CONCRETE TECHNOLOGY
By Kim Basham, PhD PE FACI
ANCHoR BoLT
Tolerances
Misplaced or misaligned anchor bolts are a common field problem. ut-of-position anchor bolts occur when the measured deviation or variation from the specified location or alignment exceeds the specified construction tolerance. To avoid schedule delays and costs associated with out-ofposition anchor bolts, you must understand what tolerances your work will be judged by and take the necessary actions to position, align and secure the anchor bolts.
O
Table 1. Tolerances for Anchor Bolts and Column Base Plates Item
Steel Tolerances
Concrete Tolerances AISC 303 (Ref 1) ACI 117 (Ref 2)
1
Dimension between centers of any two anchor bolts in a group
≤ 1/8 in.
---
2
Dimension between centers of adjacent anchor bolt groups
≤ 1/4 in.
---
3
Elevation of tops of anchor bolts
± 1/2 in.
± 1/2 in.
4
Accumulated variation between centers of anchor bolt groups
≤ 1/4 in. per 100 ft 1 in. maximum
---
5
Variation in dimension from center or anchor bolt group to column line
≤ 1/4 in.
---
INDUSTRY-STANDARD TOLERANCES
6
Centerline of individual anchor bolts from specified location
---
3/4 in. & 7/8 in. bolts … ± 1/4 in. 1, 1¼ & 1½ in. bolts … ± 3/8 in 1¾, 2, & 2½ in. bolts … ± 1/2 in.
The concrete and steel industries both provide anchor bolt tolerances. Unfortunately, the tolerances are not compatible. Tolerances provided by the American Institute of Steel Construction (AISC) are more restrictive than anchor bolt tolerances provided by the American Concrete Institute (ACI). For the steel erector, Division 5 of the specifications references AISC 303 “Code of Standard Practice for Steel Buildings and Bridges” while Division 3 of the specifications for the concrete contractor typically references ACI 117 “Specifications for Tolerances for Concrete Construction and Materials”.(1, 2) Even though the architect/engineer should clarify and resolve these tolerance differences, concrete contractors sometimes become involved in construction disputes because of the non-compatible tolerances. As shown in Table 1 and Figure 1,
7
Bolt variation from plumb, (perpendicular to base plate)
0 degrees (no tolerance)
---
8
Position of base plate holes
± 1/8 in.
---
9
Top of concrete foundation
---
+ 1/2 in., - 2 in.
10
Top surface of drilled piers
---
+1 in., - 3 in.
11
Elevation of bearing devices
± 1/8 in.
---
AISC tolerances place limits on the variation between centers of any two bolts in a group (Item 1), centerlines of bolt groups(Item 2, 4), and centers of anchor bolt group from column lines (Item 5). ACI 117 only specifies elevation (Item 3) and a location tolerance for the centerline of individual anchor bolts (Item 6). As shown for (Item 7), AISC has a very strict or zero plump tolerance for bolts whereas ACI does not specify a plumb tolerance. The only compatible tolerance between AISC and ACI is the top elevation of the bolts (Item 3). AISC’s tolerances combined with oversize holes in the base plates ensure the steel structure
can be easily erected without modifying the structure. For anchor bolts to satisfy both AISC and ACI’s tolerances, Suprenant and Malisch claim anchor bolts must be located within ± 1/16 inch of their specified location.(3) A tolerance of ± 1/16 inch is neither realistic nor achievable. Therefore, it is critical for concrete contractors to understand what tolerance applies to the concrete work – AISC or ACI tolerances? AISC recommends that Division 3 of the specifications require anchor bolts to be set in accordance with the AISC 303 “Code of Standard Practice”.(4) Even using plywood or steel templates to position,
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