ENGINEERING ETN-C-3-14 Reinforcing Institute Technical Note to dropstructural barsfor use as support for assemblyofreinforcing steel matsis consideration thatthepreferred method design professional (LDP)shouldtakeinto “means andmethods”issue, thelicensed and/orcontractor ered aconstructability tural barsasburiedcontractbars. Placers commonlyrefer todroppedstruc- inforcing andreinforcing bar Detailers bar time, uniformityandquality. Notethatre- Placers andFabricators intermsofcost, cal solutionthatisbeneficial for Contractors, and slabsonground,becauseitisapracti- ment whenconstructing foundation mats ferred methodfor supportingthereinforce- dropping mainreinforcing barsisthepre- gional practicesandPlacerpreferences, structural bars).Dependingonvarious re- elevation for useassupportbars(dropped the mainreinforcing barscanbelowered in to themainreinforcing barsor2)aportionof tural supportbarscanbeprovided inaddition plished intwoways: 1)additional,non-struc- inforcing bars. thedesignedlocationofre- maintaining Placer towork efficiently andsafely, while placed andtied. This rigid frameallows the frame” on which a mat of reinforcing bars is Support barsareusedtoconstruct a “rigid port barsareplacedonthebarsupports. is initiallyplaced.Inturn, sup- thestarter the outermostlayer of reinforcement asit bars atarelatively widespacingtoholdup support requirestarter in-place typically later inthisTechnical Note. support. Bararediscussed supporttypes chairunit support,oralinetype orbolster vary,types andcouldconsistofasingle ed onthedesigndrawings. The barsupport thespecifiedconcretecovertain designat- would beplacedonbarsupportstomain- For thelatter condition,theoutermostbars as aunitwiththeaidofadequatehoisting. tied inadifferent locationandthenplaced ment mat can be tied (built) in-place or pre- Introduction Bars for UseasSupport Bars Dropping MainReinforcement While supportbarsare often consid- The useofsupportbarscanbeaccom- Reinforcement mats thataretied(built) Depending onseveral factors, areinforce- discussed inthefollowing examples. effective depthandconcretecover willbe the concretecover as well. The impacton In some cases, dropping bars can affect the effective depthwillslightlydecrease. crease the effective depth, in other cases some cases,droppingbarswillslightlyin- of thereinforcement inthestructure. In will have animpacton theeffective depth Design ImpactofDropping Bars Technical asaguide. Noteserve on aplacingdrawing.Ineithercase,letthis request for information (RFI)oraquestion ted totheLDPfor approval eitherthrougha ings for approval. This option will besubmit- dropping barspriortosubmitting thedraw Fabricator willusuallydiscusstheoptionof theContractor,tion. Otherwise, Placerand drawings priortobeingissuedfor construc- should beincorporatedintothestructural of structural bars for use as supportbars showing allowing ornotations thedropping bars. Therefore, whenever possible,details approval areneededtoaddresswhere the Placer andFabricator and theEngineer’s of thedroppedbars. the ground will be controlled by the spacing supports supportingthebottom matoff of spacing (inonedirection) of individualbar shown 1. inFig. Ifthismethodisused,the bottom layer for useassupport bars,as the reinforcing barsfromtheinner-most Considerations Dropped (Buried) Bottom Bar with Dropped Structural BarsinBottom Layer Figure 1— Typical Foundation Mat/Footing Section supports usedandapplicationvaries. Note: Bar supportsomitted for clarity. Actual It is important tonotethatdroppingbars It isimportant Conversation betweentheContractor, It iscommonpracticetodropsomeof - required concrete cover will be measured from. There Depending on the overall thickness of the member and are two options concerning the concrete cover on the the percentage of reinforcement, this slight reduction reinforcement in the bottom layer. may be minor; alternately, the LDP may need to verify if the slight reduction in effective depth still maintains the flexural capacity of the section, in both directions.

Note: Bar supports omitted for clarity. Actual supports used and application varies.

Figure 2 — Effect of Dropping Structural Bars on Concrete Cover - Note: Bar supports omitted for clarity. Actual supports used and Option #1 application varies. Note: Bottom reinforcement not shown for clarity.

