Appendix DDevelopment of the 2018 Design Guide#
The 2018 Design Guide for Roadside Signs has been revised from the 2001 Design Guide. The primarily structural revisions are:
- use of the current Standard, AS/NZS 1170.2-2011 Wind Actions terminology, with an appropriate pressure reduction factor used to derive similar support structure sizes to the 2001 design methodology; the reduction factor is subject to a Department of Transport and Main Roads-approved design exemption, and
- removal of Trusses as a support option.
D1Background#
The 2001 Guide was developed from the 1996 draft edition which was released in response to negative feedback from users of the 1991 Guide and a perceived deficiency in the treatment and explanation of breakaway posts. This feedback was confirmed in a user survey, with many respondents indicating that the steel posts and footings derived from the 1991 Guide to be excessive to that required to support road signs. The survey indicated that users were adopting alternatives to use of the Guide including:
- factoring of the 1991 Guide Figure B2 to give less conservative post sizes
- use of the pre-1987 Guide drawing TC9043, which uses steel yield as the allowable stress, 25-year return period, old shape factor of 1.2 and no cyclone factor, and
- avoidance of breakaway posts by only using CHS for which there are no breakaway details.
Users indicated a strong preference for accepting that some signs may be blown over in a cyclone or storm rather than using larger posts which present an increased traffic hazard. It was seen to be a relatively simple operation to bend smaller posts (particularly CHS) back into position if blown over. The survey also indicated a preference for use of CHS posts (as discussed further) for which the range of sizes and details are limited in the 1991 Guide.
A limited field survey of road signs in Metropolitan District indicated that users are adopting post sizes and details in variance to those prescribed by the Guide.
The incorporation of a rational method in accordance with Australian Standards for derivation of acceptable post sizes was considered critical for the success of the revised Guide.
Since 1996 there has been multiple cyclonic weather conditions experienced in far north Queensland, including the Category 5 Cyclone Yasi in 2011. Following these high wind events, there has been no reported occurrences of widespread sign face detachment; most signs that did fail, failed in the desired failure mechanism of post bending failure.
D2Design wind pressure#
D2.1Previous Guide#
The 1996 / 2001 Guides were based on a reduced structural importance multiplier, Mi. The structural importance multiplier, Mi in AS/NZS 1170.2-1989 represented a probability of exceedance of a design wind speed. For Mi = 1.0, there is a 5% chance of exceedance of the ultimate wind speed in a 50-year return period. The adopted importance multiplier which related to the maximum acceptable chance of exceedance was Mi = 0.75. For Mi = 0.75, the chance of exceedance in 50-year and one-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.
AS/NZS 1170.2 allows a directionality factor of 0.95 on wind speed in non-cyclonic regions for overturning calculations to account for the reduced probability of the design wind speed occurring in the critical direction for a structure. This directionality factor was reduced further to 0.9 to account for the fact that (in non-cyclonic regions) the design wind speed will generally come from one direction for a particular locality. As the design wind speed for a particular locality comes from one direction, depending on the road orientation, some signs will never experience the design wind speed in their critical direction. Viewing the performance of the road sign structures globally, rather than designing for directionality in each individual sign, justified the use of the reduced directionality factor of 0.9.
Support sizes were chosen based on wind loads at locations which were assumed to be in the general sheltered terrain categories, TC3 and TC4. For Exposed locations, the method required use of one wind region higher; for example, for an exposed (TC2) location in Region B, Region C design charts were to be used.
A safe failure mode was achieved by post bending prior to stiffener rails and panel fixing failure to prevent flying sign panels presenting a hazard. Stiffener rails were designed for the maximum design wind pressure with an additional safety factor of 1.67 to ensure signs are not blown off before the poles bent over. The factor 1.67 derived from the combination of load factor and capacity reduction factor on the pole (1.5/0.9).
D2.22018 Design Guide#
The proposed method to be adopted in the revised Design Guide is based on the following:
Safe failure mode#
The design strategy is based on a safe failure mode of support post yielding and bending prior to failure of the sign face and its attachment to the posts. To ensure that signs are not blown off before the poles bend, aluminium stiffener rails and sign face are designed to accommodate peak edge pressures generated by the critical 45 degree wind approach angle, Terrain Category 2 (fully exposed) and with an additional safety factor of 1.5 applied to the post design pressure.
