Street Light Pole Foundation Design and Drawing: a practical guide

What a street light pole foundation does

A pole foundation has two jobs. It carries the weight of the pole and luminaire down into the ground, and it stops the pole from tipping over when wind pushes on the shaft, the arm and the fixture. The second job is the harder one. Wind acts high above the ground, so a small sideways force becomes a large turning effect at the base. That turning effect is called the overturning moment, and the foundation has to resist it.

A foundation can fail in four ways. The soil can give way under the pad, so the whole foundation tilts. The anchor bolts can stretch or pull out. The concrete around the bolts can crack. Or the steel at the base can rust away over time. A good foundation design deals with all four, and a good drawing tells the site team exactly what to build.

Height matters here. A taller pole puts the fixture further above the ground, which increases the turning effect for the same wind. Our street light pole height and spacing guide explains how to choose the mounting height, and that choice should be fixed before the foundation is designed.

Types of street light pole foundation

Most street light poles stand on one of a few foundation types. The right one depends on the soil, the space around the pole and how heavy the load is.

The most common choice for a single pole is a pad. In engineering terms a shallow foundation is one where the width is greater than the depth, and an isolated footing under a single column is the usual form. A pad under a street light pole follows the same idea, with anchor bolts in place of a column. A drilled pier goes the other way: it is narrow and deep, so it is useful where there is little room at the surface.

Reading the foundation section drawing

The section drawing cuts through the foundation and shows what is inside. It is the most important drawing for the site team, because it shows the depth, the width, the reinforcement and the position of the bolts in one view. The drawing below shows the parts you should expect to see, with the dimension symbols used in this guide.

The cover to the reinforcement deserves a note. Concrete cover is the minimum distance between the steel reinforcement and the outer surface of the concrete. It protects the steel from corrosion and fire and gives the bars enough embedment to work properly. The drawing should state the cover, and the site team should use cover blocks so the cage does not sit on the soil.

Plan drawing: bolt pattern, orientation and conduit

The plan view looks straight down on the foundation. It answers three questions that the section cannot: where exactly are the bolts, which way does the pole face, and where does the cable come in. Getting any of these wrong is hard to fix once the concrete has set.

Three details deserve attention. First, the bolt pattern, that is the PCD and the number of bolts, must match the holes in the base plate exactly. Second, the drawing must mark the arm direction against a fixed reference, such as the kerb line, because bolts on the windward side take the pull and the pole must face the right way when it is bolted down. Third, the conduit entry must be shown with its size and position, so the cable path is built into the concrete and not cut into it later.

If you already know your pole type, the section sizes and weights in our 410 SP tubular pole table guide help you describe the pole to the designer. The base plate and bolt details still come from the drawing of the pole you actually order.

Anchor bolts, template and base plate

The anchor bolts are the link between the steel pole and the concrete. If they are in the wrong place, the pole cannot be fitted, and if they are loose the load cannot pass into the foundation. So the bolts are normally fixed in a rigid cage before the concrete is poured.

The template is a plate drilled to the same pattern as the base plate. It holds all the bolts at the correct spacing and keeps them straight. Without it, bolts drift while the concrete is poured and the pole will not fit. After the concrete has gained strength, the template is removed and the pole is lifted onto the bolts, levelled with nuts and washers, and tightened as the pole supplier specifies.

Ask your supplier two simple questions. Are the anchor bolts and the template supplied with the pole, or do you arrange them? And which drawing do the bolt size, length and projection come from? Our street light pole price guide lists base plate, bolts and foundation as items that are often priced separately, which is why the answer matters before you compare quotes.

Soil, wind and how the size is decided

The size of a foundation is the result of a calculation, not a rule of thumb. The designer starts from the load the pole puts on the base, then works out a pad or pier that the soil and the concrete can carry safely. The inputs below are what the calculation needs.

The soil input is the one most often guessed. A geotechnical investigation is a study that learns the physical properties of the soil before foundations are built, using soil sampling and laboratory or in-situ tests. From it the designer takes the bearing capacity, which is the ability of the soil to carry the loads placed on it. The safe, or allowable, value is found by dividing the ultimate bearing capacity by a factor of safety.

For a project with many poles on similar ground, one soil report may cover the lot. For a single pole in unknown ground, the designer may ask for a trial pit. Designers commonly refer to Indian standards for wind load, plain and reinforced concrete, and shallow and pile foundations, for example IS 875 Part 3, IS 456, IS 6403 and IS 2911. Check the current edition and what your tender specifies, because we do not quote design values here.

Concrete, reinforcement and site checks

A correct drawing can still be built badly. Most foundation problems start on site: a hole that is too shallow, bolts out of position, concrete that is poorly compacted, or a pole erected before the concrete is ready. The table below turns the drawing into a pour-day checklist.

Concrete is placed around steel reinforcement, and when the concrete is dense and the cover is adequate, its alkalinity helps protect the steel from rust. That is why compaction and cover matter as much as the size of the pad. At the top, a slight slope and a slightly raised finish help rain water drain away from the base plate, because standing water at the base is a common cause of rust.

Finally, think about what stands above the concrete. A galvanised pole on a poorly drained foundation can still rust at the plate. A routine inspection of the base, covering rust, bolts and standing water, is easier on a well-built foundation.

Foundation drawing checklist, common mistakes and what to send

Before you approve a foundation drawing, check that it shows the following. If something is missing, ask for it before work starts.

• Title, pole type, pole height and the project or location it is made for.
• Section and plan views with the dimensions B, D, the height above ground and the bolt details.
• Base plate size, bolt number, diameter, PCD, embedment and projection.
• Concrete grade, reinforcement details, cover and the PCC layer.
• Orientation of the arm against a fixed reference, and the conduit entry.
• The soil capacity and wind load the design assumed, and who prepared and checked it.
• Notes on curing and the strength needed before the pole is erected.

When you ask us for a quote, send the pole height, the arm type, the fixture weight, the quantity, the site location and any foundation drawing or BOQ you already have. Our site says customers can share drawings, BOQs and pole schedules, and our team in Bhopal will reply in writing. The foundation design itself is normally prepared by your civil or structural engineer for your ground conditions.

Frequently asked questions