ACI-staggered-lap-splices.pdf

  e    t Staggered Lap Splices   o    N    l   a   c    i   n    h   c   e    T Engineering Technical Note ETN-D-2-13

Introduction

Lapped splices are probably the most common means of joining two lengths of reinforcing bars. Staggering the lap splices is sometimes required by the designer, basically for two reasons: (1) to reduce reinforcement congestion in locations where there is a relatively heavy amount of reinforcement, such as in a lower story column of a multi-story building, and (2) to reduce a concentration of bond stresses at the bar ends of the lap splices. A staggered arrangement of the lap splices subsequently reduces the localized stresses at each lap location, and lowers the possibility of concrete cracking (splitting) or reduces the crack widths. Figure 1 shows a photo of a staggered lap splice arrangement that has been installed in the top layer of a foundation mat. In Figure 1, note that every other bar is shown lap spliced in the same general location.

Review of Options

Staggered

Lap

Splice

Staggering of splices is the longitudinal spacing offset of the spliced bars. When considering the longitudinal stagger arrangement of lap splices, there are three basic options, as shown in Figure 2: (A) no stagger, (B) stagger with zero gap, and (C) stagger with a (positive) gap.

Figure 2 — Lap splice stagger options

Figure 1 — Staggered lap splices

Stagger Requirements of the ACI 318 Code Since 1963, the ACI 318 Code has acknowledged the benets of staggering lap splices by requiring staggered congurations of bars and splices under various conditions. Table 1 (shown on page 2) summarizes the conditions under which ACI 318-11 [2011] requires the staggering of lap splices and the stagger distance. Depending on the condition, these distances are expressed in terms of either bar diameters, a dened length (in inches), or ℓ d .

 As addressed previously previously,, the ACI 318 Code has a number of references to staggering lap splices; these are presented in Table 1. However, these references do not clearly dene a standard or minimum distance for the stagger to be effective or how the lap splice stagger distance should be measured. These two pieces of information are critical to making sure the reinforcing bars are detailed and placed properly and as required.

Designer’s Responsibility It is recommended that staggered splices are only used when they are essential to the design of a structure due to the complexity they add to both the detailing and placement of the reinforcement. When they are required, it is the designer ’s responsibility to clearly dene the staggered lap

Table 1 — Stagger Requirements for Lap Splices, per ACI 318-11 Condition

Stagger Distance

Code Section

Individual bars within a bundle need to terminate at different points with at least a 40 d b stagger.

40 d b minimum

Section 7.6.6.4

Individual bar splices within a bundle shall not overlap.

No length specied

Section 12.14.2.2

Mechanical or welded splices that do not meet the 1.25 yield strength requirement.

24 in. minimum

Section 12.15.5.1

Mechanical or welded splices in tension tie members.

30 in. minimum

Section 12.15.6

Class A tension lap splices in columns where half or fewer of the bars are spliced at any section.

ℓ d  minimum

Section 12.17.2.2

End-bearing splices in columns.

No length specied

Section 12.17.4

Splices of principal tensile reinforcement in shells (with not more than 1/3 of the reinforcement spliced at any section).

ℓ d  minimum

Section 19.4.12

splice requirements on the contract drawings. In an effort to avoid ambiguity, graphical details (as illustrated in Figure 3) are preferred over general notes. The lap splice note in Figure 4 can be unclear, incomplete, and easily misinterpreted. For example, is the note in Figure 4 saying: 1. Stagger all lap splices an undisclosed distance, with the lap splice of a #6 bar being 17 in. minimum and lap splice of a #8 bar being 37 in. minimum? 2. Is the stagger distance of the #6 bar 17 in. minimum, and stagger distance of the #8 bar 37 in. minimum? The lap splice length is either not given, or presented elsewhere.

Figure 3 — Example of a typical staggered lap splice detail 

With respect to Option 2, where is the stagger measured from? Is it from the end of the bars or centerline of the spliced bars? The lap splice note in Figure 4 seemingly creates more confusion than providing clarity. The designer also needs to clearly dene how the lap splice details apply to the structure in two ways. Figure 4 — Example of a simple staggered lap splice note

1. Describe what elements need to be detailed with staggered lap splices - walls, slabs, beams, etc. This can be done through a clarifying note next to the detail. 2. Describe how the lapped reinforcing bars within each element are required to be staggered.

2

Staggered Lap Splices [ETN-D-2-13]

a. Is the stagger condition only required for lap splicing bars in a single parallel layer? This should be clearly communicated through a graphical detail similar to Figure 3.

Staggered Lap Splice Measurement  A lap splice detail should clearly dene the stagger distance and how the stagger distance should be measured.

