STRUCTURAL DESIGN FOR ARCHITECTURE

Table A2.4 Basic permissible stresses for steel
[suitable for preliminary sizing of elements only.
Not to be used for final design calculations]
(BS 449)

Stress type Basic permissible stress {Grade 43 Steel)

Tension 155 N/mm
Compression 155 N/mm
Bending 165 N/mm
Shear (average) 100 N/mm
Bearing 190 N/mm

Appendix 2

Shear stress is only likely to be critical in cases
of very heavy loading on relatively short spans.
The deflection of the beam can be checked
using the relevant formula from Table A2.3.
The critical requirement is that the maximum
deflection under the action of imposed load
only should not exceed span/360. It is possible
for this limit to be exceeded even if the section
selected is adequately strong, especially if the
load is relatively light and the span long.
Where this occurs a larger size of section than
that required to satisfy the strength criterion
must be selected.

A2.4.3 Columns
Unless there is reason to believe that they will
carry a significant amount of bending moment
columns should be regarded as axially loaded
for the purpose of approximate sizing. For the
approximate sizing of steel structures a trial-
and-error procedure similar to that outlined in
Section A2.3.4 is used and consists of:

1 The selection of a trial cross-section. If the

column is axially loaded and the conditions

of lateral restraint are the same for all

potential buckling planes, the best section

shapes are those which have similar

bending strength about all their principal

axes. The H-shaped universal column (Table

3.3) or square or circular hollow sections

therefore perform best in this situation.

Section shapes in which the bending

strength about one principal axis is signifi-

cantly greater than about the other, such as

the I-shaped universal beam, should be

used if the column is subjected to a combi-

nation of bending and axial load or if the

conditions of lateral restraint are signifi-

cantly different for different planes.

A preliminary estimate of the size of

section which will be required can be

obtained by dividing the axial load by an

estimate of the final value of the permis-

sible stress. Unless the column is very

lightly loaded this will normally be in the

upper third of Table A2.5.

The calculation of the slenderness ratio of

the trial column. As was discussed in

Section A2.3.4, a slenderness ratio applies

to a particular plane of potential buckling. If

the trial cross-section is not symmetrical

and/or the conditions of lateral restraint of

the column are different in different poten-

tial buckling planes the slenderness ratios

for these planes, will also be different. The

permissible compressive stress is deter-

mined by the highest value of slenderness

ratio which is calculated.

For a given potential plane of buckling

the slenderness ratio is calculated from:

slenderness ratio = Le/r

Le is the effective length of the column for

the plane concerned. This is determined

principally by the distance between points

at which the column is restrained against

lateral movement (normally the storey

height of the building). It is affected by the

conditions of restraint at these locations,

however, and in particular with whether or

not any restraint is provided against

rotation. This must be assessed and the

effective length adjusted in accordance with

Table A2.6. In most steel frameworks the

effective length is between 0.7 and 1.0 of the

distance between lateral restraints.

r is the radius of gyration of the cross-

section about the axis which is at right

angles to the potential plane of buckling.

Radii of gyration of cross-sections are given

in Steel Section Tables (see Tables 3.2 and

3.3). 259