Examining the problems behind sign cladding


The chance of injuring someone is probably higher from bits of broken sign cladding flying around than from a sign frame falling over. However, as neither is ideal for you or the unfortunate passer-by, let us examine a few common issues that surround this area.

Materials

Common cladding materials include aluminium, composite sheets such as Alucobond, steel and acrylic/polycarbonate. None are likely to cause problems on smaller signs, but where used on larger ones, in locations subject to high wind loads, certain factors affecting safety come into play.

Aluminium and composite sheet are easily formed, light, and have fewer corrosion issues than steel with exposed material edges. Steel has advantages where similar cladding and structural materials are required on larger signs, and it provides greater resistance to ‘tear’ or ‘pull over’ around fixings.

Acrylic and polycarbonate are not generally used as structural materials as they will become brittle over time, fracture or splinter more easily, and require a greater degree of care over expansion and contraction.

Fixings

There are obviously a myriad of methods for securing cladding, including screws, bolts, tape, adhesive, slotting. Structurally, tape and adhesive provide permanent joints and are largely dependant on the correct method and conditions for application. Manufacturer guides for loadings should be closely followed, and remember these may reflect application under ideal conditions—rarely achieved.

Mechanical fixings, such as machine screws, bolts, and self-tap screws, will provide removable cladding panels and are probably the most popular options. If drilling and tapping into a sign frame, care should be taken that, firstly, the section tapped will provide enough thread to securely fix the screw or bolt. For example, trying to fix an M6 bolt into a 2mm thick section will not provide enough ‘thread’.

Secondly, ensure the hole drilled does not weaken the strength of the section to a point that could cause failure. If using self-tap screws—tek bolts—again, ensure the screw is correct for the wall thickness and likely loads.

When securing thin materials, two particular issues should be addressed. Firstly, if a hole is too close to the edge of a material, it can ‘tear’ between the hole and the edge.

Secondly, material can ‘pull over’ the head of a securing bolt or screw where it tears around the small circumference of the bolt head.

Both these issues are of particular concern when using relatively ‘soft’ aluminium as cladding. Ensure holes are not too close to the edge and, if possible, use washers to spread the load and help prevent ‘pull over’.

Strengthening and supporting

I am sure everyone has seen a sign where a panel is ‘dished’ or curving inwards to the centre—this indicates that the panel is not sufficiently strengthened and/or supported from behind. Wind pressure will be successfully resisted over different sized areas by different materials. It will be a combination of strengthening the rear of a panel with ‘top hats’, ‘z’ or ‘c’ sections—common terms for sections usually formed from sheet material—bonded, riveted, or welded to the panel. These will certainly strengthen and usually prevent ‘dishing’, but they will not prevent the actual panel failing around the edges—either due to ‘tear’ or ‘pull over’ at the fixings, or the panel fracturing at the folds.

To prevent this, separate supports will be required attached to the sign frame itself. The design and quantity will depend upon the likely worst case wind loads. Once again, there is no standard solution, but being aware of safety issues does mean that often simple design changes can make a big difference.