some notes on...
Det etaili aili ng an d P lacing Reinfor Reinf or cing ci ng Ba r s B Y PAUL F. RICE TECHNICAL D IRECTOR CONCRETE REINFORCING STEEL I NSTITUTE
T
he 1963 ACI Code for the first time provides specifically for new steels, lightweight aggreg aggregates, struc structural systems, special large bars, and design methods. Many of the previous specifications were updated so that they will be valid throughout the whole range of application for the new code. Designers will, in certain instances, have to provide more complete information for detailing. New books on rei re inforced concrete design and details based upon the 1963 ACI Code will soon appear. The Concrete Reinforcing Steel Institute is now preparing a two-volume De sign Handbook for working w orking stre ss design and ultimate strength design. This article will consider only the problems of details for the new high strength steels and the special bars. Proper details have always been important to designers and contrac contractors, but we have been taking details for granted. A routine design or specification note that “de“d etails shall conform to the ACI Detailing Ma nual” and a routine sort of inspection have usually been sufficient. Past codes, even as late as 1956, presented us with rul rulesof-thumb for reinf orced concrete details, proven by years of successful successful use with with steel stress limited limited to 20,000 psi in combination with low working stresses in the concret rete. New materials and methods of design and construcconstruction have been developed in the past 10 years that required new code provi provisions. This has been parti particularl y y true of rei reinforcing steels. Figure 1 gives the important prope perties of new types of rei reinforcement rec recognized in the 1963 Cod Code. How will the utilization of these new steels affect designs? The 1963 ACI Code separates design into two metho hods, ultimate strength design (USD) and worki working stress design (WS (WSD). Working rkin g st ress design is the conventio conve ntional nal method . With ordi ordinary ary concret concrete, any steel stee l in standard s tandard size bars b ars (up to #11), and a working stress stress of 24,000 psi or less, our previous design proc procedure is essentially unchanged except for shear and bond. Our u sual standard details apply except for splices, end anchorag anchora ge, and edge beams with torsion. Even under the 1956 1956 ACI Code there was a
need f or special s pecial details when 75,0 00 psi yield stre ngth bars were were used in columns, and all lap splices of special #14S or #18S w ere not covered. Tension lap splices of #14S and #18S b ars are now prohibite d entire l y, and an d welded or mechanical splices splices are req required. Such splices are also pref preferred for compression, although bars not required to carry tension may be spliced by end-bearin end-beari ng. The detailed provisions must be shown s hown on design dra wings and in specifications and never left to the detailer. Ultimate strength design was first recognized in the 1956 Cod Code, but no specific proc procedures for design of connections and de tails were included. Thi s deficiency is remedied for the most part in the 1963 ACI Cod Code. In order to utilize properly yield points above 40,000 psi the new code provides for more than prop proportionately longer splices and anchorag anchorages. See Fig Figure 2. The designer is almost compelled, in any cast-in-place construction, to stagger splices. He is encouraged to enclose splices by closed ties or spirals. He is required to use mechanical connections or to weld all tension splices of #14S and #18S bars. The 1963 Code states that splices in tension in flexural members must be “fully deve loped” and shall transfer the entire stress from bar to bar. This ambiguity is a holdover fr om the 1956 AC I Co de. Welded s plices and mechanical connections must develop a definite 125 percent of the full specified yield point of the b ars or an additional 25 percent bar area must be provi prov ided. Althou gh va rious lesser les ser laps have been common commo n in practice under WSD, welded wire fabric figured at 60,000 psi under USD must be lapped a full mesh plus 2 inches plus overh overha angs. Thus, anchorage of the end cros cross- wi wire is at least one-half mesh plus 2 inches beyond the most critical crack. Fig Figure 3. Plain bars requi re double the embedment length
FIGURE I— NEW TYPES OF REINFORCEM ENT IN THE 19 63 CODE ASTM
Size
Specifi- Bars cation *
A432
A408
#3 thru #18S
Specified yield f y (ksi)
60
f s, WSD
f y , USD
Tension Compression Tension Compression
24
24
60
60
18 20 20
13.2 16 20
33 40 50
33 40 50
24
—
—
—
30(2)
— 60
—
30
75(3)
75(4)
24
60
60
30(2)
#14S Str. 33 # 18S Inter. 40 Hard. 50
A185(1) Welded wire varies fabric to 1/2” dia. A431
1963 Code Stresses ksi
ASTM
#3 thru #18s
75
A61 #3 thru (Rail) # 11
60
24 30(2) 24 30(2)
(1) When c ross wires a re not more tha n six ga uge numb er sma ller in size tha n main wires a nd spa ced no t more than 12 inches, w elded w ire fab ric ma y be used a s eq uivalent to deformed ba rs in slabs. (2) For sizes 3/8 inch d iameter or less in one-w ay s labs with spa ns no t more than 12 feet o nly. (3) Only when justified b y a n ac ce pted full-sca le loa d tes t. (4) Only when strain is not more than 0.003 at a proof stress equal to specified fy. Elsew here, the fy for des ign sha ll be 63.75 hsi. General * If reinforcing ba rs are to b e we lded, thes e ASTM Spec s mus t be s upplemented by requirements to assure weldability in conformance with AWS D12.1-61. ** In computing strength capacity of various reinforced concrete members appropriate reduction fac tors are req uired; com puted steel stresses at ultimate design load w ill conseq uently be reduced .
