The design ultimate axial force in a plain concrete wall may be calculated on the assumption that the beams and slabs transmitting forces into it are simply supported.
6.5.3.2 Effective height of unbraced plain concrete walls
The effective height le of an unbraced plain concrete wall should be taken as follows:
a) in the case of a wall supporting at its top a roof or floor slab spanning at right angles: le = 1,5 lo
b) in the case of other walls: le = 2 lo
where lo is the clear height of the wall between lateral supports; for gable walls, lo may be measured midway between eaves and ridge.
6.5.3.3 Effective height of braced plain concrete walls
The effective height of braced plain concrete walls should be taken as follows:
a) where the lateral supports provide resistance both to lateral movement and to rotation, le equals three-quarters of the clear distance between lateral supports or twice the distance between a support and a free edge, as appropriate;
NOTE - This distance is measured vertically if the lateral supports are horizontal (e.g. floors) or horizontally if the lateral supports are vertical (e.g. other walls).
b) where the lateral supports provide resistance to lateral movement only, le equals the distance between centres of supports, or two and a half times the distance between a support and a free edge, as appropriate.
6.5.3.4 Limits of slenderness
The slenderness ratio le/h should not exceed 30, whether the wall be braced or unbraced.
6.5.3.5 Minimum transverse eccentricity of forces
Whatever the arrangements of vertical or horizontal forces, the resultant force in every plain concrete wall should be assumed to have a transverse eccentricity of the greater of at least h/20 or 20 mm. In the case of a slender wall, additional eccentricity can arise as a result of deflection under load.
Procedures allowing for this are given in 6.5.3.12 and 6.5.3.13.
6.5.3.6 Eccentricity in the plane of the wall 6.5.3.6.1 In the case of a single wall in-plane
Eccentricity due to forces may be calculated by statics alone.
6.5.3.6.2 In a case where a horizontal force is resisted by two or more parallel walls
The force should be assumed to be shared between the walls in proportion to their relative stiffnesses, provided the resultant eccentricity in any individual wall does not exceed one-third of the length of that wall.
Where the eccentricity in any individual wall is found to exceed this, the wall stiffness should be regarded as zero and an adjustment made to the forces that are assumed to be carried by the remaining wall(s).
6.5.3.6.3 In the case of a shear connection being assumed between vertical edges of adjacent walls
An appropriate elastic analysis may be made, provided the shear connection is designed to resist the design ultimate forces.
6.5.3.7 Eccentricity at right angles to the wall
6.5.3.7.1 The load transmitted to a wall by a concrete floor or roof may be assumed to act at one-third of the depth of the bearing area from the loaded face. Where there is an in-situ concrete floor on either side of the wall, the common bearing area may be assumed to be shared equally by each floor.
6.5.3.7.2 Loads may be applied to walls at eccentricities exceeding half the thickness of the wall by means of special fittings (e.g. joist hangers), provided that the adequacy of such fittings against local failure is proved by testing or other means.
6.5.3.7.3 The resultant eccentricity of the total load on a braced wall at any level may be calculated on the assumption that, immediately above a lateral support, the resultant eccentricity of all the vertical loads above that level is zero.
6.5.3.8 In-plane and transverse eccentricity of resultant force on an unbraced wall
At any level, full allowance should be made for the eccentricity of all vertical loads and the overturning moments produced by any lateral forces above that level.
6.5.3.9 Concentrated loads
When loads are purely local (as at beam bearings), they may be assumed to be immediately dispersed, provided that the local design stress under the load does not exceed 0,6fcu for concrete of grade 25 or higher, or 0,5fcu for concrete of a lower grade.
6.5.3.10 Calculation of design load per unit length
The design load per unit length nw should be assessed on the basis of a linear distribution of load along the length of the wall, with no allowance for any tensile strength.
