Annex E(Clause 24.2)

NBC 2016 (SP 7:2016) Volume 1

2016 · PDF page 877

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Effective Length of Columns

E-1 In the absence of more exact analysis, the effective length of columns in framed structures may be obtained from the ratio of effective length to unsupported length $l_{\mathrm{ef}} / l$ given in Fig. 32 when relative displacement of the ends of the column is prevented and in Fig. 33 when relative lateral displacement of the ends is not prevented. In the latter case, it is recommended that the effective length ratio $l_{\mathrm{ef}} / l$ may not be taken to be less than 1.2.

NOTES

1 Figures 31 and 32 are reproduced from ‘The Structural Engineer’ No. 7, Volume 52, July 1974 by the permission of the Council of the Institution of Structural Engineers, U.K.

2 In Figs. 31 and 32, $\beta_{1}$ and $\beta_{2}$ are equal to:

\frac {\sum K _ {\mathrm{c}}}{\sum K _ {\mathrm{c}} + \sum K _ {\mathrm{b}}}

where the summation is to be done for the members framing into a joint at top and bottom respectively; and $K_{\mathrm{c}}$ and $K_{\mathrm{b}}$ being the flexural stiffness for column and beam, respectively.

E-2 To determine whether a column is a no sway or a

sway column, stability index $Q$ may be computed as given below:

Q = \frac {\sum P _ {\mathrm{u}} \varDelta_ {\mathrm{u}}}{H _ {\mathrm{u}} h _ {\mathrm{s}}}

where

$\sum P_{\mathrm{u}} =$ sum of axial loads on all column in the storey,

$\Delta_{\mathrm{u}} =$ elastically computed first order lateral deflection,

$H_{\mathrm{u}} =$ total lateral force acting within the storey, and

$h_{\mathrm{s}} =$ height of the storey.

If $Q \leq 0.04$ , then the column in the frame may be taken as no sway column, otherwise the column will be considered as sway column.

E-3 For normal usage assuming idealized conditions, the effective length $l_{\mathrm{ef}}$ of the compression member in a given plane may be assessed on the basis of Table 32.

Chart

FIG. 32 EFFECTIVE LENGTH RATIOS FOR A COLUMN IN A FRAME WITH No SWAY

Chart

FIG. 33 EFFECTIVE LENGTH RATIOS FOR A COLUMN IN A FRAME WITHOUT RESTRAINT AGAINST SWAY

PART 6 STRUCTURAL DESIGN — SECTION 5 CONCRETE: 5A PLAIN AND REINFORCED CONCRETE

Table 32Effective Length of Compression Members (Clause E-3)

Sl No.(1)	Degree of End Restraint of Compression Members(2)	Symbol(3)	Theoretical Value of Effective Length(4)	Recommended Value of Effective Length(5)
i)	Effectively held in position and restrained against rotation at both ends		0.50 l	0.65 l
ii)	Effectively held in position at both ends, restrained against rotation at one end		0.70 l	0.80 l
iii)	Effectively held in position at both ends, but not restrained against rotation		1.00 l	1.00 l
iv)	Effectively held in position and restrained against rotation at one end, and at the other restrained against rotation but not held in position		1.00 l	1.20 l
v)	Effectively held in position and restrained against rotation at one end, and at the other partially restrained against rotation but not held in position		-	1.50 l
vi)	Effectively held in position at one end but not restrained against rotation, and at the other end restrained against rotation but not held in position		2.00 l	2.00 l
vii)	Effectively held in position and restrained against rotation at one end but not held in position nor restrained against rotation at the other end		2.00 l	2.00 l
	NOTE - l is the unsupported length of compression member.

(Clause 25.1.2) Ductile Design and Detailing of Reinforced Concrete Structures Subjected to Seismic Forces

F-1 General

F-1.1 This annex covers the requirements for designing and detailing of members of reinforced concrete (RC) structures designed to resist lateral effects of earthquake shaking, so as to give them adequate stiffness, strength and ductility to resist severe earthquake shaking without collapse. This annex addresses lateral load resisting structural systems of RC structures composed of,

a) RC moment resisting frames;

b) RC moment resisting frames with unreinforced masonry infill walls;

c) RC moment resisting frames with RC structural walls; and

d) RC structural walls.

F-1.2 Provisions of this annex shall be adopted in all lateral load resisting systems of RC structures located in Seismic Zone III, IV or V. The annex is optional in Seismic Zone II.

F-1.3 The provisions for RC structures given herein apply specifically to monolithic RC construction, and not for precast RC structures. Precast and/or prestressed concrete members may be used, only if they are designed to provide similar level of ductility as that of monolithic RC structures during or after an earthquake. Likewise, flat slab structures shall have a lateral load resisting system capable of providing similar level of performance as envisioned in this Section and shall be designed for drift compatibility as per F-10. Specialist literature shall be referred to for design and construction of such structures. The adequacy of such designs shall be demonstrated by adequate, appropriate experimentation and non-linear dynamic structural analyses.

F-1.4 All RC frames, RC walls and their elements in a structure need not be designed to resist lateral loads and the designer can judiciously identify the lateral load resisting system based on relative stiffness and location in the building and design those members for full lateral force. RC monolithic members assumed not to participate in the lateral force resisting system (see F-2.7) shall be permitted provided that their effect on the seismic response of the system is accounted for. Consequence of failure of structural and non-structural members not part of the lateral force resisting system shall also be considered in design.