Torsional
CHAPTER 7: REBAR CAGES INTRODUCTION INTRODUCTION
The design of the reinforcing, or "rebar," cage for a drilled shaft is a necessary step in the engineering process. Rebar cages will be considered from two perspectives in this manual: (1) geometry of the steel necessary to resist stresses that. develop because of loads applied to the drilled shaft, which is addressed in Chapter 13, and (2) the characteristics of the cage from the perspective of constructability, which is addressed in this chapter.
A rebar cage for a drilled shaft is made up of longitudinal bars that are distributed with (usually) equal spacing around the outside of a cylinder. Transverse reinforcing is placed around and attached to the longitudinal bars, with the longitudinal and transverse steel being held together with ties, clamps, or, in special cases, with welds. Other components of a rebar cage that may be used are hoops for sizing, guides for centering the cage in the borehole and the tremie inside the cage, and stiffeners and pickup devices to aid in lifting the cage. For long cages and cages with large diameters, temporary or permanent strengthening elements should be provided to prevent permanent distortion of the cage as a result of stresses due to lifting and placing. -
Where structural requirements result in shaft cage diameters that must be different from column cage diameters, cages with sufficient lap distance can be used to transfer the load. This is not normally done when large groundline shears are expected.
The required amount of reinforcing steel to be placed in a drilled shaft must be computed carehlly from structural requirements and not selected by rule of thumb. The axial load, lateral load, and moment (taking into account the eccentricities due to accidental batter and tolerance in location) can be applied to the shaft head and the combined stresses can be computed. The placement of reinforcing steel is made in consideration of the stresses that will exist, using appropriate load factors in the computations. The buckling load for those cases where the soil is very weak or where the drilled shaft projects some distance above the groundline may also need to be considered. Buckling of a drilled shaft is not ordinarily a problem because the lateral support of the surrounding soil, even relatively weak soil, is such that the effective length of the shaft for computation of buckling is usually quite small. These issues are addressed in Chapter 13. However, when considering how the steel cage resulting from the structural computations is to be assembled and handled during construction, a number of important empirical rules
discussed in this chapter should be followed.
The assumption is made that the rebar cage is always placed in the excavation, and the concrete is then placed, during which it flows around the cage. Short rebar cages may be pushed or vibrated into fresh concrete, but such a procedure is unusual.
PROPERTIES OF STEEL
The American Society for Testing and Materials (ASTM) provides specifications for several steels that can be used for reinforcing drilled shafts. These specifications are presented in the Annual Book of ASTM Standards and are conveniently collected in Publication SP-71 of the American Concrete Institute (ACI, 1996). Most of the ASTM steels also have a designation from the American Association of State Highway and Transportation Officials (AASHTO).
The properties of steel that may be employed for building rebar cages for drilled shafts are shown in Table 7.1. The steel that is usually available is AASHTO M 3 1 (ASTM A 6 15) in either Grade 40 140 ksi (276 MPa) yield strength] or Grade 60 [60 ksi (413 MPa) yield strength]. The specifications in the table do not address the welding of the M 3 1 or M 42 steels because these bars are not to be welded in normal practice. Where the welding of the rebar cage is desirable, a weldable steel, ASTM A 706, can be specified, but availability is often limited. Galvanized or epoxy-coated steel is also available for longitudinal and transverse reinforcement for those cases where there is danger of corrosion. Epoxy-coated steel is often specified for drilled shaft rebar cages in marine environments, where the chlorides content of the ground and/or surface water is high. Alternatively, the rebar may be used without epoxy, and a dense concrete of low
permeability may be specified, as discussed in Chapter 8.
The designations of deformed bars, their weights per unit length, cross-sectional areas, and perimeters are given in Tables 7.2 and 7.3. The values shown in the tables are equivalent to those of a plain bar with the same weight per unit length as the deformed bar. Table 7.1 shows the maximum size of bar that is available for the designations of steel that are shown. Very rarely are plain bars used for the fabrication of rebar cages, and they should never be used if the cage is to be placed in a drilling slurry.
The modulus of elasticity of steel is usually taken as 199.8 GPa (29,000,000 psi). For design purposes the stress-strain curve for steel is usually assumed to be elastic-plastic, with the knee at the yield strength (Ferguson, 198 1).
LONGITUDINAL REINFORCING
The principal role of the longitudinal reinforcing steel is to resist stresses due to bending and tension. If the computed bending and tensile stresses are negligible, there may seem to be no need at all for longitudinal steel except as required by specifications. However, construction tolerances will allow nominally concentric axial loads to be applied with some amount of eccentricity, unanticipated lateral loads may occur (such as those caused by long-term lateral translation of soil), and the top portion of any drilled shafl will need to act as a short column if there is any axial load. Therefore, it is good practice to provide at least some amount of longitudinal steel reinforcing in all drilled shafts for bridge foundations.
In virtually all designs, the steel requirements will be maximum near the groundline and will
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diminish rapidly with depth. Therefore, the maximum number of longitudinal bars will be required at the top of a drilled shaft. Some of the bars can be eliminated, or "cut off," as depth increases. In some of the methods of construction, as noted in the last section of this chapter, a short rebar cage can be sometimes be used near the top of the drilled shaft, leaving the bottom unreinforced. In the casing method of construction, however, the cage should be able to stand alone on the bottom of the borehole during the placement of the concrete; thus, some of the longitudinal bars must extend over the h l l length of the shaft.