6.5.3.11 Maximum unit axial load for short braced plain walls
The maximum design ultimate axial load per unit length of wall due to ultimate loads, nw, should satisfy the following equation:
nw < 0,3 (h - 2ex) fcu (20)
where
nw is the maximum design axial load per unit length of wall due to design ultimate loads;
h is the thickness of wall;
ex is the resultant eccentricity of load at right angles to plane of wall (see 6.5.3.5 for minimum value); and
fcu is the characteristic strength of concrete.
6.5.3.12 Maximum unit axial load for slender braced plain walls
At every section of a slender braced wall, the maximum design axial load nw should satisfy equation (20) and, additionally, the following:
nw < 0,3 (h - 1,2ex - 2ea) fcu (21)
where
nw, h, ex and fcu are as in 6.5.3.11; and
ea is the additional eccentricity due to deflections, which may be taken as le2/2 500 where le is the effective height of the wall.
6.5.3.13 Maximum unit axial load for unbraced plain walls
The maximum unit axial load at every section of an unbraced plain wall should satisfy the following two conditions:
a) nw < 0,3 (h - 2ex1) fcu b) nw < 0,3 [h - 2(ex2 + ea)] fcu where
nw, h, ea, and fcu are as in 6.5.3.11 and 6.5.3.12;
ex1 is the resultant eccentricity calculated at top of wall (see 6.5.3.7); and ex2 is the resultant eccentricity calculated at bottom of wall (see 6.5.3.7).
6.5.3.14 Shear strength
The design shear resistance of plain walls need not be checked if one of the following conditions is satisfied:
a) the horizontal design shear force is less than one-quarter of the design vertical load; or
b) the horizontal design shear force is less than that required to produce an average design shear stress of 0,45 MPa over the whole wall cross-section.
NOTE - For concrete of grades lower than grade 25 and for lightweight aggregate concrete, the figure of 0,30 MPa should be used instead of 0,45 MPa.
6.5.3.15 Cracking of concrete
Reinforcement may be needed in walls to control cracking due to flexure or thermal and hydration shrinkage (see 6.5.3.16 to 6.5.3.18). Wherever reinforcement is provided, the quantity should be:
a) for reinforcement of grade 450: at least 0,25 % of the concrete cross-sectional area; and b) for reinforcement of grade 250: at least 0,30 % of the concrete cross-sectional area.
6.5.3.16 Reinforcement in plain walls for flexure
If, at any level, a length of wall exceeding one-tenth of the total length is subjected to tensile stress resulting from in-plane eccentricity of the resultant force, vertical reinforcement may be necessary to distribute potential cracking. Reinforcement need only be provided in the area of wall found to be in tension under design service loads. It should be arranged in two layers and should comply with the spacing rules given in 4.11.8.2.
6.5.3.17 Reinforcement in plain walls to counteract cracks resulting from shrinkage and temperature
6.5.3.17.1 Plain concrete walls that exceed 2 m in length and are cast in-situ, may have to be reinforced to control cracking arising from shrinkage and temperature effects, including temperature rises caused by the heat of hydration released by the cement. Reinforcement for this purpose should be considered as follows:
a) in an external plain wall directly exposed to the weather, reinforcement should be provided in both
horizontal and vertical directions; it should consist of bars of small diameter, relatively closely spaced, with adequate cover near the exposed surface (see also 6.5.3.15);
b) in an internal wall it may only be necessary to provide reinforcement in that part of the wall where junctions with floors and beams occur, in which case it should be equally dispersed between each face (see also 6.5.3.15).
6.5.3.17.2 In general, it will not be necessary to provide reinforcement to counteract shrinkage and temperature effects in walls made of no-fines concrete.
6.5.3.18 Reinforcement around openings in plain walls Nominal reinforcement should be considered.
6.5.3.19 Deflection of plain concrete walls
The deflection in a plain concrete wall will be within acceptable limits if the preceding provisions have been conformed to and if, in the case of a cantilever shear wall, the total height of the wall does not exceed ten times its length.
7 Fire resistance 7.1 General
7.1.1 When a structural concrete element is subjected to fire, it undergoes a gradual reduction in strength and rigidity. For limit state design, therefore (as stated in 3.2.4.3), there are three conditions to be considered:
a) retention of structural strength;
b) resistance to penetration of flames; and c) resistance to heat transmission.
