Detailing Corner Bar Detailing at Wall Openings I
t’s a necessary fact that any building is going to have openings in walls or structural slabs. Round or rectangular, openings are required for conduit, piping, ductwork, doors, or windows. Their locations are often the responsibility of the architect, and they can change by minor or major amounts throughout the design process (and even during construction). This Detailing Corner focuses on openings in walls, but some of the discussion could be equally applicable for openings in slabs. In a typical wall with no openings, reinforcing bars will probably consist of layers of bars (curtains) with uniform spacing. The bars will typically be continuous over the full height or width of the wall, but an opening can interrupt the bars. Also, the re-entrant corners created by rectangular openings cause stress risers that will likely lead to diagonal cracking. So, reinforcing bars are typically added in the perimeter zone of an opening. These added bars are commonly known as trim bars, opening bars, or corner bars. Trim
DETAILING CORNER
Joint ACI-CRSI Committee 315-B, Details of Concrete ReinforcementConstructibility, has developed forums dealing with constructibility issues for reinforced concrete. To assist the Committee with disseminating this information, staff at the Concrete Reinforcing Steel Institute (CRSI) are presenting these topics in a regular series of articles. If you have a detailing question you would like to see covered in a future article, please send an e-mail to Neal Anderson, CRSI’s Vice President of Engineering, at
[email protected] with the subject line “Detailing Corner.”
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bars (called trimmer or trimming bars in some regions of North America), are the added bars placed near openings to replace the reinforcement interrupted by the opening. These bars are usually considered structural and are placed so as to provide a reinforced load path around the opening. Opening bars are minimum reinforcement added around windows, doors, and similar-sized openings. Per Section 14.3.7 of ACI 318-08,1 each layer of reinforcement in the wall requires at least one No. 5 (No. 16) bar around the perimeter of an opening. Opening bars must be developed at the corners, so they extend a development length beyond the opening. Corner bars are opening bars placed on a diagonal at each corner. Although they aren’t explicitly required, it’s good practice to use them—they provide more efficient restraint of the likely cracks at the re-entrant corners.
Design Considerations
On the structural drawings, wall openings are often detailed in a “typical” fashion with a number of generic notes pointing to specific vertical, horizontal, or diagonal bars on the drawing. Figure 1 provides an example of a typical drawing detail. General structural notes for the reinforcement at openings are equally generic. Frequently, the designer’s notes specifying trim bars around openings do not clearly express the designer’s intent, potentially leaving the detailer or bar placer uncertain about what reinforcement is specifically required. The most common general note for trim bars requires that one-half the number of cut or interrupted bars be placed on each side of the opening. If an odd number of bars are cut, then the number is rounded up to an even number and one-half of the bars are placed on each side. This practice is generally based on Section 13.4.2 of ACI 318-08, which is specifically required for slabs; however, it is normally followed for walls as well. In the case of nominal-sized openings, this practice usually doesn’t create a problem. Guidance for design and reinforcing bar detailing around openings exists in other ACI documents. ACI 313-97,2 for example, provides design and construction requirements for concrete silos. In lieu of a detailed
analysis of the effects of the stress risers at an opening, the standard requires that trim bars on each side of the opening have a total area of at least one-half the area of the bars interrupted by the opening (in effect, the trim bars must replace the interrupted bars at an opening). For openings located in zones of pressure, the area of the horizontal trim bars must be 20% greater than this minimum. Vertical trim bars on each side of the opening must also be designed as column reinforcement for a narrow strip on each side of the wall. The column width is limited to 4h, where h is the wall thickness, and the unsupported length of the column is equal to the full height of the opening. The trim bars must be developed at the corners, but they must extend no less than 24 in. (600 mm) or one-half the opening dimension beyond the opening. Section 14.3.7 and the associated commentary of ACI 350-063 require that trim bars replace the interrupted bars at an opening. The trim bars must be developed (or extend at least 24 in. [600 mm]) beyond the corners of an angular opening or the intersection with other trim bars of circular openings. The commentary points out that this is minimum reinforcement—walls with lateral loads should be designed to maintain the strength of the wall and transfer load around the opening. As both ACI 313-97 and ACI 350-06 emphasize, it’s important to develop the bars beyond the opening. But as simple as the design and detailing might appear, there are recurring issues and concerns, as addressed in the following.
Trim Bars
Issue 1—Full height trim bars
location of an opening may not be available at the time the footing concrete is placed. Conversely, if the small opening is moved or the dimensions changed after the footing is cast, the dowels would no longer be properly located. It’s recommended that the designer avoid this requirement wherever possible.
