State of Play 2019/wk22

Put the store back online last weekend. Set fees to zero for those items usually downloaded by students, everything else been grouped in with the services requiring and enquiry. At the moment reluctant to reactivate the store, as been hacked twice this year, which is the first time since creating in 2014. I don’t know what the hack is, though it involves modifying my paypal details so payments are redirected elsewhere. Paypal provides little help as it doesn’t involve my account. The buyers have to request Paypal for a refund from the parties which actually received the money. I’m reasonably certain the password hasn’t been hacked, the plug-in’s seem ok. So my guess at the moment is an input field allows database commands to be executed and modifies the database. Which is strange as one of the updates, prior to the hack was wrapping input fields in commands which strip items which are not relevant to the input field.

So until fully figured it out, all items to be purchased will have to start with an email enquiry, and then invoiced and paid via EFT, before item is released.

Speaking of which been making more refinements to the pricing process. As mentioned previously the effort required is proportional to the complexity not to the size of the structure. As long as two structures have the same structural form, they require the same design effort. As a consequence small structures need to be designed once and made many times, so as to distribute the cost to many production units and lower the unit cost of the design.

Whilst there is an issue that larger structures if they fail may cause more consequential damage than a small structure, it is to be noted that the small structures may be installed into the built environment at the rate of a few hundred units per year. If a one small structure fails, it may have consequences for all such small structures. Thus on balance the risk associated with both small and large structures is similar. For the most part this risk is covered by professional indemnity insurance and other more specific insurances. If risk and hazards are an issue then they need to be directly considered. That which is catered for by codes of practice and regulations can be considered a minor risk, it may involve a major hazard, but the risk is minor as we have a means of mitigating the hazard. In short there is no real justification for design fees increasing in proportion to size of structure to accommodate higher risks. Only an increase in complexity as a consequence of increase in size should be considered, as increasing the fee.

So architects and engineers complaining about decreasing percentage fees, and their work becoming a commodity, basically have little understanding of the nature of the work they actually do compared to what they believe they do, which they typically describe with hyperbole.

Established technologies (eg. buildings, bridges, vehicles, road signs) are a matter of routine design. For certain new assemblies introduce a new sequence of tasks, which may be uncertain until finished: but each individual task is a matter of routine. There is little unknown and uncertain about the design and assessment.

Road Signs

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.

The design of a road sign is relatively routine. The wind loading code, covers signs and hoardings. So wind loads can be determined. The moments in support posts can be calculated and suitable size posts selected, welds and bolted connections designed. The local effects on support structures also assessed.

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 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.

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 web stiffeners, to prevent the flange of the gantry from rotating, and prevent the web buckling from the compressive component of force from the base moment. Whilst the gantry 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 welds would need checking for torsion, as would the bolted connection. The bolts would typically resist the torsional moment in shear.

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. 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 direction 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 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 used more for earthquake design}

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.

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.

As to, how to design the signs taking this into consideration I don’t know, as a 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.

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. First of all when the Bureau of meteorology advises a severe weather warning, when wind average 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.

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.

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

To conclude a 2 year qualified structural or mechanical engineering associate could have designed the sign, an engineer not required. However, the available information is inadequate to cover wind induced vibration: or is it? Well lets google: wind induced vibration road signs. Seems literature research maybe all that is required.

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 likely the type of references which maybe involved are not available for borrowing: hence have to keep returning to the library to read unless can photocopy all that need.

These days however can search amazon.com find out what books 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, its just 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.


Revisions:

  1. [01/06/2019] : Original