Detailing Corner Grade Beam Depth and Dowel Embedment S
hallow grade beams can cause congestion problems at the tops of drilled pier foundations or pile caps. Achieving proper embedment of the column vertical reinforcing bars, dowels projecting from the foundation, or both often requires that the reinforcement be hooked in the grade beam. The resulting congestion may make it very difficult for the grade beam longitudinal bars to pass unobstructed over the foundation. The problem is compounded if two grade beams intersect over a foundation element.
Options
Figure 1 shows an example of a grade beam-drilled pier connection as originally detailed. The drilled pier is 18 in. (460 mm) in diameter with eight No. 8 (No. 25) vertical bars. The grade beam is 2 x 2 ft (0.6 x 0.6 m) with No. 11 (No. 36) top and bottom longitudinal bars and No. 5 (No. 16) stirrups. The dowels for the columns are eight No. 8 (No. 25) bars. As shown in Fig. 1, the reinforcing bars can barely fit—even with ideal placement. If there were more bars in
DETAILING CORNER
Joint ACI-CRSI Committee 315-B, Details of Concrete Reinforcement— Constructibility, has developed forums dealing with constructibility issues for reinforced concrete. Staff at the Concrete Reinforcing Steel Institute (CRSI) are presenting these topics in a regular series of articles. CRSI staff will also respond to requests for information (RFIs) regarding design, detailing, and construction. If you’d like to suggest an article topic or submit an RFI for this feature, please send an e-mail to Neal Anderson, CRSI’s Vice President of Engineering, at nanderson@ crsi.org with the subject line “Detailing Corner.” Fig. 1: Drilled pier to grade beam connection, as designed Concrete international
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Fig. 4: Option 3—Increase grade beam depth only at the drilled pier Fig. 2: Option 1—Column dowels extended straight into the drilled pier
Fig. 3: Option 2—Increase grade beam depth
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Fig. 5: Option 4—Add a pile cap under the grade beam
the beam, drilled pier, or column or if the bars were larger, it would be impossible to fit all of the bars in the cross section. Moreover, if there was an intersecting beam, there would be no room to install the bars. One option would be to extend the column dowels straight into the drilled pier as shown in Fig. 2. This option, however, presents some difficulties: ■■ In many parts of the country, the contract for installation of piles and drilled piers is separate from the contract for construction of the remaining building structure, so different contractors normally execute each portion. While the foundation contractor’s workers will place the steel in the drilled piers, union rules may not allow them to place the column dowels. This would require coordination at a time when the building concrete contractor may not be on site yet; ■■ If they are cast in the drilled pier, the column dowels can’t be moved or adjusted to accommodate beam reinforcement or column locations; ■■ For large diameter drilled piers, the tolerance on pier location is much larger than that for the column location. If the column dowels are installed according to the foundation tolerances, they may be located farther away from their intended location than the tolerances for column location can accommodate; and ■■ If the dowels are out of tolerance, who is responsible for corrective actions—the building concrete contractor or the foundation contractor? A second option would be to design a deeper grade beam as shown in Fig. 3. Deepening the beam may eliminate the need for the drilled pier vertical reinforcing bars to be hooked, which eases the congestion to a large extent. Without the hooks, it’s possible to accommodate an intersecting grade beam. Although this option increases the volume of concrete, it may reduce the amount of steel required in the grade beam. A third option would be to deepen the grade beam only at the drilled pier as shown in Fig. 4. This thickened section would be placed concurrently with the grade beam concrete and gives results similar to Option 2. It eliminates the need for hooks on the drilled pier vertical bars, easing congestion. This option only slightly increases the volume of concrete and adds a slight amount of steel at each drilled pier. It does not significantly affect the reinforcing steel required in the grade beam. A fourth option would be to add a pile cap under the grade beam at the drilled pier as shown in Fig. 5. This would likely require a separate placement, independent of the grade beam work. In the options shown in Fig. 3 to 5, there is enough depth to achieve straight embedment of the column
