Steel Advisor

Pre-Tensioning of High Strength Holding Down Rods

Holding down bolts are typically threaded rods of varying lengths.

1 December 2020

Holding down bolts are typically threaded rods of varying lengths. Tightening of the hold down bolts to achieve a minimum level of bolt pre-tension eliminates a potential source of undesirable connection flexibility. Such as in column base plate connections in a seismic-resisting system where the frame stiffness will change if the base plate lifts off the footing, as the rigidity of this connection has a significant effect on the behaviour of the seismic-resisting system(s) and the overall building. One option for achieving the necessary pre-tensioning would be to use direct tensioning equipment but this is difficult to calibrate reliability for structural bolts if galvanised. However, a simpler option is to use the part turn concept of NZS 3404. A bolt extension associated with achieving the minimum bolt proof load applied as a specified turn of nut past snug tight. The method modifies the method previously presented in HERA Design & Construction Bulletin No 56. Included in the end of this article is a worked example for determining the calculation of required amount of nut rotation from snug tight. This article provides additional information to ERC1001 High Strength Bolting.


Kevin Cowie