Importance level#
Importance Level 1 in accordance with AS/NZS1170:0-2002 Appendix F is used, based on ‘small or moderate’ economic consequence and safe failure mode with failure unlikely to endanger human life. A design working life of 25 years is considered appropriate and this yields an annual probability of exceedance of 1 in 100 (1/100) for cyclonic and non-cyclonic wind speed.
Pressure reduction factor#
A pressure reduction factor, R = 0.5, is applied to the ultimate support post design pressure derived in accordance with AS/NZS 1170.2-2011. This factor is applied to the resultant pressure derived from wind speed and pressure co-efficient. The R = 0.5 reduction factor accounts for both wind speed increase and the increased force on the windward post under oblique wind directions which was not accounted for in the 1993 Guide. The effective design eccentricity is taken as e = 0.1b (Table D2(B) in AS/NZS 1170.2-2011) resulting in a 33% increase in force on the windward post. This eccentricity, which is less than the maximum eccentricity of e = 0.2b for wind at 45 degrees is considered applicable to the very flexible sign support structures which will distribute uneven loads.
Using the formula in AS/NZS 1170.2-2011. Table 3.1 which derives regional wind speeds: VR = 106 - 92R - 0.1 for Region B (for example) – the wind speed which is a reduction of (2/1.33) 0.5 on the regional 100-year return wind speed of V100 = 48m/s is 39m/s. For VR = 39m/s = 106 - 92R - 0.1, R = 26.7-year return period, equivalent to 3.75% annual probability of exceedance. The Region B design pressure is then calculated as p = 0.0006 * (39 * 0.83 * 0.95) 2 * 1.5 = 0.86kPa. For wind speed reduction also accommodating the 1.33 windward pole pressure factor, the wind speed which is a reduction of (2) 0.5 on the regional 100-year return wind speed of V100 = 48m/s is 34m/s. For VR = 34m/s = 106 - 92R - 0.1, R = 12.7-year return period, equivalent to 7.8 % annual probability of exceedance. The Region B design pressure is then calculated as p = 0.0006 * (34 * 0.83 * 0.95) 2 * 1.5 * 1.33 = 0.86kPa.
D3#
Design method#
D3.1Stiffener arrangement#
The number of posts and stiffener type, spacing and number are selected directly for a given sign size. Variations to the standard post spacing are catered for by an additional table for widely spaced posts and a table of maximum stiffener overhangs for reduced post spacing. Sign width limits are tabulated for the four geographic regions to be consistent with the post design method.
D3.2Post size and selection#
A graphical method of post selection has been maintained in the new Guide, similar in format to the Australian Standard AS1742.2-2009. Each geographic region is catered for with a separate set of tables for clarity and ease of use. The post size is chosen directly off the table for a given sign size, height and number of posts. An option is given for either CHS or RHS posts for the smaller signs. Guidance for choice of CHS or RHS is presented in the text. A section equivalence table has been added to the Guide for CHS posts to offer alternative Grade C250 section sizes to those Grade C350 section sizes called up by the Guide.
D4Drawings#
Standard Drawing SD1363 Traffic sign – Multiple traffic sign support – Standard and breakaway posts – Drawings 1 of 2 and 2 of 2, Standard Drawing SD1364 Traffic sign – Connection strap and erection cleat details and Standard Drawing SD1365 Traffic Sign – Traffic sign support breakaway post details (two or more supports) have superseded existing Standard Drawings SD1360, SD1361 and SD1362. The Standard Drawings have been organised to cater for breakaway and non-breakaway supports together, rather than providing separate drawings with repeated details. This assists in providing a less fragmented document. The slip base and fuseplate hinge are simply additional details incorporated as required onto the standard post. The revised drawings also present CHS and RHS posts together rather than on separate drawings with inconsistent specifications. Specification of clearances, heights, orientation etc. are referred to the Manual of Uniform Traffic Control Devices rather than trying to incorporate some of this information on the structural drawings.
Standard Drawing SD1295 Sign – Fingerboard, geographical feature and street name signs bracket details has not yet been altered while Standard Drawings SD1296, SD1297, SD1298, SD1299, SD1300 and SD1360 have been withdrawn.
D5AS4100 Steel Structures Code#
The sign support posts have been designed in accordance with the limit state steel code AS4100-1998. Ultimate design wind speeds were used.
D6Breakaway posts#
D6.1Risk#
‘Low risk’ is defined as outside the clear zone and ‘high risk’ is within half the clear zone distance of the traffic lane edge of the road.
