Advanced Structural Designs

  

ACN 097 789 87    92 Vasey Cres CAMPBELL ACT 2612

Telephone  61612171         Facsimile 61612170

Email   mal@structuraldesigns.com.au

 

 
 
                                                                                                           

                                                             

 

What do we know about cylindrical steel silos?

 

You’re probably thinking we specialise in commercial buildings rather than agricultural structures and you are right, but Mal Wilson did carry out quite a deal of research in order to try to justify some very marginal steel silos, conveyors and elevator towers in a grain handling facility at Temora in 2001.

 

The three predominant design considerations for the flat-bottomed silos were

 

  1. Bursting under hoop tension during loading
  2. Buckling of the wall plates during unloading
  3. Wind loading when empty

 

Bursting under hoop tension is a relatively easy check but care needs to be taken to ensure that discontinuities such as doorways are adequately catered for.

 

Wind loading is also a relatively simple check provided you have access to a finite element program supporting curved shell elements.  Local wind funnelling between silos is a well-known effect as is the possibility of vortices shedding from the leeward face.  We normally use NSW Universities wind engineers when model testing or expert opinions are required.

 

Buckling of the wall plates during unloading is potentially simple if you stick to the design recommendations in the many excellent papers written on the subject.  Mal has 16 design papers and a couple of good reference books that cover all aspects of design. 

 

In a typical situation of a lap bolted steel silo with vertical stiffeners account needs to be taken of

 

  1. Moment at the horizontal lap.
  2. Relative stiffness of the walls and stiffeners (remembering the Poisson’s ratio effect on the walls).
  3. Effect of bolt slippage.
  4. Restraint at the base (against horizontal movement).
  5. Effect of discontinuities such as doors and aeration ducts.
  6. Beneficial effect of the grain pressure.

 

 

Unfortunately when you do take account of all of these effects a disturbing proportion of so called standard designs brought in from overseas do not comply with the Australian Design Codes.

 

We spoke to a couple of Australian designers working in the area but sadly they informed us that they did not take any account of items 1 to 5 and were unaware of the many design papers on the subjects.  The codes unfortunately cover loads rather than design techniques and fail to point designers in the direction of any relevant research.

 

One designer Mal spoke to from Queensland said he had been designing grain silos on high platforms for years and never taken any account of the possibility of earthquake loading.  This was particularly surprising as AS 3774 is crystal clear when it comes to loading and devotes a whole section to Seismic loading. 

 

The Temora job required a great deal of strengthening work to make it code compliant and left Mal with the distinct impression that designers of agricultural structures have a long way to go before they deliver the same degree of reliability as building designers.  One reason for this may be that there are many excellent and comprehensive books around on designing various types of buildings there are many fewer on silos.  The best single book Mal can recommend on this subject is “Guide to the Economic Design of Circular Metal Silos” by J Michael Rotter 2001 Spon Press.  You will also need a copy of a few European Codes to use it.

 

Mal has developed a number of excellent design tools that take the tedium out of designing and checking circular silos, so if you require some structural engineering appraisals or designs of silo’s near Canberra ACT near we would be happy to take it on.

 

Home  FAQ's