While individual member tolerances are usually self-compensating and of minor significance in the overall structure, the possibility exists that these tolerances may accumulate and lead to misalignments that are difficult to correct in the field. As an example of the effect individual member tolerances may have on the total structure, consider the tolerances on columns and beams. Individual column and beam members are shown with their respective permissible tolerances in Figure 3.5.3-1. These tolerances come from several sources: permissible camber and sweep are specified in ASTM A6/A6M and AWS D1.1; permissible variation from detailed length for members framed to other steel parts is specified in the AISC Code of Standard Practice; mill tolerances on the cross-section are illustrated in the 2005 AISC Code of Standard Practice Figure C-5.1. The foregoing example involves a possible but highly unlikely scenario.
A case where individual members fabricated within permissible tolerances could make it impossible to erect a heavy two-story column within the plumbness tolerance of ±1:500 is illustrated in Figure 3.5.3-2. Although the condition shown would be unusual and represents the worst case with all member tolerances maximized and accumulated in one direction, it is evident that the accumulation of tolerances requires special consideration. Other possible examples include double-angle and end-plate connections to columns, attached shelf or spandrel angles, large plan dimensions in which many pieces line up, long bracing, expansion joints, and vertical systems such as stairs and multi-story wall panels. Deflections of cantilevered members and tolerance accumulation on complex framing systems involving a long series of connections before the load is in the column (causing accumulation of vertical tolerances) should also be considered.
Details for material supported by the steel framing must provide for the standard tolerances. For example, in buildings with large plans, it is beneficial to develop special details that accommodate the accumulation of fabrication tolerances. Note that building expansion joints cannot be adjusted to proper position without a provision for this adjustment.
The use of oversized holes, short-slotted holes, and long-slotted holes, provided a satisfactory method for achieving erection within tolerances as illustrated in Figures 3.5.3-3 and 3.5.3-4. Other satisfactory methods include the use of finger shims, shop layout to theoretical working lines, and recognition of tolerance accumulation in details for finishes, such as the curtain wall or stonework attachments.
last modified 1 January 2006