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Q

How can I prepare wind pressure design capacity tables for roofing sheets for use in Australian buil

A

Steel roofing sheets in Australia are commonly made from light gauge high tensile steel. It is common practice to crest-fasten the sheets to battens with self-drilling metal screws. Wind creates uplift pressures on the sheets resulting in high stress concentrations around the screw holes.  A common cause of sheets blowing off roofs is pull-through of the sheets over the head of the screws.  It is very difficult to carry out theoretical stress analysis around the screw holes and obtain meaningful results which could be used to prepare design capacity tables.  Most of the design capacity tables in Australia are prepared from the results of full-scale testing carried out in laboratories. To carry out full-scale testing, the sheets are attached to an air-box (usually 3 sheets wide) and are subject to uplift or downwards pressures.  To be able to interpolate design pressures for intermediate spans, a minimum of 3 span lengths for a sheet profile have to tested.  The NCC nor the Australian Standards allow extrapolation of results.

Q

Is Finite Element Analysis (FEA) reliable?

A

Although FEA is a very useful tool for carrying out stress analysis on structural components, the accuracy of the results depends on the skills and experience of the analyst.  Errors could be introduced during modelling and meshing the structure and in the interpretations of results. Modelling errors can occur when the geometry is prepared and the restraints and loads are applied.  Sometimes the geometry of the structure may have too many features for the software to be able to solve the problem.  The analyst has to decide which features may be removed without compromising on the accuracy of the results.  Other errors could be introduced when deciding on the restraints (boundary conditions) and in the application of the load. FEA involves breaking up the structure into small (finite) elements which is known as meshing.  Generally, smaller elements (finer mesh) give better results than coarser mesh (larger elements).  The finer the mesh, the longer the computational time.  To reduce computational time, a finer mesh is used where the stresses are critical and a coarser mesh where the results are not so critical.  There is a point where increasing the mesh density does not result in a significant improvement of the results. Interpretation of results can sometimes be complex.   Once the problem is solved on the computer, there are various outputs such as stress contours, strain and displacement contours, reaction forces, etc that be plotted.  The analyst has to establish whether the results are what he/she would expect.  This involves a combination of experience and a good understanding of the behaviour of the materials used.  Wherever possible, the analyst may carry out some hand calculations to find out whether the FEA results are within the expected range.  There may be high stresses in the plots caused by singularities and the analyst has to decide whether to neglect them or to carry out further checks.

Q

What Codes and Standards apply to the design of residential roof gutterss?

A

The design and installation of residential roof gutters are required to meet the performance requirements of the Australian National Construction Codes.  The codes cross-reference AS/NZS 3500.3 (Australian and New Zealand Standards. Plumbing and Drainage.  Storm Water Drainage).

Q

What are the advantages of steel framed houses?

A

As timber is getting more difficult to purchase, light weight steel is commonly used in residential properties.  The thickness of the steel used can vary between 0.42 to 1.0mm.  Wall panels, roof trusses and floor panels are generally pre-fabricated in factories.  Light weight steel has the following advantages over timber: The steel pre-fabricated panels are light weight and can generally be handled by 2 persons.  This saves on the cost of hiring cranes Steel will not be attacked by termites Steel will not distort or twist over time Steel will not rot or decay Pre-fabricated steel panels will reduce construction time and costs Steel is sustainable