The information below is intended to reflect the preferred practice of Main
Roads Western Australia ("Main Roads"). Main Roads reserves the right to update
this information at any time without notice. If you have any questions or comments please contact Soori Sooriakumaran by e-mail or on (08) 9323 4618.
To the extent permitted by law, Main Roads, its employees, agents, authors and contributors are not liable for any loss resulting from any action taken or reliance made by you on the information herein displayed.
The design process for single post cantilever signs and frames follows the flow chart given in Figure 4.6.
Establish Design Parameters (Refer to Section 4.6.1)
Determine whether sign should be frangible or not (Refer to Section 4.6.2)
Select design concept (Refer to Section 4.6.3)
Structure design in accordance with design concept and record structural post and frame design details and footing details on an A3 size drawing. (Refer to Section 4.6.4)
Figure 4.6 : Flow Chart for the Design of Single Post Cantilever Signs and Frames
All sign posts located within the clear zone should be frangible, unless the signposts are shielded with a road safety barrier.
The clear zone as defined in AS/NZS 3845 - 1999 is "the horizontal width of space available for the safe use of an errant vehicle which consists of the verge area and is measured from the nearside edge of the left-hand traffic lane."
Clear zones shall be determined in accordance with Chapter 3 of the AASHTO Roadside Design Guide (1996).
220.127.116.11. Frangible Post Selection Criteria
The Main Roads method for the conceptual design of posts, footings and stiffeners for single post cantilever signs is shown in Figure 4.7.
Choose wind speed region and terrain category (Refer to Section 18.104.22.168)
Calculate area of sign & detemine wind force acting on sign (Refer to Section 22.214.171.124)
Spacing of the CHS posts and Outstand length (Refer to Section 126.96.36.199)
Determine stiffener type and spacing (Refer to Section 188.8.131.52)
Determine all loads on sign and conduct structural analysis (Refer to Section 184.108.40.206)
Select appropriate post size (Refer to Section 220.127.116.11)
Determine footing details (Refer to Section 18.104.22.168)
The designer is required to examine the map shown on Drawing No. 0330-1678. The wind speed region is determined by determining the location of the sign on the map.
Terrain Category Selection
The terrain category is selected by choosing the photograph that shows terrain most similar to the terrain where the sign is to be located.
Photographs are only provided for Terrain Categories 2, 3 and 4. Terrain category 1 is for very smooth surfaces. Terrain Category 2 should adequately cover all smooth terrain situations that are likely to occur.
Record wind speed region and terrain category on the detailed sign drawing
If the area of the sign is between 0 and 10 m2 the chart shown on Drawing No. 0330-1680 is used to determine the wind force acting on the sign. If the area is between 10 and 60 m2 then the chart shown on Drawing No. 0330-1681 is used.
The charts are read by starting on the horizontal axis and locating the sign's area. A vertical line is then drawn up until the correct curve for the sign's Wind Speed Region and Terrain Category is intersected. A horizontal line is then drawn to the left and the force acting on the sign is read from the scale.
22.214.171.124. STEP 4.3 - Spacing of the CHS Posts and Outstand Length
The sign layout should follow the concept design for a cantilever sign shown on Drawing No. 200231-0054. The sign is attached to Type A or B stiffeners, which in turn are attached to the 60.3CHS posts forming part of the cantilever frame.
126.96.36.199. STEP 4.6 - Post Size Selection