The first criterion is applicable to all structural elements while the other two criteria are applicable to walls and floors, which perform a separating function.
7.1.2 The requirements for fire resistance for various elements in a structure are either checked by a standard test on a specimen or satisfied by suitable choices based on the data given in this clause.
NOTE - Standard fire tests are not intended to give information on the use of an element after it has been subjected to fire.
7.1.3 The following factors influence the fire resistance of concrete structures (some of these factors cannot be taken into account quantitatively):
a) the size and shape of the element;
b) the type of concrete;
c) the type of reinforcement or tendon;
d) the protective concrete cover provided to reinforcement or tendons (see 7.1.9);
e) the load supported; and f) the conditions of restraint.
7.1.4 Concretes made with siliceous aggregates have a tendency to spall when exposed to high temperatures but this tendency can be reduced by the incorporation of supplementary reinforcement in the concrete cover. Spalling does not generally occur with either calcareous or lightweight aggregates. The insulation properties of concrete made from lightweight aggregates are superior to those of concrete made from siliceous and calcareous aggregates. Other measures that may be taken to prevent spalling from occurring are
a) a finish of plaster, vermiculite, etc., applied by hand or sprayed;
b) the provision of a false ceiling as a fire barrier; and c) the use of sacrificial tensile steel.
7.1.5 Concrete, prestressing tendons, and reinforcement show a reduction in strength at high temperatures. At about 400 °C, tendons are likely to lose about 50 % of their strength at ambient temperature and in the case of reinforcement, a similar reduction in strength occurs at about 550 °C.
7.1.6 The fire resistance of structural elements is generally determined when the element is supporting its service load, which is taken as the sum of all the nominal self-weight and imposed loads.
Tables 43 to 46 show the minimum dimensions for various elements when these loads are to be supported; any reduction in load will be reflected by an increase in fire resistance, but there are not sufficient data available to define the relationship.
7.1.7 Recent investigations have shown that the provision of end restraint against thermal expansion can substantially increase the fire resistance of a structural element. Until this aspect is more fully investigated, it is proposed that in beams and slabs so built into a structure that restraint against thermal expansion caused by fire would be provided at two opposite ends, the amount of protective cover to reinforcement and tendons be reduced to the value shown for the next lower period in tables 43 to 46. Thermal restraint can be assumed to be provided by the surrounding structure if no gaps or combustible materials exist between the structure and the ends of the floor or beam and if the surrounding structure is capable of withstanding the thermal stresses induced by the heated floor or beam.
7.1.8 In tables 43 to 50 (inclusive), the "minimum dimension" and the "minimum thickness" quoted are all recommended dimensions that are subject to the dimensional deviations given in SABS 0100-2.
7.1.9 Where plaster or sprayed fibre is used as an applied finish to elements other than the ones in tables 43 to 50, it may be assumed that the thermal insulation provided is at least equivalent to the same thickness of concrete. Such finishes can therefore be used to remedy deficiencies in cover thickness. For selected materials, the following guidance can be given with respect to allowing the use of additional protection not exceeding 25 mm in thickness as a means of providing effective cover to steel reinforcing or prestressing elements. In each case, the equivalent thickness of concrete may be replaced by the protection named.
Mortar 0,6 x concrete thickness
Gypsum plaster
Lightweight plaster 1,0 x concrete thickness; < 2 h Sprayed lightweight
insulation 2,0 x concrete thickness; > 2 h
Vermiculite slabs 1,0 x concrete thickness; < 2 h 1,5 x concrete thickness; > 2 h
(See also table 47 for the effect of soffit treatment on the fire resistance of slabs.)
7.2 Beams
7.2.1 The fire resistance of a reinforced or prestressed concrete beam depends on the amount of protective cover, consisting of concrete with or without an insulating encasement, provided to the reinforcement or tendons. It is also necessary that the beam have a minimum width to avoid failure of the concrete before the reinforcement or tendons reach the critical temperature. For I-beams, the web thickness bw of a fully exposed beam should be at least 0,5 of the minimum width stated in tables 43 and 44 for the fire resistance of various beams.
