Welded Splices of Reinforcing Bars_tcm45-340900_2.pdf

 We  We l d e d s p l i c e s o f reinfor reinfo r cing ci ng bar s Both contractor and engineer must be aware of special requirements  B Y DAVID P. GUSTAFSON TECHNICAL DIRECTOR C ONCRETE REINFORCING STEEL I NSTITUTE

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roper rope r splic s plic ing of rei nf orcing bars bar s is crucial cruc ial to the integ rity rit y of re i nf orced concre con cre t e. The AC I Build ing Code1 Cod e1 stat es: es : “Splices “Spl ices of re i n f o rc ement shall be made only as required or permitted on the design dra w i n g s, in the specifications, or as authorized by the th e engineer.” e ngineer.” Great responsib ility for design, specification and perf o rmance of splices rests with the engineer, engineer, and only the engineer who is familiar with the structural analysis and design stress, s tress, probable construction conditions and final conditions of service can p roperly evaluate the va riables to select the most efficient and economical splice method. Three method m ethod s are ar e used u sed for splicing spli cing reinforcing bars: bar s:

• Lap splices

Thus, if lap splices splice s are not permitted permitte d or o r are impractical to use, mechanical connections connections or w elded splices must be used. Mechanical Mechanical connections are made with p rop rie t a ry spli sp lice ce devi de vice ces. s. Pe rf rf o rman rm ance ce info in form rm at io n and test data should be secured directly from manufacturers of o f the t he splice s plice devices. The purpose of this br ief article is simply to answer some of the questions concerning welded splices. Although only welded splices are discussed here, it should not be construed constru ed that welded splices are being advocated. Each splice method has its advantages and suitability for parti cular applications. For projects of all size s, manual arc welding will usually be the most costly  method, due to direct and indirect costs of proper inspection.

• Mechanical connections

Building code requirements for w elded splices

• Welded splices

The model building codes, which are the basis for many statutory building codes, have some special requirements remen ts for welded welde d splice s plices. s. The Basic Building Buildi ng Co de2 and the th e St andard Buildi Bui lding ng Code3 Code 3 adopt ado pt the ACI Building  Code Cod e by re f e re n c e. Excep t for A706 A70 6 bars, ba rs, the Uni form Building Code4 prohibits welding unless the carbon equivalent (explained (expla ined below) is known. The UBC permits the building official to w aive this requirement for f or minor deta ils or repairs, provided provide d the welding weldi ng pro ced u res are the same as those for a carbon equivalent exceeding cee ding 0.75 perc ent. Some key items included in the ACI Building Code req u i rements for welded splices are: welding must conf o rm to AWS D1.4-79;5 in a full welded splice, as required in a “tension “t ension tie t ie member” memb er” or in compression, compres sion, the bars have to be butted and the splice must develop at least 125 percent of the specified yield strength, fy, fy, of the b a r. For tension splices where the area of the bars is twice twi ce that required by stru st ru ctural analys an alysis, is, the splices spli ces can be designed for less than 125 percent fy of the bar. There are also al so rule r uless for staggeri sta ggering ng the splic sp lices, es, and a nd tack ta ck we lding  is not permitted unless authori zed by the engineer. The basic welding requirements given in AWS D1.4-7 9, “St ruc tu ral Weldin Weld ing g Co d e— Re Re inf orcing rci ng St ee l,” incl in clud ude e

The traditional lap splice, when it will satisfy all req ui re m e n t s, is generally gene rally the most economical econ omical splice, spli ce, and we lded splices splic es generall g enerally y re quire the most expenexpe nsive field fi eld labor. l abor. However, lap splices s plices cause congestion c ongestion at the splice locations, sometimes making their use imposs i b l e. The location of construction joints, provision for f u t u re construction, and the particular method of cons t ruction can also make lap splices impractical. In addition, the ACI Building Code does not permit lap splices in “tension tie members,” or in #14 and #18 bars exc e p t for compression only, only, when spliced to smaller size footing dowe ls. In column design, consideration must also be given to the fact that lapped offset bars may have to come inside of the bars above a bove and therefore reduce the moment m oment arm in bendi ng. When the amount of column vertical reinforcement is greater than 4 percent, and particularly in combination with large applied moments, the use of  butt splices—either splice s—either mechanical mechanica l connections c onnections or welded splices—should be considered to reduce congestion, and to provide for greater design moment strength of  the section at the splice locations.

p ermissib le stre s s e s, both for the strength desi gn method and the working stress method; splice details;  w orkmanship; filler metal (electrodes) req u i re m e n t s ;  welding technique; welder qualification; and inspection. These are explained in the following series of questions and answers.

Types of welded splices Q : What

types of welded splices are included in the AW S 

code? 

When or why would thermite welding be used? 

 A: Thermite welding has

been used with success in making butt-welded joints in the large #14 and #18 bars. This p rocess has been successful in joining hard - t o - we l d s te el s, because it welds the entire cross s ection at the same time and automatically provides preheat and slow  cooling. Q:

Is thermite welding popular? 

