Cantilever Signs - Sign Structural design

Document No:  67-08-4K
Revision:  2G
Date amended:  14-May-2015

Image: orange line.RCN-D13^23151823.GIF 


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.

Revision Register


Ed/Version Number Clause Number Description of Revision Date
1 All Guideline Developed 19-Mar-2003 
2 All Drawing Numbers Replaced 28-Jun-2005 
2A All Clause numbers Amended, Figure 5.6 and 5.7 Replaced. 01-Jul-2005 
2B Drawing 0330-1680 to replace the incorrect Drawing 8820-354. 20-Feb-2007 
2C Drawings 0330-1680 and 0330-1681 amended. 11-Dec-2008 
2D Drawing 8820-354 amended. 22-Jan-2009 
2E Header Contact person changed. 21-Sep-2011
2F and Drawings 0330-1678 and 0330-1684 amended. 07-Jun-2012
​2G​Header​Guideline introduction amended.​14-May-2015

Table of Content


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


4.6.1 STEP 1 - Establish Design Parameters

In addition to the design parameters contained in Section 5.5.1, the location of any underground services, which may affect footing design, should be established.  The positions of any power poles or other signs should be noted to ensure the cantilever post is no closer to the road than existing poles and posts.  It is preferable that the sign post be away from the edge of the road as indicated by Type 5 on Drawing No. 200231-0054.

4.6.2 STEP 2 - Determine Whether Cantilever Post Should Be Frangible or Not

It is the responsibility of the designer to determine whether the sign post is required to be frangible.

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). Frangible Post Selection Criteria

The following criteria must be complied with in ensuring that the selected posts are frangible:
    • If the speed limit of the road is 60 km/h or less then the Z modulus of the cantilever post should not exceed that of a 114.3 CHS 5.4 post.
    • If the speed limit of the road is 70 or 80 km/h then the Z modulus of the cantilever post should not exceed that of an 88.9 CHS 5.0 post.
    • If the speed limit of the road is greater than 80 km/h then the Z modulus of the cantilever post should not exceed that of a 60.3 CHS 4.5 post.
    • There are no approved breakaway post designs for single post cantilever signs.
    • The single post should be located to provide the maximum practical lateral clearance from the edge of the roadway, paths and services.

4.6.3 STEP 3 - Select Concept Design

To provide uniformity of appearance and aesthetics for large sign design, the use of square hollow section (SHS) and circular hollow section (CHS) posts and frames shall be in accordance with Drawing No. 200231-0054.

4.6.4 STEP 4 - Structural Design of Sign and Produce Detailed Drawings

The structural design shall be carried out by a competent structural engineer and shall be submitted to the Main Roads Structures Branch for approval.  For single post cantilever signs, separate drawings are permitted for the (a) sign layout, (b) post and frame structural drawing and (c) footing details.

The Main Roads method for the conceptual design of posts, footings and stiffeners for single post cantilever signs is shown in Figure 4.7.


STEP 4.1

Choose wind speed region and terrain category
(Refer to Section




STEP 4.2

Calculate area of sign & detemine wind force acting on sign (Refer to Section




STEP 4.3

Spacing of the CHS posts and Outstand length
(Refer to Section




STEP 4.4

Determine stiffener type and spacing
(Refer to Section




STEP 4.5

Determine all loads on sign and conduct structural analysis (Refer to Section




STEP 4.6

Select appropriate post size
(Refer to Section




STEP 4.7

Determine footing details
(Refer to Section


Figure 4.7 : Flow Chart for the Conceptual Design of Single PCantilever Signs and Frames STEP 4.1 - Wind Speed Region and Terrain Category Chart
Wind Speed Region Selection

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 STEP 4.2 - Sign Area and Wind Force Acting on the Sign
The sign area is calculated from first principles. The area of the square hollow sections may be ignored. Record the sign area 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. 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.


The spacing of the CHS posts and outstand length is calculated using the formulae and diagram shown on Drawing No. 200231-0054. STEP 4.4 - Stiffener Type and SPacing
The chart on Drawing No. 0330-1684 assists the designer to determine the type of stiffener and the required stiffener spacing. Drawing No. 8820-354 gives details of the types of stiffeners used in Western Australia.
Firstly, stiffener type A should be tried. If the Type A stiffener spacing is less then 250 mm, a Type B stiffener shall be used.
The outstand length is found on the horizontal axis of the correct chart for the stiffener type. A vertical line is then constructed up until the correct curve for the sign's Wind Speed Region and Terrain Category is intersected. From the point of intersection, a line is then drawn horizontally to the left until the vertical scale is intersected.
The maximum stiffener spacing is 450 mm for Wind Speed Regions A, B and C and 400 mm for Region D to control deflection of the sheeting between the stiffeners.
The stiffener type and spacing should be recorded on the detailed design drawing. STEP 4.5 - Determine All Loads and Conduct Structural Analysis 
In addition to the loading caused by wind, the following loads should also be included in the structural analysis: Dead load caused by the weight of the sign, stiffeners, CHS posts and SHS frame.

Live load caused by maintenance workers.
The structural analysis is to be carried out in accordance with Australian Standards and Code. The design process is necessarily an iterative one, since initial post and frame sections need to be selected to estimate the dead loads. These are then checked for structural integrity under the design loading conditions.
The sign layout should follow the concept design for a cantilever sign shown on Drawing No. 200231-0054. STEP 4.6 - Post Size Selection

The sizes of the post and frame square hollow sections are selected to withstand the forces and bending moments determined during the structural analysis.
If the posts are required to be frangible, the designer should ensure that the criteria outlined in Section are complied with. If the required SHS to support the sign cannot comply with the frangibility requirements of Section, then the designer needs to consider alternatives such that either (a) the post is located outside of the clear zone, or (b) the post is protected by a suitable safety barrier.
Record the post and frame details on the detailed design drawing. STEP 4.7 - Determine Footing Details
The required footing details, including footing size, embedment depth and reinforcing details should be established from first principles. The design process should be clearly documented and contain information on all required inputs and assumptions.
The footing details should be recorded on the detailed design drawing.