(1) Value of slenderness ratio.- The slenderness ratio of a wall shall be taken as the ration of the effective height to the effective thickness, or that of the effective length to the actual thickness, whichever ratio is smaller:
Provided that the slenderness ratio of a free-standing wall shall be taken as the ratio of the effective height to the effective thickness.
(2) The slenderness ration of a column shall be taken as the ratio of the effective height in the direction under consideration to the lateral dimension of the column in the same direction, or to the diameter, were the column is circular.
(3) Effective length.- The effective length of a wall shall be taken as two comma five times the unsupported length for a wing-wall, and as the unsupported length for any other type of wall14.
(4)Effective height.- The effective height of walls and columns for the conditions of lateral restraint indicated in the Table shall be set out in Table XVII , where H is taken as-
a) for a bearing-wall or curtain-wall, the height of the storey;
b) for a panel-wall or a partition-wall or a partition-wall or a column, the actual height from the top of the floor construction, collar-beam or frame-work above;
c) for a free-standing wall or a wing-wall, the actual height.
(5) Effective thickness.- The effective thickness of a solid wall, without piers, shall be the actual thickness of such wall.
(6) The effective thickness of a solid wall stiffened by piers bonded shall be obtained by multiplying the actual thickness of such wall by the factor given for the dimensions and spacing of the piers in Table XVII if the width of the pier parallel to the length of the wall is not less than twice the actual thickness of the wall, tw, and the total thickness of the pier (including the thickness of the wall), tp , is not less than 2tw:
Provided that bonded intersecting-walls of a thickness not less than tw may be regarded as piers of effective pier-thickness tp14 equal to half the unbroken length of the intersecting-wall plus the thickness of the stiffened wall.
TABLE XVII
EFFECTIVE HEIGHT OF WALLS AND COLUMNS
TABLE XVIII
FACTORS FOR EFFECTIVE THICKNESS OF STIFFENED MASONRY AND CONCRETE WALLS
(Effective thickness= actual thickness x factor)
13 The minimum forces for which provision must in all cases be made are set out in Chapter 3.
14 See definition of “length of wall”.
(7)When the thickness of a wall is changed between the bottom and the top of a storey or of a wall, its effective thickness shall be calculated by multiplying the thickness of each section of constant thickness by its heights and dividing the sum of their products by the total height of the wall.
(8) The effective thickness of an unstiffened cavity-wall shall be found from the expression
te = 2/3 (to – w), where-
te = effective thickness; and
to = overall thickness; and
w = width of cavity.
(9) The effective of a cavity-wall stiffened by piers shall be taken as the effective thickness of the unstiffened wall multiplied by the factors specified in Table XVIII for the size and spacing of piers concerned:
Provided that-
i) the over-all thickness of the wall, to, shall be used in place of tw in determining the minimum size and spacing of piers; and
ii) piers are bonded to one leaf of the wall.
(10) Where, in a cavity-wall, one leaf is designed to carry total superimposed load, the permissible stress for that leaf shall-
a) be based on the slenderness ration calculated from the effective thickness of the whole wall; or
b) be take as the actual thickness of the load-bearing leaf;
whichever gives the smaller value of the slenderness ratio.
(11) Permissible limits of the slenderness ratio.- The slenderness ratio of walls and columns shall not exceed the value given for the relevant conditions in Table XIX .
TABLE XIX
PERMISSIBLE LIMITS OF SLENDERNESS RATIO