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The elongated fibres in spruce wood, commonly used in wood construction, are able to withstand very high tensile stress loads. However, this only applies to tensile stress loads which act in parallel to the direction of the fibres. The tensile strength in the wood is significantly lower perpendicular to the direction of the fibres. Tension will appear perpendicular to the grain if a wall plate has halved joints at its ends so that it rests on another timber and is subjected to a vertical load. Enormous tensile forces will then occur in the corner of the halved joint, which can cause the wall plate to tear open lengthwise. To prevent this from happening, what are known as transversal tensile reinforcements consisting of screws or threaded rods are positioned on such wood connections. They are screwed into place perpendicular to the wood fibres, next to the halved joint.
What else should be kept in mind?
The screws or threaded rods used must be capable of absorbing extremely high tensile forces safely and transferring them into the wood through the positive and frictional locking of their threads. The longer and wider the fastener thread is, the greater the tensile forces are that it can bear. Only full-thread screws are used for transversal tensile reinforcement in this case; threaded rods obviously have a thread over their entire length.
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