specified for deformed bars. In addition all e nds must be hooked (except for temperature or bottom bars at an intermediate support). A plain bar tension splice for practical reasons, therefore, will seldom be seen; in fact, plain bars are just practically never used except as s lip dowels or temperature bars. The details of bar fabrication are no longer “standard” for all designs. For convenience, the new code consolidates all provisions relating to fabricating and placing bars into one chapter. Only minimum bend radii are prescribed. These radii represent the minimum that can be expected by cold bending in any recognized grade of steel. See Figure 4. Please note that the ASTM specifications do not require any bend tests for #14S and #18S, special large size bars; nor for rail steel.
Exceptions are permitted for tighter bends (1) of stirrups and ties, where bar size will usually be less than #5 and where the inside of the bend usually bears on the longitudinal steel, and (2) of structural and intermediate gr ades onl y, in #6 to #11 sizes. The latter are commonly needed at ends of shallow slabs or joist cons truction. If the design utilizes higher than 40,000 psi yield and tighter than minimum bending is necessary in some p articular location, it is now the responsibility of the engineer to check: (1) that the detail is specially noted, that the job specification permits hot bending, and that the steel is not injured by impro pe r heating and cooling pro c ed ures; (2) that the concrete strength is sufficient to prevent crushing; and (3) that splitting is prevented. The designer and contractor should clearly understand that such bending may be i mpossible without h eat and th at hot bending is special and costly. Unless these points are appreciated and suitable provisions made, extra charges are inevitable. Crushing inside a bend of too small a radius should be considered. Most of us have never even thought of this possibility or much less likely analyzed it. With the standard size bars and minimum radii as shown, 40,000 psi steel and the usual concrete stre ngt hs, say 3,000 psi, we know from experience that no check on crushing is needed. In USD, particularly where high stre ngt h steels are utilized, we may find the minimum radii too tight for low strength concretes. The trend toward higher concrete strengths may help to avoid difficulty, b ut the conservati ve approach would be not to exceed the pro portionate crushing stresses we have found satisf ac tory. See Figu re 5. This diagram is rep roduced from discussions p ublished followi ng the 1956 ACI Code adoption and illustrates a simplified two-dimensional analytic approach suitable to determine only the relative crushing
FIGURE 2 a. COMPRESSION LAP SPLICE LENGTHS (For vertical bars in which the critical design stress is compressive in spiral or tied columns) Minimum Lap Lengths (in Bar Dia.) (For f’c = 3,000 psi*) Steel yield, ksi Bars #5—#11 Bars #14S & # 18S
f y =50 f y =60 f y =75 20 D 24 D 30 D Lap Slices Not Recommended
* When f’ c is less than 3,000 psi, increase all lap lengths show n by one-third.
