SHAPE CODE HOOKS AND STIRRUPS SHEET NO. I.27 (BASED ON ACI CODES)

SHAPE CODE HOOKS AND STIRRUPS SHEET NO. I.27 (contd) (BASED ON ACI CODES)

I.9.3.6. Stirrup anchorage

There are several permissible methods for stirrup anchorage The most common is to use one of the hooks shown in Sheet No. I.27. Types S1 to S6 illustrate not only the uses of the two types of hooks, but also the directions in which the hooks can be turned. In detailing the anchorage, care must be taken that the ends of stirrup hooks that are turned outward into shallow slabs have adequate cover. If not, the hooks should be turned inward and this change brought to the A/E’s attention.

Where the free ends of stirrups cannot be tied to longitudinal bars, or where there are no longitudinal bars, stirrup support bars should be specified by the A/E.

I.9.3.7. Standard bar bends

To list the various types of bent bars in a schedule it is necessary to have diagrams of the bars with the lengths of the portions of the bars designated by letters. A chart of such standard bar bends is shown in Sheet No. I.27.

Dimensions given for Hooks A and G are the additional length of bar allowed for the hook as shown in Sheet No. I.27. For straight portions of the bar, the distance is measured to the theoretical intersection of the outside edge line extended to the outside edge line of the adjacent straight portion, or to the point of tangency to a curve, from which point the length of the latter is tabulated, as in Types 10 and 11. Truss bar dimensioning is special and is shown in large-scale detail.

I.9.3.8. Radius bending

When reinforcing bars are used around curved surfaces, such as domes or tanks, and no special requirement is established in the contract documents, bars prefabricated to a radius equal or less than those in Table I.11 are prefabricated by the reinforcing bar fabricator. In the smaller sizes, the bars are sprung to fit varying job conditions, such as location of splices, vertical bars, jack rods, window openings and other blocked out areas in the forms. The larger size bars, which are more difficult to spring into desired position, are

Table I.11. When radial prefabrication is required

Bars are to be prefabricated when either radius or bar length is less than tabulated value

Bar size no. Radius: ft (mm) Bar length: ft (mm

3 (10) 5 (1500) 10 (3000)

ordinarily employed in massive structures where placing tolerances are correspondingly larger. Table I.11 shows parameters for radial fabrication.

Radially prefabricated bars of any size tend to relax the radius originally prefabricated as a result of time and normal handling. The last few feet involved in the lap splice area often appear as a tangent rather than a pure arc, due to limitations of standard bending equipment. For these reasons, final adjustments are a field placing problem to suit conditions and tolerance requirements of a particular job. See Figures 8 and 9 for radial tolerances and Section 4.2(c)3 of the code. Bars requiring a larger radius or length than shown in Table I.11 are sprung in the field without prefabrication.

The presence of the tangent end does not create any problem on bar sizes No. 3 through 11 (No. 10 through 36) as they are generally lap spliced and tangent ends are acceptable. No. 14 and 18 (No. 43 and 57) bars cannot be lap spliced, however, and are usually spliced using a proprietary mechanical splice or a butt weld. It is a problem to place a radially bent bar when using a mechanical splice sleeve because of the tangent ends on bars bent to small radii. To avoid this problem, all No. 14 and 18 (No. 43 and 57) bars bent to a radius of 20 ft (6000 mm) or less should be furnished with an additional 18 in. (450 mm) added to each end. This 18 in. (450 mm) tangent end is to be removed in the field by flame cutting. Bars bent to radii greater than 20 ft (6000 mm) will be furnished to the detailed length with no consideration given to the tangent end. The ends of these bars generally are saw cut.

Shop removal of tangent ends can be made by special arrangement with the reinforcing bar supplier.

I.9.3.9. Slants To determine the length of the straight bar necessary to form a truss bar, the length of the slant portion of the bar must be known. The standard angle is 45°

for truss bars, with any other angles being special. Slants and increments are calculated to the closest 1/2 in. (10 mm) so that for truss bars with two slants, the total increment will be in full inches (25 mm). This makes the computation easier and is within the tolerances permitted. It is important to note that when the height of the truss is too small, 45° bends become impossible. This condition requires bending at a lesser angle and lengthens the slant portion.

I.9.3.10. Tolerances There are established, standard industry fabricating tolerances that apply unless otherwise shown in the project specifications or structural drawings.

Sheet No. I.27 define these tolerances for the standard bar bends shown in Sheet No. I.27. Note that tolerances more restrictive than these may be subject to an extra charge. For further tolerance information. see ACI 117.

II. Reinforced concrete beams and slabs

II.1. Reinforced concrete beams

Beams are structural elements carrying external loads that cause bending moments, shear forces and torsional moments along their length. The beams can be singly or doubly reinforced and can be simply supported, fixed or continuous. The structural details of such beams must resist bending, diagonal tension, shear and torsion and must be such as to transmit forces through a bond without causing internal cracking. The detailer must be able to optimize the behaviour of the beams under load. He must liaise with the structural engineer on the choice of structural details needed for particular conditions.

The shapes of the beam can be square, rectangular, flanged or tee (T).

