Details:

Creators:
Hoisington, David ; Hamel, Scott
Hoisington, David ; Hamel, Scott Less -
Corporate Creators:
University of Alaska Anchorage
Corporate Contributors:
United States. Dept. of Transportation. Research and Innovative Technology Administration
Subject/TRT Terms:
[+]
Anchor Bolts Field Studies Finite Element Method Foundations High Mast Lighting Nuts (Fasteners) Poles (Supports) Pretensioning Research Hub Strain (Mechanics)
Publication/ Report Number:
DTRT12-UTC10
Resource Type:
Tech Report
Geographical Coverage:
United States
Corporate Publisher:
Pacific Northwest Transportation Consortium
Abstract:
High mast lighting poles (HMLPs) are cost effective structures for lighting highways and intersections. They are 100 to 250 feet (30m to 76m) tall, and can hold a variety of lamp configurations. They are commonly used at highway interchanges because a single unit effectively covers more area than the typical, approximately 30 foot (10m) tall, light poles. The AKDOT&PF maintains 104 such poles in the greater Anchorage area. One issue that has been observed by the Alaska Department of Transportation and Public Facilities (AKDOT&PF) with HMLPs is anchor nut loosening. Anchor rods and their associated nuts are used to secure the HMLP base plate to the pole’s foundation. When they’re tight, they allow the rods to transfer load from the HMLP to the foundation. The anchor nuts have been loosening on many HMLPs regardless of foundation type, pole height, lamp configuration, date of installation, number of anchor rods, rod diameter, or temperature during the time of installation. Any poles that have loose nuts undergo a re- tightening procedure outlined by the American Association of Highway Transportation Officials (AASHTO). From 2007-2011, 177 inspections were done on 104 poles. 54 of these inspections revealed loose anchor nuts. This program is too costly for the Department to continue indefinitely. The need for solutions for existing and yet to be installed poles is evident. To understand the behavior of HMLP foundations during tightening, strains were monitored in the anchor rods of two HMLPs. The first was tightened according to existing AASTHO provisions. The second modified those provisions based on the conclusions drawn from the first tightening. The strains in the rods of both HMLPs were monitored after their tightening procedures to try to capture anchor nuts loosening. The tightening procedures did not result in rod pretension magnitudes below existing recommendations. Some of the rods in the initial tightening procedure resulted in rods tightened above yield. Existing literature suggests that the recommended pretension magnitudes are adequate to prevent nearly all loosening in dynamic loading scenarios of low magnitude. This is how traditional loosening manifests itself, with the nut rotating due to vibratory effects. There are reports by AKDOT&PF personnel who indicated that nuts that were “loose” didn’t rotate from a position which was marked after tightening. The loss of clamp load without rotation of either clamping nut has been quantified in previous studies which showed that to simulate this nonlinear post-yield behavior, a complex model is required. This model must allow for contact interactions, friction between parts, nonlinear behavior, displacement based tightening, and force based loading. Finite-element (FE) modelling satisfies all these requirements in the most accurate way possible. An FE model was created of several HMLP foundation configurations, including the two whose tightening was monitored in the field. In addition to these scenarios, the effects of thickening the base plates, adding stiffeners to the poles, and using high strength anchor rods were analyzed. Significant clamp load loss due to post-yield effects was recreated in all of the scenarios. One such scenario had complete clamp load loss in five rods with a single application of a design wind load. Other scenarios were highly resistant to this type of clamp load loss. A number of conclusions were drawn from these studies. It is shown that large diameter fasteners with short grip lengths are snug tightened without controlling the torque, they are likely to exceed the recommended snug tight pretension range. Final bolt pretensions would be more likely to fall within the desired range if the degree of rotation in the turn- of-the-nut method were adjusted for the grip length/rod diameter ratio. Clamp load loss due to permanent rod deformation is not affected by pretension magnitude (in F1554 grade 55 rods). The difference between the magnitude of external load required to cause complete clamp load loss in one rod, and that required to cause complete clamp load loss in several rods, is relatively small. Rods in double nut moment connections and high strength rods are less likely to experience clamp load loss due to permanent deformation. Recommendations for existing and yet to be installed HMLPs are presented based on these conclusions
Format:
PDF
Funding:
2012-S-UAF-0014
Collection(s):
US Transportation Collection University Transportation Centers
Main Document Checksum:
[+]
urn:sha256:f73f3ec1246c926322444157cb894402fa203836749caff660026ca452da9827
Download URL:
https://rosap.ntl.bts.gov/view/dot/28069/dot_28069_DS1.pdf
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