D6.2Frangible posts#
The size of posts considered to be frangible has not been modified from the 1996 / 2001 Design Guides. Crash testing of longitudinal sight boards in 2016/17 commissioned by NSW Centre for Road Safety and Queensland’s Department of Transport and Main Roads has confirmed that 65NB posts are readily frangible for vehicle speeds >80km/hr which is consistent with recommendations in the 2001 Design Guide. The AS174.2-2009 recommendation for 50NB for speeds >80km/hr is considered to be overly conservative.
D6.3Criteria#
Criteria for satisfactory performance of breakaway posts is included in the Guide. The American Association of State Highway and Transportation Officials (AASHTO) Roadside Design Guide criterion for 2.1m clearance has been incorporated. The AASHTO criteria on post weight will be satisfied for all posts in the range of the Guide. A minimum post spacing of 1.5m has been recommended as the limit for use of additional posts of ‘frangible’ size to support signs in ‘high risk’ zones. The AASHTO Guide considers all posts in a swept path of 2.1m when checking that the weight of posts is less than the recommended limit. Design in accordance with the AASHTO Guide would therefore require consideration of the combined resistance to impact of all posts within a 2.1m width, rather than designation of individual posts as frangible if smaller than the recommended size. Adoption of a 2.1m swept path would disqualify many signs from use of ‘frangible’ post support. The 1.5m minimum spacing limit is proposed as an acceptable, less conservative, limit for consideration of impact on posts in isolation. A limit on sign height relative to clearance has been incorporated to ensure that the fuse plate moment capacity is not exceeded under the design wind speed. The current fuse plate detail appears to be designed for the post capacity which relates to a sign height not greater than the clearance. Many existing signs will not conform to this criterion and will therefore potentially fail at the fuse plate at less than the design wind speed.
D6.4Post type#
Breakaway post details have been incorporated for both CHS and RHS posts.
D6.5Details#
D6.5.1Slip base#
The slip base plates have been detailed to accommodate angled impact from both traffic directions. The previous Guides (1996/2001) specifies the part-turn method of tensioning which is relevant to high strength friction grip bolts tensioned to the bolt Proof Load. AASHTO recommends clamping forces relative to post size for satisfactory slip base performance. Excess bolt tension increases the impact force transmitted to the vehicle. The clamping force recommended in AASHTO relates to very low bolt tension which may cause problems of a loose connection, unserviceable for wind loading. The proposed method is to torque the bolts to 100 Nm. This torque relates to only 1/2 and 1/3 of snug tight for M16 and M20 respectively. AS4100-1998 discourages the use of torque control of bolt tension in favour of load indicating washers due to inaccuracies from thread cleanliness, wrench calibration, thread type and various other factors; however, load indicating washers do not cater for low bolt tension and inaccuracies in torque control can be minimised by oiling the threads and assembling the baseplates in the shop prior to delivery to site. Shop assembly has the added advantage of enforcing the plumbing of the posts prior to pouring concrete rather than casting in the stub below the slip base and then using shims between the baseplates (in the critical slip zone) to plumb the posts, as is currently specified in the Guide. It is further noted that AASHTO recommends regular checking of bolt tension for signs in service.
Bolts have been sized with an additional factor of safety to ensure post failure prior to bolt failure. This also recognises the cyclic loading and fatigue regime operable on the bolts.
Additional washers have been specified under the bolt head and nut to enable uniform pressure under the head and nut with the slotted baseplates.
The fillet welds of posts to baseplates have been corrected. The previous Guide specification shows fillet weld size increasing with overall post size rather than with tube wall thickness. The weld sizes currently tabulated are generally not compatible with the post strength such that failure would occur at the welds before post bending capacity was mobilised.
D6.5.2Fuse plate hinge#
The fuse plate detail accommodates impact from both traffic directions. This has been achieved by use of a fuse plate on both sides of the post with a complete separation / cut of the post.
The fuse plate detail has bending capacity to 45% of the post capacity to enable sign height to be up to 165% of clearance (refer to discussion in Section D6.3 previously).
