STRUCTURAL DESIGN FOR ARCHITECTURE

Structural Design for Architecture

Fig. 3.47 Typical layout of floor beams in a steel framework

(a) The basic unit consists of a series of secondary beams

in a parallel arrangement at equal spacing. The spacing

depends on the span capability of the floor-slab system.

The secondary beams are used in conjunction with primar

beams to give a column grid which is more-or-less square.

(b) Variations to the basic arrangement are used to accom

modate space-planning requirements.

Fig. 3.48 A common variation on the basic floor grid

arrangement is the provision of columns for each

secondary beam at the building' perimeter. The closely

spaced columns eliminate the need for a separate cladding

support system.

the need for a secondary system of mullions in

the external wall on which to mount the

cladding.

The types of steel sections which are used

for the elements in the floor grids depend on

the span and on the intensity of the load. For

grids in the span range of 4mX6mto6mX

12 m, carrying office-type loading, universal

beam or castellated beam sections are

normally adopted; an indication of the depths

of beam which are required is given in Table

3.10.

For economy in construction and simplicity

in the detailing of the structure, standardisa-

tion of elements is desirable. This is facilitated

if the spans of the principal elements are kept

constant. Where beams with different strengths

are required, a constant floor depth is

sometimes achieved without compromising

efficiency by the use of different weights of

beam within a given serial size.

Where a larger column grid is required than

is possible with standard sections, that is a

grid with dimensions in excess of around 20 m,

various forms of built-up sections can be used

to provide stronger beams. Plate girders and

compound beams allow the span range to be

extended but for very large spans lattice

girders are required. In cases where very long

spans must be achieved it is frequently advan-

tageous to maintain a basic floor grid, based

on standard sections, and use a tertiary system

of girders to achieve the long span (Fig. 3.49).

The depth of long-span structures tends to

be large and various measures can be adopted

90 to minimise the amount of unusable volume

Primary beam

Secondary

beam

Normally around 8 m

(maximum 30 m)

Normally around 3 m

(maximum 6m)