FIGURE 2b.—MINIMUM TENSION LAP SPLICE LENGTHS* ( Applicable to deformed A 305 bars; f’c=3,000 psi; minimum cover and spacing 6 in. or 6 bar dia.)** Top Bars
Other Bars
Bar Size
f s = 20 f y = 40
f s = 24 (WSD)
f y = 60 (USD)
f s = 20 f y = 40
fs= 24 (WSD)
f y = 60 (USD)
#2 (plain) #3 #4 #5 #6 #7 #8 #9 #10 #11
12” 12” 12” 15” 21” 28” 36” 46” 58” 72”
18” 14” 18” 23” 27” 33” 43” 55” 70” 86”
18” 14” 18” 23” 31” 42” 55” 70” 88” 109”
12” 12” 12” 15” 18” 21” 27” 34” 43” 53”
18” 14” 18” 23” 27” 32” 36” 41” 49” 60”
18” 14” 18” 23” 27” 32” 38” 48” 61” 75”
#14S—# 18S
Lap splices not permitted.
* Minimums 12 in. or For f s = 20 ksi f y = 40 ksi For f s = 24 ksi
**(a) For plain bars (#2) minimum laps are doubled. Top ba rs,
L = 35.9 D2 > 24D
Other bars, L = 26.5 D2 > 24D Top ba rs,
L = 43.1 D2 > 36D
Other ba rs, L = 31.8 D2 > 36D For fy = 60 ksi
Top ba rs,
L = 54.5 D2 > 36D
Other ba rs, L = 37.8 D2 > 36D
stresses. It neglects transverse stress effects and so gives no indication of the induced tension tending to split the surrounding concrete. Nume rous recent tests show that our modern deformed bar cannot develop its full bond value unless the concrete is prevented from splitting. At all lap splices, h ook s, end anchorages in limited thickness concre t e sections, and any other tight spots, a conservative designer will consider using special ties, stirrups, spirals, or transverse reinforcement. Field welded connections (including arc and thermit welding) will be more commonly use d especially with the growing use of large bars, high strength steels, and p recast concrete. (For common details see Figure 6). Several patented mechanical connectors are on the market—consisting, in general, of high strength steel sleeves with serrated interiors, fitted loosely on the bar and filled with nonferrous metal or with wedging type sleeves with toothed interi or s. End - b ea ring connections for pure compression splices are acceptable and more economical than the fully developed (125 percent) tension-compression weld or connection. Note that t he end preparation of these bars re q uires a saw-cut. An ordin ary s he ared end will not provide adequate bearing. Pilot tests indicate no significant loss in compressive capacity
(b) For f’ c other than 3,000 psi, multiply laps shown by
√3,000 f’ c (c) Exce pt in co lumns with sp ira ls or extra ties. For la p with cover or spa cing less than 6 inches or 6D, add 20 percent to a ll tabulated values. f’ c but lap not less than minimums at left.
Figure 3—WELDED WIRE FABRIC
S plic e for 1/2 or les s tha n 1/2 permiss ible stres s
with a deviation up to 2 degrees at the contact surfaces of the two bars. A 2 degree tolerance on the ends of the individual bars is recommended. Field adjustments such as rotating the bar may be necessary to ensure a close contact. In the application of these purely compressive column splices, it is essential that splices be so staggered to maintain a symmetrical arrangement of un-
FIGURE 4.—MINIMUM RADII ON INSIDE OF BEND
BAR SIZE #3, #4, or #5 #6, #7, or #8 #9, #10, or #11 #14S* or #18S*
FIGURE 5. CRUSHING INSIDE BENDS OF BARS
MINIMUM R ADII 2 1/2 bar diameters 3 bar diameters 4 bar diameters 5 bar diameters
* AS TM Spe cifica tions for these sizes do not req uire a ny bend test.
Note Exceptions: 1. Stirrups and ties only—1 bar diameter. 2. #6 to #11 inclusive, structural and intermediate grades
FIGURE 6a . SCHEDULE OF PREFERRED JOINT DETAILS FOR ARCWELDED SPLICES IN REINFORCING BARS Joint Type
Recommended Bar Size
Square End wit h Angle Back up
No. 6 and Smaller
Single Vee wit h Angle Back up
5 to 9
Single Vee w it hout Backup
5 to 9
Remarks
Det ail below
Area of pressure, Ap Ap Max. local pressure, p
Double Bevel
8 to 18S
For vert icle bars
1 1 ,1 4 S, 1 8 S
Det ail at right
2r sin —d 2 2 C d — — T = — f s p = A 4 p = rd
df s df s — r= — 4r 4 p
E XAMPLE
Relative crushing stresses under ordinary past practice with f s = 10,000 psi at hook. Bar Size
8 to 18S
(b) Ass umed pa rabo lic stress distribution of loca l pressure on bend.
c = 2T sin— 2
p=
Double Vee
Sleeve
a) Po rtion o f a n inclined bar and ac ting forces.
