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Design Guide Design Guide for Roadside Signs

 Design Guide Design Guide for Roadside Signs

Design Guide Design Guide for Roadside Signs


The Traffic Engineering Section of Traffic & Road Use Management Division develops and provides appropriate guidance in relation to traffic engineering standards in Queensland. It also provides technical support, training and a diverse traffic engineering consultancy service.
Following the commercialisation of the Traffic & Road Use Management Division of Main Roads, these services are provided on a full commercial basis to both the public and private sectors.
Major functions include:-
  • the provision of full traffic engineering services including traffic studies and impact assessment, design of sign and pavement marking layout for design schemes, development and support of traffic management systems.
  • investigation of matters relating to traffic control devices and traffic operations in conjunction with other research organisations such as Australian Road Research Board,AUSTROADS and other State Road Authorities.
  • research and develop standards and guidelines to increase safety of all road users through investigations of speed zoning, roadside hazards, road geometry, intersection treatments and information systems.
  • monitoring and evaluation of traffic safety programmes and standards.
  • liaison within the Department of Main Roads in Queensland, Local Governments, Government Departments and the private sector.
Accredited training can be provided in a number of areas including:
  • Roadworks signing
  • Introductory level traffic engineering
  • Pavement marking.
The Manual of Uniform Traffic Control Devices, Guide to Pavement Markings, Design Guide for Roadside Signs and the Traffic Engineering Manual are some of the more well known publications developed by the Traffic Engineering Section.
Further information on the capabilities of Traffic Engineering Section may be obtained by contacting the Principal Engineer (Traffic).

It is proposed to use an appropriate probability of exceedence of the design wind speed to produce results that are acceptable to both design life and a road safety. The following clauses outline the structural assumptions made in this guide.
Steel supports
To prevent the hazard of flying sign panels, it is important that signs should fail by pole bending before failure of either stiffener rails or panel fixing. To ensure that signs are not blown off before the poles bend, stiffener rails are designed for the maximum design wind pressure, with an additional safety factor of 1.67. This factor has been derived from the combination of load factor and capacity reduction factor on the pole (1.5 and 0.9 respectively).
The sign-face pressure is reduced when the steel pole bends in plastic bending.
Timber supports
Timber, by its very nature, is a non-plastic material and therefore cannot fail by plastic pole bending. As the timber pole failure may lead to signs being blown across the carriageway,causing damage to property and people, different factors to the steel support must be used.
STRUCTURE IMPORTANCE MULTIPLIER
The structure importance multiplier, Mi in AS1170.2, represents a probability of exceedence of a design wind speed. For Mi = 1.0 there is a 5% chance of exceedence of the Ultimate Wind Speed in a 50 year return period. For Mi = 0.9, as used in the 1991 Design Guide, the chance of exceedence in 50 year and 1 year return periods is 25% and 0.5% respectively.
The proposed structure importance multiplier for steel supports, which relates to the maximum acceptable chance of exceedence, is Mi=0.75, ie the chance of exceedence in 50 year and 1 year
return periods is 96% and 6.5% respectively. That is, every year there is a 6.5% chance of the sign experiencing its design ultimate wind speed.
For timber, the desirable failure mode is different and an Mi of 1.0 has been adopted.
DIRECTIONALITY
It is improbable that the direction of the wind will always be in the critical direction for a sign
structure. To allow for this fact, in non-cyclonic regions AS 1170.2 allows a wind speed
directionality factor of 0.95 to reduce the value of design wind speed.
As the design wind for a particular locality generally blows from one direction, depending on the road orientation, some signs will never experience the design wind speed in their critical direction.
It is therefore proposed that the directionality factor is further reduced to 0.9 in non-cyclonic regions.
Viewing the performance of the road signs structures globally, rather than designing for
directionality in each individual sign, justifies use of the reduced directionality factor of 0.9.

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