The nuts and anchor rods of lighting poles

by Elaina Adams | January 1, 2013 9:00 am

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By Bill Yao, P.Eng.
It is common to see lighting poles in shopping mall parking lots, walkways, roadsides, and residential areas. Generally, they are galvanized or painted steel tubing, with the bottom end welded to a steel base plate that has holes matching the pattern of anchor bolts embedded in a concrete footing. The lighting pole is then secured atop the footing with anchor bolts and nuts.

There are two kinds of concrete footings for base-mounted lighting poles: one where the top is raised slightly above the grade (Figure 1), and another where the footing is raised 600 to 1000 mm (24 to 39 in.) above grade (Figure 2) . Lighting poles mounted to raised-concrete footings are often found in parking lots because they are strong enough to withstand possible
vehicle collisions.

A lighting pole structure can be separated into the following three components:
• cantilever column above the ground;
• concrete footing buried underground; and
• connection between the lighting pole and the concrete footing—typically a set of anchor bolts and nuts.

Lighting poles should be planned to support single or multiple lighting luminaires, and be able to withstand environmental loads, like ice and wind forces. The isolated concrete footing should be proportioned to resist factored loads and induce reactions in the ground. The footing can crack if inadequate reinforcing bars are used; an undersized footing could result in the pole tilting or leaning.

The design and capability of a lighting pole and its footing to withstand wind or ice loads is beyond the scope of this article. Instead, this author will focus on how to improve the concrete footing’s durability, and protect the anchor bolts to provide relatively maintenance-free service life.

Concrete durability
Durability of concrete is determined by its ability to resist weathering action, chemical attack, abrasion, or any other deterioration process. Concrete can protect embedded steel from corrosion through its highly alkaline nature. However, the presence of chloride ions in de-icing salts can destroy or penetrate the concrete’s film. Once the chloride corrosion threshold is reached, an electric cell is formed along the steel or between bars and the electrochemical process of corrosion begins.

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Several deterioration modes are defined in sources such as the Cement Association of Canada’s (CAC’s) Concrete Design Handbook which includes the Canadian Standards Association (CSA) A23.3–04, Design of Concrete Structures, and the Precast/Prestressed Concrete Institute (PCI) Design Handbook. Examples include:

•  freezing and thawing;
• alkali-aggregate reaction (AAR) which occurs in concrete between highly alkaline cement paste and non-crystalline silicon dioxide and is found in many common aggregates;
• chemical attack;
• corrosion of embedded metals; and
• abrasion.

These modes are also explained in American Concrete Institute’s (ACI’s) Building Code Requirements for Structural Concrete (ACI 318-05) and Commentary (ACI 318R-05).

Most lighting poles and concrete footings are close to roadways, sidewalks, or within parking lots and are exposed to de-icing chemicals in winter months. The designer, contractor, and owner should recognize the harmful effects of freeze-thaw in a wet environment, chemical attack, and corrosion of embedded metals.

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Low water-cement ratio
Water-cement (w/c) ratio plays an important role for concrete durability. Generally, the lower the w/c ratio, the better the material performs. Water-reducing admixtures (WRAs) should be used during concrete mixing to improve durability. Concrete with a low w/c ratio (0.4 or lower) is more durable than concrete with a high water-cement ratio (0.5 or higher).

Air-entrained admixtures
The most potentially destructive weathering factor is freezing and thawing while concrete is wet, particularly in the presence of de-icing chemicals. Deterioration is caused by the freezing of water and subsequent expansion in the paste and aggregate particles.

With the addition of an air-entrainment admixture, concrete is highly resistant to freezing and thawing. During freezing, the water displaced by ice formation in the paste is accommodated so it is not disruptive; the microscopic air bubbles in the paste provide chambers for water to enter and relieve the hydraulic pressure generated. Concrete with an air-entrained content of five to eight per cent and a low w/c ratio should withstand a great number of freeze and thaw cycles without distress.

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Increase concrete cover
Adequate concrete cover over steel is essential for footings exposed to the elements. As seen in Ontario Provincial Standard Drawings (OPSD) 2200.01, “Concrete Footing for Base Mounted Lighting and Signal Poles” (Figure 3), the concrete footing’s upper section is exposed to weather, while the lower portion is exposed to earth. In this case, earth is used as formwork to hold the concrete. A 75-mm (3-in.) concrete cover over steel reinforcement bars specified in concrete codes such as CSA A23.1–04, Concrete Materials and Methods of Concrete Construction and National Building Code  of Canada (NBC) should be applied.

Protection of anchor bolts
Anchor bolts are the most commonly used method to secure poles to concrete footings. Normally, anchor bolts are hot-dip galvanized steel. Although galvanized, the threads of anchor bolts and nuts are the easiest parts to corrode due to excessive surface area, acute shape, and severe exposure.

As rust has a much higher volume than the originating mass of steel, its buildup can cause failure by forcing adjacent components apart—a phenomenon sometimes known as ‘rust smacking.’ Reinforced concrete is also vulnerable to rust damage. Internal pressure caused by expanding corrosion of concrete-covered steel can cause the concrete to spall, leading to bond breakage between the steel and the concrete.