Figure 4 — Typical Foundation Mat/Footing Section with Dropped Option #1 Structural Bars in Top Layer The required concrete cover to the outer-most layer of bottom bars will be maintained, as shown in Fig. 2. This option requires the dropped support bars to encroach Dropped (Buried) Top Bar Considerations into the bottom concrete cover. This option will result in Figure 4 shows an example of a footing or foundation a slight increase in the effective depth of the reinforce- with a top or upper mat of reinforcing bars. In this figure, ment in the inner-most bottom layer, which will mean a the lower reinforcement mat has been omitted for clarity. slight increase in the moment capacity of the cross sec- For the upper mat, it may be possible to drop reinforc- tion. However, concrete cover for those dropped bars ing bars from the outermost top layer for use as support in the layer will be reduced by one bar diameter (of the bars. If this method is used, the spacing (in one direc- dropped bars). For a footing or foundation this may not tion) of individual bar supports supporting the top mat be an issue, as the concrete is likely placed on undis- will be controlled by the spacing of the dropped bars. turbed soil or a level, prepared sub-base, and the con- In this instance, the top concrete cover is maintained to crete cover may be 3 in. per ACI 318 [2011]. The intrusion the outermost layer and is likely not a concern. The ef- into the bottom cover may be viewed as minor by the fective depth of the reinforcement in that layer is slightly LDP, depending on bar size. reduced, as is the moment capacity of the section.

Note: Bar supports omitted for clarity. Actual supports used and application varies.

Figure 3 — Effect of Dropping Structural Bars on Concrete Cover - Option #2

Option #2

The required concrete cover to the outer-most layer of bottom bars is provided to the dropped support bars, as shown in Fig. 3. This option moves the main bottom bars upward by one bar diameter, which will reduce the ef- fective depth of these bar layers by one bar diameter. The section moment capacity will be reduced propor- Note: Bar supports omitted for clarity. Actual supports used and tionately in each direction. Notice that the bars in the in- application varies. ner-most layer of bottom bars will also be moved up one Figure 5 — Typical Foundation Mat/Footing Section with Dropped bar diameter and so reduce their effective depth as well. Structural Bars in Top and Bottom Layers

2 Dropping Main Reinforcing Bars for Use as Support Bars [ETN C-4-14] Details for Combined Dropped (Buried) Top cover, then these details must also be clearly shown and Bottom Bars in the RFI sketch. Similar requirements apply to de- tails shown on placing drawings. Within a single footing or foundation mat, it is common to drop some of the reinforcing bars from both the top 3. Approval should be obtained from the LDP for the and bottom layers for use as support bars, as shown in dropped bar arrangement. All field personnel and Fig. 5. The impact on the design strength of the section Inspectors need to be made aware of the support has already been discussed, with the moment capacity method. of the outer-most top layer being reduced and the mo- ment capacity of the bottom layer either being increased 4. The placing drawings must include a detail that or decreased, depending on how the concrete cover at clearly shows the approved support method and re- the bottom is maintained. inforcing bar arrangement. Reinforcing Bar Supports for Foundation Mats and Footings Figure 7 shows various types of bar supports used in foundation mats and footings. Most reinforcement sup- ports for these applications have larger bearing surfaces on their base to mitigate settlement or punching into the Note: Bar supports omitted for clarity. Actual supports used and base material. The supports may be application varies. blocks (dobies), or “sand plates” on conventional com- posite or steel supports. Figure 6 — Typical Slabs on Ground Section (Only Single Mat Required) with Dropped Structural Bars Bar supports are generally estimated and furnished (for the top bars only) for slabs up to 4 feet [1,200 mm] thick unless otherwise specified on the project drawings or in the project specifications. In certain regions of the Dropped (Buried) Slab on Ground Bar country, bar supports are generally not furnished by the Considerations reinforcing steel Supplier for bars in singly-reinforced For a slab on ground with a single mat of reinforce- slabs on ground. Because there are many ways of sup- ment, it may be possible to drop reinforcing bars from porting such bars, Suppliers generally furnish supports the top layer for use as support bars, as shown in Fig. for such purposes only by special arrangement. 6. If this method is used, the spacing (in one direction) Standees, normally fabricated from reinforcing bars, of the individual bar supports supporting the single mat are a common form of bar support, especially for heavy will be controlled by the spacing of the dropped bars. mats of reinforcement or for supporting reinforcement Whether dropping bars will affect the concrete cover de- at relatively greater heights. Commonly, #4 bar standees pends on where the layer of reinforcement is positioned are used for concrete thickness of 2 feet [600 mm] or in the slab, and the slab thickness. Important Steps for Dropping Bars Once the decision has been made to utilize dropped bars, it is important that the following steps be followed:

1. The proposed bars to be dropped and the specific re- inforcing bar arrangement needs to be agreed upon by the Contractor, Placer and Fabricator.

