Welded splices of
reinforcing bars
Both contractor and engineer must be aware of special requirements
BY DAVID P. GUSTAFSON
TECHNICAL DIRECTOR
CONCRETE REINFORCING STEEL INSTITUTE
roper splicing of reinforcing bars is crucial to
the integrity of reinforced concrete. The ACI
Building Code1 states: “Splices of re i n f o rc ement shall be made only as required or permitted on the design drawings, in the specifications, or as
authorized by the engineer.” Great responsibility for design, specification and performance of splices rests with
the engineer, and only the engineer who is familiar with
the structural analysis and design stress, probable construction conditions and final conditions of service can
properly evaluate the variables to select the most efficient and economical splice method.
Three methods are used for splicing reinforcing bars:
• Lap splices
Thus, if lap splices are not permitted or are impractical to use, mechanical connections or welded splices
must be used. Mechanical connections are made with
p ro p ri e t a ry splice devices. Pe rf o rmance information
and test data should be secured directly from manufacturers of the splice devices.
The purpose of this brief article is simply to answer
some of the questions concerning welded splices. Although only welded splices are discussed here, it should
not be construed that welded splices are being advocated. Each splice method has its advantages and suitability for particular applications. For projects of all sizes,
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 welded splices
• Welded splices
The model building codes, which are the basis for
many statutory building codes, have some special requirements for welded splices. The Basic Building Code2
and the Standard Building Code3 adopt the ACI Building
Code by re f e re n c e. Except for A706 bars, the Uniform
Building Code4 prohibits welding unless the carbon
equivalent (explained below) is known. The UBC permits the building official to waive this requirement for
minor details or repairs, provided the welding procedures are the same as those for a carbon equivalent exceeding 0.75 percent.
Some key items included in the ACI Building Code requirements for welded splices are: welding must conform to AWS D1.4-79;5 in a full welded splice, as required in a “tension tie member” or in compression, the
bars have to be butted and the splice must develop at
least 125 percent of the specified yield strength, fy, of the
bar. For tension splices where the area of the bars is
twice that required by structural analysis, the splices can
be designed for less than 125 percent fy of the bar. There
are also rules for staggering the splices, and tack welding
is not permitted unless authorized by the engineer. The
basic welding requirements given in AWS D1.4-79,
“St ru c t u ral Welding Code—Reinforcing Steel,” include
P
The traditional lap splice, when it will satisfy all req u i re m e n t s, is generally the most economical splice,
and welded splices generally require the most expensive field labor. Howe ve r, lap splices cause congestion at
the splice locations, sometimes making their use impossible. The location of construction joints, provision for
future construction, and the particular method of construction 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 except
for compression only, when spliced to smaller size footing dowels.
In column design, consideration must also be given to
the fact that lapped offset bars may have to come inside
of the bars above and therefore reduce the moment arm
in bending. 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 mechanical connections or welded
splices—should be considered to reduce congestion,
and to provide for greater design moment strength of
the section at the splice locations.
permissible stresses, both for the strength design
method and the working stress method; splice details;
workmanship; filler metal (electrodes) re q 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 AWS
code?
A: Direct and indirect butt splices, and lap-welded
splices.
Q: Is special end preparation of the bars required for direct
butt splices?
A: Yes, the particular end details—V-grooves or bevels—
depend upon whether the bars will be placed in a horizontal or vertical position.
Q: What is an indirect butt splice?
A: A splice where both bars 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 is not
welded.
Q: What types of lap-welded splices are included in the
AWS code?
A: 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, preferably #5 or smaller.
Double lap joints would be preferred if eccentricity of
the splice is a consideration. In an indirect type, the bars
are welded to a common splice plate; there is a space between the bars.
Q: Where are fillet welds used?
A: An example would be connections of rebars to structural steel members, provided sufficient stru c t u ra l
strength can be achieved.
Q: What welding processes does the AWS code cover?
A: Shielded metal arc, flux cored arc, pressure gas and
thermite welding processes.
Q: What is thermite welding?
A: 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
separate 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,
preheating them. Subsequent flow completes the
process.
Q: 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
process has been successful in joining hard - t o - we l d
steels, because it welds the entire cross section at the
same time and automatically provides preheat and slow
cooling.
Q: Is thermite welding popular?
A: Not so much in recent years for splicing rebars. It
seems to be extensively used in other applications such
as continuously welded railroad rails; the suppliers of
the proprietary molds and other materials may be concentrating their attention on the other applications.
Q: The AWS code discusses filler metal (electrodes). What
kind of electrodes are required 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 welded. It is important that the coatings of low - h yd rogen-type electrodes such as Classes E70XX and E80XX be thoroughly
dry when used. For example, E80XX electrodes taken
from hermetically sealed packages must be used within
4 hours.
Weldability and carbon equivalent
Q: When the subject of welding is discussed, the term
weldability is often mentioned. What is meant by weld ability?
