Structural Design of Traffic Signs

{This is a re-posting of commentary I originally posted in my state-of-play posts. I have given it its own post, so I can extend and revise as needed.}

Now road signs are established technologies, there must be hundreds of them all over the world. We don’t expect them to fail, and certainly don’t expect them to collapse and crush moving vehicles. As a community we expect they can be designed to be fit-for-function and also fabricated and installed to be fit-for-function. We don’t expect assessment of fitness-for-function to be some abstract esoteric theory which has not been tried and tested. So from my perspective there is no engineering involved, as engineering takes place at the frontiers or science and technology. Rather the design of a sign is a matter of routine design, with assessment of fitness-for-function based on proof-calculations.

The design of a road sign is relatively routine. The wind loading code, covers signs and hoardings, though arguably not thoroughly enough. So wind loads can be determined. The moments in the supporting posts can be calculated and suitable size posts selected, welds and bolted connections designed. The local effects on support structures can also be assessed.

Design Actions & Effects

Sign Front View and Side View. Wind Load shown coming from the right, though the wind can change direction. Whether the wind is normal to the sign or parallel, the load will be normal two the faces of the sign. The blue hatched block indicates the wind pressure over the area of the sign, the blue arrow shows the direction. The red symbols are the reactive forces and moments.
Side view with the moment reaction replaced by a force couple. Which would be provided by the bolted connection. R2 would be tension in the bolt, and R1 is compression at an hard point such as the compression flange of the post

The Tullamarine Freeway sign for example being supported on a large gantry beam spanning the highway, needs assessment of the gantry beam its connections and also its supports. The base moment at the support posts to the signs becomes a torsional twisting moment on the gantry beam.

The reactions from the sign post, applied as equal and opposite actions to the gantry beam. The ZZ-axis comes out of the page, a moment about this axis is torsion. The horizontal reaction being eccentric to the XX-axis produces a torsional moment, and the applied moment is also torsional.

The connections of the gantry unlikely too be designed for torsional moments, not the least of which torsion isn’t covered by AS4100. For that matter it contains few mandatory requirements for checking local effects on plates at the location of connections: such may be covered by AISC/ASI design manuals but few seem to be aware of such manuals.

The applied moment replaced by the actions on the bolted joint.

If the connection design manuals, or similar references are used then at the connections of the sign posts to the gantry, likely to require a flange doubler plate and support, along with web stiffeners.

The flange is likely thinner than the sign base plate, the doubler plate makes the gantry flange similar thickness: make as thick as necessary to reduce bending and deformation of the plate. The flange can rotate so a bearing stiffener connected to the flange and web will reduce such rotation.

Tilt of the Gantry Beam Flange
Plates Added to Gantry to Prevent rotation of the Top Flange. Wind can change direction so likely need one to each side.

With the plate in place, now also have a hard point for the compression force to bear against. The plate may buckle, but it is partially restrained by the gantry web: the two act in combination to resist buckling.

Whilst the gantry beam, end connections need a connection form with some torsional resistance, which wouldn’t be a cleat plate, expect something more like a welded end plate. The end plate would be bolted to the gantry support columns. The welds would need checking for torsion, as would the bolted connection. The bolts would typically resist the torsional moment in shear. A thin flexible end plate would allow the gantry beam to be considered simply supported, otherwise a moment end connection can be designed for the gantry beam. The welds and bolts would then have to resist the effects of both a torsional and bending moment. The bending moment would be resisted by tension in the bolts, whilst the torsional moment by shear in the bolts. The beam end reaction also produces shear in the bolts.

All in all a matter of routine for a steel designer. Except …

Vibration and Fatigue

There is however one issue which is not routine, and not adequately covered by the wind loading code AS1170.2, and that is the buffeting and flutter, and other wind induced vibration of plates and slender poles. The dynamic section of AS1170.2 only really covers multi-storey buildings, it is inadequate for:

  1. Pole mounted lights
  2. Glass Panel Balustrades mounted atop multi-storey buildings
  3. Signs and hoardings

This wind induced vibration, will cause fatigue of the material in the post, the welds and the bolted connection. Vibration could loosen the bolts, which would shift the bolts from being in direct tension to being in bending: circular shafts have low bending capacity. Steel has a fatigue limit of around 1 million cycles at which point its equivalent static strength has reduced to around 0.45 of its full strength.

