Mechanical vs. Lap Splice


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Mechanical vs. Lap Splices in Reinforced Concrete Construction Study Conducted by: ERICO Concrete Reinforcement Products For almost 100 years, construction practices in the building of concrete structures have focused on the use of steel reinforcement to transfer tension and shear forces. Lap splicing has become the traditional method of connecting the steel reinforcing bars, largely due to a misconception that lap splicing is “no-cost” splicing. Lap splicing requires the overlapping of two par
  Mechanical vs. Lap Splices in Reinforced Concrete Construction For almost 100 years, construction practices in the buildingof concrete structures have focused on the use of steelreinforcement to transfer tension and shear forces. Lapsplicing has become the traditional method of connectingthe steel reinforcing bars, largely due to a misconceptionthat lap splicing is “no-cost” splicing.Lap splicing requires the overlapping of two parallel bars.The overlap load transfer mechanism takes advantage ofthe bond between the steel and the concrete to transferthe load. The load in one bar is transferred to theconcrete, and then from the concrete to the ongoingbar. The bond is largely influenced by deformations(ribs) on the surface of the reinforcing bar.Continuing research, more demanding designs in concreteconstruction, new materials, hybrid concrete/structuralsteel designs and other changes in the constructionindustry are calling for the use of alternatives to lapsplicing. From the standpoint of function, laps performwell on bar sizes #6 and #8 of 40KSI yield steel and3,000 lb. concrete, with a structure of 15 stories considereda high-rise. Today, buildings taller than 15 stories areincreasingly common. A “high-rise” of reinforced concretein Kuala Lumpur, Malaysia recently topped over 100stories, and at least a half dozen other 85-plus storybuildings are under construction or planned.The use of higher strength concrete, which is moreprone to splitting, also is on the increase. Compoundingthis problem, calculations within the ACI code results inshorter lap lengths with high strength concrete.Recent presidential orders for Federal buildings in the U.S.also are increasing the focus on structural seismic safety.According to Executive Order 12941, signed in 1996, eachagency that owns or leases buildings for Federal use mustensure that the building is designed and built in accordwith proper seismic design and construction standards. Theorder mandates the consideration of seismic safety in anybuilding occupied by a Federal agency. The government isthe largest renter of office space in the nation. LAP SPLICING PROBLEMS Over the years, many structural engineers, architects andspecifiers have noted that lap splicing has few advantagesand many disadvantages. ACI R21.3.2.3 states that lapsplices are not considered reliable under conditions ofcyclic loading into the elastic range. Further, there is aquestion as to the effectiveness of laps with larger bars:#8, #9, #10 and #11. These are major structural elementsin the frame of a reinforced concrete structure, and anyquestion regarding their efficacy is cause for concern.Over the years, to counter these concerns, the requiredlap lengths in the ACI 318 Building Code have becomelonger and longer. ACI has prohibited the useof lap splices in bar sizes #14 and #18. Laps are alsoprohibited on bar sizes in tension tie members (ACI12.15.5) and within joints and locations of flexuralyielding (ACI about lap splicing go to the very principlesthat are the basis for lap splicing. Lap splicing requiresconcrete to take tension and shear loads, though concreteis notoriously poor in handling tension and shear. Fig. 1Distribution of transverse forces in concretebqlap length Study Conducted by: ERICO Concrete Reinforcement Products  MECHANICAL SPLICE ADVANTAGES Mechanical butt splices are mechanical connectionsbetween two pieces of reinforcing steel that enable thebars to behave in a manner similar to continuous lengthsof reinforcing steel bars. Mechanical splices join rebarend-to-end, providing many of the advantages of acontinuous piece of rebar. Years ago, arc welding wasthe only method of achieving continuity. Today, a myriadof mechanical splices are available to ensure that a precise,reliable connection can be quickly and easily made.Mechanical butt splices are more reliable than lap splicesbecause they do not depend on concrete for load transfer.Further, mechanical butt splices are stronger than lapsplices: ACI requirements for mechanical splices are atleast 25% higher than typical design strengths for lapsplices. Mechanical butt splices provide superior strengthduring load transfer. Superior cyclic performance andgreater structural integrity during manmade, seismic orother natural events are other advantages of mechanicalbutt splices.From the structural perspective, the most importantbenefit of using mechanical butt splices is to ensureload path continuity of the structural reinforcementindependent of the condition or existence of theconcrete. Additionally, mechanical butt splices reducecongestion of the reinforcing concrete by eliminatinglaps (See Fig. 3). Fig. 3Lap splices double thenumber of bars leading torebar congestion which canrestrict the flow of larger aggregatesMechanical couplersreduce rebar congestionand improves concreteconsolidationBeamReinforcing LAP SPLICESMECHANICAL SPLICES PLAN VIEWPLAN VIEW Mechanical Coupler Column ReinforcingFig. 4BeamReinforcingLap Bar Column Reinforcing Laps double the steel/concrete ratio and create problemswhile placing the bar and during concrete consolidation.Elimination of laps also frees space for post tensioningoperations.From the design perspective, mechanical butt splices canbe relied upon to improve steel-to-concrete ratios, whichassists in delivering a consistent ratio under 8%. Whenusing laps, working with small diameter reinforcing barsmay require the use of larger column dimensions toaccommodate a greater quantity of bars. Using mechanicalbutt splices allows the option of using larger diameterrebar in a smaller column, while minimizing congestion.Reduced column size results in a more efficient optimumuse of floor space – an extremely beneficial economicand design consideration (See Figure 4).Efforts to evaluate the comparative costs of using lapsplicing and mechanical butt splicing in concreteconstruction show that the reputation of butt splicesfor adding substantial cost to a construction budget isunfounded. A recent study conducted by Cagley &Associates of two structures showed that the additional
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