Procedimientos metodológicos

6.2.4.1 Terms relating to bearings for precast units The following terms relate to bearings for precast units:

a) simple bearing: a supported unit bears directly on a support, the effect of projecting steel or added concrete being discounted;

b) dry bearing: a bearing with no intermediate padding material;

c) bedded bearing: a bearing with contact surfaces that have an intermediate padding of cementitious material;

d) non-isolated unit: a supported unit that, in the event of loss of an assumed support, would be capable of carrying its load by transverse distribution to adjacent units;

e) bearing length: the length of support, supported unit or intermediate padding material (whichever is the least) measured along the line of support (see figure 27); and

f) bearing width: the overlap of support and supported unit, measured at right angles to the line of support (see figure 27).

6.2.4.2 Concrete corbels

6.2.4.2.1 A corbel is a short cantilever beam in which the principal load is so applied that the distance a_v between the line of action of the load and the face of the supporting element is less than d (where d is the effective depth of the corbel at the face of the supporting element), and the depth at the outer end of the beam is at least one-half of the depth at the face of the supporting element.

6.2.4.2.2 Determine the depth at the face of the supporting element from shear conditions in accordance with 4.3.4.2 but limit a_v as specified above.

6.2.4.2.3 Design the main tension reinforcement in a corbel and check the strength of the corbel on the assumption that it behaves as a simple strut-and-tie system. Ensure that the reinforcement so obtained is at least 0,4 % of the section at the face of the supporting element and is adequately anchored. At the front face of the corbel, anchor the reinforcement either by welding to a transverse bar of equal strength or by bending the bars backwards to form a loop; in the latter case, ensure that the bearing area of the load does not project beyond the straight portion of the bars forming the main tension reinforcement.

6.2.4.2.4 When the corbel is designed to resist a stated horizontal force, provide additional reinforcement to transmit this force in its entirety; weld the reinforcement to the bearing plate and anchor it adequately within the supporting element.

6.2.4.2.5 Provide shear reinforcement in the form of horizontal links distributed in the upper two-thirds of the effective depth of the corbel at the column face; ensure that this reinforcement is at least one-half of the area of the main tension reinforcement, and anchor it adequately.

6.2.4.2.6 Corbels should be designed for shrinkage and temperature stresses.

6.2.4.3 Continuous concrete nibs

Where a continuous nib less than 300 mm deep provides a bearing, as on a boot lintel, design the nib as a short cantilever slab in accordance with the provisions given below:

6.2.4.3.1 Ensure that the projection of the nib is sufficient to provide an adequate bearing width for the type of unit to be supported (see 6.2.4.4). Give the reinforcement in the nib and any reinforcement in the supported unit a minimum nominal overlap in plan of 60 mm.

6.2.4.3.2 Assume the line of action of the design load to occur at the outer edge of the loaded area, i.e. at the front edge of the nib, or at the beginning of the chamfered edge, or at the outer edge of the bearing pad, as appropriate.

6.2.4.3.3 Take the maximum design bending moment as the distance from the line of action of the load to the nearest vertical leg of the links in the beam element from which the nib projects, times the load. (Ensure that the tension reinforcement in the nib is at least that required by 4.11.4, and anchor the reinforcement adequately.)

6.2.4.3.4 Extend the tension reinforcement (the area of the reinforcement being not more than that given in 4.11.5) as near to the front face of the nib as considerations of adequate cover will allow, and anchor it there, either by welding to a transverse bar of equal strength or by bending the bars through 180° to form loops in the horizontal or vertical plane (ensure that vertical loops are of a bar diameter not exceeding 12 mm).

6.2.4.3.5 Provide links in the element from which the nib projects. The links should be capable of transmitting (in addition to any other forces they resist) the load from the nib to the compression zone of the element.

6.2.4.4 Bearings for precast units 6.2.4.4.1 General

Ensure that the bearing width (see 6.2.4.1(f)) of precast units is sufficient to provide a) a proper anchorage of the tension reinforcement (see 4.11.7), and

b) a proper restraint against loss of bearing through movement. Do not use direct bearing connections as column/column or wall/wall connections, either with or without flexible padding.