Field inspection along the Gateway Arterial Road indicated that the fillet weld size and length on the fuse plates are frequently less than specified on the Standard Drawings. Cracking was evident in some of these inadequate welds. These inadequate welds could significantly reduce the expected life of the posts, particularly as the welds are subject to cyclic wind gust loading and stress concentration effects. Apart from a recommendation for improved quality control, the detail has been revised with thicker fuse plates to improve the chance of correct weld size. Thicker fuse plates also improve the transfer of wind shear across the cut post. Welding is continuous all around the fuseplates which also alleviates stress concentrations. A smaller 3mm or 4mm weld is specified for the post below the cut to facilitate failure on impact.
Field inspection indicated that posts are susceptible to corrosion along the cut edges and the weld regions. The procedure specifies welding of the fuseplate prior to hot dip galvanising. Additionally, the
The post splice is specified to be full contact which should enable the galvanising to seal across the cut. The fuse plate is welded all around to effectively seal behind the plate such that the whole assembly can be protected with the hot dip galvanising.
D7Posts types#
The user survey indicated a strong preference for the use of CHS posts rather than RHS. Advantages identified with CHS are pregalvanised; availability; cheaper (availability in 6.5m lengths compared with 8m for RHS leading to less wastage is one consideration); readily cut and capped on site with pipe cutters so less tolerance on post length required; less requirement for alignment with sign face; availability of fittings; and more easily pushed back to alignment if bent over by wind or vehicle collision. 90 NB is included in the graphs but is not readily available.
RHS posts have been retained with further explanation to when they may be economical. The structural efficiency and hence potential cost saving has been identified particularly with the use of pregalvanised RHS. Tubemakers Duragal is only 3% more expensive than black steel and is rated at Grade 450. The cost of a Grade 450 pregalvanised RHS post is half the cost of the equivalent strength CHS. The use of pregalvanised RHS (without further hot dip galvanising) is limited to regions of low corrosion potential due to the reduced thickness of zinc coating (100g/m² compared to 300g/m² for hot dip galvanising).
RHS posts may also be cost effective for breakaway posts where the advantage of pregalvanised CHS is lost with the requirement on hot dip galvanising the breakaway posts after fabrication.
D8Stiffener rails#
A maximum stiffener spacing of 500mm with overhang of 150mm is used. The Guide specifies a pole spacing ratio of 0.15 / 0.35 / 0.35 / 0.15 and 0.2 / 0.6 / 0.2 for three and two pole signs respectively. These ratios balance both pole load and stiffener support and span moment.
D9Single post signs#
The 1996/2001 Guides included a new system and detail to accommodate the common situation of signs wider than 950mm, which cannot be supported on two posts; for example, on a narrow median strip. The sign panel is stiffened and fixed to a single post with 6mm steel brackets. The steel brackets are screwed into the post to prevent rotation.
D10Footings#
D10.1Design strategy#
The Guide specifies footings sizes for two soil strength categories for both cohesive and non-cohesive soils. Cohesive and non-cohesive soils are distinguished between, due to different mechanisms of soil resistance and hence design formulae. For non-cohesive soils, lateral resistance is dependent on overburden stress, so footings are deeper and narrower than for cohesive soils.
Table B.4 in the Appendix B design procedure enables choice of appropriate foundation strength category by use of either simple field identification procedures or parameters derived by laboratory testing.
D10.1.1Cohesive soils#
The footings for cohesive soils are designed using the method suggested by Coyle and Bierschwale, Design of Rigid Shafts in Clay for Lateral Load, ASCE J Geotech Eng, Vol 109, No 9, 1983. This method determines the ultimate lateral load that can be applied to a pile for a limiting deflection criteria. A pile rotation of two degrees is considered to be the serviceable limit beyond which loosening of the pile may occur due to plastic deformation of the soil. This method produces less conservative results than other methods which limit soil pressure.
D10.1.2Non-cohesive soils#
The SAA Piling Code (AS2159-1978) was used for design of footings in non-cohesive soils with an appropriately chosen Factor of Safety on ultimate lateral resistance.
D10.1.3Other soils#
The Guide highlights the requirement for special design of footings in very soft or swampy soils and sound rock.
D10.1.4Details#
The footings have been designed without reinforcement by utilising the combined bending resistance of the embedded post and unreinforced concrete pier. The diameter and depth of piers have been chosen with consideration to minimising pier depth.
D11Modular sign panels#
Modular sign panels have been included in the Guide to facilitate the installation of larger signs. A sketch which details the general arrangement for these signs has been included in the Guide. Each 1200mm high sign panel module is required to have three stiffeners at a set spacing of 580mm. The Guide design procedure for selection of supports for modular sign panels is exactly the same as for normal signs.