Outofplane eccentricity General

The out-of-plane eccentricity of loading on walls:

- shall be assessed,

- may be calculated:

- from the material properties given in Section 3,

- the joint behaviour,

- and from the principles of structural mechanics.

A simplified method is given in Annex C:

Assumptions:

- the joint between the wall and the floor may be simplified, by using uncracked cross sections;

- elastic behaviour of the materials;

A frame analysis or a single joint analysis may be used.

Joint analysis may be simplified as shown in figure C.1

Elastic Behaviour End Joint

Figure C.1: Simplified frame diagram

For less than four members, those not existing should be ignored.

The ends of the members remote from the junction should be taken as fixed, unless they are known to take no moment at all, when they may be taken to be hinged.

Note:

The simplified frame model is not considered to be appropriate, where timber floor joists are used.

(For such cases refer to paragraph (4) below).

The moment in member 1, Mi, may be calculated from equation (C.1): (the moment in member 2, M2, similarly but using E2I2/h2 instead of E1I1/h1 in the numerator)

nE1I1

W3I2 W4I4

12 12

h1 h2 l3 l4

Where:

n is the member stiffness factor, taken as 4 for members fixed at both ends and otherwise 3;

En is the modulus of elasticity of member n, where n = 1, 2, 3 or 4

Ij is the second moment of area of member j, where j = 1, 2, 3 or 4

(in the case of a cavity wall in which only one leaf is loadbearing, Ij should be taken as that of the loadbearing leaf only);

hi is the clear height of member 1;

h2 is the clear height of member 2;

13 is the clear span of member 3;

14 is the clear span of member 4;

w3 is the design uniformly distributed load on member 3, using the partial safety factors from table 2.2, unfavourable effect;

w4 is the design uniformly distributed load on member 4, using the partial safety factors from table 2.2, unfavourable effect.

(2) The results of such calculations will usually be conservative because to the true fixity, ie. the ratio of the actual moment transmitted by a joint to that which would exist if the joint was fully rigid, of the floor/wall junction cannot be achieved.

It will be permissible for use in design to reduce the eccentricity, obtained from the calculations above, by multiplying it by (1-k/4), provided that:

- the design vertical stress acting at the junction in question

is greater than 0,25 N/mm2

when averaged across the thickness of the wall

- and k is not taken to be greater than 2;

1

<3

'4

E1I1

, E2I2

hi

h2

(3) If the eccentricity calculated in accordance with paragraph (2) above is greater than 0,4 times the thickness of the wall, or the design vertical stress is 0,25 N/mm2 or less, the design may be based on paragraph (4) below.

(4) The eccentricity of loading to be used in design may be based on the design load being resisted by the minimum required bearing depth, but not based on a bearing depth of more than 0,2 times the wall thickness, at the face of the wall, stressed to the appropriate design strength of the material (see figure C.2);

this will be appropriate, particularly, at the roof.

Eccentricities Structures

Figure C.2: Eccentricity obtained from design load resisted by stress block

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  • rowan
    What is eccentricity in structural design?
    5 years ago

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