2. The proposal to incorporate dropped bars in the reinforcement placement needs to be sent to the LDP for approval through the use of an RFI and/or specifically called out on placing drawings submitted for review. The LDP needs to approve this support method, because a portion of the main reinforcing bars are being moved from their design location. If used, the RFI should be clearly worded and contain a sketch that clearly shows the proposed support method and reinforcing bar arrangement. If the re- Figure 7 — Typical Types of Bar Supports for Foundation Mats and inforcement arrangement will affect the concrete Footings

CRSI Technical Note 3 Glossary of Terms Effective Depth – Distance measured from the ex- treme compression fiber to the centroid of longitu- dinal tension reinforcement for the direction under consideration.

Support Bars – Reinforcing bars used to ease the placement of reinforcing bars.

Figure 8 — Typical Standees Additional Non-Structural Support Bars – Reinforcing bars that are not required per structural less, and #5 bar standees are used for concrete thick- design and are added ONLY when dropped bars are ness from 2 to 4 feet [600 to 1,200 mm]. Figure 8 shows not utilized. These reinforcing bars serve no structural typical standee configurations: bar bend Types 25 and purpose and are only intended to maintain the location 26. of reinforcing bars/mats required per structural design during concrete placement. These bars are supplied in The design of a support system for top steel for ei- addition to the reinforcing bars required per structural ther slabs on ground or foundation mats over 4 feet design. [1,200 mm] thick is generally the responsibility of the Contractor. The reinforcing bar Supplier cannot design Dropped Structural Bars – Main reinforcing bars re- the support system due to lack of knowledge and con- quired per structural design that are lowered in eleva- trol of construction loads and placement methods. The tion (dropped) with the LDP’s approval for the purpose reinforcing bar Supplier will quote the support system of supporting and maintaining the location of reinforcing based on the Customer’s/Buyer’s design. bars/mats required per structural design during concrete placement. No added support bars are supplied as de- More information on bar supports for foundations, and sign reinforcement is used. bar supports in general, can be found in Chapter 3 of CRSI’s Manual of Standard Practice [2009]. Buried Contract Bars – Common term for dropped structural bars.

Conclusion LDP – Licensed Design Professional For foundation mats and slabs on ground, the prac- tice of lowering in elevation (dropping) some of the RFI – Request for Information main reinforcing bars to serve as support bars for the entire reinforcement mat is an accepted and common References practice. However, this practice can affect the moment American Concrete Institute - ACI Committee 318 (2011), Building capacity of the member cross section and possibly the Code Requirements for Structural Concrete (ACI 318-11) and Com- mentary (ACI 318R-11), American Concrete Institute, Farmington concrete cover on the reinforcement. For these rea- Hills, Michigan, 503 pp. sons, the Designer must give his/her approval and all Concrete Reinforcing Steel Institute - CRSI (2009), Manual of concerned parties (Contractor, Placer, Inspector, and Standard Practice, 28th Edition, Concrete Reinforcing Steel Institute, Fabricator) must have a clear understanding of the Schaumburg, IL, 144 pp. specific reinforcement arrangement. Although the ex- amples in this Technical Note only highlight foundations mats and slabs on ground, dropping bars can also be employed in walls, elevated slabs, and other members.

Contributors: Robert Hall, Dale Rinehart, Chris Stowers and Neal S. Anderson, PE, SE, with review by members Concrete of the CRSI Placing Reinforcing Bars Committee. Reinforcing

Keywords: Concrete cover, contractor bar, dropped bar, effective depth, flexural capacity, reinforcing bar, rein- Steel forcement support Institute

Reference: Concrete Reinforcing Steel Institute – CRSI [2014], “Dropping Main Reinforcing Bars for Use as Sup- 933 North Plum Grove Rd. port Bars,” CRSI Technical Note ETN-C-3-14, Schaumburg, Illinois, 4pp. Schaumburg, IL 60173-4758 p. 847-517-1200 • f. 847-517-1206 Historical: None. New Technical Note. www.crsi.org Note: This publication is intended for the use of professionals competent to evaluate the significance and limitations Regional Offices Nationwide of its contents and who will accept responsibility for the application of the material. The Concrete Reinforcing Steel A Service of the Concrete Reinforcing Steel Institute Institute reports the foregoing material as a matter of information and, therefore, disclaims any and all responsibility for application of the stated principles or for the accuracy of the sources other than material developed by the Institute. ©2014 This publication, or any part thereof, may not be reproduced without the expressed written consent of CRSI.

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