Issue 2—Trim bar embedment
There can be a wide variance in how designers call out the embedment or development of trim bars. Figure 2(a) shows the embedment of the trim bars measured from the corner of the opening. This is the preferred method, as it gives the detailer and placer a precise location for the trim bar based on the bar embedment length per the structural drawings. At placement time, the opening location should be marked in the field or formwork should be in place, thus providing a “hard” geometric control point for the placer. The detail in Fig. 2(b) is also frequently used, whereby the embedment length is referenced to a reinforcing bar. Designers who use this detail may believe that the opening location will not be precisely set and that the bar is a better control point. In this example, the horizontal trim bar embedment is then conditional on the first trim bar location in the perpendicular or vertical direction. The detailer can easily determine the trim bar embedment, assuming that the first perpendicular trim bar will be located at the proper concrete cover distance from the opening. Embedments or other conditions, however, may force the perpendicular trim bar to be placed further away from the opening, resulting in the horizontal trim
Some designers require vertical trim bars located on each side of the opening to run the full height of the wall, lapped with dowels anchored in the footing and oftentimes embedded in the floor or roof above (Fig. 1). The requirement is applied to all openings, regardless of the opening size or its location relative to the bottom or top of the wall. For large openings, structural design may dictate full-height wall bars. This practice is usually unnecessary, however, for small openings near the bottom or top of the wall. Trim bars are only required to have full embedment (that is, tension development length) past the edge of the opening. Detailing and placing full-height bars can be a problem because the precise dimensions and Fig. 1: A typical detail showing reinforcement at wall openings Concrete international
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(a)
(b)
(c)
Fig. 2: Designers use different reference points for trim bar extensions: (a) measuring from the opening corner is the most precise; (b) using a trim bar as a reference point can create problems if the trim bar location must shift; and (c) measuring from the outermost trim bar may lead to excessive embedment lengths
(a)
(b)
Fig. 3: Examples of column reinforcement details incorporated into a wall: (a) column-like wall; and (b) corner column-like wall (based on Reference 4)
bar embedment length being too short beyond the vertical bar in this case. This can be corrected in the design phase by providing a set horizontal dimension that includes the development length plus a nominal width dimension to account for tolerances, a placement range for the vertical trim bar(s), and concrete cover. From a structural perspective, providing a development length slightly greater than the ACI 318 Code minimum is not harmful and could be helpful; the added cost of the extra length on the bars will be minimal. The detail in Fig. 2(c) is used occasionally in heavily reinforced walls or opening locations. This detail presents the same potential problems as the detail in Fig. 2(b). Moreover, Fig. 2(c) illustrates that if there are a large number of vertical trim bars adjacent to the opening, the horizontal trim bar length could become excessively long; this excessive length may not be structurally necessary. For these reasons, this detail is not desirable and the designer should strive to use the detail shown in Fig. 2(a).
Issue 3—Opening too wide or high?
When is an opening too wide or too tall to apply the general practice of replacing the interrupted reinforcement?
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Oftentimes, the general notes do not indicate the maximum dimension beyond which this practice is no longer valid. At some point, the size of an opening could become a structural concern necessitating beam and column design around the opening. Clearly, this is dependent on the opening size, wall thickness, and loading condition at the opening. If the horizontal dimension (span) of the opening is too great, beam-type reinforcement may be required above or below the opening. Likewise, the vertical wall on each side of the opening may necessitate a dedicated reinforced column section with specific longitudinal and tie reinforcement. Section 4.3.8.4 of ACI 313-97 provides some practical guidance for the engineer, stating that “unless determined otherwise by analysis, walls 8h in width or less between openings shall be designed as columns.” ACI 315-994 illustrates an example of how a column cross section would be incorporated within the confines of the wall thickness (Fig. 3). The designer should always show the maximum dimensions for which the general practice can be applied. All larger openings should be individually investigated by the designer and the proper reinforcement details should be shown on the structural drawings.
Issue 4—Spacing of trim bars
What is the spacing of the trim bars? This is usually not indicated in the general notes; in such cases, the detailer will usually not indicate a dimension either. Section 4.3.9 of ACI 313-972 requires that horizontal bar clear spacing to be at least 2 in. (50 mm) and center-to-center spacing to be at least five bar diameters. The section also requires that the spacing of horizontal bars in slipformed walls to be large enough to allow bars to be placed and tied during form movement. Absent any spacing information on the structural or reinforcing bar placing drawings, the bar placer will determine the spacing. The placer will arbitrarily space the bars to a personal preference or experience, usually about 3 in. (75 mm). If for any reason the spacing is critical, the designer should indicate the trim bar spacing on the structural drawings.
