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lapping of reinforcement in beams and columns

 
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k.gangadharan
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PostPosted: Mon Mar 09, 2009 4:07 pm    Post subject: lapping of reinforcement in beams and columns Reply with quote

dear sefians,
I have a  very practical field problem to practising Structural engineers.
In beams & columns when the % of reinforcement is very large( of course within the maximum limit) and esp. when we use large diameter bars like 25 & 32 mm dia bars, the space between the rods is very minimum and when we want to lap the rods there will be no spaces for easy & smooth flow of concrete between the rods at the lapping places.
To avoid congestion of rods at lap locations, we can either use welding of rods or use mechanical splices.Even when we use mechanical splices the diameter of the mech. splices will be more than the dia. of the rods and there won't be enough spaces between splices for easy and smooth flow of concrete. If anybody has used splices, then they can understand the space avilable between the splice junctions. I am referring to a case where the reinforcement is very close with min gap bet: rods
In such circumstances the other alternative is welding for extension of rods.
We have gas welding and electric arc welding ;and when welding is resorted,a large amount of heat is generated .
My question is: 1. by this very high temperature the the composition of the steel and its strength will be affected or not?
2:What type of welding rods are used for welding high strength bars of
Fe 415 or Fe500 or higher  strength bars as there are different type of welding rods available in the market ?.
There is no guidance regarding the type of welding bars to be used for renforcing bars.
Professional engineers with practical experience can give their expert opinion  and advice in this matter,
There are some guidance in the IRC codes ; but they are not very specific in details.
with regards
K.Gangadharan
structural engineer
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P.K.Mallick
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PostPosted: Mon Mar 09, 2009 5:23 pm    Post subject: Reply with quote

Please try vertical lap instead of horizontal lap. Vertical lap: reinforcement is kinked and placed over other reinforcement in lapped position. A small metalic U clip is used to keep them position. Horizontal lap: Lapped bars are placed side by side thereby resulting in reduction in space between reinforcements.
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kamleshpatel
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PostPosted: Mon Mar 09, 2009 6:20 pm    Post subject: Reply with quote

Dear Mr. Malick/Gangadharan

Vertical lap is not useful when there are more than one layers of reinforcement in both directions. In such cases the distace between the layers is increases and effective depth decreases, which again requires more reinforcement.

Many times, I use the staggered laps, which serves the purpose. Similarly Mechnical splices can be staggered to avoid congestion.

Best Regards,
K Patel
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P.K.Mallick
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PostPosted: Tue Mar 10, 2009 1:54 am    Post subject: Reply with quote

Look for


ANSI/AWS D1.4/D1.4M:2005
Structural Welding Code - Reinforced Steel, Includes Errata
Edition: 6th
American Welding Society / 08-Jul-2005 / 84 pages
ISBN: 0871710048


The review of this code says:
This code covers the requirements for welding reinforcing steel in most reinforced concrete applications. It contains a body of rules for the regulations of welding reinforcing steel and provides suitable acceptance criteria for such welds.

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Dr. N. Subramanian
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PostPosted: Tue Mar 10, 2009 2:01 pm    Post subject: lapping of reinforcement in beams and columns Reply with quote

Dear Mr. Malick/ Mr. Gangadharan,

As suggested by Mr. Patel, we should strive for staggered splicing. IS code also suggests that no more than 50% of reinforcements should be spliced. It also states that if more than 50% bars are spliced, length of lap should be increased and/or closely spaced stirrups should be used along the length of splice.(Cl. 26.2.5). It also says lap splices should not be used for bars larger than 32mm and bars larger than 32 mm shall be welded or mechanically spliced(I think 36 mm was changed to 32 mm in an amendment to the code).

Someone raised the question of welding. I am quoting below the details from ACI code

"When welding of reinforcing bars is required, the weldability of the steel and compatible welding procedures need to be considered. The provisions in ANSI/AWS D1.4 Welding Code cover aspects of welding reinforcing bars, including criteria to qualify welding procedures.

Weldability of the steel is based on its chemical composition or carbon equivalent (CE). The Welding Code establishes preheat and interpass temperatures for a range of carbon
equivalents and reinforcing bar sizes. Carbon equivalent is calculated from the chemical composition of the reinforcing bars. The Welding Code has two expressions for calculating
carbon equivalent. A relatively short expression, considering only the elements carbon and manganese, is to be used for bars other than ASTM A 706 material. A more comprehensive
expression is given for ASTM A 706 bars. The CE formula in the Welding Code for A 706 bars is identical to the CE formula in the ASTM A 706 specification. The engineer should realize that the chemical analysis, for bars other than A 706, required to calculate the carbon equivalent is not routinely provided by the producer of the reinforcing bars. For welding reinforcing bars other than A 706 bars, the design drawings or project specifications
should specifically require results of the chemical analysis to be furnished.

The ASTM A 706 specification covers low-alloy steel reinforcing bars intended for applications requiring controlled tensile properties or welding. Weldability is accomplished
in the A 706 specification by limits or controls on chemical composition and on carbon equivalent.3.4 The producer is required by the A 706 specification to report the chemical
composition and carbon equivalent.

The ANSI/AWS D1.4 Welding Code requires the contractor to prepare written welding procedure specifications conforming to the requirements of the Welding Code. Appendix A of the Welding Code contains a suggested form that shows the information required for such a specification for each joint welding procedure.

Often it is necessary to weld to existing reinforcing bars in a structure when no mill test report of the existing reinforcement is available. This condition is particularly common in
alterations or building expansions. ANSI/AWS D1.4 states for such bars that a chemical analysis may be performed on representative bars. If the chemical composition is not
known or obtained, the Welding Code requires a minimum preheat. For bars other than A 706 material, the minimum preheat required is 300 F for bars No. 6 or smaller, and 400
F for No. 7 bars or larger. The required preheat for all sizes of A 706 is to be the temperature given in the Welding Code’s table for minimum preheat corresponding to the
range of CE “over 45 percent to 55 percent.” Welding of the particular bars should be performed in accordance with ANSI/AWS D 1.4. It should also be determined if additional
precautions are in order, based on other considerations such as stress level in the bars, consequences of failure, and heat damage to existing concrete due to welding operations."

As per IS, welding has to be done as per IS 2751 & IS 9417. However, welding can reduce the fatigue life of the rebar. note that prestressing bars should never be welded.

Quench and self tempering (QST) bars, which are also referred to as TEMPCORE bars transforms only the surface of the bar to a martensitic structure, while the core remains austenitic. As a result, the bar has a composite metallurgical structure with a strong martensitic surface layer and a relatively soft, ductile ferrite-pearlite core. The quenching effect can be reduced or may be completely eliminated by welding. Equations for calculating tensile strength of the welded QST bars are developed by Degtyarev(2007)


Degtyarev, V. V., Tensile Strength of Welded Splices of QST Reinforcing Bars, ACI Materials Journal , Jan/Feb 2007

Best wishes
Subramanian





--- On Mon, 3/9/09, kamleshpatel <forum@sefindia.org> wrote:
Quote:
From: kamleshpatel <forum@sefindia.org>
Subject: [SEFI] Re: lapping of reinforcement in beams and columns
To: general@sefindia.org
Date: Monday, March 9, 2009, 11:50 PM

Dear Mr. Malick/Gangadharan

Vertical lap is not useful when there are more than one layers of reinforcement in both directions. In such cases the distace between the layers is increases and effective depth decreases, which again requires more reinforcement.

Many times, I use the staggered laps, which serves the purpose. Similarly Mechnical splices can be staggered to avoid congestion.

Best Regards,
K Patel
     



     


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