A: A metallurgist defines weldability in terms of the
chemical composition of the steel; his measure is carbon
equivalent content. A structural engineer probably
thinks of weldability in terms of the strength achieved at
a splice, while a welder or contractor considers it in
terms of cost, welding method required, 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 structure and to perform satisfactorily in the intended
service.”
Q: What is carbon equivalent?
A: 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 weldability, and it is a numerical value expressed
as a percent. The AWS code and the ASTM A706 rebar
specification have the same formula for C.E.
%Mn
%Cu
%Ni
%Cr
%Mo
%V
C.E. = %C + ––––– + ––––– + ––––– + ––––– - ––––– - –––––
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 weldability, i.e., molybdenum (Mo) and vanadium (V) are
subtracted. Not all of these elements are necessarily present in any given heat of steel.
Q: How is the carbon equivalent value established?
A: From the chemical analyses in the mill test reports.
Q: Are chemical analyses routinely included in mill test
reports?
A: This depends on which steel is specified. Since the
standard rebar specifications 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
percent for these bars). The chemical composition
(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 reported for A615
bars unless special complete analyses are requested.
Howe ve r, A706 reinforcing bars are intended for welding. 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 reported.
Q: How is the carbon equivalent used in the AWS code?
A: The minimum preheat and interpass tempera t u re s
are based upon carbon equivalent. 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.
Q: What preheat is required for larger values of carbon
equivalent?
A: 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.
Specifying bars for welding
Q: What are some practical points that one should con sider when specifying welded splices?
A: This depends on the size of the project and the
amount and importance of the welding. There is an excellent discussion in Reference 2.
Q: Besides requiring all welding to conform to AWS D1.479, what should be considered in a large, long term pro ject involving extensive welding in important structural
elements?
A: Consider use of A706 rebars; check availability before
specifying. Field inspection will be simplified since little
or no preheat will be required, and inspectors will not
have to keep track of differing preheat temperatures for
each bar size or bar shipment.
Q: What about projects which require some welding in
important elements?
A: Project specifications should be open to include both
A706 and A615 bars. Specify mill test reports. The contractor needs these reports to provide the required preheat. Inspectors should be present whenever welding is
done on important elements.
Q: What about small projects requiring little or only oc casional welding in non-critical areas?
A: Specify maximum preheat for an assumed carbon
equivalent greater than 0.75 percent.
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 temperature is less than 0° F.
Q: Little or no preheat is required for bars with lower
ranges of carbon equivalent; are there requirements when
the ambient temperature is low?
A: For bars which require no preheat at normal working
temperatures, if the temperature of the bars is below 32°
F, they must be preheated to at least 70 degrees F and this
temperature must be maintained during welding.
Some bars require little preheat at normal working
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 temperature below 50°F they
must be preheated so that the temperature of the cross
section of the bar within 6 inches on each side of the
joint is 50° F or greater.
Q: What is the tack welding which the ACI Code pro hibits?
A: Connection of 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 authorized by the engineer, the welds should be made in conformance with all
requirements of AWS D1.4-79. We do not recommend
this practice; wire ties should be used for assembly of
reinforcing steel.
Q: Why is tack welding a poor practice?
A: 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 resis-
tance and fatigue resistance.
2. Basic Building Code, 1981 Edition, Building Officials and
Code Administrators International, Inc., Homewood, Illinois.
Q: Are the harmful effects of tack welding well proven?
3. Standard Building Code, 1979 Edition, Southern Building
Code Congress International Inc., Birmingham, Alabama.
A: Yes, research reports are cited in Reference 7. See Reference 8 also.
Conclusions
4. Uniform Building Code, 1979 Edition, International Conference of Building Officials, Whittier, California.
It is evident from these questions and answers that
properly engineered and constructed welded splices require more considerations than a simple statement in
the contract documents, “All welded splices shall conform to ‘St ru c t u ral Welding Co d e — Re i n f o rcing Steel’
(AWS D1.4-79).” The welding code is a comprehensive
document. Howe ve r, other important items such as securing chemical properties of the rebars, field inspection, supervision, and quality control are required for a
project with welded reinforcement.
5. “Structural Welding Code—Reinforcing Steel (D1.4-79),”
American Welding Society, 2501 N.W. 7th Street, Miami,
Florida 33125.
References
1. “Building Code Requirements for Reinforced Concrete
(ACI 318-77),” American Concrete Institute, Detroit, Michigan, 1977.
9. ASTM standards referenced by number throughout text
are available from American Society for Testing and Materials, 1916 Race Street, Philadelphia, Pennsylvania 19103.
6. Rice, P. F. and Hoffman, E. S., Structural Design Guide to
the ACI Building Code, 2nd Edition, Van Nostrand Reinhold,
1979.
7. Reinforcement Anchorage and Splices, Concrete Reinforcing Steel Institute, Chicago, 1980.
8. Firth, M. and Williams, W. M., “Avoid Martensite When
Welding Rebar,” Metal Progress, April 1979, pages 38-40.
PUBLICATION#C810807
Copyright © 1981, The Aberdeen Group
All rights reserved