Levelling shim
Also called Packer, Levelling packer, Shim pack, Stacker Packer
A levelling shim is a thin, load-bearing plate placed under a structural element — a precast unit, a steel baseplate, a stair flight, a modular pod — to bring it to the correct level before it is fixed or packed. Shims are stacked to make up the gap, so they carry the element while it is being set, and in many details they are left in and become part of the permanent load path.
From our specifications
Our own shim is the Stacker Packer, and its published data is the shape of the answer for this class of product. The SPK range is 70 mm × 70 mm, moulded in high-impact polystyrene, and supplied in 2 mm, 3 mm, 5 mm and 10 mm thicknesses, so a stack is built to the gap rather than the gap being built to the stack. Projecting dowels interlock the shims horizontally and vertically so that the stack does not slide against itself under load, and they are designed to depress under load and leave a flat surface with no high points. The range has been independently compression tested by HCC County Highways Laboratory and by TWI Ltd over stack heights from 25 mm to 100 mm; under a 14.65 t load test the 50 mm stack recorded post-test vertical deformation of 0.38% and 0.36% respectively, and we publish a recommendation that for shim stacks up to 50 mm high the compressive strength value be taken as 30 N/mm² under factored loads. That is a published product recommendation with its own limits attached, not a design value for your bearing — the value that applies to a specific detail is the engineer’s to set.
Source: Stacker Packer (SPK) published product data, datasheet and independent load testing report
Where it matters on site
Three things decide whether shimming goes well and none of them is the shim’s strength. The first is whether the stack stays where it was put: loose steel or plywood packers slide as a unit is landed, and the adjusting is done with fingers between a suspended load and a bearing. The second is the surface the element finally sits on, because a proud dowel, a rocking pack or a stack that has spread concentrates the bearing on one corner of a unit rather than across it. The third is what happens next, and it is the one to raise with the designer: if the packs stay in and the gap is dry packed around them, they are permanently inside the load path, and what they are allowed to carry over the life of the structure is a design question rather than a site one. Ask whether the packs are to be removed; if they are, someone has to be able to get them out after the packing has gone off, and that is easier to plan than to discover.