7.2.2 Typical performances are given in table 43 for reinforced concrete beams and in table 44 for prestressed concrete beams, both for siliceous aggregate concrete and for low-density aggregate concrete.
7.2.3 The average concrete cover is determined by summing the product of the cross-sectional area of each bar or tendon and the distance from the surface of the bar to the nearest relevant exposed face, and dividing the sum by the total area of these bars or tendons. Only those bars or tendons provided for the purpose of resisting tension due to ultimate loads should be considered in this calculation. When reinforcement is used in combination with tendons, its total area should be used.
7.2.4 Tables 43 and 44 give the average concrete cover required to provide the stated fire resistance, but in no case may the nominal concrete cover to any bar or tendon be less than half this value, or less than the value given for the half-hour period appropriate to that form of construction.
7.2.5 In addition, in certain cases where siliceous aggregate concrete is used, it will be necessary to consider the provision of supplementary reinforcement to hold the concrete cover in position.
7.2.6 Supplementary reinforcement will be required in those cases indicated in tables 43 and 44 where the cover to all the bars and tendons under consideration exceeds 40 mm. When used, supplementary reinforcement shall consist of expanded metal lath or a wire fabric not lighter than 0,5 kg/m2 (2 mm diameter wires at centres not exceeding 100 mm) or a continuous arrangement of links at centres not exceeding 200 mm, incorporated in the concrete cover at a distance not exceeding 20 mm from the face.
Table 43 - Fire resistance of reinforced concrete beams
1) average concrete cover to main
reinforcement . . . .
thick, with light mesh reinforcement:
1) average concrete cover to main
reinforcement . . . . c) As in (a) with vermiculite/gypsum plaster**)
or sprayed asbestos, 15 mm thick, on light mesh reinforcement securely fixed to the beam:
1) average concrete cover to main
reinforcement . . . .
1) average concrete cover to main
reinforcement . . . .
*)Supplementary reinforcement may be necessary to hold the concrete cover in position (see 7.2.6).
**)Vermiculite/gypsum plaster should have a mix ratio in the range 1,5:1 to 2:1 by volume.
7.2.7 For I-beams, the average concrete cover determined as in 7.2.3 is adjusted by multiplying it by 0,6 to allow for the additional heat transfer through the upper flange face.
Table 44 - Fire resistance of prestressed concrete beams b) As in (a) with vermiculite concrete slabs,
15 mm thick, used as permanent d) As in (a) with gypsum plaster, 15 mm
thick, with light mesh reinforcement 1) average concrete cover to tendons . . . . e) As in (a) with vermiculite/gypsum
plaster**) or sprayed asbestos, 15 mm
*)Supplementary reinforcement may be necessary to hold the concrete cover in position (see 7.2.6).
**)Vermiculite/gypsum plaster must have a mix ratio in the range 1,5:1 to 2:1 by volume.
Table 45 - Fire resis tance of rein fo rced concre te f loors (s illiceous or calcareous aggrega te)
1234567 Floor constructionMinimum dimension of concrete mm Fire resistance h 4321,510,5 a)Solid slabAverage cover to reinforcement... Depth, overall*)... 25 150 25 150 20 125 20 125 15 100 15 100 b)Cored slabs in which the cores are circular or are higher than they are wide. Not less than 50 % of the gross cross-section of the floor should be solid material
Average cover to reinforcement... Thickness under cores... Depth, overall*)...
25 50 190
25 40 175
20 40 160
20 30 140
15 25 110
15 20 100 c)Hollow box sections having one or more longitudinal cavities, which are wider than they are highAverage cover to reinforcement... Thickness of bottom flange... Depth, overall*)...