Not so much in recent years for splicing reb ar s. It seems to be extensively used in other applications such as continuously welded ra i l road rails; the suppliers of  the proprietary molds and other materials may be concentrating their attention on the other applications.

 A:

Direct and indirect butt splices, and lap-we ld e d splices.

 A:

Q : Is special end pre paration of the bars re quired for

dire ct 

butt splices?   A: Yes,

the particular end details—V- gro oves or bevels— depend upon whether the bars will be placed in a horizontal or ve rtical position. Q : What

Q:

is an indirect butt splice? 

 A :  A

splice w here both bar s are welded to a common splice member such as a plate, angle or other shape. The bars are nearly aligned; bar ends are separated no more than 3 ⁄ 4 inch; and the cross section of the bars i s not  welded. What types of lap-welded splices are included in the   AWS code? 

Q : The

AWS code discusses filler metal (electrodes). What  kind of electrodes are re q u i red for arc welding?   A: The electrodes

should conform to AWS Specifications  A5.1 or A5.5. They should be of a classification and size appropriate to the welding conditions and to the tensile strength and analysis of the bars to be weld ed. It is imp ortant that the coatings of low - h yd rogen- type elec trodes such as Classes E70XX and E80XX be thoro ughly  d ry when used. For example, E80XX electrodes taken f rom hermetically sealed packages must be used within 4 hours.

Q:

There are two types: direct and indirect. A direct type is one in which the bars are in contact and welded together; single or double lap joints can be used; they are suitable only for small bars, pref erably #5 or smaller. Double lap joints would be preferred if eccentricity of  the splice is a consideration. In an indirect type, the bars a re welded to a common splice plate; there is a space between the bars.

 A:

Q:

Where are fillet welds used? 

 A: An

example would be connections of rebars to structu ral steel members, provid ed sufficient stru c t u ra l strength can be achieve d. Q:

What welding processes does the AWS code cover? 

Shielded metal arc, flux cored arc, pres sure gas and t he rmite welding pro ce ss es.

Weldability and carbon equivalent When the subject of welding is discussed, the term  weldability is often mentioned. What is meant by weld ability?  Q:

 A:  A

metallurgist defines weldability in terms of the chemical composition of the steel; his measure is carbon e qu ivalent content. A stru c tural engineer pro b ab ly  thinks of weldability in terms of the strength achieved at a splice, while a welder or contractor considers it in t e rms of cost, welding method re q u i red, and amount of  preheat. The AWS code defines weldability as “the capacity of a metal to be welded under the fabrication conditions imposed into a specific suitably designed structu re and to perf orm satisfactorily in the intended serv i c e.” Q:

 A:

Q:

What is thermite welding? 

It is a process in which the ends of the bars are fusion  welded. Refractory molds are assembled on the bars and sealed in place. Heat-generating powders are filled into a s ep arate cavity in the molds. The powders are ignited and burn with enough heat to form superheated molten steel. The steel flows through the gap between the bars and some flows into a second cavity beyond the bars, p reheating them. Subsequent flow completes the process.  A:

What is carbon equivalent? 

This is a quantitative measure of weldability. The carbon equivalent (C.E.) is based on the chemical composition of steel; it accounts for those chemical elements affecting we ldability, and it is a numerical value expressed as a percent. The AWS code and the ASTM A706 re b a r specification have the same formula for C.E.

 A:

C .E. = %C +

%Mn %Cu %Ni %C r %Mo %V ––––– + ––––– + ––––– + ––––– - ––––– - ––––– 6 40 20 10 50 10

Note that fractions of the percentages of manganese (Mn), copper ( Cu), nickel (Ni) and chromium (Cr) are

added to the percentage of carbon (C). Fractions of the percentages of alloying elements which enhance weldab il it y, i.e., molybdenum ( Mo) and vanad ium ( V) a re subtracted. Not all of these elements are necessarily present in any given heat of steel. Q:  A:

How is the carbon equivalent value established?  From the chemical analyses in the mill test re p ort s.

Q:  Are

chemical analyses routinely included in mill test  re p o rt s ?  This depends on which steel is specified. Since the s t and ard rebar specific ations ASTM A615, A616 and  A617 specifically state that “weldability of the steel is not part of this specification,” there are no limits on the chemical elements included in the C.E. formula, nor is there a limit on C.E. (the C.E. would typically exceed 0.55 pe rcent for these bars). The chemical compositio n (A615) and carbon range (A617) are only provided upon request. Of the chemical elements in the C.E. formula, only carbon and manganese will be re p o rted for A615 bars unless special complete analyses are requested. However, A706 re inforcing bars are intended for we lding. In addition to restrictions on chemical composition including carbon, the C.E. is limited to 0.55 percent. The chemical composition and C.E. must be re p o rt e d .  A :

Q:

How is the carbon equivalent used in the AWS code? 

The minimum preheat and interpass temperat u re s are based upon carbon eq uivalent. For instance, ASTM  A706 rebars are limited to a C.E. of 0.55 percent, and the  AWS code requires little or no preheat for bars at or below this limit.

