Use of BS 5950 - Part 1

BS5950 specifies rules for ensuring steel structures are safe

The current code of practice called "Structural Use of Steelwork in Building" is BS 5950 Part 1. This code gives specific guidance on the strength and stiffness of steel structures for buildings to allow numerical calculations to be made.

Beam capacity is checked by comparing the ultimate strength with factored loadings.

It checks the strength of a structure by ensuring ultimate strength is not less than working load x load factor

The load factor varies with the type of load.

Different load factors are applied to different types of load and load combinations. This reflects the varying degree of confidence in the values of dead, imposed and wind loads used. Values of the load factors to be used are given in Table 2 of BS 5950.

The material strength is specified in relation to steel grade.

The ultimate strength is dependent on yield stress. Stresses are given for three grades of steel called S275, S (These were formerly referred to as Grades 43, 50 and 55). Grade S275 (formerly Grade 43) is commonly used, although S355 is popular on larger projects where it can offer significant economies. Higher grades are rarely used, except for bridges, and special applications

The yield stresses py corresponding to the different grades are given in BS 5950.

Specific guidance is given for determining the maximum moment and shear capacity of a beam cross-section.

The maximum moment capacity is the lesser of:

Mc = py.Sx

and

Mc = 1.2 py.Zx

Sx is the plastic section modulus and Zx is the elastic section modulus.

The maximum shear capacity is

Pv = 0.6 py Av

where Av is the shear area.

Cross-sections are classified according to their proportions.

Open sections are classified as plastic, compact, semi-compact or slender. The classification depends on the proportions of the webs and flanges. (Note that rolled sections are seldom classified as slender).

Shear area for beam sections

Shear area for beam sections

 

The moment capacity of some sections may be reduced if shear forces are high.

The actual shear force (multiplied by the factor) is referred to by the symbol Fv. If Fv exceeds 60% of the shear capacity of the cross-section, Pv, then this is defined as a "high" shear load and the moment capacity for plastic and compact sections is reduced.

For beams which are not fully restrained, bending strength may be reduced due to lateral-torsional buckling; this is related to the slenderness ratio and cross-section details of the beam.

If the compression side of a beam is not fully restrained against lateral torsional buckling then the design stress py is reduced. This reduction depends on two factors called the slenderness ratio and the D/T ratio.

The slenderness ratio l is given by:

l = LE/ry

LE is the effective length (Table 9 of BS5950).

ry is the radius of gyration = the square root of (I/A). Standard tables give values for ry. The slenderness ratio may be reduced using the slenderness correction factor n from table 20, to allow for the shape of actual bending moment.

The beam must be stiff enough to carry the working load without exceeding the deflection limits specified.

Limiting deflections are given in Table 8 of BS 5950 as a proportion of the beam span. These limiting values are compared with calculated deflections taking account of the load, length, support conditions, and cross-section of the beam. In calculating deflections it is normal to ignore any permanent loads, and to consider conditions in service (ie without any load factors)

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