Tower Base Plate Anchor Bolts – should they have adjusting nuts?
A common detail for base plate anchor bolts for an isolated tower is to have two nuts threaded onto the bolt cast into the concrete foundation. The extra nut is placed under the base plate, providing a convenient means of plumbing the tower before the base plate is grouted.
Recently there was a collapse of a tower following corrosion fatigue in the threaded portion of the anchor bolts between the two nuts. Fortunately there were no fatalities. The tower was 40 years old, and exposed to wind-blown salty water from the bay less than half a kilometre away. The hole in the base plate had been slotted during construction, allowing ingress of water into the thread under the top nut. Also, there were significant voids in the grout.
It seems to me that the extra nuts underneath are not a good idea. If the grouting is poorly placed or shrinks on curing, there is little benefit from tightening the bolts down onto the foundation. If the extra nut was not there, retightening the bolts could be used as a crude means for checking the adequacy of the grout.
The question is not why this happened, but why was the tower base designed in this way in the first place. Does anyone out there have reference material discussing the use of adjusting nuts under base plates? Especially useful would be a 40-year-old textbook on the subject.
Russell Keays
A common detail for base plate anchor bolts for an isolated tower is to have two nuts threaded onto the bolt cast into the concrete foundation. The extra nut is placed under the base plate, providing a convenient means of plumbing the tower before the base plate is grouted.
Recently there was a collapse of a tower following corrosion fatigue in the threaded portion of the anchor bolts between the two nuts. Fortunately there were no fatalities. The tower was 40 years old, and exposed to wind-blown salty water from the bay less than half a kilometre away. The hole in the base plate had been slotted during construction, allowing ingress of water into the thread under the top nut. Also, there were significant voids in the grout.
It seems to me that the extra nuts underneath are not a good idea. If the grouting is poorly placed or shrinks on curing, there is little benefit from tightening the bolts down onto the foundation. If the extra nut was not there, retightening the bolts could be used as a crude means for checking the adequacy of the grout.
The question is not why this happened, but why was the tower base designed in this way in the first place. Does anyone out there have reference material discussing the use of adjusting nuts under base plates? Especially useful would be a 40-year-old textbook on the subject.
Russell Keays