 A:

What preheat is re quired for larger values of carbon  equivalent?  Q:

The highest preheat, 500 degrees F, is required for all bar sizes if the carbon equivalent is above 0.75 percent. If  the chemical composition of the bars to be welded is not known, the carbon equivalent is assumed to be above 0.75 percent.

 A:

h a ve to keep track of differing preheat tempera t u res for each bar size or bar shipment. What about projects which re quire some welding in  important elements?  Q:

 A: Project specifications should be open to include both  A706 and A615 bars. Specify mill test re p o rt s. The contractor needs these reports to provide the required preheat. Inspectors should be present whenever welding is done on important elements.

What about small projects requiring little or only oc casional welding in non-critical areas? 

Q:

Specify maximum preheat for an assumed carbon equivalent greater than 0.75 perc ent.

 A:

Q : What

about cooling the bars after the welding is com -

pleted?   A: The bars

must be allowed to cool naturally; accelerated cooling is prohibited. Q : Is there a

temperature restriction on field welding? 

 A:  AWS

D1.4-79 prohibits manual arc welding when the ambient tempera t u re is less than 0° F. Q : Little

or no preheat is required for bars with lower  ranges of carbon equivalent; are there re quirements when  the ambient temperature is low?  For bars which require no preheat at normal work ing  te mpe ratures, if the temperature of the bars is below 32° F, they must be preheated to at least 70 degrees F and this t emp erat ure must be maintained during we lding. Some bars requ ire little preheat at normal work i ng  t e m p e ra t u res; for example, #11 bars having a carbon equivalent from 0.46 to 0.55 percent require a preheat of  50° F. If these bars are at a tempera t u re below 50°F they  must be preheated so that the tempera ture of the cross section of the bar within 6 inches on each side of the  joint is 50° F or greater.

 A:

Q : What

is the tack welding which the ACI Code pro -

Specifying bars for welding

hibits? 

What are some practical points that one should con sider when specifying welded splices? 

 A: Connection of

Q:

This depends on the size of the project and the amount and importance of the welding. There is an excellent discussion in Re ference 2.

 A:

Besides requiring all welding to conform to AWS D1.479, what should be considered in a large, long term pro  ject involving ex tensive welding in important structural  elements?  Q:

Consider use of A706 rebars; check availability before specifying. Field inspection will be simplified since little or no preheat will be re quired, and inspectors will not

 A:

crossing rebars by small arc welds, such as in a column cage where the ties are welded to the longitudinal bars. If tack welding is authori zed by the engineer, the welds should be made in conformance with all re quirements of AWS D1.4-79. We do not re comme nd this practice; wire ties should be used for assembly of  rei nf orcing steel. Q : Why

is tack welding a poor pra ctice? 

For the column cage case, such welding can cause a metallurgical “notch” effect in the big longitudinal bars, reducing their original tensile strength and bendability. Tack welding is particularly detrimental to impact re sis-

 A:

2. Basic Building Code, 1981 Edition, Building Officials and Code Administrators International, Inc., Homewood, Illinois.

tance and fatigue re sistance. Q :   Are

the harmful effects of tack welding well proven? 

 A: Yes,

research reports are cited in Reference 7. See Reference 8 also.

Conclusions It is evident from these questions and answers that p roperly engineered and constructed welded splices require more considerations than a simple statement in the contract documents, “All welded splices shall confo rm to ‘St ru c tu ral Welding Co de — Re i nf o rcing St e el’ (AWS D1.4-79).” The welding code is a compre hensive document. Howe ve r, other important items such as securing chemical pro perties of the re bars, field inspection, supervision, and quality control are re q u i red for a project with welded reinforcement. References

1. “B uilding Co de Req uirements for Reinforce d C oncrete (ACI 318-77),” American Concrete Institute, Detroit, Michiga n, 1977.

3. Standard Building Code, 1979 Edition, Southern Building Co de Co ngress International Inc., B irmingha m, Alab am a. 4. Uniform Building C od e, 1979 Edition, Internationa l Conference of Building Officials, Whittier, California. 5. “ S tructura l Welding C od e—Reinforcing S teel (D1.4-79),” Ame ric a n Welding S oc iety, 2501 N.W. 7th S treet, Miam i, Florida 33125. 6. Rice, P. F. a nd Hoffma n, E. S ., S tructural Des ign Guide to the ACI Building C od e, 2nd Edition, Va n Nostrand Reinhold, 1979. 7. Reinforcem ent Anchorag e a nd S plices , Co ncrete Reinforcing S teel Institute, Chica go , 1980. 8. Firth, M. a nd Willia ms , W. M., “ Avoid Marte ns ite When Welding Reb ar,” Meta l Progres s, April 1979, pag es 38-40. 9. AS TM sta nda rds referenced by number throughout text a re ava ilable from America n S oc iety for Tes ting and Materials, 1916 Race S treet, P hilad elphia, P ennsylvania 19103.

PUBLICATION#C810807 Copyright © 1981, The Aberdeen Group All rights re served