Monday, 14 September 2015

Aluminium balustrades: HAZ

{Previously posted in LinkedIn group I created: Pre-Engineered manufactured structural Building products, and have since shut down. It having a membership of 30 people. Most of these people also in my group Pre-Engineered Manufactured Building Systems Group which has over 2000 members.}

Manufacturers of Aluminium balustrades use standard calculations for the balustrades they supply. Typically those balustrades are designed to be cast into the concrete floor slab. Occasionally however, it is not viable to cast-in the posts, and welded end plates and anchors are specified. This flows into council and requests for further information result for the base connection. Aluminium is not a commonly used structural material, and therefore the code not always held by consultants. With the pressures of time, it appears that little attention is given to the properties of aluminium, and the calcs-for-council requested simply size up some mechanical anchors such as Ramset Dynabolts. In some Australian states, it appears that such things are permitted to be self-certified. Thus posing an hazard.

For the welding of the post to an end-plate to produces a heat affected zone (HAZ) in the post in the vicinity of the maximum post moment. In the HAZ the strength of the aluminium is approximately half that of the parent material. Thus the post is potentially no longer adequate for purpose. The other issue is that not all grades of aluminium are suitable for welding, and the given balustrade design may not be made from suitable material for welding. So a quick fix end plate design is not appropriate.

Such product should be fully designed, giving consideration to its potential applications, and the designer should have experience in aluminium design, or at least willing to get up to speed on aluminium design.

Builders turning up once in a millennium to fix their problems doesn't produce much motivation for a consultant to specialise in a given product design and material: not the least of which is they have to go out buy all the new codes and get familiar with them for a single job: that causes delays. It is therefore preferable that the designers be employed on staff by the builder/manufacturer, or otherwise put a lot more work through the consultants they wish to use.

Design the product not the project. Then assess suitability of product for the project. Further more calc's-for-council constitutes neither design nor engineering, and produces low quality products.


Looking at a system which avoids the problem of welding aluminium, by making use of a steel bar insert.

A normal steel bar (Grade 300 plus), has lower strength than the aluminium tube, whilst a BisPlate insert has higher strength. At first it appeared that the flat bar insert was just being used as a means of connecting to the slab, but on further inspection the BisPlate insert is being used to increase the capabilities of the post. (the tube is filled with grout)

Pushed to the extremes the aluminium post could just be considered decoration slipped over a structural section.

Seems like there is scope for some more efficient design of aluminium balustrades and guard railing. Starting by designing the extruded components for strength, and connectivity first and incorporating aesthetics second.