Compression Lap Splice in Unconfined Concrete of 40 and 60 MPa (5800 and 8700 psi) Compressive Strengths

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Compression Lap Splice in Unconfined Concrete of 40 and 60 MPa (5800 and 8700 psi) Compressive Strengths Powered By Docstoc
					 ACI STRUCTURAL JOURNAL                                                                                   TECHNICAL PAPER
Title no. 107-S17


Compression Lap Splice in Unconfined Concrete of 40 and
60 MPa (5800 and 8700 psi) Compressive Strengths
by Sung-Chul Chun, Sung-Ho Lee, and Bohwan Oh

A compression lap splice may be calculated to be longer than a                            l s, ACI = 0.071f y d b           for f y ≤ 420 MPa                     (1)
tension lap splice in high-strength concrete according to current
design codes. New criteria for the compression lap splice,                                ( = ( 0.13f y – 24 ) )d b          for f y > 420 MPa
including the effects of concrete strength and end bearing, are
required. An experimental study was conducted using compressive
concrete strengths of 40 and 60 MPa (5800 and 8700 psi) without                         l s, ACI = 0.0005f y d b            for f y ≤ 60,000 psi
transverse reinforcement. Test results showed that the splice                            = ( 0.0009f y – 24 ) d b           for f y > 60, 000 psi
strength was proportional to f c′ and that the spacing between
bars had little effect on the splice strength. The bar stresses developed
by bond were nearly identical to those calculated by the ACI                where fy is the specified yield strength of a reinforcing bar
Committee 408 equation. Therefore, the strength increase in the             and db is the nominal diameter of a spliced bar.
compression splices is solely attributed to end bearing. A design              The R12.16.1 commentary of ACI 318-08 says that splice
equation was derived for compression lap splices without transverse         strengths in compression depend considerably on end
reinforcement through regression analysis.                                  bearing; however, the end bearing and bond contributions
                                                                            cannot be recognized from Eq. (1). The specified yield
Keywords: bond; compression lap splice; end bearing; splice strength;       strength for the bars is considered but the specified compressive
unconfined concrete.                                                        strength of the concrete is not included in Eq. (1). Consequently,
                                                                            a compression lap splice could be longer than a tension lap
                       INTRODUCTION                                         splice, as shown in Fig. 1.
   The compression lap splice criteria in ACI 318-081 were based               The fib code5 adopted tension splice rules as provisions of
on just 11 column tests2 conducted over 40 years ago using                  compression splices. A compression splice length is obtained
concrete with a maximum compressive strength of 29.0 MPa                    from Eq. (2), which is identical to the basic equation for a
(4190 psi). Lap requirements for compression splices have                   tension splice length. In Eq. (2), a partial safety factor for
remained the same since the 1963 Code. Due to end bearing, the              bars and a material safety factor for the bond are included
splice length in compression is shorter than the length in tension to       and, therefore, Eq. (2) is directly compared to Eq. (1).
develop the specified yield strength of reinforcing bars; however,          The fib code has two features that distinguish it from the ACI
a design compression lap splice could be longer than a design
tension lap splice according to ACI 318-08, as concrete strength
becomes higher, as shown in Fig. 1. This anomaly arises because
the provisions for compression splices do not properly consider
the effects of the compressive strength of the concrete and end
bearing. To enhance the efficiency of high-strength concrete, new
criteria for compression lap splices are required.

                RESEARCH SIGNIFICANCE
   This research examined 44 column specimens to develop a new
design equation for compression lap splices. The effects of the
compressive strength of the concrete, the clear spacing between
bars, and the splice length on compression splices were
investigated. Through regression analysis of measured splice
strengths, an equation was derived to predict splice strengths,
which was then converted into an equation for splice lengths.
Using the proposed equation, the anomaly associated with splice
lengths in tension and compression could be removed.
                                                                            Fig. 1—Comparison of calculated splice lengths with varying
           DESIGN CODES AND PREVIOUS                                        concrete compressive strengths. (Note: 1 MPa = 145 psi.)
                  RESEARCH STUDIES
Design codes for compression splices                                          ACI Structural Journal, V. 107, No. 2, March-April 2010.
  The U.S.,1 Canada,3 and New Zealand4 codes have similar                     MS No. S-2008-362.R1 received April 10, 2009, and reviewed under Institute publication
                                                                            policies. Copyright © 2010, American Concrete Institute. All rights reserved, including the
provisions for compression lap splices. Equation (1) is the design          making of copies unless permission is obtained from the copyright proprietors. Pertinent
                                                                            discussion including author’s closure, if any, will be published in the January-February
equation given in ACI 318-08 for compression splice lengths                 2011 ACI Structural Journal if the discussion is received by September 1, 2010.


170                                                                                                 ACI Structural Journal/March-April 2010
                                                                                                         Cairns and Arthur6 and Cairns7 carried out 23 rectangular
ACI member Sung-Chul Chun is a Principal Researcher at Daewoo Institute of
Construction Technology (DICT), Suwon, Korea. He received his BS, MS, and PhD in                       column tests and used Eq. (3) to calculate the splice strengths
1994, 1996, and 2007, respectively, from Seoul National University, Seoul, Korea. He                   with measured loads and adjusted concrete strength
is a member of Joint ACI-ASCE Committee 352, Joints and Connections in Monolithic
Concrete Structures. His research interests inc
				
DOCUMENT INFO
Description: A compression lap splice may be calculated to be longer than a tension lap splice in high-strength concrete according to current design codes. New criteria for the compression lap splice, including the effects of concrete strength and end bearing, are required. An experimental study was conducted using compressive concrete strengths of 40 and 60 MPa (5800 and 8700 psi) without transverse reinforcement. Test results showed that the splice strength was proportional to [the square root of]f'^sub c^ and that the spacing between bars had little effect on the splice strength. The bar stresses developed by bond were nearly identical to those calculated by the ACI Committee 408 equation. Therefore, the strength increase in the compression splices is solely attributed to end bearing. A design equation was derived for compression lap splices without transverse reinforcement through regression analysis. [PUBLICATION ABSTRACT]
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