As part of the footing’s design, its top part is dome-shaped or grooved. This allows the rapid drainage of surface water. It is also recommended the top of the footing is specified to be at least 75 mm (3 in.) above the finished grade.

Understanding OPSD
The design of a lighting pole and its concrete footing have been standardized in Ontario for many years. OPSD specifies concrete footing with parameters as follows:

• burial depth;
• size and length of reinforcement bars;
• 75-mm (3-in.) minimum concrete cover; and
• the footing’s height above the finished grade line.

In OPSD 2200.01, the footing’s height above the finished grade line is specified as 25 mm (1 in.). Four 25 x 50-mm (1 x 2-in.) deep grooves sloped on the concrete footing are required to provide drainage. These elements are used to prevent the corrosion of steel anchor bolts and nuts.

In practice, the 25 mm specified in OPSD is too small to prevent anchor bolts from corrosion. Commonly, the concrete footing of lighting poles installed several years ago have been buried in earth. This would cause the anchor bolts and nuts to rust due to exposure to excess moisture over a prolonged period.

Further, for esthetic reasons, at the pole’s bottom, a set of aluminum flash collars are used to mask the anchor bolts and nuts, which also tend to hide the problem of rusting anchor bolts. It is often surprising to see what is below the covers when they are removed.

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The revised OPSD 2200.01 (Figure 4) was updated in 2005; it includes the increase:

top of footing shall be installed at 40 mm ± 15 mm [1.6 m ± 0.6 in.]above finished grade in paved or concrete areas and 75 mm ± 25 mm above [2.9 in. ± 1 in.] finished grade in earth or granular areas.

This is a great measure to prevent anchor bolts from corrosion.
OPSD 2200.01 does not have notes specifying the concrete mix design such as the strength and durability. It is recommended everything is put into one document (i.e. drawing or specification), including:

• concrete strength after 28 days shall be 25 MPa (3500 psi);
• concrete shall be mixed in w/c ratio equalling 0.4; and
• air-entrained concrete with five to seven per cent air content should be used.

Repair of rusted anchor bolts and nuts
The lighting poles installed in the 1970s and 1980s have been standing for 30 to 40 years. After this amount of time, these anchor systems should be field-inspected and measures taken to attend to any corrosion problems.
In the early stage of anchor bolt corrosion, where embedded bolts are still secured in the concrete, only the threaded parts and nuts may be completely corroded and unable to further secure the pole. The steel base plate and concrete footing are sometimes still in good condition. It is good to re-tap the rusted anchor bolts again and put in new nuts to secure the pole. Rustproofing materials to cover the bolts and nuts should also be applied when the job is done. Black asphalt undercoating can be used.

If the anchor bolts are completely deteriorated, and the top concrete cover is also loose due to the expanding corrosion of anchors, more invasive ‘surgery’ has to be considered. The necessary repair of the concrete footing and anchors is a costly job. It roughly follows these steps:

• disconnect the electrical wires;
• remove the steel lighting pole;
• cut off the existing concrete until the fresh anchor bolt shows up;
• saw-cut the rusted bolts off;
• connect the new galvanized threaded rod extensions with hex coupling nuts;
• set-up fibre tubing formwork;
• roughen the concrete surface and apply bonding agent;
• fill concrete and raise the top of footing to 100 mm (4 in.) above grade;
• cure concrete for a minimum of seven days; and
• re-install the lighting pole and re-connect electrical wires.

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For many anchor rods, steel is not easily welded. Therefore, mechanical threading connections (e.g. coupling nuts) are generally chosen. However, a new threaded rod extension field welded to the existing anchor rod is also possible using butt welding. Butt welding two round rods together requires special detailing, as shown in Figure 5.

Many repair measures can be determined according to individual situations, but it is better to prevent corrosion of anchor bolts in advance.

Maintenance
Lighting poles are designed to be safely exposed to aggressive Canadian weather for several decades. It is important to emphasize some issues in the ongoing landscaping and property maintenance of areas surrounding concrete poles.

Not raising the grade around the footings
It is important to emphasize the finished grade around concrete footings should not be raised by adding top soil.
Shown in Figure 6, whole anchor bolts and aluminum collars are buried in earth. Water and moisture will remain regardless of measures taken. It is recommended earth around the footing be re-graded so the top can be above the newly finished grade by at least 50 mm (2 in.).

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Cutting branches around lighting poles
Sometimes, the upper part of a lighting pole can be completely immersed in branches of a nearby tree. The branches can touch the pole and produce unexpected forces in severe weather conditions. It is recommended the branches be cut off and kept at a 1-m (3.2-ft) clearance.

Conclusion
Lighting poles are common items in our daily life, so little attention is paid to them. After a long time of being ignored, however, they will become a problem. With proper attention to design, construction, and maintenance, concrete lighting poles can have a long and problem-free service life.

Bill Yao, P.Eng., has been a civil/structural designer for 20 years. He currently works as a structural engineer at an engineering firm in Markham, Ont. He can be reached via e-mail at wyao03@yahoo.com[8].

 

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