Figure 5 — Staggered lap splice detail for different layers in a foundation mat or wall 

Figure 6 — Recommended measurement of the stagger distance for staggered lap splices

The recommended manner to measure the lap splice stagger distance is shown in Figure. 6. This gure illustrates the staggered lap splice as it is measured from end-of-bar to end-of-bar, rather than from the center of the lap splice. This end-to-end dimension cannot be misintrepreted by reinforcing bar detailers during the creation of placing drawings or by ironworkers as the reinforcing bars are placed in the field. Any given stagger distance will ensure the bar ends will not line up. Figure 7 illustrates the same example shown in Figure 3, but this time incorporating this recommened manner of measurement that is shown in Figure 6. Note how much clarity this measurement method adds to the detail for reinforcing bar details and ironworkers.

Stöckl Research on Lap Splices  Alth oug h it is allo wed, the staggered lap splice layout shown in Figure 2(B) is not the most ideal from a structural standpoint. Because the bar ends of successive terminated bars are aligned, there is a strong tendency for a splitting crack to develop in the concrete, coincident with the bar ends.

Figure 7 — Example of a typical staggered lap splice detail using the recommended measurement of the stagger distance

b. Do lap splices in different layers within a single element have to stagger with respect to each other? (An example of this condition would be the top and bottom layers in a foundation mat or inside and outside faces in a wall) This situation should be clearly communicated through a graphical detail similar to Figure 5.

Stöckl [1972] studied the effect that different staggered lap splice configurations had on the width of flexural cracks at the ends of lap splices. Three configurations of lap splice stagger tested by Stöckl are shown in Figure 8. Note that the staggered lap splice layout in Figure 2(B) was studied by Stöckl, as shown in Figure 8(a). Transverse reinforcement in the region of the lap splices may provide confinement and reduce the crack width, but providing a gap between the ends of the staggered lap splices is more desirable, as shown in Figure 8(b).

 According to Stöckl, the staggering of lap splices in beams (providing a “negative” gap, as shown in Figure 8(c)) can reduce the width of exural cracks at the ends of the lap splices, provided that the stagger distance is at least one-half of the lap splice length. For a Class A lap splice, with the lap splice length equal to the tension development length ℓ d , the minimum stagger would be 0.5 ℓ d . For a Class B lap splice, with the lap splice len gth

CRSI Technical Note

3

equal to 1.3 times the tension development length, the minimum stagger would be 0.65 ℓ d . In either case, a closer stagger where the staggered regions overlap (a negative gap) provides the best structural behavior and will be consistent with the recommendations from the Stöckl report.

Summary

(a) Superimposed effects can be adverse, resulting in large crack width even when lap length exceeds tension development length

The longitudinal staggering of lap splices is important to avoid reinforcing bar congestion in the lap splice region. From a structural engineering perspective, providing a negative or positive gap is desirable to mitigate the splitting crack tendency; refer to Figure 2(C) for denition of gap. Providing no gap, as illustrated in Figure 2(B), should be avoided because of the propensity for a wide splitting crack to develop. Detailing the stagger for lap splices is equally important to properly convey the design intent. A simple, yet comprehensive staggered lap splice (typical) detail, similar to Figures 3 and 6, should be provided on the contract drawings. General notes concerning lap splices can be too ambiguous and subject to different interpretations.

References (b) Avoiding superposition reduces crack width

American Concrete Institute – ACI Committee 318 (2011), Building Code Requirements for Structural Concrete (ACI 318-11) and Commentary (ACI 318R-11),  American Concrete Institute, Farmington Hills, Michigan, 503 pp. Stöckl, S. (1972), Übergreifungsstöße von zug-

beanspruchten Bewehrungsstäben (Lap Splicing of Reinforcing Bars Subject to Tension), Beton- und Stahlbetonbau, V. 10, Ernst & Sohn, Berlin, Germany, pp. 229-234. (in German)

(c) Low superposition results in smallest crack width Figure 8 — Crack widths, as a function of splice locations (after Stöckl [1972])

Contributors: The principal authors of this publication are Robbie Hall, Greg Rohm, Michael Ugalde, Anthony L. Felder, and Neal S. Anderson, with review by members of the CRSI Reinforcement Anchorages and Splices Committee. Keywords: Contact, cracking, lap splice, reinforcing bar, stagger  Reference: Concrete Reinforcing Steel Institute - CRSI [2013], “Staggered Lap Splices,” CRSI Technical Note ETN-D-2-13 , Schaumburg, Illinois, 4 pp.

933 North Plum Grove Rd. Schaumburg, IL 60173-4758

Historical: None. New technical note Note: This publication is intended for the use of professionals competent to evaluate the signi cance and limitations of its contents and who will accept responsibility for the application of the material it contains. The Concrete Reinforcing Steel 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.

p. 847-517-1200 • f. 847-517-1206  www.crsi.org Regional Ofces Nationwide  A Service of the Concrete Reinforcing Steel Institute ©2013 This publication, or any part thereof, may not be reproduced without the expressed written consent of CRSI. Printed in the U.S.A.