Fatigue is an accumulative phenomenon: with machines regular maintenance is carried out and parts are replaced before they reach their fatigue life: especially critical for aircraft. Unlike steel, the materials used in the construction of aircraft have no fatigue limit, the more fatigue cycles experienced the lower the strength becomes: the components have to be designed for a known fatigue life and monitored.

Structural dynamics is given little consideration when it comes to structures in the built environment. However it is still not engineering, the science has been their for along time, it just hasn’t been applied to any significant extent. {NB: It is noted that it is being increasingly used to design for earthquake resistance}

Vibration During Single Wind Event

With respect to the sign there are two aspects to the vibration and fatigue. The first is during a single wind event, the wind can cause the sign to flutter, especially if it is a thin plate. This is common with signs supported on single poles at ground level. Highly noticeable in the plastic sticks with reflective strips used as road edge markers: the cross section of these sticks is an arc shape. Its amazing they don’t snap off. {I should take a photo of the things}

Fatigue From Multiple Wind Events

The second aspect, is that, most of the time the wind speed is around 5km/h and it occasionally peaks at the higher speeds, our reference design speed is 162km/h. So throughout its entire life, the sign is being loaded and unloaded and so experiencing fatigue stress cycles. Each time it is loaded the sign deflects and then elastically recovers when the load is removed: though it won’t fully recover. Some of these stress cycles are significant some are not, but none the less all contribute to the accumulation of fatigue damage.

Designing For Unknown Vibration

As to, how to design the signs taking this vibration into consideration I don’t know, as a so called engineering technologist my knowledge is limited, but so is that of the so called engineers. As I mentioned the science of structural dynamics and mechanical vibrations is not new: having studied mechanical engineering I covered some aspects of such subjects.

However, the engineering issue is a need for more research and collection of data and determination of models for wind induced vibration of plates, poles, membranes and cables. So that codes of practice such as AS1170.2, and industry manuals can otherwise be revised. We need to know the frequency of the vibration,and the frequency of the applied force. Need to know how many fatigue cycles it will experience during its expected life. Need to know if it will resonate.

Design for Stiffness Not Just Strength

However, the art of engineering is about finding solutions before all the science is complete. The flutter and vibration is mostly a consequence of a lack of stiffness. Designing such structures for ultimate strength only is likely to result in comparatively slender structures. {NB: another option is to look at the technologies used for chimney stacks to stop vibration}

First of all the Bureau of meteorology (BoM) advises a severe weather warning, when average wind speeds exceed 63km/h and instantaneous speeds exceed 90 km/h, the intent is that people take shelter not drive around. However, people may need to get home, or emergency services may be driving around, so design for some higher speed may be preferable. So will stay with the AS1170.2 derived wind speed: which typically starts at 162km//h and is adjusted to suit the site and building/structure. {NB: AS1170.2 is for buildings not structures in general}

Serviceability wise do not want the sign to deflect too much that cannot read it. On the other hand it can probably deflect a significant amount before that happens. Also don’t want it to deflect to any extent that it is noticeable and becomes a concern. So could arbitrarily restrict deflection at the ultimate strength wind speed to 30 mm, whilst deflection at 90km/h is limited to 10 mm. Then play around with these speeds, deflections and required stiffness, and develop a variable design model: such that the deflection limits at higher speeds seem more reasonable. Not looking for a span /deflection ratio, but deflection constraint related to wind speed.

This won’t stop fatigue, it will just reduce the deformation experienced with each fatigue cycle.

Who Could Design It

To conclude a 2 year traditionally qualified structural or mechanical engineering associate (I’ll call those with traditional qualifications Associate Technologists) could have designed the sign to be code compliant, an engineer is not required.

However, a code compliant sign is likely to fail, if wind induced vibration or fatigue are critical issues. This is where we look to the engineer to look beyond code compliance. The engineer reviews the proposal, identifies code compliance has been met, then identifies that there may be an issue. Now if the engineers only capability is code compliance, then they are not any use to anybody. The engineer has to find a solution and take responsibility for it: the interim and immediate solution may be as simple as doubling the load. Such simple solution however does not provide much insight or understanding of the problem, nor help predict when failure will occur.