25 50 230
25 40 205
20 40 180
20 30 155
15 25 130
15 20 105 d)Ribbed floors having hollow infill blocks of clay, or inverted T- section beams with hollow infill blocks of concrete or clay. A floor in which less than 50 % of the gross cross-section is solid material shall be provided with a 15 mm plaster coating on soffit
Average cover to reinforcement... Width of rib, or beam, at soffit... Depth, overall*)...
25 125 190
25 100 175
20 90 160
20 80 140
15 70 110
15 50 100 e)Upright T-sectionsAverage bottom cover to reinforcement... Side cover to reinforcement... Least width of downstanding leg... Thickness of flange*)...
**)65 **)65 150 150
**)55 **)55 140 150
**)45 **)45 115 125
35 35 90 125
25 25 75 100
15 15 60 90 f)Inverted channel sections with radius at intersection of soffits with top of leg not exceeding depth of sectionAverage bottom cover to reinforcement... Side cover to reinforcement... Least width of each downstanding leg..... Thickness at crown*)...
**)65 **)40 75 150
**)55 **)30 70 150
**)45 **)25 60 125
35 20 50 125
25 15 40 100
15 10 30 90 g)Inverted channel sections or U-sections with radius at intersection of soffits with top of leg exceeding depth of sectionAverage bottom cover to reinforcement... Side cover to reinforcement... Least width of each downstanding leg..... Thickness at crown*)...
**)65 **)40 70 150
**)55 **)30 60 150
**)45 **)25 50 100
35 20 40 100
25 15 35 75
15 10 25 65 *)Non-combustible screeds and floor finishes may be included in these dimensions. **)Supplementary reinforcement may be necessary to hold the concrete cover in position (see 7.3).
Table 46 - Fire resis tance of pres tressed concre te f loors (s iliceous or calcareous aggrega te)
1234567 Floor constructionMinimum dimension of concrete mm Fire resistance h 4321,510,5 a)Solid slabAverage cover to tendons... Depth, overall*)... 65 150 50 150 40 125 30 125 25 100 15 90 b)Cored slabs in which the cores are circular or are higher than they are wide. Not less than 50 % of the gross cross-section of the floor should be solid material
Average cover to tendons... Thickness under cores... Depth, overall*)...
65 50 190
50 40 175
40 40 160
30 30 140
25 25 110
15 20 100 c)Hollow box sections having one or more longitudinal cavities, which are wider than they are highAverage cover to tendons... Thickness of bottom flange... Depth, overall*)...
65 50 190
50 50 205
40 40 180
30 30 155
25 25 130
15 20 105 d)Ribbed floors having hollow infill blocks of clay, or inverted T- section beams with hollow infill blocks of concrete or clay. A floor in which less than 50 % of the gross cross-section is solid material shall be provided with a 15 mm plaster coating on soffit
Average cover to tendons... Width of rib, or beam, at soffit... Depth, overall*)...
65 125 190
50 100 175
40 90 160
30 80 140
25 70 110
15 50 100 e)Upright T-sectionsAverage bottom cover to tendons... Side cover to reinforcement... Least width of downstanding leg... Thickness of flange*)...
**)100 **)100 250 150
**)85 **)85 200 150
**)65 **)65 150 125
50 50 120 125
40 40 90 100
25 25 60 90 f)Inverted channel sections with radius at intersection of soffits with top of leg not exceeding depth of sectionAverage bottom cover to tendons... Side cover to reinforcement... Least width of each downstanding leg... Thickness at crown*)...
**)100 **)50 125 150
**)85 **)45 100 150
**)65 **)35 75 125
50 25 60 125
40 20 45 100
25 15 30 90 g)Inverted channel sections or U-sections with radius at intersection of soffits with top of leg exceeding depth of sectionAverage bottom cover to tendons... Side cover to reinforcement... Least width of each downstanding leg... Thickness at crown*)...
**)100 **)50 110 150
**)85 **)45 90 150
**)65 **)35 70 125
50 25 55 125
40 20 45 100
25 15 30 90 *)Non-combustible screeds and floor finishes may be included in these dimensions. **)Supplementary reinforcement may be necessary to hold the concrete cover in position (see 7.3).