Fig. 6: Option 5—Hold back the concrete from the pile top
dowels. This would be acceptable to most designers. Some designers, however, insist on hooked dowels because they feel this provides more accurate placement of the dowels, ensuring proper lap length projecting above the grade beam. For that reason, the column dowels are shown hooked in those figures. Another consideration may center on whether or not any moment is transferred into the foundation. A fifth option could be used if the grade beam is supported by a concrete-filled steel pile or casing, as shown in Fig. 6. In this case, the steel jacket is filled with concrete to an elevation sufficiently below the top so the column dowels could project into the pile to give the proper embedment length. This option is similar to Option 1 in Fig. 2, except in this case the column dowels are placed with the grade beam. This allows a certain amount of adjustment in placing the dowels. The top of the pile is filled with concrete when the concrete for the grade beam is placed. A sixth option would be to place a blockout in the top of the drilled pier as shown in Fig. 7. This option has advantages similar to those of Option 6. There is room for adjusting the location of the dowels for more accurate placement. As in Option 6, the blockout is filled with the grade beam concrete placement. Yet another option may be to use headed bars. Concrete international
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RFI on stainless steel reinforcing bars
RFI 09-2: I’m considering the use of stainless steel reinforcing bars on a project where I need the level of corrosion resistance this material provides. What are some of the issues that I need to understand about stainless steel?
Fig. 7: Option 6—Form a blockout in the top of the drilled pier
Design Considerations
Designers should always look carefully at the embedment lengths of the reinforcing bars passing from one member into another. The possibility of congestion should be reviewed whenever the drilled pier vertical reinforcing bars have to be hooked. Frequently, the difference between requiring a hooked bar rather than a straight bar is a matter of only a few inches. Wherever it’s possible without jeopardizing the integrity of the structure, the designer should opt for a straight bar embedment to minimize congestion. Relative advantages and disadvantages of the options discussed depend on the specifics of each case. The cost impact is usually not significantly different from one to the other if the deeper beam in Option 2 allows a reduction in the amount of beam reinforcement required. Thanks to Joint ACI-CRSI Committee 315 member Dick Birley, President of Condor Rebar Consultants, Inc., in Vancouver, BC, Canada, for providing the information in this article.
Selected for reader interest by the editors.
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Response: Stainless steel reinforcing bars are not stocked in quantities as large as for traditional reinforcing bars. Therefore, it’s recommended that the contractor coordinate early with the stainless steel reinforcing bar supplier to ensure timely delivery to the job site. The bars are typically only available in lengths up to 40 ft (12.2 m), which may require detailing changes to accommodate lap splice locations. Also, keep in mind that bar supports have to be made of stainless steel or plastic. Similarly, tie wire has to be stainless steel or plastic-covered carbon steel. Mechanical couplers, if needed, will also have to be stainless steel. Reinforcing bar fabricators and contractors need to recognize that stainless steel reinforcing bars are pickled. Pickling (cleaning with an acid mixture) is required by ASTM A9551 to remove mill scale and iron particles. This prevents the particles from corroding and damaging the stainless steel, but the bars must be handled in such a way that the surface is not recontaminated with carbon steel during fabrication, storage, transportation, and placement. In practice, this means the stainless steel bars cannot be fabricated using the same pieces of equipment used to fabricate traditional reinforcing bars, have to be handled with nylon slings, and should be stored inside or under a protective cover. For additional information, see “Stainless Steel Rebar Guidelines for Shipping, Handling, Fabrication and Placement.”2
References 1. ASTM A955/A955M-07a, “Standard Specification for Deformed and Plain Stainless-Steel Bars for Concrete Reinforcement,” ASTM International, West Conshohocken, PA, 2007, 11 pp. 2. “Stainless Steel Rebar Guidelines for Shipping, Handling, Fabrication and Placement,” Nickel Development Institute, Toronto, ON, Canada, and Specialty Steel Industry of North America, Washington, DC, 6 pp.