More detailed mathematical modelling and hypothesis is required, we look to the ingenuity of engineers to dream this up. This in turn may lead to physical experimentation: either by building models and testing in a wind tunnel, or installing sensors on existing signs and monitoring their behaviour.

My original assumption was that: the available information is inadequate to cover wind induced vibration. But then asked: or is it? And suggested a google search: wind induced vibration road signs. The results suggested that a literature search maybe all the research that is required. If that is the case then I suggest that we look to engineering technologists to transform such research into industry practice notes, which can then be used as a matter of routine by Associate Technologists.

{I should point out I expect modern Associate Technologists, Technologists and Engineers to all study the same first year.}

Researching Literature

Some of us start by writing what we know, what we don’t know which may be an issue then go do a literature research. In the past literature research may have been simply in the books we have, and checking known industry associations and their publications. Libraries being avoided as too far to travel to, and also likely that the type of references which maybe involved, are not available for borrowing: hence have to keep returning to the library to read, unless we can photocopy all that we need.

These days however can search amazon.com find out the books which are available. The problem there however is may cost some $500 or more for a collection of books and need to wait some 4 to 12 weeks, for them to arrive: so not convenient if need the answer yesterday. Therefore tend to look for websites and research papers: the basic reason the internet was born.

Needing the answer yesterday is one of the problems. People turn up expecting the item they wish to build or manufacture is common place, the only original thing is that they are making it for the first time. Problem is the local consultants they go to are also designing and assessing the product for the first time. The problem is too many don’t research the issue, and assume the 4 year B.Eng provides all the knowledge they require. In a way it does, but only if you intend designing experiments, building and testing prototypes, and otherwise reinventing the wheel. If don’t expect to being doing experimentation then literature research is required. Even if do need to do experiments, it is preferable that the reason is that have exhausted the available literature. Published literature is likely to have consolidated the results from a multitude of independent experiments: and so likely more reliable than a one off experiment. If studying for a masters or doctorate by research then likely to review the literature first. And real engineering is little different than studying for a masters and doctorate, just don’t get any award, and the real world is the final judge.


Related External Articles which resulted in my comments elsewhere including on LinkedIn. Also included articles and books which may assist with design and assessment, and understanding of wind loading.

  1. Angus J. MacDonald (1975),”Wind Loading on Buildings”, Applied Science Publishers
  2. Andy Hunt (September 1980), “Wind Engineers Save Builders’ Blows”, New Scientist
  3. Henry Wong (May 1982), “The Taming of the Vortex”, New Scientist
  4. New Scientist (December 1986), “Putting The Wind Up British Builders”, New Scientist
  5. John D Holmes (2001), “Wind Loading of Structures”, spon press
  6. Google: Melbourne freeway sign collapse
  7. Google: wind induced vibration road signs
  8. Google: wind induced vibration of chimney stacks
  9. Google: vortex shedding
  10. Emley Moor transmitting station
  11. Cooling Towers Ferrybridge C
  12. LightPole-Vibration_031306.mpg
  13. Aeolian Vibration
  14. Freak freeway sign fall seen in shocking video nearly dooms driver in Australia
  15. Tullamarine Freeway sign falls and crushes car, injuring woman
  16. ROAD SIGNAGE COLLAPSE NECESSITATES STRICTER ENFORCEMENT OF STEELWORK COMPLIANCE
  17. Victoria moves closer to mandatory engineers registration

NB: Take note of the government highway departments getting the reports on wind induced vibration of traffic signals.

Found more recently:

  1. Tullamarine Freeway sign may have fallen due to sustained wind pressure, engineering experts say
  2. Melbourne overhead sign fell due to missing steel plate, investigation finds

WARNING: Code compliance does not equate to fit-for-function, as most codes are deficient, so ensuring greater compliance does not make our environment safer. Knowing the limitations of codes helps, doing the fundamental experimental research helps. Not taking things for granted, having curiosity, and seeking understanding helps.


Revisions:

  1. [01/06/2019] : Original
  2. [02/06/2019] : Given own posting, minor rephrasing.
  3. [03/06/2019] : Added Diagrams, some rewrite and extension