How to Design the Cantilever Sign Structure

Cantilever signs may be multistory as long as the lowest sign meets clearance regulations.

Signs are used along most roads and highways to inform drivers of dangerous conditions, mark upcoming destinations and provide current coordinates. To enable drivers to read signs, the signs are shaped and sized for maximum visibility; this makes them large and ungainly. A variety of sign structures are used to meet the needs of different roads and signs. The cantilever sign structure is shaped like a "7" and designed to hold overhead signs closer to one side of the road. Cantilever structures are particularly weak against torsional loading, the twisting of the concrete foot caused by wind pressure against the sign.

Things You'll Need

• Paper
• Pencil
• Ruler

Instructions

1. Write down the minimum head clearance and other cantilever restrictions. Request this information from your local Highway and Transportation Commission.

2. Note the material used for construction (for example, steel, aluminum alloy). Maintain consistency throughout the design, including the welded parts. Include details about special coatings and treatments, because untreated metals are subject to accelerated corrosion from moisture and pollution.

3. Draw a two-dimensional sketch of the cantilever structure from the front and back. Sketch the sign on the cantilever lightly in pencil.

4. Calculate the minimum possible length of the mast arm to reduce wind-induced vibration. Note the figure on the sketch.

5. Label the mast of the cantilever with the minimum height for maintaining head clearance per state guidelines.

6. Add additional mast arms to fortify the cantilever, prevent galloping and decrease susceptibility to truck gusts.

7. Review the design for areas that are overly rigid and subject to torque.

Tips & Warnings

• Base sign designs off of particular roads whenever possible.

• Never use a cantilever design where a butterfly or bridge structure would be more suitable.

References

• Federal Highway Administration: Guidelines for the Installation, Inspection, Maintenance and Repair of Structural Supports
• National Transportation Library: Alternative Support Systems for Cantilever Signal/Sign Structures
• Missouri Highways and Transportation Commission: Butterfly and Cantilever Overhead Sign Trusses