(a)
Issue 5—Effectiveness of trim bars
How far from the opening can the trim bars be placed before they lose their effectiveness? As was addressed in the section regarding Issue 3, there are times when the maximum opening width is not given in the general notes. By default, the detailer must assume that all of the openings are to be trimmed with one-half of the interrupted bars on each side. If a large amount of bars are cut, however, there will potentially be a large quantity of additional trim bars on each side of the opening (Fig. 4(a)). As they are evenly spaced out from the opening, some of these bars may be too far away from the opening to be considered fully effective. Section 13.4.2 of ACI 318-08 requires that “an amount of reinforcement equivalent to that interrupted by an opening” be added to the sides of the opening. It doesn’t require that the same bar size be used for the trim bars, only that half the area be placed on each side. Using the same bar size is simpler and avoids confusion in the field, but this is not always practical from a potential congestion standpoint. A solution to this problem would be to use fewer but larger bars (Fig. 4(b)). For example, say 20 No. 5 (No. 16) bars are interrupted by an opening. Rather than placing 10 No. 5 (No. 16) bars on each side (As = 3.10 in.2 [2000 mm2]), seven No. 6 (No. 19) bars (As = 3.08 in.2 [1987 mm2]) or six No. 7 (No. 22) bars (As = 3.60 in.2 [2323 mm2]) can be used. The designer should consider this possibility and account for this option in the general note. Alternatively, specific trim bar details could be indicated on the structural drawings for each affected opening.
Diagonal Bars
The minimum opening bars required per ACI 318-08, Section 14.3.7, are illustrated in Fig. 5 for a typical opening.
(b) Fig. 4: Trim bars may be located too far from the opening if cut bars are replaced one-for-one: (a) for a large opening, matching the cut bar size can result in a wide band of trim bars; and (b) using larger replacement bars (in this case, using a trim bar with about twice the area of the cut bar) can allow trim bars to be concentrated close to the opening
These bars are intended to restrain cracking, but trim bars will also help keep any cracks tight. Diagonal bars have the primary purpose of arresting cracks at re-entrant corners of wall openings, and they deserve two more comments.
Issue 6—Diagonal bar location in wall thickness
Are diagonal bars effective on multi-layer wall curtains? Due to cover requirements and the thickness of the reinforcing bar curtains, the diagonal bars will likely be placed closer to the center of the wall rather than near the concrete faces. Figure 6 shows a section of a wall with multiple layers of reinforcement and diagonal bars. The diagonal bars are less effective in controlling cracks Concrete international
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radiating from the opening corners because the bars are so far from the concrete faces. The vertical and horizontal trim bars should provide the requisite reinforcement near the surface to keep the cracks tight. Because the corner crack will likely propagate through the wall thickness, the diagonal bars will provide supplemental restraint near the midsection of the wall.
Issue 7—Diagonal bar development
For openings close to the top of a wall or near a slab, the normal straight diagonal bar detail may not be possible. In such cases, the diagonal bar detail needs to be modified in one of two ways (Fig. 7). The bars can extend past the opening with a standard hook at the end, or the diagonal bar can be bent to avoid the obstruction.
Fig. 5: A typical detail for opening and corner bars used for crack control (based on Reference 5)
address the conditions
Designers should be cautious of generic notes for trim bars around openings. Trim bars should be carefully considered on a project-by-project basis to determine if special conditions exist that may require nonstandard details. These special conditions should be addressed in the general notes or special details should be included in the structural drawings. The designer needs to make his or her intent clear for all aspects of the trim bars around openings.
References
Fig. 6: Likely diagonal bar location in multi-layered wall
1. ACI Committee 318, “Building Code Requirements for Structural Concrete (ACI 318-08) and Commentary,” American Concrete Institute, Farmington Hills, MI, 2008, 473 pp. 2. ACI Committee 313, “Standard Practice for Design and Construction of Concrete Silos and Stacking Tubes for Storing Granular Materials (ACI 313-97) and Commentary,” American Concrete Institute, Farmington Hills, MI, 1997, 39 pp. 3. ACI Committee 350, “Code Requirements for Environmental Engineering Concrete Structures (ACI 350-06) and Commentary,” American Concrete Institute, Farmington Hills, MI, 2006, 488 pp. 4. Joint ACI-CRSI Committee 315, “Details and Detailing of Concrete Reinforcement (ACI 315-99),” American Concrete Institute, Farmington Hills, MI, 1999, 44 pp. 5. ACI Committee 315, ACI Detailing Manual, SP-66 (04), American Concrete Institute, Farmington Hills, MI, 2004, 212 pp. Thanks to Greg Birley of Condor Rebar Consultants and a member of Joint ACI-CRSI Committee 315, Details of Concrete Reinforcement, and Neal Anderson and Anthony Felder of CRSI for providing the information in this article.
Fig. 7: Development of diagonal bars in close proximity to another opening will require bends or hooks
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Selected for reader interest by the editors.
