Detailing Corner Altrnating Bar siz esigners often use alternating bar sizes for reinforcement in walls and slabs. While at first glance this may seem to be a simple method to enhance design efficiency in terms of total steel weight, it usually doesn’t optimize the overall cost of the project. Before the decision is made to specify alternating bar sizes, several issues should be considered.
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Issues Which bar tart a rn? Whether to start a run of bars with the larger or smaller bar (Fig. 1) may seem like a trivial issue, but this
Fig. 1: Should a run of bars start with the larger or smaller bar? (Note: 1 in. = 25.4 mm)
DeTAILING CORNeR Joint ACI-CRSI Committee 315-B, Details of Concrete Reinforcement— Constructibility, 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
[email protected] with the subject line “Detailing Corner.”
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Fig. 2: Cover thickness will vary if alternating bar sizes are used in the outside layer of wall or slab reinforcement, and the effective depth of the section may be smaller than anticipated
Fig. 3: The difference in splice lengths for the different bar sizes causes the location of splices to change relative to one another when stock length bars are used
simple decision has been the root of countless disagreements between inspectors and contractors, especially when there are an odd number of bars. Designers and inspectors naturally prefer to maximize the amount of steel and tend to start with the larger bar, while fabricators and placers prefer to minimize steel congestion and tend to start with the smaller bar. Consequently, the size of the starting bar in the run becomes a matter of debate and needs to be coordinated in advance. Concrt covr may vary The vertical bars in a wall are tied to the horizontal bars. If the horizontal bars are inside the vertical bars and the vertical bars alternate in size, the specified cover must be met on the larger bar and the cover on the smaller bars will be larger (Fig. 2). If the bars differ by two bar sizes, the smaller bars will have 1/4 in. (6 mm) greater concrete cover than the larger bars. Designers must be aware that this situation will result in a smaller effective depth to the reinforcement and, therefore, a smaller nominal moment capacity than would occur if both bar sizes had the same cover.
Fig. 4: Alternating bar sizes can complicate the distribution of reinforcing steel around openings
Lap location can hift Alternating bar sizes can create problems with lap locations, particularly in long runs of horizontal wall bars or temperature reinforcement in a slab (Fig. 3). If stock length bars are used, the difference in lap length for the two bar sizes will cause the lap locations to move apart relative to each other. This means the final bar in the run is a different length for each of the bar sizes. How ar trim bar allocatd? Typically, half of the reinforcing bars interrupted by a slab or wall opening must be placed as trim or framing bars on each side of the opening. If the bar sizes alternate and six bars are interrupted by an opening, for example,
Fig. 5: The difference in hook dimensions for vertical wall bars with different bar sizes affects the location of the outermost slab or beam reinforcement Concrt intrnational
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three large bars and three small bars must be distributed at the sides of the opening. The configuration and layout could be issues for dispute, so the “field fix” shown in Fig. 4 could result in the need for even more bars. Hook dimnion will vary If the tops of vertical bars with different sizes are terminated with standard hooks, the bend diameters will be different for the two bar sizes, as shown in Fig. 5. Very often, especially along perimeter walls, the designer requires a continuous bar tied in the corner of the vertical hook. The continuous bar touches only every other vertical bar and appears to be too low, causing concern from some inspectors. Additionally, if the vertical bars are hooked into a continuous beam at the top of the wall, the different arcs of the hooks may result in one or more bars positioned lower in the cross section than expected. This may become an issue concerning the effective depth of the member. Maintaining accrat intallation i mor difficlt Slab bars are typically placed on supports known as slab bolsters. When the bottom bars are tied in place, the smaller bars are lifted off the slab bolsters and secured against the layer of bars above, as depicted in Fig. 6. Later, as workers walk on the reinforcing steel to place conduits, inserts, and other embedded items or during concrete placement, the ties holding the small bars could be stretched or broken so the bars are no longer secured against the reinforcing steel mat. They must be re-tied
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Fig. 6: Alternating slab bar sizes can lead to broken or loose tie wires
or they are likely to be displaced by the concrete as it is placed. Fabrication xpn can incra It’s more economical to cut a quantity of bars of a single bar size than it is to cut half the quantity for each of two different bar sizes. Furthermore, if the designer used only the larger bar with a wider spacing or an intermediate bar size with a wider spacing, there would be fewer total bars required, thus reducing fabrication costs. Placing xpn can alo incra Sorting and placing bars of alternating sizes requires extra labor and handling. To avoid getting the alternating bars out of sequence, ironworkers must start placing the reinforcing bars at a single point on a slab or wall, rather than in several locations simultaneously. The previously mentioned tendency for lap locations to shift will also increase labor costs.
TOTAL COsT Is MORe THAN A FuNCTION OF WeIGHT Designers should understand that what they may perceive as cost efficiency might actually add to the overall cost of a project. While they may be trying to save reinforcing steel, they are increasing the potential difficulties for the placers and the inspectors. Even better cost efficiencies and reduced potential for placing and inspection difficulties can often be achieved simply by adjusting the bar spacing and using a single size.
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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