6.2.4.4.2 Calculation of net bearing width

For non-isolated units (see 6.2.4.1(d)), the net bearing width should be the greater of 40 mm and the value calculated from the equation:

net bearing width = design ultimate support reaction per unit (design effective bearing length x design ultimate bearing stress)

where the design effective bearing length is as in 6.2.4.4.3 and the design ultimate bearing stress is as in 6.2.4.4.4. For isolated units, the net bearing width should exceed that of non-isolated units (see 6.2.4.1(d)) by 20 mm.

6.2.4.4.3 Design effective bearing length

In the equation given in 6.2.4.4.2, the effective bearing length is the least of a) bearing length per element,

b) one-half of bearing length per element plus 100 mm, and c) 600 mm.

6.2.4.4.4 Design ultimate bearing stress

The design ultimate bearing stress is based on the weaker of the bearing surfaces and has the following value:

a) for dry bearing on concrete: 0,4 fcu (an allowance for γm included);

b) for bedded bearing on concrete: 0,6 fcu (an allowance for γ_m included);

c) for the concrete face of a steel bearing plate cast into a unit or support and not exceeding 40 % of the bearing length: 0,8 f_cu (an allowance for γ_m included).

Bearings using flexible padding may be designed using stresses intermediate between those for dry and for bedded bearings.

6.2.4.5 Spalling at supports

The outer edges of the concrete interface of precast units and the bearings are subject to spalling.

Chamfers occurring within areas subject to spalling may be ignored when the outer edge of a supporting unit or the end of a supported unit is being determined (see figure 27). The recommendations for allowances for effects of spalling at supports and at the end edges of supported units are given below.

6.2.4.5.1 The distances to be assumed ineffective as bearing surfaces for the outer edges of supports in relation to the material of the support:

a) steel: nil;

b) concrete grade 30 or higher, plain or reinforced: 15 mm;

c) brickwork or masonry: 25 mm;

d) concrete of a grade lower than grade 30, plain or reinforced: 25 mm;

e) reinforced concrete less than 300 mm deep at the outer edge: not less than the nominal cover to reinforcement on the outer face of the support; and

f) reinforced concrete where vertical-loop reinforcement exceeds 12 mm diameter: nominal end cover plus inner radius of bend. Where unusual spalling characteristics are known to apply when particular constituent materials are being used, adjustment should be made to the distances recommended.

Figure 27 — Schematic arrangement of allowance for bearing

6.2.4.5.2 The distances to be assumed ineffective as bearing surfaces for the end edges of supported units in relation to the reinforcement at bearing of the supported unit:

a) straight bars, horizontal loops or vertical loops not exceeding 12 mm in diameter, close to end of element: the greater of 10 mm or cover;

b) tendons or straight bars exposed at end of element: nil; and

c) vertical-loop reinforcement of bar size exceeding 12 mm: nominal end cover plus inner radius of bend.

6.2.4.6 Allowance for construction inaccuracies

The allowance for construction inaccuracies should cover deviations that can occur during the assembling of components, site construction, manufacture and erection, and may be assessed from a statistical analysis of measured or predicted deviation. Alternatively, for supported members of span up to 15 m and with average standards of accuracy, the allowance may be taken as the greatest of:

a) 15 mm, or 3 mm per metre of distance between the faces of steel or precast concrete supports;

b) 20 mm, or 4 mm per metre of distance between the faces of masonry supports; and c) 25 mm, or 5 mm per metre of distance between the faces of in-situ concrete supports.

6.2.4.7 Horizontal forces or rotation at a bearing

The presence of horizontal forces at a bearing can reduce the load-carrying capacity of the supporting unit considerably by causing premature splitting or shearing. These forces may be due to creep, shrinkage, and temperature effects, or may result from misalignment, lack of plumb or other causes.

When they are likely to be significant, consider these forces in designing and detailing the joints by providing

a) either sliding bearings or suitable lateral reinforcement in the top of the supporting unit, and b) continuity reinforcement to tie together the ends of the supported units.

Where, owing to large spans or other reasons, large rotations are likely to occur at the end supports of flexural units, use bearings that are capable of accommodating these rotations.