Although it is more economical to use concrete in compression, it is not always possible to obtain an adequate sectional area of concrete owing to restrictions imposed on the size of the beam (such as restrictive head room).

The flexural capacity of the beam is increased by providing compression reinforcement in the compression zone of the beam which acts with tensile reinforcement. It is then called a doubly reinforced concrete beam. As beams usually support slabs, it is possible to make use of the slab as part of a T-beam.

In this case the slab is generally not doubly reinforced.

Where beams are carried over a series of supports, they are called continuous beams. A simple beam bends under a load and a maximum positive bending moment exists at the centre of the beam. The bottom of the beam which is in tension is reinforced. The bars are cut off where bending moments and shear forces allow it. This aspect was discussed in Section I. In a continuous beam the sag at the centre of the beam is coupled with the hog at the support. A negative bending moment exists at the support. Where a positive moment changes to a negative moment, a point of contraflexure or inflection occurs at which the bending moment is zero. An adequate structural detailing is required to cater for these changes. Again this aspect is discussed in Section I. The reinforcement bars and their cut-off must follow the final shape of the final bending moment diagram.

Where beams, either straight or curved, are subjected to inplane loading, they are subjected to torsional moments in addition to flexural bending and shear. The shape of such a moment must be carefully studied prior to detailing of reinforcement. The codes including BS 8110 give a comprehensive treatment on the provision of shear reinforcement, namely links and bent bars.

Again, whether the beams are simply supported, rigid or continuous the shear force diagram will give a proper assessment of the number and spacing of such bars.

In circumstances where the bars are given lap lengths, they must be in line with the provisions of a code. As discussed in Section I, all bars are checked for bond using standard formulae, so that it should be possible to transfer stresses from one material to the other. The structural detailing of reinforcing bars must prevent relative movement or slip between them and the concrete.

As discussed earlier, the increased compressive area of concrete obtained by using a T-beam is not available at the support. Over the support, the

compression zone lies below the neutral axis. In order to strengthen the beam at the support a greater depth with a haunch is provided. The beam will have a different section at the support from that at the centre. Special care is needed to design and detail such a beam.

II.1.1. Detailing based on British codes and practices

Since beams are reinforced in the longitudinal direction against bending, Sheet No. II.1(a) shows structural detailing of simply supported reinforced concrete beams for light loading (II.1(a)(i)) and for heavy loading (II.1(a)(iii)) together with an isometric view (II.l(a)(ii)) indicating how main bars and links are placed. The reinforcement layouts are self-explanatory, for example under II.1(a)(i), 28R110-03-175 means 28 numbers of round 10 mm diameter mild steel bars of identification number 3 are placed at 175 mm centre to centre. All such bar sizes and spacings are determined from the loading and secondary conditions such as fire and corrosion. Where down-stand and upstand beams in construction become necessary, an optimum reinforcement layout should be devised. One such layout is shown in Sheet No. II.1(b). A typical doubly reinforced concrete beam layout is given in II.1(c)(i) and II.1(c)(ii).

Sheet No. II.2 demonstrates how links and bent bars are placed in relation to main reinforcement. The examples chosen are for rectangular, L- and T-beams with a single system of links. A double system of links is specifically included in II.2(b)(iv). Straight and inclined bars for resisting shear are detailed under II.2(c).

Sheet No. II.3(a) gives reinforcement layouts for both inverted and upright T-beams. A composite bending moment diagram is given in II.3(b)(i) with cut-off positions along with two types of reinforcement layouts. A single continuous beam is shown in II.3(b)(ii). A continuous beam with bent bars in a frame is detailed with several cross-sections in II.3(c). Continuous beams with slabs and columns are detailed separately in Section IV.

Sheet No. II.4 shows some details of interconnected beams with and without holes and shear bars.

(a) Beam grid (Sheet No. II.4). Cases (i) and (ii) show the layout and a typical detailing of primary and secondary beams. Main reinforcement, shear links or stirrups and connecting U-bars are clearly indicated.

(b) Beam monolithic with a wall. When a beam is monolithic with a wall, the minimum lap or bond length of a hook shall be 0·1 times beam length or 45 times the diameter of the bar. The total steel area of the top bars with hooks shall not be less than half of the total area of main steel. This is shown in case (b) on Sheet No. II.4.

(c) Cantilever beams. A cantilever beam shall be reinforced in a manner shown in case (c) on Sheet No. II.4. Again, the top hooked bars of total steel area A_sshall have a bond length not less than half of the effective span length. Where bars are extended beyond 0·5 times length or 45 times diameter, the area of steel shall not be less than half the steel area A_s.

(d) Holes in a beam. There are several ways of reinforcing holes in a beam.

The most well known are the square and the orthogonal layouts which are shown in case (d) on Sheet No. II.4. In all cases the bond length beyond the hole shall not be less than 45 times the diameter of the bar.

(e) Bent-up bars. Sometimes shear links and their shear amount cannot resist enormous shear forces. The bent-up bars are introduced to resist these shear forces. A typical layout is shown in case (e) on Sheet No. II.4.

In document Structural Detailing in Concrete (Page 71-79)