Radius of bend to Relative Crushing Stress axis of bar* (Max.) p=
#3, #4, #5
r=3d
2,600 psi
#6, #7, #8
r=3 1 ⁄ 2d
2,250 psi
#9, #10, #11
r=41 ⁄ 2d
1,750 psi
*Add 1 ⁄ 2d to minimum radii as specified in code.
FIGURE 6c. FIGURE 6 b.
S LEEVE S P LICE FOR C OMPRES S ION ONLY
FIGURE 7. RECOMMENDED SIZES—STIRRUP & TIE HOOKS
D= 1 1/2 in. for #2, #3, #4, and #5 bars Structural and Intermediate Grades RECOMMENDED HOOKS (D= 11 ⁄ 2 in.) Bar Size d
90° Hook A or G
#2
135° J
Hook A or G
Approx. H
3
31 ⁄ 2
31 ⁄ 2
21 ⁄ 4
#3
3
33 ⁄ 4
4
21 ⁄ 2
#4
31 ⁄ 2
41 ⁄ 4
41 ⁄ 2
31 ⁄ 4
#5
41 ⁄ 2
51 ⁄ 2
5
33 ⁄ 4
Bend s a nd hooks in stirrups o r ties ma y be b ent to the diamete r of the p rincipal reinforcing b a r enclosed therein.
FIGURE 8.—ALTERNATE COLUMN TIE LAYOUTS
spliced verticals in each face of the column. The percentage of bars which may be spliced at each staggered position, usually 25 to 50 percent, must be determined by design requirements and shown on design dra wings. The code implies that the remaining unspliced bars provid e 125 percent, at working stress or ultimate load, of the maximum tension at the splice point. Ties just ab ove and be low any such splices are a precaution which prudent designers will require. Another c ritical p oint in the de-
sign of detail connections of columns is just below the supported slab or drop panel. Additional ties at the top of a column are always desirable especially where the framing beams are not balanced on four s id e s, and such tra ns verse rei nf orcement is specifically re qu ired just below the sloping portion of offset column ve rtic a l s, where no framing beam is available to confine the lateral thrust. Some of the 1963 ACI Cod e changes produce substantial economies in details. For instance
the new standard 90 degree stirrup and tie hooks with 6 bar diameter extensions (tails) have long been preferred by the steel setter and fabricator to the 1956 Code hooks which we re either 135 degrees or had 12 bar diameter tails. Figure 7. The new provisions require a separate tie corner only at alternate inner column v erticals a nd permit a 45 degree bend at the vertical bar instead of 90 degrees, which results in considerable economies. For example, in a square column with 20 vertical bars the 1956 Code required 5
ties per set; under the 1963 Code only 2 ties per set are requi red, one square tie enclosing all the bars and one special octagonal tie for alternate inner bars. Figure 8. Not only is th ere a material economy, but the conc rete placing is facilitated t hrough the clear core area with much less likelihood of honeycomb since there is plenty of space for vibration and no obstructing ties. Fi nal l y, the more elaborate our
design, the more exact our analysis, the more we should never forget the i mportance of proper bar placing. The new code for the first time specifies placing tolerances for the final position of rei nfo rcement. These to le rances we re first published in the book, CRSI Re comm end e d Practice for Placing Re i nf orcin g Bars, 1959. Bar supports do not figure in a design and they don’t come under code jurisdiction —unless the
PUBLICATION #C650005 Copyright © 1965, The Aberdeen Group All rights reserved
bar support fails its purpose. A designer or inspector checking details should not accept less than the 1963 CRSI Industry Standard for Bar Supp orts. This standard for bar supports plus much other information on proper details req uired by the 1963 ACI Code will also appear in the 1964 edition of the ACI De tail ing Manual.
