Article and photos by Gregory Havel
When a steel-framed structure is built, it usually features a steel frame that rests on concrete foundations. The structural steel must be connected to the foundation or the building will collapse before it is completed. The traditional way to set anchor bolts in concrete is to embed long L-shaped bolts in the concrete foundation while it is being poured. When the concrete has set and cured, these bolts cannot be pulled out of the concrete without destroying the foundation.
The structural engineer designs the concrete foundation’s dimensions and the length, diameter, and spacing of the bolts; he also specifies the size of the steel columns, flanges, and bolt holes.
Photo 1 shows four large anchor bolts prepared to be set in a concrete foundation. The bolts have been assembled onto piece of oriented strand board (OSB) that was made from the template the steel fabricator supplied to match the bolt holes on the column’s bottom flanges. Wood bridging on the concrete forms holds the assembly at the proper elevation; sometimes tie wires to the steel rebar are used. Although the concrete foundation can be poured so that it is close to the elevation needed for the steel column, it often isn’t precise.
Photo 2 shows the anchor bolts after the concrete foundation has set. The OSB template has been removed and replaced with a steel “setting plate” from the structural steel fabricator. Nuts threaded onto the anchor bolts support this plate. These nuts will be adjusted so that the setting plate is level and at precisely the elevation the structural steel columns require. The nuts on top will lock the setting plate in place.
Photo 3 shows the setting plate after the space between it and the concrete foundation has been packed with quick-setting Portland-cement grout for added strength. After the grout has cured, the top nuts are loosened. The nuts will be removed completely to set the column, replaced, and tightened to hold the column in place. Usually, this completed connection is not visible in the completed building because it is below floor level and covered by a poured concrete floor, or embedded inside a masonry wall.
This system provides a strong connection between column and foundation. Its disadvantage is that it leaves little room for error. The bolt holes in column flanges are usually large enough to compensate for an anchor bolt that is 1/8-inch out of alignment. If the error is large enough so that the column flange does not fit, either the flange or the anchor bolt must be modified. Any modification of anchor bolts for structural steel requires the written approval of the design structural engineer, according to consensus standards and the requirements of the Occupational Safety and Health Administration (OSHA). If the anchor bolts are too far from their proper location or of the wrong size, the engineer may order them to be cut off and replaced with new anchor bolts epoxied or grouted into holes drilled deep into the foundation.
In the past, a too-frequent cause of structural collapse during construction was the setting of structural steel before the concrete foundations or masonry walls were sufficiently cured. OSHA’s 2001 revision of the Steel Erection Standard (29 CFR 1926 Subpart R (1926.750 through 1926.761, plus Appendixes A-H) requires that contractors follow the procedures outlined to reduce the possibility of building collapse during construction, providing a safe place for contractor employers to work, and a less-hazardous environment for emergency responders who may be called.
OSHA’s complete Steel Erection Standard can be found at http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=10939
Section 1926.752 outlines the written notifications required before any steel is set, including the strength of foundations and masonry piers, and may be found at http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=10788
Section 1926.755 discusses strength requirements for and modification of anchor bolts, and may be found at http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=12746
Subjects: Anchor bolts in concrete, building construction for firefigher operations