and-beam wooden frame building may fall out of the foundation or beams, and to prevent this, reinforcement hardware called hold-down hardware is used. Hold-down hardware is attached to both the lower part of the column, the column foot, and the upper part, the column head.
Double threaded bolts, which are threaded on both sides, are used to tighten and secure the hold-down hardware attached to the columns above and below the beams and other structural members from the top and bottom of the structural members.
Uses of Double Threaded Bolts
The importance of hold-down hardware was reaffirmed during the Hanshin-Awaji earthquake, when there was almost no damage to houses with such hardware installed. The subsequent laws made it mandatory to install hold-down hardware regardless of the number of floors in a building.
However, after the experiences of the East Japan earthquake and the Kumamoto earthquake, both of which were much larger in scale, there is a possibility that earthquake resistance standards will become even stricter in the future, and building materials such as hold-down hardware and double threaded bolts will be utilized more frequently.
Principle of Double Threaded Bolts
Commercially available double threaded bolts have thread sizes of M12 and M16. In wood-frame construction, seismic forces transmitted from the ground to the vertical framing of beams and columns, and to the horizontal framing of the horizontal framing of the horizontal framing are much greater in the vertical framing. For this reason, M16 size Double threaded bolts should be used in the vertical direction to attach the hold-down hardware. On the other hand, M12 size bolts are used when connecting the transverse framing members laterally across the columns with fittings other than hold-down hardware.
The lengths of commercially available double threaded bolts range from 180mm to 600mm for M12 size and from 300mm to 1,100mm for M16 size, depending on the manufacturer.
Double threaded bolts are made of carbon steel conforming to a strength class and are chromated or zinc plated on the surface.