STRUCTURAL DESIGN FOR ARCHITECTURE

Table A2.6 Effective lengths of steel compressive elements for different conditions of end restraint

(after BS 449)

Conditions of end restraint Effective length of element (L = distance between restraints)

Effectively held in position and restrained in direction at both ends 0.7L

Effectively held in position at both ends and restrained in 0.85L

direction at one end

Effectively held in position at both ends but not restrained in L

direction

Effectively held in position and restrained in direction at one end I.5L

and at the other partially restrained in direction but not held in

position

Effectively held in position and restrained in direction at one 2.0L

end but not held in position or restrained in direction at the

other end

Structural Design for Architecture

The internal geometry of triangulated girders

is arranged so that very small internal angles

(less then 30°) are avoided. The ideal arrange-

ment is one of equilateral triangles.

The individual sub-elements of triangulated

structures can be regarded as carrying either pure

axial tension or pure axial compression. The

magnitudes of the internal forces on any particu-

lar sub-element can be determined by using the

method of sections. This is a version of the 'imaginary

cut' technique. Once the magnitudes of the axial

forces in the sub-elements are known the size is

determined by using the techniques outlined in

Sections A2.4.2 or A2.4.3.

Axial internal forces are best resisted by cross-

section shapes which are symmetrical, such as

circular or square hollow sections. Angle and

channel sections are also commonly used.

The feasibility of a particular structural

proposal can be checked by sizing only the

most heavily loaded of the sub-elements. In

parallel-chord arrangements these are the

horizontal sub-elements at mid-span and the

inclined or vertical sub-elements close to the

supports.

A2.4.5 Elements subjected to combined

stress

The basic rule for elements which are

subjected simultaneously to more than one

type of internal force is that the following

equation must be satisfied:

fa/fpa + fb/fpb<1 (A2.12)

where: fa = actual axial stress

f pa = permissible axial stress

fb - actual bending stress

fpb = permissible bending stress

The equation must be satisfied at all locations

in the element. If the internal forces vary along

the length of the element it may be necessary

to check that the equation is satisfied at more

than one location.

A2.5 Reinforced concrete structures

In reinforced concrete structures the elements

are subjected to primary load actions which

are either of the bending type (beams and

slabs) or of the axial compression type

(columns and walls). In the case of bending-

type elements there are two considerations

which affect the overall sizes of the cross-

sections which must be adopted - the provi-

sion of adequate bending strength and the

prevention of excessive deflection. In the case

262 of compressive elements, the prevention of