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World J of Engineering and Pure and Applied Sci. 2012;2222((((3333):):):):98989898 ISSN 2249-0582
SSSSarkiarkiarkiarki et alet alet alet al., 2012. Safety of Lap Lengths Prediction in Reinforced Concrete Structures
1 Department of Civil Engineering, Faculty of Engineering, Ahmadu Bello University, ZARIA
2 Department of Civil Engineering, Faculty of Engineering, Ahmadu Bello University, ZARIA
3 Department of Civil Engineering, Faculty of Engineering, Ahmadu Bello University, ZARIA
Address for Correspondence/ Adresse pour la Correspondance: [email protected]
Accepted/Accepté: March, 2011
Citation: Sarki YA, Murana AA, Abejide SO. Assessing safety in predictions of lap lengths in reinforced concrete structures. World Journal of Engineering and Pure and Applied Sciences 2012;2(3):98-106.
The recommendations for lap lengths of steel bars in reinforced concrete as specified by the pseudo-
probabilistic design codes the British Standards Institution in about the past four decades and the current
relevant Eurocode have been reviewed and evaluated for their safety implications in the bar curtailment and
detailing. Results indicate that the implied safety is not uniform especially when the bar size factor in the
prevailing specified equations for lap length design is about half its value for all the codes. Also, the earlier
provision of the British codes gave the best safety indices in all the cases evaluated. However, it is suggested
herein that an adequate lap length of bars in reinforced concrete design could be about 50 times the smallest
bar size in the reinforced concrete member as opposed to the Eurocode’s provisions of 40times the smallest
bar size
Keywords: Reinforcement bars, lap lengths, concrete
Les recommandations pour une longueur de recouvrement des barres d'acier dans le béton armé tels que
spécifiés par les codes de conception pseudo-probabilistes de laBritish Standards Institution dans
environ les quatre dernières décennies et l'Eurocodeactuelle pertinents ont été examinés et évalués pour
leurs répercussions sur la sécuritédans la réduction bar et détaillant . Les résultats indiquent que la
sécurité implicite n'est pas uniforme en particulier lorsque le facteur de taille de la barre dans les
équations en vigueur spécifiées pour la conception longueur de recouvrement est d'environ la moitié de
sa valeur pour tous les codes. En outre, la disposition antérieure des codesbritanniques ont donné
les meilleurs indices de sécurité dans tous les cas évalués.Cependant, il est suggéré ici qu'une longueur de
recouvrement suffisante de barres debéton armé dans la conception pourrait être d'environ 50 fois la
taille plus petite barredans l'élément de béton armé, par opposition aux dispositions de
la Eurocode de40times de la taille plus petite barre
Mots-clés: Des barres d'armature, les longueurs de recouvrement, en béton
INTRODUCTIONINTRODUCTIONINTRODUCTIONINTRODUCTION In the construction of reinforced concrete, due to the
limitations in available length of bars and due to
constraints in construction, there are numerous occasions
when bars have to be joined, some of which are detailed
and hence the essence of overlapping two bars over at
least a minimum specified length called lap length [1, 2,
3]. This lap length varies depending on the bars sizes as
there are various bar sizes and where the bars are lapped
and/or which structural member or element the lapping
occurs.
As a result of load transfer, the steel bars maybe either in
axial tension or axial compression [4, 5]. Flexure, shear
and torsion may occur as effects [6], but due to
limitations of results and other factors, this paper would
focus solely on tension and compression and how they
vary with different lap lengths and bar sizes. Hence, the
distribution of tensile stresses in the concrete normal to
the axis of the bars is relevant. The overlap on the other
hand transfers or generates additional forces in the
concrete which tend to push the bars apart, so concrete
cover must be strong enough to overcome this “bursting
force” [2, 7].
This paper attempts to deal basically with the variables
of the lap splice which include lap length, the head-size
and shape, and the bar spacing.
MATERIALS AND METHODSMATERIALS AND METHODSMATERIALS AND METHODSMATERIALS AND METHODS Lap Splice Design Equations A revised equation developed resulting from
advancement in technology, submitted to building codes
ISSN 2249-0582 World J of Engineering and Pure and Applied Sci. 2012;2222((((3333):):):):99999999
SSSSarkiarkiarkiarki et alet alet alet al., 2012. Safety of Lap Lengths Prediction in Reinforced Concrete Structures
yield stresses in the algorithms to get the various safety
levels. These safety criteria are plotted against the
various respective diameters of reinforced concrete bars.
Some of the results of the safety values against their
respective diameters are shown in figures 1 to 18.
