Column bases
Packing and shim stack under a base plate
- where
- pack height needed, equal to the grout gap (mm)
- specified level of top of base plate (m)
- surveyed concrete level (m)
- base plate thickness (mm)
- number of main packs
- main pack thickness (mm)
- number of shims
- shim thickness (mm)
- achieved stack height (mm)
- stack error (mm)
Explain this equation
Works out the height of steel packing needed under a base plate and how to make it up from thick packs and thin shims. Use it when setting out bases before landing a column.
L_top is the specified level of the top of the base plate and L_c the surveyed concrete level where the pack sits. t_p is the plate thickness. H is the pack height needed, which is also the grout gap. t₁ and t₂ are the pack and shim thicknesses, n₁ and n₂ how many of each. S is the stack you achieve and E how far it is from H.
The level difference in metres is turned into millimetres, then the plate thickness is taken off to find the space under the plate. As many thick packs as fit are used, with the floor brackets rounding down, and the remainder is made up with the nearest whole number of shims.
Sense check: a 65 mm level difference with a 25 mm plate leaves 40 mm, four 10 mm packs.
Watch out: concrete is rarely flat, so survey at each pack position, and keep packs clear of where the grout has to flow.
Height of steel packing needed under a base plate to bring the top of plate to the specified level, split into thick packs and thin shims.
Enter your values to see the result.
Good to know
The NSSS requires steel packs to be supplied so the frame can be lined and levelled, and base packs must not stop full grouting. Place packs near the bolts and under the column flanges, away from where grout must flow. Packs may be left in place if the engineer agrees. NSSS tolerance on top of base plate level is ±5 mm (as quoted in NSSS-based checklists), so the shim size controls how close you get.
Common mistake
Taking the level of the concrete at the centre of the base when packs sit at the corners, or forgetting the base plate thickness.
Limits
One pack position. Levels in metres to a common datum.
Sources
- BCSA National Structural Steelwork Specification for Building Construction, 7th edition, 1st revision (2023) – Section 9 accuracy of erected steelwork; Section 7 cambers; 8.6.2 reference temperature 20 °C
- BCSA National Structural Steelwork Specification, 5th edition (cl. 3.4.5 HD bolt holes 6 mm over bolt diameter; 8.1.6 steel packs; 9.2 foundation and HD bolt check before erection; 9.6 erected components)
- SCI P178 Design for construction, column bases (typical 25 mm to 50 mm grout gap with non-shrink cementitious grout; sleeve about 3 × bolt diameter; top of bolt within +25 mm to −5 mm of specified level)
- AJB Setting Out: QA checklist, structural steelwork (citing the NSSS: column plumb ±h/600 or 5 mm per storey, max ±25 mm single storey and ±50 mm overall up to 10 storeys; beam level ±10 mm; level at each end of a beam ±5 mm; top of base plate ±5 mm)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Always work to the engineer’s drawings and bending schedules, BS 8666, BS EN 1992, BS EN 1090-2, BS 5531/the BCSA National Structural Steelwork Specification and manufacturers’ data. Lifting must be planned under LOLER by a competent person.