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Re: Anchor cluster tolerance

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The positional tolerance has to be compatible with the hole clearance. So
here AS4100 steel structures code permits the hole to be 6mm larger in
diameter than the anchor bolt diameter. That is equivalent to 3mm greater in
radius. The bolt can thus move 3mm in any direction and still fit in the
hole. Thus move two bolts move away from each other, and get +6mm on the
nominal dimension. Move the two bolts together and get -6mm on the nominal.
However such ignores positional tolerance on both the bolts and the holes.
AS4100 says the positional tolerance is +/- 3mm (approx. 1/8"). Suggesting a
equal split between tolerance on the anchor bolt position and the position
of holes in the base plate. At extreme place anchor bolts 3mm closer
together, and the holes 3mm further apart and the bolts should still fit in
the holes, and vice versa. This is assuming the variation in bolt diameter
and hole diameter is insignificant.

If want larger positional tolerance on the hole and bolt positions, then the
holes have to be larger, which then creates problems sizing a thick washer
plate which doesn't bend too much. Here AS4100 says the washer plate shall
be minimum thickness of 4mm, if the hole is more than 3mm larger in diameter
than the bolt. For other than an anchor bolt the hole is only permitted to
be 2mm larger in diameter.

The point is that the 1/8" Steve mentioned from the AISC is likely to be
compatible with the base plate hole size, whilst the 1" mentioned by Paul to
the ACI is unlikely to be compatible with the base plate hole size.

If the anchor bolts are installed individually on site then the ACI
tolerances are probably a more realistic expectation of the construction
process. If the bolt cluster is installed as a single fabricated unit (bolt
cage), then the AISC tolerances should be achievable. To AS4100 the centre
of the bolt group is only permitted +/- 6mm off ideal position, the maximum
accumulation +/- 25mm.

Tolerances and clearances should be determined from the functional
requirements of the design. Ideally the specified variation should be
greater than the variation achievable from the production process. This
unlikely to be the case if AISC criteria imposed on concrete contractors.

Which suggests either need to explicitly place required tolerances in
project specification. Or that may be there is a need for the steel
structures codes/specification to be revised to be more accommodating at the
interface with other trades.

But how to accommodate a 1" variation in position without need for an
extremely large bolt hole? One expensive option: custom make each base plate
to fit. The best solution is usually to avoid the problem: but chemical
anchors introduce other problems, as does welding the column to the base
plate on site.

Also the construction process can always make use of plywood or sheet steel
templates to locate bolts in cluster, and achieve better than 1" variation
in position. The templates should be capable of being made to same
tolerances as the base plates, therefore positional variation on anchor
bolts within cluster can comply with AISC requirements. Also given that the
base plates could probably be made with +/- 1mm variation between holes, a
greater proportion of the combined deviation could be allocated to the
anchor bolt position, to allow for wear of templates on site and loose fit
of bolts in template holes. So concrete industries tolerances on size of
elements and position of reinforcement not appropriate to apply to placement
of anchor bolts: can achieve better and function of base plate and anchor
bolts requires better.

If adopt the ASCC criteria, then need to ensure steel fabricator can achieve
the compatible associated tolerance on hole position in the base plate. The
more deviation permitted in position of the bolts, the less deviation
permitted in position of base plate holes unless much larger holes used.
Noting that ASCC statement #14 seems to be suggesting adopting larger holes
and plate washers in the first place. Therefore steel fabricator needs to be
advised of variation to what they would routinely adopt from ASIC criteria:
otherwise may have to enlarge some holes on site. (eg. Would tend to adopt
smallest hole diameter, and associated drill and reamer, and eliminate need
to fabricate plate washers.)

Conrad Harrison
B.Tech (mfg & mech), MIIE, gradTIEAust
South Australia

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