DISCUSSIONDISCUSSIONDISCUSSIONDISCUSSION RECOMMENDATIONRECOMMENDATIONRECOMMENDATIONRECOMMENDATION With regards to the safety values in the figures 1 - 18 as
shown, the lap lengths in bars can be said to be safest at
the lowest possible values of the variables (that is, bar
diameter, yield stress and adherence factor). It would be
observed that at a constant yield strength and adherence
factor, the optimum safety (i.e. the maximum β value)
occurs at small diameters of reinforced concrete bars
when the specified compressive strength of concrete is
high. But the safety of bar lap lengths reduces as the bar
diameter increases and the compressive strength of
concrete decreases simultaneously.
Therefore it could be seen that the safety of lap lengths of
reinforced concrete bars is inversely proportional to the
diameter of reinforced concrete bars and directly
proportional to the specified compressive strength of
concrete at constant yield stress of reinforcement bars
and adherence factor.
The results also indicate that the British [12, 13] and
European [14] codes for the design of reinforced concrete
structures all have similar results. Also the provision of
the American Concrete Institute [15] is similar to the
three codes used as examples.
Results show that among the building codes
investigated, the provisions for bar lap lengths in
concrete of the British code [12] gave the highest safety.
This implies that the provisions, which stipulates lap
lengths of reinforced concrete bars to be equal to
(25 , gives the best provisions for lap lengths of
steel in tension. But the provision for lap length of bars in
compression is smaller than that for tension. Also,
separate formulations will have to be remembered in
design or construction of reinforced concrete structures.
However, it is necessary to prescribe a format as in the
current European code [13] which will be very easy to
comprehend while still achieving both safety and
economy when considering laps of bars in reinforced
concrete structures.
CONCLUSIONCONCLUSIONCONCLUSIONCONCLUSION There are separate formulations for bar lap lengths in
compression and tension of reinforced concrete members
in the British codes, while only one provision is made in
the current Eurocode [14] for design and construction of
reinforced concrete members. One of the British codes
[13] has attempted a single value provision. However,
the safety intrinsic in the single value provisions in the
Eurocode and British code is not commensurate with the
economy in bar curtailment achieved. Thus, it is
necessary to prescribe a single value provision that will
be safe while at the same time achieving the required
economy, which will be applicable to all codes for the
design and construction of reinforced concrete structures.
This is a bid to reduce failure of structures which have
occurred in recent times in many parts of the world;
hence the review of the current international codes of
practices.
Figure 1:Figure 1:Figure 1:Figure 1:: This figure shows effect of Bar Size on Safety
of Lap Lengths at fy = 250 and α = 0.5 (BS8110)
Figure 2:Figure 2:Figure 2:Figure 2:: This figure shows effect of Bar Size on Safety
of Lap Lengths at fy = 250 and α = 2 (BS8110)
ISSN 2249-0582 World J of Engineering and Pure and Applied Sci. 2012;2222((((3333):):):):103103103103
SSSSarkiarkiarkiarki et alet alet alet al., 2012. Safety of Lap Lengths Prediction in Reinforced Concrete Structures
[8] Gollwittzer S, Abdo T, Rackwitz R. First Order
Reliability Method,(FORM) Manual, RCP
Gmbh, Nymphenburger Str. 134, MÜNCHEN;
Germany. 1988.
[9] Ditlevsen O, Madsen HO. Structural Reliability
Method. First edition. John Wiley & Sons Ltd;
Chichester. 1996. Retrieved from:
http://www.mek.dtu.dk/staff/od/books.htm.
[10] Ayyub B, McCuen RH. Probability, Statistics,
and Reliability for Engineers CRC Press LLC;
New York. 1997.
[11] Hasofer, AM, Lind, NC. Exact and Invariant
Second – moment Code Format. Journal of
Engineering Mechanics 1974;1:111–21.
[12] British Standards Institution. The Structural Use
of Concrete, Part 1: Code of Practice for Design
and Construction. British Standards
Institution; London. 1972.
[13] British Standards Institution. The Structural Use
of Concrete, Part 1: Code of Practice for
Design and Construction. British Standards
Institution; London. 1997.
[14] European Committee, Design of Concrete
Structures, Part 1.1. European Committee for
Standardization; Brussels. 2008.
[15] American Concrete Institute. Building Code
Requirement for Structural Concrete and
Reinforced Concrete. American Concrete
Institute; Michigan. 2005.
ACKNOWLEDGEMENT ACKNOWLEDGEMENT ACKNOWLEDGEMENT ACKNOWLEDGEMENT / / / / SOURCE OF SUPPORTSOURCE OF SUPPORTSOURCE OF SUPPORTSOURCE OF SUPPORT Nil
CONFLICT OF INTERESTCONFLICT OF INTERESTCONFLICT OF INTERESTCONFLICT OF INTEREST No conflicts of interests were declared by authors.
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