Table 7.1. Properties of reinforcing steel for concrete reinforcement
Table 7.2. Weights and dimensions of deformed bars (Customary) Max.
Bar Size
55 M
(No. 18)
35 M (No. 11)
55 M
(No. 18)
*
ASTM Designation
A 615
A 616
A 706
Bar No.
3 4 5 6 7 8 9 10 1 1 14 18
Weldable?
No
No
Yes AASHTO
No.
M 31
M 42
-
Weight,
Nlm (lblft)
5.49 (0.376) 9.75 (0.668) 15.22 (1.043) 2 1.92 (1.502) 29.83 (2.044) 38.97 (2.670) 49.63 (3.400) 62.91 (4.303) 77.55 (5.3 13)
1 1 1.7 (7.650) 198.5 (13.60)
Description
Deformed and plain billet-steel bars Deformed and plain rail-steel bars Deformed low-alloy steel bars
Diameter,
mm (in.)
9.53 (0.375) 12.7 (0.500) 15.9 (0.625) 19.1 (0.750) 22.2 (0.875) 25.4 (1,000) 28.7 (1.128) 32.3 (1.270) 35.8 (1.410) 43 .O (1.693) 57.3 (2.257)
Yield Strength, MPa (ksi)
276 (40) 413 (60)
345 (50) 413 (60)
413 (60)
Cross- Sectional
Area, mm2 ( i n 3
71.3 (0.11) 126.7 (0.20) 198.6 (0.3 1) 286.5 (0.44) 387.1 (0.60) 506.7 (0.79) 646.9 (1.00) 8 19.4 (1.27) 1006 (1.56) 1452 (2.25) 2579 (4.00)
Perimeter, mm (in.)
29.9 (1.178) 39.9 (1.571) 49.9 (1.963) 59.8 (2.356) 69.8 (2.749) 79.8 (3.142) 90.0 (3.544) 10 1 (3.990) 1 13 (4.430) 135 (5.320) 180 (7.090)
Table 7.3. Weights and dimensions of deformed bars (Metric)
Deformed bars are invariably selected for the reinforcement even though there could be some loss of bond in the slurry method of construction. As the concrete rises to displace the slurry around the rebar steel, there is a possibility that some of the bentonite or polymer will be trapped under the deformations. As discussed in Chapter 6, there is no evidence at present to indicate that any loss of bond that may occur because of such action presents a problem if the slurry meets appropriate specifications at the time the concrete is poured.
Bar No.
10M 15M 20M 25M 30M 35M 45M 55M
It is conceptually possible to vary the spacing of the longitudinal bars and to orient the cage in a specific direction in the case where the main forces causing bending have a preferential direction.
However, the savings that would be gained by such a procedure might be more than offset by the delays that would be inevitable in the inspection and construction. Therefore, the longitudinal bars are recommended to be spaced equally around the cage, except in cases where there are compelling reasons for nonsymmetrical spacing. The minimum number of bars in a symmetrical cage should be five or six so that the bending resistance be virtually equal in any direction. A view of the longitudinal steel in a rebar cage that is being assembled on a job site is shown in Figure 7.1.
The No. 8 bar is usually the minimum size of the longitudinal steel in a drilled shaft. The minimum spacing between longitudinal bars (and between transverse bars or spiral loops, as well) must be sufficient to allow free passage of the concrete through the cage and into the space between the cage and the borehole wall without resorting to vibrating the concrete. Various authorities recommend that the minimum clear space between bars range from three to five times the size of the largest of the coarse aggregate in the concrete mix. Although this spacing is somewhat dependent upon other characteristics of the fluid concrete mix, a good rule to follow is to use a minimum spacing of five times the size of the largest coarse aggregate in the mix or 76 rnm (3 in.), whichever is larger. The bar size that is selected for the longitudinal steel must be such that the proper clear spacing between bars is maintained. If a very large amount of reinforcing steel is required, two rebar cages, one inside the other, may be required.
Weight, kglm (Iblft)
0.784 (0.526) 1.568 (1.052) 2.352 (1 378) 3.920 (2.629) 5.488 (3.681) 7.840 (5.259) 1 1.76 (7.888) 19.60 (13.15)
Diameter, mm (in.)
1 1.3 (0.455) 16.0 (0.630) 19.5 (0.768) 25.2 (0.992) 29.9 (1.177) 35.7 (1.406) 43.7 (1.720) 56.4 (2.220)
Cross- Sectional Area, mm'
( i n 3 100 (0.155) 200 (0.3 10) 300 (0.466) 500 (0.777) 700 (1.088) 1000 (1.554) 1500 (2.332) 2500 (3.886)
Corresponding Customary Bar Designation (Approximate)
NO. 3
-
NO. 4No. 5 No. 6 No. 8 NO. 9 -NO. 10
No. 11 No. 14 No. 18
Figure 7.1. View of a rebar cage being assembled, showing longitudinal steel
In some instances, two or three bars can be clustered, or "bundled," together in order to increase the steel percentage while maintaining a cage with appropriate rebar spacing. Bundling of bars does not degrade the bond between the steel and the concrete by trapping bleed water or slurry, as long as the bars are vertical. A photograph of a cage with bundles of two No. 18 bars is shown in Figure 7.2.
Figure 7.2. View of bundles of No. 18 rebar in a drilled shaft cage
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